Photoluminescence and heteroepitaxy of ZnO on sapphire substrate (0001) grown by rf magnetron sputtering

ZnO thin films were epitaxially grown on alpha -Al2O3 (0001) single-crystal substrates by rf magnetron sputtering. The films were grown at substrate t emperatures of 550-600 degreesC for 1 h at a rf power of 60-120 W and Ar/O- 2 ratios of 1-4. The crystalline structure of the ZnO films was analyzed by four-circle x-ray diffraction (XRD) and Rutherford backscattering (RBS)/ch anneling. For the ZnO films deposited at 550 degreesC, the full width at ha lf maximum (FWHM) of the XRD theta -rocking curve of the ZnO (0002) plane w as found to be increased from 0.16 degrees to 0.3 degrees as the rf power w as increased from 80 to 120 W. The in-plane epitaxial relationship of the Z nO film on alpha -Al2O3 (0001) substrates was found to be ZnO [10 (1) over bar0]parallel to alpha -Al2O3[11 (2) over bar0], indicating a 300 rotation of the ZnO unit cell with respect to the alpha -Al2O3 (0001) substrate. For the specimen grown at 600 degreesC, the FWHM of the XRD theta -rocking cur ve was 0.13 degrees, In RBS/channeling studies, the films, which were depos ited at 600 degreesC and 120 W, showed good crystallinity, with a channelin g yield minimum (chi (min)) of only 3.5%. whereas chi (min) for the films d eposited at 550 degreesC was as high as 50%-60%, indicating poor crystallin e quality. In the case of photoluminescence (PL) measurements, sharp near-b and-edge emission was observed at room temperature. The FWHM of the PL peal ; decreased from 133 to 89 meV at a growth temperature 550 degreesC by incr easing the rf power. For the films deposited at 600 degreesC, a FWHM of the PL peak of 75-90 meV was observed, which is the lowest value reported to d ate. From the results of both XRD and PL measurement, it was found that the crystallinity of the films grown at 550 degreesC improved, but its optical property degraded. With increasing rf power, the films show a deep-level e mission in the presence of higher mixtures of Ar:O-2 because a considerable amount of activated oxygen was supplied in the ZnO films with an increase of rf power. From transmission electron microscopy and atomic force microsc ope analyses, the grain size and defects were found to affect the PL proper ties. The relationship between optical propel ties and crystal quality is d iscussed in terms of crystalline structure and grain size. (C) 2000 America n Vacuum Society. [S0734-2101(00)04306-2].