@article{callsen_wagner_kure_reparaz_buegler_brunnmeier_nenstiel_hoffmann_hoffmann_tweedie_et al._2012, title={Optical signature of Mg-doped GaN: Transfer processes}, volume={86}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.86.075207}, abstractNote={Mg doping of high quality, metal organic chemical vapor deposition grown GaN films results in distinct traces in their photoluminescence and photoluminescence excitation spectra. We analyze GaN:Mg grown on sapphire substrates and identify two Mg related acceptor states, one additional acceptor state and three donor states that are involved in the donor-acceptor pair band transitions situated at 3.26–3.29 eV in GaN:Mg. The presented determination of the donor-acceptor pair band excitation channels by photoluminescence excitation spectroscopy in conjunction with temperature-dependent photoluminescence measurements results in a direct determination of the donor and acceptor binding, localization, and activation energies, which is put into a broader context based on Haynes’s rule. Furthermore, we analyze the biexponential decay dynamics of the photoluminescence signal of the acceptor and donor bound excitons. As all observed lifetimes scale with the localization energy of the donor and acceptor related bound excitons, defect and complex bound excitons can be excluded as their origin. Detailed analysis of the exciton transfer processes in the close energetic vicinity of the GaN band edge reveals excitation via free and bound excitonic channels but also via an excited state as resolved for the deepest localized Mg related acceptor bound exciton. For the two Mg acceptor states, we determine binding energies of 164 ± 5 and 195 ± 5 meV, which is in good agreement with recent density functional theory results. This observation confirms and quantifies the general dual nature of acceptor states in GaN based on the presented analysis of the photoluminescence and photoluminescence excitation spectra.}, number={7}, journal={PHYSICAL REVIEW B}, author={Callsen, G. and Wagner, M. R. and Kure, T. and Reparaz, J. S. and Buegler, M. and Brunnmeier, J. and Nenstiel, C. and Hoffmann, A. and Hoffmann, M. and Tweedie, J. and et al.}, year={2012}, month={Aug} }
 @article{akyildiz_casper_ayguen_lam_maria_2011, title={Hydrothermal BaTiO3 thin films from nanostructured Ti templates}, volume={26}, ISSN={["0884-2914"]}, DOI={10.1557/jmr.2010.102}, abstractNote={Abstract}, number={4}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Akyildiz, Hasan and Casper, Michelle D. and Ayguen, Seymen M. and Lam, Peter G. and Maria, Jon P.}, year={2011}, month={Feb}, pages={592–599} }
 @article{aygun_daniels_borland_maria_2010, title={In situ methods to explore microstructure evolution in chemically derived oxide thin films}, volume={25}, ISSN={["0884-2914"]}, DOI={10.1557/jmr.2010.0066}, abstractNote={In situ residual gas analyzer techniques were used to identify process-property relationships that regulate microstructure evolution in chemical solution-deposited BaTiO3films. In situ analysis of furnace exhaust gasses enabled quantitative exploration of thermolysis and crystallization reactions and an ability to identify processing parameters that influence the temperature ranges over which they occur. The atmospheric analysis was instrumental in identifying heat treatments that produced optimally consolidated precursor gels that crystallized into BaTiO3layers with optimized structure and properties. Slow ramp rates resulted in higher porosity, larger grain size, and a dramatic drop in the capacitor yield. Fast ramp rates produced similar trends; however, the mechanisms were distinct. The effects of oxygen partial pressure were also explored. BaTiO3grain size increased with increasing pO2, whereas there was no appreciable influence on density and capacitor yield. Optimal firing parameters, i.e., 20 °C/min ramp rate at a pO2of 10−13atm, were identified as those that produced an overlap in the temperature ranges of thermolysis and crystallization reactions and thus a precursor gel with a density and compliance that supports crystallization and densification while tolerating the associated volume contraction. This in situ approach to analyze downstream furnace gas is shown to be a generically applicable means to understand synthesis methods that are complicated by simultaneous mechanisms of precursor decomposition, extraction of volatile components, and crystallization.}, number={3}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Aygun, Seymen M. and Daniels, Patrick and Borland, William J. and Maria, Jon-Paul}, year={2010}, month={Mar}, pages={427–436} }
 @article{ihlefeld_daniels_aygun_borland_maria_2010, title={Property engineering in BaTiO3 films by stoichiometry control}, volume={25}, ISSN={["2044-5326"]}, DOI={10.1557/jmr.2010.0151}, abstractNote={BaTiO3 thin films were prepared on metallic foil substrates using chemical solution deposition. The impact of A to B site cation ratios on the phase assemblage and microstructural and dielectric properties was investigated by characterizing a sample set that includes stoichiometric BaTiO3 and 1, 2, 3, 4, and 5 mol% excess BaO. Each composition was subjected to a high-temperature anneal step with maximum dwell temperatures of 1000, 1100, and 1200 °C for 20 h. Excess barium concentrations greater than 3% lead to dramatic grain growth and average grain sizes exceeding 1 μm. Despite the large deviations from stoichiometry and the 20 h dwell time at temperature, x-ray diffraction, and high-resolution electron microscopy analysis were unable to detect secondary phases until films with 5% excess barium were annealed to 1200 °C. Thin films with 3% excess barium were prepared on copper substrates and annealed at 1060 °C, the practical limit for copper. This combination of BaO excess and annealing temperature produced an average lateral grain size of 0.8 μm and a room-temperature permittivity of 4000. This is in comparison to a permittivity of 1800 for stoichiometric material prepared using identical conditions. This work suggests metastable solubility of BaO in BaTiO3 that leads to enhanced grain growth and large permittivity values. This technique provides a new solid-state means of achieving grain growth in low thermal budget systems.}, number={6}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Ihlefeld, J. F. and Daniels, P. R. and Aygun, S. M. and Borland, W. J. and Maria, J-P.}, year={2010}, month={Jun}, pages={1064–1071} }
 @article{paisley_losego_aygun_craft_maria_2008, title={Barrier layer mechanism engineering in calcium copper titanate thin film capacitors through microstructure control}, volume={104}, ISSN={["1089-7550"]}, DOI={10.1063/1.3033166}, abstractNote={A peak permittivity greater than 10 000 has been achieved for calcium copper titanate (CCT) thin films by engineering a thin film microstructure that maximizes space charge contributions to polarizability. This permittivity is an order of magnitude greater than previous polycrystalline thin film efforts. This unique microstructure control is accomplished using a chemical solution deposition process flow that produces highly dense parallel layers ∼100 nm in thickness. We observe a thickness dependent permittivity where the entire film thickness constitutes the conducting region of a barrier layer capacitor despite the presence of multiple grain boundaries within that thickness. The model predictions are in good agreement with experimental data and are consistent with existing literature reports. These trends in permittivity with dielectric thickness raise new questions regarding the nature of barrier layers in CCT—and specifically, these results suggest that grain boundaries may not always participate as high resistance interlayers.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Paisley, E. A. and Losego, M. D. and Aygun, S. M. and Craft, H. S. and Maria, J. -P.}, year={2008}, month={Dec} }
 @article{aygun_daniels_borland_maria_2008, title={Hot sputtering of barium strontium titanate on nickel foils}, volume={103}, ISSN={["0021-8979"]}, DOI={10.1063/1.2909920}, abstractNote={The relationships linking temperature and voltage dependent dielectric response, grain size, and thermal budget during synthesis are illustrated. In doing so, it was found that maximizing thermal budgets within experimental bounds leads to electrical properties comparable to the best literature reports irrespective of the processing technique or microstructure. The optimal film properties include a bulk transition temperature, a room temperature permittivity of 1800, a voltage tuning ratio of 10:1 at 450 kV/cm, and a loss tangent less than 1.5% at 450 kV/cm. The sample set illustrates the well-known relationship between permittivity and crystal dimension, and the onset of a transition temperature shifts at very fine grain sizes. A brick wall model incorporating a high permittivity grain and a low permittivity grain boundary is used to interpret the dielectric data. However, the data show that high permittivity and tunability values can be achieved at grain sizes or film thicknesses that many reports associate with dramatic reductions in the dielectric response. These differences are discussed in terms of crystal quality and maximum processing temperature. The results collectively suggest that scaling effects in ferroelectric thin films are in many cases the result of low thermal budgets and the consequently high degree of structural imperfection and are not from the existence of low permittivity phases at the dielectric-electrode interface.}, number={8}, journal={JOURNAL OF APPLIED PHYSICS}, author={Aygun, Seymen M. and Daniels, Patrick and Borland, William and Maria, Jon-Paul}, year={2008}, month={Apr} }
 @article{nath_fathelbab_lam_ghosh_aygun_gard_maria_kingon_steer_2006, title={Discrete Barium Strontium Titanate (BST) thin-film interdigital varactors on alumina: Design, fabrication, characterization, and applications}, ISBN={["978-0-7803-9541-1"]}, ISSN={["2576-7216"]}, DOI={10.1109/mwsym.2006.249652}, abstractNote={Discrete barium strontium titanate (BST) thin-film capacitors in industry standard 0603 footprint are introduced and characterized. BST capacitors have a voltage-dependent permittivity, enabling BST thin-film capacitors to be used as tuning elements in frequency agile devices. The capacitance changed by 1.5:1 at 35 V (116 kV/cm) bias. The temperature dependence of the capacitance was measured to be less than plusmn 20 % from -100 degC to +100 degC. A 2nd-order tunable combline bandpass filter on FR4 substrate has been implemented using the discrete BST varactors. The filter showed a center frequency tuning of 22% from 2.14 GHz to 2.61 GHz upon application of 130 V (433 kV/cm) bias. The zero-bias insertion loss was 4.9 dB which decreased to 2.9 dB at the high bias state. The return loss was better than 11 dB over the tuning range. Nonlinear characterization of the filter using two-tone test and a digitally-modulated CDMA 2000 signal showed an IP3 of +32 dBm and an ACPR of better than -50 dBc up to 26 dBm of input power, respectively}, journal={2006 IEEE MTT-S INTERNATIONAL MICROWAVE SYMPOSIUM DIGEST, VOLS 1-5}, author={Nath, Jayesh and Fathelbab, Wael M. and Lam, Peter G. and Ghosh, Dipankar and Aygun, Seymen and Gard, Kevin G. and Maria, J. -P. and Kingon, Angus I. and Steer, Michael B.}, year={2006}, pages={552–555} }
 @inproceedings{victor_nath_ghosh_aygun_nagy_maria_kingon_steer_2006, title={Voltage controlled GaN on Si HFET power oscillator using thin?film ferroelectric varactor tuning}, DOI={10.1109/eumc.2006.281206}, abstractNote={A 1.6 GHz power oscillator with a GaN-on-Si heterostructure field effect transistor (HFET) is reported. The voltage-controlled oscillator used a thin-film barium strontium titanate (BST) interdigital varactor as the tuning element. The surface-mount varactor was fabricated using sputtered BST film and copper metallization on alumina. An output power of 1.6 W (32 dBm) is obtained with a DC conversion efficiency of 25.5%. Flat tuning sensitivity of 500 kHz/V, 49 MHz linear frequency tuning, and power flatness of better than 0.5 dB are obtained with 0-100 V tuning voltage. The maximum oscillator phase noise is -81.4 dBc/Hz at 100 kHz offset}, booktitle={Proceedings of the 36th European Microwave Conference, (EuMW2006), Manchester, UK}, publisher={London: Horizon House}, author={Victor, A. and Nath, J. and Ghosh, D. and Aygun, S. and Nagy, W. and Maria, J.-P. and Kingon, A. I. and Steer, M. B.}, year={2006}, pages={87–90} }
 @article{kwon_aygun_cann_2005, title={Capacitive response of doped CuO/ZnO heterocontacts to hydrogen}, volume={3}, number={3}, journal={Sensor Letters}, author={Kwon, S. and Aygun, S. and Cann, D. P.}, year={2005}, pages={258–262} }