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 Cr 2 O 3 layers on the polar ZnO surfaces were characterized to determine how polar substrates can influence the properties of non-polar films. Photoelectron spectroscopy (XPS and UPS), high resolution transmission electron microscopy (HRTEM), electron diffraction (RHEED and LEED), and x-ray diffraction and reflectivity (XRD and 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 Cr 2 O 3 0001 . Despite the initial disorder, HRTEM and XRD/XRR measurements on thicker films revealed an abrupt interface with the Cr 2 O 3 lattice extending all the way to the interface. This indicates that above a critical thickness of 10 -15 Cr-O 3 -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 2 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 Cr 2 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 Cr 2 O 3 and thin Cr 2 O 3 layers on ZnO 0001 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. PACS number(s): 68.47.Gh,

Orientation-dependent chemistry and band-bending of Ti on polar ZnO surfaces

“…A similar chemistry is observed upon annealing the Ti adlayers, although with very different activation temperatures, 500 K on O-ZnO and 700 K on Zn-ZnO. Those orientation-dependent behaviours are expected to strongly affect applications relying either on rather thin Ti/ZnO films or on surface properties that depend on the charge compensation mechanism at the interface of the adlayer with polar ZnO substrates 63 . The present results also partly explain why Ti/ZnO electrical contact properties are quite scattered and depend on annealing treatments and crystal orientation 1 .…”

Monolayer Intermixed Oxide Surfaces: Fe, Ni, Cr, and V Oxides on Rutile TiO2(011)

“…The Cr 3+ may be attributed to the Cr 2 O 3 islands and the minority of Cr 2+ to the formation of an intermixed chromium–titanium monolayer oxide. Comparison with the band edge for thicker Cr 2 O 3 films allows us to assign the band edge at ∼1.5 eV binding energy to Cr 2 O 3 and the edge at ∼2.2 eV to the intermixed oxide monolayer.…”

Epitaxial NixPd1–x (111) Alloy Substrates with Continuously Tunable Lattice Constants for 2D Materials Growth

“…For all films, polished α-Al 2 O 3 (0001) growth substrates (>99.99%, ±0.5° tolerance, MTI) were first ultrasonically cleaned in isopropanol and deionized water and then annealed in air at 1075 K for 12 h. The substrates were then loaded into a UHV chamber configured for MBE with <1 × 10 –9 Torr base pressure and then cleaned in the effluent of an O 2 plasma source at 475 K in the high 10 –6 Torr range for 1 h. Chromium oxide films were grown as described previously: , Cr (99.997%, Alfa Aesar) was heated in an electron beam evaporator and deposited in 2 × 10 –6 Torr O 2 at a rate of ∼15 Å/min, while holding the sample at 625 K. After deposition, the films were moved to a tube furnace and annealed for 12 h at 875 K under ambient pressure air flow and then reloaded into the UHV chamber and cleaned again with the O 2 plasma at 475 K in 6–8 × 10 –6 Torr O 2 for 1 h. Nickel and Pd were codeposited via twin high-temperature effusion cells, with a total growth rate of 2–3 Å min –1 and a substrate temperature of 525 K. To promote adhesion and reduce the driving force for dewetting, a 10 Å Ni seed layer was first deposited before codeposition of the metals to a final thickness of 500 Å, with the exception of the 100% Pd film ( a = 3.89 Å). Palladium was also deposited via an electron beam evaporator for the a = 3.86 Å film on α-Al 2 O 3 (0001); however, the effusion cell afforded greater rate control and was used for the remaining samples.…”