Surface and interface properties of polar thin films on a ferroelectric substrate: ZnO on LiNbO3 (0001) and (0001¯)

Abstract

The impact of ferroelectric polarization on film growth has been studied through the deposition of polar ZnO thin films on ferroelectric LiNbO3 (0001) and (0001¯) surfaces. The growth was monitored by reflection high energy electron diffraction and characterized by x-ray photoelectron spectroscopy (XPS), x-ray diffraction, and x-ray reflectivity. The evolution of the XPS peak intensities and x-ray reflectivity data suggest that the growth was Stranski–Krastanov with a two-dimensional to three-dimensional transition and a small degree of roughness at the interface. The film maintained the substrate surface crystallography initially, then transitioned to an ordered ZnO (0001) phase after passing through a disordered regime. Higher Zn 2p XPS core level binding energies were observed on positively poled LiNbO3 and were attributed to the negative compensating charges at the positive surface n doping the ZnO film, thereby the Fermi level is pinned at the bottom of the ZnO conduction band. In addition, the reaction of 2-propanol was used as a probe to identify the polarity of thick ZnO films. The results indicate that ZnO films grown on either LiNbO3 (0001) or (0001¯) polar surfaces ultimately develop a negative polarization. Therefore, it is concluded that the LiNbO3 polar substrate has a more obvious impact over a short range near the ZnO/LiNbO3 interface but this does not translate into directing the polarization direction of thicker ZnO films.