Growth, structure, and electronic properties of nonpolar thin films on a polar substrate:Cr2O3on ZnO (0001) and ZnO (0001¯)

Abstract

The growth and geometric and electronic structures of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ layers on the polar ZnO surfaces were characterized to determine how polar substrates can influence the properties of nonpolar films. X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), high resolution transmission electron microscopy (HRTEM), reflection high energy electron diffraction, low energy electron diffraction, x-ray diffraction (XRD), and x-ray reflectivity (XRR) were employed to characterize the growth mode, film quality, and interfacial electronic properties. Chromium oxide growth on ZnO $(000\overline{1})$ and $(0001)$ followed the same trends: two-dimensional growth with initial disorder followed by the formation of epitaxial ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3} (0001)$. Despite the initial disorder, HRTEM and XRD/XRR measurements on thicker films revealed an abrupt interface with the ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ lattice extending all the way to the interface. This indicates that above a critical thickness of 10--15 $\mathrm{Cr}\ensuremath{-}{\mathrm{O}}_{3}\ensuremath{-}\mathrm{Cr}$ repeat units, the entire film reorganizes into an ordered structure. It is postulated that the oxygen remained ordered throughout the growth but that the chromium initially filled interstices randomly in the oxygen sublattice, which allowed the film to eventually grow with a well-defined epitaxial relationship with the substrate. The polar interfaces showed a small band offset that decayed with increasing film thickness, suggesting that the compensating charges at the interface may partially migrate to the film surface. No evidence of formal changes in the Cr oxidation state at the interfaces was seen. On the other hand, statistical analyses of UPS valence band spectra revealed an enhanced density of states near the valence band edge for ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ on ZnO $(0001)$, consistent with stabilization of the positive interface by filling surface electronic states. In contrast, no significant valence band differences were observed between bulk ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and thin ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ layers on ZnO $(000\overline{1})$, suggesting a different charge compensation mechanism on the negative surface. The potential impact of these findings on the surface properties of chromium oxide thin films is discussed.