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) transition occurring in hydrogenated amorphous Si (a-Si:H) analogue memory devices as a function of temperature. The de resistance of the devices undergoes a continuous change in the range 65-100 K from semiconductor-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 MNM 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 with a resistance which has a positive temperature coefficient of resistance equivalent to that of the de resistance in this temperature range. This anomalous behaviour is explained in terms of a percolation-like critical behaviour of the dielectric constant epsilon(eff), which diverges at the threshold p(c).