Study of Au–Si(1 0 0) interface by means of Si 2p core-level photoemission spectroscopy

Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 97–100

Study of Au–Si(1 0 0) interface by means of Si 2p core-level photoemission spectroscopy Yuichi Haruyama∗ , Kazuhiro Kanda, Shinji Matsui Graduate School of Science, Laboratory of Advanced Science and Technology for Industry, Himeji Institute of Technology, 3-1-2 Kouto, Kamigori, Ako 678-1205, Japan Available online 2 April 2004

Abstract We have investigated the Au–Si(1 0 0) interface as a function of the Au coverage by means of the Si 2p core-level photoemission spectroscopy. At the 1 ML deposition, the spectral feature changed remarkably. This indicates that the deposited Au atoms interact with the surface Si atoms in this deposition range. With increasing the Au coverage at ∼3 ML the spectral feature become three-peak structures at 100.0, 99.4, and 99.0 eV. The appearance of the three-peak structures indicates the Au–Si alloy or Au silicide formation. With further increasing the Au coverage, two peaks at 100.3 and 99.7 eV become dominant. From the curve fitting analysis, the photoemission spectrum at the higher Au coverage was decomposed by three components. Two components at higher binding energy side were assigned to the surface Si atoms while a component at lower binding energy side was assigned to Si atoms in the amorphous silicide layer. © 2004 Elsevier B.V. All rights reserved. Keywords: Si(1 0 0); Au–Si interface; Electronic structure; Photoemission spectroscopy; Synchrotron radiation

1. Introduction The study on the Au–Si(1 0 0) interface has attracted significant interest in the field of the surface science and for the purpose of the application in electronic devices [1–9]. In order to understand the Au–Si interaction at the interface formation, many studies on the electronic and geometric structures of Au–Si(1 0 0) interface have been performed so far by means of the low energy electron diffraction (LEED), Auger electron spectroscopy (AES), ion scattering, photoemission spectroscopy (PES) and the scanning tunneling microscopy [1–9]. From these previous studies, the Au deposited Si(1 0 0) surface at room temperature has mainly been classified into the four regions, depending on the Au coverage. At the initial region I of the Au coverage less than ∼1 ML, the 2×1 LEED pattern was observed [1] and the intensity of the peak assigned to the surface state in the valence band PES decreased rapidly with the Au coverage [6]. At the region II where the Au coverage spans from ∼1 to 3–4 ML, the 1 × 1 LEED pattern was observed [1]. In the regions I and II, the deposited Au grew on the Si(1 0 0) substrate in a layer-by-layer fashion [8]. At the region III where the Au ∗

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coverage spans from 3–4 to ∼30 ML,1 the Au–Si alloy or Au silicide was formed by the intermixing between Au and Si atoms [1–3]. At the region IV of the Au coverage more than ∼30 ML, the Au(1 1 1) film was formed on the Si(1 0 0) surface [1]. In spite of these previous studies, a lot of uncleared issues still remain on the electronic and geometric structures of Au–Si(1 0 0) interface. Particularly, the electronic structures such as the chemical components and the charge state of Au–Si(1 0 0) interface are not clarified. The core-level photoemission spectroscopy is a useful method to investigate the information on the chemical components and the charge state. In the previous PES measurements, the Si 2p core-level photoemission spectra did not resolve the chemical components due to moderate energy resolution [4]. In this study, we performed the Si 2p core-level photoemission experiments with the improved energy resolution to investigate the electronic structure of the Au–Si(1 0 0) interface. As a result of it, the fine structures in Si 2p core-level photoemission spectra were newly observed. In addition, the observed Si 2p core-level photoemission spectra changed remarkably with increasing the Au coverage. Using the 1 Concerning the boundary of the Au coverage, there is some ambiguity by the literature.

Y. Haruyama et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 97–100

curve fitting analysis, the chemical components of the Au deposited Si(1 0 0) surface were determined. Based on the Si 2p core-level photoemission results, the change of the electronic structure as a function of the Au coverage is discussed.

2. Experimental Photoemission measurements were performed at BL7B of the NewSUBARU, Himeji Institute of Technology. At the beamline, a varied line spacing grating monochromator, which spans a photon energy (hν) range from 50 to 280 eV, was installed [10]. Photoemission spectra were measured by using the conventional photoelectron spectroscopy apparatus, which was mounted with the X-ray source of the Mg K␣ line (hν = 1253.6 eV) and the hemispherical electrostatic analyzer (VSW Ltd, CL150). The Si 2p core-level photoemission measurements were carried out with a total energy resolution of ∼43.5 meV full width at half maximum (FWHM) at hν = 130 eV. The base pressure in the photoemission analysis chamber was ∼ 1 × 10−8 Pa. All photoemission spectra shown in this work were recorded at ∼50 K. The temperature was measured by Si diode attached to the sample holder. The p- (B-doped, ∼5  cm) Si(1 0 0) wafers were used. The clean surface was obtained by flashing of the sample ∼1470 K at several times, subsequent to annealing at ∼1070 K for several hours. The annealing temperature of the sample was measured with an optical pyrometer. The cleanliness was checked by X-ray photoemission spectroscopy (XPS) for the absence of extra peaks arising from the contaminations. Ta ribbons attached to the sample and Au films evaporated on Ta ribbons were used for the Fermi level measurement and the total energy resolution reference. Au was deposited from a heated W filament onto Si(1 0 0) substrates at room temperature. The Au deposition was monitored with a quartz thickness monitor. The Au coverage was mainly obtained from the thickness and calibrated by the XPS intensity ratio I(Au 4f)/I(Si 2p).2

3. Results and discussion Fig. 1 shows the normal-emission Si 2p core-level spectra as a function of the Au coverage on the Si(1 0 0) surface measured with hν = 130 eV. In the bottom of the figure, the Si 2p core-level photoemission spectrum of the clean Si(1 0 0) surface is positioned. The fine structures at 99.7 and 99.1 eV were well-resolved by comparison with the previous Si 2p core-level photoemission spectra of the clean Si(1 0 0) surface [11,12]. This indicates that the well-ordered 2 Since it is very difficult to estimate the exact Au coverage due to the Si diffusion, the Au deposition was mainly used as the Au coverage in this study.

hν =130 eV

Si(100)-Au

50 ML 40 ML

Intensity (arb. units)

98

30 ML 20 ML 10 ML 5 ML 3 ML 2 ML 1 ML 0.6 ML clean 101

100

99

98

Binding Energy (eV) Fig. 1. Normal-emission Si 2p core-level spectra as a function of the Au coverage on the Si(1 0 0) surface measured with hν = 130 eV. The observed photoemission spectra were normalized to the maximum intensity.

Si(1 0 0) surface was obtained in our experiments. With increasing the Au coverage, the intensity of a peak at 98.4 eV decreased rapidly and disappeared at ∼1 ML. In addition, the fine structures at 99.7 and 99.1 eV also disappeared and the intensity around 99.3 and 98.7 eV in the photoemission spectrum increased. The spectral feature at 1–2 ML become two-peak structures with tails at the higher and lower binding energy sides. With increasing the Au coverage at ∼3 ML a peak at 100.0 eV appeared and the spectral feature become three-peak structures at 100.0, 99.4, and 99.0 eV. The intensity of the peak at 99.0 eV decreased gradually with increasing the Au coverage while the intensity of the peaks at 100.3 and 99.7 eV increased. At 50 ML, the peaks at 100.3 and 99.7 eV become dominant. In order to discuss the details of the observed Si 2p photoemission spectra, the curve fitting analysis was performed for the Si 2p photoemission spectra of the clean surface and those of the higher Au coverage using a least square method and the Voigt functions3 . The bottom spectrum of Fig. 2 shows that the photoemission spectrum of the clean surface was decomposed by five components positioned at 99.145, 98.953, 98.923, 98.712, and 98.435 eV with the Gaussian FWHM of 54, 64, 50, 64, and 64 meV, respectively. The Lorentzian FWHM and the spin-orbit splitting were 60 and 605 meV. According to the previous photoemission studies, the five components were assigned to the subsurface, down-dimmer, bulk, defect and up-dimmer components, from the higher binding energy side [11,12]. Since the fitting results were essentially identical to the previous

3 We did not perform the curve fitting analysis at the photoemission spectra of Au deposition between 0.6 and 10 ML because there were some ambiguities due to many components.

Y. Haruyama et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 97–100

hν = 130 eV Normal

Intensity (arb. units)

Si(100)-Au

50 ML

99

Table 1 Area intensity ratio of each components for the 22, 40, and 50 ML Au deposited Si(1 0 0) surfaces Au coverage

Component at 99.77 eV (%)

Component at 99.62 eV (%)

Component at 99.40 eV (%)

50 ML 40 ML 22 ML

45.0 32.9 26.3

36.3 35.5 24.9

18.7 31.6 48.8

40 ML

clean 101

100

99

98

Binding Energy (eV) Fig. 2. Si 2p core-level photoemission spectra (dots) of the clean Si(1 0 0) surface and of 22, 40, and 50 ML Au deposited Si(1 0 0) surfaces. The fitting results (lines) are also plotted in each photoemission spectrum.

ones [11,12], we used the same assignments in the previous photoemission results. At the initial stage of the Au coverage, the observed rapid decrease in intensity of the peak at 98.4 eV and the disappearance of the fine structures at 99.7 and 99.1 eV indicate that the deposited Au atoms affect the dimmer and the subsurface atoms in the substrate. This corresponds to the decrease in intensity of the peak assigned to the surface state in the previous valence band photoemission spectra [6]. The increase in intensity around 99.3 and 98.7 eV in the photoemission spectrum indicates that the deposited Au atoms interact with the Si atoms and that there are Si atoms in the different charge state with the clean surface. With increasing the Au coverage at ∼3 ML, three-peak structures at 100.0, 99.4, and 99.0 eV was observed. The peak at 99.0 eV is assigned to the bulk component as the bulk component of the clean surface is positioned at 99.0 eV. With increasing the Au coverage, the photoelectrons emitted from the bulk component decrease due to the escape depth of the photoelectron. Therefore, the gradual decrease in intensity of the peak at 99.0 eV with the Au coverage is observed. In the previous Si 2p core-level photoemission studies [4,5], the component positioned at higher binding energy side of the bulk component was assigned to the component in Si–Au alloy or Au silicide. It is considered that the chemical shift is caused by the electronegativity difference between Au and Si atoms. That is, the charge transfer from Si atoms to Au atoms occurs since the electronegativity of the Au (2.54) is larger than that of the Si (1.90). Therefore, two structures at 100.0 and 99.4 eV observed in this study are ascribed to the component in Si–Au alloy or Au silicide.

hν =130 eV

Si(001)-Au Intensity (arb. units)

22 ML

As shown in Fig. 1, the appearance of the component shifted to higher binding energy side at more than ∼3 ML Au coverage indicates that the Au silicide was formed. This result is consistent with the previous studies [1–3]. The spectral feature derived from the Au silicide changed with increasing the Au coverage from 3 ML. This indicates that the intensity ratio of the some chemical components in the Au silicide changes with the Au coverage. In Fig. 2, the best fitting results for the higher Au coverages indicate that the Si 2p photoemission spectra were basically decomposed by three components positioned at 99.77, 99.62, and 99.40 eV except the bulk derived components around 99.0 eV. The Gaussian FWHM of each component was 70 meV. The Lorentzian FWHM and the spin-orbit splitting were 60 and 605 meV. The area intensity of each component is shown in Table 1. The area intensity ratio of the component at the higher binding energy side seems to increase as compared with that of the component at the lower binding energy side with increasing the Au coverage. In Fig. 3, the Si 2p core-level photoemission spectra of the 20 and 40 ML Au deposited Si(1 0 0) surfaces measured at the normal emission (open circle) are compared with that measured at the emission angle of 60◦ to the surface normal (filled circle),

Normal 60 ˚ 40 ML

20 ML

101

100

99

98

Binding Energy (eV) Fig. 3. Si 2p core-level photoemission spectra of 20 and 40 ML Au deposited Si(1 0 0) surfaces measured at the normal emission (open circle) and at the emission angle of 60◦ to the surface normal (filled circle), respectively. The photoemission spectra were normalized by the maximum peak intensity.

100

Y. Haruyama et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 97–100

respectively. The photoemission spectra were normalized by the maximum peak intensity. When the emission angle is 60◦ , the intensity of the peaks at 100.3 and 99.7 eV for the 20 ML Au deposited surface enhanced while the intensity of the peak at 99.4 eV for the 40 ML Au deposited surface decreased. These results indicate that the components at 99.77 and 99.62 eV are derived from the surface. The photoelectron electron escape depth (∼3 Å) in the used hν is considerably smaller than the Au coverage. This indicates that the intermixing between Au and Si atoms occurs. It was reported in the previous studies that the Si atoms in the substrate diffuse to the surface upon the Au deposition and that the interface was constituted by the amorphous silicide layer [1,2]. Therefore, the components at 99.77 and 99.62 eV are derived from the surface Si atoms diffused from the substrate. The component at 99.40 eV is derived from the Si atoms in the amorphous silicide layer. These assignments can explain the decrease in intensity of the component at 99.40 eV with the Au coverage because the Au(1 1 1) film is formed on the amorphous silicide layer. It is noted that the absolute intensity of the photoemission spectra is not discussed in this study. Since Narusawa et al. [2] pointed out that the composition of the amorphous Au silicide layer is Au5 Si in the MeV ion scattering experiment, there is a possibility that the component at 99.4 eV corresponds to Au5 Si. Finally, we comment on the Au 4f core-level photoelectron spectra as a function of the Au coverage (not shown). The Au 4f core-level photoelectron spectra as a function of the Au coverage were essentially identical to those by Lu et al. [5]. Lu et al. [5] pointed out that the Au–Au component in the clusters (3D islands) appeared in addition to the Au silicide component with increasing the Au coverage. The appearance of the Au–Au component would correspond to the initial state of the Au(1 1 1) film formation. Moreover, their photoemission spectra suggest that there is Au silicide layer even at 50 ML Au deposition [5]. This is consistent with our result by Si 2p core-level photoemission spectra. 4. Conclusions The electronic structure of the Au–Si(1 0 0) interface as a function of the Au coverage up to 50 ML was investigated by means of the Si 2p core-level photoemission spectroscopy.

The remarkable change of the spectral feature at the 1–2 ML Au deposition indicates that the deposited Au atoms interact with the surface Si atoms. At the 3–5 ML Au coverage, the spectral feature become three-peak structures at 100.0, 99.4, and 99.0 eV. Two peaks at 100.0 and 99.4 were ascribed to the silicide component while the peak at 99.0 eV was to the bulk component. With increasing the Au coverage up to 40–50 ML, the spectral feature at 100.3 and 99.7 eV become dominant. From the curve fitting analysis, the photoemission spectrum at the higher Au coverage was decomposed by three components. From the comparison between the photoemission spectra measured at 0 and 60◦ emission angles, two components at higher binding energy side were assigned to the surface Si atoms while a component at lower binding energy side was Si atoms in the amorphous silicide layer. It is considered that the Si atoms diffuse from the substrates upon the Au deposition.

Acknowledgements We are pleased to thank the staff of the NewSUBARU facility for excellent support.

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