Growth of nano size grains in CuInSe2 thin films using E-beam deposition technique

Crystalline defects, such as the density of voids, grain boundaries and dislocations in copper based ternary/multinary semiconducting compounds/alloys such as CulnSe2 and Culn(x)Ga(1-x)Se2 absorber layers depend on the fabrication conditions and determine to a large extent the efficiency of photo-voltaic devices. The material properties, however, can be improved significantly by using the optimized deposition conditions. This paper reports the results of studies carried out on growth and characterisation of CulnSe2 thin films. Coatings of thickness less than one micron were grown using an electron-beam evaporation technique onto glass slides at various substrate temperatures. The structure, surface morphology and electro-optical properties of the films have been investigated using a number of analytical techniques. The effects of the substrate temperature deposition rate, deposition time and type of the target material (a loose powder and single crystal) on the properties of the films have also been examined. In the as-grown films, X-ray diffraction (XRD) analysis revealed a strong preferred orientation with the (112) plane parallel to the substrate. Results from energy dispersive analysis with X-rays (EDAX) indicated a deficiency of selenium and/or copper in some of the samples, otherwise the composition was comparable with the starting polycrystalline material. Scanning electron micrographs showed almost no grains in the films prepared at deposition temperature less than 150 degrees C with small deposition rate of 10 angstroms/sec. However, a nano-scale grain structure (approximately 60-80 nm) was observed in the films grown at elevated temperatures (> or = 200 degrees C) with larger deposition rate of 20 angstroms/sec. The values of the crystallite size calculated from the profile of the main (112) peak using Scherrer formula were in the range of 10-20 nm. Electrical measurements revealed both n- and p-type conductivities with surface resistivity values in the range of 10(-1) to 10(4) omega-cm.