Core-Level Photoemission Study of Thallium Adsorbed on a Si(111)-(7×7) Surface: Valence State of Thallium and the Charge State of Surface Si Atoms

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Core-level photoemission study of thallium adsorbed on a Si(111)-(7×7) surface: Valence state of thallium and the charge state of surface Si atoms

Kazuyuki Sakamoto, Johan Eriksson, Seigi Mizuno, Nobuo Ueno, Hiroshi Tochihara and Roger Uhrberg

Linköping University Post Print

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Kazuyuki Sakamoto, Johan Eriksson, Seigi Mizuno, Nobuo Ueno, Hiroshi Tochihara and Roger Uhrberg, Core-level photoemission study of thallium adsorbed on a Si(111)-(7×7) surface: Valence state of thallium and the charge state of surface Si atoms, 2006, Physical Review B. Condensed Matter and Materials Physics, (74), 7, 075335. http://dx.doi.org/10.1103/PhysRevB.74.075335 Copyright: American Physical Society http://www.aps.org/

Postprint available at: Linköping University Electronic Press http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-36299 PHYSICAL REVIEW B 74, 075335 ͑2006͒

Ã Core-level photoemission study of thallium adsorbed on a Si„111…-„7 7… surface: Valence state of thallium and the charge state of surface Si atoms

Kazuyuki Sakamoto,1,* P. E. J. Eriksson,2 Seigi Mizuno,3 Nobuo Ueno,1 Hiroshi Tochihara,3 and R. I. G. Uhrberg2 1Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan 2Department of Physics, Chemistry and Biology, Linköping University, S-581 83 Linköping, Sweden 3Department of Molecular and Material Sciences, Kyushu University, Fukuoka 816-8580, Japan ͑Received 24 May 2006; published 31 August 2006͒ The coverage-dependent valence state of Tl adsorbed on a Si͑111͒-͑7ϫ7͒ surface and the coverage dependence of the charge states of surface Si atoms have been investigated by high-resolution core-level photoelectron spectroscopy. Although two different reconstructions were observed in low-energy electron diffraction at different coverages, a ͑1ϫ1͒ pattern at a Tl coverage of 1 monolayer ͑ML͒ and a ͑ͱ3ϫͱ3͒ pattern at a coverage of 1/3 ML, the binding energy of the Tl 5d core-level was the same at Tl coverages up to 1 ML. Taking the valence state on a ͑1ϫ1͒ surface reported in the literature into account, we conclude that the valence state of Tl is 1+, and that the 6s2 electrons of Tl are inactive as an inert pair in the Tl-Si bonding on aSi͑111͒ surface at a coverage of 1 ML and below. In the Si 2p core-level spectra, one surface component was observed on the ͑1ϫ1͒ surface, and three surface components were observed on the ͑ͱ3ϫͱ3͒ surface. The binding energies and intensities of the Si 2p surface components indicate that the charge state of the surface Si atoms on Tl/Si͑111͒-͑1ϫ1͒ is the same as that of the ͑ͱ3ϫͱ3͒ surfaces induced by the other group III metals, but they are different on the Tl/Si͑111͒-͑ͱ3ϫͱ3͒ surface.

DOI: 10.1103/PhysRevB.74.075335 PACS number͑s͒: 73.20.Ϫr, 79.60.Ϫi, 61.14.Hg, 68.43.Ϫh

I. INTRODUCTION neling microscopy ͑STM͒ depending on the adsorption 15 ͑ ϫ ͒ Motivated by the technological importance of nanometer- conditions. The 3 1 reconstruction is a typical phase ͑ ͒ scale electronic devices, self-assembling one-dimensional formed by monovalent atoms alkali metals or Ag at a cov- ͑ͱ ϫͱ ͒ and two-dimensional superstructures, formed on semicon- erage of 1/3 ML, and the 3 3 reconstruction is a typi- ductor surfaces by the adsorption of metal atoms, are of in- cal phase formed by the other group III trivalent atoms at a creasing interest. From a scientiﬁc point of view, these one- coverage of 1/3 ML. Based on the observation of these three and two-dimensional structures have attracted much atten- phases a variable valency for Tl on a Si͑111͒ surface has 15,16 tion due to the possibility of observing various exotic low- been proposed. However, so far, there is no investigation dimensional physical phenomena.1,2 The low-dimensional on the valence state of Tl on a Si͑111͒ surface, and therefore structures formed by the adsorption of group III metals on no strong evidence on the variable valence of Tl. semiconductor surfaces are candidates for such systems. So In this paper, we present a high-resolution core-level pho- far, numerous studies have been performed on the interaction toelectron spectroscopy study performed to determine the between the metals Al, Ga, and In in group III and Si sur- coverage-dependent valence state of Tl adsorbed on a faces. On a Si͑111͒-͑7ϫ7͒ surface, these three group III Si͑111͒-͑7ϫ7͒ surface and the charge states of surface Si metals have been reported to form common surface struc- atoms. In the low-energy electron diffraction ͑LEED͒ study, 3–7 a ͑1ϫ1͒ pattern was observed at a Tl coverage of 1 ML and tures such as magic cluster arrays at modest temperature, ͱ ͱ and a ͑ͱ3ϫͱ3͒ reconstruction ͓Fig. 1͑a͔͒ with an adsorbate a ͑ 3ϫ 3͒ pattern was observed at a coverage of 1/3 ML. coverage of 1/3 monolayer ͑ML͒ at higher temperatures.8–12 Recently, the study on group III metals on Si͑111͒- ͑7ϫ7͒ has been expanded to the heaviest element of this group, thallium ͑Tl͒, whose behavior differs from those of the lighter group III metals ͑Al, Ga, and In͒, i.e., Tl has a peculiar behavior in the form of the so-called “inert pair effect.” In fact, the adsorption of Tl on Si͑111͒-͑7ϫ7͒ has been reported to form a ͑1ϫ1͒ phase at a coverage of 1 ML.13–18 This phase has not been observed by the adsorption of other group III metals. On the ͑1ϫ1͒ surface, Tl atoms are adsorbed on the T4 sites of the bulk terminated unrecon- ͑ ͒ 16–18 ͑ ͒ structed Si 111 surface as shown in Fig. 1 b , and the FIG. 1. ͑Color online͒ Schematic illustrations of ͑a͒ a group III 2 6s electrons of Tl have been reported to be inactive as an metal induced Si͑111͒-͑ͱ3ϫͱ3͒ surface, and ͑b͒ the Si͑111͒- 16,18 inert pair in the Tl-Si bonding. This means that Tl be- ͑1ϫ1͒ surface formed by the adsorption of 1 ML of Tl. Large ﬁlled haves as a monovalent atom on the ͑1ϫ1͒ surface. Together circles are metal atoms, which are adsorbed on the T4 site, and the with the well-ordered ͑1ϫ1͒ surface, small ͑3ϫ1͒ and other circles are Si atoms. The thick dashed lines indicate the unit ͑ͱ3ϫͱ3͒ domains have been observed using scanning tun- cell of each surface.

FIG. 2. LEED patterns of ͑a͒ the Tl/Si͑111͒-͑1ϫ1͒ surface and ͑b͒ the Tl/Si͑111͒-͑ͱ3ϫͱ3͒ surface. The primary electron energies are ͑a͒ 53 eV and ͑b͒ 43 eV.

The binding energy of the Tl 5d core level was the same not only at these two coverages but also at coverages up to 1 ML. This result indicates that the valence state of Tl is constant at coverages of 1 ML and below. In the Si 2p core-level spectra, one surface component was observed on the ͑1ϫ1͒ surface, and three surface components on the ͑ͱ3ϫͱ3͒ surface. The binding energy and intensity of the Si 2p surface component indicate that the charge states of the surface Si atoms are almost the same on the Tl induced ͑ ͒ ͑1ϫ1͒ surface and on the Al, Ga, and In induced ͑ͱ3ϫͱ3͒ FIG. 3. Color online Coverage-dependent Tl 5d core-level spectra obtained with a photon energy of 50 eV. The uppermost surfaces. Regarding the Tl induced ͑ͱ3ϫͱ3͒ surface, the spectrum is the Tl 5d core level of the highest coverage ͑1ϫ1͒ Si 2p core-level spectra indicate that the charge states of the surface, and Tl coverage decreases as going downward. The third surface Si atoms are different from those of other group III spectrum from the bottom is the Tl 5d core level of the ͑ͱ3ϫͱ3͒ ͑ͱ ϫͱ ͒ metal induced 3 3 surfaces. surface. primary electron energy of 53 eV. Only strong and sharp ϫ1 II. EXPERIMENTAL DETAILS spots are observed with a remarkably low background inten- The high-resolution photoemission measurements were sity, and thus this pattern warrants the high quality of the ͑ ϫ ͒ performed at beamline 33 at the MAX-I synchrotron radia- 1 1 surface used in the present study. The LEED pattern ͑ ͒ tion facility in Lund, Sweden. The Tl 5d and Si 2p core-level shown in Fig. 2 b was obtained by reducing the Tl coverage ͑ ͒ ͑ ϫ ͒ spectra were obtained with an angle-resolved photoelectron by annealing the Tl/Si 111 - 1 1 surface at a temperature spectrometer with an angular resolution of ±2°. The total between 600 and 650 K. The primary electron energy used in ͑ ͒ experimental energy resolutions were ϳ180 meV for the Fig. 2 b was 43 eV. Although the background intensity is ͑ ͒ ϫͱ Tl 5d measurements and ϳ80 meV for the Si 2p measure- slightly higher than that in Fig. 2 a , the strong 3 spots ͑ ͒ ments. The Si͑111͒ sample, cut from an Sb-doped ͑n-type, observed in Fig. 2 b indicate the formation of a surface ͑ͱ ϫͱ ͒ 3 ⍀ cm͒ Si wafer, was preoxidized chemically before it was covered almost entirely with 3 3 domains, instead of small domains as reported by STM.15 The formation of a inserted into the vacuum system. In order to obtain a clean ͱ ͱ surface, we annealed the sample at 1230 K by direct resistive well-deﬁned Tl induced ͑ 3ϫ 3͒ surface was also observed heating in the vacuum chamber to remove the oxide layer, in a recent LEED study.16 and at 1520 K to remove carbon contamination from the Figure 3 shows Tl 5d core-level spectra obtained at dif- surface. After the annealing, a sharp ͑7ϫ7͒ LEED pattern ferent Tl coverage with a photon energy ͑h␯͒ of 50 eV and ͑␪ ͒ was observed, and neither the valence-band spectra nor the an emission angle e of 0°, i.e., normal emission. The Tl 5d Si 2p core-level spectra showed any indication of contami- core-level spectra were normalized using the background in- nation. Thallium was deposited from a Knudsen cell onto a tensity, which is proportional to the photon ﬂux. The upper- clean Si͑111͒-͑7ϫ7͒ surface at a substrate temperature of most spectrum shows the Tl 5d core-level emission of the 570 K. The base pressure was below 4ϫ10−11 Torr during Tl/Si͑111͒-͑1ϫ1͒ surface with a coverage of 1.0 ML, and the measurements, and below 1ϫ10−10 Torr during the Tl the spectra below were obtained by reducing the Tl coverage evaporation. by annealing the ͑1ϫ1͒ surface at temperatures between 600 and 700 K. The third spectrum from the bottom is the Tl 5d core-level spectrum of the ͑ͱ3ϫͱ3͒ surface shown in Fig. III. RESULTS AND DISCUSSION 2͑b͒. Based on the Tl coverage of the ͑1ϫ1͒ surface and the Figure 2͑a͒ shows the LEED pattern of the Tl/Si͑111͒- fact that the intensity of the ͑ͱ3ϫͱ3͒ spectrum is 1/3 of the ͑1ϫ1͒ surface obtained at a Tl coverage of 1.0 ML with a ͑1ϫ1͒ spectrum, we conclude that the Tl coverage of the

075335-2 CORE-LEVEL PHOTOEMISSION STUDY OF THALLIUM¼ PHYSICAL REVIEW B 74, 075335 ͑2006͒

͑ͱ ϫͱ ͒ 3 3 surface is 1/3 ML. As shown in Fig. 3, the Tl 5d5/2 and 5d3/2 core levels are observed at binding energies of approximately 13 and 15.2 eV in the spectrum of the ͑1 ϫ1͒ surface, and they do not shift as the Tl coverage decreases.19 Further, no structure except these two peaks is observed in any of the spectra. Taking into account that the difference in binding energies of the 5d core levels for the Tl1+ and Tl3+ is reported to be 0.58 eV,20 the result shown in Fig. 3 indicates that the valence state of Tl is identical in the coverage range investigated in the present study. Therefore, based on the valence state of Tl on the ͑1ϫ1͒ surface reported in previous studies,16,18 we conclude that the valence state of Tl is 1+ and thus the 6s2 electrons of Tl are inactive as an inert pair in the Tl-Si bonding on a Si͑111͒ surface at coverages up to 1 ML. The core-level result is not consistent with the variable valency of Tl on Si͑111͒ proposed in the literature. In order to obtain more detailed information about the Tl/Si͑111͒-͑1ϫ1͒ and ͑ͱ3ϫͱ3͒ surfaces, we have measured the Si 2p core-level spectra of these two surfaces. Fig- ure 4 shows the results obtained with h␯=135 eV at two ͑␪ ͒ different emission angles e that give a difference in the surface sensitivity. Without any processing, the experimental spectra ͑open circles͒ of the ͑1ϫ1͒ surface reveal the presence of one surface component that contributes to the shape of the spectra. That is, both the Si 2p3/2 and 2p1/2 components of S are observed as peaks in the spectrum obtained ␪ using e =60°, and the Si 2p3/2 component of S is clearly ␪ observed as a shoulder in the spectrum obtained using e =0°. Concerning the ͑ͱ3ϫͱ3͒ surface, the broad spectral feature and the presence of the long tails on both the high and low energy sides of the main peak suggest that the number of surface components of the ͑ͱ3ϫͱ3͒ surface is larger than that of the ͑1ϫ1͒ surface. Quantitative information about the components that con- tribute to the shape of the spectra is obtained by analyzing the Si 2p spectra by a standard least-squares-fitting method using spin-orbit split Voigt functions. The solid lines overlapping the data in Fig. 4 are the fitting curves. We used 608 meV for the spin-orbit splitting and an 80 meV full width at half maximum ͑FWHM͒ for the Lorentzian contri- bution for all components in the fitting procedure. The FIG. 4. ͑Color online͒ Si 2p core-level spectra of the Gaussian width of the bulk component is 210 meV ͑FWHM͒ Tl/Si͑111͒-͑1ϫ1͒ and ͑ͱ3ϫͱ3͒ surfaces measured with h␯ ͑ ϫ ͒ ͑ ͒ in the spectra of the 1 1 surface, and 335 meV FWHM =135 eV at ␪e =0° and 60°. The open circles are the experimental in those of the ͑ͱ3ϫͱ3͒ surface. The Gaussian width of the data, and the solid lines overlapping the open circles are the fitting surface component is 195 meV on the ͑1ϫ1͒ surface, and curves. Each component is indicated by different shading. The dif- the widths of the surface components are between 335 and ference between the experimental data and fitting result is shown by 400 meV on the ͑ͱ3ϫͱ3͒ surface. A polynomial back- a solid line at the bottom of each spectrum. ground was subtracted before the decomposition of each the binding energy of S and S1 are −300 meV relative to the spectrum, and each component is indicated by different shad- B component. The relative binding energies of the S2 and S3 ing. The difference between the experimental data and fitting components, which are observed on the ͑ͱ3ϫͱ3͒ surface, result is indicated by a solid line at the bottom of each spec- are −630 and 285 meV. The intensity ratio of the B and S trum. ␪ components is approximately 1:0.91 at e =60°, and approxi- From the result of the fitting procedure, we can conclude mately 1:0.24 at ␪ =0° on the ͑1ϫ1͒ surface. On the ͑ ϫ ͒ e that the 1 1 surface has one surface component and the ͑ͱ3ϫͱ3͒ surface, the intensities of S1, S2, and S3 relative to ͑ͱ3ϫͱ3͒ surface has at least three surface components. ␪ the bulk one are 0.25, 0.09, and 0.11 at e =60°, and 0.10, ␪ Among the surface components, the S component of the 0.06, and 0.07 at e =0°. ͑1ϫ1͒ surface and the S1 component of the ͑ͱ3ϫͱ3͒ sur- By comparing the Si 2p core-level spectra of the Al, Ga, face are observed at the same relative binding energy, i.e., and In induced Si͑111͒-͑ͱ3ϫͱ3͒ surfaces21–25 and the re-

075335-3 SAKAMOTO et al. PHYSICAL REVIEW B 74, 075335 ͑2006͒ sults shown in Fig. 4, one notices that the spectral shapes and tional surface components ͑S2 and S3͒ and their binding en- the fitting results of the Tl/Si͑111͒-͑1ϫ1͒ surface closely ergies, suggests that the charge transfer between surface Si resemble those of the ͑ͱ3ϫͱ3͒ surfaces induced by the atoms might lead to a removal of dangling bonds.29 We pro- other group III metals. That is, a surface component, whose pose that the charge redistribution in the Si surface layer relative binding energy is similar to that observed in Fig. 4 is contributes to the stability of the monovalent Tl induced observed on the Al, Ga, and In induced Si͑111͒-͑ͱ3ϫͱ3͒ Si͑111͒-͑ͱ3ϫͱ3͒ surface. surfaces. The origin of the surface component of Al, Ga, and In induced ͑ͱ3ϫͱ3͒ has been reported to be the first layer Si IV. CONCLUSION atoms that are negatively charged due to charge transfer from In conclusion, we have investigated the coverage- metal atoms. Since the unreconstructed Si͑111͒-͑1ϫ1͒ dependence of the valence state of Tl adsorbed on a structure is common to both the Tl/Si͑111͒-͑1ϫ1͒ surface Si͑111͒-͑7ϫ7͒ surface and the charge states of surface Si and the other group III metal induced ͑ͱ3ϫͱ3͒ surfaces ͑the atoms by high-resolution core-level photoelectron spectros- only difference in structure is the adsorbate coverage͒, the copy. The binding energy of the Tl 5d core level of the similarity in binding energies of the surface components sug- ͑1ϫ1͒ surface obtained at a Tl coverage of 1 ML was the gests that the charge states of the first layer Si atoms are same as that of the ͑ͱ3ϫͱ3͒ surface obtained at a coverage almost the same on these surfaces. On the unreconstructed of 1/3 ML. Taking into account that the binding energy of Si͑111͒-͑1ϫ1͒ clean surface, the high density of half-filled the Tl 5d core level was the same at not only these two dangling bonds makes this surface unstable ͑there is one dan- coverages but at coverages up to 1 ML, we conclude that the gling bond per one surface Si atom͒. The charge transfer valence state of Tl is +1 in this coverage range. This means from metal atoms might be the origin of the stabilization of that the 6s2 electrons of Tl are inactive as an inert pair in the the unreconstructed Si͑111͒-͑1ϫ1͒ structure underneath the Tl-Si bonding on a Si͑111͒ surface for coverages up to 1 ML, group III metal overlayer.26 A semiquantitative correlation and thus that there is no variable valency for Tl on a Si͑111͒ between the charge transfer and the adsorbate-induced core- surface. Regarding the Si 2p core-level spectra, one surface level shifts has been derived empirically for the Si 2p core component was observed on the ͑1ϫ1͒ surface. By compar- level as ␦E/A eV=␦q,27,28 where A is a constant with a value ing the binding energy and intensity of this surface compo- between 2 and 3.4, ␦q is the charge transfer, and ␦E is the nent with those of the surface components of the Al/ ,Ga/, core level shift. Taking this relation and the relative binding and In/Si͑111͒-͑ͱ3ϫͱ3͒ surfaces, the origin of this compo- energies of the surface components into account, we propose nent is concluded to be the surface Si atoms that are nega- that a charge transfer of approximately 0.1 electron per sur- tively charged due to charge transfer from Tl atoms. Further, face Si atom stabilizes the unreconstructed Si͑111͒-͑1ϫ1͒ the quite similar binding energies of the surface components structure on which the group III metal atoms are located. suggest that the charge states of the first layer Si atoms are Regarding the Tl/Si͑111͒-͑ͱ3ϫͱ3͒ surface, the core- almost the same on the Tl/Si͑111͒-͑1ϫ1͒ and Al/, Ga/, and level result is completely different from that of the trivalent In/Si͑111͒-͑ͱ3ϫͱ3͒ surfaces. Based on a semiquantitative Al, Ga, or In atoms induced ͑ͱ3ϫͱ3͒ surface. This result correlation between the charge transfer and the adsorbate- supports that the valence state of Tl on a Si͑111͒-͑ͱ3ϫͱ3͒ induced core-level shifts, we propose that a charge transfer surface is +1, a value that is different from the valence state of approximately 0.1 electron per surface Si atom stabilizes of the other group III metal atoms. The relative binding en- the unreconstructed Si͑111͒-͑1ϫ1͒ structure underneath the ergy of the S1 component is the same as that of the S com- group III metal overlayers. On the Tl/Si͑111͒-͑ͱ3ϫͱ3͒ sur- ponent, but the relative intensity of S1 is approximately 1/3 face, three surface components were observed in the Si 2p of that of S. Based on this result and the 1+ valence state of core-level spectra. The binding energies and relative intensi- Tl at a coverage of 1/3 ML, we conclude that Tl atoms affect ties of these surface components indicate that the charge dis- only 1/3 of the first layer Si atoms on the ͑ͱ3ϫͱ3͒ surface. tribution in the surface Si layer on Tl/Si͑111͒-͑ͱ3ϫͱ3͒ is However, although two dangling bonds per unit cell are ex- different from that on the other group III metal induced pected to remain on the surface, the Tl/Si͑111͒-͑ͱ3ϫͱ3͒ ͑ͱ3ϫͱ3͒ surfaces. surface is quite stable. A ͑ͱ3ϫͱ3͒ LEED pattern and Si 2p core-level spectra with the same qualities as those shown in ACKNOWLEDGMENTS Figs. 2 and 4 were observed more than 12 h after the sample Experimental support from T. Balasubramanian and the preparation. One of the origins for this stability might be MAX-lab staff, and suggestions on the sample preparation the charge distribution in the surface layer of Tl/Si͑111͒- ͱ ͱ from S. Hatta and A. Visikovskiy are gratefully acknowl- ͑ 3ϫ 3͒ that is different from those of the trivalent metal edged. This work was financially supported by the Ministry induced ͑ͱ3ϫͱ3͒ surfaces. The difference in charge distri- of Education, Culture, Sports, Science and Technology of the bution, which is obvious from the presence of the two addi- Japanese Government, and the Swedish Research Council.

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