Difference in optical bandgap between zinc-blende and wurtzite ZnO structure formed on sapphire (0001) substrate

Two photoluminescence peaks were observed from ZnO film formed by the oxidation of zinc blende ZnS film on sapphire (0001) substrate. The emission peaks correspond to hexagonal and columnar ZnO nanocrystallites. X-ray diffraction patterns show that the hexagonal and columnar crystals indicate c-axis oriented wurtzite and zinc blende ZnO structure, respectively. The difference in the emission energies was 0.10 eV, which is very close to the calculated difference in energy gap between zinc blende and wurtzite ZnO structure. With increasing thickness of ZnO, only the hexagonal crystallites were observed on the film surface and therefore the intensity of the emission peak of 3.27 eV from wurtzite structure dominates the spectrum.