(The) effect of heterogeneity on transport mechanisms in hydrogenated amorphous silicon

Abstract

Studies on electrical conduction mechanism and its relationship with sample heterogeneity are carried out for the DC glow discharge (GD) a-Si:H using the results of electrical conductivity, infrared (IR) absorption, hydrogen evolution, scanning and transmission electron microscopy (SEM and TEM). In particular, the high temperature conductivity downward kink in Arrhenius plot ($\log\,\sigma$ versus 1/T plot) is presented as a powerful tool for studying conduction mechanism. Firstly, it is shown that the temperature ($T_k$) at which kink occurs can be used to determine the thickness of band bending for both sides of top surface and substrate-sample interface. That is, $T_k$ is thickness-sensitive and the rapid increase in $T_k$ is observed around the sample thickness of $0.24 \mu m$ which is well in agreement with the critical thickness reported by others. Secondly, comparing the electrical conductivity data with the hydrogen evolution spectra, the IR absorption spectra and the SEM and TEM micrographs, it is proposed that the downward kink could be interpreted by the series-connection of tightly and loosely hydrogen-bonded (micro-structural and intermicrostructural) regions with different band structures and the upward kinks in heavily doped samples by the model of two transport path. Thirdly, it is reported for the first time that the downward kink can appear or disappear after successive annealing depending on samples, and the upward shift in $T_k$ by annealing occurs for the samples showing kinks. These results are interpreted to be due to the microstructural and compositional change in heterogeneity caused through the diffusion and rearrangement of atoms (coalescence). Finally, the morphological studies of sample growing are carried out using SEM and TEM micrographs. It is observed that the dimension of microstructures in DC GD samples is several ten times larger than the one in RF GD samples. Here the formation of microstructure is explained in terms of hydr...