Thermal conductivity of wide and ultra-wide bandgap semiconductors

Abstract

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 nand 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 PREFACE This Ph.D. thesis is written based on the research results accumulated during the graduate studies of Tran Quoc Dat at the Center for III-Nitride Technology (C3NiT-Janzen), the Department of Physics, Chemistry and Biology in the Linköping University from September 2018 to August 2023. The thesis is based on scientific papers and contains two main parts: the first part provides an introduction to the research field and the second part presents the main research results summarized in six scientific papers. The dissertation is a continuation of the work presented in my Licentiate thesis (No. 1908; Linköping Studies in Science and Technology, ISBN 978-91-7929-629-2).The graduate studies were accomplished under the financial supports (i) the Swedish