Growth optimization of ZnxCd1 − xS thin films by radio frequency magnetron co-sputtering for solar cell applications

Abstract

Zinc cadmium sulfide (ZnxCd1−xS) thin films (0≤x≤1) were deposited by the radio frequency (RF) co-sputtering of cadmium sulfide (CdS) and zinc sulfide (ZnS). The RF powers of CdS and ZnS were varied to control the composition of the films, which was confirmed using energy dispersive X-ray analysis. The structural properties of the films were investigated using X-ray diffraction, which showed that the films have a hexagonal (wurtzite) structure with a strong preferential orientation along the (002) plane. The values of lattice constants ‘a’ and ‘c’ decreased as ‘x’ increased. The surface morphology, topology and optical properties were investigated using field emission scanning electron microscopy (FESEM), atomic force microscopy and ultraviolet spectrophotometry. The FESEM studies revealed an increase in grain size for zinc (Zn) contents up to x=0.62, followed by a decrease in the grain size until ‘x’ reached 0.81, above which the films were amorphous. The optical band gaps of the films were obtained from optical absorption measurements and shifted to a higher energy as the content of ‘x’ increased. The presence of a small amount of zinc in CdS strongly influenced the optical band gap and transmittance of ZnxCd1−xS thin films. The electrical sheet resistance of the films was also found to be relatively high. Among the range of ZnxCd1−xS compositions tested, the samples with a Zn content of 0.17 to 0.43 showed a better film quality, making them suitable as the window layer in ZnxCd1−xS/CdTe thin film solar cells.