Application of X-ray diffraction and reflectometry methods for analysis of damaged layers on polar faces of ZnO after surface chemical-mechanical treatment

Abstract

ZnO single crystals are used for the fabrication of laser targets for high-energy electron irradiated UV laser cathode-ray tubes and homoepitaxial substrates for lasers. The technology of ZnO based UV LEDs imposes strict requirements to surface quality. Chemical-mechanical polishing delivers good surface quality but it is known that polishing of ZnO polar faces may yield different results. Surface-sensitive high-resolution X-ray diffraction (HRXRD) and X-ray reflectometry (XRR) methods have been used for studying the structure of (0001) and (000–1) polar faces of ZnO after chemical-mechanical polishing. Two double-sided polished (0001) ZnO substrates have been cut out from different hydrothermally grown ingots. The damage and density depth profiles for the Zn and O faces of the specimens have been retrieved from the X-ray diffraction curves and the specular reflection curves, respectively. Intensity distributions in the vicinity of the [0002] and [0000] reciprocal lattice sites have been taken on a D8 Discover X-ray diffractometer (Bruker-AXS, Germany) in a triple-crystal setup. For separating the coherent and incoherent scattering components, the intensity profiles have been analyzed along sections perpendicular to the diffraction vector and located at different distances from the reciprocal lattice sites. The HRXRD and XRR data have been compared with atomic force microscopy (AFM) data. The HRXRD method has revealed damaged layers at both faces of the specimens, with the layer thicknesses differing for the Zn and O faces, i.e., 5–7 nm for the Zn face and 10–11 nm for the O face. The XRR method has shown that both faces are sufficiently smooth. These results have been confirmed by AFM (RMS roughness ~ 0.23 ± 0.07 nm). However, the concentration of electrons in the superficial layers has been found to change. The layer thickness proves to be greater for the O face. We have hypothesized that the phenomena observed are caused by the difference in the chemical interaction of the Zn and O faces with the polishing agents.