Surface-driven degeneracy and charge transport in ultrathin SnO ₂ films deposited by atmospheric spatial atomic layer deposition

Abstract SnO 2 is a critical wide-bandgap oxide semiconductor that is widely used in photovoltaics, transparent electrodes, and sensors, where intrinsic defects strongly influence its electronic properties. Understanding the origin of high carrier densities in ultrathin SnO 2 remains a fundamental challenge, particularly under atmospheric growth conditions where defect formation differs from conventional vacuum processes. In this study, we investigate the thickness-dependent electronic properties of SnO 2 thin films grown by atmospheric-pressure spatial atomic layer deposition (AP-SALD) in the ultrathin regime (≈7–60 nm). A noticeable increase in the electron concentration was observed with decreasing thickness, reaching degenerate levels (∼10 20 cm −3 ) without intentional doping. Combined cryogenic temperature-dependent Hall measurements and density functional theory reveal that this behavior originates from oxygen vacancies ( V O ) preferentially formed at surfaces and grain boundaries. Charge transport is governed by the interplay between extended and localized band-tail states, resulting in a weak temperature dependence of carrier density and mobility, limited by thermally assisted tunneling across grain-boundary barriers. These results provide new insight into the origin of carriers and transport mechanisms in degenerate SnO 2 thin films, highlighting the importance of defective interfaces in governing electronic properties in ultrathin oxide semiconductors. They also suggest that AP-SALD may offer opportunities for tuning electrical properties without intentional doping, which is of interest for transparent conductive materials.