Abstract

We present a phenomenological model for the photocurrent transient relaxation observed in ZnO-based metal-semiconductor-metal (MSM) planar photodetector devices based on time-resolved surface band bending. Surface band bending decreases during illumination, due to migration of photogenerated holes to the surface. Immediately after turning off illumination, conduction-band electrons must overcome a relatively low energy barrier to recombine with photogenerated holes at the surface; however, with increasing time, the adsorption of oxygen at the surface or electron trapping in the depletion region increases band bending, resulting in an increased bulk/surface energy barrier that slows the transport of photogenerated electrons. We present a complex rate equation based on thermionic transition of charge carriers to and from the surface and numerically fit this model to transient photocurrent measurements of several MSM planar ZnO photodetectors at variable temperature. Fitting parameters are found to be consistent with measured values in the literature. An understanding of the mechanism for persistent photoconductivity could lead to mitigation in future device applications.

Highlights

  • Bulk and thin-film ZnO materials have attracted a great deal of attention, primarily due to their potential applications as photodetectors, gas sensors and as electronic and light-emitting nano-devices.Zinc oxide-based devices have the potential to outperform other wide band gap materials and, in the future, may replace nitride-based semiconductors [1]

  • An understanding of the mechanism for persistent photoconductivity could lead to mitigation in future device applications

  • After turning off illumination, conduction band electrons must overcome a relatively low energy barrier to recombine with photogenerated holes at the surface; with increasing time, the adsorption of oxygen at the surface increases band bending, resulting in an increased bulk/surface energy barrier that slows transport of photogenerated electrons

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Summary

Introduction

Bulk and thin-film ZnO materials have attracted a great deal of attention, primarily due to their potential applications as photodetectors, gas sensors and as electronic and light-emitting nano-devices.Zinc oxide-based devices have the potential to outperform other wide band gap materials and, in the future, may replace nitride-based semiconductors [1]. Bulk and thin-film ZnO materials have attracted a great deal of attention, primarily due to their potential applications as photodetectors, gas sensors and as electronic and light-emitting nano-devices. A technique for low-cost growth has been developed, where Zinc-metal films are grown and, thermally oxidized to produce polycrystalline ZnO films [2,10]. Resistive films with polycrystalline structure have resulted when starting with approximately 200 nm-thick Zn-metallic films [11,12,13]. These ZnO films have been utilized as a base for ZnO ultraviolet photodetectors, with various metals applied as contacts for photodetectors having a metal-semiconductor-metal (MSM) planar structure [14]. Zheng et al fabricated ZnO photodetectors on glass substrates with Al contacts, which showed a large UV photoresponse [15]

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