Electronic structure of bismuth terminated InAs(1 0 0)

Surface Science 603 (2009) 190–196

Contents lists available at ScienceDirect

Surface Science journal homepage: www.elsevier.com/locate/susc

Electronic structure of bismuth terminated InAs(1 0 0) Karolina Szamota-Leandersson *, Mats Leandersson, Pål Palmgren, Mats Göthelid, Ulf O. Karlsson Materials Physics, MAP, ICT, Royal Institute of Technology, Electrum 229, SE-164 40 Kista, Sweden

a r t i c l e

i n f o

Article history: Received 11 July 2008 Accepted for publication 31 October 2008 Available online 9 November 2008 Keywords: Adatoms Indium arsenide Bismuth Photoemission 2 DEG Reconstruction

a b s t r a c t Deposition of Bi onto (4  2)/c(8  2)-InAs(1 0 0) and subsequent annealing results in a (2  6) surface reconstruction as seen by low electron energy diffraction. The Bi condensation eliminates the original (4  2) surface reconstruction and creates a new structure including Bi-dimers. This surface is metallic and hosts a charge accumulation layer seen through photoemission intensity near the Fermi level. The accumulation layer is located in the bulk region below the surface, but the intensity of the Fermi level structure is strongly dependent on the surface order. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Indium arsenide is a narrow band-gap semiconductor with unique properties such as very high electron mobility and high carrier densities. An important characteristic for this semiconductor is that it forms a charge accumulation layer in the surface region. The (1 1 0) surface has been found to form such layers after adsorption of various metals [1–3] while they are directly found on the pristine (1 0 0) and (1 1 1) surfaces [4]. It was suggested that an accumulation layer on the (1 0 0) surface is induced by donor-like intrinsic surface states whose density is determined by the surface reconstruction [5,6]. Theoretical calculations confirm the importance of native defects and the proportionality between host anion and cation covalent radii on the ‘averaged band edge’ for accumulation layer formation [7]. An experimental study of InAs(1 0 0) has shown that the charge density is an order of magnitude higher on the As-terminated surface compared to the In-terminated surface, due to differences in the Fermi level pinning [4]. The accumulation layer may also be induced by impurity ad-atoms or through adsorbed ordered overlayers. In Ref. [8], it was shown that one monolayer of Pb on the In-rich InAs(1 0 0) surface leads to a (2  1)/ (1  4)-Pb/InAs(1 0 0) surface reconstruction, and a strong accumulation layer is present. The observed intensity was much stronger than from the pristine surface, although there was no additional band bending. This was explained as Pb acting as sub-surface donors giving an improved debye screening that moves the total charge closer to the surface.

A general property of III-V (1 0 0) surfaces is that their surface reconstructions depend on the surface composition and thermal treatment [9,10]. An example is the indium-rich (4  2)/c(8  2) reconstruction, recently solved by a surface X-ray diffraction (SXDR) study by Kumpf [11] and the arsenic-rich (2  4)InAs(1 0 0) surface described by the b(2  4) reconstruction (including adsorbed As-dimer in the top layer and another As-dimer in the third layer) [12]. For both surfaces, angle-resolved photoemission studies have revealed several surface states [13,14]. A Bi-induced (2  6)/c(2  12)-Bi/InAs(1 0 0) reconstruction was recently observed above 1 ML Bi coverage, and a structural model was proposed including two layers with dimers [15]. In the present study we show a direct dependence between the surface order and the bismuth induced accumulation layer on the In-terminated InAs(1 0 0), using low energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES). Evaporating a thin layer of bismuth is not sufficient to create the accumulation layer, while removing excess bismuth and creating the Bi-induced (2  6)/c(2  12) surface reconstruction, the accumulation layer is seen as a weak photoemission peak above the conduction band minimum (CBM). The presence of an accumulation layer at the surface is important to further the understanding of the surface stabilization and development of proper surface models. ARPES was also used to study the bismuth induced surface states in the valence band, with special interest in the top layer dimer bismuth state. 2. Experimental

* Corresponding author. E-mail address: [email protected] (K. Szamota-Leandersson). 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.10.042

The experiments were performed at Beamline 33, MAX-lab synchrotron radiation facility in Lund, Sweden. The photon energy

191

K. Szamota-Leandersson et al. / Surface Science 603 (2009) 190–196

range of the beamline is 15–200 eV and the end station is equipped with a goniometer-mounted angle-resolving analyzer with a variable angular resolution [16,17]. InAs(1 0 0) samples were cut from n-type single crystal wafers (Wafer Technology Ltd., UK) doped with S with carrier concentration of 4.4  1016 cm3. In situ cleaning comprised initial degassing, cycles of simultaneous argon ion bombardment (Ar+, 0.7 keV) and annealing (400 °C). The cleanliness and long-range order of the surface was monitored by low energy electron diffraction (LEED), valence band (VB) and core level

(CL) spectroscopy. The surface preparation produced a sharp and well-defined (4  2)/c(8  2) structure in LEED. Bi was deposited at room temperature (RT) from a well outgassed source, and the pressure in the chamber during evaporation was around 2  109 Torr. The evaporation rate was calibrated with a quartz-crystal microbalance (QCM) and the thickness of the deposited layer was estimated from In 4d, As 3d and Bi 5d spectra. After condensation of 3.5 monolayer (ML), a weak blurry (4  1/1  3) pattern was observed in LEED. Annealing at 290 °C

Fig. 1. (a) LEED (64 eV) from the clean (4  2)/c(8  2)-InAs(1 0 0) surface, (b) LEED (58 eV) from (4  1)/(1  3)-Bi/InAs(1 0 0) surface with about 3.5 ML of Bi, (c) LEED (65 eV) from (2  6)/c(2  12)-Bi/InAs(1 0 0). Post annealing was done at 290 °C.

Bi 5d, hν =60eV

x3 C

60o

B A

2x6 x3

Intensity / a.u.

0o

60

o

4x1/1x3 0o

29

28

27

26

25

24

23

22

21

Binding energy / eV Fig. 2. The 5d core level of the Bismuth; hm = 60 eV. The deconvolution parameters are given in the text.

K. Szamota-Leandersson et al. / Surface Science 603 (2009) 190–196

for 2 min produced a (2  6)/c(2  12) reconstruction, with 1 ML of Bi left on the surface, where one monolayer of bismuth is equal to 4.14  1018 atoms/m2. Angle resolved valence band spectra were measured along the C  J [1 1 0] and C  J0 [1 1 0] azimuths using photon energies between 21 and 24 eV. The samples were oriented using LEED. The structure at the Fermi level (M) was mapped along the C  X  C direction in the Brillouin zone, using photon energies between 20 eV and 33 eV. CL spectra were measured in normal emission (NE) and 60° off normal emission, using 60 eV photons. The binding energies are given with respect to the Fermi level (EF), measured on a clean Ta foil in good electrical contact with the sample. The overall resolution for structure M was 40 meV and 70 meV for CL and VB. The angular resolution was ±0.8° for M and ±1.2° for VB and CL.

3. Results and discussion After preparation, the clean InAs(1 0 0) surface produced a sharp and well-defined (4  2)/c(8  2) LEED pattern as shown in Fig. 1a, in agreement with previous observations [6,18]. After condensation of 3.5 ML Bi, the LEED pattern was weak and blurry (4  1)/(1  3) (Fig. 1b), with a three-symmetry along the [0 1 1] direction (the extra spots are marked by arrows), and a weak but visible foursymmetry along the [0 1 1] direction. The 4 symmetry was observed around 43 eV electron energy and is not shown here. After annealing at 290 °C, the LEED pattern showed a sharp (2  6)/ c(2  12) reconstruction (Fig. 1c), (with two-symmetry along the [0 1 1] direction and six-symmetry along the [0 1 1] direction). Fig. 2 shows Bi 5d spectra collected with 60 eV photons at normal and 60° emission angles from the (4  1)/(1  3) and (2  6) surfaces. A numerical fitting with three components was successful. The following parameters were used: Lorentzian width (WL) 0.23 eV, spin-orbit splitting (SO) 3.05 eV, Branching ratio (BR) 1.42 and Gaussian width (WG) 0.42 eV. First, we turn to the spectra from (2  6). The shape of the (2  6) is very similar to Ref. [15], what allows us to interpret structures in spectra according to the structural model proposed by Laukkanen [15]. According to this, the bismuth in the surface is arranged in two layers with two kinds of dimers: Bi-dimers along [0 1 1] direction in the topmost layer and Bi-dimers along [0 1 1] in the second layer. Based on this, the following assignment in our spectra was given: structures at binding energies 26.68 eV, 27.15 eV and 27.50 eV were assigned to emission from second layer Bi bonded to first layer Bi-dimers (A), second layer Bi-dimers (B) and first layer Bi-dimer (C), respectively. The intensity variations induced by the changed emission angle confirms the multilayer character of this surface reconstruction and underlines the assignment of the different components. It should be noted that the changes in relative intensity we observe differs somewhat from those presented by Laukkanen et al. [15]. We use a slightly different photon energy (60 eV instead of 70 eV as in Ref. [15]), leading to differences in photoelectron diffraction which is known to be strong for Bi. Therefore, we conclude that our CL spectra are in line with the model proposed by Laukannen [15] for the top layer arrangement. As the model is limited and does not describe the character of back bonds, we do not draw any further conclusions. The (4  1)/(1  3) spectra (bottom of Fig. 2), display narrow symmetric peaks. The absence of a structural model makes it difficult to analyze the bismuth arrangement from Bi 5d spectra. The absence of different structures in the spectra indicates that all Bi atoms have a similar chemical bonding. This is not surprising however because of the large size of Bi atoms. Therefore this (4  1)/ (1  3) structure has only short range order as earlier indicated by LEED.

Valence band spectra recorded at normal emission using 21 eV photon energy, from the InAs(1 0 0) (4  2), Bi/InAs(1 0 0) (1  3) and Bi/InAs(1 0 0) (2  6) surfaces are shown in Fig. 3. Fig. 4 shows angular series along the C  J 0 direction from the (4  1)/(1  3) and the (2  6) surfaces. The shape of spectra from the clean InAs(1 0 0) (4  2) agree well with previously published results [13,14]. In the NE-spectrum A1 and So are marked. The first of these was earlier identified as a surface state [13], and So (weak intensity at the Fermi level) was earlier observed and interpreted as emission from the accumulation layer on the clean surface [6]. Several Bi-induced features in the spectra are seen in Figs. 3 and 4. The vertical line (Fig. 4) at 3 eV marks emission the from Bi 6p state, observed on Bi(1 1 1) samples [19]. The decrease in A1 intensity after evaporation (see Fig. 3) is pronounced but not surprising, since A1 was only observed on the In-terminated surface and not on the As-terminated clean surface. In our spectrum (Fig. 4) the character of Sx is similar to that of S on the As-terminated InAs(1 0 0) surface with respect to dispersion [13,2] (the dispersion

00 B1 B2 A1

(4x2) Intensity / a.u.

192

Sx So Sn

(4x1)/ (1x3) M

(2x6)

0,5

8

0,0

6

-0,5

4

2

0

-2

Binding energy / eV Fig. 3. Normal emission valence band spectra collected at RT using 21 eV photon energy from the (4  2)/c(8  2)-InAs(1 0 0) clean surface, the (4  1)/(1  3)-Bi/ InAs(1 0 0) surface with about 3.5 ML of Bi, and the post-evaporation annealed (2  6)/c(2  12)-Bi/InAs(1 0 0) surface. The main peaks are marked in the spectra. The labeled peaks are described in the text.

193

K. Szamota-Leandersson et al. / Surface Science 603 (2009) 190–196

Γ - J' Sx

θe

o

57

46o Sx S2

Intensity / a.u.

36o

25o

o

21

Sx

14o

10o Bi6p

(2x6) (4x1)/(1x3)

9

8

o

0

Sn

3.5 ML + Ann. 3.5 ML

7

6

5

4

3

2

1

0

-1

-2

-3

-4

Binding energy / eV Fig. 4. Comparison of RT ARPES spectra from the (4  1)/(1  3)-Bi/InAs(0 0 1) and (2  6)/c(2  12)-Bi/InAs(1 0 0) surfaces along (C  J 0 ). Photon energy is 21 eV. The surface states are describe in the text. Dotted lines marks Bi 6p and Sn respectively.

of Sx is marked in Fig. 4). From this we suggest that Sx originates from the bismuth induced dimer, and that, supporting the Laukannen model, it originates from second layer dimers. Sx appears al-

ready on the (1  3) but becomes stronger on the (2  6) surface, hence the (1  3) order is tentatively assigned to Bi-dimerization already after deposition (Fig. 4). S2, at binding energy 3.4 eV on

K. Szamota-Leandersson et al. / Surface Science 603 (2009) 190–196

the (4  2) surface [13], was assigned as a surface state with small dispersion, originating either from the back bonds or dimer-bonds according to Ref. [13]. In Fig. 4, S2 is observed at 4 eV binding energy and 36° emission angle (kII = 1.077 Å1) along the C  J 0 -direction, both for (2  6) and (1  3). From earlier studies [13], the As(S20 ) on the As-terminated surface has been assigned to similar character as In(S2). However, on the As-terminated surface it was observed at higher binding energy and was pronounced at 36° off normal emission. We do not see any emission from S2 along (C  J) for (1  3) (not shown) and at present we do not have enough data for (2  6) to conclude the explicit character of S2. However similarities between S2 and As(S20 ) indicates that the back bonds are already influenced by bismuth at room temperature. In the NE-emission spectrum for the (4  2) there is a shoulder marked Sn close to the valence band maximum (VBM). On the clean surface, the VBM is located round 0.5 eV below the EF, (which is higher than the 0.57 eV given in Ref. [6]. Earlier studies have shown a surface state in this binding energy region, referred to as n2 [14]. Bi condensation increases the Sn intensity (Fig. 3) but it decreases to a weak shoulder after the subsequent annealed surface (2  6). On the Bi/InAs(1 1 0) (1  1) surface, the highest occupied state at binding energy 0.38 eV was identified as the topmost occupied dangling bonds, and in Ref. [14] n2 was assigned to the topmost occupied structure. It is thus not impossible that the (4  1)/(1  3) surface is terminated with Bi-dangling bonds, even though the emission from this state is located at about 0.2 eV higher binding energy than the emission from the (1  1)-Bi/InAs(1 1 0) dangling bonds. The similarities between the valence band from (1  3) and from (2  6) suggest that the (1  3) structure bears similarities with the more stable (2  6)/c(2  12) structure. As mentioned earlier, the structural model for (2  6) [15] does not discuss the third layer arrangement but our data suggest that the third layer is influenced by bismuth as the S2 shifts its binding energy compared to the clean surface. Finally, we turn our attention to the region closest to the Fermi level. In a previous study of the clean InAs(1 0 0) (4  2) surface, the surface exhibited a strong downward band bending with the Fermi level EF pinned 250 meV above CBM [13], or, as shown in Ref. [6], 210 meV above CBM. Our results show an EF pinning; around 150 meV above CBM and despite this small band bending, similar to earlier results [6], the accumulation layer is clearly observed. Upon deposition and formation of the (1  3) phase, EF shifts and is pinned 310 meV above CBM, and there the weak metallic edge, however it is intensity is weaker than So. The thermal treatment to form the (2  6) reconstruction changes the line shape and a clear emission, M, is observed. The emission from M is confined in a narrow phase range around the C point (Fig. 5), ±2°, which corresponds to ±0.07 Å1. The intensity display a strong photon energy dependence, as shown in Fig. 6 with an apparent maximum at 21 eV. Below 20 eV, higher-order contributions obscure the spectra making it difficult to determine the exact behavior. The resonant behavior of the cross section matches the maxima for the accumulation layer state on InAs(1 1 1)A [4], but with an apparent maximum at 20 eV instead of 21 eV. Using a free electron like model for the final bands and the symmetries of the bulk and surface Brillouin zone, similar to Ref. [4], we can state that the observed emission does not come from surface states but originates from excitations at the center of the bulk Brillouin zone. We can therefore conclude that the emission is associated with conduction band states within the potential well formed by the band bending. The changes in the cross section maxima of the charge accumulation peak on InAs(1 0 0) (around 20 eV) [4] compared to 21 eV observed in our case, must indicate a slight change in the surface potential. The fact that the structure is detectable only in a narrow

part of the Brillouin zone, i.e. strong band dispersion at CBM, suggests a low density of states. The intensity variation with respect to the photon energy shows that the excitations are of direct inter band type. These observations show that M originates from a charge accumulation layer. The apparent increase in photoemission intensity of M from Bi/ InAs(1 0 0) (2  6) compared to InAs(1 0 0) (4  2) is intriguing. Previous photoemission studies of charge accumulation layers have indicated that the accumulation extended up to few tens of nm in the bulk and that the weak signal in photoemission is meanfree-path limited [1]. Theoretical calculations put forward an explanation for the origin of the accumulation layer on clean surfaces, as due to the native defect-nature of surface state [7]. The experimentally found accumulation layer on the S/InAs(1 0 0)

(2x6)-Bi/InAs(100)

hν =21eV

θe /deg

+2.0

+1.0 Intensity / a.u.

194

+0.5

Γ 0.0

-0.5

-1.0

-2.0 0.50

0.25

0.00

-0.25

-0.50

Binding energy / eV Fig. 5. ARPES spectra of the M feature. Spectrum from clean InAs(1 0 0) as dotted line.

K. Szamota-Leandersson et al. / Surface Science 603 (2009) 190–196

(2  1) surface, terminated by 1 ML of sulphur, was explained by sulphur-induced ionized states related to defects [20]. For a large band bending of 540 meV, the authors calculated that for the (2  1) a surface density of states is equals to of 6.7  1012 cm2 (for a surface density of atoms of 5.4  1014 cm2), meaning that the surface donor density is only about 1.2% of the sulphur-atom density [20]. This means that the fraction of sites with ionized donors in the surface is about 1.2%. Moreover, the authors calculated that the fraction of sites with ionized donors for the (2  4) and (4  2) reconstructions are 0.2%, and 0.1%, when band bending is about 175 meV and 100 meV, respectively [20]. For the bismuth– terminated (2  6) surface, the bismuth coverage was estimated to about 1 ML in our experiments and 1.3 ML by Laukannen [15]. The band bending observed in our case is much lower than for the sulphur-terminated surface. One would thus expect to see that a small fraction of Bi ought to give a strong band bending. It is thus

(2x6)-Bi/InAs(100)

hν 33eV 31eV 30eV

Intensity / a.u.

29eV

195

striking that we do not observe emission from an accumulation layer for the Bi-terminated (4  1)/(1  3) surface. Instead, we observe a strong requirement in surface order for the accumulation layer. Therefore, at this stage we will adopt the explanation of appearance of accumulation layer due to changes in the surface potential. It was suggested [21] that surface preparation by additional surface donors or N vacancies on InN can change the downward band bending due to narrowing of the accumulation layer without changing it’s charge density i.e. varying the so-called trapping potential. In our case, increasing the ordering at the surface is directly connected to the appearance of the accumulation layer. Therefore we suggest that the bismuth ordered overlayer acts as an additional source of electrons, similar to a previous study of Pb/ InAs(1 0 0) (2  1)/(1  4) [8]. These electrons will increase the charge density within the accumulation layer and shorten the screening length, thus moving the potential well and the electrons closer to the surface. The charge transfer to the accumulation layer and the charge arrangement must be connected to the sharpness of the interface, which is at its highest when the surface reconstruction is fully developed. Furthermore, it has been put forward by Getzlaff et al. [22], that the donor level from the metal varies with coverage due to hybridization. This hybridization would then be most favorable within ordered monolayers and less favorable when the metal coverage increases or the surface order is poor. It is known that electrons trapped in the accumulation layer are confined in the direction perpendicular to the surface, while they can be described by free electron waves parallel to the surface, approximately and in the simplest cases. Recent experimental results for InN [21] show a non-parabolicity of the states in the accumulation layer around C, mimicking the characteristic shape of the conduction band. The experimental results have shown two states in the quantum well, which is consistent with previous observation for InAs e.g. for Cs/InAs(1 1 0) [23], where also two states were observed. In our experiment, we do not observe such states, due to the low density of states in the accumulation layer. Also the conditions of the experiments, such as RT and insufficient resolution might be a reason. The low density of states in the accumulation layer might be connected to changes in the surface coulomb potential as the Bi–Bi-dimer is created.

28eV 4. Conclusion

23eV

22eV

We have studied the electronic structure of the Bi-induced (2  6) reconstruction on InAs(1 0 0). A surface state related to dimer bond state has been observed, and the experimental data supports the dimer model proposed by Laukkanen [15]. Deposited Bi layers form a weak (4  1)/(1  3) structure with electronic similarities to the (2  6) structure, indicating an early dimer formation. We also observed the creation of a charge accumulation layer below the (2  6) reconstruction. Acknowledgments

21eV

20eV 0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

Binding energy / eV Fig. 6. Spectra of the Fermi level feature M for different photon energies along the C  X direction.

We would like to thank Dr. Balasubramanian Thiagarajan and Stefan Singer for their assistance during the experiments and helpful discussions, as well as to the assistance from the MAX-lab staff. We kindly acknowledge financial support from the Swedish Research Council (VR) and the Göran Gustafsson Foundation.

References [1] M. Morgenstern, M. Getzlaff, D. Haude, R. Wiesendanger, R.L. Johnson, Phys. Rev. B 61 (2000) 13805. [2] M.G. Betti, V. Corradini, U. Del Pennino, V. De Renzi, P. Fantini, C. Mariani, Phys. Rev. B 58 (1998) R4231.

196

K. Szamota-Leandersson et al. / Surface Science 603 (2009) 190–196

[3] V.Yu. Aristov, G. Le Lay, V.M. Zhilin, G. Indlekofer, C. Grupp, A. Taleb-Ibrahimi, P. Soukiassian, Phys. Rev. B 60 (1999) 7752. [4] L.Ö. Olsson, C.B.M. Andersson, M.C. Håkansson, J. Kanski, L. Ilver, U.O. Karlsson, Phys. Rev. Lett. 76 (1996) 3626. [5] M. Noguchi, K. Hirakawa, T. Ikoma, Phys. Rev. Lett. 66 (1991) 2243. [6] P. De Padova, C. Quaresima, P. Perfetti, R. Larciprete, R. Bronchier, C. Richter, I. Ilakovac, P. Bemcok, C. Teodorescu, V.Yu. Aristov, R.L. Johnson, K. Hricovini, Surf. Sci. 482–485 (2001) 587. [7] A. Höglund, C.W.M. Castleton, M. Göthelid, B. Johansson, S. Mirbt, Phys. Rev. B 74 (2006) 0753332. [8] J.M. Layet, M. Carrere, H.J. Kim, R.L. Johnson, R. Belkhou, V. Zhilin, V.Yu. Aristov, G. Le Lay, Surf. Sci. 402–404 (1998) 724. [9] C.F. McConville, T.S. Jones, F.M. Leibsle, S.M. Driver, T.C.Q. Noakes, M.O. Schweitzer, N.V. Richardson, Phys. Rev. B 50 (1994) 14965. [10] M.T. Sieger, T. Miller, T.-C. Chiang, Phys. Rev. B 52 (1995) 8256; G. Franklin, D.H. Rich, A. Samsavar, E.S. Hirschorn, F.M. Leibsle, T. Miller, T.-C. Chiang, Phys. Rev. B 41 (1990) 12619; D.K. Biergelsen, R.D. Bringans, J.E. Northrup, L.-E. Swartz, Phys. Rev. B 41 (1990) 5701. [11] C. Kumpf, D. Smilgies, E. Landemark, M. Nielsen, R. Feidenhans’l, O. Bunk, J.H. Zeysing, Y. Su, R. L Johnson, L. Cao, J. Zegenhagen, B.O. Fimland, L.D. Marks, D. Ellis, Phys. Rev. B 64 (2001) 075307.

[12] M. Göthelid, Y. Garreau, M. Sauvage-Simkin, R. Pinchaux, A. Cricenti, G. Le Lay, Phys. Rev. B 59 (1999) 15285. [13] M.C. Håkansson, L.S.O. Johansson, C.B.M. Andersson, U.O. Karlsson, L.Ö. Olsson, J. Kanski, L. Ilver, P.O. Nilsson, Surf. Sci. 374 (1997) 73. [14] P. De Padova, P. Perfetti, C. Quaresima, C. Richter, O. Heckmann, M. Zerrouki, R.L. Johnsson, K. Hricovini, Surf. Sci. 532–535 (2003) 837. [15] P. Laukkanen, M. Ahola, M. Kuzmin, R.E. Perälä, I.J. Väyrynen, J. Sadowski, Surf. Sci. 598 (2005) L361. [16] B.N. Jensen, S.M. Butorin, T. Kaurila, R. Nyholm L.I. Johansson, Nucl. Instr. Meth. Phys. Res. A 394 (1997) 243. [17] P. Butcher, R. Uhrberg, Phys. World Sept. (1995) 48. [18] C. Kendrick, G. Le Lay, A. Kahn, Phys. Rev. B 54 (1996) 17877. [19] F. Patthey, W.-D. Schneider, H. Micklitz, Phys. Rev. B 49 (1994) 11293. [20] M.J. Lowe, T.D. Veal, A.P. Mowbray, C.F. McConville, Surf. Sci. 544 (2003) 320. [21] L. Colakerol, T.D. Veal, H.-K. Jeong, L. Plucinski, A. DeMasi, T. Learmonth, P.-A. Glans, S. Wang, Y. Zhang, L.F.J. Piper, P.H. Jefferson, A. Fedorov, T.-C. Chen, T.D. Moustakas, C.F. McConville, K. Smith, Phys. Rev. Lett. 97 (2006) 237601. [22] M. Getzlaff, M. Morgenstern, C. Meyer, R. Brochier, R.L. Johnson, R. Wiesendanger, Phys. Rev. B 63 (2001) 205305. [23] V.Yu. Aristov, M. Grehk, V.M. Zhilin, A. Taleb-Ibrahimi, G. Indlekofer, Z. Hurych, G. Le Lay, P. Soukiassian, Appl. Surf. Sci. 104/105 (1996) 73.