Thermal stability of α-Ga2O3 films grown on c-plane sapphire substrates via mist-CVD

The thermal stability of α-Ga2O3 films grown on c-plane sapphire substrates was investigated. A strong correlation was found between thermal stability and film thickness: the more the α-Ga2O3 films maintained the α-phase upon heating at higher annealing temperature, the thinner they were. Transmission electron microscopy observations revealed that the phase transition of the α-Ga2O3 film to the thermodynamically most stable β-phase had the orientation relationship of β-Ga2O3 [2̄ 01] || sapphire [0001]. High-temperature x-ray diffraction measurement for the α-Ga2O3 film showed the relationship of β-Ga2O3 [4̄ 01]/[301] || sapphire [0001] as well. The dependence of the stability boundary on the film thickness originates from a thermal stress caused by a larger thermal expansion coefficient of α-Ga2O3 than that of sapphire. Relaxation of residual stress by introducing a selective area growth technique enhanced the thermal stability of α-Ga2O3 so that α-Ga2O3 maintained the corundum structure upon heating at 800 °C, although a small diffraction peak from β-Ga2O3 was detected by x-ray diffraction measurement. The enhanced thermal stability of α-Ga2O3 widens device process windows as well as growth windows

High-Frequency and Below Bandgap Anisotropic Dielectric Constants in \u3cem\u3eα\u3c/em\u3e-(Al\u3csub\u3ex\u3c/sub\u3eGa\u3csub\u3e1-x\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e (0≤x≤1)

A Mueller matrix spectroscopic ellipsometry approach was used to investigate the anisotropic dielectric constants of corundum α-(AlxGa1-x)2O3 thin films in their below bandgap spectral regions. The sample set was epitaxially grown using plasma-assisted molecular beam epitaxy on m-plane sapphire. The spectroscopic ellipsometry measurements were performed at multiple azimuthal angles to resolve the uniaxial dielectric properties. A Cauchy dispersion model was applied, and high-frequency dielectric constants are determined for polarization perpendicular (ε∞,⟂) and parallel (ε∞,∥) to the thin film c-axis. The optical birefringence is negative throughout the composition range, and the overall index of refraction substantially decreases upon incorporation of Al. We find small bowing parameters of the high-frequency dielectric constants with b⟂=0.386 and b∥=0.307

Anisotropic Dielectric Functions, Band-to-Band Transitions, and Critical Points in \u3cem\u3eα\u3c/em\u3e-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e

We use a combined generalized spectroscopic ellipsometry and density functional theory approach to determine and analyze the anisotropic dielectric functions of an α-Ga2O3 thin film. The sample is grown epitaxially by plasma-assisted molecular beam epitaxy on m-plane sapphire. Generalized spectroscopic ellipsometry data from multiple sample azimuths in the spectral range from 0.73 eV to 8.75 eV are simultaneously analyzed. Density functional theory is used to calculate the valence and conduction band structure. We identify, for the indirect-bandgap material, two direct band-to-band transitions with M0-type van Hove singularities for polarization perpendicular to the c axis, E0,⊥=5.46(6) eV and E0,⊥=6.04(1) eV, and one direct band-to-band transition with M1-type van Hove singularity for polarization parallel to E0,∥=5.44(2) eV. We further identify excitonic contributions with a small binding energy of 7 meV associated with the lowest ordinary transition and a hyperbolic exciton at the M1-type critical point with a large binding energy of 178 meV

Infrared Dielectric Functions and Brillouin Zone Center Phonons of \u3cem\u3eα\u3c/em\u3e-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e compared to \u3cem\u3eα\u3c/em\u3e-Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e

We determine the anisotropic dielectric functions of rhombohedral α-Ga2O3 by far-infrared and infrared generalized spectroscopic ellipsometry and derive all transverse optical and longitudinal optical phonon mode frequencies and broadening parameters. We also determine the high-frequency and static dielectric constants. We perform density functional theory computations and determine the phonon dispersion for all branches in the Brillouin zone, and we derive all phonon mode parameters at the Brillouin zone center including Raman-active, infrared-active, and silent modes. Excellent agreement is obtained between our experimental and computation results as well as among all previously reported partial information from experiment and theory. We also compute the same information for α-Al2O3, the binary parent compound for the emerging alloy of α-(AlxGa1−x)2O3, and use results from previous investigations [Schubert, Tiwald, and Herzinger, Phys. Rev. B 61, 8187 (2000)] to compare all properties among the two isostructural compounds. From both experimental and theoretical investigations, we compute the frequency shifts of all modes between the two compounds. Additionally, we calculate overlap parameters between phonon mode eigenvectors and discuss the possible evolution of all phonon modes into the ternary alloy system and whether modes may form single-mode or more complex mode behaviors

サファイア基板上高品質コランダム構造酸化ガリウムの成長に関する研究

京都大学0048新制・課程博士博士(工学)甲第22450号工博第4711号新制||工||1736(附属図書館)京都大学大学院工学研究科電子工学専攻(主査)教授 藤田 静雄, 教授 山田 啓文, 准教授 船戸 充学位規則第4条第1項該当Doctor of Philosophy (Engineering)Kyoto UniversityDFA

Infrared-active phonon modes and static dielectric constants in -(AlxGa 1-x)2O3(0.18 ≤ x ≤ 0.54) alloys

We determine the composition dependence of the transverse and longitudinal optical infrared-active phonon modes in rhombohedral α-(AlxGa1-x)2O3 alloys by far-infrared and infrared generalized spectroscopic ellipsometry. Single-crystalline high quality undoped thin-films grown on m-plane oriented α-Al2O3 substrates with x = 0.18, 0.37, and 0.54 were investigated. A single mode behavior is observed for all phonon modes, i.e., their frequencies shift gradually between the equivalent phonon modes of the isostructural binary parent compounds. We also provide physical model line shape functions for the anisotropic dielectric functions. We use the anisotropic high-frequency dielectric constants for polarizations parallel and perpendicular to the lattice c axis measured recently by Hilfiker et al. [Appl. Phys. Lett. 119, 092103 (2021)], and we determine the anisotropic static dielectric constants using the Lyddane-Sachs-Teller relation. The static dielectric constants can be approximated by linear relationships between those of α-Ga2O3 and α-Al2O3. The optical phonon modes and static dielectric constants will become useful for device design and free charge carrier characterization using optical techniques