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Atomic hydrogen modification of copper surfaces studied by helium atom scattering
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Applied Surface Science 121/122 (1997) 138-141
Atomic hydrogen modification of copper surfaces studied by helium atom scattering T. Miyake *, H. Petek Adcanced Research Laboratory" Hitachi Ltd., Hatovama Saitama 350-03, Japan
Received 15 October 1996; accepted 24 February 1997
Abstract The structure and bonding of single crystal copper surfaces under irradiation by H atoms is investigated by helium atom scattering under UHV conditions. The effect of H atoms on Cu-Cu bonding is deduced from the temperature dependence of the intensity of the specular He reflection from a clean Cu(110) surface and for irradiation with an effusive H atom beam. The Debye temperature deduced from the attenuation of the specular reflection decreases from 340 _+ 10 K to 287 _+ l0 K upon exposure to hydrogen atoms, implying a decrease in the surface Rayleigh phonon frequency by the H-Cu interaction. Also, the chemical reduction of a Cu(100)-(2~- × ~-)R45°-O surface by irradiation with H atoms at 723 K is investigated. H atoms are shown to be an effective means for removing oxide impurities from Cu surfaces. © 1997 Elsevier Science B.V. Keywords: 07.77.Gx; 61.18.Bn; 61.82.Bg; 68.; 81.65.- b; 81.65.cf
1. Introduction This work presents a He atom scattering (HAS) study of H atom effects on Cu surfaces. Copper is a potentially important material for replacing A1 alloys for metallization in advanced ultra-large scale integration (ULSI) circuits due to its low resistivity and high durability against electromigration [1-3]. In the fabrication of Cu interconnects, the chemical processing of Cu surfaces is of interest for catalytic enhancement of diffusion, control of adhesion, and removal of surface impurities. Although surface chemistry of Cu has been a subject of many experi-
* Corresponding author. Tel.: 4-81-492-966111 ; fax: + 81-492966006; e-mail: [email protected].
mental and theoretical studies, most of these did not address the issues of interest to the electronic industry. Of current interest is technology for low temperature diffusion (reflow) of Cu for fabrication of < 0.2 /xm interconnects. The investigation of H atom effect on Cu surfaces, is motivated by recent theoretical work by Stumpf that predicts a reduction in the barrier for self-diffusion on the Be(0001) surface by a factor of 3 in the presence of surface atomic hydrogen [4]. If H atoms have similar effect on other metals, this could be a general method for inducing low temperature surface diffusion. The presence of surface impurities at sub-monolayer concentration also can influence surface diffusion rates. Effective cleaning of a Cu surface is demonstrated by H atom removal of an oxide layer from a Cu(100) surface.
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S01 69-4332(97)00273-0
7~ Miyake, 14. Petek /Applied Surface Science 121 / 122 (1997) 138-14l
2. Experimental Helium atom scattering gives atomic level, Fourier space information on structure, binding, diffusion, and energy relaxation at the uppermost atomic or molecular layer at the surface-vacuum interface [5]. The experiments are performed using the high angular and energy resolution HAS spectrometer described in detail in Ref. [6]. The vacuum system consists of five main units: doubly differentially pumped beam source chamber, the scattering chamber, the loadqock chamber, the two-chamber timeof-flight unit, and the two-chamber detector unit. The scattering geometry is fixed, namely, 0 i + 0r = 90 °, where 0 i and Or are incident and scattered angles, respectively. The scattering chamber is equipped with a sample manipulator, a reflection high-energy electron diffraction apparatus, a cylindrical mirror analyzer type Auger electron spectrometer (AES), an atomic hydrogen source, a residual gas quadrupole mass spectrometer, and an ion gun. The effusive H source consists of a 0.15 mm diameter W filament, which is wrapped into a coil within a 3 mm inner diameter alumina tube, and heated to 2000 K (monitored by optical pyrometer). He diffraction measurements are performed by rotating the sample manipulator perpendicular to the scattering plane with 0.01 ° resolution. The sample temperature is regulated with an electron bombardment heater and a liquid nitrogen cryostat, which allow sample temperature control in the range of 110-1500 K. Copper crystal surfaces are oriented to better than 0.5 °, mechanically polished, and a clean surface is prepared by repeated cycles of sputtering (500 eV Ar + ions at 500 K), and annealing at 800 K for 30 rain. No contamination above 1% could be detected by Auger analysis.
139
surface temperature plots for three conditions: (a) under vacuum, (b) exposed to H 2 , and (c) exposed to H / H 2. Analysis of the attenuation curves gives the following Debye temperatures: (a) 340 4- 10; (b) 331 + 10; and (c) 287 4- 10 K. The Debye temperature for (a) and (b) is the same as previously reported for Cu(110) [7]. By contrast, exposure to H atoms reduces the surface Debye temperature of Cu(110) by about 50 K. This can be interpreted by a reduction in the surface Rayleigh phonon frequency from the effect of H on C u - C u bonding. However, other mechanisms for attenuation of the specular intensity include an increased density of surface defects and surface roughening. Weaker C u - C u interaction, higher density of defects, and surface roughening all imply a higher mobility of surface Cu atoms, and therefore, a lower activation energy for self-diffusion. Such effect is reported for the first time in Cu; however, there are precedents for similar H atom effects on other metals. Density functional theory calculations on Be(0001) indicate that surface H - B e bonding transfers the electron density from the Besubstrate bond, resulting in reduction of the barrier for self-diffusion to 1/3 of the clean surface value [4]. Also, annealing of Nb thin films in H atmosphere has been shown to greatly improve crystalinity [8]. Fig. 2 shows the half-order (0, T~/2) diffraction peak intensity vs. surface temperature for
''i
.......
I .......
~-~ 1 0 6 ~
o
I
I .......
I .......
..... I'
I .......
I'''
....
I ....
He/Cu(110)
~
010]
{ 3. Results and discussion Attenuation of the specular beam with temperature due to inelastic scattering of He with surface phonons is often used to measure the surface Debye temperature [7]. Effect of H atom irradiation on the surface bonding at the Cu(110) surface is deduced from the specular He beam attenuation with temperature. Fig. 1 shows the He specular intensity vs.
>"
:));;Wtth h ° H : ( ~ ~ ~ 105 c); with H, H2(H2:5x10-8 torr) '%"%. 400
440
480
520
560
600
640
Surface Temperature [K] Fig. 1. Helium atom specular intensity vs. surface temperature under the following conditions: (a) in vacuum; (b) H2 atmosphere at 5 × 1 0 -8 Torr; and (c) H / H 2 atmosphere at H 2 ~ 5 × 10 -8 Tort.
T. Mivake H. Petek / Applied Surface Science 121 / 122 (1997) 138-141
140
Cu(110)-2 X l-H surface. The rise in the intensity at ~ 240 K may be due to subsurface H migration to the surface, which requires thermal activation [9]. Since, the desorption temperature of H from single crystal Cu surfaces is ~ 300 K, for the Debye temperature measurements, H can be present on the surface only on the time scale of associative desorption. Fig. 3 shows the He diffraction scans along the [010] direction with incident energy of 64 meV for three conditions: (a) a Cu(100)-(2~-X f2-)R45°-O surface, (b) after exposing the oxide surface to H 2, and (c) after exposing to H / H 2. The surface is prepared by exposing the Cu(100) surface at 573 K to 1000 L O 2. H 2 is introduced into the scattering chamber through the H atom source resulting in a pressure rise to 1 × 10 7 Torr at 723 K. The surface is exposed to hydrogen for 30 rain with the H atom source off (Fig. 3b) or on (Fig. 3c). In the case of exposure to only molecular hydrogen, there is negligible reduction of the oxide. However, H atom irradiation results in effective reduction of the oxide layer to recover intense specular peak of the clean Cu(100) surface (corrugation of this surface is too small to see diffraction). The most probable reaction induced by H atoms is: Cu20 + 2H ~ 2Cu + H201'. The present data are not sufficient to determine the mechanism for reduction. The initial step for the
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~
. . . .
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Surface Temperature [K] Fig. 2. Intensity of the (0, ~ diffraction peak due to the Cu(110)-2 X I-H surface as a function of surface temperature. The inset shows the He-diffraction along the [001] azimuth at 220 K.
- I l l , l r l l , [ l i , l l l , f I [ l l l l l l l f l l J , I , -
(a) _
He/Cu(lO0)
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era
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.
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.
.
.
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Scattered Angle [deg] Fig. 3. He-diffraction scans along the [010] azimuth with incident energy of 64 meV for three conditions: (a) a Cu(100)-(2~/2 X ~ ' ) R 4 5 ° - O surface, (b) after exposing the oxide surface to H2, and (c) after exposing to H / H 2.
reaction probably involves formation of a partial hydroxide layer. Formation of H 2 0 ( s ) may occur by reaction of OH(s) by highly mobile surface H atoms (Langmuir-Hinschelwood mechanism) or by direct abstraction by the incident H atom beam (Elay-Reidel mechanism). The mechanism and kinetics for the oxide reduction process will be the subject of a future work. The reduction of oxide by H 2 is highly inefficient due to a ~ 1 eV barrier to dissociative chemisorption on Cu surfaces, even though the diffusive source is expected to produce translationally, rotationally, and vibrationally excited H 2 [10]. Surface cleaning may have a significant effect on surface diffusion, since there is evidence that contamination or defects at l0 4 monolayer level can drastically reduce the step flow diffusion rate of Cu [11]. The surface cleaning effect of H atoms also has been investigated by AES for sputtered films [3]. For polycrystalline samples removal of oxygen and carbon impurities to below AES sensitivity level ( < 1%) occurs at < 200°C in the presence of H atoms. Thus the effect of H atoms on surface diffusion also may be due to removal of impurities that act as pinning
T. Miyake, H. Petek /Applied Surface Science 121/122 (1997) 138-141
sites for the Cu surface transport. Furthermore, low temperature cleaning of Cu is useful for reducing the electrical resistance at interfaces in multi-layer metallization.
4. Conclusion He atom scattering technique is used to study H atom modification of Cu surfaces related to low temperature reflow of Cu. Irradiation of a Cu(110) surface by an effusive H beam reduces the surface Debye temperature by ~ 50 K, which may be due to weaker Cu-substrate bonding in the presence of H. H atoms also are shown to efficiently remove impurities from Cu surfaces. The reduction of oxide on a Cu(100)-(2~- × ¢2-)R45°-O surface, which results in a clean, highly ordered Cu(100) surface, occurs in the presence of atomic H but not with H 2. H atom processing of metal surfaces may offer significant advantages in metallization technology for ULSI devices.
141
References [1] C.W. Park, R.W. Vook, Appl. Phys. Lett. 59 (1991) 175. [2] G. Bai, C. Chiang, J.N. Cox, S. Fang, D.S. Gardner, A. Mack, T. Marieb, X.C. Mu, V. Ochoa, R. Villasol, J. Yu, Symposium on VLSI Technology, Digest of Technical Papers, 1996, p. 48. [3] T. Miyake, H. Petek, K. Takeda, K. Hinode, Appt. Phys. Lett. 70 (1997) 1239. [4] R. Stumpf, Phys. Rev. B 53 (1996) R4253. [5] E. Hulpke (Ed.), Springer Series in Surface Sciences, Vol. 27, Springer, Heidelberg, 1993. [6] T. Miyake, E.S. Gillman, I. Oodake, H. Petek, Jpn. J. Appl. Phys. 36 (1997) 4531. [7] G. Armand, J. Lapujoulade, Y. Lejay, in: Proc. 7th Int. Vac. Congr. and 3rd Intern. Conf. Solid Surfaces, Vienna, 1977, p. 1361. [8] P.M. Reimer, H. Zabel, C.P. Flynn, J.A. Dura, Phys. Rev. B 45 (1992) 11426. [9] K.H. Rieder, W. Stocker, Phys. Rev. Lett. B 57 (1986) 2548. [10] H.A. Michelsen, C.T. Rettner, D.J. Auerbach, in: Springer Series in Surface Sciences, Vol. 34, Springer-Verlag, Berlin, 1994. [11] M. Ritter, M. Stindtmann, M. Farle, K. Baberschke, Surf. Sci. 348 (1996) 243.
Applied Surface Science 121/122 (1997) 138-141
Atomic hydrogen modification of copper surfaces studied by helium atom scattering T. Miyake *, H. Petek Adcanced Research Laboratory" Hitachi Ltd., Hatovama Saitama 350-03, Japan
Received 15 October 1996; accepted 24 February 1997
Abstract The structure and bonding of single crystal copper surfaces under irradiation by H atoms is investigated by helium atom scattering under UHV conditions. The effect of H atoms on Cu-Cu bonding is deduced from the temperature dependence of the intensity of the specular He reflection from a clean Cu(110) surface and for irradiation with an effusive H atom beam. The Debye temperature deduced from the attenuation of the specular reflection decreases from 340 _+ 10 K to 287 _+ l0 K upon exposure to hydrogen atoms, implying a decrease in the surface Rayleigh phonon frequency by the H-Cu interaction. Also, the chemical reduction of a Cu(100)-(2~- × ~-)R45°-O surface by irradiation with H atoms at 723 K is investigated. H atoms are shown to be an effective means for removing oxide impurities from Cu surfaces. © 1997 Elsevier Science B.V. Keywords: 07.77.Gx; 61.18.Bn; 61.82.Bg; 68.; 81.65.- b; 81.65.cf
1. Introduction This work presents a He atom scattering (HAS) study of H atom effects on Cu surfaces. Copper is a potentially important material for replacing A1 alloys for metallization in advanced ultra-large scale integration (ULSI) circuits due to its low resistivity and high durability against electromigration [1-3]. In the fabrication of Cu interconnects, the chemical processing of Cu surfaces is of interest for catalytic enhancement of diffusion, control of adhesion, and removal of surface impurities. Although surface chemistry of Cu has been a subject of many experi-
* Corresponding author. Tel.: 4-81-492-966111 ; fax: + 81-492966006; e-mail: [email protected].
mental and theoretical studies, most of these did not address the issues of interest to the electronic industry. Of current interest is technology for low temperature diffusion (reflow) of Cu for fabrication of < 0.2 /xm interconnects. The investigation of H atom effect on Cu surfaces, is motivated by recent theoretical work by Stumpf that predicts a reduction in the barrier for self-diffusion on the Be(0001) surface by a factor of 3 in the presence of surface atomic hydrogen [4]. If H atoms have similar effect on other metals, this could be a general method for inducing low temperature surface diffusion. The presence of surface impurities at sub-monolayer concentration also can influence surface diffusion rates. Effective cleaning of a Cu surface is demonstrated by H atom removal of an oxide layer from a Cu(100) surface.
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S01 69-4332(97)00273-0
7~ Miyake, 14. Petek /Applied Surface Science 121 / 122 (1997) 138-14l
2. Experimental Helium atom scattering gives atomic level, Fourier space information on structure, binding, diffusion, and energy relaxation at the uppermost atomic or molecular layer at the surface-vacuum interface [5]. The experiments are performed using the high angular and energy resolution HAS spectrometer described in detail in Ref. [6]. The vacuum system consists of five main units: doubly differentially pumped beam source chamber, the scattering chamber, the loadqock chamber, the two-chamber timeof-flight unit, and the two-chamber detector unit. The scattering geometry is fixed, namely, 0 i + 0r = 90 °, where 0 i and Or are incident and scattered angles, respectively. The scattering chamber is equipped with a sample manipulator, a reflection high-energy electron diffraction apparatus, a cylindrical mirror analyzer type Auger electron spectrometer (AES), an atomic hydrogen source, a residual gas quadrupole mass spectrometer, and an ion gun. The effusive H source consists of a 0.15 mm diameter W filament, which is wrapped into a coil within a 3 mm inner diameter alumina tube, and heated to 2000 K (monitored by optical pyrometer). He diffraction measurements are performed by rotating the sample manipulator perpendicular to the scattering plane with 0.01 ° resolution. The sample temperature is regulated with an electron bombardment heater and a liquid nitrogen cryostat, which allow sample temperature control in the range of 110-1500 K. Copper crystal surfaces are oriented to better than 0.5 °, mechanically polished, and a clean surface is prepared by repeated cycles of sputtering (500 eV Ar + ions at 500 K), and annealing at 800 K for 30 rain. No contamination above 1% could be detected by Auger analysis.
139
surface temperature plots for three conditions: (a) under vacuum, (b) exposed to H 2 , and (c) exposed to H / H 2. Analysis of the attenuation curves gives the following Debye temperatures: (a) 340 4- 10; (b) 331 + 10; and (c) 287 4- 10 K. The Debye temperature for (a) and (b) is the same as previously reported for Cu(110) [7]. By contrast, exposure to H atoms reduces the surface Debye temperature of Cu(110) by about 50 K. This can be interpreted by a reduction in the surface Rayleigh phonon frequency from the effect of H on C u - C u bonding. However, other mechanisms for attenuation of the specular intensity include an increased density of surface defects and surface roughening. Weaker C u - C u interaction, higher density of defects, and surface roughening all imply a higher mobility of surface Cu atoms, and therefore, a lower activation energy for self-diffusion. Such effect is reported for the first time in Cu; however, there are precedents for similar H atom effects on other metals. Density functional theory calculations on Be(0001) indicate that surface H - B e bonding transfers the electron density from the Besubstrate bond, resulting in reduction of the barrier for self-diffusion to 1/3 of the clean surface value [4]. Also, annealing of Nb thin films in H atmosphere has been shown to greatly improve crystalinity [8]. Fig. 2 shows the half-order (0, T~/2) diffraction peak intensity vs. surface temperature for
''i
.......
I .......
~-~ 1 0 6 ~
o
I
I .......
I .......
..... I'
I .......
I'''
....
I ....
He/Cu(110)
~
010]
{ 3. Results and discussion Attenuation of the specular beam with temperature due to inelastic scattering of He with surface phonons is often used to measure the surface Debye temperature [7]. Effect of H atom irradiation on the surface bonding at the Cu(110) surface is deduced from the specular He beam attenuation with temperature. Fig. 1 shows the He specular intensity vs.
>"
:));;Wtth h ° H : ( ~ ~ ~ 105 c); with H, H2(H2:5x10-8 torr) '%"%. 400
440
480
520
560
600
640
Surface Temperature [K] Fig. 1. Helium atom specular intensity vs. surface temperature under the following conditions: (a) in vacuum; (b) H2 atmosphere at 5 × 1 0 -8 Torr; and (c) H / H 2 atmosphere at H 2 ~ 5 × 10 -8 Tort.
T. Mivake H. Petek / Applied Surface Science 121 / 122 (1997) 138-141
140
Cu(110)-2 X l-H surface. The rise in the intensity at ~ 240 K may be due to subsurface H migration to the surface, which requires thermal activation [9]. Since, the desorption temperature of H from single crystal Cu surfaces is ~ 300 K, for the Debye temperature measurements, H can be present on the surface only on the time scale of associative desorption. Fig. 3 shows the He diffraction scans along the [010] direction with incident energy of 64 meV for three conditions: (a) a Cu(100)-(2~-X f2-)R45°-O surface, (b) after exposing the oxide surface to H 2, and (c) after exposing to H / H 2. The surface is prepared by exposing the Cu(100) surface at 573 K to 1000 L O 2. H 2 is introduced into the scattering chamber through the H atom source resulting in a pressure rise to 1 × 10 7 Torr at 723 K. The surface is exposed to hydrogen for 30 rain with the H atom source off (Fig. 3b) or on (Fig. 3c). In the case of exposure to only molecular hydrogen, there is negligible reduction of the oxide. However, H atom irradiation results in effective reduction of the oxide layer to recover intense specular peak of the clean Cu(100) surface (corrugation of this surface is too small to see diffraction). The most probable reaction induced by H atoms is: Cu20 + 2H ~ 2Cu + H201'. The present data are not sufficient to determine the mechanism for reduction. The initial step for the
'
r-1 o
3
~ 2
o
'
'
,,
~
t"-
~ 13_
~
'
'
I
. . . .
t
0
,
'2
0
'
'
'
310cajOd
. . . .
'
I
[001]
Ts=220K (o,o)
~
. . . .
I
He/Cu (110)
I \
.o. C
I
;n~iOe
~~_
[deg~ I'O
. . . .
Surface Temperature [K] Fig. 2. Intensity of the (0, ~ diffraction peak due to the Cu(110)-2 X I-H surface as a function of surface temperature. The inset shows the He-diffraction along the [001] azimuth at 220 K.
- I l l , l r l l , [ l i , l l l , f I [ l l l l l l l f l l J , I , -
(a) _
He/Cu(lO0)
3
[ol o]
%2 -(2~/2x~/2)R45o-o
Ts=300K Ei=e4meV
era
0 .,
o
2 7- H2 irradiation
o
! ! i i i i • . .~--.4-4~,, .
ii
r<
]
ii ....
i
, , , , i .... .
(b) -
~- H2; lx10-7 torr = at Ts=723K, 30min W filament; OFF
¢--
0
o 6 13. ('0%4 )<
.
H,Hirradiation 2 f
.
.
.
.
ii
.
t
/C)
i
w
'~.
xlooo
2 0 ~-(. . . . 2'0 . . . . 3()
4[) ~ "-'5'0 . . . . 6(i . . . . 7() . . . . 80
Scattered Angle [deg] Fig. 3. He-diffraction scans along the [010] azimuth with incident energy of 64 meV for three conditions: (a) a Cu(100)-(2~/2 X ~ ' ) R 4 5 ° - O surface, (b) after exposing the oxide surface to H2, and (c) after exposing to H / H 2.
reaction probably involves formation of a partial hydroxide layer. Formation of H 2 0 ( s ) may occur by reaction of OH(s) by highly mobile surface H atoms (Langmuir-Hinschelwood mechanism) or by direct abstraction by the incident H atom beam (Elay-Reidel mechanism). The mechanism and kinetics for the oxide reduction process will be the subject of a future work. The reduction of oxide by H 2 is highly inefficient due to a ~ 1 eV barrier to dissociative chemisorption on Cu surfaces, even though the diffusive source is expected to produce translationally, rotationally, and vibrationally excited H 2 [10]. Surface cleaning may have a significant effect on surface diffusion, since there is evidence that contamination or defects at l0 4 monolayer level can drastically reduce the step flow diffusion rate of Cu [11]. The surface cleaning effect of H atoms also has been investigated by AES for sputtered films [3]. For polycrystalline samples removal of oxygen and carbon impurities to below AES sensitivity level ( < 1%) occurs at < 200°C in the presence of H atoms. Thus the effect of H atoms on surface diffusion also may be due to removal of impurities that act as pinning
T. Miyake, H. Petek /Applied Surface Science 121/122 (1997) 138-141
sites for the Cu surface transport. Furthermore, low temperature cleaning of Cu is useful for reducing the electrical resistance at interfaces in multi-layer metallization.
4. Conclusion He atom scattering technique is used to study H atom modification of Cu surfaces related to low temperature reflow of Cu. Irradiation of a Cu(110) surface by an effusive H beam reduces the surface Debye temperature by ~ 50 K, which may be due to weaker Cu-substrate bonding in the presence of H. H atoms also are shown to efficiently remove impurities from Cu surfaces. The reduction of oxide on a Cu(100)-(2~- × ¢2-)R45°-O surface, which results in a clean, highly ordered Cu(100) surface, occurs in the presence of atomic H but not with H 2. H atom processing of metal surfaces may offer significant advantages in metallization technology for ULSI devices.
141
References [1] C.W. Park, R.W. Vook, Appl. Phys. Lett. 59 (1991) 175. [2] G. Bai, C. Chiang, J.N. Cox, S. Fang, D.S. Gardner, A. Mack, T. Marieb, X.C. Mu, V. Ochoa, R. Villasol, J. Yu, Symposium on VLSI Technology, Digest of Technical Papers, 1996, p. 48. [3] T. Miyake, H. Petek, K. Takeda, K. Hinode, Appt. Phys. Lett. 70 (1997) 1239. [4] R. Stumpf, Phys. Rev. B 53 (1996) R4253. [5] E. Hulpke (Ed.), Springer Series in Surface Sciences, Vol. 27, Springer, Heidelberg, 1993. [6] T. Miyake, E.S. Gillman, I. Oodake, H. Petek, Jpn. J. Appl. Phys. 36 (1997) 4531. [7] G. Armand, J. Lapujoulade, Y. Lejay, in: Proc. 7th Int. Vac. Congr. and 3rd Intern. Conf. Solid Surfaces, Vienna, 1977, p. 1361. [8] P.M. Reimer, H. Zabel, C.P. Flynn, J.A. Dura, Phys. Rev. B 45 (1992) 11426. [9] K.H. Rieder, W. Stocker, Phys. Rev. Lett. B 57 (1986) 2548. [10] H.A. Michelsen, C.T. Rettner, D.J. Auerbach, in: Springer Series in Surface Sciences, Vol. 34, Springer-Verlag, Berlin, 1994. [11] M. Ritter, M. Stindtmann, M. Farle, K. Baberschke, Surf. Sci. 348 (1996) 243.