Recombination traffic in highly crystallized undoped microcrystalline Si films studied by steady state photoconductivity

The dependences of photoconductivity (sigma(ph)) Of highly crystallized dense undoped hydrogenated microcrystalline silicon (mu c-Si:H) thin films on the temperature and the intensity of light were measured in a temperature range 15-325 K. The variation in light intensity exponent (gamma) is found to be 0.5 <= gamma <= 0.9 for thin samples, and 0.15 <= gamma <= 0.8 in a thick fully crystallized sample. Based on our simulation and experimental results, we have been able to construct density of state map for both the samples of different microstructure. In particular, we found that the fully crystallized thick sample has a very steep conduction band tail compared to thin sample. The valence band tails (VBT) are classified into two parts: one with sharper tail near the edge originating from grain boundary defects and other one with less steep tail associated with the defects in columnar boundary regions. Capture cross section for the deeper tail is smaller than the shallower one. The shallower VBT states extend up to more depth in thin sample. Our simulation results for thick sample show the transformation of recombination traffic taking place from shallower states to deeper tail states in the. temperature region where gamma < 0.5. Therefore, these deeper tail in thick sample works as a "safe hole traps". (c) 2005 Elsevier B.V. All rights reserved.