Carrier transport and recombination in hydrogenated amorphous silicon

Abstract

A study on the transport and recombination processes of electrons in hydrogenated amorphous silcon has been done employing the time-of-flight (TOF) technique. At room temperature TOF transient photocurrent(TPC) shows the normal non-dispersive behavior and the drift mobility of the electron is about 0. 3$cm^2$/V$\cdot$sec. As the temperature is lowered TOF TOF exhibits the despersive decay showing the famous power-law decay. Thetemperatuere dependence of the drift mobility us approximately an activated one with the activation energy of 0.13 eV. This results are well interpreted by the multiple trapping with the exponential tail and demarcation level approximation. The characteristic temperature of the exponential tail is about 300K. The temperature dependence of the drift mobility is well fitted with the attemptto-escape frequency of $2.4\times10^{11}sec^{-1}$ and the band mobility of 7.5$cm^2$/V$\cdot$sec. At low temperatures and in the early stage of the transient photocurrent, the decay faster than $t^{-(1-\alpha)}$ was observed as was in other``s. The Transition time from the intial fast decay to the normal power-law decay shows the activated behavior with the activation energy of 0.13 eV which is the same as that of the drift mobility. In order to interpret this initial fast decay, a tail stste distribution which is linearly decreasing and followed by an exponential tail(DLE) was assumed with the transition from the linear to the exponentila tail at 0.13eV. It is assumed that the initial fast decay is a result of the thermalization of carriers through the linear part of tail states. With this assumption and demarcation level approximation, the transition time obtained experimentally is well interpreted with the attampt-to-escape frequency of $2.6\times10^{11}sec^{-1}$. Moreover, an analytical expression of the pre-transit TOF TPC is developed for the DLE using the multiple trapping and demarcation level approximation. This expression well describles the TPC...