title: Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires.
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.
authors.affiliation: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA. capasso@seas.harvard.edu.
authors.forename: Jiming
authors.initials: J
authors.lastname: Bao
authors.forename: Ilan
authors.initials: I
authors.lastname: Shalish
authors.forename: Zhihua
authors.initials: Z
authors.lastname: Su
authors.forename: Ron
authors.initials: R
authors.lastname: Gurwitz
authors.forename: Federico
authors.initials: F
authors.lastname: Capasso
authors.forename: Xiaowei
authors.initials: X
authors.lastname: Wang
authors.forename: Zhifeng
authors.initials: Z
authors.lastname: Ren