Abstract
An experimental method for determining the depletion depth in thin films of cadmium selenide is used to measure simultaneous variation of conductivity and the space-charge region. The temperature dependence of the conductance reveals donor levels distributed about 0.10 eV and 0.50 eV. Total depletion of films a few thousand angstroms thick produced by oxygen adsorption occurs by exhaustion of the shallow donors. Following exposure to the atmosphere at room temperature, the film conductance decreases by two orders of magnitude when heated in vacuum above 70°C. The subsequent time dependence of the current is shown to follow an Elovich-type relation. This effect is interpreted as conversion of physically adsorbed oxygen to chemisorbed oxygen. The role of evaporated silicon oxide layers in the prevention of the conversion process is discussed. The instability in thin film transistor threshold voltage above 200°C is described in terms of an interface desorption model.
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