Growth, structure, and electronic properties of nonpolar thin films on a polar substrate: Cr2O3 on ZnO (0001) and ZnO (0001¯)

作     者：X. Zhu M. D. Morales-Acosta J. Shen F. J. Walker J. J. Cha E. I. Altman 

作者机构：Center for Research on Interface Structures and Phenomena (CRISP) Yale University New Haven Connecticut 06520 USA Department of Chemical and Environmental Engineering Yale University New Haven Connecticut 06520 USA Department of Applied Physics Yale University New Haven Connecticut 06520 USA Department of Mechanical Engineering and Material Science Yale University New Haven Connecticut 06520 USA 

出 版 物：《Physical Review B》 (物理学评论B辑：凝聚态物质与材料物理学)

年 卷 期：2015年第92卷第16期

页      面：165414-165414页

核心收录：

学科分类：07[理学] 0702[理学-物理学] 

基　　金：National Science Foundation [CHE 1213751, DMR 1119826] Direct For Mathematical & Physical Scien Division Of Chemistry Funding Source: National Science Foundation Division Of Materials Research Direct For Mathematical & Physical Scien Funding Source: National Science Foundation 

主　　题：CRYSTAL growth CRYSTAL structure ELECTRONIC structure THIN films CHROMIUM oxide ZINC oxide 

摘      要：The growth and geometric and electronic structures of Cr2O3 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 (0001¯) and (0001) followed the same trends: two-dimensional growth with initial disorder followed by the formation of epitaxial Cr2O3 (0001). Despite the initial disorder, HRTEM and XRD/XRR measurements on thicker films revealed an abrupt interface with the Cr2O3 lattice extending all the way to the interface. This indicates that above a critical thickness of 10–15 Cr−O3−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 Cr2O3 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 Cr2O3 and thin Cr2O3 layers on ZnO (0001¯), suggesting a different charge compensation