Anisotropic dielectric function, direction dependent bandgap energy, band order, and indirect to direct gap crossover in α-(AlxGa1−x)2O3 (≤x≤1)

Abstract

Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure α-(Al xGa1− x)2O3 thin films (0 [Formula: see text] x [Formula: see text] 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular ([Formula: see text] = 1.31 eV) and parallel ([Formula: see text] = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between α-Ga2O3 and α-Al2O3, respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in α-Ga2O3 whereas for α-Al2O3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M1 type in α-Ga2O3 to M0 type van Hove singularity in α-Al2O3.

Determination of anisotropic optical properties of MOCVD grown m-plane α-(Al x Ga1−x )2O3 alloys

“…Some attempts to alloy the stable β-Ga 2 O 3 phase with Al have been made, [28][29][30][31][32][33] but this remains challenging since α-Al 2 O 3 does not share the same crystal structure and monoclinic Al 2 O 3 (θ-Al 2 O 3 ) remains obscure. 34) In contrast, alloying α-Ga 2 O 3 with Al by CVD, 35) pulsed laser deposition (PLD), 36) and MBE, 37) can provide single-crystal films especially when grown on m-plane sapphire. 38) c-plane Al 2 O 3 as substrate seems to lead to the formation of a few monolayers α-Ga 2 O 3 followed by β-Ga 2 O 3 , due to the large in-plane lattice mismatch between layer and substrate, observed in metalorganic CVD (MOCVD), MBE and PLD, but not in mist CVD or halide vapour phase epitaxy.…”

“…Studies of the optical properties of α-(Al x Ga 1−x ) 2 O 3 taking into account the anisotropy are rare. Hilfiker et al investigated the optical absorption onset 34) and dielectric limits ε ∞ 51) by spectroscopic ellipsometry. Additionally Stokey et al 52) determined IR-active phonon modes and static dielectric constants with the same technique in the IR.…”

“…They are in turn used to determine the anisotropic bowing parameters. To underline the novelty of this study, we emphasize that a different approach is used to evaluate the DF compared to Hilfiker et al 34) Consequently, this leads to deviating results in the bowing parameters and the order of Γ-point transitions.…”

Formation energy crossings in Ga2O3-Al2O3 quasibinary system: ordered structures and phase transitions in (Al x Ga1−x )2O3

“…In addition, the bandgap of Ga 2 O 3 can be tuned at a wide range using substitutional alloying on the Ga site by Group III elements. [6][7][8][9][10][11][12][13][14][15][16] For instance, it was shown theoretically that increasing the Al doping composition of Ga 2 O 3 linearly increases the bandgap of the semiconducting crystal. 6,7) On the other hand, In the case of In substitution, the bandgap decreases with increasing doping concentration.…”

Oxygen vacancies in α-(Al x Ga1−x )2O3 alloys: a first-principles study

“…6) The formation of α-(Al x Ga 1−x ) 2 O 3 alloys between isostructural α-Al 2 O 3 and α-Ga 2 O 3 allows for band gap engineering in a wide range up to 8.6 eV. 7,8) The crystal growth and fundamental properties of α-(Al x Ga 1−x ) 2 O 3 alloys have been studied both experimentally 5,[7][8][9][10][11][12] and theoretically [13][14][15] towards in-depth understanding and full utilization of their functionalities.…”