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 700°C by plasma-assisted molecular beam epitaxy was investigated. Zinc beam flux (F_Zn) was varied from 2.2≤F_Zn≤8.3 Å/s with an O₂ 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 Å/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 (1010) 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 cm²V^-1s^-1; and a residual carrier concentration of 1.2×10¹⁷ 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.