Visible-infrared spectral response of microcrystalline hydrogenated silicon hetero-junctions

Microcrystalline p-i-n devices with an increased infrared sensitivity infrared sensitivity are prepared by the closed-chamber cyclic technique. The spectral response for different applied bias, the current-voltage characteristics at different wavelengths and the photocurrent delivered by the device are analysed. The spectral response is extended far beyond the amorphous limit of about 750 nm and even at a wavelength of 1000 nm, the response is still at a level of about 5% of the maximum. The good near infrared sensitivity is considered to be a result of the large optical absorption ascribed to the crystalline phase. Under reverse bias, the spectral response is high and essentially unchanged, reflecting a good collection efficiency. Under increasing forward bias, up to values near the open-circuit voltage, it decreases continuously, but even at much higher bias (up to about 0.8 V), the device produces a photocurrent. A heterojunction model based on the growth mechanism of the device and supported by a numerical simulation is presented to explain this behaviour.