Investigation of the surface band structure and the evolution of defects in -(Al_xGa_1-x)₂O₃

This study examines the electronic and luminescent properties of beta-(AlxGa1-x)(2)O-3 (0 <= x <= 0.42) thin films grown on (0001) sapphire using laser-MBE, with a focus on the evolution of defect energy levels and their impact on surface Fermi level pinning and luminescence. X-ray photoelectron spectroscopy (XPS) and cathodoluminescence (CL) have been employed to analyze surface band bending and defect evolution as a function of aluminum content. The results have revealed a pinned Fermi level at 3.6 eV above the valence band maximum despite the increase in the bandgap. The consequent upward band bending has been confirmed by a peak shift in the core level XPS. The defects that lead to the Fermi level pinning effect are attributed to E2*, which is related to a Ga vacancy or Ga vacancy-O vacancy complex. In addition, CL spectroscopy and depth-resolved CL have demonstrated consistent blue and ultraviolet emissions across the Al content range and a similar suppression of electron concentration on blue and ultraviolet emissions in beta-(AlxGa1-x)(2)O-3 and beta-Ga2O3. Based on the observed evolution of defects with Al content, the blue band emission is attributed to electron transition in the donor-accepter pair.