Electronic characterisation and computer modelling of thin film materials and devices for optoelectronic applications
Double beam collection efficiency measurements have been carried out on hydrogenated amorphous silicon p-i-n devices. Apparent collection efficiencies higher than unity were observed, and explained by a process identified as photogating, in which a low intensity weakly absorbed probe beam m o d u l a t e s the photocurrent produced by a high intensity strongly absorbed bias beam. Computer simulations were used to gain insight into the photogating phenomenon. It was found that the gating effect operates by the modulation of the internal field profile in a device, via deeply trapped space charge introduced by the probe beam. Conditions for high collection efficiencies were identified by modelling and by experiment. Collection efficiencies of 100 or greater could be achieved, much higher than any previously reported in the literature. The effects of external parameters including bias and probe beam wavelength and intensity, and applied voltage were studied. Additionally, the effects of internal parameters, such as the density and spatial distribution of defects, were investigated. The photogating phenomenon proved a sensitive and potentially useful indicator of defect density. The time response of the photogating effect revealed slow components to the response, associated with the need to involve deeply trapped space charge in the effect. Measurements of this time response explain in part the much lower values of collection efficiency reported earlier, which were made using ac lock-in techniques. A comparison was made of the two-beam photogating experiment, with a single beam current-voltage measurement, which is also influenced by trapped space charge, as indicators of defect distributions. It was found that the photogating measurement is a more accurate indicator of the distribution of space charge, and hence defects, within a device. Application of the photogating effect in a colour detector is introduced and a detector structure proposed. The simple structure and the thin film technique of a-Si:H deposition suggests the possibility of a low cost photodetector with high colour resolution.
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