Electronic states near surfaces and interfaces of β-Ga2O3 and κ-Ga2O3 epilayers investigated by surface photovoltage spectroscopy, photoconductivity and optical absorption

Transient surface photovoltage (SPV) spectroscopy, optical absorption, and photoconductivity (PC) were applied to study electronic transitions in (-201) 13-Ga 2 O 3 and (001) x-Ga 2 O 3 epitaxial layers on c -plane sapphire substrates. SPV signals were distinguished for charge separation near the surface and near the layer/substrate interface. The bandgaps of 13-Ga 2 O 3 and x-Ga 2 O 3 epitaxial layers were found to be, respectively, about 4.75 and 4.79 eV from optical absorption measurements, about 4.8 and 4.9 eV from PC, and 4.65 and 4.9 (near the surface) from SPV measurements. Near the x-Ga 2 O 3 layer/substrate interface SPV instead gives a value of 4.75 eV, possibly related to the presence of an interlayer. Defect -related transitions with onset energies at about 4.5, 4.1, and 3.5 eV were observed along the whole 13-Ga 2 O 3 layer cross-section. In contrast, defects related to different transitions were differently distributed in x-Ga 2 O 3 epitaxial layers: transitions setting on at about 4.3 eV originate from defects uniformly distributed across the layer, while transitions starting at 2.4 eV and 1.7 eV were respectively connected with defects located near the surface or near the layer/substrate interface. The PC measurements at photo -excitation energy above the bandgap indicated that defect densities and recombination losses were much larger in x-Ga 2 O 3 than in 13-Ga 2 O 3 .