CAPACITANCE ANOMALY NEAR THE METAL-NON-METAL TRANSITION IN CR-HYDROGENATED AMORPHOUS SI-V THIN-FILM DEVICES

We present experimental results that show a metal-non-metal (MNM) tran sition occurring in hydrogenated amorphous Si (a-Si:H) analogue memory devices as a function of temperature. The de resistance of the device s undergoes a continuous change in the range 65-100 K from semiconduct or-like behaviour to metallic behaviour, as the temperature increases. The ac conductivity, measured over the frequency range 1-3.1 x 10(7) Hz, shows an anomalous change as the temperature is varied over the MN M transition. Ac characteristics were modelled using multicomponent RC and RL equivalent circuits below and above the MNM transition region respectively. It is found that the capacitance increases markedly when the temperature approaches the MNM transition from the semiconductor side. Near the transition temperature this capacitance disappears, and the equivalent circuit now requires an inductive component together w ith a resistance which has a positive temperature coefficient of resis tance equivalent to that of the de resistance in this temperature rang e. This anomalous behaviour is explained in terms of a percolation-lik e critical behaviour of the dielectric constant epsilon(eff), which di verges at the threshold p(c).