Surface structures and growth mode of the CuSi(100)2 × 1 surface depending on heat treatment

ELZVIER

Surface Science 336 (1995) 76-84

Surface structures and growth mode of the Cu/Si(100)2 surface depending on heat treatment T. Ikeda, Y. Kawashima, H. Itoh, T. Ichinokawa

X

1

*

Department of Applied Physics, Waseda Uniuersity, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169, Japan

Received 24 December 1994; accepted for publication 1 March 1995

Abstract New surface structures of the Cu/Si(lOO) surface were found by low-energy electron diffraction after quenching from 500°C for Cu-deposited surfaces at 0.3 to 10 monolayers (ML). At room temperature, Cu was intermixed with Si and no defined interface structure was observed at coverages higher than a few ML. However, the growth mode in the Cu/Si(100)2 X 1 surface at 500°C is of the Volmer-Weber type and a part of the Cu atoms deposited on the surface diffuses into the bulk at 500°C in addition to island formation on the surface. The Cu atoms diffused into the bulk are segregated to the surface by quenching and the segregated Cu atoms form several superstructures depending on cooling speed and annealing time. As a result, 2 X 2 + 6 X 2, 2 X 2 + 6 X 5, 2 X 2 + 6 X 5 + 10 X 5, and 2 X 2 + 10 X 5 structures appear in the ranges of the Cu/Si Auger ratio between 0.05 and 0.08, 0.08 and 0.14, 0.14 and 0.18, and 0.18 and 0.2, respectively. There are one-to-one correspondences between surface structures and surface Cu concentrations. The growth mode and surface structures of the Cu/Si surface are characterized by a large diffusion coefficient of interstitial Cu atoms in Si as similar to that of the Ni/Si surface. Auger electron spectroscopy; Copper; Epitaxy; Low energy electron diffraction (LEED); Metal-semiconductor Silicon; Surface segregation

Keywords:

1. Introduction The structures and growth modes of silicon-metal interfaces have been investigated for basic interest of epitaxial growth and technical importance in the semiconductor industry. Particularly, intensive attention has been devoted to the Cu/Si(lll) system recently [l-22], leading to an increasing number of papers. For the Cu on Si(111)7 X 7 system, intermixing of Cu with Si occurs at room temperature at an early

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Corresponding author.

interfaces;

stage of deposition. However, details on the depth profile of the intermixing layer have not been revealed. At high temperature (200-6OO”C), a quasi-5 x 5 layer is formed by annealing at the surface of the one monolayer (ML) of Cu on the Si(111)7 X 7 system which has an incommensurate structure and has been studied in a number of papers [12-221. Above 1 ML, three-dimensional (3D) islands nucleate on the quasi-5 X 5 layer with the StranskiKrastanov mode. However, there are many varieties in the models of the incommensurate structure and this structure has not been revealed. Compared to the Cu/Si(lll) interface, experiments on the Cu/Si(lOO) surface are very few

0039-6028/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0039-602X(95)00400-9

T. Ikeda et al. /Surface

[4,23-261 and no well defined structure has been reported at any coverage and any temperature. Above 500°C it was reported [23] that deposited Cu forms three-dimensional islands by the Volmer-Weber mode and the islatds are covered by a Si layer with a thickness of 50 A based on ex situ observations by scanning electron microscopy @EM) and transmission electron microscopy (TEM). In the present paper, new phases of 6 X 2, 6 X 5, and 10 X 5 mixed with 2 X 2 have been found for quenched surfaces from above 300°C depending on the Cu coverage measured by Auger electron spectroscopy. The deposited Cu atoms diffuse into the bulk above 300°C and segregate to the surface by quenching. These facts are similar to the behavior of Ni/Si surfaces reported by Ichinokawa et al. [27-301. The formation of surface structures in the Cu/Si(lOO) surface studied by low-energy electron diffraction (LEED) is presented in this paper as a function of Cu coverage and heat treatment, and the mechanism of impurity segregation depending on the quenching rate and annealing time is explained by the trapping of segregated Cu atoms to the Cu islands while annealing.

2. Experiment

The experiments have been performed in an ultrahigh vacuum (UHV-SEM) chamber with a base pressure of 2 X lo- lo Torr, which was equipped with a LEED system for observation of diffraction patterns and a cylindrical mirror electron energy analyzer for measurements of Auger and electron energy loss spectra. Cu was evaporated from a tungsten filament at a rate of 0.4 ML/min on the Si(lOOj2 X 1 clean surface. The thickness was measured by a quartz thickness oscillator with converting frequency differences into a monolayer scale. Using the density of Si atoms in an ideal surface, a coverage of 1 ML (0 = 1) corresponds to a Cu density of 6.78 X 1014 cm -2 for Si(100). The pressure during Cu evaporation was always better than 5 X 10-‘” Torr. As a substrate we employed boron-doped p-type Si(100) wafers (20-30 R. cm) which prior to mounting in the UHV chamber were cleaned in ethanol and deionized water. In situ preparation consists in heating the samples by a direct electric

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77

current to 1100°C for several minutes. The surface concentration of carbon could then be detected in the AES spectra and the intensity ratio between Si L,,W and C KLL was larger than 200. The LEED pattern showed a well developed 2 X 1 reconstruction. The Auger electrons were excited by electron bombardment at an energy of 2.5 keV and a beam current of 10 PA. For analysis of the growth process, peak-to-peak values of Si L,W (92 eV) and Cu M,,W (61 eV) transitions have been measured by a lock-in amplifier with the dN(E)/dE mode. The peak intensities of Auger electrons are normalized by the Si L,,W Auger peak of the Si clean surface. Cu was deposited up to 8 = 10 ML onto the Si clean surface, which was previously flashed to 1100°C and controlled by AES and LEED for sufficient cleanliness and structural order. The EELS spectra were measured with the second derivative mode d2N(E)/dE2 at a primary electron energy of 100 eV. The surface temperature was measured by a direct electric current passing through the substrate calibrated by an optical pyrometer. The accuracy of the temperature measurements is about f 10°C. The island formation on the Cu-deposited surface above 500°C was observed by a scanning electron microscope with ultrahigh vacuum (JAMP-30) as a function of Cu thickness with checking island formation at high temperature.

3. Experimental results

The Auger electron peak ratios of Si L,,W

and Cu M,,W transitions to Si L23W from the clean surface were measured as a function of Cu thickness and deposition temperature. Fig. 1 shows the variation of Auger electron ratios with Cu coverage for deposited surfaces at room temperature. The Si L,,W and Cu M23W Auger electron ratios change continuously with Cu coverage and reach 0.3 and 0.4, respectively, at a Cu coverage of 10 ML. These values agree with those of Mathiez et al. [25]. They reported that saturated values of Si L23W and Cu M23W Auger ratios at 50 ML are 0.2 and 0.5, respectively, and correspond to the composition of Cu,Si. The Si L2,W Auger peak splits into two peaks above a few ML and shows an intermixing

T. Ikeda et al. /Surface

Science 336 (1995) 76-84

500°C or the annealed surfaces at 500°C after deposition at RT, the Cu Auger signals measured at 500°C are very low as shown in Fig. 2 by a solid curve, and LEED patterns of those surfaces show the 2 X 1 structure at 500°C as similar to the clean surface. These results indicate that the Cu growth at 500°C is the Volmer-Weber mode and 3D islands grow on the clean surface. However, we found that the Auger intensity measured at 500°C changes by quenching as shown in Fig. 2 by dashed curves. The quenching rate was approximately 2OO”C/s. The curves of the quenched surfaces in Fig. 2 have a break point at around 1 ML and the growth mode is like the Stranski-Krastanov type. The LEED patterns observed at RT for the quenched surfaces from 500°C show various structures, e.g., 6 X 2, 6 X 5, and 10 X 5 mixed with 2 X 2 with increasing Cu thickness. For submonolayer thickness, the Cu Auger ratio increases linearly with Cu coverage and becomes a constant above 1 ML. The increase of the Cu Auger peak by quenching suggests that Cu atoms diffused into the bulk at 500°C segregate to the surface by quenching. The segregated Cu atoms form various superstructures depending on the thickness. Cu atoms, besides diffusing into the bulk, form islands on the surface.

RT

SiLVV CuMVV

splitting

(6

1 eV)

0 [ML) Thickness Fig. 1. Auger intensities of Cu M,,W and Si LasW transitions (normalized to the intemsity of the Si L,W for a clean Si) versus Cu thickness deposited at room temperature. The continuous curve corresponds to layer-by-layer growth. The Si L,,W peak splits into two peaks above 3-4 ML and shows intermixing between Cu and Si.

layer between Cu and Si even at room temperature

CRT). On the other hand, for &-deposited

surfaces at

1.

‘bn,~‘_..,___.___.._______________~________~_ -__-_ SiLVV

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CuMVV (6 2 ,%%-

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0 IMLJ Thickness Fig. 2. The solid curves are Auger intensities of Cu M,W and Si L,.W transitions (normalized to the intensity of clean Si) measured at 500°C versus Cu thickness for deposited surfaces at 500°C (0) and annealed surfaces at 500°C (A ). The dashed curves show the Auger intensities for surfaces quenched from 500°C to RT at a Cu thickness between Cu intensities increase by quenching and the growth mode for the quenched surface shown by dashed curves looks like the type.

the Si L2sW for a after RT deposition 0 and 10 ML. The Stranski-Krastanov

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T. Ikeda et al. /Surface Science 336 (1995) 76-84

increases with Cu coverage up to 1 ML and becomes a constant above a break point. This fact shows that the increase of the Cu Auger intensity in the submonolayer range is caused by the increase of the 2 X 2 domains in the mixed structures. Similar LEED experiments were carried out for the vicinal surfaces

The LEED patterns of the surfaces quenched from 500°C are summarized in Figs. 3a-3d, showing the structures of 2X2+6X2, 2x2+6x5, 2X2+ 6 X 5 + 10 X 5, and 2 X 2 + 10 X 5, corresponding to the deposition thickness of 0.3, 0.6, 1.0, and 10 ML, respectively. The intensity of the 2 X 2 structure

2x2+6x5

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Fig. 3. LEED patterns of (a) 2 X 2 + 6 X 2 at 0.3 ML, (b) 2 X 2 + 6 X 5 at 0.6 ML, (c) 2 X 2 + 6 X 5 + 10 X 5 at 1.0 ML, and (d) 2 x 2 + 10 X 5 at 10 ML. (e), (f), and (9> show the schematic diagrams of 6 X 2, 6 X 5, and 10 X 5 LEED patterns with two domains. CO), CO), and (0) show the 1 X 1, 2 X 2, and 6 X 2, 6 X 5 or 10 X 5 LEED spots, respectively.

80

T. Ikeda et al. /Surface

of Si(100)2 X 1 with single domains and showed that the six-fold axis of the 6 X 2 and 6 X 5 structures is parallel to the direction normal to the dimer row in the Si(100)2 X 1 surface and the 10 X 5 structure cannot be formed on the vicinal surface, because the terrace width of the vicinal surface is too narrow to form the 10 X 5 structure. The electron energy loss peak of the bulk plasmon measured at 500°C for the Cu 1 ML surface shifts 0.5 eV towards higher energy compared to that of the clean surface as shown in Fig. 4c, and that of the quenched surface shown in Fig. 4d does not shift from the clean surface. Since the bulk plasmon energy of the clean surface does not shift by a

:=lOOeV

I

1

I

20

15

10

Energy

Loss

5

(eV)

Fig. 4. The electron energy loss spectra at primary electron energy of 100 eV for Si(100) clean and Cu/Si(lOO) surfaces. (a) Clean surface measured at RT, (b) clean surface measured at 5OO”C, (c) Cu 1 ML surface measured at SOO”C, (d) surface quenched immediately after Cu 1 ML deposition at 500°C and (e) Cu 1 ML surface quenched atIer 1 min annealing at 500°C before quenching. The shifts of surface and bulk plasmon are explained by the effect of Cu diffusion and segregation by heat treatment (see text for explanation).

Science 336 (1995) 76-84

temperature effect, the shift of the bulk plasmon peak at 500°C should be attributed to a diffusion effect of Cu from the surface to the bulk. On the other hand, the surface plasmon energy for quenched surfaces from 500°C increases 1.0 eV from that of the clean surface as shown in Fig. 4d. These facts agree with the AES results in this experiment and confirm that the Cu atoms which diffused into the bulk at 500°C segregate to the surface by quenching. Furthermore, annealing of the Cu 1 ML surface at 500°C for 1 min before quenching induces a decrease of the Cu Auger intensity from 0.16 to 0.12 compared to that of the surface quenched from 500°C immediately after deposition, and the LEED pattern changes from 2X2+6x5+10x5 to 2X2+6 X 5. Such an effect was also observed by EELS. The surface plasmon energy of the surface annealed at 500°C for 1 min before the quenching is 1.0 eV lower than that of the surface quenched immediately after the deposition as shown in Fig. 4e, and nearly the same as that of the clean surface. However, the bulk plasmon energy does not shift by annealing. These facts suggest that the annealing at 500°C before quenching does not only induce Cu diffusion into the bulk, but also decreases the amount of surface segregation by annealing. The effect of heat treatment on Cu diffusion into the bulk was measured quantitatively by AES for the Cu 1.5 ML surface. The Cu Auger intensity decreases with increasing temperature as shown in Fig. 5 by a solid curve and increases with quenching as shown by a dashed curve. The surface concentration becomes maximum by quenching from 500°C and then decreases with increasing quenching temperature above 550°C. From our thermal desorption experiment, it was found that the decrease of Cu segregation above 550°C is due to a thermal desorption effect of Cu from the surface. Here, it should be noticed that there is a one-to-one correspondence between the surface structures and the Cu Auger intensity ratios, the 2 X 2 + 10 X 5 structure appears from 0.2 to 0.18, the 2 X 2 + 10 X 5 + 6 X 5 structure from 0.18 to 0.14, the 2 X 2 + 6 X 5 structure from 0.14 to 0.08, the 2 X 2 + 6 X 2 structure from 0.08 to 0.05 and the 2 X 1 structure from less than 0.05 regarding the Cu Auger ratios as shown in Fig. 5. These behaviors are similar to those of the Ni/Si(lOO) surfaces, except for a difference between

T. lkeda et al. /Surface

CuMVV

(6 1 eV),

0=1.

81

Science 336 (1995) 76-84

5ML

2 t

.-4

-2x2+10x5 2+6x5+10x5

0

200

400 Temperature

600 IT

800

1

Fig. 5. The Cu Auger intensity for a Cu 1.5 ML surface (normalized to the Si Auger peak of a clean Si) decreases with increasing specimen temperature as shown by a solid curve and increases by quenching as shown by a dashed curve for quenching immediately after deposition at various temeratures. The increase of the Cu Auger intensity by quenching is a function of quenching temperature. The decrease at higher temperature than 550°C is due to the desorption of Cu atoms from the surface. The check was performed by a thermal desorption experiment using a quadrapole mass spectrometer. Moreover, the increase of Cu Auger intensity with quenching is a function of annealing time and cooling rate. The surface structures corresponding to the Cu Auger intensities are shown. Details are described in the text.

the surface segregation temperatures. The diffusion and segregation temperatures in the Cu/Si(lOO) system are 200°C lower than those of the Ni/Si(lOO) system and it is attributed to a temperature dependence of the diffusion coefficients. Furthermore, the surface concentration of Cu changes with the cooling rate or the annealing time before quenching. Fig. 6 shows the variation of the Cu Auger intensity with increasing annealing time at 500°C before quenching for the Cu 1.5 ML surface. The Auger intensities were measured for the surfaces quenched after every annealing for one minute. The surface segregation decreases exponentially with the increase of the annealing time at 500°C. For long-time annealing, the Cu Auger ratio becomes less than 0.05 and the structure is 2 X 1. The decrease of the Cu Auger intensity with annealing time may be caused by trapping of Cu atoms to the centers during annealing. Fig. 7 is a phase diagram showing the surface structures as a function of deposition thickness and quenching temperature for immediately quenched surfaces after deposition. It should be noticed that the phase diagram depends on the cooling rate and the annealing time before quenching. The island formation on the Cu/Si(lOO) system at 500°C was observed by the W-IV-SEM as a

function of Cu thickness, and Fig. 8 shows an example of a quenched surface from 500°C after Cu deposition at 10 ML. The islands of a 0.5 pm size and a pyramid type are uniformly distributed in the substrate surface. From our Auger electron microanalysis, it was found that the islands consist of silicide and besides the islands a flat surface is

500°C.

1. 5ML

‘, ->

I 0

I 2 annealing

I

I 4 time

I 6 [mini

Fig. 6. Cu Auger intensity ratios as a function of annealing time at 500°C before quenching for the Cu 1.5 ML surface. Every annealing for one minute was performed before quenching. The Cu Auger intensity decreases exponentially with increasing annealing time.

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T. Ikeda et al. /Surface

Science 336 (199.5) 76-84

2x1

2x2+ 6X5+

2x2+10x5

10x5

( 2x2+6x5+1ox5 1

T+6x2i 2x2+6x5

\

6 300

0

I 0. 2

0. 4

I 0. 6

I 0. 8

I 1

I 3

I 8

10

e [ML1 Thickness Fig. 7. Phase diagram showing the surface structures for the Cu/Si(lOO) system as a function of deposited thickness and quenching temperature immediately after Cu deposition at each temperature. It should be noticed that the phase diagram changes by cooling rate from temperatures higher than 300°C.

reconstructed depending on the coverage. Here, we should notice that almost all signals of Cu should be emitted from the flat surface and that the signals from the islands are negligible for macroscopic analysis of Auger electron spectroscopy.

4. Discussion The Cu Auger intensity ratios for the Cu/Si(lOO) surfaces measured at 500°C are very small and less than 0.02 at a thickness of 10 ML. This fact suggests that the growth mode of the Cu/Si(lOO) surface at 500°C is the Volmer-Weber type, because signals from islands are negligible small. However, the rela-

tionship between the Cu Auger ratio and the Cu thickness for quenched surfaces from 500°C suggests that the growth mode of Cu for the quenched surfaces looks like the Stranski-Krastanov type and the increase of the Cu signals by quenching is due to the segregation of Cu from the bulk to the surface. Thus, the quenched surfaces from 500°C for a thickness above 1 ML should consist of Cu islands with the Cu 1 ML surface. It should be noticed that the Cu 1 ML surface is not homogeneous and has a mixed structure of 2 X 2 + 10 X 5. The existence of a mixed structure at 1 ML coverage is strange to our knowledge on epitaxial growth. The mixed structure of 2 x 2 + 10 X 5 is formed by segregation of Cu 1 ML by quenching. For deposition at high temperature,

Fig. 8. UHV-SEM micrographs of the Cu 10 ML surface quenched from 500°C. Almost all islands have regular pyramid shapes and do not show anisotropy for the growth on the Si(100) dimer structure. The anisotropic growth of islands was observed at coverages higher than 10 ML [26].

T. Ikeda et al. /Surface

most of the deposited Cu atoms form islands by the Volmer-Weber growth, but part of the Cu atoms diffuse into the bulk. Such a phenomenon occurs due to the large diffusion coefficient of Cu in Si. The 2 X 2 + 10 X 5 structure appears at the maximum coverage of 1 ML, and the 2 X 2 domains increase with a Cu coverage less than 1 ML by changing the structure in the order of 6 X 2, 6 X 5, and 10 X 5. The increase of the 2 X 2 domains within the mixed structures was estimated by the increase of the 2 X 2 spot intensity in the LEED pattern. Now, we should discuss the mechanism of surface segregation depending on the quenching rate. If a temperature gradient exists in a specimen in the cooling process, impurities diffuse in the direction from low to high temperature. The surface temperature of the specimen is to be lower than that of the bulk while cooling; therefore the impurities should segregate in the direction from the surface to bulk. Therefore, the origin of the surface segregation in this experiment is not caused by a temperature gradient and should be due to a supersaturation effect by quenching. According to the present experiment, the faster is the cooling rate, the larger is the surface segregation in the range between 1 and 2OO”C/s. This is contrary to the common knowledge of impurity segregation. The important factor in surface segregation is the annealing time rather than the cooling rate. The growth mode in the Cu/Si(lOO) surface at high temperature is the Volmer-Weber type, however part of the Cu atoms deposited onto the surface diffuse into the bulk in addition to island formation. The quantity of Cu atoms diffusing into the bulk is a function of diffusion coefficient and solubility. The interstitial Cu atoms with a high diffusion coefficient may be trapped somewhere during annealing. The trapping centers are probably islands and the quantity of trapped atoms increases with annealing time and annealing temperature. Therefore, the ideal Volmer-Weber growth is impossible in this system and layer growth due to the surface segregation occurs always together with island formation. Variation of the island size during surface segregation was reported for the Ni/Si system [30]. The diffusion coefficients of Cu and Ni in Si are extremely much larger than those of other metals and have values from 10-l to 1 m*/s at temperatures

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from 500 to 900°C for Cu and from 800 to 1300°C for Ni. Thus, a metal with a high diffusion coefficient in Si has a complex growth mode due to the surface segregation. Superstructures formed by quenching in the Ni/Si system were reported for several planes [27-301. The mechanisms of surface segregation and formation of superstructures in the Cu/Si surfaces are similar to those in the Ni/Si surfaces. However, the quantity of surface segregation in the Cu/Si surface is much larger than that of the Ni/Si surfaces. The formation of superstructures in the Ni/Si surface occurs at a thickness less than 0.1 ML, but for the Cu/Si(lOO) surface it takes place in the range of l/3 to 1 ML. This is explained by a solubility difference between Ni and Cu in Si. In fact, the solubility of Cu in Si is 2 X 1021/m3 at 600°C and ten times larger than that of Ni in Si. So far, we have not discussed on the atomic arrangements of the superstructures, but we can say that the Cu coverage of superstructures increases with an order of 6 X 2, 6 X 5, and 10 X 5. Moreover, we observed a difference of segregation temperatures depending on crystallographic orientations. For a Ni impurity, the surface segregation along [ill] occurs above 9OO”C,but it takes place above 700°C along [loo]. This fact shows that diffusion or segregation of Ni along [loo] takes place at a temperature lower by 200°C than that along [ill]. Such experiments on the Cu/Si surface are being studied at present for several surface planes. Island formation for the Cu/Si(lOO) surface from 1 to 100 ML at 500°C was investigated in a previous paper 1261using micro-Auger electron spectroscopy. Anisotropic growth of islands was observed depending on 2 X 1 and 1 X 2 domain structures. In the present experiment at coverages lower than 10 ML, anosotropic growth of islands depending on the dimerization direction of the substrate surface has not been observed. Therefore, it can be said that anisotropic growth of islands occurs at a thickness higher than 10 ML in the Cu/Si(lOO) surface.

5. Conclusion The surface structures and the growth mode of the Cu/Si(lOO) surface were studied by LEED, AES, and EELS as a function of Cu coverage between 0

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T. Ikeda et al. /Surface

and 10 ML. From the present experiment, it was found that the deposited layer of Cu is intermixed with Si at room temperature and no defined interface structure is formed at coverages higher than a few ML. However, the growth mode in the Cu/Si(100)2 x 1 surface at 500°C is of Volmer-Weber type and part of the Cu atoms diffusing into the bulk at 500°C segregates to the surface by quenching. Cu-segregated surfaces show several superstructures depending on Cu coverage. The surface structures of 2 X 2 +6X2,2X2+6X5,2X2+6X5+10X5,and 2 X 2 + 10 X 5 appeared in the ranges of 0.05 to 0.08, 0.08 to 0.14, 0.14 to 0.18, and 0.18 to 0.2, respectively, with respect to the Cu Auger intensities normalized by that of the Si clean surface. Surface segregation by quenching is interpreted by impurity diffusion, and the decrease of Cu segregation due to annealing or slow cooling is explained by trapping of Cu atoms into islands during annealing. The characteristic property of the growth mode in the Cu/Si(lOO) surface is originated by a large diffusion coefficient of interstitial Cu atoms in Si similarly to those in the Ni/Si surfaces.

Acknowledgement

The present work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture.

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