The Study Of Novel Liquid Crystal Light Valve And Ultraviolet Photodiode Array

The research and development of ultraviolet(UV) photodiode and liquid crystal light valve(LCLV) were reviewed in this thesis. Detailed descriptions were given of the structure and operation, material characteristics, preparation processing and applications of the two devices.UV photodetectors with high responsivities for wavelengths shorter than 400 nm are very important for applications in the fields of UV astronomy, environmental and biological science as well as in medical instrumentation. High quality single-crystal ZnS -based II-VI thin films have been prepared on GaAs and GaP substrate using molecular beam epitaxy (MBE) technique. Successful n-type doping of ZnSxSe1-x alloy using aluminum source has been carried out. A different approach, named "two step growth approach" has been applied to fabricate an 8x8 photodiode array in the first time. The micro-processing procedures of this photodiode array including standard photolithography, a number of metallisation, wet-chemical etching and SiC2 deposition for insulation were developed in this study. The detector was characterized to have a cutoff wavelength at 340 nm and the photo-responsivity measurements on the pixels result a UV response as high as 0.15 A/W, corresponding to an external quantum efficiency of 54.8% in the visible-blind spectral ranging from 400 down to 250 nm. The highest responsivity of 0.2 A/W was obtained at wavelength of 330 nm, corresponding to an external quantum efficiency of 75.2%. Imaging tests indicated that this array is able to capture the intensity profile of a given UV light source with reasonably good capability.As one of the key devices in the opto-electronic information technology, LCLV can be used as image amplifiers for large screen projection display, image wavelength converters and optical data processing and correlation. However, limited by the responsivity of the photo-sensitive layer in the device, nearly all of the currently used LCLVs only worked in the visible to infrared spectrum region. To the best of our knowledge, there is no report so far on studies of visible blind UV-LCLV using MBE-grown ZnSxSe1-x thin film with tunable response wavelength as the photo-sensitive layer. In this thesis, detailed description was given of the structure and operation of the novel ZnSxSe1-x visible blind UV-LCLV. The opto-electronic requirements of ZnSxSe1-x photosensor for the device were derived and discussed from the UV-LCLV electrical model and equivalent circuits.The pre-growth treatment of ITO coated glass is essential for achieving high quality ZnSxSe1-x thin films. ITO substrate with an smooth surface of 0.2nm RMS roughnessmeasured by AFM was obtained by the developed pre-cleaning processing procedure. MBE growth of ZnSxSe1-x thin films on ITO coated glass substrates were carried out using ZnS and Se sources. The XRD 9/29 spectra resulted from these films indicated that the as-grown polycrystalline ZnSxSe1-x thin films had a preferred orientation along the (111) planes. XPS depth measurement suggested that the films were compositionally homogeneous along the growth direction; that the interdiffusion between ZnSxSe1-x and ITO was not serious in this deposition process; and a sharp interface had developed. High quality ZnSxSe1-x thin film grown at the optimized temperature had the smoothest surface with lowest RMS value of 1.2 nm and TEM cross-sectional micrograph showing a well defined columnar structure. The dependence of substrate temperature, deposition rate and alloy composition to the structure of the film was discussed in the thesis. The developed theory named "quasi-structure area mode" can successfully explain the film growth mechanism of polycrystalline ZnSxSe1-x thin films deposited on ITO substrate by MBE.Suitable polycrystalline ZnSxSe1-x film with zinc sulfide cubic structure and (111) preferred orientation that provided a good matching with the requirements of LCLV were deposited on ITO coated glass by MBE method. Room temperature photo-responsivity measurements performed on these thin films...