Cubic Zirconia Crystalline Surface Oxide Epitaxial Formation on ZrB 2 ( 0001 ) Confirmed by Circularly-Polarized-Light Photoelectron Diffraction

Pure cubic zirconia (c-ZrO2) is unstable at room temperature. We achieved the epitaxial formation of c-ZrO2 crystalline surface oxide islands on ZrB2(0001) by annealing the substrate without sample cleaning at 950 ◦C under ultrahigh-vacuum conditions. The interface structure at the c-ZrO2 islands and the ZrB2(0001) substrate was investigated using element-specific circularly-polarized-light photoelectron diffraction, angle-resolved X-ray photoelectron spectroscopy, and reflection high-energy electron diffraction (RHEED). The ZrO2(111) islands was a twin crystal oriented in ZrO2[11̄0]//ZrB2[21̄1̄0], and was stable up to around 1500 ◦C. The Zr-Zr distance of ZrB2 bulk and that of ZrO2(111) agree with at the ratio of 8 to 7. [DOI: 10.1380/ejssnt.2015.111]


I. INTRODUCTION
Cubic zirconia (c-ZrO 2 ) is used as a diamond simulant because of its high refractive index.ZrO 2 is a promising candidate for high-k materials and gas sensors [1][2][3].However, pure c-ZrO 2 does not exist at room temperature (RT).Pure ZrO 2 has three phases in different ranges of temperature at normal atmospheric pressure: monoclinic phase (space group P 21/c) from RT to 1170 • C, tetragonal (P 42/nmc) from 1170 to 2370 • C, and cubic fluorite structure (F m3m) from 2370 to 2706 • C [4,5].The addition of solutes such as MgO, CaO, Y 2 O 3 , HfO 2 is necessary to stabilize c-ZrO 2 at RT. c-ZrO 2 is the hardest among the three structures.c-ZrO 2 is useful materials as a heatresistant hard-coating.
ZrB 2 , which was used as a substrate crystal in this study, has unique features such as high hardness, high melting point, high corrosion resistance, and metallic conductivity.It has an AlB 2 -type crystal structure consisting of alternative stacking of a close-packed Zr layer and a graphene-like B layer.The clean (0001) surface is terminated with a Zr layer [6].The oxidization of ZrB 2 has been studied for a powder ZrB 2 [7,8] and a single crystal as a substrate for GaN epitaxial formation [9,10], as well as for a basic study of surface oxidation [11,12].Recently, the oxidization of ZrB 2 nanoparticle has been reported by G. Zhao, et al. in 2014 [13].However, the oxide surface layer and interface structures on ZrB 2 (0001) have been still unclear.Here we report the fabrication and confirmation of a stable pure c-ZrO 2 epitaxially grown on ZrB 2 (0001) without additive solutes simply by annealing at 950 • C under ultrahigh-vacuum (UHV) condition after exposed to the air.This c-ZrO 2 was stable from RT to about 1500 • C, which is sufficient for a heat-resistant hard coating.
In this study, we investigated the ZrO 2 atomic struc-ture and the interface relation between ZrO 2 and ZrB 2 (0001) using element-specific two-dimensional photoelectron diffraction (2D-PED) and X-ray photoelectron spectroscopy (XPS) measured by the display-type spherical mirror analyzer (DIANA) [14][15][16] and reflection high-energy electron diffraction (RHEED).The acceptance angle of DIANA was ±60 • .By scanning the sample azimuth over 360 2D-PED is a useful element-selective atomic structure analysis method that enables us to observe threedimensional atomic configurations without surface destruction.A photoelectron from a localized core level is an excellent element selective probe for surface structure analysis.Forward focusing peaks (FFPs) appearing in the photoelectron intensity angular distribution (PIAD) indicate the directions of surrounding atoms seen from the photoelectron emitter atom.When we use circularlypolarized light as an excitation source, the atomic distance between the emitter and the scatterer atoms can be deduced from the circular-dichroism shift of FFP direction [17][18][19][20].In this study, we determined the structure of surface oxides on ZrB 2 (0001) as a cubic zirconia by this unique method.

II. EXPERIMENT
The experiments were performed at the circularly polarized soft-X-ray beamline BL25SU at SPring-8, Japan [21].2D-PED and RHEED experiments were performed in an UHV system at RT.A single crystalline ZrB 2 (0001) surface was used as the substrate of c-ZrO 2 .The ZrB 2 single crystal was grown using the rf (radio frequency) heated floating-zone method [22].A 1-mm-thick sample of 7-8 mm diameter was cut from the crystal rod after orientating to a (0001) plane using the X-ray Laue method.One side of the sample was mirror-polished with diamond and alumina paste [12].We prepared an atomically clean ZrB 2 (0001) surface with the method reported in ref. [6].The sample was at first heated up to 1000 • C for degassing.After waiting for the vacuum to recover, the sample was flash heated at 1400-1500 • C several times by electron bombardment heating.Here we call this sample S1.For a c-ZrO 2 formation, the ZrB 2 substrate was once exposed to atmosphere.We annealed the sample at 950 • C for 10 min without cleaning by direct current injection after transferring into an ultra-high vacuum below 3×10 −8 Pa.We call this sample S2.We measured angle-resolved constant-final-state (CFS) mode XPS spectra, RHEED patterns, and PIADs for S1 and S2.

III. RESULTS AND DISCUSSION
The XPS spectra of S1 showed no contaminants of C and O.The surface structure was characterized by RHEED.We confirmed a sharp (1×1) pattern coming from the clean surface.Figure 1(a) shows the RHEED pattern from the ZrB 2 (0001) clean surface after flashheating to 1400 • C. The incident kinetic energy was 15 keV.The intervals of 1×1 fundamental diffraction spots appeared were 3.97 ± 0.07 Å−1 (3.17 ± 0.09 Å) matching with the Zr-Zr atomic distance 3.169 Å of the ZrB 2 substrate.For S2, we observed O 1s, Zr 3d, and Zr 3p peaks from the surface oxide on ZrB 2 (0001) in the XPS spectra.RHEED patterns from ZrB 2 surface with and without oxygen were compared.Figure 1(b) shows the RHEED pattern from the ZrB 2 (0001) surface after oxidation.The transmission diffraction pattern was observed, which is similar to the observation by Armitage et al. [9].The intervals of diffraction spots were 3.45 ± 0.06 Å−1 (3.64 ± 0.06 Å) matching with the lattice constant 3.631 Å of the ZrO 2 substrate.Note that seven times the length of Zr-Zr in-plane distance in ZrO 2 lattice matches with eight times the length of Zr-Zr distance of ZrB 2 substrate lattice.We clarified that the ratio of Zr-Zr distance parallel to the surface between ZrB 2 bulk and that of ZrO 2 (111) was 8 to 7.
The RHEED result in Fig. 1(b) shows that the surface oxide forms small islands.The size of the islands was about 1 nm.This size was much smaller than the size of the excitation light spot of about 0.3 mm.We considered that the CFS-mode XPS was effective to estimate the average thickness of the oxide islands.We measured CFS-mode photoelectron spectra for the estimation of the oxide islands thickness of S2. Figure 2 shows the angle resolved CFS mode photoelectron spectra of Zr 3d at a kinetic energy of 600 eV.We measured the emission angle dependence of CFS-XPS from 0 • to 90 • relative to the surface normal.Mean free path of photoelectron was kept constant.The peaks at photon energy of 786.5 and 789.0 eV shown in Fig. 2 correspond to Zr atoms bonded to B atoms (Zr-B) and O atoms (Zr-O), respectively.The Zr-O peak intensity increases with the increasing emission polar angle.The estimated chemical shift of Zr-O relative to Zr-B was 3.6 eV.This value was larger than that of O atom adsorbed on the ZrB 2 surface forming (2×2) superstructure but smaller than that of fully oxidized state Zr oxides [12].We estimated the average thickness of the oxide islands using the following formula.
≈ λ O cosθ ln I O and I S are the photoelectron intensities from the oxide island and the substrate, respectively.N O and N S are the atom densities of the surface oxide and substrate, respectively.λ O and λ S are the attenuation lengths in the surface oxide and the substrate.We used the common value of 17 Å deduced by simulation code SESSA [23,24] as the attenuation lengths for λ O and λ S .The thickness of the oxide islands was determined to be 11.0 ± 0.7 Å, which corresponds to the thickness of four Zr layers in c-ZrO 2 (111).
Then 2π-steradian Zr 3d and O 1s PIADs from the clean ZrB 2 (0001) surface and the oxide islands on ZrB 2 (0001) were measured using DIANA installed at BL25SU.A set of 2π-steradian PIADs excited by σ+ and σ− helicity lights was measured by switching the path of storage ring electrons in twin helical undulators at 0.1 Hz [21].
As shown in Fig. 3 as the top and side views in Fig. 4(a) and (c).These FFPs were also observed in the Zr 3d PIADs from the oxidized surface as shown in Fig. 3(b) indicating that the substrate structure was also observed.FFP of B close to Zr could not be detected because of its small scattering cross-section.Figure 3(b) is a PIAD pattern for the ZrB 2 surface with oxide islands.Figure 3(b) does not have strong effect of oxide islands.We consider that the oxide forms small islands and does not form uniform layer, and the substrate ZrB 2 (0001) appears on the surface in some extent, which is inferred from the transmission pattern of RHEED in Fig. 1(b).The arrangement of the peak (1) in Fig. 3(b) and the peak A in Fig. 3(c) were seen in the same azimuthal directions.These peaks correspond to the arrangement of Zr atoms in a Zr layer just above the emitter atom as indicated by arrows (1) and A in Fig. 4(c).From this comparison of FFP arrangement, we concluded that the arrangements of Zr atoms were the same in the substrate and the oxide islands forming triangular lattice in the two-dimensional horizontal plane.Aizawa et al. reported that ZrB 2 (0001) surface has a metallic character with high reactivity to gas adsorption and oxygen is adsorbed dissociatively onto the three-fold hollow site at RT [11].According to their work, we propose that the ZrB 2 (0001) substrate for c-ZrO 2 (111) formation was terminated with O atoms at the hollow sites.3(c) leads us to conclude that there were twin domains of ZrO 2 on ZrB 2 (0001).As shown in Fig. 3(c), the directions of B and B' peaks of the first nearest Zr from O atoms were three-fold symmetric and their polar angles were 71.5 • corresponding to the atomic structure of ZrO 2 .Among the three structures of ZrO 2 ; monoclinic, tetragonal, and cubic fluorite structure, only the cubic structure has this configuration.From these peaks, we easily and directly concluded that the c-ZrO 2 formed on ZrB 2 (0001).
Furthermore, the distances between O atom and Zr atoms were determined from the analysis of O 1s FFP shifts in a circularly-polarized-light PIAD. Figure 3(d ∆ϕ which is well described by the Daimon's formula: where m * f (θ out ) and k are the angular momentum and the wave number of photoelectron, respectively [16].θ out is the angle between the incident photon axis and the outgoing direction of the emitted photoelectrons.The shift is inversely proportional to the interatomic distance R between the photoelectron emitter and the scatterer atoms.The interatomic distance from each O atom to the first and second nearest Zr atoms were determined to be 2.7 ± 0.7 Å and 4.9 ± 1.7 Å, respectively, which are in good agreements with the value of c-ZrO 2 crystal; 2.22 Å and 4.25 Å.Note that there are six FFPs for the first and second nearest Zr atoms due to the twin crystal structure of ZrO 2 islands [9].

IV. CONCLUSIONS
In conclusion, we achieved the epitaxial formation of c-ZrO 2 crystalline oxide islands on ZrB 2 (0001) by annealing the substrate without sample cleaning at 950 • C under ultrahigh-vacuum conditions.We investigated the surface oxide and interface structure of ZrO 2 on the ZrB 2 (0001) substrate by 2D-PES and RHEED.We found that the 11nm pure c-ZrO 2 (111) grew epitaxially on the ZrB 2 (0001) substrate with a commensurate condition.The Zr-Zr distance of ZrB 2 bulk and that of ZrO

Figure 3 (
c) is O 1s PIADs from the ZrB 2 with oxide after annealing at 950 • C. The open circles labeled as B and B' at the polar angle of 71.5 • in Fig. 3(c) correspond to the FFP directions of the first nearest Zr atoms seen
FIG. 4. Structure models for a ZrB2 substrate and a c-ZrO2 thin film.(a) and (b) Top views of the ZrB2 substrate and ZrO2 film, respectively.(c) Side view.The smaller Zrs are located in the back.