Photoluminescence excitation spectroscopy of hydrogenated amorphous silicon using a free electron laser

Abstract

Photoluminescence excitation spectroscopy (PLE) has been used to study the role of the defect states in the gap of hydrogenated amorphous silicon (a-Si:H). This material is used in many areas of technology including solar cell devices and thin film transistors. Because of the sometimes limiting feature of defect states in the gap, a clear method of probing these states is essential to understand what affect they may have on device performance. In this thesis, PLE has been used to determine that the defect states are in fact two well-defined distributions of states consisting of D⁰/D ⁺ and D⁻ transition energies separated by the correlation energy, U. The D⁻ state is higher in energy than the D⁰/D⁺ states, and the transition energy to that state is in fact the limiting transition in the photoluminescence process. This is consistent with the dangling bond model, which predicts well-defined states characterized by the neutral dangling bond. However, this is in contrast to the defect pool model, which predicts that the distribution of defect states is a large pool of defects dominated by charged states. These measurements are the first to determine the position and shape of the defect state distributions.