UV photoconductors based on Ga-doped ZnO films
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0891-EE08-07.1
UV photoconductors based on Ga-doped ZnO films Leelaprasanna J. Mandalapu, Faxian Xiu, Zheng Yang, and Jianlin Liu Quantum Structures Laboratory, Department of Electrical Engineering, University of California, Riverside, CA 92521, U.S.A. ABSTRACT Ga-doped n-type ZnO films were grown by molecular-beam epitaxy (MBE) on R-plane sapphire substrates. Material characterizations such as photoluminescence (PL), X-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS) and Hall measurements were carried out to characterize the optical, structural, and electrical properties of the film. Photoconductor devices for ultraviolet (UV) light detection were fabricated by depositing Al (250 nm)/Ti (20 nm) ohmic metal contacts. Linear characteristics were obtained from current-voltage (I-V) measurements that showed response to UV and visible illumination. Voltage dependent photocurrent (PC) spectra and responsivity spectrum were obtained to characterize the detection capability of the device in the UV region.
INTRODUCTION ZnO is a II-VI oxide semiconductor that is being explored widely in recent years for UV optoelectronic applications because of its attractive and promising properties. The properties such as direct wide bandgap, high radiation hardness and high exciton binding energy make ZnO a good candidate for UV detection and UV emission [1-3]. Attempts are being continuously made to grow high quality ZnO films and fabricate various kinds of ZnO based photodetectors such as photoconductors, heterojunction and homojunction photodiodes, thin film transistors, etc [4-7]. Reproducible p-type doping of ZnO is a challenge to obtain reliable high performances from some of these devices and active research is carried out in this area. The exploration of n-type ZnO and n-type ZnO based devices has also been constantly increasing, as eventually, both the p-type and n-type materials are necessary. While employing a single type of semiconductor for devices such as photodetectors, n-type is preferred as the electrons have a higher mobility and saturation velocity. ZnO has mainly found its application as transparent conductive electrodes and protective coatings for other devices in the past, which led to detailed study of electrical properties of the material. The study of optical properties, specifically photoelectric responses emerged much later but the fabrication of these materials into self-sustaining devices is just recently on the rise. Different methods have been employed to enhance photoconductivity in undoped ZnO films [8-10]. In this study, we have reported the successful fabrication and characterization of photoconductors based on Ga-doped ZnO films grown by MBE. These devices exhibit better photoresponse than the undoped ZnO.
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EXPERIMENTAL DETAILS ZnO films were grown using plasma-assisted MBE on R-plane sapphire substrates. Effusion cells were sources of Zn and Ga, operated at the temperatures of 350˚C and 320˚C, respectively. Oxygen was from pure oxygen gas and oxygen plasma was generated
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