Observation of a 2D Electron Gas and the Tuning of the Electrical Conductance of ZnO Nanowires by Controllable Surface Band-Bending

Direct experimental evidence for the existence of a 2D electron gas in devices based on ZnO nanowires (NWs) is presented. A two-channel core/shell model is proposed for the interpretation of the temperature-dependent current-voltage (I-V) characteristics of the ZnO NW, where a mixed metallic-semiconducting behavior is observed. The experimental results are quantitatively analyzed using a weak-localization theory, and suggest that the NW is composed of a "bulk" semiconducting core with a metallic surface accumulation layer, which is basically a 2D electron gas in which the electron- phonon inelastic scattering is much weaker than the electron-electron inelastic scattering. A series of I-V measurements on a single NW device are carried out by alternating the atmosphere (vacuum, H-2, vacuum, O-2), and a reversible change in the conductance from metallic to semiconducting is achieved, indicating the surface accumulation layer is likely hydroxide- induced. Such results strongly support the two-channel mode and demonstrate the controllable tuning of the ZnO NW electrical behavior via surface band-bending.