Effect of chromium percentage doping on the optical, structural, morphological and electrical properties of ZnS:Cr thin films

Abstract

Zinc sulfide (ZnS) and chromium-doped zinc sulfide (ZnS:Cr) thin films were grown on glass and Si substrates by cathodic radio-frequency (RF) sputtering. The deposition time and RF power were fixed respectively at 90 min and 200 W while chromium content was varied between 3 and 7%. The effect of Cr percentage on the structural, morphological and optical properties of the deposited thin films have been studied. X-ray diffraction analysis indicated that all the sputtered ZnS films were monophasic with a preferred growth orientation along the (111) plane of the zinc-blend (ZB) phase. The Cr-doped films, on the other hand, showed the (111), (2 2 2), and (311) peaks of the same phase that was confirmed by TEM . The crystallite size was about 17.75 nm for undoped ZnS films for RF power of 200 W and increased from 16.3 nm to 25.5 nm for ZnS:Cr. The strain was shown to decrease from 2.2 × 10 −3 to1.45 × 10 −3 . The optical properties (mainly the thicknesses, refractive index , absorption coefficient and optical band gap) were determined from optical transmission measurements in the ultraviolet–visible-near-Infrared wavelength range. Scanning electron microscopy observations revealed smooth film surfaces. Energy Dispersive Spectroscopy (EDS) revealed that the chromium sputtering rate is smaller than the Zn and S ones. UV–Visible transmission measurements revealed an average transmittance on the order 70% for the ZnS films while that of the ZnS:Cr was less than 60% in the visible wavelength region. Moreover, it was observed that the optical band-gap of the ZnS films is 3.72 eV, and slightly decreases from 3.68 to 3.44 eV as the chromium percentage increases, while the films thickness increased. The electrical resistivity of thin films has decreased from 5.2 × 10 5 Ω cm to 1.1 × 10 5 Ω cm as the chromium percentage increases from 3 to 7%. • XRD patterns showed that the particles crystallized in cubic lattice. • The crystallite size was observed to increase with increasing the chromium percentage. • It is observed that the energy band gap decreases from 3.7eV to 3.44 eV. • The resistivity decrease as Cr contents increasing.