Compositional and temperature dependence of the energy band gap of CuxInySe2 epitaxial layers

The compositional and temperature dependence of the energy band gap of CuxInySe2 epitaxial layers grown by metal-organic chemical vapour deposition on GaAs(1 0 0) substrates was studied by photoreflectance (PR) spectroscopy. Investigation of the compositional dependence at 20 K showed a slight increase in the energy gap with the increase in the [Cu]/[In] fraction. Line-shape analysis of PR spectra indicated that near stoichiometric and slightly Cu-rich CuInSe2 layers have a better crystallinity than In-rich ones. Investigation of the temperature dependence for both Cu-rich and In-rich modifications, in the range 300-20 K, demonstrated a red-shift of band-gap energies with the decrease in temperature below 70 K and provided experimental evidence of the anomalous temperature dependence of the energy gap in this material. Moreover, a combination of modulation power-dependent PR and photoluminescence studies at 20 K revealed that the optical emission interfering in the PR spectra at energies below the band-gap energy originates from donor-acceptor pair recombination and diminishes at low modulation powers