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High performance II-VI visible-blind UV photodetectors

Authors Ma, Zhaohui
Issue Date 1999
Summary UV photo-detectors with high responsivities for wavelengths shorter than 400nm are very important for applications that require detection against a strong visible or infrared backgrounds. This study focuses on the development of a novel visible-blind UV photodector in five main areas: (i) Growth of ZnS-based II-VI thin films by the molecular beam epitaxy technique; (ii) n-type doping of the thin films by Al and various characterizations to reveal a number of its physical properties relevant to device applications; (iii) fabrication and characterization of ZnS, ZnSSe and ZnSTe Schottky barrier photodiodes; (iv) investigation of the role of isoelectronic centers in the photoresponse characteristics of these diodes; (v) examination of the lattice thermal expansion effects upon the spectral response of the diodes through detailed studies of the temperature dependence of their response performance. Various ZnS-based II-VI thin films have been successfully grown on GaAs, Si and GaP substrates by molecular beam epitaxy for the first time. The approach first developed by us for the growth of ZnSTe alloy using ZnS and ZnTe compound sources was proved to be very effective in giving high quality single component alloy for the entire Te composition range. This approach was also applied in the growth of ZnSSe films. Successful n-type doping of ZnSTe alloy using elemental aluminum source has been carried out. We have performed detailed studies on the transport and optical properties using Hall and photoluminescence techniques. We also investigate the thermal stability of Al dopant in ZnSTe matrix through the study of its thermal diffusion characteristics using secondary ion mass spectroscopy together with computer modeling based on Fick's second law of diffusion. Following the achievement in Al doping studies, a "two-step MBE growth approach " has been developed to fabricate ZnSTe-based Schottky-barrier photovoltaic UV detectors. UV sensitive and visible blind response of these detectors has been confirmed. The built-in potential of the device was determined to be 1.7 V. Transient measurements indicated that a response time of 1.2ns could be achieved in 400x400μm2 detectors. The gradual turn-on characteristic in the photoresponse of low Te containing ZnSTe diodes is proved to be attributed to the Te isoelectronic trapping effect. On the other hand, with the advantage of being free of isoelectronic traps, ZnSSe is proved to be a good candidate as the active material for UV photodetection applications, which require excellent visible rejection and tunable turn-on wavelength capabilities. In characterizing the performance of the diodes as a function of temperature, it was found that in general the responsivity at higher temperatures would shift to longer wavelengths because of band gap narrowing. A remarkable observation is that the overall responsivity increases at higher temperature. We suggest that this observation is attributed to the change of the density of state distribution due to the lattice expansion at high temperature. This experimental study has revealed some of the interesting issues about using ZnS-based II-VI thin films as new UV detection materials. The results show that the developed Schottky-barrier diode structures are suitable for UV detector applications that require fast and visible-blind response.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 1999
Language English
Format Thesis
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