Bidirectional Control of the Band Bending at the (2̅01) and (010) Surfaces of β-Ga₂O₃ Using Aryldiazonium Ion and Phosphonic Acid Grafting

Beta gallium oxide (β-Ga2O3) is an ultrawide-bandgap semiconductor with one of the highest-known breakdown strengths (∼8 MV/cm) making it a strong candidate for high-efficiency power electronics, deep-ultraviolet photodetectors, and transparent electronic devices. Bare β-Ga2O3 surfaces exhibit a strong upward band bending and electron depletion that is reduced by the formation of a hydroxyl termination on exposure to the atmosphere, although this effect varies with exposure conditions and the processing history of different samples. This work investigates the covalent modification of (2̅01) and (010) β-Ga2O3 surfaces with organic layers, as a mechanism for more effectively controlling their surface band bending. We compare the different electronic effects of grafted layers formed by the electrochemical reduction of 4-nitrobenzenediazonium ions and the spontaneous grafting of octadecylphosphonic acid (ODPA). Atomic force microscopy, synchrotron X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy confirmed the presence of few-nanometer layers of covalently attached nitrophenyl (NP) and ODPA molecules. Valence band XPS showed that NP modification produced upward band bending shifts of +0.51 and +0.53 eV on (2̅01) and (010) β-Ga2O3, respectively, with further increases of +0.35 and +0.20 eV after X-ray-induced reduction of the nitro substituent to amino-like moieties. In contrast, ODPA modification produced downward shifts in surface band bending of −0.36 and −0.07 eV on (2̅01) and (010) β-Ga2O3, respectively. Our study demonstrates that covalently bound NP and ODPA molecules can be used to significantly modify the electronic properties of β-Ga2O3 surfaces, a finding that should prove useful for electronic applications of this material.