Determinution of surface structures is if very difficult problem because of the lack of periodicity in the direction normal to the surface. One of the most promising techniques is photoelectron ho- lography. which was recently proposed by Barton [ 11 basal on the X-ray photoelectron diffraction (Xl’liD) tcchniquc and the idea of photoelectron holography [2]. In this method. a three-dimen- sional surface structure can be deduced from the data of the two-dimensional angular dependence of the intensity of ph~~toelectrons emitted from the inner shell of ordered adsorbed atoms. The emitted electron is scattered by nearby atoms, and the intcnaity dihtrihution contains information on the geometry around the emitting atom. This measure- mcnt is. howcvcr, very time-consuming when one List’s c~~~~vc~~ti(~~~~Il and c~~rnr~lerci~lly available im- glc-resolved deflection-type analyzers. Hence. no experiments on thia work have been made so far. In recent years acveral approaches were tried to overcome this trouble with multichannel detection analyzers [3-61.

One approach is to use retarding grids [3.4]. Kanayama et al. [4] obtained a t~vo-dimensioilal photoelectron diffraction pattern from bulk CiaAs(O01) by using this retarding field type analyzer. This type of analyzer i:, suitable to mea- bure the electrons which have a kinetic energy which is almost the highest among all the electrons emitted from the surface. For the measurement of electrons of the inner core level. this analyzer is not effective and gives a high background because outcr-shell electrons of higher kinetic energy can also pass the grids and be detected. Those inncr- shell electrons are suitable for the ph~)t~)elcctr~)ll diffraction experiment, because they are confined to the nucleus and the emitting point is fixed. 7‘~) avoid this problem, a more sophisticated energy selecting two-dimensional analyzer must be used.

Eastman et ai. [5] realized such an energy sriecting display-type analyzer using an ellipsoidal mirror grid and an electrode. and Rieger et al. [6] has built the same type of analyzer. This type of analyzer is powerful but has the disadvantage that the pattern obtained is distorted and difficult tc~ construct. No experiments on photoelectron dif- fraction were reported with this type of ~inaiyzrr so far.

We have developed a new type of energy select-

H. Daimon e! al. / JD XPED patterns .from .!A(1 I I) and TaC(I Ii) 289

ing two-dimensional display-type spherical mirror analyzer ]7-9] using a he~spherical grid and elec- trode. It has been used for surface studies effec- tively to observe one-dimensional diffraction [lo] or ESDIAD (electron stimulated desorption ion angular distribution) [ll] patterns. In the present study, it was applied to photoelectron diffraction patterns.

Because of the difficulty of measuring the two- dimensional angular distributions (2DAD) of core

X-ray-photoelectron (XPS) peak intensities, only two works have been reported so far for bulk crystal, and none for surface structures. One is that reported by Baird et al. [12] for AufOOl) crystals with a point-by-point method. The second work is that for GaAs(OO1) by Kanayama et al. [4] using as mentioned above a retarding grid

analyzer. The present work offers new examples of 2DAD patterns for Si(ll1) and TaC(111) crystals.

In the present work, the 2DAD phot~~e~tron diffraction pattern from an ordered adsorbate, which is necessary for photoelectron holography, has been observed for the first time for a Si(lll)fi X a-Ga(3d) surface.

2. Experiment

The experiment was made at BL-7A in PF (Photon Factory, a 2.5 GeV positron storage ring)

-

of the National Laboratory for High Energy Physics. The new two-dimensional display-type spherical mirror analyzer [7-9] was used for the measurement of the kinetic energy of photoelec- trons as well as their angular distributions. Pho- tons of 10 to 1000 eV from an ultra-high vacuum plane-grating grazing-incidence monochromator [13] were focused on the sample to a spot of 1 mm diameter. The base pressure of the chamber was 2 X lO-‘O Torr. The acceptance cone of the analyzer is variable. and was about 95” in the present work.

Fig. 1 shows an example of a photoelectron spectrum obtained from a clean Si( 111)7 x 7 surface. The photon energy was about 240 eV. In this spectrum, the valence band peak (indicated as VB), Si 2p core, and Si LVV Auger peaks are seen. First, the angular distribution of the strong peak of the Si2p core was measured.

Fig. 2 shows the bulk photoelectron diffraction pattern of the Si2p core from a clean Si(lll)7 x 7 surface at a kinetic energy of 500 eV. This figure was obtained from the original pattern by sub- tracting the pattern which was obtained by the same process with a 30°-rotated sample. This subtraction was necessary because the non-uni- formity of the detector was greater than the con- trast of the photelectron diffraction pattern. This subtraction enhances the contrast if there is a

Si(111) 7x7

L-

100 150 200 250

KINETIC ENERGY (eV)

Fig. 1. Photoelectron spectrum From Si(l11)7 x 7 surface at the photon energy of about 240 eV

fold synmetry in the pattern. If there is only normal peak (or clip) has disappeared in thih s

x-fold symmetry in the original pattern, this traction, although this peak is almost always

traction produces also only a three-fold pat- petted in the bulk photoelectron diffraction 1 1. One must notice that the central. or surface tern.

H. Daimon et al. / 20 XPED patterns from Sl(1 I I) und TaC(I II) 291

Fig. 4. Ga(3d) photoelectron diffraction pattern from Si(l ll)fi X t/3 -Ga surface at the kinetic energy of 200 eV. The orientation of

the Si(l11) surface is the ame as that of fig. 2.

The typical time to obtain one original pattern was about 30 min. The number of pixels of the pattern is 256 X 256. The maximum intensity in the pattern was typically 150 counts/pixel in the original pattern, and 20 counts/pixel in the dif-

ference pattern. Six-fold symmetrical peaks are clearly observed

in fig. 2. The azimuths of these peaks are the same as those for which one might observe the Kikuchi bands in an electron diffraction pattern. In other words, the directions of them are near the crystal- lographic axes, [112], [121], [211], [llO], [loll. and [Oil]. The direction of the peak at the top of the figure corresponds to the [112] direction, and that at the bottom corresponds to about the [l lo] direction. Although the Si(ll1) surface has a three-fold symmetry, this figure shows that the pattern has almost a six-fold symmetry.

Second, the angular distribution of the Ta4f core emission from the TaC(111) surface was mea- sured for comparison as another example of the (111) surface. The clean TaC(111) surface was obtained by repeated annealing of the sample at a temperature above 2000 o C for a period of about 2 s. Fig. 3 shows the pattern of Ta4f emission at a kinetic energy of 500 eV. A three-fold symmetrical

pattern is clearly observed in this pattern. This pattern was obtained by subtracting the 60” rotated pattern. The symmetry of this pattern is

C,“, but a C, symmetric pattern was obtained

when the 30” rotated pattern was subtracted. Hence, this surface produces intrinsically a three- fold C,,. pattern. By the way, in the case of Si(lll), the pattern almost disappears when the 60” rotated pattern was subtracted. The direc- tions of the very strong peaks correspond to the [llO], [loll, and [Oil] directions.

The photoelectron diffraction pattern from an adsorbate of the ordered Si( Ill)& X &-Ga(3d) structure is shown in fig. 4. The kinetic energy of Ga3d core photoelectron was set to 200 eV. The orientation of the Si(ll1) surface is the same as that of fig. 2. Because of the weakness of the signal, the S/N of the pattern is low. The six-fold symmetry is, however. observable.

3. Discussion

It is well known [14] from bulk photoelectron diffraction experiments that the intensity of the core XPS photoelectron is high around the low index crystallographic axes and modulated by dif-

fraction effects. These studies are usually being made with polar angle scans in one azimuthal ii11gk.

The observed six-fold symmetry of the pattern

from the Si( 111) surface is somewhat strange. Al- though the direction of the peaks is near the cryst~ll~~gr~phic axes, the axes are not arranged in hexagon and not six-fold symmetry. Hence. the observed pattern cannot be understood satisfacto- rily by explanation of the crystallographic axes only. Although the kinetic energy dependence is not mentioned, these systems have little kinetic energy dependence [ 151. Hence, the diffraction effect is not important in this case.

Fig. 5 shows the unit cell of the diamond struc- ture of Si. Around the (111) plane shown by a dotted line. thcrc is a quasi-regular hexagon, which includes atoms indicated by 11121, 1121). and 12111. Tlw directions of the ohservcd peaks corresponds

clctscly to the directions. which faces to these atoms from the origin. The concentration of the electron behind the scatterer has often been ob- served in photoelectron diffraction [ 141 or electron diffraction pattern [IO]. Hence. the quasi-six-fold symmetry of the pattern of Si( 111) surface can be understood by this quasi-regular hexagon.

The strong three-fold symmetric Ta4f peaks in the pattern from the T&(111) surface can also be understood by the directions of the strong scatterers. TaC’ has an N&I type structure, and

the Ta atoms are arranged in ;I fee lattice. ‘I’hc carbon atoms are light and negligible in the

scattering process compared with the vrr> hcav> Ta atoms. The directions of the nearost neighbor

Ta atoms from the emitting Ta :rtom are the 11 10). [loll. and [Ol l] directions. These directiona corru-

spond to the observed strong peaks. In the case of Si(lll)fi x 43 -Ga(3d), the pat-

tern should also be a three-fold one, hccausc thu surface is three-fold and a quasi-regular hexagon above the adsorbed Ga atoms ih unimagim~ble. I1 the Ga atoms are embedded below the first layor of Si atoms, the environn~ent ahnve the <;;I ztom.

which is the emitter of the ph~~t~~electr~)tis. must IX ;I three-fold symmetry. However. \vhcn the <ia atoms are adsorbed above the first Si layer, the main scatterers are not the substrate Si atoms hut the nearest neighbor Ga atoms. The adsorh;itc layer of Ga atoms must have :I six-fold bynrmctr> because the periodicity is ~5 X {3 and the (iii coverage is l/3 monolayer. Hence, the six-I’cU symmetr? of the observed pattern suggest,\ that the Ga atoms are adsorbed above the first Si layer. This suggestion agrees well with recent L.t;,f”I) analysis [ 161 and photoelectron diffraction ;innlv- sis 1171.

4. Conclusions

Two-dimensional photoelectron diffraction pat- terns from Si( 1 I 1). and TaC’( 111) surfaces have been obtained for the first time. The observed patterns from these surfaces arc very different from each other. The pattern of the Si 2p core emissions from the TaC(lI1) has three-fold xym- metry. They were explained qu;~litatively considur- ing the directions of the strong scatterers.

Photoelectron diffraction patterns from an ordered adsorbate have been obtained for the first time. The obtained six-fold pattern from Si( 111 )fi X d! -Ga( 3d) Suggests that the Gu atoms are adsorbed above the first Si layer, and this is in good agreement with the existing model.

More quantitative theoretical stud&x will he necessary to fully characterize these effects.

H. Daimon er 01. / 2D XPED patterns from .%(I I I) and T&(1 I I) 293

Acknowledgments

We are grateful to Dr. S. Otani for the offer of a TaC crystal. The support by the staff of the Photon Factory of the National Laboratory for High Energy Physics, is gratefully acknowledged. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Edu-

cation. Science and Culture.

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