Effect of O/Zn flux ratio on crystalline quality of ZnO films grown by plasma-assisted molecular beam epitaxy

The effect of O/Zn flux ratio on the crystalline quality of ZnO films grown at 700degreesC by plasma-assisted molecular beam epitaxy was investigated. Zinc beam flux (F-zn) was varied from 2.2 less than or equal to F-zn less than or equal to 8.3 Angstrom/s with an O-2 flow rate of 3 sccm and RF power of 300 W. The surface morphology of the ZnO layers strongly depended on F-zn. ZnO epilayers grown under stoichiometric flux conditions (i.e., F-zn = 5.1 Angstrom/s) had high crystalline quality, as was confirmed by using X-ray diffraction, photoluminescence (PL), and Hall-effect measurements: the full width at half maximum (FWHM) of a skew symmetric (1 0(1) over bar 0) X-ray rocking curve was 720 arcsec; the dominant neutral donor bound exciton emission intensity in the PL spectra became maximum with the narrowest FWHM; the electron mobility was a maximum of 130 cm(2)V(-1)s(-1); and a residual carrier concentration of 1.2 x 10(17)cm(-3) was achieved. We demonstrated that stoichiometric ZnO films have the lowest dislocation density and the highest electron mobility compared with ZnO films grown under nonstoichiometric flux conditions.