Thermal post-deposition treatment effects on nanocrystalline hydrogenated silicon prepared by PECVD under different hydrogen flow rates

In this paper, hydrogenated nanocrystalline silicon (nc-Si: H) thin films were deposited on monocrystalline silicon substrate by plasma enhanced chemical vapor deposition (PECVD) under different hydrogen flow rates followed by a thermal treatment in an infrared furnace at different temperature ranging from 300 to 900 degrees C. The investigated structural, morphological and optoelectronic properties of samples were found to be strongly dependent on the annealing temperature. Raman spectroscopy revealed that nc-Si: H films contain crystalline, amorphous and mixed structures as well. We find that post deposition thermal treatment may lead to a tendency for structural improvement and a decrease of the disorder in the film network at moderate temperature under 500 degrees C. As for annealing at higher temperature up to 900 degrees C induces the recrystallization of the film which is correlated with the grain size and volume fraction in the layer. We demonstrate that high annealing temperature can lead to a decrease of silicon-hydrogen bonds corresponding to a reduction of the amorphous matrix in the layer promoting the formation of covalent Si-Si bonds. The effusion of the hydrogen from the grown film leads to increase its density and therefore induces a decrease in the thickness of the layer. For post-deposition thermal treatment in temperature range under 700 degrees C, the post-deposition anneal seems to be crucial for obtaining good passivation quality as expressed by a minority carrier lifetime of 17 mu s, as it allows a significant reduction in defect states at the layer/substrate interface. While for a temperature higher than 900 degrees C, the lifetime reduction is obtained because of hydrogen effusion phenomenon, thus a tendency for crystallization in the grown film. (C) 2015 Elsevier B.V. All rights reserved.