Optical properties of flash-evaporated CuIn 0.75Ga 0.25Se 2 thin films by photoacoustic spectroscopy

Abstract

A high-resolution near-infrared photoacoustic spectrometer of the gas-microphone type is used for room-temperature analysis (in the subgap region of the spectrum) of non-radiative defect levels in as-grown CuIn 0.75Ga 0.25Se 2 thin films. Films were grown by flash evaporation onto glass substrates at 200 °C. The absorption coefficient has been derived from the photoacoustic spectra to determine the gap energy and establish the activation energies for several defect-related energy levels. We also present preliminary results relating to the influence of post-deposition heat treatments in a selenium atmosphere on the photoacoustic spectral response. The improvements in the photoacoustic spectrum following annealing are directly correlated with the sample compositional, structural and electrical properties. Finally, the effect of interference on the photoacoustic spectra is discussed.