Diameter-dependent surface photovoltage and surface state density in single semiconductor nanowires.

Based on single-nanowire surface photovoltage measurements and finite-element electrostatic simulations, we determine the surface state density, N(s), in individual n-type ZnO nanowires as a function of nanowire diameter. In general, N(s) increases as the diameter decreases. This identifies an important origin of the recently reported diameter dependence of the surface recombination velocity, which has been commonly considered to be independent of the diameter. Furthermore, through the determination of the surface carrier lifetime, we suggest that the diameter dependence of the surface state density accounts for the rather abrupt transition from bulk-limited to surface-limited carrier transport over a narrow nanowire diameter regime (~30-40 nm). These findings are supported by the comparison between bulk-limited and surface-dependent minority carrier diffusion lengths measured at various diameters.