Conference-ALC ’ 15-Auger Intensity Anomalies from ZnO ( 0001 ) Surface Excited by RHEED Incident Beam

Anomalous behaviors of Auger intensities have been observed for ZnO(0001) surface depending on the glancing angle of incident electron beam of RHEED. In the beam rocking Auger electron spectroscopy, BRAES, the profiles of Zn(LMM) and O(KLL) were compared with each other. It was found that their profiles differed in some aspects. This suggests that their Auger intensities are influenced by the wave field of incident electron beam of RHEED. In this study, RHEED rocking curves were also measured and analyzed by dynamical calculations. It has been found that the determination of surface polarity is possible by RHEED rocking curves. [DOI: 10.1380/ejssnt.2016.92]


I. INTRODUCTION
Zinc oxide (ZnO) is a wide bandgap semiconductor with a large exciton binding energy of 60 meV.Because of these outstanding properties, ZnO is a hoped material as a transparent conducting film and photo-electronic devices [1,2].Since ZnO crystal has wurtzite structure, the polarity of ZnO surface, Zn-polar and O-polar, influences the crystal growth process, surface morphology and electrical properties.Therefore, it is important to determine the polarity of ZnO.Several methods such as conver- gent beam electron diffraction (CBED), etching, coaxial impact collision ion scattering spectroscopy (CAICISS), and X-ray photoelectron spectroscopy (XPS) have been used for determining the polarity of ZnO [3][4][5][6].Among these methods, CBED and etching are destructive methods.XPS is a non-destructive method, but synchrotron radiation is needed to determine the polarity of ZnO from XPS spectra.CAICISS is a popular method used to determine the polarity of material surfaces.However, CAICISS requires samples with relatively large dimensions.Compared with these methods, reflection high-energy electron diffraction (RHEED) [7] is a simple technique.We can obtain RHEED patterns during crystal growth.However, the RHEED patterns of the ZnO surfaces have not been discussed so much.It is necessary to analyze the surface structure including the polar surface determination.
This study aims at the structural analysis of the ZnO(0001) surface using RHEED and Auger electron spectroscopy (AES).Rocking curves were measured and examined whether the curves could be explained by the calculated ones based on the bulk truncated surface structure.Auger electrons excited by incident electron beam of RHEED were particularly paid attention.Auger intensity changes depending on the incident glancing angle, which is named "beam rocking Auger electron spectroscopy (BRAES)", has been measured for the ZnO(0001) surface.Up to now, BRAES profiles for Si(111)7×7 [8], MgO(001) [9], Si(111) [11], Si(001)2×2-Al [12] and Si(001)2×1 [13] surfaces have been measured.Auger intensity anomalies in the BRAES profiles are related to the distributions of the wave fields of incident electron beam.It is expected that the BRAES technique can be used for the structure analysis.In this study, BRAES profiles of a binary crystal ZnO(0001) was studied for the first time.whether the wave field localizes on Zn or O atomic rows.

II. EXPERIMENTAL
Experiments were carried out using an ultrahigh vacuum RHEED apparatus combined a cylindrical mirror analyzer (CMA) for AES measurement.Figure 1 shows the RHEED-AES system.The entrance plane of the CMA was set about 10mm above the sample surface, which can detect Auger electrons during the RHEED observation.Acceleration voltage of the incident electron beam of RHEED was fixed to 10 kV.The base pressure of the chamber was under 1 × 10 −7 Pa.
Experimental rocking curves of the diffraction spot intensity were measured by a CCD camera while changing the glancing angle of incident electron beam mechanically in about 0.05 • increments.The BRAES profiles of Zn(LMM) and O(KLL) were measured by CMA, that is the peak-to-peak intensity of the energy derivative spectra while changing the glancing angle.
The used sample was ZnO(0001) single crystal (Tokyo Denpa Co., Ltd.) of wurtzite structure with a size of 5 × 10 × 0.5 mm 3 .Polished Zn face was used as the sample surface, but with off-angle of 0.5 ± 0.2 • along ⟨1 100⟩ direction in specification.The sample was set on a Ta sheet with 0.05 mm thickness in the sample holder.The sample was heated by passing electric current through the Ta sheet.The correlation between the sample temperature and the passing current was preliminarily measured.The correlation was approximately linear, and the heating temperature showed 650 • C by an electric current of 10 A.
RHEED incident azimuth was set at [2 11 0] in four index notation which corresponds to [100] direction in three index notation as shown in Fig. 2. Since this incident azimuth is asymmetric direction because of three times symmetry of the sample surface, observed RHEED pattern should be different in intensity between the corresponding spots in right and left side patterns.When the step height is c/2 (c is a lattice constant of surface normal direction, 5.21 Å), the terrace surfaces which are separated by the step have anti-phase atomic arrangement to each other.If there are many steps on the surface, RHEED pattern becomes symmetrical because of the mixture of the anti-phase terrace surfaces, so-called two domains A and B.

III. RESULTS AND DISCUSSIONS
In order to clean the sample surface, the sample was heated to 450 • C progressively.However, it was observed from Auger spectrum that contamination carbon still re- mained on the sample surface as shown in Fig. 3. Further heating with higher temperature than that was not done to avoid the change of the surface stoichiometry.It is roughly estimated that carbon coverage is less than one monolayer considering the Auger sensitivities of C(KLL), O(KLL) and Zn(LMM) based on Zn terminated surface.Since any additional spots in RHEED pattern did not observed even at very low glancing angle, it is considered that carbon atoms randomly remained on the surface.Although the sample surface was not clean enough, RHEED patterns were relatively clear as shown in Fig. 4. The contamination effect may not be serious, however, it raises the background intensity and makes the diffraction intensity weak at very low glancing angle.This sample surface was used for the experimental measurements.
RHEED patterns for ZnO(0001) at several glancing angles from (a) 1.8 • to (h) 5.3 • with 0.5 • intervals are shown in Fig. 4. Indices of diffraction spots are indicated in (h).A characteristic pattern is seen at θ = 4.3 • in Fig. 4(f), where both side spots of 0 1 and 0 1 on the 0 th Laue zone are very intense and several spots on the 1 st Laue zone are visible relatively well.
Figure 5 shows the experimental results of rocking curves and BRAES profiles.Rocking curves were measured for specular spot of 0 0 and both side spots of 0 1 and 0 1.Taking into account of the positions of the shadow edge and Kikuchi lines, it was determined for the sample to have a vicinal angle of 0.55 • toward step down along the incident beam direction.The glancing angle θ was corrected as that measured from the just (0001) surface.Supposing the regular step surface with the step height of c/2, the terrace length is estimated to be relatively wide 27 nm.Then, the interference between the reflected electron waves from the terrace domains A and B is not important, but the intensity sum from the both domains should be considered.It is worth to notice that the rocking curves of the both side spots, 0 1 and 0 1, are very similar to each other in spite of the asymmetric incidence.This suggests that the experimental RHEED pattern consists of the intensity mixture from many step terraces of domains A and B, and the all step terraces have mono-polar surfaces.sity anomalies are seen at some points.Especially, the profiles at θ = 4.3 • shown by arrows should be noticed.Auger intensity of O(KLL) increases, however, that of Zn(LMM) decreases to objection.The opposite behavior of the BRAES profiles is very interesting, which means localization of the wave field of incident electron beam.In other words, the incident electron density is high on atomic rows of O and low on those of Zn.This incident condition causes Auger intensity anomalies and strong intensity peaks of the both side 0 1 and 0 1 spots.It is considered that the interference between the both side evanescent beams of 02 and 0 2 forms intense wave field parallel to the incident azimuth.
BRAES profile of Zn(LMM) monotonically increases with glancing angle, while that of O(KLL) has a peak at about 4.5 • and decreases for larger glancing angle than that.This is considered to be raised by the escape depth of Auger electron.Since the kinetic energies of O(KLL) and Zn(KLL) Auger electrons are 503 eV and 994 eV, respectively, the escape depth of Zn(KLL) Auger electron is deeper than that of O(KLL) one.When the glancing angle becomes larger, the penetration depth of the incident beam becomes deeper.Therefore, increasing the glancing angle, Zn(KLL) Auger intensity keeps increasing, however, O(KLL) one becomes saturated.It sometimes occurs that when the electron beam axis is very slightly deviated from the center of the sample surface, the background intensity of BRAES profile decreases at large glancing angle.In this paper, anomalous behavior of the BRAES profile is noticed and the background is disregarded.
Figure 6 shows the calculated rocking curves for bulk truncated surface without surface relaxation, where (a) and (b) are calculated results based on Zn and O terminated surface models, respectively.The multi-slice method [14] was used for the calculation of diffraction intensities, and the crystal surface was sliced in a parallel manner up to a depth of about 100 Å with a thickness interval of 0.1 Å.An atomic scattering potential was obtained from the analytical formula [15] and the value was multiplied by 0.86 for correction.Here, 10% of the potential value was adopted as an imaginary part for the incident beam absorption effect.The Debye-Waller factor was calculated using the Debye temperature of 510 K.For the incidence azimuth [2 11 0], the number of reciprocal lattice rods that were introduced into the calculation was seven in the 0-th Laue zone, i.e., 0 0, 0 1, 0 1, 0 2, 0 2, 0 3 and 0 3.
It is found that the two rocking curves of the specular 0 0 spot for (a) Zn terminated surface and (b) O terminated surface considerably differ.This is considered to be effective for the discrimination of the surface polarity.It is seen that the rocking curve profiles of both side spots of 0 n and 0 n (n = 1, 2, 3) are different because of asymmetric incidence.As seen in Fig. 5, however, experimental rocking curves of both side 0 1 and 0 1 spots are similar to each other because of the existence of double terrace domains of A and B. Therefore, it is necessary to use the averaged profile of the both side rocking curves in order to compare with the experimental result.Furthermore, the used sample has vicinal surface with step down along the incident azimuth.The beam shadow area at the step edges should be considered.The rate of beam irradiation area decreases at lower glancing angle.
Figure 7 shows the comparison between the experimental (broken lines) and calculated (solid lines) rocking curves for the two polar surfaces.The calculated rocking curves of 0 0 and 0 1 spots took into account of the rate of beam irradiation area.The calculated rocking curve of 0 1 spot was averaged for those of the both side 0 1 and 0 1 spots.It is seen that the calculated rocking curve of 0 0 spot for Zn terminated surface agrees with the experimental one relatively well as shown in Fig. 7(a), however, that for O terminated surface is not so as shown in Fig. 7(b).For the experimental rocking curve of specular 0 0 spot, a peak appears at θ = 3.1 • .In this condition, the specular spot is very close to the horizontal 0003 Kikuchi line as shown in Fig. 4(d).On the other hand, the calculated rocking curves of 0 1 spot for the both polar surfaces have a similar sharp peak, which is also seen in experimental results at θ = 4.3 • .In this condition, the both side spots cross the horizontal 0006 Kikuchi line and are also very close to oblique Kikuchi lines as shown in Fig. 4(f).It is found that the calculated rocking curves for Zn-polar surface as a whole reproduce the experimental ones relatively well.The result is consistent with the sample specification.For the calculated peak of 0 0 spot at about θ = 1.5 • and that of 0 1 spot at about θ = 3.1 • , the experimental results are very weak.This disagreement may be due to the surface structure model without relaxation and the remained carbon atoms on the surface.A preliminary calculated result taking into account of surface relaxation does not affect the decision of surface polarity.

IV. CONCLUSION
It was proved that the polarity of ZnO(0001) surface could be evaluated by RHEED rocking curve.The calculated rocking curve of specular spot is considerably different between Zn-polar and O-polar surfaces.The experimental rocking curves of the both side spots are similar to each other even at asymmetric incident azimuth.It is considered that this fact comes from double domains of the step terrace surfaces.Calculations of the rocking curves taking into account of the surface relaxation are now carried out and the results will be reported.
When the intensity of the both side 0 1 and 0 1 beams became maximum at θ = 4.3 • , BRAES profile of O(KLL) increased and that of Zn(LMM) decreased.The opposite behavior was obtained for the first time in this study.The opposite behavior between the BRAES profiles of Zn and O suggests that Auger electrons are excited by wave field of incident electron beam of RHEED.Binary crystal is effective for the study of the wave field in this way.Dynamical calculation of the wave field will be also performed after the determination of the surface structure.
Contaminant carbon atoms remained on the sample surface in this study.For exact study, it is necessary to examine the cleaning method without changing the stoichiometry of Zn and O.The heat treatment in the oxygen introduced vacuum will be tried in order to remove the contaminant carbon atoms on the sample surface.