Solid phase recrystallization of ZnS thin films on sapphire

Abstract

High quality ZnS thin films are important for light emitting diodes based on ZnS, which is a very efficient phosphor. To improve as grown, molecular beam epitaxial, (111)-oriented cubic ZnS films, where defects were introduced due to the large mismatch between ZnS and a sapphire substrate (~ 20%), the ZnS was recrystallized by annealing at temperatures in the 825–1000 °C range, and sulfur pressures of 10 atm. The films have been structurally characterized by high-resolution x-ray diffraction, and electron diffraction by electron channeling patterns. Structural properties of the films annealed at temperatures above 900° have improved significantly. Tilting in the recrystallized films has been reduced more than tenfold, with the recrystallized grains being defect-free. Most films were recrystallized in the as-grown, cubic form, as shown by electron channeling patterns. The surfaces of the films have been inspected with scanning electron microscope, and on most samples they have been found to remain smooth, although on some of the films annealed at elevated temperatures we have observed hexagonal pits. The role of sulfur gas overpressure in the recrystallization has been discussed, and possible effects on film evaporation, grain boundary migration and compliancy of sapphire substrate have been analyzed.