Optical characterizations of CuInSe2 epitaxial layers grown by molecular beam epitaxy

CuInSe2 (CIS) films with Cu/In ratios of γ=0.82–1.79 have been grown on a GaAs (001) substrate by molecular beam epitaxy. Piezoelectric photoacoustic (PPA) measurements were carried out from liquid helium to room temperature to investigate nonradiative carrier recombination processes in comparison with photoluminescence (PL) measurements which directly detected radiative carrier recombination processes. Three PPA signal peaks which corresponded to band gap energies of the CIS (AB and C bands) and the GaAs substrate, were clearly obtained between liquid helium and room temperatures. A free-exciton emission line was observed up to 200 K in the PL spectra. Two additional peaks on intrinsic defects which are Cu vacancy (VCu) and interstitial In (Ini) were observed in the In-rich CIS samples. The PPA measurements were useful in investigating the defect levels and the band gap energy in the CIS/GaAs thin films.