Zinc Oxysulfide Thin Films Grown by Pulsed Laser Deposition

Pulsed laser deposition was used to produce thin films of zinc oxysulfide (ZnO (x) S(1-x) ) on quartz substrates. The target was a sintered pellet (ZnO(0.39)S(0.61)) made of a solution precipitate. The film composition obtained by electron probe microanalysis (EPMA) was ZnO(0.41)S(0.59), ZnO(0.44)S(0.56), and ZnO(0.37)S(0.63) for substrate temperatures of 450A degrees C, 540A degrees C, and 630A degrees C, respectively. X-ray diffraction (XRD) showed that samples deposited at 450A degrees C and at 540A degrees C had a prominent cubic sphalerite phase, whereas samples deposited at 630A degrees C consisted of three phases, viz. hexagonal wurtzite and cubic sphalerite (ZnS), and hexagonal zincite (ZnO). With respect to the tabulated lattice spacings for sphalerite (cell constant 0.5406 nm), distinct shifts were observed for the low temperature samples, yielding cell constants around 0.533 nm. Transmission electron microscopy (TEM)-selected area electron diffraction studies support the XRD data. Patterns of films deposited at 540A degrees C could be indexed as sphalerite, with similar lattice shifts as in XRD, resulting in a cell constant of 0.53. Locally highly resolved chemical analysis by TEM-energy dispersive x-ray analysis revealed a stoichiometry that was consistent with the EPMA results. Ultraviolet (UV)-visible transmission measurements of the films led to bandgap energies around 3.3 eV, which is well below the reported bandgap energies of ZnS.