Surface properties of MOCVD grown (Al1−xGax)2O3 thin films on c-plane sapphire via scanning Kelvin probe microscopy

We report on the surface properties of aluminum-gallium-oxide [(AlxGa1−x)2O3] thin films grown on c-plane sapphire substrates, which form the building block of heterojunction semiconductor devices, by metal–organic chemical vapor deposition (MOCVD). The (AlxGa1−x)2O3 films with aluminum mole fraction (composition) ranging from x = 0 to x = 0.36 were analyzed using x-ray diffraction (XRD) and atomic force microscopy (AFM). The increase in the aluminum composition resulted in the shift in XRD peaks to larger 2θ angles due to the decrease in the lattice constants and broadening due to the degradation in crystallinity. By utilizing the AFM in scanning Kelvin probe microscopy (SKPM) configuration, we report the surface work function of (AlxGa1−x)2O3 decreased from 6.13 to 5.62 eV as aluminum composition increased from 0% to 36%. Direct measurements of epitaxial surface potential in (AlxGa1−x)2O3 thin films show downward band bending ranging from 1.19 to 0.65 eV. The SKPM measurements with trap filling under UV illumination suggest the presence of a relatively high density of donor surface states in (AlxGa1−x)2O3 samples. By modeling the surface states as a spatially localized and energetically discrete level in the bandgap, the density of surface states is found to be ∼2 × 1012 to ∼4 × 1013 cm−2.