RESUMO
The ability to control conductivity in semiconductor nanostructures is often challenged by surface states trapping the majority of the charge carriers. Addressing this challenge requires a reliable method for assessing electrical properties such as carrier concentration and mobility. Unfortunately, here we are facing another challenge, as the Hall effect is geometrically inapplicable to nanowires while the field effect model is also challenged by the geometry of the common nanowire field effect transistor, and can only yield channel mobility which is very different from Hall mobility. In this paper, we propose a method that combines resistivity and photovoltage measurements with a chemical perturbation to the surface to measure carrier concentration and mobility, as a function of wire diameter, and also to measure the surface state density and the surface band bending before and after the chemical treatment. We apply this method to CVD grown GaN nanowires, before and after a mild HCl etch. Using transmission electron microscope and x-ray photoelectron spectrometry we find that HCl removes the native gallium oxide. The etch is found to reduce the surface state density from 1 × 10(12) to 5.3 × 10(11) cm(2), which is calculated from a reduction of the critical radius for full depletion from 7.6 to 4 nm, and a reduction of the surface band bending from 0.53 to 0.29 eV, observed using surface photovoltage. On the average, the values of carrier concentration we obtain are about ten times smaller, and the mobility about ten times greater, than values obtained using field effect transistors. Interestingly, the weak size dependence of the mobility disappears after etching, suggesting a causal linkage between the as-grown size dependence of the mobility and the density of surface states. The proposed method provides an experimental handle to the study of surface states and their effects on the electrical properties of nanowires.
RESUMO
Photoconductivity is studied in individual ZnO nanowires. Under ultraviolet (UV) illumination, the induced photocurrents are observed to persist both in air and in vacuum. Their dependence on UV intensity in air is explained by means of photoinduced surface depletion depth decrease caused by oxygen desorption induced by photogenerated holes. The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase. After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained. Once vacuum is broken and air is let in, the photocurrent quickly decays down to the typical air-photoresponse values. The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity. The adsorption-desorption balance is fully recovered after the ZnO surface is exposed to air, which suggests that under UV illumination, the ZnO surface is actively "breathing" oxygen, a process that is further enhanced in nanowires by their high surface to volume ratio.
