Grain-size effect on the optical properties of vacuum deposited nanocrystalline AS2S3 thin films

The microstructure and optical properties of nanocrystalline and amorphous AS(2)S(3) films with thicknesses in the range 15-70 nm are followed. It is found that within the spectral interval 200-600 nm, the thickness dependencies of the refractive indices n and extinction coefficients k have different trends for both kinds of sample. It is established that the optical band gap energy, E-g, for indirect transitions is almost independent of the thickness of the amorphous films, while a strong reduction of this parameter is observed for the thickest nanocrystalline samples. Also, a fairly strong grain-size effect on the values of Eg for direct optical transitions in crystalline films is found. On the basis of the results obtained, the impact of a nanocrystalline structure on the optical behaviour of vacuum deposited AS(2)S(3) films is discussed.