Two-dimensional photoelectron diffraction patterns by display-type spherical mirror analyzer

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 holography. 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-dimensional 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 measuremcnt is. howcvcr, very time-consuming when one List’s c~~~~vc~~ti(~~~~Il and c~~rnr~lerci~lly available imglc-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 meabure 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 inncrshell 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 diffraction 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(1I I) and TaC(I Ii)

289

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 photoelectrons as well as their angular distributions. Photons 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 subtracting the pattern which was obtained by the same process with a 30°-rotated sample. This subtraction was necessary because the non-uniformity of the detector was greater than the contrast of the photelectron diffraction pattern. This subtraction enhances the contrast if there is a

ing two-dimensional display-type spherical mirror analyzer ]7-9] using a he~spherical grid and electrode. It has been used for surface studies effectively 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 twodimensional 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) -

Si(111)

7x7

L-

100

150 KINETIC

Fig. 1. Photoelectron spectrum From Si(l11)7

x

200

250

ENERGY (eV) 7 surface at the photon energy of about 240 eV

fold

synmetry

x-fold traction

in the pattern.

symmetry produces

in

If

the original

also

only

there

is only

pattern,

a three-fold

1. One must notice that the central.

normal

this

traction,

pat-

petted

or surface

tern.

peak (or clip) has disappeared although in the bulk

this

peak

is almost

photoelectron

in thih s always

diffraction

1

H. Daimon et al. / 20

Fig. 4. Ga(3d)

photoelectron

diffraction

291

XPED patterns from Sl(1 I I) und TaC(I II)

pattern from Si(l ll)fi X t/3 -Ga surface at the kinetic energy 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 difference 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 crystallographic 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 measured 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

of 200 eV. The orientation

of

pattern is clearly observed in this pattern. This pattern was obtained by subtracting the 60” rotated pattern. The symmetry of this pattern is pattern was obtained C,“, but a C, symmetric when the 30” rotated pattern was subtracted. Hence, this surface produces intrinsically a threefold C,,. pattern. By the way, in the case of Si(lll), the pattern almost disappears when the 60” rotated pattern was subtracted. The directions 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

made

effects.

with

polar

These studies angle scans

are usually being in one azimuthal

ii11gk.

The observed six-fold symmetry of the pattern the Si( 111) surface is somewhat strange. Although 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 satisfactorily 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 structure 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. from

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 observed 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 1110). [loll. and [Ol l] directions. These directiona corruspond to the observed strong peaks. In the case of Si(lll)fi x 43 -Ga(3d), the pattern 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 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 ;innlvsis 1171.

4. Conclusions

Two-dimensional photoelectron diffraction patterns 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 xymmetry. They were explained qu;~litatively considuring 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)

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 Education. Science and Culture.

References [l] J.J. Barton, Phys. Rev. Lett. 61 (1988) 1356. [2] A. Szoke. in: Short Wavelength Coherent Radiation: Generation and Applications, Eds. D.T. Attwood and J. Boker, AIP Conference Proceedings No. 147 (American Institute of Physics, New York, 1986). [3] S.P. Weeks, J.E. Rowe, S.B. Christman and E.E. Chaban, Rev. Sci. Instrum. 50 (1979) 1249. [4] S. kanayama, M. Owari. E. Nakamura and Y. Nihei, Rev. Sci. Instrum. 60 (1989) 2231.

293

[5] D.E. Eastman, J.J. Donelon. N.C. Hien and F.J. Himpsel, Nucl. Instrum. Methods 172 (1980) 327. [6] D. Rieger, R.D. Schnell. W. Steinmann and V. Saile, Nucl. Instrum. Methods 208 (1983) 777. [7] H. Daimon. Rev. Sci. Instrum. 59 (1988) 545. [8] H. Daimon and S. Ino, J. Vacuum Sot. Jpn. 31 (1988) 954. [9] H. Daimon and S. Ino, Rev. Sci. Instrum. 61 (1990) 57. [lo] H. Daimon and S. Ino, Surf. Sci. 222 (1989) 274. [ll] H. Daimon and S. Ino, Vacuum 41 (1990) 215. [12] R.J. Baird. C.S. Fadley and L.F. Wagner. Phys. Rev. B 15 (1977) 666. [13] H. Namba. H. Daimon. Y. Idei. N. Kosugi, H. Kuroda, M. Taniguchi, S. Suga. Y. Murata, K. Ueyama and T. Miyahara, Rev. Sci. Instrum. 60 (1989) 1909. [14] C.S. Fadley, in: Synchrotron Radiation Research: Advances in Surface Science, Eds. R.Z. Bachrach (Plenum. New York, 1990). and references therein. [15] H. Daimon, Y. Tezuka, N. Kanada. A. Otaka. S.K. Lee, S. Ino. H. Namba and H. Kuroda, Proc. 3rd Int. Conf. on Structure of Surfaces, Milwaukee. 1990. [16] A. Kawazu and H. Sakama, Phys. Rev. B 37 (1988) 2704. [17] T. Hanada. Thesis. University of Tokyo (1988).