Temperature effect on zinc oxysulfide-Zn(O,S) films synthesized by atomic layer deposition for Cu(In,Ga)Se₂ solar cells

Thin films of Zn(O,S) were deposited by atomic layer deposition from diethylzinc, water (H2O), and hydrogen sulfide (H2S). First, a study on the influence of the H2S/(H2O+H2S) pulse ratio from pure ZnO to pure ZnS was performed at deposition temperature Tdep=120 and 200 °C. Zn(O,S) films had higher S content than expected, and this effect was stronger at Tdep=200 °C. Then, Zn(O,S) films have been synthesized over the range of temperature 120-220 °C at the constant H2S/(H2O+H2S) pulse ratio of 9%. For Tdep<180 °C, high and almost constant S content has been measured in the films. The significant increase of the S/(O+S) atomic ratio for Tdep>180 °C confirmed that exchange reactions occurred between the Zn(O,S) growing films and H2S. The grazing incidence x-ray diffraction patterns showed Zn(O,S) films with hexagonal wurtzite structures and with an optimum crystallization for temperatures Tdep=160-180 °C. Indeed, in this temperature range, well crystallized and large grains were obtained which was in good correlation with the film morphology determined by scanning electron microscope; and Hall effect measurements revealed low resistivities, high carrier concentrations (>1019 cm-3), and low mobilities. From these results, the authors propose the existence of a temperature range where the properties undergo significant changes while the atomic composition remains constant.