The oxidized layer on ZrB2(0 0 0 1)
Introduction
A ZrB2(0 0 0 1) surface is recently attracting wide interest because its in-plane lattice constant and thermal-expansion coefficient reasonably matches those of GaN(0 0 0 1) [1]. The GaN films with low defect densities reportedly grow on it [2].
It has an “AlB2”-type crystal structure consisting of alternate stacking of a close-packed Zr layer and a graphene-like B layer. The clean (0 0 0 1) surface is terminated with the Zr layer [3]. The surface therefore has a metallic character, with high reactivity to gas adsorption. Oxygen is adsorbed dissociatively onto the threefold hollow site at room temperature (RT) [4].
Zirconium oxide is an important material among those with a high dielectric constant (high- materials). Additionally, it is often used as a support material for C1 catalysts. Well-controlled oxidation of the ZrB2 surface is important for application of ZrB2 in these fields. Because ZrB2 and its composites are promising ultra-high temperature ceramics [5], [6], oxidation and corrosion processes have been studied intensively on a macroscopic scale. Initial oxidation of a pure metal Zr(0 0 0 1) is examined experimentally using surface science techniques [7], [8], [9], [10] and theoretical investigation [11], [12]. A surface structure was reported at a half monolayer oxygen coverage, in which subsurface adsorption was confirmed using low-energy electron diffraction (LEED) crystallography [7], [10].
In this work, initial oxygen reaction on ZrB2(0 0 0 1) at high temperatures is examined on the atomic level under in situ observation using reflection high-energy electron diffraction (RHEED). A surface reconstruction is found, as characterized using Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS).
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Experiment
For this study, AES and HREELS experiments were performed in an ultra-high vacuum (UHV) system consisting of two chambers: one is made of a high-permeability alloy equipped with an HREELS spectrometer (Delta 0.5; Specs GmbH); the other is a sample preparation chamber equipped with an RHEED system, a cylindrical mirror analyzer for AES, and a load-lock system. The base pressures of these chambers were, respectively, Pa and Pa. The AES was measured using the RHEED gun for
Results
The clean ZrB2(0 0 0 1) sample was heated at 1600–1800 K in Pa of high-purity O2 gas (99.99%). The in situ observed RHEED pattern changed from to in the first 30 s; then it turned gradually diffuse. Fig. 2(a) shows the clear RHEED pattern taken at RT after reacting with O2 at 1600 K for 25 s. AES spectrum of the surface is shown in Fig. 1(c) in comparison with the O-adsorbed ZrB2(0 0 0 1) shown in Fig. 1(b), which is almost saturated after Pa s O2 exposure at RT. The spectra
Discussion
The AES in Fig. 1 shows that the peak at 181 eV (an overlap of B and Zr Auger peaks) reduces on the surface compared with the clean or RT-O-saturated surfaces, which indicates B decrease in the vicinity of the surface upon heating. Because boron oxides: BOx have high vapor pressure at 1600 K, boron might evaporate in oxygen gas, leaving Zr oxides. This is merely the initial step in the macroscopic oxidation of ZrB2 ceramics [5], [6].
The thermal instability of the structure suggests a
Summary
In summary, initial oxidization of ZrB2(0 0 0 1) was investigated using RHEED, AES, HREELS, and XPS. A well-ordered reconstruction was found after heating a sample at 1600 K for 30 s in Pa of oxygen gas. Near the surface, boron content was decreased, leaving Zr suboxide, indicating Zr3+ from the Zr 3d chemical shift in the XPS. The HREELS results suggest an insulating character of the structure.
Acknowledgement
The authors acknowledge Dr. S. Suehara for fruitful discussions and for allowing us to use his XPS data analyzing program.
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