OPTICAL CHARACTERIZATION OF VACUUM-EVAPORATED CADMIUM-SULFIDE FILMS

The optical constants (n,k) of cadmium sulfide (CdS) thin films were d etermined in the spectral range of 0.55 mu m to 1.80 mu m from the opt ical absorption and transmittance measurements. The optical band gap ( E-g) was determined for the films deposited at different substrate tem peratures. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and X-ray diffraction (XRD) techniques were used to de termine the morphology, composition, crystalline structure and crystal lite size of the films. Evaporated CdS films show a predominant hexago nal phase with small crystallites. The optical band gap values of the films varied from 2.38 to 2.41 eV depending on the substrate temperatu re. It has been observed that the band gap and refractive index of the films have a close relationship with the size of the crystallites. Th e lower estimated value of band gap of the films is explained consider ing the effects of excitons and (or) some impurities. (C) 1997 Elsevie r Science S.A.