This PhD thesis presents experimental and theoretical studies of the thermal conductivity of wide and ultra-wide bandgap semiconductors including GaN, AlN, β-Ga_{2}O_{3} binary compounds, and Al_{x}Ga_{1−x}N, Sc_{x}Al_{1−x}N, Y_{x}Al_{1−x}N ternary alloys. Thermal conductivity measurements are conducted using the transient thermoreflectance (TTR) technique and the results are interpreted using analytical models based on the solution of the Boltzmann transport equation (BTE) within the relaxation time approximation (RTA).
The study is motivated by the increasing research interest in these material systems due to their potential for the development of high-power (HP) and high-frequency (HF) electronic devices. Due to its wide bandgap, high electrical field, and high electronic saturation velocity, GaN is an excellent material for fast-switching HP electronic devices. Al_{x}Ga_{1−x}N is considered a natural choice for next-generation HP electronic devices since by tuning the bandgap from 3.4 eV to 6 eV a significant increase of the critical electric field and thus the device breakdown voltage, can be achieved. Furthermore, both n- and p-type conductivity can be realized in AlGaN allowing flexible device design. β-Ga_{2}O_{3} is also promising for HP electronics because of its ultra-wide bandgap (4.8 eV) and a very high Baliga’s figure of merit (FOM) exceeding by far that of GaN. Moreover, the mature growth techniques of bulk β-Ga_{2}O_{3} can enable low-cost substrates with high crystal quality. Sc_{x}Al_{1−x}N and Y_{x}Al_{1−x}N have recently emerged as a new class of III-nitride semiconductors. Due to the large piezoelectric coefficients and spontaneous polarization in these alloys, a very large density of two-dimensional electron gas (2DEG) can be achieved at (Sc,Y)_{x}Al_{1−x}N/GaN heterostructures enabling high mobility transistors (HEMTs) with an enhanced HF performance as compared with the common Al_{x}Ga_{1−x}N/GaN HEMTs.
For any HP and HF device, the thermal conductivity of the constituent materials in the device structures is of crucial importance. Such devices operate at high currents, high voltages, and/or high frequencies, so a high Joule heat is generated in the device’s active region. This heat must be effectively dissipated in order to ensure high device performance and reliability. Therefore, understanding the materials’ thermal conductivity is essential for the device’s thermal management.
We have investigated different bulk materials and epitaxial layers and have established the effects of dislocation density, doping, alloying, layer thickness, and crystal orientation on thermal conductivity. The results presented in this Ph.D. thesis give new insights into the thermal properties of wide and ultra-wide semiconductors and could be useful for the design, optimization, and thermal management of electronic devices based on these materials.
The main research results presented in this Ph.D. thesis are summarized in six scientific papers.
Paper I is focused on studying the thermal conductivity of high Al-content Al_{x}Ga_{1−x}N and β-Ga2O_{3} thin layers. For β-Ga_{2}O_{3} layers the effect of Sn doping on their thermal conductivity is also studied. The experimental measurements are performed in a temperature range of 280-350 K. A modified Callaway’s model is employed for the interpretation of the results. Calculations of the thickness-dependent thermal conductivity reveal quite different transport mechanisms of the two materials.
Paper II presents experimental results of the thermal conductivity of thick Al_{x}Ga_{1−x}N layers. A detailed discussion of the phonon-alloy scattering which is the main mechanism limiting the thermal conductivity of Al_{x}Ga_{1−x}N is presented. Analyzing the interplay between the phonon-alloy scattering and the phonon-boundary scattering the experimentally observed thickness dependence of the thermal conductivity is explained.
Paper III is devoted to studying the role of defects on the thermal conductivity of Al_{x}Ga_{1−x}N alloys with 0 ≤ x ≤ 1. The effect of dislocations, impurities, free carriers, and alloying have been separately studied and discussed. The thermal conductivity of samples with various concentrations of the defect is measured and the results are interpreted using a theoretical model based on the solution of the BTE equation within the RTA.
Paper IV focuses on the thermal conductivity study of Sc_{x}Al_{1−x}N and Y_{x}Al_{1−x}N alloys. The experimental measurements are performed for layers having compositions in the range of 0 ≤ x ≤ 0.22. The effect of phonon-alloy scattering in these alloy materials is discussed and compared with other phonon scattering processes. The experimental results are interpreted within the frame of a modified Callaway’s model in combination with ab-initio calculation for phonon dispersions and mode Grüneisen parameters.
Paper V investigates the thermal conductivity anisotropy of bulk GaN. The thermal conductivity along the c- and m-axis crystallographic directions of wurtzite GaN is measured in a temperature range of 80-400 K. Experimental observations are elaborated by an analysis of the anisotropy of the phonon group velocity, the Debye temperature and the mode Grüneisen parameters.
Paper VI explores the effects of doping and free carriers on the thermal conductivity of bulk GaN and homoepitaxial layers and heteroepitaxial GaN layers via both experimental and theoretical approaches. The impurities considered include Si, O, Mg, and Fe. The experimental results are analyzed using a non-Debye RTA model in combination with ab-initio calculations of the phonon dispersion.