- Email: [email protected]
Atomic diffusion at the Cu–Au–Si multilayers interface
Microelectronic Engineering 81 (2005) 349–352 www.elsevier.com/locate/mee
Atomic diffusion at the Cu–Au–Si multilayers interface S. Iaiche a, N. Benouattas b, A. Bouabellou a
a,*
, L. Osmani b, L. Salik
b
Laboratoire Couches Minces et Interfaces, Universite´ Mentouri Constantine, Campus Chaabet Erssas, Constantine 25000, Alge´rie b Laboratoire Surfaces et Interfaces des Mate´riaux Solides, Universite´ Ferhat Abbas, Setif 19000, Alge´rie Available online 7 April 2005
Abstract Si(1 0 0) and (1 1 1) oriented silicon wafers were used as a substrate for metallic bilayers deposition of copper and gold. Cu/Au/Si structures were obtained by thermal evaporation and then heated below 400 C in vacuum. These solid-state reactions occurred in the samples have been studied using X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), scanning electronic microscopy (SEM) and X-ray dispersive energy analyzer (XDE). The study shows that heat treatment at 200 C of the multilayered Cu/Au/Si structure leads to the formation and the co-existence of both Cu3Si and Cu4Si copper rich-silicides with the expansion of their respective cells, independently of the orientation of the substrate. The increasing of the annealing temperature until 400 C leads to the growth of well-oriented crystallites corresponding to Cu3Si and Cu4Si silicides on Si(1 1 1) but only Cu4Si crystallites with square and rectangular shapes on Si(1 0 0). The thermal stability of formed copper silicides after heat treatment at 400 C during 30 min for both Cu/Au/Si(1 0 0) and Cu/Au/Si(1 1 1) systems is analyzed. 2005 Elsevier B.V. All rights reserved. Keywords: Copper silicides; Gold; Atomic interdiffusion; Epitaxy
1. Introduction The phase composition, the phase sequence, the morphology, and the temperatures of silicides formation during the interactions of thin metal films with silicon substrates have long been explored, with both scientific interest and technological importance [1–4]. And the continuing interest in *
Corresponding author. Tel.: +213 31 61 4711; fax: +213 31 61 4711/4342. E-mail address: [email protected] (A. Bouabellou).
studies of silicides formation stems from the central importance of these reactions in vast area of applications in which new metallization schemes of integrated circuits [5]. For ULSI devices, for example, it is crucial that the layers of silicides are very uniform, have a low resistivity and a good thermal stability [6,7] and that epitaxial silicides prove to be good candidates for this role [8]. The big use of copper silicides in microelectronics is due to its lower electrical resistivity and higher electromigration resistance over aluminium [9,10]. Copper silicides are formed easily and offer
0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.03.030
350
S. Iaiche et al. / Microelectronic Engineering 81 (2005) 349–352
a good adherence on Si and SiO2 in comparison with other metal silicides. But the principal concern in Cu metallization is the thermal stability of Cu/Si, Cu/metals and Cu/silicides contacts, which is of great importance to the device performance [11]. That is why we introduce a new layer of Au to the Cu/Si system to control silicides formation and/or extend the thermal stability of silicide/Si contacts to higher temperatures.
2. Experiment Single-crystal wafers silicon of (1 0 0) and (1 1 1) orientation were used as substrates for metallic bilayers. The substrates were previously degreased in acetone, trichloroethylene and methanol baths, and then etched during 30 s in a diluted hydrofluoric acid (HF) solution to remove native oxide layer before to loading into the evaporator. High purity Au and Cu thin layers of 1 0 0 and 120 nm thick, respectively, were alternatively evaporated without breaking vacuum under pressure of ˚ /s. The 2 · 107 Torr. The deposition rate was 6 A obtained samples with layered geometry of Cu/ Au/Si were annealed in a vacuum furnace at temperature range between 200 and 400 C during one time of 30 min. The samples were analyzed by several techniques. X-ray diffraction (XRD) in the h 2h mode using Cu Ka radiation was carried out to identify the formed phases. Rutherford backscattering measurements (RBS) were performed using 2 MeV alpha beam (4 He+) and fitted with the help of the RUMP program. This analyze constitutes a powerful means in the examination of the chemical composition according to the thickness of the growing layer. The surface morphology of formed compounds were visualized by scanning electronic microscopy (SEM) operating at 9 keV and equipped with an X-ray dispersive energy analyzer. From the examination of the Cu–Si phases diagram [12], shown in Fig. 1, three low temperature equilibrium compounds could be formed: Cu3Si(g 0 ), Cu15Si4 or Cu4Si(e) and Cu5Si(c). We note that the stoechiometric range of these equilibrium alloys is rather narrow.
Fig. 1. Cu–Si equilibrium phases diagram [12].
3. Results and discussion Shown above, Fig. 2 represents a superposition of the backscattering spectra of the Cu/Au/Si(1 1 1) structure before and after annealing. It is easy to observe that the RBS spectrum of the as-prepared structure is composed of three well distinct parts: a peak corresponding to the gold film, towards high energies, a very spread out plate corresponding to the Si substrate towards low energies, and an inter-
Fig. 2. RBS spectra of Cu/Au/Si(1 1 1) multilayers: before annealing (————), annealed at 200 C (+++) and 400 C for 30 min (-Æ-Æ-Æ-Æ-Æ-).
S. Iaiche et al. / Microelectronic Engineering 81 (2005) 349–352
800 Cu 3Si Cu3Si
80
400˚C 30min
450
Cu4Si Cu3Si Cu3Si Cu 3Si
600
200
(b)
400˚ C 30min
150
100
50 Cu 3Si
150 0
Cu4Si
Cu3Si
300
0 40
50
60
2θ (deg)
70
40
50
60
70
2θ (deg)
Fig. 4. X-ray diffraction spectra of the Cu/Au/Si(1 0 0) structure annealed for 30 min at 200 C (a) and 400 C (b).
(b)
0
Cu3Si
Cu4Si
Cu3Si
Cu4Si
40 Cu4Si
400
200
(a)
(a)
750 200˚C 30min
60
Cu3Si
600
Cu4Si
Intensity (arb. unit)
200˚C 30min
among all the peaks recorded after annealing at 200 C, only two peaks corresponding to Cu3Si and Cu4Si persist as it is shown from the results of Fig. 3(b). For a similar structure using an Si(1 0 0) substrate, the compounds formed at 200 C are typical of those observed for the first structure (see Fig. 4(a)). But after annealing at higher temperature about 400 C (Fig. 4(b)), one notes the disappearance of the peaks corresponding to Cu3Si. It persists only the peak corresponding to the Cu4Si phase. The morphology of the surface of the Cu/Au/ Si(1 0 0) sample, shown on Fig. 5, is very heterogeneous. It indicates the growth of well-oriented
Intensity (arb. unit)
mediate signal corresponding to the copper layer deposited. The Au/Si and Cu/Au interfaces are very abrupt, which indicates that no reaction and/or formation of phase occurred during the preparation of the samples. The simulation of the spectra RBS allows to evaluate the thicknesses of the Cu ˚ , respectively. and Au layers to 1200 and 1030 A According to the RBS profiles corresponding to the Cu/Au/Si(1 1 1) samples annealed at 200 and 400 C (see Fig. 2), it is clear that the heat treatments induce an important interdiffusion of the three elements. The signals of the metal layers exhibit interfaces Cu/Au and Au/Si with different sides from those of the abrupt interfaces of the reference sample. Indeed, one observes a displacement towards high energies of the higher and lower energy limits of the signals Si and Cu, respectively. This is due to the diffusion of silicon towards surface and copper through the gold layer towards the substrate. Indeed, analysis of the X-ray diffraction profiles (Fig. 3) reflects structural changes in Cu/Au/ Si(1 1 1) due to the thermal reactions indicating the formation of new phases. Fig. 3(a) shows the presence of an abundance of peaks corresponding to Cu3Si and Cu4Si copper silicides after annealing at 200 C during 30 min. This reveals the polycrystalline character of the phases resulting from the reaction between the Cu layer and the Si substrate. By increasing the annealing temperature to 400 C,
351
20
0 40
60
2θ (deg)
80
40
60
80
2θ (deg)
Fig. 3. X-ray diffraction spectra of Cu/Au/Si(1 1 1) multilayers annealed at 200 C (a) and 400 C (b).
Fig. 5. SEM micrography of Cu/Au/Si(1 0 0) sample annealed at 400 C for 30 min.
352
S. Iaiche et al. / Microelectronic Engineering 81 (2005) 349–352
crystallites on the gray background which is composed mainly of silicon (96.27 at.%) originated from substrate. These crystallites of square (15 · 15 lm) and rectangular (25 · 23 lm) shapes are composed of an Au–Cu layer with a composition of 56.74 at.% Cu and 20.83 at.% Au interdiffused with silicon. One can affirm that the interdiffusion of different elements leads to the formation of a mixture Cu–Au–Si with a thickness equivalent to the sum of the evaporated metal layers thicknesses. The formation of the two Cu3Si and Cu4Si silicides at 200 C is in agreement with the equilibrium phases at low temperatures. In the same way, it is reported in many works on Cu/Si contacts [3], that the first growing phase on the interface is the Cu3Si silicide starting from the temperature threshold of 170 C, but on the Si(1 0 0) substrate, in particular, the reaction can take place at 125 C [13]. However, it is surprising to see that in the presence of the gold layer, the interfacial reactions between copper and silicon are also important for the structures with Si(1 0 0) and Si(1 1 1) substrates. But, in order to study the first stages of the reaction and to better determine the Si orientation effect on the kinetics of formation of this silicide, it would be desirable to go down even more in temperature. The increase in the annealing temperature to 400 C in the case of Cu/Au/Si(1 0 0) ternary system, shows a greater thermal stability of the Cu4Si phase compared to the Cu3Si phase at high temperature. This result was also found for the binary Cu/Si system [11]. On this fact, the Cu4Si phase can become more interesting for its employment in the microelectronic devices. It is also known that the formation of an SiO2 oxide layer in the Cu/Cu3Si/Si system at low temperature occurs at the expense of the Cu3Si compound until its disappearance [14]. For the structure with Si(1 1 1) substrate, the two silicides are formed and grow at the same time. This co-existence allows to suggest that on Si(1 1 1) one must further increase annealing at a
temperature higher than 400 C to see disappearing the Cu3Si phase.
4. Conclusion We have shown that for Cu/Au/Si structures, the Cu3Si and Cu4Si copper silicides are formed and remain stable at low temperature. With an anneal of 30 min around 400 C the growth sequence of these compounds is not the same for the two Cu/Au/Si(1 0 0) and Cu/Au/Si(1 1 1) multilayered structures. The annealing temperature increase leads to the well-oriented growth of crystallites corresponding to Cu3Si and Cu4Si silicides on Si(1 1 1), but only Cu4Si crystallites with square and rectangular shapes are observed on Si(1 0 0) substrate.
References [1] A.L. Barg, B.S. Bokstein, L.M. Klinger, J. Appl. Phys. 72 (1992) 1359. [2] M. Nicolet, S.S. Lau, VLSI Applications, Academic Press, London, 1983, p. 15. [3] Chin-An Chang, J. Vac. Sci. Technol. A 9 (1991) 98. [4] G. Rossi, I. Lindan, Phys. Rev. B 28 (1983) 98. [5] K. Barmak, C. Michaelsen, G. Lucadamo, J. Mater. Res. 12 (1997) 133. [6] R. Pretorius, J.W. Mayer, J. Appl. Phys. 81 (1997) 2442. [7] S.P. Murarka, Silicides for VLSI Application, Academic Press, London, 1983, p. 15. [8] N.G. Galkin, A.V. Konchenko, S.V. Vavanova, A.M. Malso, A.O. Talanov, Appl. Surf. Sci. 175–176 (2001) 299. [9] Jian Li, W. Straine, S.W. Russell, S.Q. Hong, J.W. Mayer, J. Appl. Phys. 72 (1992) 2810. [10] K.N. Tu, J.W. Mayer, Thin Films Interdiffusion and Reactions, Vielvey, London, 1978, p. 359. [11] K. Maex, Properties of Metal Silicides, INSPEC, London, 1995, p. 144. [12] D.T. Hawkins, R. Hultgren, Metals Handbook ‘‘constitution of Binary Alloys’’, vol. 8, American Society For Metals, 1973, p. 251. [13] M. Hanbucken, J.J. Me´tois, P. Matthiez, F. Salvan, Surf. Sci. 162 (1985) 622. [14] J. Li, H. Yonezawa, T. Shigematsu, J. Appl. Phys. 31 (1992) 210.
Atomic diffusion at the Cu–Au–Si multilayers interface S. Iaiche a, N. Benouattas b, A. Bouabellou a
a,*
, L. Osmani b, L. Salik
b
Laboratoire Couches Minces et Interfaces, Universite´ Mentouri Constantine, Campus Chaabet Erssas, Constantine 25000, Alge´rie b Laboratoire Surfaces et Interfaces des Mate´riaux Solides, Universite´ Ferhat Abbas, Setif 19000, Alge´rie Available online 7 April 2005
Abstract Si(1 0 0) and (1 1 1) oriented silicon wafers were used as a substrate for metallic bilayers deposition of copper and gold. Cu/Au/Si structures were obtained by thermal evaporation and then heated below 400 C in vacuum. These solid-state reactions occurred in the samples have been studied using X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), scanning electronic microscopy (SEM) and X-ray dispersive energy analyzer (XDE). The study shows that heat treatment at 200 C of the multilayered Cu/Au/Si structure leads to the formation and the co-existence of both Cu3Si and Cu4Si copper rich-silicides with the expansion of their respective cells, independently of the orientation of the substrate. The increasing of the annealing temperature until 400 C leads to the growth of well-oriented crystallites corresponding to Cu3Si and Cu4Si silicides on Si(1 1 1) but only Cu4Si crystallites with square and rectangular shapes on Si(1 0 0). The thermal stability of formed copper silicides after heat treatment at 400 C during 30 min for both Cu/Au/Si(1 0 0) and Cu/Au/Si(1 1 1) systems is analyzed. 2005 Elsevier B.V. All rights reserved. Keywords: Copper silicides; Gold; Atomic interdiffusion; Epitaxy
1. Introduction The phase composition, the phase sequence, the morphology, and the temperatures of silicides formation during the interactions of thin metal films with silicon substrates have long been explored, with both scientific interest and technological importance [1–4]. And the continuing interest in *
Corresponding author. Tel.: +213 31 61 4711; fax: +213 31 61 4711/4342. E-mail address: [email protected] (A. Bouabellou).
studies of silicides formation stems from the central importance of these reactions in vast area of applications in which new metallization schemes of integrated circuits [5]. For ULSI devices, for example, it is crucial that the layers of silicides are very uniform, have a low resistivity and a good thermal stability [6,7] and that epitaxial silicides prove to be good candidates for this role [8]. The big use of copper silicides in microelectronics is due to its lower electrical resistivity and higher electromigration resistance over aluminium [9,10]. Copper silicides are formed easily and offer
0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.03.030
350
S. Iaiche et al. / Microelectronic Engineering 81 (2005) 349–352
a good adherence on Si and SiO2 in comparison with other metal silicides. But the principal concern in Cu metallization is the thermal stability of Cu/Si, Cu/metals and Cu/silicides contacts, which is of great importance to the device performance [11]. That is why we introduce a new layer of Au to the Cu/Si system to control silicides formation and/or extend the thermal stability of silicide/Si contacts to higher temperatures.
2. Experiment Single-crystal wafers silicon of (1 0 0) and (1 1 1) orientation were used as substrates for metallic bilayers. The substrates were previously degreased in acetone, trichloroethylene and methanol baths, and then etched during 30 s in a diluted hydrofluoric acid (HF) solution to remove native oxide layer before to loading into the evaporator. High purity Au and Cu thin layers of 1 0 0 and 120 nm thick, respectively, were alternatively evaporated without breaking vacuum under pressure of ˚ /s. The 2 · 107 Torr. The deposition rate was 6 A obtained samples with layered geometry of Cu/ Au/Si were annealed in a vacuum furnace at temperature range between 200 and 400 C during one time of 30 min. The samples were analyzed by several techniques. X-ray diffraction (XRD) in the h 2h mode using Cu Ka radiation was carried out to identify the formed phases. Rutherford backscattering measurements (RBS) were performed using 2 MeV alpha beam (4 He+) and fitted with the help of the RUMP program. This analyze constitutes a powerful means in the examination of the chemical composition according to the thickness of the growing layer. The surface morphology of formed compounds were visualized by scanning electronic microscopy (SEM) operating at 9 keV and equipped with an X-ray dispersive energy analyzer. From the examination of the Cu–Si phases diagram [12], shown in Fig. 1, three low temperature equilibrium compounds could be formed: Cu3Si(g 0 ), Cu15Si4 or Cu4Si(e) and Cu5Si(c). We note that the stoechiometric range of these equilibrium alloys is rather narrow.
Fig. 1. Cu–Si equilibrium phases diagram [12].
3. Results and discussion Shown above, Fig. 2 represents a superposition of the backscattering spectra of the Cu/Au/Si(1 1 1) structure before and after annealing. It is easy to observe that the RBS spectrum of the as-prepared structure is composed of three well distinct parts: a peak corresponding to the gold film, towards high energies, a very spread out plate corresponding to the Si substrate towards low energies, and an inter-
Fig. 2. RBS spectra of Cu/Au/Si(1 1 1) multilayers: before annealing (————), annealed at 200 C (+++) and 400 C for 30 min (-Æ-Æ-Æ-Æ-Æ-).
S. Iaiche et al. / Microelectronic Engineering 81 (2005) 349–352
800 Cu 3Si Cu3Si
80
400˚C 30min
450
Cu4Si Cu3Si Cu3Si Cu 3Si
600
200
(b)
400˚ C 30min
150
100
50 Cu 3Si
150 0
Cu4Si
Cu3Si
300
0 40
50
60
2θ (deg)
70
40
50
60
70
2θ (deg)
Fig. 4. X-ray diffraction spectra of the Cu/Au/Si(1 0 0) structure annealed for 30 min at 200 C (a) and 400 C (b).
(b)
0
Cu3Si
Cu4Si
Cu3Si
Cu4Si
40 Cu4Si
400
200
(a)
(a)
750 200˚C 30min
60
Cu3Si
600
Cu4Si
Intensity (arb. unit)
200˚C 30min
among all the peaks recorded after annealing at 200 C, only two peaks corresponding to Cu3Si and Cu4Si persist as it is shown from the results of Fig. 3(b). For a similar structure using an Si(1 0 0) substrate, the compounds formed at 200 C are typical of those observed for the first structure (see Fig. 4(a)). But after annealing at higher temperature about 400 C (Fig. 4(b)), one notes the disappearance of the peaks corresponding to Cu3Si. It persists only the peak corresponding to the Cu4Si phase. The morphology of the surface of the Cu/Au/ Si(1 0 0) sample, shown on Fig. 5, is very heterogeneous. It indicates the growth of well-oriented
Intensity (arb. unit)
mediate signal corresponding to the copper layer deposited. The Au/Si and Cu/Au interfaces are very abrupt, which indicates that no reaction and/or formation of phase occurred during the preparation of the samples. The simulation of the spectra RBS allows to evaluate the thicknesses of the Cu ˚ , respectively. and Au layers to 1200 and 1030 A According to the RBS profiles corresponding to the Cu/Au/Si(1 1 1) samples annealed at 200 and 400 C (see Fig. 2), it is clear that the heat treatments induce an important interdiffusion of the three elements. The signals of the metal layers exhibit interfaces Cu/Au and Au/Si with different sides from those of the abrupt interfaces of the reference sample. Indeed, one observes a displacement towards high energies of the higher and lower energy limits of the signals Si and Cu, respectively. This is due to the diffusion of silicon towards surface and copper through the gold layer towards the substrate. Indeed, analysis of the X-ray diffraction profiles (Fig. 3) reflects structural changes in Cu/Au/ Si(1 1 1) due to the thermal reactions indicating the formation of new phases. Fig. 3(a) shows the presence of an abundance of peaks corresponding to Cu3Si and Cu4Si copper silicides after annealing at 200 C during 30 min. This reveals the polycrystalline character of the phases resulting from the reaction between the Cu layer and the Si substrate. By increasing the annealing temperature to 400 C,
351
20
0 40
60
2θ (deg)
80
40
60
80
2θ (deg)
Fig. 3. X-ray diffraction spectra of Cu/Au/Si(1 1 1) multilayers annealed at 200 C (a) and 400 C (b).
Fig. 5. SEM micrography of Cu/Au/Si(1 0 0) sample annealed at 400 C for 30 min.
352
S. Iaiche et al. / Microelectronic Engineering 81 (2005) 349–352
crystallites on the gray background which is composed mainly of silicon (96.27 at.%) originated from substrate. These crystallites of square (15 · 15 lm) and rectangular (25 · 23 lm) shapes are composed of an Au–Cu layer with a composition of 56.74 at.% Cu and 20.83 at.% Au interdiffused with silicon. One can affirm that the interdiffusion of different elements leads to the formation of a mixture Cu–Au–Si with a thickness equivalent to the sum of the evaporated metal layers thicknesses. The formation of the two Cu3Si and Cu4Si silicides at 200 C is in agreement with the equilibrium phases at low temperatures. In the same way, it is reported in many works on Cu/Si contacts [3], that the first growing phase on the interface is the Cu3Si silicide starting from the temperature threshold of 170 C, but on the Si(1 0 0) substrate, in particular, the reaction can take place at 125 C [13]. However, it is surprising to see that in the presence of the gold layer, the interfacial reactions between copper and silicon are also important for the structures with Si(1 0 0) and Si(1 1 1) substrates. But, in order to study the first stages of the reaction and to better determine the Si orientation effect on the kinetics of formation of this silicide, it would be desirable to go down even more in temperature. The increase in the annealing temperature to 400 C in the case of Cu/Au/Si(1 0 0) ternary system, shows a greater thermal stability of the Cu4Si phase compared to the Cu3Si phase at high temperature. This result was also found for the binary Cu/Si system [11]. On this fact, the Cu4Si phase can become more interesting for its employment in the microelectronic devices. It is also known that the formation of an SiO2 oxide layer in the Cu/Cu3Si/Si system at low temperature occurs at the expense of the Cu3Si compound until its disappearance [14]. For the structure with Si(1 1 1) substrate, the two silicides are formed and grow at the same time. This co-existence allows to suggest that on Si(1 1 1) one must further increase annealing at a
temperature higher than 400 C to see disappearing the Cu3Si phase.
4. Conclusion We have shown that for Cu/Au/Si structures, the Cu3Si and Cu4Si copper silicides are formed and remain stable at low temperature. With an anneal of 30 min around 400 C the growth sequence of these compounds is not the same for the two Cu/Au/Si(1 0 0) and Cu/Au/Si(1 1 1) multilayered structures. The annealing temperature increase leads to the well-oriented growth of crystallites corresponding to Cu3Si and Cu4Si silicides on Si(1 1 1), but only Cu4Si crystallites with square and rectangular shapes are observed on Si(1 0 0) substrate.
References [1] A.L. Barg, B.S. Bokstein, L.M. Klinger, J. Appl. Phys. 72 (1992) 1359. [2] M. Nicolet, S.S. Lau, VLSI Applications, Academic Press, London, 1983, p. 15. [3] Chin-An Chang, J. Vac. Sci. Technol. A 9 (1991) 98. [4] G. Rossi, I. Lindan, Phys. Rev. B 28 (1983) 98. [5] K. Barmak, C. Michaelsen, G. Lucadamo, J. Mater. Res. 12 (1997) 133. [6] R. Pretorius, J.W. Mayer, J. Appl. Phys. 81 (1997) 2442. [7] S.P. Murarka, Silicides for VLSI Application, Academic Press, London, 1983, p. 15. [8] N.G. Galkin, A.V. Konchenko, S.V. Vavanova, A.M. Malso, A.O. Talanov, Appl. Surf. Sci. 175–176 (2001) 299. [9] Jian Li, W. Straine, S.W. Russell, S.Q. Hong, J.W. Mayer, J. Appl. Phys. 72 (1992) 2810. [10] K.N. Tu, J.W. Mayer, Thin Films Interdiffusion and Reactions, Vielvey, London, 1978, p. 359. [11] K. Maex, Properties of Metal Silicides, INSPEC, London, 1995, p. 144. [12] D.T. Hawkins, R. Hultgren, Metals Handbook ‘‘constitution of Binary Alloys’’, vol. 8, American Society For Metals, 1973, p. 251. [13] M. Hanbucken, J.J. Me´tois, P. Matthiez, F. Salvan, Surf. Sci. 162 (1985) 622. [14] J. Li, H. Yonezawa, T. Shigematsu, J. Appl. Phys. 31 (1992) 210.