Optical characterization of polycrystalline CuInSe₂ films on scattering substrates by fourier transform photothermal deflection spectroscopy

Fourier transform photothermal deflection spectroscopy is used to obtain values for the optical band gaps of relatively rough polycrystalline films of CuInSe₂ on scattering Al₂O₃ or Mo/Al₂O₃ substrates and to estimate absorption coefficients at photon energies close to the band gap. Heating of the films in air generally caused a slight shift to lower photon energies of the band gap. Absorption edges in Cu₂Se-rich films were absent or poorly resolved. Results obtained for a single-crystal sample agree well with literature values. In this case it was possible to determine from the variation in the phase angle of the signal with photon energy that the sub-band-gap absorption originated from the bulk rather than the surface. We did not find conclusive evidence for indirect transitions in either the crystal or the thin films. The integration of this method into a semiconductor-liquid junction system for semiconductor characterization is discussed. In order to illustrate this possibility, simultaneous photothermal deflection and photocurrent measurements are performed for the single crystal and for a thin film immersed in an electrolyte.