- Email: [email protected]
Si(100) interface by photoelectron spectroscopic methods
applied surface science ELSEVIER
Applied Surface Science 123/124 (1998) 550 554
In situ investigation of the formation of an intermixed phase at the Ni/Si(100) interface by photoelectron spectroscopic methods R. Kilper a,*, S. Teichert b, p. Oelhafen a a lnstitut.[~r Phvsik. der Unit:ersitiit Basel, KlingelbergstrqBe 82, CH-4056 Basel, Swit=erland b lnstitutfiir Physik, TU Chemnit:, D-09107 Chemnit=, Germany
Abstract Photoemission experiments were performed systematically on N i / S i ( 1 0 0 ) interfaces and amorphous alloys with different nickel coverages and content, respectively. Both valence-band spectra and analysis of core level spectroscopy of the Ni2P3/= indicate the formation of a homogeneous intermixed, amorphous interface layer. The nickel content in this layer varies in a wide range with the nominal nickel coverage as long as the respective amorphous alloys exist. The interface reaction is terminated at a Ni concentration of about 75 at%, which coincides with the known upper stability limit for the amorphous alloy. © 1998 Elsevier Science B.V. PACS." 61.43Dq; 68.35.Dv; 71.20.Be; 82.80Pv
1. Introduction Interfaces of 3d-transition metal/silicon have been extensively studied due to their importance as the initial stage in the silicide formation process ([1] and references therein). The interest was focused on the structural and compositional behaviour of those interfaces formed after the deposition of a few monolayers (ML) of a transition metal onto clean silicon surfaces. The interface of N i / S i prepared at room temperature was characterized by structural as well as spectroscopic methods [2-6]. The authors have shown that even at room temperature a reaction between the metal and the silicon atoms occurs which leads to an intermixed metal/silicon interface layer. A still open question in this context concerns
': Corresponding author. Tel.: +41-61-2673714: fax: +41-612673784; e-mail: [email protected].
both the determination of the composition and the thickness of the reacted metal-silicon layer dependent on the deposited metal thickness. In this paper we propose a method based on analysis of photoelectron spectroscopic data for the determination of composition and structure of the reacted metal-silicon interface layer. Following the suggestions in [7] for the description of nobel metal/metaloid interfaces we will compare the electronic structure of the interfaces with these of the corresponding alloys. We demonstrate the application of this method to the Ni/Si(100) interface.
2. Experimental The in situ photoemission investigations were performed on a Fisons ESCALAB 210 electron spectrometer equipped with two electron beam evapora-
0169-4332/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 7 ) 0 0 5 6 9 2
551
R. Kilper et al./Applied Surface Science 123 / 124 (1998) 550-554
substrates at room temperature with a resulting film thickness of about 20 nm. During all deposition processes the pressure was kept below 5 × 10 t0 mbar. In order to record the valence-band structure as well as the core levels of the interfaces and the alloys an UPS resonance discharge lamp (He I, h u = 21.2 eV) and monochromatized X-ray source (A1K ~, h u = 1486.6 eV) were used. The measurements have been performed in the pass energy mode giving a constant energy resolution of about 80 meV (UPS) and 370 meV (MXPS), respectively.
tors (Omicron EMF-3). Prior to the metal deposition the Si substrates were prepared by ion beam cleaning and a subsequential thermal recrystallization. Typical surface states (labeled with S1 and $2 in Fig. 1) in the valence-band spectrum confirm the presence of a crystalline and partially reconstructed Si(100) surface [8]. The nickel metal was deposited at room temperature at rates of about 0.3 M L / m i n , whereas the rates were adjusted using a quartz crystal monitor. The amorphous Ni,.Si~00 ~ films with different stoichiometry were prepared by coevaporation of nickel and silicon at room temperature. (To our knowledge there are no data containing crystallization temperatures or the range of stability of the amorphous Ni~Silo 0 ., alloys. Chemically similar systems, like amorphous Pd~Si~00 ~ and Ni~B100 ~, exist at a metal content x < 80 at% and have crystallization temperatures well above 500 K [9]; see also Ref. [17].) These alloys were deposited onto sapphire
3. Results and discussion
The basic idea of our method is the comparison of the photoemission spectra taken from the interfaces with those from m e t a l - s i l i c o n alloys with a known metal content for determination of the composition
amorphous alloys [
] Ni/Si(100) - interface [
i
i
r
I
h'L Iii
>25
100
12
85
"2".
10
"5 .=_
7.0
75
5.0 2.0 1.4 1.0
68 60
0.5
45
0.3 0.2
41
52
27 18 4
3
2
1
binding energy (eV)
0
4
3
2
l
0
binding energy (eV)
Fig. I. Valence-band spectra of the Ni/Si(100) interface (left panel) m comparison to amorphous Ni ~SiL0o ~ (right panel, solid lines) and polycrystalline alloys (dashed lines, see text). The spectra were fixed at the maximum of the intensity.
552
R. Kilper et al. / Applied SurJace Science 123 / 124 (1998) 550-554
of the reacted metal-silicon interface [7]. The main point of this method is the extraction of significant features from the UPS as well as the MXPS spectra in order to perform a quantitative comparison. The evaluation of the photoemission data gives a set of parameters depending on the thickness t (in ML) of the nominal Ni coverage of the N i / S i interface and on the composition x in Ni~Sil00 x of the alloys. In a first step we compare the valence-band spectra of nickel-silicon interfaces with those of amorphous nickel-silicon alloys (Fig. 1, not all measured spectra shown here). With increasing Ni coverage on silicon the valence-band spectra of the interface is more and more dominated by a structure near 1.8 eV binding energy, which can be assigned to Ni 3d states. These states shift towards E v above coverages of about 2 ML Ni. For our question the most interesting qualitative features of these interface spectra are the c o n t i n u o u s changes in the shape of
the valence-band and the energetic position of the Ni 3d states with increasing Ni thickness. A similar behaviour of the UPS data can be observed for the amorphous alloys with rising Ni content x. It has to be emphasized that the continuous changes at the interface cannot be explained taking into account the UPS spectra obtained from the different crystalline bulk nickel silicides as described in the literature [10] solely. The valence-band spectra for low Ni coverages (below 1 ML) at the interface are unsuitable for the desired comparison because of the undeterminated contribution of the substrates to the spectra. However, for higher coverages this comparison seems to be justified. In order to characterize only the reacted interface layer we use the Ni 2p3/2 core level. This core level is sufficiently separated from other spectral features such as plasmons and Auger electrons and therefore the opportunity to a full quantitative
Ni/Si(100) - interface ]
amorphous alloys ] r
/
0.0
iI)
-1.0
T
T
0.20
/
-0.5
r
0.0
3.20 -0.5
0.15
-6
=
3.15
-1.0
[]
0.10 D
-1.5
/
2
4
68
1
2
4
Ni coverage (ML)
68
10
0.10
-1.5
O
0.05 0.1
/
~ 0
0 20
. 40
0 60
5 80
100
NixSil00-x (at%)
Fig. 2. The line shape parameters asymmetry a and AEbind (respective to pure nickel) of the Ni/Si(lO0) interface (left panel) in comparison to the amorphous alloys. The lines are to guide the eyes.
R. Kilper et al. / Applied Surface Science 123/124 (1998) 550-554
analysis of the core level with the Doniach-Sunjic line shape [11], convoluted with a Gaussian broadening, is given. Additionally a multiplet-splitting has been included, known from the analysis of the Fe 2p3/2 core level of ferromagnetic iron [12]. The independent determination of the parameters F L and F G (FWHM of the Lorentzian and Gaussian, respectively), the asymmetry parameter c~ and the binding energy Ebind w e r e performed by a least square fitting procedure [13]. A more detailed description and discussion of the reliability and accuracy of this method is also given in [7,14]. Results of the deconvolution of the line position A Ebind (respective to pure nickel) and asymmetry c~ of the Ni 2 p v 2 core level are shown in Fig. 2 for the N i / S i interfaces and the amorphous Ni,Sil0 0 , alloys, respectively. Briefly, these parameters are determined by the local chemical environment and local electronic structure [15] of the Ni atoms. A continuous change of the binding energy Ebind as well as the asymmetry c~ is observed as a function of the nominal Ni overlayer thickness t at the interface 100
,
o
,
,
, , , , , i
.
.
.
.
.
Ni3d position ] bind. energy Ni2p3/2 I asymmetry Ni2p3/2 I
,,,i
,
.g e©
60
//
40
0.1
1 Ni coverage (ML)
.
.
.
.
.
.
.
i
,
,
,
,
,i
x,"
3.0 x
®
2.5
.1
x x
2.0
®
x x
1.5
x
1.0 g
x
x x lg x
®
x
0.5
0.0 0.1
x x
amorphous interface layer
~ ,;,a,~,,
9
pure nickel
;_ , ; , 1
10
Ni coverage (ML) Fig. 4. Amount of Si which participates in the interface reaction as function of Ni coverages (a homogeneous interface layer assumed).
"/~ ff]~
80 z
.
553
l0
Fig. 3. From Fig. 2 determinated Ni concentration for the intermixed amorphous layer as function of the Ni coverages.
and of the Ni content x in the amorphous alloys. The general behaviour of the absolute values of the parameters are similar for both alloys and interfaces. An exact analysis of the trend of the lineshape parameters will be given elsewhere [16]. Here we use these parameters only for the comparison of the interface and the amorphous alloy. Using the asymmetry o~ and the binding energy Ebind obtained from the Ni 2p3/2 core level and, additionally, the position of the d-band maximum in the valence-band spectra for coverages higher than 1 ML, we can find a correlation between the composition of the reacted interface layer and the deposited nickel thickness (Fig. 3). Despite of the uncertainty in the determination of the position of the d-band maximum at the interface the results for the thickness dependence of the composition of the interface layers deduced from the UPS and MXPS analysis are in remarkable agreement for the three independent parameters of the photoelectron spectra. Taking into account the differences in the escape depth of the
554
R. Kilper et al. /Applied Surface Science 123 / 124 (1998) 550 554
photoelectrons for different kinetic energies [15] this behaviour suggests (i) the metal-silicon interface layers might be a nearly homogeneous amorphous mixture, and (ii) the concentration in the interface layer varies in a wide range of nickel content. Obviously, a certain amount of the silicon substrate has to participate in the interface reaction. A homogeneous mixture assumed the amount of the reacted silicon layer has to be a monotonous or constant function of the nominal nickel coverage as long as the reaction occurs. The actually incorporated Si thickness can be derived from the known deposited metal thickness and the determined composition of the interface layer (Fig. 4). At a coverage of about 6-7 ML Ni the thickness of the reacted silicon layer appears to decrease. We interpret this behaviour as that of an inhomogeneous system. It is fair to assume that this consists of the amorphous interface layer and a pure nickel film on the top. The latter was also observed in [5,6] for similar nickel coverages. An answer to the question why the reaction at room temperature terminates at that specific metal thickness could be found through a discussion of the composition of the resulting amorphous layer. The metal content in this amorphous layer is approximately 75 at% nickel (see Figs. 3 and 4) which coincidences well with the highest metal content x of other stable amorphous M%Si~00 x alloys [17]. Therefore, the reaching of the amorphous concentration seems to be responsible for the termination of the interface reaction.
4. Summary and outlook In summary, we have shown by means of a systematic study with photoelectron spectroscopic methods that a determination of the composition of reacted transition metal-silicon interfaces can be achieved. The described method has been applied to the Ni-Si interface. It has been shown that the quantitative and qualitative results of photoemission
measurements of the reacted interface layers accord with those of amorphous metal-silicon alloys. Further investigations are necessary both to estimate the range of the stability of the amorphous alloys and to study the interface reaction at different temperatures.
Acknowledgements Financial support by Swiss National Science Foundation is gratefully acknowledged.
References [l] A.H. Reader. A.H.v. Ommen, P.J.W. Weijs, R.A.M. Wolters, D.J. Oostra, Rep. Prog. Phys. 56 (1992) 1397. [2] K.L.I. Kobayashi, S. Sugaki, A. Ishizaka, Y. ShirakL H. Daimon, Y. Murata, Phys. Rev. B 25 (1982) 1377. [3] I. Abbati, L. Braicovich, B. d. Michelis, U. del Pennino, S. Valeri, Solid State Commun. 43 (1982) 199. [4] F. Comin, J.E. Rowe, P.H. Citrin, Phys. Rev. Lett. 51 (1983) 2402. [5] E.J.v. Loenen, J.W.M. Frenken, J.F.v.d. Veen, Appl. Phys. Lett. 45 (1984) 41. [6] E.J.v. Loenen, J.F.v.d. Veen, F.K. LeGoues, Surf. Sci. 157 (1985) 1. [7] H.-G. Boyen, A. Cossy-Favre, G. Gantner, H. Stupp, C. Lauinger, P. Oelhafen, J. Non-Cryst. Solids 205 207 (1996) 776. [8] K. Fujiwara, Phys. Rev. B 26 (1982) 2036. [9] J. Niebuhr, R. Gerber, A. Schaller, H.-W. Miiller, Physical Data of Amorphous Metals, FIZ, Karlsruhe, 1991. [10] A. Franciosi, J.H. Weaver, F.A. Schmidt, Phys. Rev. B 26 (1982) 546. [11] S. Doniach, M. Sunjic, J. Phys. C 3 (1970) 285. [12] F.U. Hillebrecht, C. Roth, H.B. Rose, W.G. Park, E. Kisker. N.A. Cherepkov, Phys. Rev. B 53 (1996) 12182. [13] G.K. Wertheim, S.B. Dicenzo, J. Electron Spectrosc. Rel. Phen. 37 (1985) 57. [14] R. Kilper, Dissertation, Technische Universit~it ChemnitzZwickau, 1996. Also available at: http://archiv.tuchemnitz.de/pub/1997/0008. [15] S. Hiifner, Photoelectron Spectroscopy, Springer, Berlin, 1995. [16] R. Kilper, H.-G. Boyen, S. Teichert, P. Oelhafen, to be published. [17] P. H~iussler, Phys. Rep. 222 (1992) 65.
Applied Surface Science 123/124 (1998) 550 554
In situ investigation of the formation of an intermixed phase at the Ni/Si(100) interface by photoelectron spectroscopic methods R. Kilper a,*, S. Teichert b, p. Oelhafen a a lnstitut.[~r Phvsik. der Unit:ersitiit Basel, KlingelbergstrqBe 82, CH-4056 Basel, Swit=erland b lnstitutfiir Physik, TU Chemnit:, D-09107 Chemnit=, Germany
Abstract Photoemission experiments were performed systematically on N i / S i ( 1 0 0 ) interfaces and amorphous alloys with different nickel coverages and content, respectively. Both valence-band spectra and analysis of core level spectroscopy of the Ni2P3/= indicate the formation of a homogeneous intermixed, amorphous interface layer. The nickel content in this layer varies in a wide range with the nominal nickel coverage as long as the respective amorphous alloys exist. The interface reaction is terminated at a Ni concentration of about 75 at%, which coincides with the known upper stability limit for the amorphous alloy. © 1998 Elsevier Science B.V. PACS." 61.43Dq; 68.35.Dv; 71.20.Be; 82.80Pv
1. Introduction Interfaces of 3d-transition metal/silicon have been extensively studied due to their importance as the initial stage in the silicide formation process ([1] and references therein). The interest was focused on the structural and compositional behaviour of those interfaces formed after the deposition of a few monolayers (ML) of a transition metal onto clean silicon surfaces. The interface of N i / S i prepared at room temperature was characterized by structural as well as spectroscopic methods [2-6]. The authors have shown that even at room temperature a reaction between the metal and the silicon atoms occurs which leads to an intermixed metal/silicon interface layer. A still open question in this context concerns
': Corresponding author. Tel.: +41-61-2673714: fax: +41-612673784; e-mail: [email protected].
both the determination of the composition and the thickness of the reacted metal-silicon layer dependent on the deposited metal thickness. In this paper we propose a method based on analysis of photoelectron spectroscopic data for the determination of composition and structure of the reacted metal-silicon interface layer. Following the suggestions in [7] for the description of nobel metal/metaloid interfaces we will compare the electronic structure of the interfaces with these of the corresponding alloys. We demonstrate the application of this method to the Ni/Si(100) interface.
2. Experimental The in situ photoemission investigations were performed on a Fisons ESCALAB 210 electron spectrometer equipped with two electron beam evapora-
0169-4332/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 7 ) 0 0 5 6 9 2
551
R. Kilper et al./Applied Surface Science 123 / 124 (1998) 550-554
substrates at room temperature with a resulting film thickness of about 20 nm. During all deposition processes the pressure was kept below 5 × 10 t0 mbar. In order to record the valence-band structure as well as the core levels of the interfaces and the alloys an UPS resonance discharge lamp (He I, h u = 21.2 eV) and monochromatized X-ray source (A1K ~, h u = 1486.6 eV) were used. The measurements have been performed in the pass energy mode giving a constant energy resolution of about 80 meV (UPS) and 370 meV (MXPS), respectively.
tors (Omicron EMF-3). Prior to the metal deposition the Si substrates were prepared by ion beam cleaning and a subsequential thermal recrystallization. Typical surface states (labeled with S1 and $2 in Fig. 1) in the valence-band spectrum confirm the presence of a crystalline and partially reconstructed Si(100) surface [8]. The nickel metal was deposited at room temperature at rates of about 0.3 M L / m i n , whereas the rates were adjusted using a quartz crystal monitor. The amorphous Ni,.Si~00 ~ films with different stoichiometry were prepared by coevaporation of nickel and silicon at room temperature. (To our knowledge there are no data containing crystallization temperatures or the range of stability of the amorphous Ni~Silo 0 ., alloys. Chemically similar systems, like amorphous Pd~Si~00 ~ and Ni~B100 ~, exist at a metal content x < 80 at% and have crystallization temperatures well above 500 K [9]; see also Ref. [17].) These alloys were deposited onto sapphire
3. Results and discussion
The basic idea of our method is the comparison of the photoemission spectra taken from the interfaces with those from m e t a l - s i l i c o n alloys with a known metal content for determination of the composition
amorphous alloys [
] Ni/Si(100) - interface [
i
i
r
I
h'L Iii
>25
100
12
85
"2".
10
"5 .=_
7.0
75
5.0 2.0 1.4 1.0
68 60
0.5
45
0.3 0.2
41
52
27 18 4
3
2
1
binding energy (eV)
0
4
3
2
l
0
binding energy (eV)
Fig. I. Valence-band spectra of the Ni/Si(100) interface (left panel) m comparison to amorphous Ni ~SiL0o ~ (right panel, solid lines) and polycrystalline alloys (dashed lines, see text). The spectra were fixed at the maximum of the intensity.
552
R. Kilper et al. / Applied SurJace Science 123 / 124 (1998) 550-554
of the reacted metal-silicon interface [7]. The main point of this method is the extraction of significant features from the UPS as well as the MXPS spectra in order to perform a quantitative comparison. The evaluation of the photoemission data gives a set of parameters depending on the thickness t (in ML) of the nominal Ni coverage of the N i / S i interface and on the composition x in Ni~Sil00 x of the alloys. In a first step we compare the valence-band spectra of nickel-silicon interfaces with those of amorphous nickel-silicon alloys (Fig. 1, not all measured spectra shown here). With increasing Ni coverage on silicon the valence-band spectra of the interface is more and more dominated by a structure near 1.8 eV binding energy, which can be assigned to Ni 3d states. These states shift towards E v above coverages of about 2 ML Ni. For our question the most interesting qualitative features of these interface spectra are the c o n t i n u o u s changes in the shape of
the valence-band and the energetic position of the Ni 3d states with increasing Ni thickness. A similar behaviour of the UPS data can be observed for the amorphous alloys with rising Ni content x. It has to be emphasized that the continuous changes at the interface cannot be explained taking into account the UPS spectra obtained from the different crystalline bulk nickel silicides as described in the literature [10] solely. The valence-band spectra for low Ni coverages (below 1 ML) at the interface are unsuitable for the desired comparison because of the undeterminated contribution of the substrates to the spectra. However, for higher coverages this comparison seems to be justified. In order to characterize only the reacted interface layer we use the Ni 2p3/2 core level. This core level is sufficiently separated from other spectral features such as plasmons and Auger electrons and therefore the opportunity to a full quantitative
Ni/Si(100) - interface ]
amorphous alloys ] r
/
0.0
iI)
-1.0
T
T
0.20
/
-0.5
r
0.0
3.20 -0.5
0.15
-6
=
3.15
-1.0
[]
0.10 D
-1.5
/
2
4
68
1
2
4
Ni coverage (ML)
68
10
0.10
-1.5
O
0.05 0.1
/
~ 0
0 20
. 40
0 60
5 80
100
NixSil00-x (at%)
Fig. 2. The line shape parameters asymmetry a and AEbind (respective to pure nickel) of the Ni/Si(lO0) interface (left panel) in comparison to the amorphous alloys. The lines are to guide the eyes.
R. Kilper et al. / Applied Surface Science 123/124 (1998) 550-554
analysis of the core level with the Doniach-Sunjic line shape [11], convoluted with a Gaussian broadening, is given. Additionally a multiplet-splitting has been included, known from the analysis of the Fe 2p3/2 core level of ferromagnetic iron [12]. The independent determination of the parameters F L and F G (FWHM of the Lorentzian and Gaussian, respectively), the asymmetry parameter c~ and the binding energy Ebind w e r e performed by a least square fitting procedure [13]. A more detailed description and discussion of the reliability and accuracy of this method is also given in [7,14]. Results of the deconvolution of the line position A Ebind (respective to pure nickel) and asymmetry c~ of the Ni 2 p v 2 core level are shown in Fig. 2 for the N i / S i interfaces and the amorphous Ni,Sil0 0 , alloys, respectively. Briefly, these parameters are determined by the local chemical environment and local electronic structure [15] of the Ni atoms. A continuous change of the binding energy Ebind as well as the asymmetry c~ is observed as a function of the nominal Ni overlayer thickness t at the interface 100
,
o
,
,
, , , , , i
.
.
.
.
.
Ni3d position ] bind. energy Ni2p3/2 I asymmetry Ni2p3/2 I
,,,i
,
.g e©
60
//
40
0.1
1 Ni coverage (ML)
.
.
.
.
.
.
.
i
,
,
,
,
,i
x,"
3.0 x
®
2.5
.1
x x
2.0
®
x x
1.5
x
1.0 g
x
x x lg x
®
x
0.5
0.0 0.1
x x
amorphous interface layer
~ ,;,a,~,,
9
pure nickel
;_ , ; , 1
10
Ni coverage (ML) Fig. 4. Amount of Si which participates in the interface reaction as function of Ni coverages (a homogeneous interface layer assumed).
"/~ ff]~
80 z
.
553
l0
Fig. 3. From Fig. 2 determinated Ni concentration for the intermixed amorphous layer as function of the Ni coverages.
and of the Ni content x in the amorphous alloys. The general behaviour of the absolute values of the parameters are similar for both alloys and interfaces. An exact analysis of the trend of the lineshape parameters will be given elsewhere [16]. Here we use these parameters only for the comparison of the interface and the amorphous alloy. Using the asymmetry o~ and the binding energy Ebind obtained from the Ni 2p3/2 core level and, additionally, the position of the d-band maximum in the valence-band spectra for coverages higher than 1 ML, we can find a correlation between the composition of the reacted interface layer and the deposited nickel thickness (Fig. 3). Despite of the uncertainty in the determination of the position of the d-band maximum at the interface the results for the thickness dependence of the composition of the interface layers deduced from the UPS and MXPS analysis are in remarkable agreement for the three independent parameters of the photoelectron spectra. Taking into account the differences in the escape depth of the
554
R. Kilper et al. /Applied Surface Science 123 / 124 (1998) 550 554
photoelectrons for different kinetic energies [15] this behaviour suggests (i) the metal-silicon interface layers might be a nearly homogeneous amorphous mixture, and (ii) the concentration in the interface layer varies in a wide range of nickel content. Obviously, a certain amount of the silicon substrate has to participate in the interface reaction. A homogeneous mixture assumed the amount of the reacted silicon layer has to be a monotonous or constant function of the nominal nickel coverage as long as the reaction occurs. The actually incorporated Si thickness can be derived from the known deposited metal thickness and the determined composition of the interface layer (Fig. 4). At a coverage of about 6-7 ML Ni the thickness of the reacted silicon layer appears to decrease. We interpret this behaviour as that of an inhomogeneous system. It is fair to assume that this consists of the amorphous interface layer and a pure nickel film on the top. The latter was also observed in [5,6] for similar nickel coverages. An answer to the question why the reaction at room temperature terminates at that specific metal thickness could be found through a discussion of the composition of the resulting amorphous layer. The metal content in this amorphous layer is approximately 75 at% nickel (see Figs. 3 and 4) which coincidences well with the highest metal content x of other stable amorphous M%Si~00 x alloys [17]. Therefore, the reaching of the amorphous concentration seems to be responsible for the termination of the interface reaction.
4. Summary and outlook In summary, we have shown by means of a systematic study with photoelectron spectroscopic methods that a determination of the composition of reacted transition metal-silicon interfaces can be achieved. The described method has been applied to the Ni-Si interface. It has been shown that the quantitative and qualitative results of photoemission
measurements of the reacted interface layers accord with those of amorphous metal-silicon alloys. Further investigations are necessary both to estimate the range of the stability of the amorphous alloys and to study the interface reaction at different temperatures.
Acknowledgements Financial support by Swiss National Science Foundation is gratefully acknowledged.
References [l] A.H. Reader. A.H.v. Ommen, P.J.W. Weijs, R.A.M. Wolters, D.J. Oostra, Rep. Prog. Phys. 56 (1992) 1397. [2] K.L.I. Kobayashi, S. Sugaki, A. Ishizaka, Y. ShirakL H. Daimon, Y. Murata, Phys. Rev. B 25 (1982) 1377. [3] I. Abbati, L. Braicovich, B. d. Michelis, U. del Pennino, S. Valeri, Solid State Commun. 43 (1982) 199. [4] F. Comin, J.E. Rowe, P.H. Citrin, Phys. Rev. Lett. 51 (1983) 2402. [5] E.J.v. Loenen, J.W.M. Frenken, J.F.v.d. Veen, Appl. Phys. Lett. 45 (1984) 41. [6] E.J.v. Loenen, J.F.v.d. Veen, F.K. LeGoues, Surf. Sci. 157 (1985) 1. [7] H.-G. Boyen, A. Cossy-Favre, G. Gantner, H. Stupp, C. Lauinger, P. Oelhafen, J. Non-Cryst. Solids 205 207 (1996) 776. [8] K. Fujiwara, Phys. Rev. B 26 (1982) 2036. [9] J. Niebuhr, R. Gerber, A. Schaller, H.-W. Miiller, Physical Data of Amorphous Metals, FIZ, Karlsruhe, 1991. [10] A. Franciosi, J.H. Weaver, F.A. Schmidt, Phys. Rev. B 26 (1982) 546. [11] S. Doniach, M. Sunjic, J. Phys. C 3 (1970) 285. [12] F.U. Hillebrecht, C. Roth, H.B. Rose, W.G. Park, E. Kisker. N.A. Cherepkov, Phys. Rev. B 53 (1996) 12182. [13] G.K. Wertheim, S.B. Dicenzo, J. Electron Spectrosc. Rel. Phen. 37 (1985) 57. [14] R. Kilper, Dissertation, Technische Universit~it ChemnitzZwickau, 1996. Also available at: http://archiv.tuchemnitz.de/pub/1997/0008. [15] S. Hiifner, Photoelectron Spectroscopy, Springer, Berlin, 1995. [16] R. Kilper, H.-G. Boyen, S. Teichert, P. Oelhafen, to be published. [17] P. H~iussler, Phys. Rep. 222 (1992) 65.