Substoichiometric Atomically Thin Gallium Oxide as a Defect-Tunable Multifunctional Oxide

Abstract

Atomically thin β-Ga2O3 represents an emerging class of two-dimensional oxides whose functional properties arise from a combination of ultrawide band gap electronic structure, mixed ionic–covalent bonding, and defect-sensitive local symmetry. Here, we systematically probe the tribological, electronic, and electromechanical behavior of ultrathin nonstoichometric monoclinic β-Ga2O3–δ and show that all three responses exhibit features consistent with defect-mediated interactions. Friction force microscopy reveals that nanoscale friction is reversibly modulated by external bias and displays pronounced asymmetry between positive and negative voltages, suggesting a role for field-induced charge trapping that alters interfacial electrostatic forces. Hall measurements from 100 to 400 °C show negligible current and no measurable mobility, indicating strong carrier localization and deep-level trapping that suppress free-carrier transport in the ultrathin limit. Dual-AC-resonance piezoresponse force microscopy measurements yield a reproducible out-of-plane electromechanical response (d33eff ≈ 4–5 pm/V), consistent with local symmetry breaking that may originate from surfaces, strain, or point defects. While X-ray photoelectron spectroscopy confirms the presence of oxygen-related states, we do not attribute the observed functionalities to vacancies alone; rather, the combined data indicate that the defect- and symmetry-sensitive electronic structure plays a central role across the tribological, electronic, and electromechanical domains. These findings position atomically thin β-Ga2O3–δ as a multifunctional oxide platform in which interfacial mechanics, charge localization, and emergent electromechanical coupling can be sensitively tuned for next-generation nanoscale devices.