a-Si1-xCx:H-based transistor performance and the relationship to electrical and optical properties

The results of measurements of (1) n- and p-channel a-Si_1-x C_x inverted gate field effect transistors, (2) the photoconductivity of those alloys, and (3) the optical absorption spectra, including subband gap absorption, are discussed. Measurements of the transistor characteristics clearly show a monotonic falloff in electronic mobility with carbon addition and a somewhat slower increase in threshold voltage. The electron mobility, in the range of 0.5 cm²/V-s for a-Si:H, is still at usable levels (>0.1 cm²/V-s) even at 40% carbon concentration (gas phase). Optical absorption measurements (photothermal deflection spectroscopy) indicate that the slope of the optical absorption edge decreases with increasing carbon concentration, suggesting an increase in the tail density of states. These data, taken together with the μτ product, can be explained entirely by the decrease in mobility. The drop in the effective mobility is believed to be due to the effective widening of the band-tail state distribution