Surface and bulk electronic structures of unintentionally and Mg-doped In_0.7Ga_0.3N epilayer by hard X-ray photoelectron spectroscopy

The surface and bulk electronic structures of In_0.7Ga_0.3N epilayers are investigated by angle-resolved hard X-ray photoelectron spectroscopy (HX-PES) combined with soft X-PES. The unintentionally and Mg-doped In_0.7Ga_0.3N (u-In_0.7Ga_0.3N and In_0.7Ga_0.3N:Mg, respectively) epilayers are grown by radio-frequency plasma-assisted molecular beam epitaxy. Here three samples with different Mg concentrations ([Mg] = 0, 7 × 10¹⁹, and 4 × 10²⁰ cm⁻³) are chosen for comparison. It is found that a large downward energy band bending exists in all samples due to the formation of a surface electron accumulation (SEA) layer. For u-In_0.7Ga_0.3N epilayer, band bending as large as 0.8 ± 0.05 eV occurs from bulk to surface. Judged from the valence band spectral edge and numerical analysis of energy band with a surface quantum well, the valence band maximum (VBM) with respect to Fermi energy (E_F) level in the bulk is determined to be 1.22 ± 0.05 eV. In contrast, for In_0.7Ga_0.3N:Mg epilayers, the band bending increases and the VBM only in the bulk tends to shift toward the E_F level owing to the Mg acceptor doping. Hence, the energy band is considered to exhibit a downward bending structure due to the coexistence of the n⁺ SEA layer and Mg-doped p layer formed in the bulk. When [Mg] changes from 7 × 10¹⁹ to 4 × 10²⁰ cm⁻³, the peak split occurs in HX-PES spectra under the bulk sensitive condition. This result indicates that the energy band forms an anomalous downward bending structure with a singular point due to the generation of a thin depleted region at the n⁺ p interface. For In_0.7Ga_0.3N:Mg epilayers, the VBM in the bulk is assumed to be slightly lower than E_F level within 0.1 eV.