Role of Cu+ on ZnS:Cu p-type semiconductor films grown by sputtering: influence of substitutional Cu in the structural, optical and electronic properties

Abstract

In this work, ZnS:Cu thin films were synthetized by RF magnetron co-sputtering using ceramic ZnS and metallic Cu targets. The power applied to Cu target was changed to vary the Cu content in the films while other experimental parameters remain constant. Whatever the copper content, only the hexagonal ZnS phase was detected by X-ray diffraction and the films are oriented along the c-axis. For films with a Cu content higher than 10.6 at%, the a and c lattice parameters of the crystal cell decrease leading to a shrinkage of the ZnS:Cu cell. Changes in the microstructural, optical and electronic properties of the films are also observed. Regarding the optical properties, the energy bandgap determined by optical absorption decreases with the Cu content. Moreover, an absorption in the near infrared region appears probably due to a plasmon resonance coming from an increase of the holes carrier concentration. From electrical measurements it was found that above a copper concentration of 10 at%, the films are conductive and exhibit a p-type behavior. These results were discussed taking into account the substitution of Zn atoms by Cu ones. Finally, electron energy loss spectrometry measurements clearly showed that copper atoms are present in the ZnS lattice with an oxidation state of +I.