Ultraviolet Photodetection Properties of ZnO Microtubes
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Ultraviolet Photodetection Properties of ZnO Microtubes Jiping Cheng, Ming Fu, Yunjin Zhang, and Ruyan Guo Pennsylvania State University, University Park, PA, 16802 Abstract Photodetectors based on wide-bandgap semiconductors have demonstrated several advantages over traditional ultraviolet (UV) detectors (photomultiplier tubes and Si-based UV detectors) such as low power consumption, high stability, and no need of other optical filters. ZnO stands a good chance of being a candidate material for solar-blind UV detection because of its direct bandgap of 3.37eV and high photoresponse. In this work, single crystal ZnO microtubes synthesized using a microwave-heating growth method and their UV photodetection properties were studied. The ZnO microtubes exhibited relatively fast UV photoresponse with a cut-off wavelength ~370 nm, indicating their potential applications as high efficient and low cost UV detectors. Introduction Ultraviolet (UV) detectors present a wide range of civil and military applications including chemical and biological analysis, flame detection, satellite-based communications, emitter calibration, and astronomical studies [1]. The earliest UV detection was accomplished using photomultiplier tubes that were fragile, bulky and heavy, and costly. A successful solid-state alternative has been achieved using siliconbased UV photodiodes, offering the advantages of lightweight, low cost, good sensibility and capability for high-speed operation [2]. Some other II-IV and III-V semiconductor materials, such as Ge, GaAs, and SiC, have been pursued to realize more efficient UV detectors as well. More recently, wide bandgap III-nitride semiconductors, in particular, AlxGa1-xN have emerged as the most promising material systems for such a device due to their exceptional material properties [3]. Similar to GaN-based materials, ZnO exhibits excellent UV photosensitivity to be used for UV photodetection. ZnO is closely lattice-matched to GaN and has same bandgap of about 3.4 eV. Meanwhile, ZnO is more resistant to radiation damage and much more inexpensive than GaN. Some earlier researches have demonstrated the capability of ZnO UV photodetectors, such as photoconductors, Schottky barrier photodetectors, and MSM (metal-semiconductor-metal) structural UV detectors, which presented a sharp cutoff at 365 nm with UV/visible contrast of two to three decades [4-6]. In our earlier work, ZnO single crystal microtube revealed excellent UV photoresponse properties [7]. A more detailed study on the UV photodetection properties of ZnO microtube is carried out in this work. Experimental ZnO single crystal microtubes were prepared by using a microwave-heating growth method at a temperature range of 1300-1350°C [7]. SEM and XRD were used for characterization the morphology and crystallinity of ZnO microtubes. The setup for the photoresponse measurements is shown in Figure 1. 100 nm thick Au layers were coated
on the both ends of ZnO microtubes to from electrodes. A linear I-V characteristic indicates an ohmic contact
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