Effects of low-temperature-buffer, rf-power, and annealing on structural and optical properties of ZnO/Al2O3(0001) thin films grown by rf-magnetron sputtering

ZnO thin films were epitaxially grown on Al2O3 (0001) substrates in a radio-frequency (rf) magnetron sputtering chamber. The surface morphology of ZnO was remarkably affected by the incorporation of a low-temperature grown ZnO buffer as well as the changes in rf-power. X-ray diffractions, combined with the surface micropits, revealed strain relaxations in the ZnO epilayers grown with higher rf-powers, which in turn caused a redshift to the intrinsic exciton absorption peak. Strain relaxations were also observed in the ZnO epilayers upon thermal annealing, which led to a redshift in the E-2(high) Raman mode. A factor of similar to 0.7 cm(-1) GPa(-1), i.e., a biaxial stress of 1 GPa can shift the E-2(high) mode by 0.7 cm(-1), was obtained. The point defects related absorptions and the exciton localizations were suppressed by annealing, which, in conjunction with the strain-relaxation induced redshift in the intrinsic-exciton absorptions, steepened the absorption edge and increased the optical bandgap energy of the ZnO epilayer. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3176497]