Photoreflectance of a High Quality CuInSe2 Single Crystal

Abstract

A melt growth CuInSe2 bulk single crystal was shown to be very high quality from its sharp and strong photoluminescence (PL) emissions. Photoreflectance (PR) measurement on a selected sample were performed from 12 K to room temperature. The PR line shape depends on surface treatment of the sample. Sharp features of PR spectra were obtained from bromine-methanol etched surface. PR spectra measured from 12 K to 140 K were analyzed by using both the well known Aspnes' relation and the confluent hypergeometric function suitable for exciton with Gaussian line shape. Result of the line-shape analysis shows that transitions involved are derived from two excitons with Gaussian line shape. By comparison with excitonic emission from its PL result, the two excitons are identified as the A- and the B-exciton which result from the splitting of the uppermost of the valence band. The energy difference of the two excitons computed from the PR spectrum at 12 K is 3.7 meV which agrees fairly well with 3.2 meV from PL result. The Gaussian exciton line shape at low temperature also indicates that the exciton-phonon coupling in a high quality CuInSe2 bulk single crystal is strong. PR spectra measured from an as-polished surface are weak and exhibit broader spectral lines which are well fitted by two excitons with the Lorentzian line shape. In addition, by carefully etching and polishing, the PR spectrum at 40 K exhibits a spectral line which can be interpreted as a superposition of the two spectra of different line shapes. The spectrum is well fitted by two Gaussian excitons and two Lorentzian excitons with two sets of transition energies. The presence of Lorentzian excitons indicates that excitons near the as-polished surface are weakly coupled to phonon which may be a result of residual strain left over by mechanical polishing.