Transmission photoacoustic spectroscopy of flash-evaporated CuIn0.75Ga0.25Se2 thin films

Photoacoustic spectroscopy (PAS) has proved to be an effective technique for the evaluation of inherent defect population in a wide range of materials for various applications. This paper demonstrates the use of this technique in transmission mode and hence evaluates the optical properties of flash evaporated CuIn0.75Ga0.25Se2 (CIGS) thin films. Both the photoacoustic and transmission spectra were recorded at room temperature using high resolution near-infrared of the gas-microphone type PAS which revealed a very broad transmission region (about 300 meV) near the fundamental band edge in the as-grown CIGS thin films due to the presence of several shallow defect levels. The post-deposition heat treatment of the samples under selenium ambient followed by annealing under inert and forming gas ambient showed significant changes in the behavior of the PAS spectra particularly near the fundamental band edge. The absorption coefficient has been derived from these spectra to determine the band gap energy values and the activation energies for several defect related energy levels. Using the PAS, the energy band gap values were in the range of 1.197 to 1.202 eV. The optical transmission spectra were also recorded from the spectrophotometer. The transmission data was used to determine the energy band gap values which were calculated to be in the range of 1.159 to 1.194 eV. These values were found to be in good agreement to each other as well as to those reported in the literature.