Effects of substrate temperature on the optical properties of polycrystalline CuInSe2 thin films

Polycrystalline CuInSe₂ (CIS) thin films ~100 nm thick were deposited using the co-evaporation method onto fused quartz substrates held at three different substrate temperatures (T_s): 450°C, 525°C, and 600°C. The films were characterized with spectroscopic ellipsometry (SE) both from the film side and through the glass. These SE data with both measurement geometries were analyzed together, based on an optical model containing a bulk CIS layer and a thin surface over-layer. The resulting optical properties of the CIS films were analyzed with the critical point (CP) parabolic band approximation. CP parameters indicate that CIS grain size increases significantly from 450°C to 525°C, but the difference between 525°C and 600°C is small. This is consistent with the scanning electron microscopy measurements. A steady blue shift in CP energies with increasing T_s was also observed, possibly due to the tensile strain in the CIS films, which was confirmed by the X-ray diffraction characterizations.