@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{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{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{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{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} }