Optical properties of epitaxial CuGaS2 layers on Si(111)

CuGaS2 films epitaxially grown on Si(111) substrates by molecular beam epitaxy from elemental sources were investigated by various optical methods. Photoluminescence studies at low temperatures reveal a strong dependence of the near-bandgap emission on the composition. Ga-rich films show predominantly donor-acceptor pair recombination, while additionally free-to-bound and excitonic transitions are found for stoichiometric and Cu-rich films. Low lineshape broadening of the free excitonic transitions FXA and FXB/C in the photoreflectance spectra up to room temperature demonstrates the high crystal quality. The assignment of these transitions is confirmed by polarisation dependent photocurrent measurements making use of the optical selection rules and the three different growth directions for the c-axis of CuGaS2 relative to the Si-substrate. A fit to the photocurrent measurements yields the relative volume fraction of each c-axis orientation. (C) 2003 Elsevier Ltd. All rights reserved.