Structural And Optoelectronic Properties Of ZnOS And S-N Codoping P-type ZnO Thin Films

ZnO is a very promising material for optoelectronic applications in the blue and ultraviolet region, owing to its wide band gap (3.37eV) and large binding energy of excitons (60meV). The large bowing parameter of ZnOS could lead to a considerable change in band gap with low faction of S in ZnO. Theoretical calculations reveal that the valence-band offset increase strongly for small S content, whereas the conduction-band edge increase only weakly, which can be utilized to enhance p-type doping with shallower acceptor state in ZnO-like alloys. In this paper, ZnS, ZnOS and S-N codoping ZnO films were prepared by reactive sputtering. The structure as well as optical and electrical properties was investigated.High quality ZnS films were deposited by optimizing the deposition pressure, with a Zn/S atomic ratio of1:1. The obtained films were crystalline in hexagonal structure with preferred (0002) orientation. The band gap of ZnS films was estimated to be3.64eV.Dependence of S content on oxygen partial pressure and substrate temperature were investigated and ZnOS alloy thin films with various S contents were prepared. The structure of ZnOS alloy films was similar to ZnO in O-rich side and to ZnS in S-rich side. While in the intermediate S content, instead, phase segregation occurs. The lattice constant c increases approximately linearly as the S content increases, and the valence of S incorporated was found to be-2. Thus, we concluded that S atoms have successfully substituted in O site. The composition dependence of the band gap energy in the ternary system was determined by optical transmission and the optical bowing parameter was found to be about1.45eV. The photoluminescence spectra showed band gap related emission and the peak shifted toward lower energy with increasing S content.S-N codoping ZnO films were deposited with N2O served as N source. The N2O partial pressure and substrate temperature were optimized and the films were annealed in N2for1h at600℃. We have observed a room temperature resistivity of21.46Ωcm and a hole concentration of4.482×10cm-3. The p-type activity is enhanced by codoping S with N, which was attributed to the shallower acceptor state in ZnOS alloys.