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Optimization of the photoresponse of ZnS-based UV detectors

Authors Guo, Zhen
Issue Date 2002
Summary Currently, Il-VI materials are dominating as active semiconductor materials in the field of photon detection, such as CdZnTe in gamma ray and x-ray detection, CdS in visible detection and HgCdTe in infrared detection. So it is not surprising at all to expect the Il-VI materials will also play the dominating role in the detection of ultra-violet radiation. Since 1997, our research group has developed two novel Il-VI alloy systems for this specific application. The ZnSSe-based detectors can be tailored to have a cutoff wavelength anywhere between 400 and 340 nm, while the cutoff wavelength of ZnMgS-based detectors lies between 340 down to 270 nm. These detectors are ideal UV sensors for use in numerous visible-blind and solar-blind applications. Our previous approaches used a thin pure Au layer as the Schottky metal in all reported photovoltaic device structures. In this work, the photoresponse of Il-VI based UV detectors was further studied with focus on two new areas: (1) antireflection coating and (2) the material issues of Schottky metals. Based on the previously adopted theories developed by Cox and Hass for antireflection coating (ARC), a set of devices with ARCs of different thicknesses and compositions were fabricated. The results are in general in good agreement with the prediction from the theories. It was found that the single-layer ARC with a 460Å-SiO2 offers the best result; a +30% enhancement of the photoresponse in the UV region can be achieved with this antireflection coating. We have also studied a double-layer ARC structure consisting of MgF2-AI2O3 bilayer, its measured antireflection characteristics also fit quite well with the theories. However, it offers a relatively lower performance than that of the single-layer SiO2 structure, which is believed to be due to the lack of the optimization of the deposition conditions. Different Schottky metal deposition techniques and surface treatment on the Il-VI thin films prior to metallization, as well as different Schottky metals, were used to construct devices to investigate their effects on the photoresponse of the devices. Built-in voltages of these devices were measured and the results clearly indicate that the lowering of the built-in voltage is related to the imperfection of the Schottky metal or the interface between the Il-VI epilayer and the Schottky metal. However, it was shown that the lowering of the built-in voltage alone cannot fully explain the observed difference in the photoresponse of the devices. A model considering the defect states generated at the interface or the neighborhood of the Schottky contact due to deposition damage, oxide formation, Schottky metal alloying and diffusion was proposed to explain the observed difference in the photoresponse characteristics of the devices. It is concluded that in order to reduce the formation of the unwanted defects in the process of Schottky metal deposition, the thermal evaporation technique is more preferred than the sputtering technique, the surface of the Il-VI MBE epilayer should be kept as clean as possible prior to metal deposition to avoid oxide/other contaminant formation and pure Au is of advantageous over Au/Ni and Au/Cr as the Schottky metal since the latter two were found to have serious diffusion into the Il-VI layer or even metal alloying with Au in the Au/Cr case.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2002
Language English
Format Thesis
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