Growth and characterization of ZnO film on sapphire by the helicon wave oxygen-plasma-assisted evaporation process

The growth of ZnO films on [0001] sapphire substrates was studied using the high-density helicon wave oxygen-plasma (>10¹¹/cm ²)-assisted evaporation process. The crystallinity and optical properties of the ZnO films fabricated with and without a grounded grid installed at the end of the plasma source were studied by X-ray diffraction (XRD) analysis, atomic force microscopy (AFM) and photoluminescence (PL) measurements. XRD measurements showed that both samples were highly oriented films along the c-axis perpendicular to the substrate surface. The full widths at half maximum of the θ-rocking curve at (0002) diffraction were 0.53 and 0.63° for the ZnO films grown with and without the grid, respectively. AFM images show that the rms surface roughness of the films grown with the grid is ∼4 times smaller than that without the grid. These indicate that the quality of the ZnO films was considerably improved by adopting the grounded grid. The PL spectra of the ZnO films deposited without the grid showed the prominent deep-level emissions that were not observed from the films grown with the grid. The deep-level emissions might be attributed to the impurities and native defects in the films. These strongly suggest that the grounded grid effectively eliminated the charged ionic species, in the plasma, that can deteriorate the crystalline structure of the ZnO films.