Abstract
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.
Highlights
Semiconductor nanowires provide a natural, ready-made structure in applications where small dimensions are required [1,2,3]
A scanning electron microscopy (SEM) image of a typical ZnO nanowire device is shown in the inset of Figure 1
In summary, we proposed a model to account for the observed persistent photoconductivity in ZnO nanowires, which ties together several previously suggested explanations of different facets of the problem into a single comprehensive picture
Summary
Semiconductor nanowires provide a natural, ready-made structure in applications where small dimensions are required [1,2,3] Their small diameter (≤100 nm) implies that a host of surface effects can influence their electrical and optical properties, which is important for the functionality and performance of nanowire-based devices [4,5,6]. The element, proposed here to tie together the various pieces of the puzzle into a single comprehensive model, is a fully reversible carbon-catalyzed photolysis, where carbon is omnipresent due to the pervasiveness of surface hydrocarbons, capable of exposing zinc on ZnO surfaces upon ultraviolet (UV) exposure This surface effect is more pronounced and observed in structures of high surface-to-volume ratio, such as nanowires.
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