Ellipsometric determination of anisotropic optical constants of single phase Ga2O3 thin films in its orthorhombic and monoclinic phases
The anisotropic optical constants of high-quality, single crystalline Ga2O3 films in both orthorhombic (ε) and monoclinic (β) phases were obtained from ellipsometry study. The (0001)-oriented ε-Ga2O3 and a (-201) β-Ga2O3 epilayers were grown on (0001) α-Al2O3 substrates by mist-chemical vapor deposition and pulsed laser deposition, respectively. The experimental bandgaps derived from optical constants were found to be related with the allowed optical transitions from valence sub-bands to conduction band minimum in terms of the polarization-selection rules in such asymmetric structures. The pseudo-ordinary (a-b plane) and extraordinary (c axis) optical bandgaps of the ε-Ga2O3 film were estimated to be 4.85 ± 0.02 and 4.76 ± 0.02 eV, respectively. The pseudo ordinary (b axis) and extraordinary (a-c plane) optical bandgaps of the β-Ga2O3 film were estimated to be 4.92 ± 0.02 and 4.57 ± 0.02 eV, respectively. The energy difference between the two bandgaps of the ε-Ga2O3 is about 0.09 eV, which matches the theoretically calculated energy difference (about 0.06 eV) of the highest two valence bands.