Optical analysis of nanocrystalline thin layers deposited by the ion layer gas reaction method

Abstract

Nanocrystalline ZnS thin layers are prepared by ion layer gas reaction, a low cost deposition method for compound semiconductor thin films. They are investigated by optical spectroscopy, x-ray fluorescence and energy dispersive x-ray analysis. It is seen that the absorption edges are shifting towards lower photon energies with increasing layer thickness. Also, the amount of unconverted precursor salt is decreased. A simulation procedure for fitting the determined absorption edges of these layers is introduced. This leads to particle size distribution functions showing an increasing broadening with increasing layer thickness. Moreover, the distribution maximum is moving to larger crystallite radii with larger film thickness. Both results indicate a growth in crystallite size during the layer deposition. Additionally, the increase in strength of the phonon-photon-coupling confirms the conclusions of transmission electron micrographs. Later investigations show that the noncrystalline precursor matrix which surrounds the ZnS crystallites is vanishing with increasing layer thickness. Therefore, the nanocrystallites are more frequently in contact with each other. This again favors the propagation of phonons.