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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

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2 Author(s)
Liu, H.F. ; Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore ; Chua, S.J.

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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 E2high Raman mode. A factor of ∼0.7 cm-1GPa-1, i.e., a biaxial stress of 1 GPa can shift the E2high 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.

Published in:

Journal of Applied Physics  (Volume:106 ,  Issue: 2 )