UV photoelectron spectroscopy of band alignment at ohmic ITO/β-Ga2O3 interfaces grown by pulsed laser deposition

The energy band alignment of indium-tin-oxide (ITO) ohmic contacts to pulsed laser-deposited epitaxial β-Ga2O3 was examined using optical and ultraviolet photoelectron spectroscopies to determine bandgap values, Fermi-level position, and work function. The results reveal a Type II heterojunction with a favorable conduction band offset of 0.31 eV for electron injection from the ITO contact in its as-deposited state, which increased to 0.45 eV after annealing for 1 min in N2 at 550 °C. Annealing reduced the Fermi energy separation from the conduction band edge from 1.04 to 0.6 eV and from 0.74 to 0.15 eV in the ITO and Ga2O3 films, respectively. The corresponding change in the specific contact resistivity was a decrease from 1.5 × 10−1 Ω cm2 to 1.9 × 10−2 Ω cm2. X-ray photoelectron spectroscopy (XPS) of the Ga2O3 films revealed the presence of Ga–Ga bonding and uncoordinated oxygen, indicating the coexistence of Ga interstitials and oxygen vacancies. Although Ga interstitials are known to be shallow donors, the presence of both types of donors prevents unambiguous assignment of the intrinsic n-type conductivity. XPS also confirmed Ga diffusion into the ITO contact upon annealing, consistent with the measured decrease in the Fermi level separation from the conduction band edge and an increase in free electron concentration. The results show that forming a mixed, defective ITO/Ga2O3 layer is crucial for reducing contact resistance and enhancing the ohmicity of ITO contacts to β-Ga2O3. Schematic energy band diagrams of the ohmic ITO/Ga2O3 heterojunctions are proposed.