SOLID-PHASE RECRYSTALLIZATION OF ZNS THIN-FILMS ON SAPPHIRE

High quality ZnS thin films are important for light emitting diodes ba sed on ZnS, which is a very efficient phosphor, To improve as grown, m olecular beam epitaxial, (111)-oriented cubic ZnS films, where defects were introduced due to the large mismatch between ZnS and a sapphire substrate (similar to 20%) the ZnS was recrystallized by annealing at temperatures in the 825-1000 degrees C range, and sulfur pressures of IO atm. The films have been structurally characterized by high-resolut ion x-rap diffraction. and electron diffraction by electron channeling patterns. Structural properties of the films annealed at temperatures above 900 degrees have improved significantly. Tilting in the recryst allized films has been reduced more than tenfold, with the recrystalli zed grains being defect-free, Most films were recrystallized in the as -grown. cubic form. as shown by electron channeling patterns. The surf aces of the films have been inspected with scanning electron microscop e, and on most samples they have been found to remain smooth, although on some of the films annealed at elevated temperatures we have observ ed hexagonal pits. The role of sulfur gas overpressure in the recrysta llization has been discussed, and possible effects on film evaporation , grain boundary migration and compliancy of sapphire substrate have b een analyzed. (C) 1998 American Institute of Physics.