Elsevier

Applied Surface Science

Volume 256, Issue 4, 30 November 2009, Pages 1120-1123
Applied Surface Science

The 2×2 oxidized layer on ZrB2(0 0 0 1)

https://doi.org/10.1016/j.apsusc.2009.03.100Get rights and content

Abstract

Initial oxidation of a ZrB2(0 0 0 1) surface at high temperature yields a new 2×2 ordered surface structure, as characterized using reflection high-energy electron diffraction (RHEED), Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS). During high-temperature treatment in O2, the surface becomes boron-deficient. In the surface oxide layer, Zr is not fully oxidized, but in a Zr3+ state. The HREELS shows strong loss peaks, showing insulating characteristics of the surface.

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- Kmaterials). 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 2×2 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 2×2 surface reconstruction is found, as characterized using Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS).

Section snippets

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, 14×109 Pa and 2×108 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 1×104 Pa of high-purity O2 gas (99.99%). The in situ observed RHEED pattern changed from 1×1 to 2×2 in the first 30 s; then it turned gradually diffuse. Fig. 2(a) shows the clear 2×2 RHEED pattern taken at RT after reacting with O2 at 1600 K for 25 s. AES spectrum of the 2×2 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 10×104 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 2×2 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 2×2 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 2×2 reconstruction was found after heating a sample at 1600 K for 30 s in 1×104 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 2×2 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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