Abstract
The oxygen adsorption and photodesorption of r.f. sputtered ZnO films having the preferred growth orientation with the c-axis parallel to the substrate have been investigated. Electric-field-induced oxygen adsorption was observed on these polycrystalline films. Strong photoresponses were obtained when samples were irradiated with UV light, and were assumed to arise from oxygen photodesorption on both the surface and crystallite interfaces. Surface photodesorption produced a large, slow photoresponse, while the photoresponse produced by oxygen photodesorption of the crystallite interfaces was relatively weak and fast. Doping the surface layer of ZnO films with nitrogen could stabilize the surface structure, subsequently eliminating surface adsorption. The fast photoresponse originating from photodesorption of crystallite interfaces could appear as a result. The above results indicate that the conductivity of sputtered polycrystalline ZnO films is controlled by surface and intercrystallite depletion barriers, ascribed to the presence of ionized adsorbates on the surface and crystallite interfaces.
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