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MBE growth and characteristic of II-VI thin film structures for visible blind imaging

Authors Au, Suk Yee
Issue Date 2001
Summary This thesis study presents the work on two related projects: a) the fabrication and characterization of ZnSSe/ITO thin films which are the potential candidate as the photoconductor layer of a novel UV liquid crystal light valve device; b) the development of an array detector based on ZnS Schottky photodiodes. The molecular beam epitaxial (MBE) growth of five ZnSSe/lTO thin films was handled using a ZnS and a Se source with the substrate temperature ranging from 270 to 330 °C. The structural perfection of the as-grown thin films was examined using various techniques including X-ray diffraction (XRD), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). The MBE-grown ZnSSe/ITO thin films were found to have a preferred orientation along the normal direction of the (111) planes. A linear relationship between the lattice constant and the Se composition was revealed from the results of both XRD and XPS studies. The evaluated crystal sizes by XRD indicate a strong growth temperature dependence with the optimized temperature at about 290°C. Both AFM and TEM measurements done on these thin films also indicate a similar growth temperature dependence. The sample grown at the optimized temperature has the smoothest surface with the lowest root mean square (rms) value of 1.2 nm and its TEM cross-sectional micrograph shows a well defined columnar structure whose nucleation seems to be highly correlated with the ITO grains. Photoresponse measurements done on these thin films show that UV responsivity as high as 0.01A/W at 390 nm and more than three orders of magnitude in rejection power for wavelength longer than 450 nm can be achieved. The results of both the dc resistivity and ac impedance measurements indicate that the electrical parameters of the thin film grown at the optimized temperature can provide a good matching with the liquid crystal layer of a Liquid Crystal Light Valve (LCLV) device. Regarding to the other approach, a novel 8x8 photodiode array based on a MBE-grown ZnS Schottky-barrier diode structure was fabricated using a newly developed micro-electronic processing scheme. The discrete detector was characterized to have a cutoff wavelength at 340 nm and a responsivity as high as 0.18A/W at 330 nm in the UV spectral range from 340 down to 250 nm. The photocurrent signals generated from all the 64 pixels were measured upon the illumination of a UV source at 330 nm with a "L" shape as the testing pattern. The results show that the array is able to capture the intensity profile of a given UV light source with reasonably good capability.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2001
Language English
Format Thesis
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