Measurements of Electrostatic Potential Across p–n Junctions on Oxidized Si Surfaces by Scanning Multimode Tunneling Spectroscopy

We investigated the variation in contact potential difference (CPD) voltage across p–n junctions on oxygen-passivated Si(110) surfaces by scanning multimode tunneling spectroscopy, which detects probe–sample interaction force simultaneously with tunneling current. The enhancement of sensitivity to electrostatic force was achieved with a small amplitude of probe vibration (0.3 nm) when the tip–sample gap was adjusted to reduce short-range interactions by maintaining the tunneling current at a specified bias voltage. At the optimal tip–sample gap, the CPD voltage, derived from force gradient spectra, agrees with the expected built-in potential across the p–n junction. The CPD voltage showed a standard deviation of ∼30 mV on atomically flat terraces. Larger fluctuations were ascribed to structural and charge variations on the oxidized surfaces.