@article{tran_paskova_darakchieva_paskov_2023, title={On the thermal conductivity anisotropy in wurtzite GaN}, volume={13}, ISSN={["2158-3226"]}, DOI={10.1063/5.0167866}, abstractNote={GaN-based power devices operating at high currents and high voltages are critically affected by the dissipation of Joule heat generated in the active regions. Consequently, knowledge of GaN thermal conductivity is crucial for effective thermal management, needed to ensure optimal device performance and reliability. Here, we present a study on the thermal conductivity of bulk GaN in crystallographic directions parallel and perpendicular to the c-axis. Thermal conductivity measurements are performed using the transient thermoreflectance technique. The experimental results are compared with a theoretical calculation based on a solution of the Boltzmann transport equation within the relaxation time approximation and taking into account the exact phonon dispersion. All factors that determine the thermal conductivity anisotropy are analyzed, and the experimentally observed small anisotropy factor is explained.}, number={9}, journal={AIP ADVANCES}, author={Tran, Dat Q. and Paskova, Tania and Darakchieva, Vanya and Paskov, Plamen P.}, year={2023}, month={Sep} }
 @article{tran_delgado-carrascon_muth_paskova_nawaz_darakchieva_paskov_2021, title={Erratum: "Phonon-boundary scattering and thermal transport in AlxGa1<bold>-</bold>xN: Effect of layer thickness" [Appl. Phys. Lett. <bold>117</bold>, 252102 (2020)]}, volume={118}, ISSN={["1077-3118"]}, DOI={10.1063/5.0054625}, abstractNote={First Page}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Tran, Dat Q. and Delgado-Carrascon, Rosalia and Muth, John F. and Paskova, Tania and Nawaz, Muhammad and Darakchieva, Vanya and Paskov, Plamen P.}, year={2021}, month={May} }
 @article{blumenschein_kadlec_romanyuk_paskova_muth_kadlec_2020, title={Dielectric and conducting properties of unintentionally and Sn-doped beta-Ga2O3 studied by terahertz spectroscopy}, volume={127}, ISSN={["1089-7550"]}, DOI={10.1063/1.5143735}, abstractNote={Dielectric and conducting properties of unintentionally doped bulk and Sn-doped thin film β-Ga2O3 samples were studied using time-domain terahertz spectroscopy. Complex permittivity and optical conductivity spectra from 0.25 to 2.5 THz were obtained experimentally over a broad temperature range. The low-temperature spectra of the unintentionally doped sample were fit using a model involving two oscillators. The parameters of one of them show an unusual temperature dependence, in particular, a pronounced increase in the oscillator strength upon heating above 50 K. This is interpreted as an absorption due to thermally activated charge carriers moving in localized potential minima linked to the unintentional doping. Upon heating, the influence of this optical conductivity mechanism strongly increases, and the sample becomes opaque in the THz range near 100 K. The nanocrystalline Sn-doped Ga2O3 thin film sample exhibits a much higher optical conductivity than the unintentionally doped bulk sample, and its spectra are remarkably stable over a broad temperature range (4–750 K). This first study of β-Ga2O3 based on phase-sensitive THz spectroscopy reveals how the impurities influence the high-frequency conductive properties of the material.}, number={16}, journal={JOURNAL OF APPLIED PHYSICS}, author={Blumenschein, Nick and Kadlec, Christelle and Romanyuk, Oleksandr and Paskova, Tania and Muth, John F. and Kadlec, Filip}, year={2020}, month={Apr} }
 @article{tran_delgado-carrasco_muth_paskova_nawaz_darakchieva_paskov_2020, title={Phonon-boundary scattering and thermal transport in AlxGa1-xN: Effect of layer thickness}, volume={117}, ISSN={["1077-3118"]}, DOI={10.1063/5.0031404}, abstractNote={Thermal conductivity of AlxGa1−xN layers with 0≤x≤0.96 and variable thicknesses is systematically studied by combined thermoreflectance measurements and a modified Callaway model. We find a reduction in the thermal conductivity of AlxGa1−xN by more than one order of magnitude compared to that of GaN, which indicates a strong effect of phonon-alloy scattering. It is shown that the short-mean free path phonons are strongly scattered, which leads to a major contribution of the long-mean free path phonons to the thermal conductivity. In thin layers, the long-mean free path phonons become efficiently scattered by the boundaries, resulting in a further decrease in the thermal conductivity. Also, an asymmetry of thermal conductivity as a function of Al content is experimentally observed and attributed to the mass difference between Ga and Al host atoms.}, number={25}, journal={APPLIED PHYSICS LETTERS}, author={Tran, Dat Q. and Delgado-Carrasco, Rosalia and Muth, John F. and Paskova, Tania and Nawaz, Muhammad and Darakchieva, Vanya and Paskov, Plamen P.}, year={2020}, month={Dec} }
 @article{blumenschein_paskova_muth_2019, title={Effect of Growth Pressure on PLD-Deposited Gallium Oxide Thin Films for Deep-UV Photodetectors}, volume={216}, ISSN={["1862-6319"]}, DOI={10.1002/pssa.201900098}, abstractNote={Pulsed laser deposition (PLD) is used to grow ‐oriented single‐crystalline β‐gallium oxide (β‐Ga2O3) thin films on c‐plane sapphire substrates by optimized growth temperature and pressure. The morphology and crystallinity of the thin films are examined using X‐ray diffraction and atomic force microscopy. The thin films are used as the semiconductor layer for metal–semiconductor–metal (MSM) photodetector (PD) devices with various electrode designs. The ultraviolet photodetectors are characterized under 250 nm illumination, showing a high current amplitude increase over dark current conditions that approaches three orders of magnitude at a 6 V bias for an optimized growth pressure of 1 × 10−3 torr. The photodetectors' transient response is also measured, allowing for the defect analysis to be performed. A peak spectral responsivity of 30.45 A W−1 is measured at 250 nm incident illumination.}, number={20}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Blumenschein, Nicholas and Paskova, Tania and Muth, John F.}, year={2019}, month={Oct} }
 @article{gleco_romanyuk_gordeev_kuldova_paskova_ivanisevic_2019, title={Modification of the Surface Properties of AlxGa1-xN Substrates with Gradient Aluminum Composition Using Wet Chemical Treatments}, volume={4}, ISSN={["2470-1343"]}, DOI={10.1021/acsomega.9b01467}, abstractNote={The surface properties of biomolecular gradients are widely known to be important for controlling cell dynamics, but there is a lack of platforms for studying them in vitro using inorganic materials. The changes in various surface properties of an AlxGa1–xN film (0.173 ≤ x ≤ 0.220) with gradient aluminum content were quantified to demonstrate the ability to modify interfacial characteristics. Four wet chemical treatments were used to modify the surface of the film: (i) oxide passivation by hydrogen peroxide, (ii) two-step functionalization with a carboxylic acid following hydrogen peroxide pretreatment, (iii) phosphoric acid etch, and (iv) in situ functionalization with a phosphonic acid in phosphoric acid. The characterization confirmed changes in the topography, nanostructures, and hydrophobicity after chemical treatment. Additionally, X-ray photoelectron spectroscopy was used to confirm that the chemical composition of the surfaces, in particular, Ga2O3 and Al2O3 content, was dependent on both the chemical treatment and the Al content of the gradient. Spectroscopic evaluation showed red shifts in strain-sensitive Raman peaks as the Al content gradually increased, but the same peaks blue-shifted after chemical treatment. Kelvin probe force microscopy measurements demonstrated that one can modify the surface charge using the chemical treatments. There were no predictable or controllable surface charge trends because of the spontaneous oxide-based nanostructured formations of the bulk material that varied based on treatment and were defect-dependent. The reported methodology and characterization can be utilized in future interfacial studies that rely on water-based wet chemical functionalization of inorganic materials.}, number={7}, journal={ACS OMEGA}, author={Gleco, Sara and Romanyuk, Oleksandr and Gordeev, Ivan and Kuldova, Karla and Paskova, Tania and Ivanisevic, Albena}, year={2019}, month={Jul}, pages={11760–11769} }
 @article{adams_vinueza_romanyuk_gordeev_paskova_ivanisevic_2019, title={Nanostructured GaOOH modified with reactive yellow, red and blue water-soluble dyes}, volume={9}, ISSN={["2158-3226"]}, DOI={10.1063/1.5080353}, abstractNote={Water soluble reactive dyes are used to modify nanostructured GaOOH. The resulting particles showed excellent stability in water solutions. The materials were characterized by Scanning electron microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) to assess changes due to the dye functionalization. SEM revealed changes in size after dye modification. XPS confirmed the presence of the dyes on the nanostructured materials and assessed changes in functional groups due to use of different type of modification and concentrations of dyes. The reported approach to stabilize the nanostructured GaOOH provides a simple and environmentally friendly route to tune the properties of wide band gap semiconductor materials.}, number={2}, journal={AIP ADVANCES}, author={Adams, W. Taylor and Vinueza, Nelson R. and Romanyuk, Oleksandr and Gordeev, Ivan and Paskova, Tania and Ivanisevic, Albena}, year={2019}, month={Feb} }
 @article{blumenschein_moser_heller_miller_green_popp_crespo_leedy_lindquist_moule_et al._2020, title={Self-Heating Characterization of $\beta$ -Ga2O3 Thin-Channel MOSFETs by Pulsed ${I}$ - ${V}$ and Raman Nanothermography}, volume={67}, ISSN={["1557-9646"]}, DOI={10.1109/TED.2019.2951502}, abstractNote={$\beta $ -Ga2O3 thin-channel MOSFETs were evaluated using both dc and pulsed ${I}$ – ${V}$ measurements. The reported pulsed ${I}$ – ${V}$ technique was used to study self-heating effects in the MOSFET channel. The device was analyzed over a large temperature range of 23 °C–200 °C. A relationship between dissipated power and channel temperature was established, and it was found that the MOSFET channel was heating up to 208 °C when dissipating 2.5 W/mm of power. The thermal resistance of the channel was found to be 73 °C-mm/W. The results are supported with the experimental Raman nanothermography and thermal simulations and are in reasonable agreement with pulsed ${I}$ – ${V}$ findings. The high thermal resistance underpins the importance of optimizing thermal management in future Ga2O3 devices.}, number={1}, journal={IEEE TRANSACTIONS ON ELECTRON DEVICES}, author={Blumenschein, Nicholas A. and Moser, Neil A. and Heller, Eric R. and Miller, Nicholas C. and Green, Andrew J. and Popp, Andreas and Crespo, Antonio and Leedy, Kevin and Lindquist, Miles and Moule, Taylor and et al.}, year={2020}, month={Jan}, pages={204–211} }
 @article{tran_blumenschein_mock_sukkaew_zhang_muth_paskova_paskov_darakchieva_2020, title={Thermal conductivity of ultra-wide bandgap thin layers - High Al-content AlGaN and beta-Ga2O3}, volume={579}, ISSN={["1873-2135"]}, DOI={10.1016/j.physb.2019.411810}, abstractNote={Transient thermoreflectance (TTR) technique is employed to study the thermal conductivity of β-Ga2O3 and high Al-content AlxGa1-xN semiconductors, which are very promising materials for high-power device applications. The experimental data are analyzed with the Callaway's model taking into account all relevant phonon scattering processes. Our results show that out-of-plane thermal conductivity of high Al-content AlxGa1-xN and (−201) β-Ga2O3 is of the same order of magnitude and approximately one order lower than that of GaN or AlN. The low thermal conductivity is attributed to the dominant phonon-alloy scattering in AlxGa1-xN and to the strong Umklapp phonon-phonon scattering in β-Ga2O3. It is also found that the phonon-boundary scattering is essential in thin β-Ga2O3 and AlxGa1-xN layers even at high temperatures and the thermal conductivity strongly deviates from the common 1/T temperature dependence.}, journal={PHYSICA B-CONDENSED MATTER}, author={Tran, Dat Q. and Blumenschein, Nicholas and Mock, Alyssa and Sukkaew, Pitsiri and Zhang, Hengfang and Muth, John F. and Paskova, Tania and Paskov, Plamen P. and Darakchieva, Vanya}, year={2020}, month={Feb} }
 @article{snyder_davis_berg_pearce_romanyuk_jiricek_paskova_ivanisevic_2019, title={Passivation of semipolar (10-1-1) GaN with different organic adsorbates}, volume={236}, ISSN={["1873-4979"]}, DOI={10.1016/j.matlet.2018.10.109}, abstractNote={Expanding the functionality of electronic materials is often reliant on the attachment of organic molecules to the surface. In this study, several methods for passivating the surface of semipolar (10-1-1) GaN with different chemistries were examined; this included simple physisorption, in-situ functionalization using phosphonic acids, and a two-step carboxylic acid attachment with a peroxide pretreatment. The binding of a variety of different organic adsorbates on semipolar substrates was quantified using X-ray photoelectron spectroscopy (XPS). Our results confirm binding at the organic-inorganic interface through the presence of amine/amide species on the surface. The wide variety of organic molecules, and their binding to inorganic electronic materials, can be exploited to impart specific functionalities in technologies spanning fields of energy, biomedical science, and chemical sensing.}, journal={MATERIALS LETTERS}, author={Snyder, Patrick J. and Davis, Hailey and Berg, Nora G. and Pearce, Brady and Romanyuk, Oleksandr and Jiricek, Petr and Paskova, Tania and Ivanisevic, Albena}, year={2019}, month={Feb}, pages={201–204} }
 @article{blumenschein_slomski_paskov_kaess_breckenridge_muth_paskova_2018, title={Thermal conductivity of bulk and thin film beta-Ga2O3 measured by the 3 omega technique}, volume={10533}, ISSN={["1996-756X"]}, DOI={10.1117/12.2288267}, abstractNote={Thermal conductivity of undoped and Sn-doped β-Ga2O3 bulk and single-crystalline thin films have been measured by the 3ω technique. The bulk samples were grown by edge-defined film-field growth (EFG) method, while the thin films were grown on c-plane sapphire by pulsed-laser deposition (PLD). All samples were with (-201) surface orientation. Thermal conductivity of bulk samples was calculated along the in-plane and cross-plane crystallographic directions, yielding a maximum value of ~ 29 W/m-K in the [010] direction at room temperature. A slight thermal conductivity decrease was observed in the Sn-doped bulk samples, which was attributed to enhanced phonon-impurity scattering. The differential 3ω method was used for β-Ga2O3 thin film samples due to the small film thickness. Results show that both undoped and Sndoped films have a much lower thermal conductivity than that of the bulk samples, which is consistent with previous reports in the literature showing a linear relationship between thermal conductivity and film thickness. Similarly to bulk samples, Sn-doped thin films have exhibited a thermal conductivity decrease. However, this decrease was found to be much greater in thin film samples, and increased with Sn doping concentration. A correlation between thermal conductivity and defect/dislocation density was made for the undoped thin films.}, journal={OXIDE-BASED MATERIALS AND DEVICES IX}, author={Blumenschein, N. and Slomski, M. and Paskov, P. P. and Kaess, F. and Breckenridge, M. H. and Muth, J. F. and Paskova, T.}, year={2018} }
 @article{slomski_blumenschein_paskov_muth_paskova_2017, title={Anisotropic thermal conductivity of beta-Ga2O3 at elevated temperatures: Effect of Sn and Fe dopants}, volume={121}, ISSN={["1089-7550"]}, DOI={10.1063/1.4986478}, abstractNote={The thermal conductivity of undoped, Sn-doped, and Fe-doped β-Ga2O3 bulk crystals was measured by the 3ω technique in the temperature range of 295–410 K. A unique approach for extracting the thermal conductivity along the lateral and transverse heat flow directions was used in order to determine the thermal conductivity along different crystallographic directions. The data analysis at room temperature confirmed the expected anisotropy of the thermal conductivity of β-Ga2O3, revealing the highest value of ∼29 W/m K in the [010] direction. The thermal conductivity of the Sn-doped and Fe-doped β-Ga2O3 samples was found to be lower than that of the undoped samples due to the enhanced phonon-impurity scattering contribution, which reduces the thermal conductivity. This tendency was maintained for the thermal conductivity at elevated temperatures. The thermal conductivity in all samples decreased with increasing temperature, but the slope of the temperature dependence was found to depend on both the doping and the crystallographic orientation.}, number={23}, journal={JOURNAL OF APPLIED PHYSICS}, author={Slomski, M. and Blumenschein, N. and Paskov, P. P. and Muth, J. F. and Paskova, T.}, year={2017}, month={Jun} }
 @article{paskov_slomski_leach_muth_paskova_2017, title={Effect of Si doping on the thermal conductivity of bulk GaN at elevated temperatures - theory and experiment}, volume={7}, ISSN={["2158-3226"]}, DOI={10.1063/1.4989626}, abstractNote={The effect of Si doping on the thermal conductivity of bulk GaN was studied both theoretically and experimentally. The thermal conductivity of samples grown by Hydride Phase Vapor Epitaxy (HVPE) with Si concentration ranging from 1.6×1016 to 7×1018 cm-3 was measured at room temperature and above using the 3ω method. The room temperature thermal conductivity was found to decrease with increasing Si concentration. The highest value of 245±5 W/m.K measured for the undoped sample was consistent with the previously reported data for free-standing HVPE grown GaN. In all samples, the thermal conductivity decreased with increasing temperature. In our previous study, we found that the slope of the temperature dependence of the thermal conductivity gradually decreased with increasing Si doping. Additionally, at temperatures above 350 K the thermal conductivity in the highest doped sample (7×1018 cm-3) was higher than that of lower doped samples. In this work, a modified Callaway model adopted for n-type GaN at high temperatures was developed in order to explain such unusual behavior. The experimental data was analyzed with examination of the contributions of all relevant phonon scattering processes. A reasonable match between the measured and theoretically predicted thermal conductivity was obtained. It was found that in n-type GaN with low dislocation densities the phonon-free-electron scattering becomes an important resistive process at higher temperatures. At the highest free electron concentrations, the electronic thermal conductivity was suggested to play a role in addition to the lattice thermal conductivity and compete with the effect of the phonon-point-defect and phonon-free-electron scattering.}, number={9}, journal={AIP ADVANCES}, author={Paskov, P. P. and Slomski, M. and Leach, J. H. and Muth, J. F. and Paskova, T.}, year={2017}, month={Sep} }
 @article{bartos_romanyuk_paskova_jiricek_2017, title={Electron band bending and surface sensitivity: X-ray photoelectron spectroscopy of polar GaN surfaces}, volume={664}, ISSN={["1879-2758"]}, DOI={10.1016/j.susc.2017.07.003}, abstractNote={The role of electron band bending and surface sensitivity in determining the core level binding energies by X-ray photoelectron spectroscopy is investigated. A dominating contribution of surface atomic layers to photoemission intensity is confirmed for normal photoemission. The energy of the photoelectron core level peak does not deviate from core level peak energies of electrons photoemitted from the surface atomic layers of the crystal. The higher surface sensitivity regime, achieved e.g. at off-normal photoelectron detection angle, can be used to study the surface potential barrier in just a few topmost atomic layers. In addition, it is demonstrated that core level binding energy measured by angle-resolved X-ray photoelectron spectroscopy reflect the electron attenuation anisotropy. In particular, core level binding energy changes with emission angle and correlates with the forward focusing directions in a crystal. This effect is demonstrated by measuring the polar angle dependence of Ga 3d core levels on clean GaN(0001) and GaN(0001¯) surfaces with a higher and a lower band bending, respectively. The effect is explained by variation of emission depth in a crystal for normal and off-normal photoelectron emission angles.}, journal={SURFACE SCIENCE}, author={Bartos, I. and Romanyuk, O. and Paskova, T. and Jiricek, P.}, year={2017}, month={Oct}, pages={241–245} }
 @article{slomski_paskov_leach_muth_paskova_2017, title={Thermal conductivity of bulk GaN grown by HVPE: Effect of Si doping}, volume={254}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.201600713}, abstractNote={The thermal conductivity of bulk GaN grown by Hydride Phase Vapor Epitaxy with intentional Si doping was measured using the 3ω method. The effect of Si concentration ranging from 1.6 × 1016 to 7 × 1018 cm−3 on the thermal conductivity was studied over the temperature range of 295–470 K. The room temperature thermal conductivity was found to decrease with increasing Si doping from 245 to 210 W/m · K. Also, for each Si doped sample the thermal conductivity decreases with increasing temperature. The experimental data were analysed by a modified Callaway model and the contribution of different resistive phonon scattering process was examined. It was found that in n‐type GaN the phonon‐free‐electron scattering became an important resistive process that leads to a reduction of the thermal conductivity at high temperatures. At the highest free electron concentrations, electronic thermal conduction was found to play a role in addition to lattice thermal conduction and compete with the effects of phonon‐free‐electron scattering.}, number={8}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Slomski, Michael and Paskov, Plamen P. and Leach, Jacob H. and Muth, John F. and Paskova, Tania}, year={2017}, month={Aug} }
 @article{bartos_romanyuk_houdkova_paskov_paskova_jiricek_2016, title={Electron band bending of polar, semipolar and non-polar GaN surfaces}, volume={119}, ISSN={["1089-7550"]}, DOI={10.1063/1.4943592}, abstractNote={The magnitudes of the surface band bending have been determined by X-ray photoelectron spectroscopy for polar, semipolar, and non-polar surfaces of wurtzite GaN crystals. All surfaces have been prepared from crystalline GaN samples grown by the hydride-vapour phase epitaxy and separated from sapphire substrates. The Ga 3d core level peak shifts have been used for band bending determination. Small band bending magnitudes and also relatively small difference between the band bendings of the surfaces with opposite polarity have been found. These results point to the presence of electron surface states of different amounts and types on surfaces of different polarity and confirm the important role of the electron surface states in compensation of the bound surface polarity charges in wurtzite GaN crystals.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bartos, I. and Romanyuk, O. and Houdkova, J. and Paskov, P. P. and Paskova, T. and Jiricek, P.}, year={2016}, month={Mar} }
 @article{bartoš_romanyuk_houdkova_paskov_paskova_jiříček_2016, title={Erratum: “Electron band bending of polar, semipolar and non-polar GaN surfaces” [J. Appl. Phys. 119, 105303 (2016)]}, volume={119}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4947183}, DOI={10.1063/1.4947183}, abstractNote={First Page}, number={15}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Bartoš, I. and Romanyuk, O. and Houdkova, J. and Paskov, P. P. and Paskova, T. and Jiříček, P.}, year={2016}, month={Apr}, pages={159901} }
 @article{romanyuk_bartos_brault_de mierry_paskova_jiricek_2016, title={GaN quantum dot polarity determination by X-ray photoelectron diffraction}, volume={389}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsusc.2016.07.169}, abstractNote={Growth of GaN quantum dots (QDs) on polar and semipolar GaN substrates is a promising technology for efficient nitride-based light emitting diodes (LED). The QDs crystal orientation typically repeats the polarity of the substrate. In case of non-polar or semipolar substrates, the polarity of QDs is not obvious. In this article, the polarity of GaN QDs and of underlying layers was investigated nondestructively by X-ray photoelectron diffraction (XPD). Polar and semipolar GaN/Al0.5Ga0.5N heterostructures were grown on the sapphire substrates with (0001) and (11¯00) orientations by molecular beam epitaxy (MBE). Polar angle dependence of N 1s core-level photoelectron intensities were measured from GaN QDs and compared with the corresponding experimental curves from free-standing GaN crystals. It is confirmed experimentally, that the crystalline orientation of polar (0001) GaN QDs follows the orientation of the (0001) sapphire substrate. In case of semipolar GaN QDs grown on (11¯00) sapphire substrate, the (112¯2) polarity of QDs was determined.}, journal={APPLIED SURFACE SCIENCE}, author={Romanyuk, O. and Bartos, I. and Brault, J. and De Mierry, P. and Paskova, T. and Jiricek, P.}, year={2016}, month={Dec}, pages={1156–1160} }
 @article{qiu_caneau_paskova_2016, title={Preface}, volume={452}, ISSN={["1873-5002"]}, DOI={10.1016/s0022-0248(16)30490-0}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Qiu, Roger and Caneau, Catherine and Paskova, Tania}, year={2016}, month={Oct}, pages={VIII-VIII} }
 @article{freitas_paskova_bockowski_fujioka_2016, title={Preface: Bulk nitride workshop 2015}, volume={456}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2016.09.034}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Freitas, Jaime A., Jr. and Paskova, Tania and Bockowski, Michal and Fujioka, Hiroshi}, year={2016}, month={Dec}, pages={1–1} }
 @article{berg_paskova_ivanisevic_2017, title={Tuning the biocompatibility of aluminum nitride}, volume={189}, ISSN={["1873-4979"]}, DOI={10.1016/j.matlet.2016.11.041}, abstractNote={High-quality, electronic-grade, aluminum nitride thin films grown by reactive sputtering were studied in vitro. The semiconductor material showed high degree of stability in cell culture with very little Al leaching over time. Unlike other III-nitride materials, clean AlN does not promote the adhesion of cells to its surface. The work demonstrates that functionalization with peptides can be used to reverse this behavior. The presence of AlN in cell culture does not have any adverse effects on neurotypic cell behavior as confirmed by cell viability and reactive oxygen species assays.}, journal={MATERIALS LETTERS}, author={Berg, Nora G. and Paskova, Tania and Ivanisevic, Albena}, year={2017}, month={Feb}, pages={1–4} }
 @misc{pearce_wilkins_paskova_ivanisevic_2015, title={A review of in situ surface functionalization of gallium nitride via beaker wet chemistry}, volume={30}, ISSN={["2044-5326"]}, DOI={10.1557/jmr.2015.132}, abstractNote={Abstract}, number={19}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Pearce, Brady L. and Wilkins, Stewart J. and Paskova, Tania and Ivanisevic, Albena}, year={2015}, month={Oct}, pages={2859–2870} }
 @article{wilkins_paskova_reynolds_ivanisevic_2015, title={Comparison of the Stability of Functionalized GaN and GaP}, volume={16}, ISSN={["1439-7641"]}, DOI={10.1002/cphc.201500105}, abstractNote={AbstractSurface functionalization via 1 H,1 H,2 H,2H‐perfluoro octanephosphonic acid was done in the presence of phosphoric acid to provide a simplified surface passivation technique for gallium nitride (GaN) and gallium phosphide (GaP). In an effort to identify the leading causes of surface instabilities, hydrogen peroxide was utilized as an additional chemical modification to cap unsatisfied bonds. The stability of the surfaces was studied in an aqueous environment and subsequently characterized. A physical characterization was carried out to evaluate the surface roughness and water hydrophobicity pre and post stability testing via atomic force microscopy and water goniometry. Surface‐chemistry changes and solution leaching were quantified by X‐ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry. The results indicate a sensitivity to hydroxyl terminated species for both GaN and GaP under aqueous environments, as the increase of the degree of leaching was more significant for hydrogen peroxide treated samples. The results support the notion that hydroxyl species act as precursors to gallium oxide formation and lead to subsequent instability in aqueous solutions.}, number={8}, journal={CHEMPHYSCHEM}, author={Wilkins, Stewart J. and Paskova, Tania and Reynolds, C. Lewis, Jr. and Ivanisevic, Albena}, year={2015}, month={Jun}, pages={1687–1694} }
 @article{wilkins_slomski_paskova_weyher_ivanisevic_2015, title={Modulated optical sensitivity with nanostructured gallium nitride}, volume={106}, ISSN={["1077-3118"]}, DOI={10.1063/1.4918739}, abstractNote={Surface functionalization via etching of high aspect ratio gallium nitride (GaN) nanostructures provides a way to modulate the optical properties in addition to properties gained from unique topographical formations. In this study, planar layered (heteroepitaxy) and bulk free-standing gallium nitride were modified via a phosphonic acid (1H,1H,2H,2H-perfluorooctanephosphonic acid) assisted phosphoric acid etch in conjunction with an aqueous KOH + K2S2O8 formed gallium nitride nanostructured surface. Despite the high defect concentrations in the thin planar and nanostructured GaN layer, the nanostructured GaN sample produced improved photoluminescence intensities versus the high quality bulk free-standing gallium nitride. Subsequent treatments with additive and additive-free phosphoric etches provided a means of additional optical manipulation in the form of red-shifting the near-band-edge (NBE) emission of the nanostructured GaN sample and increasing the maximum NBE photoluminescence intensity.}, number={15}, journal={APPLIED PHYSICS LETTERS}, author={Wilkins, S. J. and Slomski, M. J. and Paskova, T. and Weyher, J. L. and Ivanisevic, A.}, year={2015}, month={Apr} }
 @article{redwing_lences_xie_paskova_2015, title={NITRIDES AND OXYNITRIDE MATERIALS Introduction}, volume={30}, ISSN={["2044-5326"]}, DOI={10.1557/jmr.2015.303}, abstractNote={In my earlier work on the Metaphysics of Sri R¡m¡nuja's ár¢ Bh¡Àya I dealt among other topics with the nature of the soul and incidentally with the nature of Consciousness.The soul is a sentience-point or an intelligence that is utterly finite, that is capable of being aware of itself without the mediation or functional activity of its consciousness, as is seen to be the case in states other than the waking and the dreaming. The soul's consciousness is inevitably used when it knows objects other than its own soul-nature. This is true even in relation to the soul knowing its own body. This indeed is the reason for considering the body to be other than the soul which possesses it and utilises it for its own purposes, This consciousness is to the soul what the rays of the sun are to the Sun, which reveals at any moment the objects to its own substrate and reveals itself along with them. Anubh£titvam n¡ma vartam¡nada¿¡y¡m svasattayaiva sv¡¿rayam prati prak¡¿am¡natvam. Svasattayaiva svaviÀayas¡dha natvam v¡. Thus it is svayam-praka¿a but not svasmaipraka¿a. As a function of the soul or knower it is known as jμ¡na. It is unlike a quality for it is deemed to be a dravya for it is capable of expansion and contraction or in other words capable of modification (avasth¡vad dravyam) even as the rays of the Sun. But it is not a substance in the sense in which the soul or Atman is a substance.}, number={19}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Redwing, Joan and Lences, Zoltan and Xie, Rong-Jun and Paskova, Tania}, year={2015}, month={Oct}, pages={2845–2845} }
 @article{romanyuk_fernandez-garrido_jiricek_bartos_geelhaar_brandt_paskova_2015, title={Non-destructive assessment of the polarity of GaN nanowire ensembles using low-energy electron diffraction and x-ray photoelectron diffraction}, volume={106}, ISSN={["1077-3118"]}, DOI={10.1063/1.4905651}, abstractNote={We investigate GaN nanowire ensembles spontaneously formed in plasma-assisted molecular beam epitaxy by non-destructive low-energy electron diffraction (LEED) and x-ray photoelectron diffraction (XPD). We show that GaN nanowire ensembles prepared on AlN-buffered 6H-SiC(0001¯) substrates with well-defined N polarity exhibit similar LEED intensity-voltage curves and angular distribution of photo-emitted electrons as N-polar free-standing GaN layers. Therefore, as in the case of GaN layers, LEED and XPD are found to be suitable techniques to assess the polarity of GaN nanowire ensembles on a macroscopic scale. The analysis of GaN nanowire ensembles prepared on bare Si(111) allows us to conclude that, on this non-polar substrate, the majority of nanowires is also N-polar.}, number={2}, journal={APPLIED PHYSICS LETTERS}, author={Romanyuk, O. and Fernandez-Garrido, S. and Jiricek, P. and Bartos, I. and Geelhaar, L. and Brandt, O. and Paskova, T.}, year={2015}, month={Jan} }
 @article{romanyuk_jiricek_paskova_bartos_2015, title={Polarity of GaN with polar {0001} and semipolar {10(1)over-bar1}, {20(2)over-bar1}, {11(2)over-bar2} orientations by x-ray photoelectron diffraction}, volume={30}, ISSN={["2044-5326"]}, DOI={10.1557/jmr.2015.153}, abstractNote={Abstract}, number={19}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Romanyuk, Oleksandr and Jiricek, Petr and Paskova, Tania and Bartos, Igor}, year={2015}, month={Oct}, pages={2881–2892} }
 @article{wilkins_greenough_arellano_paskova_ivanisevic_2014, title={In Situ Chemical Functionalization of Gallium Nitride with Phosphonic Acid Derivatives during Etching}, volume={30}, ISSN={0743-7463 1520-5827}, url={http://dx.doi.org/10.1021/LA404511B}, DOI={10.1021/LA404511B}, abstractNote={In situ functionalization of polar (c plane) and nonpolar (a plane) gallium nitride (GaN) was performed by adding (3-bromopropyl) phosphonic acid or propyl phosphonic acid to a phosphoric acid etch. The target was to modulate the emission properties and oxide formation of GaN, which was explored through surface characterization with atomic force microscopy, X-ray photoelectron spectroscopy, photoluminescence (PL), inductively coupled plasma-mass spectrometry, and water contact angle. The use of (3-bromopropyl) phosphonic acid and propyl phosphonic acid in phosphoric acid demonstrated lower amounts of gallium oxide formation and greater hydrophobicity for both sample sets, while also improving PL emission of polar GaN samples. In addition to crystal orientation, growth-related factors such as defect density in bulk GaN versus thin GaN films residing on sapphire substrates were investigated as well as their responses to in situ functionalization. Thin nonpolar GaN layers were the most sensitive to etching treatments due in part to higher defect densities (stacking faults and threading dislocations), which accounts for large surface depressions. High-quality GaN (both free-standing bulk polar and bulk nonpolar) demonstrated increased sensitivity to oxide formation. Room-temperature PL stands out as an excellent technique to identify nonradiative recombination as observed in the spectra of heteroepitaxially grown GaN samples. The chemical methods applied to tune optical and physical properties of GaN provide a quantitative framework for future novel chemical and biochemical sensor development.}, number={8}, journal={Langmuir}, publisher={American Chemical Society (ACS)}, author={Wilkins, Stewart J. and Greenough, Michelle and Arellano, Consuelo and Paskova, Tania and Ivanisevic, Albena}, year={2014}, month={Feb}, pages={2038–2046} }
 @article{wilkins_paskova_ivanisevic_2015, title={Modified surface chemistry, potential, and optical properties of polar gallium nitride via long chained phosphonic acids}, volume={327}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsusc.2014.11.179}, abstractNote={Surface potential, chemistry, topography, and optical properties were modulated utilizing the attachment of phosphonic acids (11-mercaptoundecylphosphonic acid, 1H,1H,2H,2H-perfluorooctanephosphonic acid, and 1,8-octanediphosphonic acid) with phosphoric acid to polar (c-plane) GaN. These changes were identified using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) with kelvin probe force microscopy (KPFM), photoluminescence (PL), and water contact angle. The results indicated that the attachment of phosphonic groups to gallium nitride strongly depends on the formation of a native oxide layer and subsequent passivation. It was seen that a fluorine terminated phosphonic acid increased the overall surface oxide versus other groups, as well as reduced the surface potential and improved the photoluminescence relative to other treatments. Sulfur terminated phosphonic acid demonstrated a similar reduction in surface potential and oxide formation to fluorine based phosphonic acid; however, improvements of optical luminescence on the same scale were not achieved.}, journal={APPLIED SURFACE SCIENCE}, author={Wilkins, Stewart J. and Paskova, Tania and Ivanisevic, Albena}, year={2015}, month={Feb}, pages={498–503} }
 @article{wilkins_paskova_ivanisevic_2014, title={Modulated optical properties of nonpolar gallium nitride via surface in-situ functionalization with cysteamine assisted phosphoric acid}, volume={295}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsusc.2014.01.035}, abstractNote={In-situ functionalization of nonpolar a-plane gallium nitride (GaN) surface was achieved by adding cysteamine to phosphoric acid, aiming to modulate its optical properties. The emission properties and oxide formation were explored through surface characterization with atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and water contact angle. Nonpolar a-plane bulk GaN sample sliced from a GaN boule and nonpolar a-plane GaN thin layer heteroepitaxially grown on r-plane sapphire were used to elucidate the effects of in-situ functionalization of identical surface orientation of GaN crystals with different defect ensembles. The addition of cysteamine to the phosphoric acid solution was found to result in: (i) increased surface roughness, (ii) no change to hydrophobicity, (iii) decreased oxygen content at high solution temperatures and increased gallium and nitrogen content versus phosphoric acid solutions at similar temperatures without cysteamine. The in-situ functionalization resulted in enhanced PL intensity from the nonpolar bulk GaN, while the PL intensity from the nonpolar heteroepitaxially grown GaN layer on sapphire was significantly reduced. The opposite PL modulation was explained by the effects of different defects present in the two samples on the nonradiative recombination.}, journal={APPLIED SURFACE SCIENCE}, author={Wilkins, Stewart J. and Paskova, Tania and Ivanisevic, Albena}, year={2014}, month={Mar}, pages={207–213} }
 @article{romanyuk_jiricek_paskova_bartos_2014, title={Polarity of semipolar wurtzite crystals: X-ray photoelectron diffraction from GaN{10(1)over-bar1} and GaN{20(2)over-bar1} surfaces}, volume={116}, ISSN={["1089-7550"]}, DOI={10.1063/1.4894708}, abstractNote={Polarity of semipolar GaN(101¯1) (101¯1¯) and GaN(202¯1) (202¯1¯) surfaces was determined with X-ray photoelectron diffraction (XPD) using a standard MgKα source. The photoelectron emission from N 1s core level measured in the a-plane of the crystals shows significant differences for the two crystal orientations within the polar angle range of 80–100° from the 〈0001〉 normal. It was demonstrated that XPD polar plots recorded in the a-plane are similar for each polarity of the GaN{101¯1} and GaN{202¯1} crystals if referred to 〈0001〉 crystal axes. For polarity determinations of all important GaN{h0h¯l} semipolar surfaces, the above given polar angle range is suitable.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Romanyuk, O. and Jiricek, P. and Paskova, T. and Bartos, I.}, year={2014}, month={Sep} }
 @article{freitas_meissner_paskova_miyake_2014, title={Preface}, volume={403}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2014.07.001}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Freitas, Jaime A., Jr. and Meissner, Elke and Paskova, Tania and Miyake, Hideto}, year={2014}, month={Oct}, pages={1–2} }
 @article{berg_nolan_paskova_ivanisevic_2014, title={Surface Characterization of Gallium Nitride Modified with Peptides before and after Exposure to Ionizing Radiation in Solution}, volume={30}, ISSN={0743-7463 1520-5827}, url={http://dx.doi.org/10.1021/la5040245}, DOI={10.1021/la5040245}, abstractNote={An aqueous surface modification of gallium nitride was employed to attach biomolecules to the surface. The modification was a simple two-step process using a single linker molecule and mild temperatures. The presence of the peptide on the surface was confirmed with X-ray photoelectron spectroscopy. Subsequently, the samples were placed in water baths and exposed to ionizing radiation to examine the effects of the radiation on the material in an environment similar to the body. Surface analysis confirmed degradation of the surface of GaN after radiation exposure in water; however, the peptide molecules successfully remained on the surface following exposure to ionizing radiation. We hypothesize that during radiation exposure of the samples, the radiolysis of water produces peroxide and other reactive species on the sample surface. Peroxide exposure promotes the formation of a more stable layer of gallium oxyhydroxide which passivates the surface better than other oxide species.}, number={51}, journal={Langmuir}, publisher={American Chemical Society (ACS)}, author={Berg, Nora G. and Nolan, Michael W. and Paskova, Tania and Ivanisevic, Albena}, year={2014}, month={Dec}, pages={15477–15485} }
 @article{ishmael_slomski_luo_white_hunt_mandzy_muth_nesbit_paskova_straka_et al._2014, title={Thermal conductivity and dielectric properties of a TiO2-based electrical insulator for use with high temperature superconductor-based magnets}, volume={27}, ISSN={["1361-6668"]}, DOI={10.1088/0953-2048/27/9/095018}, abstractNote={Quench protection is a remaining challenge impeding the implementation of high temperature superconductor (HTS)-based magnet applications. This is due primarily to the slow normal zone propagation velocity (NZPV) observed in Bi2Sr2CaCu2OX (Bi2212) and (RE)Ba2Cu3O7 − x (REBCO) systems. Recent computational and experimental findings reveal significant improvements in turn-to-turn NZPV, resulting in a magnet that is more stable and easier to protect through three-dimensional normal zone growth (Phillips M 2009; Ishmael S et al 2013 IEEE Trans. Appl. Supercond. 23 7201311). These improvements are achieved by replacing conventional insulation materials, such as Kapton and mullite braid, with a thin, thermally conducting, electrically-insulating ceramic oxide coating. This paper reports on the temperature-dependent thermal properties, electrical breakdown limits and microstructural characteristics of a titanium oxide (TiO2) insulation and a doped-TiO2-based proprietary insulation (doped-TiO2) shown previously to enhance quench behavior (Ishmael S et al 2013 IEEE Trans. Appl. Supercond. 23 7201311). Breakdown voltages at 77 K ranging from ∼1.5 kV to over 5 kV are reported. At 4.2 K, the TiO2 increases the thermal conductivity of polyimide by about a factor of 10. With the addition of a dopant, thermal conductivity is increased by an additional 13%, and a high temperature heat treatment increases it by nearly an additional 100%. Similar increases are observed at 77 K and room temperature. These results are understood in the context of the various microstructures observed.}, number={9}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Ishmael, S. A. and Slomski, M. and Luo, H. and White, M. and Hunt, A. and Mandzy, N. and Muth, J. F. and Nesbit, R. and Paskova, T. and Straka, W. and et al.}, year={2014}, month={Sep} }
 @article{bain_jewett_mukund_bedair_paskova_ivanisevic_2013, title={Biomolecular Gradients via Semiconductor Gradients: Characterization of Amino Acid Adsorption to InxGa1–xN Surfaces}, volume={5}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/AM4015555}, DOI={10.1021/AM4015555}, abstractNote={The band gap of indium gallium nitride can be tuned by varying the compositional ratio of indium to gallium, spanning the entire visible region and extending into the near-infrared and near-ultraviolet. This tunability allows for device optimization specific to different applications, including as a biosensor or platform for studying biological interactions. However, these rely on chemically dependent interactions between the device surface and the biostructures of interest. This study presents a material gradient of changing In:Ga composition and the subsequent evaluation of amino acid adsorption to this surface. Arginine is adsorbed to the surface in conditions both above and below the isoelectric point, providing insight to the role of electrostatic interactions in interface formation. These electrostatics are the driving force of the observed adsorption behaviors, with protonated amino acid demonstrating increased adsorption as a function of native surface oxide buildup. We thus present a gradient inorganic substrate featuring varying affinity for amino acid adhesion, which can be applied in generating gradient architectures for biosensors and studying cellular behaviors without application of specialized patterning processes.}, number={15}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Bain, Lauren E and Jewett, Scott A and Mukund, Aadhithya Hosalli and Bedair, Salah M and Paskova, Tania M and Ivanisevic, Albena}, year={2013}, month={Jul}, pages={7236–7243} }
 @article{wilkins_paskova_ivanisevic_2013, title={Effect of etching with cysteamine assisted phosphoric acid on gallium nitride surface oxide formation}, volume={114}, ISSN={["1089-7550"]}, DOI={10.1063/1.4817899}, abstractNote={In-situ functionalization of polar GaN was performed by adding cysteamine to a phosphoric acid etchant in order to study its effect on photoluminescence and oxide formation on the surfaces. The functionalization was characterized by atomic force microscopy, x-ray photoelectron spectroscopy, photoluminescence (PL), and water contact angle measurements. Two sets of polar GaN samples with different dislocation densities were evaluated, thin GaN layers residing on sapphire and thick free-standing GaN separated from sapphire substrate aiming to reveal the effect of material quality on in-situ functionalization. The addition of cysteamine to the phosphoric acid solution was found to result in: (i) decreased surface roughness, (ii) no change to hydrophobicity, (iii) decreased oxygen content especially at high-temperature treatments. The effect of the in-situ functionalization on the PL efficiency was more pronounced in the free-standing sample than in the film residing on the sapphire, which was attributed to a higher crystal quality free from strain.}, number={6}, journal={JOURNAL OF APPLIED PHYSICS}, author={Wilkins, S. J. and Paskova, T. and Ivanisevic, A.}, year={2013}, month={Aug} }
 @article{romanyuk_jiricek_paskova_bieloshapka_bartos_2013, title={GaN polarity determination by photoelectron diffraction}, volume={103}, ISSN={["1077-3118"]}, DOI={10.1063/1.4819761}, abstractNote={A nondestructive approach to determine the wurtzite GaN crystal polarity based on X-ray photoelectron diffraction is proposed. The approach, utilizing the ratio of photoemitted electron currents excited by a standard laboratory X-ray source from the N 1s level in the (101¯0) plane at polar angles of 20° and 25°, is tested on GaN crystals. The photoelectron intensity ratio I20/I25 is larger or smaller than unity for GaN(0001) or GaN(0001¯), respectively. The approach can be used for polarity determination of other binary wurtzite crystals. The atom with the smaller electron scattering cross-section should be used as the emitter.}, number={9}, journal={APPLIED PHYSICS LETTERS}, author={Romanyuk, O. and Jiricek, P. and Paskova, T. and Bieloshapka, I. and Bartos, I.}, year={2013}, month={Aug} }
 @inbook{bain_hosalli_bedair_paskova_ivanisevic_2014, place={New York}, series={Conference Proceedings of the Society for Experimental Mechanics Series}, title={Molecular Interactions on InxGa1−xN}, volume={5}, ISBN={9783319007793 9783319007809}, ISSN={2191-5644 2191-5652}, url={http://dx.doi.org/10.1007/978-3-319-00780-9_14}, DOI={10.1007/978-3-319-00780-9_14}, abstractNote={Atomic force microscopy in solution offers a platform for assessing interactions on chemically modified surfaces. In this study a biologically relevant molecule, an amino acid, is adsorbed onto a compositionally varied semiconductor substrate. AFM is used to assess the effect of the substrate composition on the adhesion of the amino acid. We report adsorption of L-arginine to an indium-gallium-nitride (InGaN) substrate with a gradient of In:Ga composition. Data are collected above and below the isoelectric point of arginine to highlight the effect of protonation on the adhesive behavior across the InGaN. Characterization is also performed using X-ray photoelectron spectroscopy to establish the presence of amino acid on the surface and determine the general composition of a given region of the substrate both with and without amino acid. Combining these factors, we are able to better evaluate the significance of substrate properties in influencing the behavior of surface molecules. Determining the dynamics of amino acid behavior as a function of both the substrate and the environment provides new insight into the preparation of semiconductor materials for biological applications.}, booktitle={MEMS and Nanotechnology}, publisher={Springer International Publishing}, author={Bain, L. E. and Hosalli, A. M. and Bedair, S. M. and Paskova, T. and Ivanisevic, A.}, editor={Shaw, Gordon, III and Prorok, Barton C. and Starman, LaVern and Furlong, CosmeEditors}, year={2014}, pages={109–114}, collection={Conference Proceedings of the Society for Experimental Mechanics Series} }
 @article{frajtag_nepal_paskova_bedair_el-masry_2013, title={Multifacet semipolar formation by controlling the groove depth via lateral sidewall epitaxy}, volume={367}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2012.12.039}, abstractNote={We demonstrate InxGa1−xN/GaN light emitting diode structures with different sets of multifacet semipolar formation grown laterally on m-plane sidewalls formed by stripe patterning on preliminary grown c-plane GaN template. It was found that regrowth on shallow side walls within the GaN template resulted in a single semipolar (11¯01) facet, while deeper side walls led to multifacet semipolar formation. Very deep etching through the entire GaN template reaching the underlying sapphire substrates resulted in a combination of semipolar (11¯01) and nonpolar (11¯00) facets. The results indicate that the depth of the groove patterning can be used as a tool for controlling the set of semipolar facet formation. In addition, the growth rate in different crystallographic directions was studied and possible factors affecting the growth rates are discussed.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Frajtag, P. and Nepal, N. and Paskova, T. and Bedair, S. M. and El-Masry, N. A.}, year={2013}, month={Mar}, pages={88–93} }
 @article{killat_montes bajo_paskova_evans_leach_li_oezguer_morkoc_chabak_crespo_et al._2013, title={Reliability of AlGaN/GaN high electron mobility transistors on low dislocation density bulk GaN substrate: Implications of surface step edges}, volume={103}, ISSN={["1077-3118"]}, DOI={10.1063/1.4829062}, abstractNote={To enable gaining insight into degradation mechanisms of AlGaN/GaN high electron mobility transistors, devices grown on a low-dislocation-density bulk-GaN substrate were studied. Gate leakage current and electroluminescence (EL) monitoring revealed a progressive appearance of EL spots during off-state stress which signify the generation of gate current leakage paths. Atomic force microscopy evidenced the formation of semiconductor surface pits at the failure location, which corresponds to the interaction region of the gate contact edge and the edges of surface steps.}, number={19}, journal={APPLIED PHYSICS LETTERS}, author={Killat, N. and Montes Bajo, M. and Paskova, T. and Evans, K. R. and Leach, J. and Li, X. and Oezguer, Ue and Morkoc, H. and Chabak, K. D. and Crespo, A. and et al.}, year={2013}, month={Nov} }
 @article{liu_huang_kamaladasa_picard_preble_paskova_evans_davis_porter_2014, title={Site-specific comparisons of V-defects and threading dislocations in InGaN/GaN multi-quantum-wells grown on SiC and GaN substrates}, volume={387}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/J.JCRYSGRO.2013.10.026}, DOI={10.1016/J.JCRYSGRO.2013.10.026}, abstractNote={Systematic site-specific comparisons of the densities of threading dislocations (TDs) and associated V-defects in six-period InxGa1−xN/GaN multi-quantum wells (MQWs) with In mole fractions of x=0.1 and 0.2 have been conducted using several electron microscopy techniques including electron channeling contrast imaging. The MQWs were grown on GaN epitaxial films deposited on either (0001) GaN substrates or on (0001)AlN/6H-SiC heterostructures. The densities of TDs and V-defects in the surfaces of the homoepitaxial GaN films and the In0.1Ga0.9N/GaN MQWs were both 5.5×106 cm−2. By contrast, the V-defect densities in the In0.2Ga0.8N/GaN MQWs exceeded by two and three times the densities of TDs measured in the surfaces of the underlying GaN films grown on GaN and AlN/SiC substrates, respectively. Similar observations of V-defects not associated with threading dislocations have been previously reported to occur in InGaN MQWs grown on GaN template films on sapphire substrates and having either an initially high In mole fraction or grown at the end of a series of MQWs with an increasing In mole fraction. From a synthesis of the results of this previous work using GaN/sapphire substrates with the present results for bulk GaN substrates, it is hypothesized that increases in stress generated by the larger mismatches in lattice parameters between the GaN film and the In0.2Ga0.8N/GaN MQWs and the additional stresses generated between the GaN barriers and the wells within these MQWs generated new TDs within the MQWs, which then acted as nucleation centers for new V-defects.}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Liu, Fang and Huang, Li and Kamaladasa, Ranga and Picard, Yoosuf N. and Preble, Edward A. and Paskova, Tanya and Evans, Keith R. and Davis, Robert F. and Porter, Lisa M.}, year={2014}, month={Feb}, pages={16–22} }
 @article{roberts_mohanta_everitt_leach_broeck_hosalli_paskova_bedair_2013, title={Spectroscopic investigation of coupling among asymmetric InGaN/GaN multiple quantum wells grown on non-polar a-plane GaN substrates}, volume={103}, ISSN={["1077-3118"]}, DOI={10.1063/1.4827536}, abstractNote={Low defect density asymmetric multiple quantum wells (MQWs) of InGaN/GaN grown on non-polar a-plane GaN substrates were investigated using time-integrated and time-resolved photoluminescence spectroscopy. Comparison of these spectra with the predicted emission energies reveals that these QWs may be spectrally resolved at low temperatures. However, a combination of thermal activation and resonant tunneling of carriers increasingly coupled the QWs, favoring emission from the lowest energy QWs with increasing temperature in a manner analogous to MQWs composed of other non-polar semiconductor materials but unlike most InGaN MQWs grown on polar substrates and influenced by the strong polarization-dependent effects.}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Roberts, A. T. and Mohanta, A. and Everitt, H. O. and Leach, J. H. and Broeck, D. and Hosalli, A. M. and Paskova, T. and Bedair, S. M.}, year={2013}, month={Oct} }
 @article{bliss_dupuis_wang_paskova_qiu_bhat_caneau_2014, title={The 19th American Conference on Crystal Growth and Epitaxy in conjunction with The 16th US Biennial Workshop on Organometallic Vapor Phase Epitaxy Preface}, volume={393}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2013.12.025}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Bliss, David and Dupuis, Russell and Wang, Chris and Paskova, Tania and Qiu, Roger and Bhat, Raj and Caneau, Catherine}, year={2014}, month={May}, pages={1–1} }
 @article{storm_deen_katzer_meyer_binari_gougousi_paskova_preble_evans_smith_2013, title={Ultrathin-barrier AlN/GaN heterostructures grown by rf plasma-assisted molecular beam epitaxy on freestanding GaN substrates}, volume={380}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/J.JCRYSGRO.2013.05.029}, DOI={10.1016/J.JCRYSGRO.2013.05.029}, abstractNote={We report the structural and electrical properties of ultrathin-barrier AlN/GaN heterostructures grown on freestanding GaN substrates by rf plasma-assisted molecular beam epitaxy. Structures with barrier thicknesses between 1.5 nm and 7.5 nm were grown and characterized. We observe that AlN/GaN structures with barriers of 3.0 nm exhibit the highest Hall mobility, approximately 1700 cm2/Vs. Furthermore, the Hall mobility is much diminished in heterostructures with AlN barriers thicker than 4.5 nm, coincident with the onset of strain relaxation.}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Storm, D.F. and Deen, D.A. and Katzer, D.S. and Meyer, D.J. and Binari, S.C. and Gougousi, T. and Paskova, T. and Preble, E.A. and Evans, K.R. and Smith, David J.}, year={2013}, month={Oct}, pages={14–17} }
 @article{ščajev_jarašiūnas_özgür_morkoç_leach_paskova_2012, title={Anisotropy of free-carrier absorption and diffusivity in m-plane GaN}, volume={100}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3674306}, DOI={10.1063/1.3674306}, abstractNote={Polarization-dependent free-carrier absorption (FCA) in bulk m-plane GaN at 1053 nm revealed approximately 6 times stronger hole-related absorption for E⊥c than for E||c probe polarization both at low and high carrier injection levels. In contrast, FCA at 527 nm was found isotropic at low injection levels due to electron resonant transitions between the upper and lower conduction bands, whereas the anisotropic impact of holes was present only at high injection levels by temporarily blocking electron transitions. Carrier transport was also found to be anisotropic under two-photon excitation, with a ratio of 1.17 for diffusivity perpendicular and parallel to the c-axis.}, number={2}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Ščajev, P. and Jarašiūnas, K. and Özgür, Ü. and Morkoç, H. and Leach, J. and Paskova, T.}, year={2012}, month={Jan}, pages={022112} }
 @article{gladkov_hulicius_paskova_preble_evans_2012, title={Below band-gap optical absorption and photoluminescence excitation spectroscopy at room temperature in low-defect-density bulk GaN:Fe}, volume={100}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3678195}, DOI={10.1063/1.3678195}, abstractNote={We present a detailed study of the below band-gap optical absorption at room temperature in bulk semi-insulating GaN:Fe versus the Fe-doping. It was established that the 1.24 eV photoluminescence band at 300 K consists of only vibrational replicas of the Fe3+ 4T1(G)→6A1(S) internal transition. We also studied the below band-gap photoluminescence excitation of the 1.24 eV band. The identical exponential rise of the photoluminescence excitation and the optical absorption coefficient identify the Fe3+/2+ charge-transfer as the main contributor to the 300 K optical absorption in the range 400-500 nm. Practical implications of these results for Fe-doping determination are discussed.}, number={3}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Gladkov, P. and Hulicius, E. and Paskova, T. and Preble, E. and Evans, K. R.}, year={2012}, month={Jan}, pages={031908} }
 @article{dashdorj_zvanut_harrison_udwary_paskova_2012, title={Charge transfer in semi-insulating Fe-doped GaN}, volume={112}, ISSN={["1089-7550"]}, DOI={10.1063/1.4732352}, abstractNote={Charge transfer kinetics is studied in free-standing Fe-doped GaN using photo-induced electron paramagnetic resonance (EPR). Samples with Fe concentrations of 1017 cm−3 reveal an increase in Fe3+ during exposure with photon energy greater than 0.8 eV, while samples with higher Fe concentrations exhibit a decrease in the Fe3+ under the same conditions. Steady-state photo-EPR measurements of the most lightly doped sample imply the existence of an Fe2+/3+ defect level within 0.8 eV of the conduction band edge consistent with earlier work, but time-dependent measurements of more heavily doped crystals indicate a multi-step charge transfer process. Analysis of time-dependent photo-EPR data reveals that charge exchange may be separated into two processes, one that is temperature independent and one that depends monotonically on temperature. While a physical model for the charge transfer is not apparent, likely scenarios involve charge trapping at extended defects and phonon interactions.}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Dashdorj, J. and Zvanut, M. E. and Harrison, J. G. and Udwary, K. and Paskova, T.}, year={2012}, month={Jul} }
 @article{mcnamara_foussekis_baski_li_avrutin_morkoç_leach_paskova_udwary_preble_et al._2012, title={Electrical and optical properties of bulk GaN substrates studied by Kelvin probe and photoluminescence}, volume={10}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.201200662}, DOI={10.1002/pssc.201200662}, abstractNote={AbstractWe have investigated the N‐ and Ga‐polar faces of bulk GaN substrates with photoluminescence (PL) and the surface photovoltage (SPV) technique using a Kelvin probe attached to an optical cryostat. Experiments were conducted in vacuum. Some of the surfaces were mechanically polished (MP), while others were epi‐ready after a chemical‐mechanical polish (CMP). From the SPV measurements, the band bending in a sample having both surfaces treated with the CMP method was calculated to be about 0.83 and 0.70 eV for the Ga‐ and N‐polar surfaces, respectively. The restoration of the SPV after ceasing the UV illumination showed that the SPV from CMP‐treated surfaces behaved as predicted by a thermionic model, whereas the SPV from MP‐treated surfaces restored with a much faster‐than‐predicted rate. This result can be interpreted by the hopping of charge carriers in the highly‐defective near‐surface layer of the MP‐treated samples. Remarkably, removing the top 700 nm defective layer by dry etching restored the quality of the electrical and optical properties of GaN. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={3}, journal={physica status solidi (c)}, publisher={Wiley}, author={McNamara, J. D. and Foussekis, M. A. and Baski, A. A. and Li, X. and Avrutin, V. and Morkoç, H. and Leach, J. H. and Paskova, T. and Udwary, K. and Preble, E. and et al.}, year={2012}, month={Dec}, pages={536–539} }
 @article{stark_detchprohm_zhao_paskova_preble_wetzel_2012, title={Fish scale terrace GaInN/GaN light-emitting diodes with enhanced light extraction}, volume={101}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4769442}, DOI={10.1063/1.4769442}, abstractNote={Non-planar GaInN/GaN light-emitting diodes were epitaxially grown to exhibit steps for enhanced light emission. By means of a large off-cut of the epitaxial growth plane from the c-plane (0.06° to 2.24°), surface morphologies of steps and inclined terraces that resemble fish scale patterns could controllably be achieved. These patterns penetrate the active region without deteriorating the electrical device performance. We find conditions leading to a large increase in light-output power over the virtually on-axis device and over planar sapphire references. The process is found suitable to enhance light extraction even without post-growth processing.}, number={23}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Stark, Christoph J. M. and Detchprohm, Theeradetch and Zhao, Liang and Paskova, Tanya and Preble, Edward A. and Wetzel, Christian}, year={2012}, month={Dec}, pages={232106} }
 @article{huang_liu_zhu_kamaladasa_preble_paskova_evans_porter_picard_davis_2012, title={Microstructure of epitaxial GaN films grown on chemomechanically polished GaN(0001) substrates}, volume={347}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2012.03.002}, DOI={10.1016/j.jcrysgro.2012.03.002}, abstractNote={An organometallic vapor phase epitaxy-based process route has been developed to achieve homoepitaxial deposition of GaN(0001) films via step-flow growth on substrates having <1° off-cut. Atomic force microscopy of the surfaces of 0, 1, 2, 5 and 10 nm thick films revealed steps and terraces as the only features; three-dimensional GaN islands were not observed. Film–substrate interfaces were not present in cross-sectional samples using high-resolution TEM. This indicated that continuous film growth occurred from steps on the substrate without re-nucleation and defect formation on the terraces. This process route also mitigated the generation of additional dislocations, as validated by the exact matches of the density and positions of dislocations that reached the substrate surface and those observed in a subsequently grown 600 nm film. The influence of grain boundaries in the interior of the GaN substrates was manifest in variations in terrace width and step orientation across the substrates and the films. A grain orientation map generated across a representative substrate revealed highly disoriented grains on the periphery. The disorientation angles between these adjacent grains were centered around ∼35°, ∼70° and ∼90°.}, number={1}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Huang, Li and Liu, Fang and Zhu, Jingxi and Kamaladasa, Ranga and Preble, Edward A. and Paskova, Tanya and Evans, Keith and Porter, Lisa and Picard, Yoosuf N. and Davis, Robert F.}, year={2012}, month={May}, pages={88–94} }
 @article{edmunds_tang_li_cervantes_gardner_paskova_manfra_malis_2012, title={Near-Infrared Absorption in Lattice-Matched AlInN/GaN and Strained AlGaN/GaN Heterostructures Grown by MBE on Low-Defect GaN Substrates}, volume={41}, ISSN={0361-5235 1543-186X}, url={http://dx.doi.org/10.1007/S11664-011-1881-9}, DOI={10.1007/S11664-011-1881-9}, number={5}, journal={Journal of Electronic Materials}, publisher={Springer Science and Business Media LLC}, author={Edmunds, C. and Tang, L. and Li, D. and Cervantes, M. and Gardner, G. and Paskova, T. and Manfra, M. J. and Malis, O.}, year={2012}, month={Feb}, pages={881–886} }
 @article{romanyuk_jiricek_paskova_2012, title={Quantitative low-energy electron diffraction analysis of the GaN (1×1) reconstruction}, volume={606}, ISSN={0039-6028}, url={http://dx.doi.org/10.1016/j.susc.2012.01.002}, DOI={10.1016/j.susc.2012.01.002}, abstractNote={Abstract The atomic structure of the GaN ( 000 1 ¯ ) (1 × 1) reconstruction was investigated by quantitative low-energy electron diffraction (LEED). The GaN ( 000 1 ¯ ) surface was annealed in an NH3 atmosphere and cooled down without any NH3 flux. LEED patterns with 6-fold symmetry were measured after annealing. The diffraction symmetry was explained by the presence of two domains on the GaN surface. LEED intensity–voltage (I–V) curves were acquired up to 500 eV and calculated in the framework of the dynamical theory of electron scattering. Good agreement between the experimental and theoretical curves was achieved for the bare GaN ( 000 1 ¯ ) (1 × 1) surface. Relaxation of the surface atomic layers decreased Pendry's R-factor to 0.21 ± 0.03. The plane relaxations derived from the LEED analysis were found to be in qualitative agreement with ab initio calculations. By using previously suggested models with adlayers on the bare GaN ( 000 1 ¯ ) (1 × 1) surface, much worse R-factors were obtained. It was concluded that the measured LEED I–V curves could be used to determine the bare GaN polarity.}, number={7-8}, journal={Surface Science}, publisher={Elsevier BV}, author={Romanyuk, O. and Jiricek, P. and Paskova, T.}, year={2012}, month={Apr}, pages={740–743} }
 @article{killat_montes_pomeroy_paskova_evans_leach_li_oezguer_morkoc_chabak_et al._2012, title={Thermal Properties of AlGaN/GaN HFETs on Bulk GaN Substrates}, volume={33}, ISSN={["1558-0563"]}, DOI={10.1109/led.2011.2179972}, abstractNote={Micro-Raman thermography, microphotoluminescence spectroscopy, and thermal simulation were used to study the thermal properties of AlGaN/GaN heterostructure field-effect transistors grown on semi-insulating bulk GaN substrates. A bulk GaN thermal conductivity of 260 was determined from temperature measurements on operating devices in combination with finite-difference thermal simulations. This is significantly higher than typical thin GaN epilayer thermal conductivities, due to a lower dislocation density in bulk GaN. Despite the thermal conductivity of bulk GaN being lower than that of SiC, transistors on bulk GaN exhibited a similar thermal resistance as GaN-on-SiC devices, attributed to the absence of a thermal boundary resistance between the device epilayers and substrate for GaN-on-GaN devices.}, number={3}, journal={IEEE ELECTRON DEVICE LETTERS}, author={Killat, N. and Montes, M. and Pomeroy, J. W. and Paskova, T. and Evans, K. R. and Leach, J. and Li, X. and Oezguer, Ue and Morkoc, H. and Chabak, K. D. and et al.}, year={2012}, month={Mar}, pages={366–368} }
 @article{meissner_schweigard_friedrich_paskova_udwary_leibiger_habel_2012, title={Cathodoluminescence imaging for the determination of dislocation density in differently doped HVPE GaN}, volume={340}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2011.12.043}, DOI={10.1016/j.jcrysgro.2011.12.043}, abstractNote={In this study we report the potential and limitations of the cathodoluminescence dark spot (DS) counting as a method for the determination of dislocation density and distribution in GaN, produced by the hydride vapour phase epitaxy (HVPE). Different GaN sample series (s.i. GaN:Fe and n-type GaN:Si) were used, in order to study the dependence of the results of the DS-counting on the dopant type and concentration. By the direct comparison of these results to classical defect selective etching, the DS-measurements were validated. It could be shown that each of the both methods have their particular restrictions, which must be considered in their application.}, number={1}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Meissner, E. and Schweigard, S. and Friedrich, J. and Paskova, T. and Udwary, K. and Leibiger, G. and Habel, F.}, year={2012}, month={Feb}, pages={78–82} }
 @article{dumcenco_levcenco_huang_reynolds_reynolds_tiong_paskova_evans_2011, title={Characterization of freestanding semi-insulating Fe-doped GaN by photoluminescence and electromodulation spectroscopy}, volume={109}, ISSN={["1089-7550"]}, DOI={10.1063/1.3592343}, abstractNote={A detailed optical characterization of a freestanding wurtzite semi-insulating Fe-doped GaN (SI-GaN:Fe) grown by hydride-vapor-phase-epitaxy was carried out by photoluminescence (PL) and contactless electroreflectance (CER) at 10 and 300 K, respectively, and photoreflectance at 300 K. Low-temperature PL spectrum of the Ga-face consisted of the Fe3+ impurity characteristic series of IR peaks with a sharp zero-phonon line (ZPL) at 1.299 eV, yellow and blue broad emission bands, and near-band-edge (NBE) emission in the ultraviolet region. The narrow linewidth of 135 μeV of the ZPL and the measured energy difference of ∼70 meV between the ZPL and E2 (high) phonon replica, which is sensitive to the lattice strain, shows good crystal quality and a strain-free incorporation of iron. The obtained transition energies of A, B, and C excitonic features in the CER spectra and the n = 2 excited states (2s) of the A and B excitons enable the estimation of the exciton binding energies. In addition to the free excitonic recombination, the PL spectrum of the Ga-face exhibited clear donor and acceptor related features in the NBE region, while the N-face exhibited a broad emission band related to the free-to-bound recombination only. The differences were explained by the presence of impurity-induced band-tail states in the N-face SI-GaN:Fe due to an increased impurity density and the incorporation of large volume vacancy-type defects.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Dumcenco, D. O. and Levcenco, S. and Huang, Y. S. and Reynolds, C. L., Jr. and Reynolds, J. G. and Tiong, K. K. and Paskova, T. and Evans, K. R.}, year={2011}, month={Jun} }
 @article{romanyuk_jiricek_zemek_tougaard_paskova_2011, title={Dielectric response functions of the (0001¯), (101¯3) GaN single crystalline and disordered surfaces studied by reflection electron energy loss spectroscopy}, volume={110}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.3622674}, DOI={10.1063/1.3622674}, abstractNote={Polar GaN(0001¯) (1×1), semipolar GaN(101¯3) surfaces prepared in NH3 vapor, and their disordered counterparts are investigated by reflection electron energy loss spectroscopy (REELS) and low-energy electron diffraction. The REELS spectra are measured in a range of polar angles at electron kinetic energies of 200 and 1000 eV. The electron energy loss function is determined from the REELS within the framework of the semiclassical approach. Good agreement between experimental and theoretical functions is achieved at all angles for the disordered GaN surfaces and for the ordered surfaces measured at a kinetic energy of 1000 eV. The agreement is worse for the crystals measured at 200 eV, which is explained by the coherent scattering contributions at low energies. The optical constants of the GaN surfaces are derived from the computed dielectric functions: the optical properties of the (0001¯) and (101¯3) surfaces are similar, except for differences in bandgap values, which may be due to observed steps on the (101¯3) surface. The surface optical properties of a disordered GaN surface are found to be different from the GaN crystals. There are pronounced changes in the electronic band structure for disordered GaN due to the preferential sputtering of nitrogen.}, number={4}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Romanyuk, O. and Jiricek, P. and Zemek, J. and Tougaard, S. and Paskova, T.}, year={2011}, month={Aug}, pages={043507} }
 @article{eichfeld_won_trumbull_labella_weng_robinson_snyder_redwing_paskova_udwary_et al._2011, title={Dual temperature process for reduction in regrowth interfacial charge in AlGaN/GaN HEMTs grown on GaN substrates}, volume={8}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.201001059}, DOI={10.1002/pssc.201001059}, abstractNote={AbstractThe effects of growth temperature and Mg compensation doping on the structural and electrical properties of AlGaN/AlN/GaN high electron mobility transistor (HEMTs) structures grown on low threading dislocation density bulk GaN substrates by metalorganic chemical vapor deposition were investigated. The background electron concentration in the regrown GaN was found to decrease from 1.5x1018 cm‐3 to 2x1016 cm‐3 as the growth temperature was reduced from 1100 °C to 950 °C. A dual temperature process was then employed for growth of the GaN base layer and AlGaN/AlN/GaN top heterostructure in the HEMT along with Mg doping of the GaN base to compensate residual Si donors at the regrown interface. Using this approach, AlGaN/AlN/GaN HEMTs were produced that had a sheet carrier density as high as 1.1x1013 cm‐3 with a room temperature mobility of 1600 cm2/Vs. The incorporation of a thin AlN layer at the regrown interface was found to reduce the sheet carrier density of the overall structure (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7-8}, journal={physica status solidi (c)}, publisher={Wiley}, author={Eichfeld, Sarah M. and Won, Dongjin and Trumbull, Kathy and Labella, Michael and Weng, Xiaojun and Robinson, Joshua and Snyder, David and Redwing, Joan M. and Paskova, Tanya and Udwary, Kevin and et al.}, year={2011}, month={May}, pages={2053–2055} }
 @article{wang_xu_alur_sharma_tong_gartland_issacs-smith_ahyi_williams_park_et al._2011, title={Electrical characteristics of the vertical GaN rectifiers fabricated on bulk GaN wafer}, volume={8}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.201001158}, DOI={10.1002/pssc.201001158}, abstractNote={AbstractVertical Schottky rectifiers were fabricated on two GaN wafers synthesised by hydride vapor phase epitaxy (HVPE) process: n‐ epilayer on n+ bulk and n‐ bulk. Full backside ohmic contact and circular Schottky contacts were deposited on the N face and Ga face of the substrate, respectively. The devices showed excellent electrical characteristics. The device fabricated on n‐ epilayer on n+ bulk substrate showed a breakdown voltage of 348 V, a specific on‐state resistance of 4.2 mΩ·cm2 and a leakage current density of 9.2×10‐6 A/cm2 at ‐90 V. The device fabricated on n‐ bulk substrate showed a breakdown voltage of 600 V, a specific on‐state resistance of 1.3 mΩ·cm2 and a leakage current density of 8.1×10‐8 A/cm2 at ‐100 V. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7-8}, journal={physica status solidi (c)}, publisher={Wiley}, author={Wang, Yaqi and Xu, Hui and Alur, Siddharth and Sharma, Yogesh and Tong, Fei and Gartland, Patrick and Issacs-Smith, Tamara and Ahyi, Claude and Williams, John and Park, Minseo and et al.}, year={2011}, month={Jun}, pages={2430–2432} }
 @article{lai_paskova_wheeler_chung_grenko_johnson_udwary_preble_evans_2012, title={Indium incorporation in InGaN/GaN quantum wells grown on m-plane GaN substrate and c-plane sapphire}, volume={209}, ISSN={["1862-6319"]}, DOI={10.1002/pssa.201127345}, abstractNote={AbstractInGaN/GaN quantum wells (QWs) grown at identical conditions on m‐plane GaN and c‐plane sapphire substrates were characterized by several techniques, aiming to clarify the reason for different emission wavelengths often observed in similar LED structures with m‐ and c‐plane surface orientations. Cathodoluminescence (CL) spectra of m‐plane QWs revealed shorter wavelength and no blueshift with increasing CL probe current in accordance with previous reports. Relative indium compositions were estimated by high‐resolution X‐ray diffraction to be 5.1 and 13.9% for m‐plane and c‐plane QWs, respectively. Cross‐sectional transmission electron microscopy images revealed that the well widths of the m‐plane QWs were noticeably thicker than those of the c‐plane QWs. The lower indium compositions and thicker well widths of the m‐plane QWs indicated that different indium incorporation and diffusion occurred in the structures grown on the GaN substrate, which is attributed to its surface off‐cut toward [0001] and the higher thermal conductivity with respect to that of sapphire.}, number={3}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Lai, K. Y. and Paskova, T. and Wheeler, V. D. and Chung, T. Y. and Grenko, J. A. and Johnson, M. A. L. and Udwary, K. and Preble, E. A. and Evans, K. R.}, year={2012}, month={Mar}, pages={559–564} }
 @article{liu_zhang_li_huang_paskova_evans_zhao_tansu_2012, title={Metalorganic vapor phase epitaxy and characterizations of nearly-lattice-matched AlInN alloys on GaN/sapphire templates and free-standing GaN substrates}, volume={340}, ISSN={["0022-0248"]}, DOI={10.1016/j.jcrysgro.2011.12.037}, abstractNote={The epitaxy optimization studies of high-quality n-type AlInN alloys with different indium contents grown on two types of substrates by metalorganic vapor phase epitaxy (MOVPE) were carried out. The effect of growth pressure and V/III molar ratio on growth rate, indium content, and surface morphology of these MOVPE-grown AlInN thin films were examined. The surface morphologies of the samples were characterized by scanning electron microscopy and atomic force microscopy. By varying the growth temperatures from 860 °C to 750 °C, the indium contents in AlInN alloys were increased from 0.37% up to 21.4% as determined by X-ray diffraction (XRD) measurements. The optimization studies on the growth conditions for achieving nearly-lattice-matched AlInN on GaN templates residing on sapphire and free-standing GaN substrates were performed, and the results were analyzed in a comparative way. Several applications of AlInN alloy for thermoelectric and light-emitting diodes are also discussed.}, number={1}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Liu, Guangyu and Zhang, Jing and Li, Xiao-Hang and Huang, G. S. and Paskova, Tanya and Evans, Keith R. and Zhao, Hongping and Tansu, Nelson}, year={2012}, month={Feb}, pages={66–73} }
 @article{li_ni_liu_zhang_liu_lee_avrutin_özgür_paskova_mulholland_et al._2011, title={On the reduction of efficiency loss in polar c -plane and non-polar m -plane InGaN light emitting diodes}, volume={8}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.201000893}, DOI={10.1002/pssc.201000893}, abstractNote={AbstractWe have undertaken a series of experiments in InGaN light emitting diode (LED) structures both on polar (c‐plane) and non‐polar (m‐plane) GaN substrates with and without magnesium doped AlGaN electron blocking layers (EBLs) on the p‐side of the p‐n junction to shed the much needed light on the carrier injection and transport. The LEDs grown on c‐plane bulk GaN substrates without EBL show 40% peak electroluminescence (EL) intensity, while the EL peak intensity of the LEDs grown on m‐plane bulk GaN substrates without EBL is 30% of that with EBL. However, optical measurements for internal quantum efficiency (IQE) reveal that the IQE values of LEDs with and without EBL are comparable for both cases, which are in the range of 80‐85% for m‐plane variety and 50% for c‐plane variety. Furthermore, with varying Al composition (15%, 8%, 0%) in the EBL, the EL intensity for both m‐pane and c‐plane LEDs decrease progressively as the Al composition decreases. When highly conductive and transparent Ga doped ZnO layers are used as the current spreading layers for LEDs, the degradation of the efficiency with injection is significantly reduced to 27% compared to LEDs with semitransparent Ni/Au contacts having 50% efficiency degradation up to the same current density ∼3500 A/cm2. This can be due to presumably the reduction in current filamentation or current crowding. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={5}, journal={physica status solidi (c)}, publisher={Wiley}, author={Li, X. and Ni, X. and Liu, H. Y. and Zhang, F. and Liu, S. and Lee, J. and Avrutin, V. and Özgür, Ü. and Paskova, T. and Mulholland, G. and et al.}, year={2011}, month={Mar}, pages={1560–1563} }
 @article{frajtag_hosalli_samberg_colter_paskova_el-masry_bedair_2012, title={Overgrowth of GaN on GaN nanowires produced by mask-less etching}, volume={352}, ISSN={["0022-0248"]}, DOI={10.1016/j.jcrysgro.2011.12.055}, abstractNote={We report on the generation of GaN nanowires (NWs) using mask-less reactive ion etching (RIE). The NWs are believed to be the result of a high etching rate in regions where a high dislocation density is present in the GaN films grown on sapphire substrates. We have studied the effect of defect densities in the original GaN films and its relation to the generation of these NWs. We show that defect reduction in the overgrown GaN is related to the presence of a network of embedded voids generated between these nanowires during the regrowth on the etched nanowires. We show that further reduction in dislocation density can be achieved by repeating the process of nanowire generation and overgrowth. Also we report on the residual strain and curvature in GaN after the first and second embedded voids approach (EVA).}, number={1}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Frajtag, P. and Hosalli, A. M. and Samberg, J. P. and Colter, P. C. and Paskova, T. and El-Masry, N. A. and Bedair, S. M.}, year={2012}, month={Aug}, pages={203–208} }
 @article{monemar_paskov_pozina_hemmingsson_bergman_lindgren_samuelson_ni_morkoç_paskova_et al._2011, title={Photoluminescence of Mg-doped m-plane GaN grown by MOCVD on bulk GaN substrates}, volume={208}, ISSN={1862-6300}, url={http://dx.doi.org/10.1002/pssa.201001036}, DOI={10.1002/pssa.201001036}, abstractNote={AbstractPhotoluminescence (PL) properties are reported for a set of m‐plane GaN films with Mg doping varied from mid 1018 cm−3 to above 1020 cm−3. The samples were grown with MOCVD at reduced pressure on low defect density bulk GaN templates. The sharp line near bandgap bound exciton (BE) spectra observed below 50 K, as well as the broader donor–acceptor pair (DAP) PL bands at 2.9–3.3 eV give evidence of several Mg related acceptors, similar to the case of c‐plane GaN. The dependence of the BE spectra on excitation intensity as well as the transient decay behaviour demonstrate acoustic phonon assisted transfer between the acceptor BE states. The lower energy donor–acceptor pair spectra suggest the presence of deep acceptors, in addition to the two main shallower ones at about 0.23 eV. Similar spectra from Mg‐doped GaN nanowires (NWs) grown by MOCVD are also briefly discussed.}, number={7}, journal={physica status solidi (a)}, publisher={Wiley}, author={Monemar, Bo and Paskov, Plamen and Pozina, Galia and Hemmingsson, Carl and Bergman, Peder and Lindgren, David and Samuelson, Lars and Ni, Xianfeng and Morkoç, Hadis and Paskova, Tanya and et al.}, year={2011}, month={Jun}, pages={1532–1534} }
 @article{mareš_hubík_krištofik_prušáková_uxa_paskova_evans_2011, title={Radial space-charge-limited electron flow in semi-insulating GaN:Fe}, volume={110}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.3609071}, DOI={10.1063/1.3609071}, abstractNote={Semi-insulating iron-doped gallium nitride (GaN:Fe), a promising material for microwave electronics and optoelectronics, is characterized by means of current-voltage curves measured under non-equilibrium charge carrier injection conditions. To increase the electric field and enable local characterization of GaN:Fe, measurements are performed using a two-electrode configuration including a point contact with a diameter of 30 μm and large-area alloyed counter contact. Experimental evidence for various radial flow regimes corresponding to the universal 3/2 and quadratic power laws and to the so-called trap-filled-limit is presented. Furthermore, it is shown that the space-charge-limited current injected from the point contact may serve as a probe to measure both local and average scale density of electrically active traps, i.e., parameters that are necessary for GaN:Fe to be applied and optimized.}, number={1}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Mareš, Jiří J. and Hubík, Pavel and Krištofik, Jozef and Prušáková, Lucie and Uxa, Štěpán and Paskova, Tanya and Evans, Keith}, year={2011}, month={Jul}, pages={013723} }
 @article{leach_wu_ni_li_xie_özgür_morkoç_paskova_preble_evans_et al._2010, title={Carrier velocity in InAlN/AlN/GaN heterostructure field effect transistors on Fe-doped bulk GaN substrates}, volume={96}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3358192}, DOI={10.1063/1.3358192}, abstractNote={We report microwave characteristics of field effect transistors employing InAlN/AlN/GaN heterostructures grown on low-defect-density bulk Fe-doped GaN substrates. We achieved unity current gain cutoff frequencies of 14.3 and 23.7 GHz for devices with gate lengths of 1 and 0.65 μm, respectively. Measurements as a function of applied bias allow us to estimate the average carrier velocity in the channel to be ∼1.0×107 cm/sec for a 1 μm device. Additionally, we found nearly no gate lag in the devices, which is considered a precondition for good performance under large signal operation.}, number={10}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Leach, J. H. and Wu, M. and Ni, X. and Li, X. and Xie, J. and Özgür, Ü. and Morkoç, H. and Paskova, T. and Preble, E. and Evans, K. R. and et al.}, year={2010}, month={Mar}, pages={102109} }
 @article{detchprohm_zhu_you_li_zhao_preble_paskova_hanser_wetzel_2010, title={Cyan and green light emitting diode on non-polar m -plane GaN bulk substrate}, volume={7}, ISSN={1862-6351 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200983611}, DOI={10.1002/pssc.200983611}, abstractNote={AbstractWe report the development of non‐polar light emitting diodes (LEDs) covering the emission spectra from 480 to 520 nm (dominant wavelength), i.e. from cyan to the green region. The devices are obtained via GaInN‐based homoepitaxy on non‐polar m ‐plane GaN bulk substrate. Owing to the absence of piezoelectric polarization, these LEDs exhibit stable emission color with a wavelength shift less than 3 nm when the drive current density is changed from 0.1–30 A/cm2. However, we observe a decreasing electroluminescence efficiency as the emission wavelengths increases from cyan to green. We tentatively attribute this to a higher density of defects in the longer wavelength structures. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7-8}, journal={physica status solidi (c)}, publisher={Wiley}, author={Detchprohm, Theeradetch and Zhu, Mingwei and You, Shi and Li, Yufeng and Zhao, Liang and Preble, Edward A. and Paskova, Tanya and Hanser, Drew and Wetzel, Christian}, year={2010}, month={Apr}, pages={2190–2192} }
 @article{kadlec_kadlec_paskova_evans_2010, title={Effect of Fe doping on the terahertz conductivity of GaN single crystals}, volume={43}, ISSN={0022-3727 1361-6463}, url={http://dx.doi.org/10.1088/0022-3727/43/14/145401}, DOI={10.1088/0022-3727/43/14/145401}, abstractNote={Bulk single crystals of GaN with different degrees of Fe doping were studied using time-domain terahertz spectroscopy at high temperatures. Features due to free carriers were observed in the complex permittivity spectra with a pronounced dependence on both doping and temperature. Fitting the spectra using the Drude model made it possible to deduce a defect ionization energy of 16 meV in the undoped sample while the spectra of doped samples are consistent with an ionization energy of 60 meV. Also, the free carrier concentrations at temperatures from 300 to 900 K were estimated.}, number={14}, journal={Journal of Physics D: Applied Physics}, publisher={IOP Publishing}, author={Kadlec, Filip and Kadlec, Christelle and Paskova, Tanya and Evans, Keith}, year={2010}, month={Mar}, pages={145401} }
 @article{leach_zhu_wu_ni_li_xie_özgür_morkoç_liberis_šermukšnis_et al._2010, title={Effect of hot phonon lifetime on electron velocity in InAlN/AlN/GaN heterostructure field effect transistors on bulk GaN substrates}, volume={96}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3358392}, DOI={10.1063/1.3358392}, abstractNote={We report on electron velocities deduced from current gain cutoff frequency measurements on GaN heterostructure field effect transistors (HFETs) with InAlN barriers on Fe-doped semi-insulating bulk GaN substrates. The intrinsic transit time is a strong function of the applied gate bias, and a minimum intrinsic transit time occurs for gate biases corresponding to two-dimensional electron gas densities near 9.3×1012 cm−2. This value correlates with the independently observed density giving the minimum longitudinal optical phonon lifetime. We expect the velocity, which is inversely proportional to the intrinsic transit time, to be limited by scattering with non equilibrium (hot) phonons at the high fields present in the HFET channel, and thus, we interpret the minimum intrinsic transit time in terms of the hot phonon decay. At the gate bias associated with the minimum transit time, we determined the average electron velocity for a 1.1 μm gate length device to be 1.75±0.1×107 cm/sec.}, number={13}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Leach, J. H. and Zhu, C. Y. and Wu, M. and Ni, X. and Li, X. and Xie, J. and Özgür, Ü. and Morkoç, H. and Liberis, J. and Šermukšnis, E. and et al.}, year={2010}, month={Mar}, pages={133505} }
 @article{lai_paskova_wheeler_grenko_johnson_udwary_preble_evans_2010, title={Effect of m-plane GaN substrate miscut on InGaN/GaN quantum well growth}, volume={312}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2010.01.020}, abstractNote={The effect of m-plane GaN substrate miscut on the growth of InGaN/GaN quantum wells (QWs) was investigated. It was found that the miscut toward [0 0 0 1] c+-axis resulted in an increase of In incorporation efficiency and in a green-shift of the QW emission, while the miscut toward [1 1 2 0] a-axis resulted in even higher In compositions but it also led to an increased epitaxial surface roughness and deterioration of the QW structures. The results indicated that miscut toward a-axis is undesirable while miscut toward c+-axis is beneficial for achieving longer wavelength emission in QWs grown on m-plane GaN substrates.}, number={7}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Lai, K. Y. and Paskova, T. and Wheeler, V. D. and Grenko, J. A. and Johnson, M. A. L. and Udwary, K. and Preble, E. A. and Evans, K. R.}, year={2010}, month={Mar}, pages={902–905} }
 @article{ni_li_lee_liu_avrutin_özgür_morkoç_matulionis_paskova_mulholland_et al._2010, title={InGaN staircase electron injector for reduction of electron overflow in InGaN light emitting diodes}, volume={97}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3465658}, DOI={10.1063/1.3465658}, abstractNote={Ballistic and quasiballistic electron transport across the active InGaN layer are shown to be responsible for electron overflow and electroluminescence efficiency droop at high current levels in InGaN light emitting diodes both experimentally and by first-order calculations. An InGaN staircase electron injector with step-like increased In composition, an “electron cooler,” is proposed for an enhanced thermalization of the injected hot electrons to reduce the overflow and mitigate the efficiency droop. The experimental data show that the staircase electron injector results in essentially the same electroluminescence performance for the diodes with and without an electron blocking layer, confirming substantial electron thermalization. On the other hand, if no InGaN staircase electron injector is employed, the diodes without the electron blocking layer have shown significantly lower (three to five times) electroluminescence intensity than the diodes with the blocking layer. These results demonstrate a feasible method for the elimination of electron overflow across the active region, and therefore, the efficiency droop in InGaN light emitting diodes.}, number={3}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Ni, X. and Li, X. and Lee, J. and Liu, S. and Avrutin, V. and Özgür, Ü. and Morkoç, H. and Matulionis, A. and Paskova, T. and Mulholland, G. and et al.}, year={2010}, month={Jul}, pages={031110} }
 @article{xu_alur_wang_cheng_kang_sharma_park_ahyi_williams_gu_et al._2010, title={In Situ Raman Analysis of a Bulk GaN-Based Schottky Rectifier Under Operation}, volume={39}, ISSN={0361-5235 1543-186X}, url={http://dx.doi.org/10.1007/S11664-010-1304-3}, DOI={10.1007/S11664-010-1304-3}, number={10}, journal={Journal of Electronic Materials}, publisher={Springer Science and Business Media LLC}, author={Xu, Hui and Alur, Siddharth and Wang, Yaqi and Cheng, An-Jen and Kang, Kilho and Sharma, Yogeshkumar and Park, Minseo and Ahyi, Claude and Williams, John and Gu, Chaokang and et al.}, year={2010}, month={Jun}, pages={2237–2242} }
 @article{monemar_paskov_pozina_hemmingsson_bergman_amano_akasaki_figge_hommel_paskova_et al._2010, title={Mg-related acceptors in GaN}, volume={7}, ISSN={1862-6351 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200983436}, DOI={10.1002/pssc.200983436}, abstractNote={AbstractPhotoluminescence spectra of c ‐plane Mg doped GaN samples grown by MOVPE on bulk GaN templates reveal previously unknown properties, like the presence of several Mg‐related acceptors. The use of unstrained samples allows a study of both bound exciton (BE) and donor‐acceptor pair (DAP) spectra. Two main acceptors A1 and A2 are observed strongly in BE spectra as well as in DAP spectra, they have similar binding energies, i.e. about 220 meV. The common assignment of the deeper blue PL emission at 2.8–3.0 eV to a deep donor‐shallow acceptor transition is questioned, and discussed in connection with the compensation problem in p‐GaN. It seems like the Fermi level in p‐GaN is controlled by a set of Mg‐related acceptors at energies 0.2–0.6 eV from the valence band top. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7-8}, journal={physica status solidi (c)}, publisher={Wiley}, author={Monemar, B. and Paskov, P. P. and Pozina, G. and Hemmingsson, C. and Bergman, J. P. and Amano, H. and Akasaki, I. and Figge, S. and Hommel, D. and Paskova, T. and et al.}, year={2010}, month={Jun}, pages={1850–1852} }
 @article{grenko_ebert_reynolds_duscher_barlage_johnson_preble_paskova_evans_2010, title={Optimization of homoepitaxially grown AlGaN/GaN heterostructures}, volume={207}, ISSN={["1862-6300"]}, DOI={10.1002/pssa.200925508}, abstractNote={AbstractWe report on the growth of Al0.25Ga0.75N/GaN heterostructures on low dislocation density semi‐insulating c‐axis GaN substrates by metalorganic vapor phase epitaxy (MOVPE). A room temperature (RT) Hall mobility (µRT) up to 2065 cm2 V−1 s−1 at sheet density (ns) of 8.25 × 1012 cm−2 has been measured. This work compliments prior studies in which we observed a buffer‐induced modulation of the RT two‐dimensional electron gas (2DEG) ns and µRT by varying the GaN buffer layer thickness. Here, we focus on the optimization of the heterostructure 2DEG properties by elimination of silicon doping in the Al0.25Ga0.75N barrier and unintentional Al in the not‐intentionally doped (n.i.d.) GaN buffer layer. The 15% improvement in µRT and ns relative to previous results is consistent with those predicted by Poisson solver calculations. Use of thick GaN buffers has minimized the theoretical mobility reduction based on intersubband scattering and has enabled us to determine the 2DEG sheet density associated with the polarization field ($n_{{\rm s}}^{{\rm polar}} $) to be ∼5 × 1012 cm−2.}, number={10}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Grenko, J. A. and Ebert, C. W. and Reynolds, C. L., Jr. and Duscher, G. J. and Barlage, D. W. and Johnson, M. A. L. and Preble, E. A. and Paskova, T. and Evans, K. R.}, year={2010}, month={Oct}, pages={2292–2299} }
 @article{grenko_reynolds_barlage_johnson_lappi_ebert_preble_paskova_evans_2010, title={Physical Properties of AlGaN/GaN Heterostructures Grown on Vicinal Substrates}, volume={39}, ISSN={0361-5235 1543-186X}, url={http://dx.doi.org/10.1007/S11664-010-1153-0}, DOI={10.1007/S11664-010-1153-0}, number={5}, journal={Journal of Electronic Materials}, publisher={Springer Science and Business Media LLC}, author={Grenko, J. A. and Reynolds, C. L., Jr. and Barlage, D. W. and Johnson, M. A. L. and Lappi, S. E. and Ebert, C. W. and Preble, E. A. and Paskova, T. and Evans, K. R.}, year={2010}, month={Mar}, pages={504–516} }
 @article{storm_katzer_deen_bass_meyer_roussos_binari_paskova_preble_evans_2010, title={Proximity effects of beryllium-doped GaN buffer layers on the electronic properties of epitaxial AlGaN/GaN heterostructures}, volume={54}, ISSN={0038-1101}, url={http://dx.doi.org/10.1016/j.sse.2010.05.041}, DOI={10.1016/j.sse.2010.05.041}, abstractNote={AlGaN/GaN/Be:GaN heterostructures have been grown by rf-plasma molecular beam epitaxy on free-standing semi-insulating GaN substrates, employing unintentionally-doped (UID) GaN buffer layers with thicknesses, dUID, varying between 50 nm and 500 nm. We have found that the heterostructures with UID buffers thicker than 200 nm exhibit much improved Hall properties and inter-device isolation current compared to heterostructures with dUID < 200 nm. The output conductance of devices fabricated on these heterostructures increases as dUID decreases below 200 nm, and devices with gate lengths of 240 nm and 1 μm exhibited no significant difference in output conductance. Evidence of buffer trapping is observed in devices for which dUID ⩽ 100 nm. The observed effects are tentatively explained by the presence of parallel conduction paths in samples with non-optimized UID buffer thickness.}, number={11}, journal={Solid-State Electronics}, publisher={Elsevier BV}, author={Storm, D.F. and Katzer, D.S. and Deen, D.A. and Bass, R. and Meyer, D.J. and Roussos, J.A. and Binari, S.C. and Paskova, T. and Preble, E.A. and Evans, K.R.}, year={2010}, month={Nov}, pages={1470–1473} }
 @article{son_hemmingsson_morishita_ohshima_paskova_evans_usui_isoya_monemar_janzén_2010, title={Radiation-induced defects in GaN}, volume={T141}, ISSN={0031-8949 1402-4896}, url={http://dx.doi.org/10.1088/0031-8949/2010/T141/014015}, DOI={10.1088/0031-8949/2010/T141/014015}, abstractNote={Radiation-induced defects in n-type GaN irradiated by 2 MeV electrons at room temperature were studied by electron paramagnetic resonance (EPR). Four EPR spectra, labelled D1–D4, were observed in irradiated n-type GaN. The D1 spectrum is a broad line (∼10–12 mT in line width) with an isotropic g-value g∼2.03 and can be detected in all the studied samples in the temperature range of 4–300 K. The D2 centre has an electron spin S=1/2 and shows a clear hyperfine structure due to interaction with three equivalent 14N. The g-values of the axial configuration are determined to be g∥=2.001 and g⊥=1.999. On the basis of the observed hyperfine structure, formation conditions and annealing behaviour, the D2 defect was assigned to the gallium vacancy–oxygen pair in the negative charge state, (VGaON)–.}, journal={Physica Scripta}, publisher={IOP Publishing}, author={Son, N T and Hemmingsson, C G and Morishita, N and Ohshima, T and Paskova, T and Evans, K R and Usui, A and Isoya, J and Monemar, B and Janzén, E}, year={2010}, month={Nov}, pages={014015} }
 @article{weyher_łucznik_grzegory_smalc-koziorowska_paskova_2010, title={Revealing extended defects in HVPE-grown GaN}, volume={312}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2010.04.021}, DOI={10.1016/j.jcrysgro.2010.04.021}, abstractNote={In this communication we will summarize the results of a complementary study of structural and chemical non-homogeneities that are present in thick HVPE-grown GaN layers. It will be shown that complex extended defects are formed during HVPE growth, and are clearly visible after photo-etching on both Ga-polar surface and on any non-polar cleavage or section planes. Large chemical (electrically active) defects, such as growth striations, overgrown or empty pits (pinholes) and clustered irregular inclusions, are accompanied by non-uniform distribution of crystallographic defects (dislocations). Possible reasons of formation of these complex structures are discussed. The nature of defects revealed by selective etching was subsequently confirmed using TEM, orthodox etching and compared with the CL method. The non-homogeneities were studied in GaN crystals grown in different laboratories showing markedly different morphological characteristics.}, number={18}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Weyher, J.L. and łucznik, B. and Grzegory, I. and Smalc-Koziorowska, J. and Paskova, T.}, year={2010}, month={Sep}, pages={2611–2615} }
 @article{ni_li_lee_liu_avrutin_özgür_morkoç_matulionis_paskova_mulholland_et al._2010, title={The effect of ballistic and quasi-ballistic electrons on the efficiency droop of InGaN light emitting diodes}, volume={4}, ISSN={1862-6254}, url={http://dx.doi.org/10.1002/pssr.201004147}, DOI={10.1002/pssr.201004147}, abstractNote={AbstractThe effect of hot electrons on electroluminescence of m ‐plane double heterostructure light emitting diodes (with a single 6 nm active layer of In0.20Ga0.80N) is investigated. Diodes with an electron blocking layer (Al0.15Ga0.85N) demonstrate from 3 to 5 times higher electroluminescence efficiency than those without a blocking layer. The lower electroluminescence efficiency in devices without the blocking layer is ex‐ plained in terms of electron overflow caused by ballistic and quasi‐ballistic transport of injected electrons across the InGaN active region. The same mechanism explains the decrease, observed at high current densities, of the electroluminescence efficiency (efficiency droop) in the In0.20Ga0.80N diodes with the Al0.15Ga0.85N blocking layer. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={8-9}, journal={physica status solidi (RRL) - Rapid Research Letters}, publisher={Wiley}, author={Ni, X. and Li, X. and Lee, J. and Liu, S. and Avrutin, V. and Özgür, Ü. and Morkoç, H. and Matulionis, A. and Paskova, T. and Mulholland, G. and et al.}, year={2010}, month={Jun}, pages={194–196} }
 @article{wang_alur_sharma_tong_thapa_gartland_issacs-smith_ahyi_williams_park_et al._2010, title={Ultra-low leakage and high breakdown Schottky diodes fabricated on free-standing GaN substrate}, volume={26}, ISSN={0268-1242 1361-6641}, url={http://dx.doi.org/10.1088/0268-1242/26/2/022002}, DOI={10.1088/0268-1242/26/2/022002}, abstractNote={Vertical Schottky diodes were fabricated on the bulk GaN substrate with decreasing impurity concentration from N-face to Ga-face. An array of circular Pt Schottky contacts and a full backside Ti/Al/Ni/Au ohmic contact were prepared on the Ga-face and the N-face of the n-GaN substrate, respectively. The Schottky diode exhibits a minimum specific on-state resistance of 1.3 mΩ cm2 and a maximum breakdown voltage of 600 V, resulting in a figure-of- merit of 275 MW cm−2. An ultra-low reverse leakage current density of 3.7 × 10−4 A cm−2 at reverse bias of 400 V was observed. Temperature-dependent I–V measurements were also carried out to study the forward and reverse transportation mechanisms.}, number={2}, journal={Semiconductor Science and Technology}, publisher={IOP Publishing}, author={Wang, Yaqi and Alur, Siddharth and Sharma, Yogesh and Tong, Fei and Thapa, Resham and Gartland, Patrick and Issacs-Smith, Tamara and Ahyi, Claude and Williams, John and Park, Minseo and et al.}, year={2010}, month={Dec}, pages={022002} }
 @article{zhu_you_detchprohm_paskova_preble_wetzel_2010, title={Various misfit dislocations in green and yellow GaInN/GaN light emitting diodes}, volume={207}, ISSN={1862-6300}, url={http://dx.doi.org/10.1002/pssa.200983645}, DOI={10.1002/pssa.200983645}, abstractNote={AbstractWe report the growth and structural characteristics of green and yellow (529–576 nm) GaInN/GaN light emitting diodes (LEDs) grown on two types of c‐plane substrates – bulk GaN and sapphire. In this longer wavelength range, depending on the substrate, we find different strain relaxation mechanisms within the GaInN/GaN quantum well (QW) region. In optimized epitaxy, structures on sapphire that contain a low density of threading dislocations (TDs) within the n‐GaN show virtually no generation of additional misfit dislocations (MDs) (<108 cm−2) or V‐defects within the QW region for emission wavelengths up to 571 nm. On bulk GaN substrate, however, where much fewer TDs reach the QWs, strain relaxation is observed by inclined dislocation pairs in green emitters and a high density of edge‐type MDs in yellow emitters. The electroluminescence line width, as well as the efficiency droop, was found to increase with dislocation density in the QWs.}, number={6}, journal={physica status solidi (a)}, publisher={Wiley}, author={Zhu, Mingwei and You, Shi and Detchprohm, Theeradetch and Paskova, Tanya and Preble, Edward A. and Wetzel, Christian}, year={2010}, month={May}, pages={1305–1308} }
 @article{detchprohm_zhu_li_zhao_you_wetzel_preble_paskova_hanser_2010, title={Wavelength-stable cyan and green light emitting diodes on nonpolar m-plane GaN bulk substrates}, volume={96}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3299257}, DOI={10.1063/1.3299257}, abstractNote={We report the development of 480 nm cyan and 520 nm green light emitting diodes (LEDs) with a highly stable emission wavelength. The shift is less than 3 nm when the drive current density is changed from 0.1 to 38 A/cm2. LEDs have been obtained in GaInN-based homoepitaxy on nonpolar m-plane GaN bulk substrates. For increasing emission wavelength we find a large number of additional dislocations generated within the quantum wells (2×108 to ∼1010 cm2) and a decrease in the electroluminescence efficiency. This suggests that the strain induced generation of defects plays a significant role in the performance limitations.}, number={5}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Detchprohm, Theeradetch and Zhu, Mingwei and Li, Yufeng and Zhao, Liang and You, Shi and Wetzel, Christian and Preble, Edward A. and Paskova, Tanya and Hanser, Drew}, year={2010}, month={Feb}, pages={051101} }
 @article{lai_johnson_paskova_hanser_udwary_preble_evans_2009, title={Cathodoluminescence evaluation of subsurface damage in GaN substrate after polishing}, volume={6}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.200880891}, DOI={10.1002/pssc.200880891}, abstractNote={AbstractThe surface of HVPE grown GaN substrates were treated with two different polishing procedures. Both procedures were successful in producing highly smooth and featureless surfaces. However, subsurface damage was observed in the sample treated by one of the procedures. The subsurface damage was revealed by cathodoluminescence (CL) spectroscopy imaging but was not visible by scanning electron microscopy (SEM) or Atomic force microscopy (AFM). Thermal annealing at 950 °C with different gases was performed in an attempt to remove subsurface damages. Annealing in gas mixtures containing H2 increased both the presence of surface scratches and overall surface roughness. On the other hand, annealing in mixtures of NH3 and N2 (with no H2) led to the surfaces with significantly reduced subsurface damage. The surface roughness and optical properties of the sample after the annealing with such gas mixtures were slightly sacrificed. In order to evaluate subsurface damage depth, CL images were taken from the annealed surface for different acceleration voltages. The results suggest that the observed subsurface damages were within 1.48 μm of the surface. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={S2}, journal={physica status solidi (c)}, publisher={Wiley}, author={Lai, K. Y. and Johnson, M. A. L. and Paskova, T. and Hanser, A. D. and Udwary, K. and Preble, E. A. and Evans, K. R.}, year={2009}, month={Jan}, pages={S325–S328} }
 @article{gladkov_humlíček_hulicius_šimeček_paskova_evans_2010, title={Effect of Fe doping on optical properties of freestanding semi-insulating HVPE GaN:Fe}, volume={312}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2009.11.032}, DOI={10.1016/j.jcrysgro.2009.11.032}, abstractNote={Systematic study of optical properties of undoped and Fe-doped
substrates grown by hydride vapor phase epitaxy has revealed a
strong dependence of the photoluminescence, transmission,
reflectivity and ellipsometric spectra on the Fe-doping level.
The changes of the near-band-gap transmission, reflectivity and
photoluminescence has been observed and ascribed to the
absorption introduced by the density of states tails, and the
Fe3+ -ions incorporated in the GaN-lattice. Several approaches
towards quantifying the Fe-doping level are suggested.}, number={8}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Gladkov, P. and Humlíček, J. and Hulicius, E. and Šimeček, T. and Paskova, T. and Evans, K.}, year={2010}, month={Apr}, pages={1205–1209} }
 @article{li_ni_lee_wu_özgür_morkoç_paskova_mulholland_evans_2009, title={Efficiency retention at high current injection levels in m-plane InGaN light emitting diodes}, volume={95}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3236538}, DOI={10.1063/1.3236538}, abstractNote={We investigated the internal quantum efficiency (IQE) and the relative external quantum efficiency (EQE) of m-plane InGaN light emitting diodes (LEDs) grown on m-plane freestanding GaN emitting at ∼400 nm for current densities up to 2500 A/cm2. IQE values extracted from intensity and temperature dependent photoluminescence measurements were consistently higher, by some 30%, for the m-plane LEDs than for reference c-plane LEDs having the same structure, e.g., 80% versus 60% at an injected steady-state carrier concentration of 1.2×1018 cm−3. With increasing current injection up to 2500 A/cm2, the maximum EQE is nearly retained in m-plane LEDs, whereas c-plane LEDs exhibit approximately 25% droop. The negligible droop in m-plane LEDs is consistent with the reported enhanced hole carrier concentration and light holes in m-plane orientation, thereby enhanced hole transport throughout the active region, and lack of polarization induced field. A high quantum efficiency and in particular its retention at high injection levels bode well for m-plane LEDs as candidates for general lighting applications.}, number={12}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Li, X. and Ni, X. and Lee, J. and Wu, M. and Özgür, Ü. and Morkoç, H. and Paskova, T. and Mulholland, G. and Evans, K. R.}, year={2009}, month={Sep}, pages={121107} }
 @article{lai_paskova_wheeler_grenko_johnson_barlage_udwary_preble_evans_2009, title={Excitation current dependent cathodoluminescence study of InGaN/GaN quantum wells grown on m-plane and c-plane GaN substrates}, volume={106}, ISSN={["1089-7550"]}, DOI={10.1063/1.3264729}, abstractNote={InGaN/GaN quantum wells (QWs) grown with identical conditions on m-plane and c-plane GaN substrates were studied by cathodoluminescence spectroscopy. At a low current of 10 nA, the emission intensity and wavelength of the m-plane aligned QWs were found to be about two times stronger and 19.5 nm blueshifted with respect to that of the c-plane aligned QWs. An increase in the current over three orders of magnitude was found to result in an increase in the emission intensities, with faster saturation in the m-plane aligned QWs. This was explained by the screening of quantum-confined Stark effect in the emission efficiency of the c-plane aligned QWs.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Lai, K. Y. and Paskova, T. and Wheeler, V. D. and Grenko, J. A. and Johnson, M. A. L. and Barlage, D. W. and Udwary, K. and Preble, E. A. and Evans, K. R.}, year={2009}, month={Dec} }
 @article{ni_lee_wu_li_shimada_özgür_baski_morkoç_paskova_mulholland_et al._2009, title={Internal quantum efficiency of c-plane InGaN and m-plane InGaN on Si and GaN}, volume={95}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3224192}, DOI={10.1063/1.3224192}, abstractNote={We investigated internal quantum efficiency (IQE) of polar (0001) InGaN on c-sapphire, and (11¯00) nonpolar m-plane InGaN on both m-plane GaN and specially patterned Si. The IQE values were extracted from the resonant photoluminescence intensity versus the excitation power. Data indicate that at comparable generated carrier concentrations the efficiency of the m-plane InGaN on patterned Si is approximately a factor of 2 higher than that of the highly optimized c-plane layer. At the highest laser excitation employed (∼1.2×1018 cm−3), the IQE of m-plane InGaN double heterostructure on Si is approximately 65%. We believe that the m-plane would remain inherently advantageous, particularly at high electrical injection levels, even with respect to highly optimized c-plane varieties. The observations could be attributed to the lack of polarization induced field and the predicted increased optical matrix elements in m-plane orientation.}, number={10}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Ni, X. and Lee, J. and Wu, M. and Li, X. and Shimada, R. and Özgür, Ü. and Baski, A. A. and Morkoç, H. and Paskova, T. and Mulholland, G. and et al.}, year={2009}, month={Sep}, pages={101106} }
 @article{grenko_ebert_reynolds_johnson_hanser_preble_paskova_evans_2009, title={Modulation of mobility in homoepitaxially-grown AlGaN/GaN heterostructures}, volume={6}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.200880907}, DOI={10.1002/pssc.200880907}, abstractNote={AbstractWe report on the growth of Al0.25Ga0.75N/GaN heterostructures on low dislocation density semi‐insulating c‐axis GaN substrates. Room temperature Hall mobilities up to 1805 cm2/Vs at sheet carrier densities of 0.77x1013 cm–2 have been measured. By varying the GaN buffer layer thickness in these homoepitaxially‐grown Al0.25Ga0.75N/GaN heterostructures, we observed a buffer‐induced modulation of the room temperature 2DEG sheet carrier densities and Hall mobilities. The increase in sheet carrier density and corresponding decrease in mobility as the GaN buffer layer thickness is reduced below 0.75 μm is related to the presence of Si impurities at the bulk GaN substrate/epitaxial interface. Capacitance‐voltage measurements and SIMS analysis confirm the presence of Si impurities at the surface prior to and after epitaxial growth. The factor of 2 reduction in the room temperature mobility is consistent with a predicted theoretical mobility reduction based on intersubband scattering. We have also been able to separate the contributions to the 2DEG carrier density from the ionized donors and the polarization field; the magnitude of each is ∼5x1012 cm–2. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={S2}, journal={physica status solidi (c)}, publisher={Wiley}, author={Grenko, J. A. and Ebert, C. W. and Reynolds, C. L., Jr. and Johnson, M. A. L. and Hanser, A. D. and Preble, E. A. and Paskova, T. and Evans, K. R.}, year={2009}, month={Jan}, pages={S1037–S1040} }
 @article{lee_li_ni_özgür_morkoç_paskova_mulholland_evans_2009, title={On carrier spillover in c- and m-plane InGaN light emitting diodes}, volume={95}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3266833}, DOI={10.1063/1.3266833}, abstractNote={The internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) in InGaN light-emitting diodes (LEDs) emitting at 400 nm with and without electron blocking layers (EBLs) on c-plane GaN and m-plane GaN were investigated in order to shed some light on any effect of polarization charge induced field on efficiency killer carrier spillover. Without an EBL the EQE values suffered considerably (by 80%) for both orientations, which is clearly attributable to carrier spillover. Substantial carrier spillover in both polarities, therefore, suggests that the polarization charge is not the major factor in efficiency degradation observed, particularly at high injection levels. Furthermore, the m-plane variety with EBL did not show any discernable efficiency degradation up to a maximum current density of 2250 A cm−2 employed while that on c-plane showed a reduction by ∼40%. In addition, IQE of m-plane LED structure determined from excitation power dependent photoluminescence was ∼80% compared to 50% in c-plane LEDs under resonant and moderate excitation condition. This too is indicative of the superiority of m-plane LED structures, most probably due to relatively larger optical matrix elements for m-plane orientation.}, number={20}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Lee, J. and Li, X. and Ni, X. and Özgür, Ü. and Morkoç, H. and Paskova, T. and Mulholland, G. and Evans, K. R.}, year={2009}, month={Nov}, pages={201113} }
 @article{paskov_monemar_paskova_preble_hanser_evans_2009, title={Optical characterization of bulk GaN substrates with c -, a -, and m -plane surfaces}, volume={6}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.200880918}, DOI={10.1002/pssc.200880918}, abstractNote={AbstractThick free‐standing GaN grown by hydride vapour phase epitaxy and epi‐ready substrates with c ‐, a ‐, m ‐plane surfaces are examined by variable‐temperature photoluminescence (PL), polarized PL and spatially resolved micro‐PL. Both as‐grown samples and polished substrates exhibit linewidth of the donor‐bound exciton emission below 0.7 meV at 2 K indicative of a high structural quality of the material. For as‐grown samples the relative intensity of green (2.4 eV) and red (1.8 eV) deep‐level‐defect emissions are found to decrease with increasing sample thickness. Based on plentiful two‐electron‐transition spectra measured in the samples the electronic fine structure of the donors and their bound‐exciton complexes was examined and discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={S2}, journal={physica status solidi (c)}, publisher={Wiley}, author={Paskov, P. P. and Monemar, B. and Paskova, T. and Preble, E. A. and Hanser, A. D. and Evans, K. R.}, year={2009}, month={Jan}, pages={S763–S766} }
 @article{paskova_preble_hanser_evans_kröger_paskov_cheng_park_grenko_johnson_2009, title={Polar and nonpolar HVPE GaN substrates: impact of doping on the structural, electrical and optical characteristics}, volume={6}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.200880912}, DOI={10.1002/pssc.200880912}, abstractNote={AbstractPolar and nonpolar bulk GaN substrates with low defect density and high structural and optical quality are demonstrated. The effect of doping by silicon, oxygen and iron within moderate doping levels on the properties of the polar GaN substrates was found uncompromised, as confirmed by high resolution X‐ray diffraction and low temperature photoluminescence spectroscopy. In contrast, the lattice parameters were affected significantly, which has to be considered in the subsequent homoepitaxial device growth. The boule growth and respectively the nonpolar substrate homogeneity were found to be hampered by the doping, due to surface microcracking and higher impurity incorporation, while n‐type undoped nonpolar substrates were demonstrated of superior quality. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={S2}, journal={physica status solidi (c)}, publisher={Wiley}, author={Paskova, T. and Preble, E. A. and Hanser, A. D. and Evans, K. R. and Kröger, R. and Paskov, P. P. and Cheng, A. J. and Park, M. and Grenko, J. A. and Johnson, M. A. L.}, year={2009}, month={Feb}, pages={S344–S347} }
 @article{paskova_2008, title={Development and prospects of nitride materials and devices with nonpolar surfaces}, volume={245}, ISSN={0370-1972 1521-3951}, url={http://dx.doi.org/10.1002/pssb.200743274}, DOI={10.1002/pssb.200743274}, abstractNote={AbstractThe quest to use nonpolar surfaces of nitride materials and devices started a few years ago with the aim to avoid the strong internal electric fields in active regions of optoelec‐tronic devices and to improve their efficiency. Starting with the growth optimizations, the progression via thorough understanding of new physical properties of the materials has led to significant improvement of device performance and to novel device concepts. In this review a historical survey of nonpolar nitride growth achievements is made. Along the way new challenges in material growth and characterization have been encountered and more sophisticated methods have been developed, which are briefly summarized. The main properties of the nitride materials grown in nonpolar directions are discussed with emphasis on the deviations from those of nitrides grown along the polar direction. Physical phenomena such as inherently present anisotropic in‐plane strain and optical polarization anisotropy have been proposed for the realization of novel polarized light‐emitting diodes, polarization‐sensitive detectors and modulators. The present status of the nitride devices with nonpolar and semipolar surfaces is discussed, and an outlook of the future trends is presented. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={6}, journal={physica status solidi (b)}, publisher={Wiley}, author={Paskova, Tanya}, year={2008}, month={Jun}, pages={1011–1025} }
 @article{lai_wheeler_grenko_johnson_hanser_preble_liu_paskova_evans_2008, title={Enlargement of bulk non-polar GaN substrates by HVPE regrowth}, volume={5}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200778743}, DOI={10.1002/pssc.200778743}, abstractNote={AbstractM‐plane non‐polar bulk GaN substrates were regrown by hydride vapour phase epitaxy (HVPE), aiming to enlarge their lateral size. The non‐polar GaN substrates were synthesized by growing thick GaN boules by HVPE along the c‐axis, and wafering transversely to expose non‐polar (m‐ or a‐plane) surfaces. Non‐polar GaN substrates obtained in this manner contains lower defect density comparing to those produced by heteroepitaxial growth. After the regrowth, substantial lateral expansion in [0001], [1120] and [1120] was observed. Threading dislocation densities were estimated by cathodoluminescence measured at the cross‐section areas. The dislocation densities in the substrate and the regrown areas were approximately 5×106 cm–2 and 1×106 cm–2, respectively, indicating a comparable structural quality achieved by the regrowth. Non‐polar GaN substrates prepared by this process provide a promising method to fabricate devices for next generation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={6}, journal={physica status solidi (c)}, publisher={Wiley}, author={Lai, K. Y. and Wheeler, V. D. and Grenko, J. A. and Johnson, M. A. L. and Hanser, A. D. and Preble, E. A. and Liu, L. and Paskova, T. and Evans, K. R.}, year={2008}, month={May}, pages={1886–1888} }
 @article{hanser_liu_preble_udwary_paskova_evans_2008, title={Fabrication and characterization of native non-polar GaN substrates}, volume={310}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2008.06.029}, DOI={10.1016/j.jcrysgro.2008.06.029}, abstractNote={Thick c-plane (0 0 0 1)-oriented native GaN boules that have been produced by hydride vapor phase epitaxy and non-polar native m-plane (1 1 0 0) and a-plane (1 1 2 0) GaN substrates have been sliced from these crystals using a multiwire saw. An optimized polishing procedure was used to achieve a smooth epi-ready surface morphology on the finished substrates, with an RMS roughness of 0.43 nm. The non-polar substrates had two types of structural characteristics in appearance : one group was uniform, transparent and nearly colorless; while the second group had regions of varying coloration resulting from transecting V-shape pitting defects in the bulk GaN crystal. These regions had different cathodoluminescence properties but similar dislocation densities of <106 cm−2. The native non-polar GaN substrates had orders of magnitude lower defect densities, including stacking faults, in comparison to heteroepitaxially grown quasi-substrates in non-polar direction on foreign substrates and subsequently delaminated. The structural characteristics demonstrated the current state-of-the-art in non-polar GaN substrate quality and additionally point at remaining improvement opportunities in substrate size and uniformity.}, number={17}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Hanser, D. and Liu, L. and Preble, E.A. and Udwary, K. and Paskova, T. and Evans, K.R.}, year={2008}, month={Aug}, pages={3953–3956} }
 @article{detchprohm_zhu_li_xia_wetzel_preble_liu_paskova_hanser_2008, title={Green light emitting diodes on a-plane GaN bulk substrates}, volume={92}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2945664}, DOI={10.1063/1.2945664}, abstractNote={We report the development of 520–540nm green light emitting diodes (LEDs) grown along the nonpolar a axis of GaN. GaInN∕GaN-based quantum well structures were grown in homoepitaxy on both, a-plane bulk GaN and a-plane GaN on r-plane sapphire. LEDs on GaN show higher, virtually dislocation-free crystalline quality and three times higher light output power when compared to those on r-plane sapphire. Both structures show a much smaller wavelength blue shift for increasing current density (<10nm for 0.1to12.7A∕cm2) than conventional LEDs grown along the polar c axis.}, number={24}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Detchprohm, Theeradetch and Zhu, Mingwei and Li, Yufeng and Xia, Yong and Wetzel, Christian and Preble, Edward A. and Liu, Lianghong and Paskova, Tanya and Hanser, Drew}, year={2008}, month={Jun}, pages={241109} }
 @article{paskova_becker_böttcher_hommel_paskov_monemar_2007, title={Bending in HVPE grown GaN films: origin and reduction possibilities}, volume={4}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200674819}, DOI={10.1002/pssc.200674819}, abstractNote={AbstractWe have studied the effects of film and substrate thicknesses on the bending and strain of thick GaN films grown by HVPE. Both experimental and simulation data, being in a very good agreement, show the highest bending at a critical film thickness slightly smaller than the substrate thickness, while the strain remains decreasing with only a small increase at much higher film thicknesses. Temperature dependent measurements allows a separation of the thermally induced component and an extrapolation of the bending to the growth temperature. The latter was found to be dependent on both the film and substrate thickness and to have a strong effect on the remaining bending of freestanding films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (c)}, publisher={Wiley}, author={Paskova, T. and Becker, L. and Böttcher, T. and Hommel, D. and Paskov, P. P. and Monemar, B.}, year={2007}, month={Jun}, pages={2256–2259} }
 @article{tuomisto_paskova_kröger_figge_hommel_monemar_kersting_2007, title={Defect distribution in a-plane GaN on Al2O3}, volume={90}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2715128}, DOI={10.1063/1.2715128}, abstractNote={The authors studied the structural and point defect distributions of hydride vapor phase epitaxial GaN film grown in the [11−20] a direction on (1−102) r-plane sapphire with metal-organic vapor phase deposited a-GaN template using transmission electron microscopy, secondary ion mass spectrometry, and positron annihilation spectroscopy. Grown-in extended and point defects show constant behavior as a function of thickness, contrary to the strong nonuniform defect distribution observed in GaN grown along the [0001] direction. The observed differences are explained by orientation-dependent and kinetics related defect incorporation.}, number={12}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Tuomisto, F. and Paskova, T. and Kröger, R. and Figge, S. and Hommel, D. and Monemar, B. and Kersting, R.}, year={2007}, month={Mar}, pages={121915} }
 @article{kröger_paskova_monemar_figge_hommel_rosenauer_2007, title={Defect structure ofa-plane GaN grown by hydride and metal-organic vapor phase epitaxy onr-plane sapphire}, volume={4}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200674909}, DOI={10.1002/pssc.200674909}, abstractNote={AbstractTo obtain a deeper insight into the mechanism of defect formation in a‐plane GaN the defect nature in such films grown by metal‐organic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) was investigated by means of transmission electron microscopy. The films showed, beside the frequently found basal plane stacking faults and threading dislocations, prismatic stacking faults originating at the film/substrate interface and facetted voids. The density of basal plane stacking fault was about 105 cm–1 and the density of partial dislocations was in the range of 1010 cm–2, accordingly. Using selected area diffraction the epitaxial relationship between film and layer was determined. Based on these findings the impact of lattice mismatch on the observed defect characteristic is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (c)}, publisher={Wiley}, author={Kröger, R. and Paskova, T. and Monemar, B. and Figge, S. and Hommel, D. and Rosenauer, A.}, year={2007}, month={Jun}, pages={2564–2567} }
 @article{paskova_becker_böttcher_hommel_paskov_monemar_2007, title={Effect of sapphire-substrate thickness on the curvature of thick GaN films grown by hydride vapor phase epitaxy}, volume={102}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.2817955}, DOI={10.1063/1.2817955}, abstractNote={The effect of sapphire-substrate thickness on the curvature and stress in thick hydride vapor phase epitaxial GaN films was studied by high-resolution x-ray diffraction at variable temperatures. The curvature was found to have the maximum value for comparable thicknesses of the film and the substrate, while the stress at the film surface decreases with increasing film thickness and increases with increasing substrate thickness, which is in very good agreement with the simulation results. The curvature at the growth temperature was found to be strongly influenced by the value of the intrinsic tensile strain, which is determined by the film/substrate thickness ratio.}, number={12}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Paskova, T. and Becker, L. and Böttcher, T. and Hommel, D. and Paskov, P. P. and Monemar, B.}, year={2007}, month={Dec}, pages={123507} }
 @article{kröger_paskova_figge_hommel_rosenauer_monemar_2007, title={Interfacial structure of a-plane GaN grown on r-plane sapphire}, volume={90}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2696309}, DOI={10.1063/1.2696309}, abstractNote={The interface between a-plane GaN, grown by metal organic vapor phase epitaxy and hydride vapor phase epitaxy, and r-plane sapphire was investigated by transmission electron microscopy in [1−100] and [0001] zone axis orientations. The interfacial structure was well defined allowing a direct observation of the misfit dislocations in both orientations. An analysis of these dislocations revealed for the respective Burgers vectors a 1∕3⟨2−1−10⟩ component in the {0002} planes and a 1∕2⟨0001⟩ component in the {1−100} planes. In addition, the relative atomic column configurations in the GaN and sapphire were determined based on Bloch-wave simulations in comparison with the experimental images.}, number={8}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Kröger, R. and Paskova, T. and Figge, S. and Hommel, D. and Rosenauer, A. and Monemar, B.}, year={2007}, month={Feb}, pages={081918} }
 @article{pozina_monemar_paskov_hemmingsson_hultman_amano_akasaki_paskova_figge_hommel_et al._2007, title={Metastability of the UV luminescence in Mg-doped GaN layers grown by MOVPE on quasi-bulk GaN templates}, volume={401-402}, ISSN={0921-4526}, url={http://dx.doi.org/10.1016/j.physb.2007.08.173}, DOI={10.1016/j.physb.2007.08.173}, abstractNote={Optical metastability in Mg-doped GaN layers grown by metal–organic vapor phase epitaxy on thick GaN templates grown by halide vapor phase epitaxy has been studied by photoluminescence (PL) and cathodoluminescence (CL). The total Mg concentration varies from 1×1019 to 1×1020 cm−3. Both PL and CL spectra change the initial shape within a few minutes exposure to the excitation source. The effect is permanent at low temperatures but the emission spectrum can be recovered to its initial shape after heating to room temperature. The difference in PL and CL spectra is explained by activation of different regions in the sample by laser and electrons, respectively. In CL the increase of the defect-related luminescence seems to be connected with an enhanced structural defect reaction under electron irradiation, which competes with the regular radiative recombination, while in PL the main effect is related to dissociation of Mg–H complexes with consequent passivation of the residual shallow acceptors.}, journal={Physica B: Condensed Matter}, publisher={Elsevier BV}, author={Pozina, G. and Monemar, B. and Paskov, P.P. and Hemmingsson, C. and Hultman, L. and Amano, H. and Akasaki, I. and Paskova, T. and Figge, S. and Hommel, D. and et al.}, year={2007}, month={Dec}, pages={302–306} }
 @article{paskova_kroeger_hommel_paskov_monemar_preble_hanser_williams_tutor_2007, title={Nonpolara- andm-plane bulk GaN sliced from boules: structural and optical characteristics}, volume={4}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200674818}, DOI={10.1002/pssc.200674818}, abstractNote={AbstractBulk GaN substrates grown by HVPE in the [0001] direction and sliced along the (11‐20) a‐plane and the (1‐100) m‐plane have been studied by SEM, TEM, HRXRD, CL and PL. Smooth morphology, low defect density, and narrow linewidths of the main XRD reflections and PL peaks are revealed and thus all the results demonstrate superior structural and optical quality of both a‐ and m‐plane bulk GaN material achieved by HVPE growth of boules in the conventional [0001] direction and subsequently sliced along nonpolar planes, implying the approach as the most promising to produce nonpolar GaN substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (c)}, publisher={Wiley}, author={Paskova, T. and Kroeger, R. and Hommel, D. and Paskov, P. P. and Monemar, B. and Preble, E. and Hanser, A. and Williams, N. M. and Tutor, M.}, year={2007}, month={Jun}, pages={2536–2539} }
 @article{darakchieva_monemar_paskova_einfeldt_hommel_lourdudoss_2007, title={Phonons in strained AlGaN/GaN superlattices}, volume={4}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200673582}, DOI={10.1002/pssc.200673582}, abstractNote={AbstractPhonons in strained AlGaN/GaN superlattices (SLs) with constant periods but different Al composition have been studied using a combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The following SL modes were identified: i) AlGaN localized E 1(TO) modes identified for the first time and AlGaN localized A 1(LO) phonons; ii) GaN localized E 2, E 1(TO) and A 1(LO) phonons; iii) delocalized E 1(LO) phonons; iv) A 1(TO) phonon; v) two modes around 660 cm–1 and 594‐625 cm–1, respectively, not predicted by theory. The effects of strain and composition on the phonon frequencies were established and discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={1}, journal={physica status solidi (c)}, publisher={Wiley}, author={Darakchieva, V. and Monemar, B. and Paskova, T. and Einfeldt, S. and Hommel, D. and Lourdudoss, S.}, year={2007}, month={Jan}, pages={170–174} }
 @article{tuomisto_paskova_figge_hommel_monemar_2007, title={Vacancy defect distribution in heteroepitaxial -plane GaN grown by hydride vapor phase epitaxy}, volume={300}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2006.11.040}, DOI={10.1016/j.jcrysgro.2006.11.040}, abstractNote={We have used positron annihilation spectroscopy to study the native vacancy distribution in a-plane heteroepitaxial GaN. We show that the Ga vacancy concentration is independent of the layer thickness in the range from 5 to 25μm. This is strikingly different from the behavior in c-plane GaN, where the Ga vacancy concentration decreases dramatically with the distance from the GaN/sapphire interface. This difference in the native vacancy profiles is tentatively correlated with the differences in the O impurity and dislocation density profiles in the polar and non-polar materials.}, number={1}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Tuomisto, F. and Paskova, T. and Figge, S. and Hommel, D. and Monemar, B.}, year={2007}, month={Mar}, pages={251–253} }
 @article{darakchieva_paskova_paskov_arwin_schubert_monemar_figge_hommel_haskell_fini_et al._2006, title={Assessment of phonon mode characteristics via infrared spectroscopic ellipsometry ona-plane GaN}, volume={243}, ISSN={0370-1972 1521-3951}, url={http://dx.doi.org/10.1002/pssb.200565400}, DOI={10.1002/pssb.200565400}, abstractNote={AbstractGeneralized infrared spectroscopic ellipsometry was applied to study the vibrational properties of anisotropically strained a‐plane GaN films with different thicknesses. We have established a correlation between the phonon mode parameters and the strain, which allows the determination of the deformation potentials and strain‐free frequency of the GaN A1(TO) mode. These results are compared with previous theoretical and experimental findings and discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (b)}, publisher={Wiley}, author={Darakchieva, V. and Paskova, T. and Paskov, P. P. and Arwin, H. and Schubert, M. and Monemar, B. and Figge, S. and Hommel, D. and Haskell, B. A. and Fini, P. T. and et al.}, year={2006}, month={Jun}, pages={1594–1598} }
 @article{paskova_darakchieva_paskov_monemar_bukowski_suski_ashkenov_schubert_hommel_2006, title={Bending in HVPE GaN free-standing films: effects of laser lift-off, polishing and high-pressure annealing}, volume={3}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200565412}, DOI={10.1002/pssc.200565412}, abstractNote={AbstractWe have studied the effects of laser lift‐off and polishing processes on the bending of free‐standing HVPE grown GaN thick films. Their structural characteristics were accessed by reciprocal space mapping and lattice parameters measurements as well as by Raman scattering and photoluminescence. The in‐plane strain difference between the two faces was found to have determining effect on the bending of the free‐standing films. Removing the high‐defect‐density near‐interface region either by melting caused by laser lift‐off, or by polishing, or by point defects dissociation caused by high‐pressure annealing was found to lead to flattening of the strain distribution along the film thickness and a significant reduction of the bending of the free‐standing films. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={6}, journal={physica status solidi (c)}, publisher={Wiley}, author={Paskova, T. and Darakchieva, V. and Paskov, P. P. and Monemar, B. and Bukowski, M. and Suski, T. and Ashkenov, N. and Schubert, M. and Hommel, D.}, year={2006}, month={Jun}, pages={1475–1478} }
 @article{tuomisto_hautakangas_makkonen_ranki_puska_saarinen_bockowski_suski_paskova_monemar_et al._2006, title={Dissociation of VGa–ON complexes in HVPE GaN by high pressure and high temperature annealing}, volume={243}, ISSN={0370-1972 1521-3951}, url={http://dx.doi.org/10.1002/pssb.200565109}, DOI={10.1002/pssb.200565109}, abstractNote={AbstractWe have used positron annihilation spectroscopy to study the high‐pressure annealing induced thermal recovery of vacancy defects in free‐standing GaN grown by hydride vapor phase epitaxy (HVPE). The results show that the in‐grown Ga vacancy complexes recover after annealing at 1500–1700 K. Comparison of the experimental positron data with ab‐initio calculations indicates that the Doppler broadening measurement of the electron momentum distribution is sensitive enough to distinguish between the N and O atoms surrounding the Ga vacancy. We show that the difference between the isolated VGa in electron irradiated GaN and the VGa–ON complexes in highly O‐doped GaN is clear, and the Ga vacancy related defect complexes that start dissociating at 1500 K can be identified as VGa–ON pairs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (b)}, publisher={Wiley}, author={Tuomisto, F. and Hautakangas, S. and Makkonen, I. and Ranki, V. and Puska, M. J. and Saarinen, K. and Bockowski, M. and Suski, T. and Paskova, T. and Monemar, B. and et al.}, year={2006}, month={Jun}, pages={1436–1440} }
 @article{darakchieva_paskova_schubert_paskov_arwin_monemar_hommel_heuken_off_haskell_et al._2007, title={Effect of anisotropic strain on phonons in a-plane and c-plane GaN layers}, volume={300}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2006.11.023}, DOI={10.1016/j.jcrysgro.2006.11.023}, abstractNote={We have studied phonons in two types of anisotropically strained GaN films: c-plane GaN films grown on a-plane sapphire and a-plane GaN films grown on r-plane sapphire. The anisotropic strain in the films is determined by high-resolution X-ray diffraction (HRXRD) in different measuring geometries and the phonon parameters have been assessed by generalized infrared spectroscopic ellipsometry (GIRSE). The effect of strain anisotropy on GaN phonon frequencies is presented and the phonon deformation potentials aA1(TO), bA1(TO), cE1(TO) and cE1(LO) are determined.}, number={1}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Darakchieva, V. and Paskova, T. and Schubert, M. and Paskov, P.P. and Arwin, H. and Monemar, B. and Hommel, D. and Heuken, M. and Off, J. and Haskell, B.A. and et al.}, year={2007}, month={Mar}, pages={233–238} }
 @article{paskova_hommel_paskov_darakchieva_monemar_bockowski_suski_grzegory_tuomisto_saarinen_et al._2006, title={Effect of high-temperature annealing on the residual strain and bending of freestanding GaN films grown by hydride vapor phase epitaxy}, volume={88}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2192149}, DOI={10.1063/1.2192149}, abstractNote={The effect of high-temperature high-pressure annealing on the residual strain, bending, and point defect redistribution of freestanding hydride vapor phase epitaxial GaN films was studied. The bending was found to be determined by the difference in the in-plane lattice parameters in the two faces of the films. The results showed a tendency of equalizing the lattice parameters in the two faces with increasing annealing temperature, leading to uniform strain distribution across the film thickness. A nonmonotonic behavior of structural parameters with increasing annealing temperature was revealed and related to the change in the point defect content under the high-temperature treatment.}, number={14}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Paskova, T. and Hommel, D. and Paskov, P. P. and Darakchieva, V. and Monemar, B. and Bockowski, M. and Suski, T. and Grzegory, I. and Tuomisto, F. and Saarinen, K. and et al.}, year={2006}, month={Apr}, pages={141909} }
 @article{arnaudov_paskova_evtimova_monemar_lu_schaff_2006, title={Electron concentration and mobility profiles in InN layers grown by MBE}, volume={203}, ISSN={1862-6300 1862-6319}, url={http://dx.doi.org/10.1002/pssa.200565265}, DOI={10.1002/pssa.200565265}, abstractNote={AbstractWe have studied depth distributions of the electrical parameters in MBE grown InN films with two types of AlN and GaN buffers. Using independently determined Hall effect electron concentration and mobility profiles, as well as electron concentration profile by photoluminescence measurements, we model the real depth profile of carrier mobility, assuming graded inhomogeneity of the sample. The obtained profiles follow power dependences of the same order for layers grown on the two buffers with a small difference in the function coefficients attributed to a contribution of the interface charge in layers grown on AlN buffers. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (a)}, publisher={Wiley}, author={Arnaudov, B. and Paskova, T. and Evtimova, S. and Monemar, B. and Lu, H. and Schaff, W. J.}, year={2006}, month={May}, pages={1681–1685} }
 @article{paskova_kroeger_figge_hommel_darakchieva_monemar_preble_hanser_williams_tutor_2006, title={High-quality bulk a-plane GaN sliced from boules in comparison to heteroepitaxially grown thick films on r-plane sapphire}, volume={89}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2236901}, DOI={10.1063/1.2236901}, abstractNote={Thick GaN bars with [112̱0] orientation have been sliced from GaN boules grown on freestanding films by hydride vapor phase epitaxy (HVPE) in the [0001] direction. High-resolution x-ray diffraction and transmission electron microscopy have been used to study the structural quality and defect distribution in the material in comparison to heteroepitaxially grown thick HVPE-GaN films grown in the [112̱0] direction on (11̱02)-plane sapphire. It is demonstrated that while the heteroepitaxial material possesses a high density of stacking faults and partial dislocations, leading to anisotropic structural characteristics, the (112̱0)-plane bulk GaN, sliced from boules, exhibits low dislocation density and narrow rocking curves with isotropic in-plane character.}, number={5}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Paskova, T. and Kroeger, R. and Figge, S. and Hommel, D. and Darakchieva, V. and Monemar, B. and Preble, E. and Hanser, A. and Williams, N. M. and Tutor, M.}, year={2006}, month={Jul}, pages={051914} }
 @article{paskov_paskova_monemar_figge_hommel_haskell_fini_speck_nakamura_2006, title={Optical properties of nonpolar -plane GaN layers}, volume={40}, ISSN={0749-6036}, url={http://dx.doi.org/10.1016/j.spmi.2006.06.014}, DOI={10.1016/j.spmi.2006.06.014}, abstractNote={We have studied optical properties of nonpolar a-plane GaN layers grown on r-plane sapphire by metalorganic chemical vapor deposition and hydride vapor phase epitaxy using different nucleation schemes. Several emission bands, which are not typical for c-plane GaN, are observed in the photoluminescence spectra and their excitation-intensity, temperature, and polarization dependencies are examined. In addition, the spatial distribution of the emissions was examined by cathodoluminescence imaging and relations of the different emissions with particular structural features in the layers are revealed. The results are discussed with emphasis on the origin of the emission line and particular recombination mechanisms.}, number={4-6}, journal={Superlattices and Microstructures}, publisher={Elsevier BV}, author={Paskov, P.P. and Paskova, T. and Monemar, B. and Figge, S. and Hommel, D. and Haskell, B.A. and Fini, P.T. and Speck, J.S. and Nakamura, S.}, year={2006}, month={Oct}, pages={253–261} }
 @article{monemar_paskov_bergman_toropov_shubina_figge_paskova_hommel_usui_iwaya_et al._2006, title={Optical signatures of dopants in GaN}, volume={9}, ISSN={1369-8001}, url={http://dx.doi.org/10.1016/j.mssp.2006.01.075}, DOI={10.1016/j.mssp.2006.01.075}, abstractNote={The characteristic optical spectra for shallow donors and acceptors in GaN are discussed. The most accurate photoluminescence (PL) data are obtained from samples grown on freestanding GaN substrates, where strain shifts are absent and a low spectroscopic line width is obtained. Recent PL data for excitons bound to the O and Si donors are discussed in some detail, giving accurate values for the binding energies and excited bound donor states. The Mg-acceptor is the most important one for p-doping, but the related optical spectra are controversial. We show that there are two acceptors present in Mg-doped GaN, with two different acceptor bound exciton peaks, and also two corresponding lower energy donor–acceptor pair spectra. We give tentative evidence for their interpretation.}, number={1-3}, journal={Materials Science in Semiconductor Processing}, publisher={Elsevier BV}, author={Monemar, B. and Paskov, P.P. and Bergman, J.P. and Toropov, A.A. and Shubina, T.V. and Figge, S. and Paskova, T. and Hommel, D. and Usui, A. and Iwaya, M. and et al.}, year={2006}, month={Feb}, pages={168–174} }
 @article{paskov_schifano_malinauskas_paskova_bergman_monemar_figge_hommel_haskell_fini_et al._2006, title={Photoluminescence ofa -plane GaN: comparison between MOCVD and HVPE grown layers}, volume={3}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200565416}, DOI={10.1002/pssc.200565416}, abstractNote={AbstractWe report on the emission properties of nonpolar a ‐plane GaN layers grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE). Together with the typical band edge exciton emission, the low‐temperature photoluminescence spectra reveal several emission bands in the spectral range 3.29‐3.42 eV. These emissions appear with different intensities in layers grown by different techniques and show different thermal quenching and recombination dynamics. The 3.42 eV and 3.35 eV emissions are found to be of an intrinsic origin and are associated with carriers localized at stacking faults. The emission at 3.29 eV, which is present only in MOCVD layers, shows a donor‐acceptor pair behavior suggesting that impurities attached to structural defects are involved. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={6}, journal={physica status solidi (c)}, publisher={Wiley}, author={Paskov, P. P. and Schifano, R. and Malinauskas, T. and Paskova, T. and Bergman, J. P. and Monemar, B. and Figge, S. and Hommel, D. and Haskell, B. A. and Fini, P. T. and et al.}, year={2006}, month={Jun}, pages={1499–1502} }
 @article{roder_einfeldt_figge_hommel_paskova_monemar_haskell_fini_speck_nakamura_2006, title={Strain ina-plane GaN layers grown onr-plane sapphire substrates}, volume={203}, ISSN={1862-6300 1862-6319}, url={http://dx.doi.org/10.1002/pssa.200565447}, DOI={10.1002/pssa.200565447}, abstractNote={AbstractThe strain in a ‐plane GaN layers of different thickness grown on r ‐plane sapphire substrates by hydride vapor phase epitaxy was studied by X‐ray diffraction. The layers are found to be under compression in the growth plane and under tension in the growth direction. Therefore, the symmetry of the GaN unit cell is no longer hexagonal but orthorhombic. With increasing layer thickness the strain relaxes and the curvature of the wafer increases. Wafer bending is proposed to be the major strain relaxation mechanism. The anisotropic in‐plane strain relaxation is attributed to the elastic and thermal anisotropy of GaN and sapphire. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (a)}, publisher={Wiley}, author={Roder, C. and Einfeldt, S. and Figge, S. and Hommel, D. and Paskova, T. and Monemar, B. and Haskell, B. A. and Fini, P. T. and Speck, J. S. and Nakamura, S.}, year={2006}, month={May}, pages={1672–1675} }
 @article{roder_einfeldt_figge_paskova_hommel_paskov_monemar_behn_haskell_fini_et al._2006, title={Stress and wafer bending of a-plane GaN layers on r-plane sapphire substrates}, volume={100}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.2386940}, DOI={10.1063/1.2386940}, abstractNote={The stress and wafer bending of (112¯0) a-plane GaN layers of different thicknesses grown on (11¯02) r-plane sapphire substrates by hydride vapor phase epitaxy were studied by high-resolution x-ray diffraction and photoluminescence and photoreflectance spectroscopies. The layers are found to be under compression in the growth plane and under tension in the growth direction. The elastic and thermal anisotropies of the GaN and the sapphire crystal result in an in-plane stress and a wafer curvature, both of which are different in the two in-plane directions parallel and perpendicular to the GaN c axis. The GaN unit cell is no longer hexagonal but orthorhombic. The stress relaxes with increasing GaN layer thickness while the curvature of the wafer increases. Different stress relief mechanisms are considered, and the stresses in the layer and the curvature of the wafer are calculated using standard models on wafer bending. The results suggest that the wafer bending is the dominant stress relief mechanism. In addition, the redshift of the near-band-edge photoluminescence and the free exciton photoreflectance peaks with increasing layer thickness is correlated with the strain data determined by x-ray diffraction.}, number={10}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Roder, C. and Einfeldt, S. and Figge, S. and Paskova, T. and Hommel, D. and Paskov, P. P. and Monemar, B. and Behn, U. and Haskell, B. A. and Fini, P. T. and et al.}, year={2006}, month={Nov}, pages={103511} }
 @article{paskov_schifano_paskova_malinauskas_bergman_monemar_figge_hommel_2006, title={Structural defect-related emissions in nonpolar a-plane GaN}, volume={376-377}, ISSN={0921-4526}, url={http://dx.doi.org/10.1016/j.physb.2005.12.121}, DOI={10.1016/j.physb.2005.12.121}, abstractNote={We have studied the optical emission properties of a-plane GaN layers grown on r-plane sapphire by metalorganic chemical vapor deposition. Together with the typical band edge exciton emission, the photoluminescence (PL) spectra reveal three low-energy emissions peaked at 3.42, 3.34 and 3.29 eV, which are related to structural defects. Temperature and excitation dependent stationary PL and the time-resolved PL have been employed in order to understand the exact origin of these emissions. The 3.42 and 3.34 eV emissions are found to be of an intrinsic origin and are associated with carriers localized at stacking faults. The emission at 3.29 eV shows a donor–acceptor pair behavior suggesting that impurities attached to structural defects most likely partial dislocations terminating stacking faults are involved.}, journal={Physica B: Condensed Matter}, publisher={Elsevier BV}, author={Paskov, P.P. and Schifano, R. and Paskova, T. and Malinauskas, T. and Bergman, J.P. and Monemar, B. and Figge, S. and Hommel, D.}, year={2006}, month={Apr}, pages={473–476} }
 @article{monemar_paskov_bergman_paskova_figge_dennemarck_hommel_2006, title={The dominant shallow 0.225 eV acceptor in GaN}, volume={243}, ISSN={0370-1972 1521-3951}, url={http://dx.doi.org/10.1002/pssb.200565425}, DOI={10.1002/pssb.200565425}, abstractNote={AbstractWe have studied the optical signatures of the Mg acceptor in GaN, using samples that are doped with Mg during MOCVD growth. In order to reduce the defect density in the material and thus achieve narrow linewidths in optical spectra we have used thick HVPE grown GaN layers as templates in the MOCVD growth. The photoluminescence (PL) spectra show two acceptor‐related bound exciton peaks at 3.466 eV and 3.455 eV respectively. In the lower photon energy range the 3.27 eV emission with its LO‐phonon replicas is dominant, riding on a broad background emission peaking at about 3.1 eV. These results, together with previous data in the literature, indicate that there are two acceptors in Mg‐doped GaN, one dominating the optical spectra (the 3.466 eV and the 3.27 eV emissions) and another related to the 3.455 eV and the 3.1 eV emissions. We suggest that the latter is related to the Mg acceptor, while the former is a H‐related complex, not necessarily involving Mg. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (b)}, publisher={Wiley}, author={Monemar, B. and Paskov, P. P. and Bergman, J. P. and Paskova, T. and Figge, S. and Dennemarck, J. and Hommel, D.}, year={2006}, month={Jun}, pages={1604–1608} }
 @article{tuomisto_saarinen_paskova_monemar_bockowski_suski_2006, title={Thermal stability of in-grown vacancy defects in GaN grown by hydride vapor phase epitaxy}, volume={99}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.2180450}, DOI={10.1063/1.2180450}, abstractNote={We have used positron annihilation spectroscopy to study the thermal behavior of different native vacancy defects typical of freestanding GaN grown by hydride vapor phase epitaxy under high pressure annealing at different annealing temperatures. The results show that the VGa-ON pairs dissociate and the Ga vacancies anneal out from the bulk of the material at temperatures 1500–1700K. A binding energy of Eb=1.6(4)eV can be determined for the pair. Thermal formation of Ga vacancies is observed at the annealing temperatures above 1700K, indicating that Ga vacancies are created thermally at the high growth temperature, but their ability to form complexes such as VGa-ON determines the fraction of vacancy defects surviving the cooling down. The formation energy of the isolated Ga vacancy is experimentally determined.}, number={6}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Tuomisto, F. and Saarinen, K. and Paskova, T. and Monemar, B. and Bockowski, M. and Suski, T.}, year={2006}, month={Mar}, pages={066105} }
 @article{monemar_paskov_bergman_paskova_hemmingsson_malinauskas_jarasiunas_gibart_beaumont_2006, title={Time-resolved spectroscopy of excitons bound at shallow neutral donors in HVPE GaN}, volume={376-377}, ISSN={0921-4526}, url={http://dx.doi.org/10.1016/j.physb.2005.12.123}, DOI={10.1016/j.physb.2005.12.123}, abstractNote={Time-resolved photoluminescence (TRPL) data for temperatures 2–150 K are presented for two thick HVPE samples grown in two different laboratories. The samples both have residual O and Si shallow donor concentrations in the 1016 cm−3 range. The radiative decay time for neutral donor-bound excitons (DBEs) related to these donors is found to be about 300 ps. The decay of the DBEs at longer decay times is found to be related to feeding from the free exciton–polariton states. At elevated temperatures the decay of the DBE is very similar to the free exciton decay.}, journal={Physica B: Condensed Matter}, publisher={Elsevier BV}, author={Monemar, B. and Paskov, P.P. and Bergman, J.P. and Paskova, T. and Hemmingsson, C. and Malinauskas, T. and Jarasiunas, K. and Gibart, P. and Beaumont, B.}, year={2006}, month={Apr}, pages={482–485} }
 @article{paskova_arnaudov_paskov_goldys_hautakangas_saarinen_södervall_monemar_2005, title={Donor-acceptor pair emission enhancement in mass-transport-grown GaN}, volume={98}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.1994943}, DOI={10.1063/1.1994943}, abstractNote={A dominating donor-acceptor pair (DAP) emission at about 3.27eV is observed in the photoluminescence and cathodoluminescence spectra of intentionally undoped mass-transport (MT)-grown GaN, while only a weak presence of the DAP emission is recorded in the as-grown hydride vapor phase epitaxial GaN. A comparative study of impurity and native defect incorporation in the as-grown and MT GaN was performed, showing a significant increase of oxygen and empty clusters involving Ga vacancy and oxygen in the MT GaN. Based on the observed results as well as on doping analysis of the structure and kinetic analysis of the emission intensities, we propose an acceptorlike complex, creating a state as a semiclassical potential well near the valence-band top due to the local tensile strain caused by the empty clusters to be responsible for the dominating behavior of the DAP emission.}, number={3}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Paskova, T. and Arnaudov, B. and Paskov, P. P. and Goldys, E. M. and Hautakangas, S. and Saarinen, K. and Södervall, U. and Monemar, B.}, year={2005}, month={Aug}, pages={033508} }
 @article{paskov_schifano_monemar_paskova_figge_hommel_2005, title={Emission properties of a-plane GaN grown by metal-organic chemical-vapor deposition}, volume={98}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.2128496}, DOI={10.1063/1.2128496}, abstractNote={We report on the emission properties of nonpolar a-plane GaN layers grown on r-plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resolved microphotoluminescence and cathodoluminescence are employed in order to clarify the nature of the different emission bands in the 3.0–3.5eV spectral range. In the near band-edge region the emission lines of the donor-bound excitons (3.472eV) and free excitons (3.478eV) are resolved in the polarized low-temperature spectra, indicating a good quality of the layers. At low energies two other emissions bands with intensity and shape varying with the excited area are observed. The 3.42eV emission commonly attributed to the excitons bound to basal plane stacking faults shows thermal quenching with two activation energies (7 and 30meV) and an S-shaped temperature dependence of the peak position. This behavior is analyzed in terms of hole localization in the vicinity of the stacking faults. The emission band that peaked at 3.29eV is found to blueshift and saturate with increasing excitation intensity. The spatially resolved cathodoluminesence measurements show that the emission is asymmetrically distributed around the triangular-shaped pits occurring at the surface. The 3.29eV emission is suggested to involve impurities, which decorate the partial dislocation terminating the basal stacking faults.}, number={9}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Paskov, P. P. and Schifano, R. and Monemar, B. and Paskova, T. and Figge, S. and Hommel, D.}, year={2005}, month={Nov}, pages={093519} }
 @article{paskova_darakchieva_paskov_birch_valcheva_persson_arnaudov_tungasmita_monemar_2005, title={Nonpolara-plane HVPE GaN: growth and in-plane anisotropic properties}, volume={2}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200461481}, DOI={10.1002/pssc.200461481}, abstractNote={Nonpolar GaN thick films with [11-20] orientation were grown on [1-102] oriented sapphire by hydride vapour phase epitaxy (HVPE) utilizing reactively sputtered AlN buffers. Growth rate and microstructure of such films were investigated and compared to those in HVPE [0001] oriented GaN thick films. The structural parameters show an angular azimuth dependence implying in-plane non-homogeneity. The lattice parameters and strain components were determined by using plane and edge symmetric measurement geometries. A red shift observed in the near band edge photoluminescence is explained by the specific strain distribution studied independently by X-ray diffraction. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (c)}, publisher={Wiley}, author={Paskova, T. and Darakchieva, V. and Paskov, P. P. and Birch, J. and Valcheva, E. and Persson, P. O. �. and Arnaudov, B. and Tungasmita, S. and Monemar, B.}, year={2005}, month={May}, pages={2027–2031} }
 @article{figge_böttcher_dennemarck_kröger_paskova_monemar_hommel_2005, title={Optoelectronic devices on bulk GaN}, volume={281}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2005.03.017}, DOI={10.1016/j.jcrysgro.2005.03.017}, abstractNote={The homoeptaxial fabrication of GaN-based devices has advantages against heteroepitaxial realization on substrates such as sapphire or SiC, since heteroepitaxy implies a lot of problems like lattice mismatch, different thermal expansion coefficients, and needs an extensive optimization of the growth at the heterointerface. In this paper we will discuss GaN-based light-emitting devices grown by homoepitaxy in comparison to devices grown on sapphire. A special emphasis is laid on the pretreatment of the GaN substrate and the device characteristics on different substrates. In detail will be discussed the advantages of the higher thermal conductivity of GaN and how this effects the device performance.}, number={1}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Figge, S. and Böttcher, T. and Dennemarck, J. and Kröger, R. and Paskova, T. and Monemar, B. and Hommel, D.}, year={2005}, month={Jul}, pages={101–106} }
 @article{paskov_bergman_darakchieva_paskova_monemar_iwaya_kamiyama_amano_akasaki_2005, title={Photoluminescence of GaN/AlN superlattices grown by MOCVD}, volume={2}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200461367}, DOI={10.1002/pssc.200461367}, abstractNote={We report on the emission properties of GaN/AlN superlattice (SL) structures grown by MOCVD. A set of samples with AlN thickness below 3 nm and different well/barrier thickness ratios has been studied. The SLs with a period between 3 and 8 nm show a photoluminescence (PL) peak in the range 3.3–3.9 eV with a full width at half maximum of about 100 meV. The difference in the transition energies, linewidths and recombination dynamics is described in terms of varying polarization field and electron filling in the wells. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (c)}, publisher={Wiley}, author={Paskov, P. P. and Bergman, J. P. and Darakchieva, V. and Paskova, T. and Monemar, B. and Iwaya, M. and Kamiyama, S. and Amano, H. and Akasaki, I.}, year={2005}, month={May}, pages={2345–2348} }
 @article{paskova_darakchieva_paskov_birch_valcheva_persson_arnaudov_tungasmitta_monemar_2005, title={Properties of nonpolar a-plane GaN films grown by HVPE with AlN buffers}, volume={281}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2005.03.013}, DOI={10.1016/j.jcrysgro.2005.03.013}, abstractNote={The influence of high temperature AlN buffer layers on the morphology, structural and optical characteristics of a-plane GaN grown by hydride vapour phase epitaxy on r-plane sapphire was investigated. While the morphology of the a-GaN was found to be significantly improved by using a-plane AlN buffer layer similarly to the effect observed in c-plane hydride vapour phase epitaxy GaN growth, the microstructure ensemble was revealed to be more complicated in comparison to that of the c-plane GaN. Higher dislocation density and prismatic stacking faults were observed. Moreover, in-plane anisotropic structural characteristics were revealed by high resolution X-ray diffraction employing azimuthal dependent and edge X-ray measurement symmetric geometry. In addition, the near band edge photoluminescence peaks, red-shifted with respect to that in c-plane GaN were observed. The latter were explained by the influence of the higher defect density and more complex strain distribution.}, number={1}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Paskova, T. and Darakchieva, V. and Paskov, P.P. and Birch, J. and Valcheva, E. and Persson, P.O.A. and Arnaudov, B. and Tungasmitta, S. and Monemar, B.}, year={2005}, month={Jul}, pages={55–61} }
 @article{strocov_schmitt_rubensson_blaha_paskova_nilsson_2004, title={Band structure effects in nitrogen K-edge resonant inelastic X-ray scattering from GaN}, volume={241}, ISSN={0370-1972 1521-3951}, url={http://dx.doi.org/10.1002/pssb.200409040}, DOI={10.1002/pssb.200409040}, abstractNote={AbstractSystematic experimental data on resonant inelastic X‐ray scattering (RIXS) in GaN near the N K‐edge are presented for the first time. Excitation energy dependence of the spectral structures manifests the band structure effects originating from momentum selectivity of the RIXS process. This finding allows obtaining k‐resolved band structure information for GaN crystals and nanostructures. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (b)}, publisher={Wiley}, author={Strocov, V. N. and Schmitt, T. and Rubensson, J.-E. and Blaha, P. and Paskova, T. and Nilsson, P. O.}, year={2004}, month={Jun}, pages={R27–R29} }
 @article{paskova_paskov_goldys_valcheva_darakchieva_södervall_godlewski_zielinski_hautakangas_saarinen_et al._2004, title={Characterization of mass-transport grown GaN by hydride vapour-phase epitaxy}, volume={273}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2004.09.025}, DOI={10.1016/j.jcrysgro.2004.09.025}, abstractNote={A comprehensive study of the morphological, optical and microstructural properties of mass-transport (MT) and conventionally grown GaN by hydride vapour-phase epitaxy is presented. Spatially resolved techniques have been utilized to reveal in a comparative way, the characteristics of the material grown either in predominant vertical or lateral growth modes. A strong donor–acceptor pair (DAP) emission is observed from the MT regions with a distinctive intensity contrast between the exciton and DAP emission bands from MT and nontransport regions. Secondary ion mass spectroscopy and imaging were employed to investigate the impurity incorporation into different regions. An increase of residual oxygen and aluminium impurity concentrations was found in the MT areas. In addition, positron annihilation spectroscopy showed a strong signal of Ga vacancy clusters in the MT grown material. The increase of the point defect concentrations of both Ga vacancy and oxygen impurity, most likely forming defect complexes, is related to the enhancement of the DAP emission.}, number={1-2}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Paskova, T. and Paskov, P.P. and Goldys, E.M. and Valcheva, E. and Darakchieva, V. and Södervall, U. and Godlewski, M. and Zielinski, M. and Hautakangas, S. and Saarinen, K. and et al.}, year={2004}, month={Dec}, pages={118–128} }
 @article{darakchieva_paskov_valcheva_paskova_monemar_schubert_lu_schaff_2004, title={Deformation potentials of the E1(TO) and E2 modes of InN}, volume={84}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.1738520}, DOI={10.1063/1.1738520}, abstractNote={The deformation potentials of the E1(TO) and E2 modes of InN are determined by combining infrared spectroscopic ellipsometry, Raman scattering, and x-ray diffraction measurements, and using a reported value of the mode Grüneisen parameter. The deformation potentials are obtained for two sets of stiffness constants. Strain-free values of the InN E1(TO) mode of 477.9 cm−1 and of the E2 mode of 491.1 cm−1 have been determined.}, number={18}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Darakchieva, V. and Paskov, P. P. and Valcheva, E. and Paskova, T. and Monemar, B. and Schubert, M. and Lu, H. and Schaff, W. J.}, year={2004}, month={May}, pages={3636–3638} }
 @article{arnaudov_paskova_paskov_magnusson_valcheva_monemar_lu_schaff_amano_akasaki_2004, title={Free-to-bound radiative recombination in highly conducting InN epitaxial layers}, volume={36}, ISSN={0749-6036}, url={http://dx.doi.org/10.1016/j.spmi.2004.09.013}, DOI={10.1016/j.spmi.2004.09.013}, abstractNote={We present a theoretical simulation of near-band-edge emission spectra of highly conducting n-InN assuming the model of 'free-to-bound' radiative recombination (FBRR) of degenerate electrons from the conduction band with nonequilibrium holes located in the valence band tails. We also study experimental photoluminescence (PL) spectra of highly conducting InN epitaxial layers grown by MBE and MOVPE with electron concentrations in the range (7.7×1017–6×1018) cm−3 and find that the energy positions and shape of the spectra depend on the impurity concentration. By modeling the experimental PL spectra of the InN layers we show that spectra can be nicely interpreted in the framework of the FBRR model with specific peculiarities for different doping levels. Analyzing simultaneously the shape and energy position of the InN emission spectra we determine the fundamental bandgap energy of InN to vary between Eg=692 meV for effective mass mn0=0.042m0 and Eg=710 meV for mn0=0.1m0.}, number={4-6}, journal={Superlattices and Microstructures}, publisher={Elsevier BV}, author={Arnaudov, B. and Paskova, T. and Paskov, P.P. and Magnusson, B. and Valcheva, E. and Monemar, B. and Lu, H. and Schaff, W.J. and Amano, H. and Akasaki, I.}, year={2004}, month={Oct}, pages={563–571} }
 @article{paskova_darakchieva_valcheva_paskov_ivanov_monemar_böttcher_roder_hommel_2004, title={Hydride vapor-phase epitaxial GaN thick films for quasi-substrate applications: Strain distribution and wafer bending}, volume={33}, ISSN={0361-5235 1543-186X}, url={http://dx.doi.org/10.1007/S11664-004-0189-4}, DOI={10.1007/S11664-004-0189-4}, number={5}, journal={Journal of Electronic Materials}, publisher={Springer Science and Business Media LLC}, author={Paskova, T. and Darakchieva, V. and Valcheva, E. and Paskov, P. P. and Ivanov, I. G. and Monemar, B. and Böttcher, T. and Roder, C. and Hommel, D.}, year={2004}, month={May}, pages={389–394} }
 @article{darakchieva_paskov_valcheva_paskova_schubert_bundesmann_lu_schaff_monemar_2004, title={Infrared ellipsometry and Raman studies of hexagonal InN films: correlation between strain and vibrational properties}, volume={36}, ISSN={0749-6036}, url={http://dx.doi.org/10.1016/j.spmi.2004.09.014}, DOI={10.1016/j.spmi.2004.09.014}, abstractNote={The vibrational properties of InN films with different strain have been studied using Infrared ellipsometry and Raman scattering spectroscopy. We have established a correlation between the phonon mode parameters and the strain, which allows the determination of the deformation potentials and the strain-free frequencies of the InN E1(TO) and E2 modes. The LO phonons and their coupling to the free-carrier plasmon excitations are also discussed in relation to the carrier concentration in the films.}, number={4-6}, journal={Superlattices and Microstructures}, publisher={Elsevier BV}, author={Darakchieva, V. and Paskov, P.P. and Valcheva, E. and Paskova, T. and Schubert, M. and Bundesmann, C. and Lu, H. and Schaff, W.J. and Monemar, B.}, year={2004}, month={Oct}, pages={573–580} }
 @article{monemar_paskov_haradizadeh_bergman_valcheva_darakchieva_arnaudov_paskova_holtz_pozina_et al._2004, title={Optical investigation of AlGaN/GaN quantum wells and superlattices}, volume={201}, ISSN={0031-8965 1521-396X}, url={http://dx.doi.org/10.1002/pssa.200404849}, DOI={10.1002/pssa.200404849}, abstractNote={We report a detailed optical study of several sets of multiple quantum wells (MQWs) in the AlGaN/GaN system, as well as AlN/GaN superlattice (SL) structures. In this study all materials were grown by MOCVD, as opposed to most previous studies where MBE was employed. In undoped MQWs discrete photoluminescence (PL) peaks related to discrete well width fluctuations by one full c lattice parameter are clearly observed. In doped samples this effect appears to be screened. While the recombination process in undoped samples is excitonic, in MQWs doped with Si above about 5 × 10 18 cm -3 free electrons (a 2DEG) are dominant, and the PL process is a free electrons-localized hole transition at low temperatures. The hole localization prevails up to very high n-doping, as was previously observed in bulk GaN. The hole localization is demonstrated via several experiments, including results on PL transient decay times and LO phonon coupling. Near surface band bending, due mainly to dopant depletion in doped structures or interaction with surface states in case of higher Al content in barriers, influences the distribution of electron filling among the QWs, making a detailed modeling of the spectral shape somewhat ambiguous. It is found that AlN barriers promote a strong room temperature PL signal from the QWs, as opposed to the case with AlGaN barriers.}, number={10}, journal={physica status solidi (a)}, publisher={Wiley}, author={Monemar, B. and Paskov, P. P. and Haradizadeh, H. and Bergman, J. P. and Valcheva, E. and Darakchieva, V. and Arnaudov, B. and Paskova, T. and Holtz, P. O. and Pozina, G. and et al.}, year={2004}, month={Aug}, pages={2251–2258} }
 @article{paskova_paskov_valcheva_darakchieva_birch_kasic_arnaudov_tungasmita_monemar_2004, title={Polar and nonpolar GaN grown by HVPE: Preferable substrates for nitride-based emitting devices}, volume={201}, ISSN={0031-8965 1521-396X}, url={http://dx.doi.org/10.1002/pssa.200404818}, DOI={10.1002/pssa.200404818}, abstractNote={We report on hydride vapor phase epitaxial growth of thick nonpolar GaN films on r-plane sapphire in comparison with polar GaN films on c-plane sapphire substrates with AlN buffer layers, aiming at developing of their quasi-substrate application. Both the thick films and the buffers were identified to have single crystalline structures. The microstructure of the films was studied by transmission electron microscopy. High resolution X-ray diffraction mapping and photoluminescence measurements were employed to characterize the strain present in both polar and nonpolar GaN films. In contrast to c-plane GaN, which is always characterised by isotropic in-plane properties, the a-plane GaN shows a strong in-plane anisotropy of the growth rate, morphology and strain distribution. Different defect, impurity and free carrier concentrations were observed in the polar and nonpolar material.}, number={10}, journal={physica status solidi (a)}, publisher={Wiley}, author={Paskova, T. and Paskov, P. P. and Valcheva, E. and Darakchieva, V. and Birch, J. and Kasic, A. and Arnaudov, B. and Tungasmita, S. and Monemar, B.}, year={2004}, month={Aug}, pages={2265–2270} }
 @article{paskov_paskova_holtz_monemar_2004, title={Polarized photoluminescence of exciton-polaritons in free-standing GaN}, volume={201}, ISSN={0031-8965 1521-396X}, url={http://dx.doi.org/10.1002/pssa.200304093}, DOI={10.1002/pssa.200304093}, abstractNote={We report on the polarization properties of the exciton-polariton modes in GaN. The dispersion curves and the expected emission lineshape of polaritons for all polarization configurations are calculated taking into account the spatial dispersion and the simultaneous exciton-photon coupling of all optically active states. An experimental study of the exciton-polariton luminescence in a free-standing GaN layer is also performed. The spectra reveal a clear difference between the emissions polarized perpendicular and parallel to the c-axis of the crystal. The experimental results are discussed in terms of optical selection rules and population of the polariton states.}, number={4}, journal={physica status solidi (a)}, publisher={Wiley}, author={Paskov, P. P. and Paskova, T. and Holtz, P. O. and Monemar, B.}, year={2004}, month={Mar}, pages={678–685} }
 @article{darakchieva_paskova_paskov_monemar_ashkenov_schubert_2005, title={Structural characteristics and lattice parameters of hydride vapor phase epitaxial GaN free-standing quasisubstrates}, volume={97}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.1823024}, DOI={10.1063/1.1823024}, abstractNote={We have studied the lattice parameters of hydride vapor phase epitaxy (HVPE)-GaN quasisubstrates in relation to their structural properties. Layers grown on single-layer metalorganic vapor phase epitaxy (MOVPE) templates and on epitaxial lateral overgrown MOVPE templates are characterized by Raman scattering, high-resolution x-ray diffraction, and reciprocal space mapping. The strain relaxation in the films versus their thickness was found to proceed similarly in the GaN samples grown using the two types of templates but the strain saturates at different nonzero levels. The lattice parameters of relatively thin HVPE-GaN free-standing quasisubstrates indicate that no total strain relaxation is achieved after the sapphire removal. The lattice parameters of the thick quasisubstrates grown on different templates are not affected by the separation process and are found to have values very close to the reference strain-free lattice parameters of GaN powder.}, number={1}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Darakchieva, V. and Paskova, T. and Paskov, P. P. and Monemar, B. and Ashkenov, N. and Schubert, M.}, year={2005}, month={Jan}, pages={013517} }
 @article{paskova_valcheva_paskov_monemar_roskowski_davis_beaumont_gibart_2004, title={HVPE-GaN: comparison of emission properties and microstructure of films grown on different laterally overgrown templates}, volume={13}, ISSN={0925-9635}, url={http://dx.doi.org/10.1016/j.diamond.2003.10.072}, DOI={10.1016/j.diamond.2003.10.072}, abstractNote={We report on a comparative study of defect and emission distributions in thick hydride vapor phase epitaxial (HVPE) GaN films grown on two different patterned template structures separately produced by multi-step procedures using metalorganic vapor phase epitaxy (MOVPE). The observed differences in the microstructures and emission distributions at the early stages of the growth in both cases were related to the change of the dominating growth mode sequence and point defects incorporation. Both template structures were found to favor formation of voids in the coalescence regions, which leads to a partial strain relaxation and allows overgrowth of thicker films without cracks.}, number={4-8}, journal={Diamond and Related Materials}, publisher={Elsevier BV}, author={Paskova, T. and Valcheva, E. and Paskov, P.P. and Monemar, B. and Roskowski, A.M. and Davis, R.F. and Beaumont, B. and Gibart, P.}, year={2004}, month={Apr}, pages={1125–1129} }