Irradiation-induced structural changes in hydrogenated amorphous silicon as measured by X-ray photoemission spectroscopy

The pertinent experimental literature on light- and X-ray-induced changes in a-Si:H, as measured by X-ray photoelectron spectroscopy are reviewed. Illumination causes both the Si2p and Si2s spectra to shift to lower binding energies by equal amounts. At least to the depth probed by X-ray photoemission spectroscopy (XPS), the shift, and the structural changes that lead to them, occur uniformly. We consider the relatively new finding of photo-induced metastable expansion in a-Si:H, and the likelihood that this phenomenon is related to the XPS shifts. In order to provide an answer to this question, we undertake preliminary density functional calculations on a simple model of a-Si:H; these calculations include effects that may occur during the metastable expansion, such as elongations of the length of compressed Si-Si bonds and variations in the Si-Si-Si dihedral angle. Our results indicate that, were the metastable expansion to be associated with the XPS shifts, unidirectional bond angle changes would have to occur uniformly throughout the bulk, and their effect would have to dominate those of all other structural changes. (C) 2002 Elsevier Science B.V. All rights reserved.