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Atomic phenomena on Si surfaces at adsorption of transition metals
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ELSEVIER
Applied Surface Science 104/105 (1996) 130-136
Atomic phenomena on Si surfaces at adsorption of transition metals B.Z. Olshanetsky Institute of Semiconductor Physics, Russian Academy of Sciences, Siberian Branch, Nol,osibirsk 630090, Russian Federation
Received 28 June 1995; accepted 14 November 1995
Abstract The effect of Ni and Co adsorption on Si surfaces with different orientations has been studied by LEED and AES. It has been established that the transport of Ni and Co atoms at Si surfaces is actually by their diffusion in the bulk of Si. There are two states of adsorbed Ni and Co atoms on Si surfaces. Nickel and cobalt adsorption may cause the reversible phase transitions on silicon surfaces at varying temperatures due to the change of their solubility in silicon and the redistribution of adsorbed atoms between the two possible states. The conditions for the existence of each of these states are different for Ni and Co atoms.
1. Introduction The nature of atomic processes on crystal surfaces has been attracting much attention in surface science. At the end of the nineteen-seventies and early eighties we discovered a number o f reversible o r d e r - o r d e r phase transitions on germanium and silicon surfaces with different orientations by LEED in the Institute of Semiconductor Physics in Novosibirsk [1-8]. W e observed several types of such reversible transitions: (1) reconstruction of one surface plane structure to another ( S i ( l l 0 ) surface); (2) reconstruction of atomically fiat surface to faceted one ( G e ( l l 0 ) and Si(320) surfaces); (3) reconstruction of faceted surface to stepped one (vicinal Ge(110) surfaces); (4) reconstruction of one step-terrace configuration to another (vicinal Si and Ge(111) surfaces). Later on Ichinokawa and coauthors had shown that the phase transitions on Si(110) surfaces discovered by us were due to the presence of nickel impurity and the variations of its surface concentra-
tion with temperature [9,10]. This was one of the reasons, which prompted us to carry out a systematical study of nickel surface heterodiffusion and of the effect of nickel adsorption on the structure and phase transitions on silicon surfaces with different orientations. They were described in our papers [11-14], which contain also the references to the related studies carried out in other laboratories. As a result we have found some interesting peculiarities of nickel behaviour on silicon surfaces, which, in turn, led us on to study the adsorption of other transition metal atoms on silicon surfaces. It was reasonable to begin such studies with Co adsorption, because the properties of Co atoms are close to those of Ni atoms.
2. Experimental The silicon samples were of p-type with resistivities between 5 - 1 0 f~ cm and the dimensions of
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0169-4332(96)00133-X
B.Z. Olshanetsky/ Applied Surface Science 104/105 (1996) 130-136
20 × 5 × 0.4 mm. Samples were oriented using the Laue X-ray technique with the precision of ___1°. The experiments were performed in an UHV system with the base pressure about 1 × 10 -~° Torr. The surface cleaning procedure was carried out by heating in vacuum at 1200°C for l - 2 min. The sources of Ni and Co atoms consisted of a tantalum ribbon to which a piece of Ni or Co plate was welded. The surface structure and the surface chemical composition were studied by LEED and AES in situ. To measure the Ni and Co surface concentrations the Auger Ni LMM (848 eV) and Co LMM (775 eV) electron peaks were used. The spatial resolution of the Auger spectrometer was several tens of microns.
3. Results a n d d i s c u s s i o n
3.1. Surface diffusion The unexpected outcome of our studies of the transition metals atoms adsorption on Si surfaces was the discovery of peculiarities of their transport along Si surfaces. In order to study surface diffusion of transition metals on silicon surfaces, a metal strips several nanometers thick were deposited perpendicular to the longer side of a sample (Fig. 1). In these experiments Co was evaporated from a BeO crucible. Then samples were annealed at certain temperatures by passing an alternating current. As it is known from literature, Ni and Co layers of several nanometer thickness fully react to the disilicides at 300-350°C. At about 960°C nickel disilicide strips melted and the films became uncontinuous. The same happened with Co disilicide strips at about 1240°C.
131
¢.%
lO
1
2
3
4
5
Fig. 2. Concentration distributions C(x) of nickel on S i ( l l l ) surface after annealing at 860°C for: (1) 15 s, (2) 60 s, (3) 240 s.
After annealing the concentration distributions of metal atoms along Si surfaces were measured by AES. Surface structures were studied by LEED. The examples of Ni concentration distributions measured in our experiments [11] are shown in Figs. 2 and 3. It was commonly supposed, and we also expected, that the surface diffusion coefficients are well in excess of those in the bulk, though there is a modest number of experimental data on surface diffusion in literature at present. It is known that the process of surface diffusion may depend on the surface orientation and structure, atomic steps, concentration of adsorbed atoms and so on. However, we did not observe the relationship between the measured nickel diffusion coefficients and the features of silicon surfaces. 10
C,% AAA ~
•
LEED, AES I
2
3
2 •
o • •
Fig. 1. Schematic of experimental setup. (1) Ni strip; (2) nickel disilicide islands; (3) Ni induced surface structures; (4) Ni atoms.
•
•
•
~ •
•
o
Fig. 3. Typical nickel C(x) distributions on silicon surfaces after annealing at 1050°C for 80 s: (1) (111); (2) (110); (3) (100).
132
B.Z Olshanetsky/ Applied Surface Science 104 / 105 (1996) 130-136
If nickel concentration did not exceed a certain value for given surface orientation, the concentration distributions, which we measured along the specimen, obeyed the equation C ( x ) = C Oe r f c ( x / 2 x / ~ ) , where x is the distance from the edge of the disilicide strip (the Ni concentration on the strip is not shown), D the diffusion coefficient and t the annealing time (Fig. 2). Such distributions are typical for one-dimensional surface diffusion from the source of constant strength. At nickel concentrations exceeding certain values (different for different planes) the C ( x ) curves deviate from those which are typical for the diffusional distributions: they become either steady-state or nonmonotonous and are badly reproducible [1]. As can be seen from Fig. 3, on S i ( l l l ) surfaces it takes place with nickel concentrations above 6%, and on Si(100) and (110) surfaces it is observed with concentrations above 1% and 2.5%, respectively. These deviations are due to the formation of three-dimensional silicide islands which could be observed by scanning electron microscope. The area occupied by three-dimensional islands of nickel silicide is much lower than that occupied by uniformly distributed nickel. And this causes either a decrease or a stabilization of the Auger signal from nickel on the surface with islands. However, the surprising thing was that nickel could not be detected at the surface by Auger spectroscopy during the sample heating. Nickel concentration distributions appeared on surfaces only after the sample cooling. So we drew the conclusion that the nickel transport along silicon surfaces is not by way of surface diffusion, but by its diffusion through the bulk with subsequent segregation to the surface at sample cooling due to the decrease of nickel solubility in silicon (Fig. 1). This conclusion agrees with the absence of correlation between the nickel diffusion coefficients and the surface orientation and structure, which was mentioned before. Besides, the diffusion coefficients deduced from our data are close to those of interstitial nickel diffusion in silicon ( 1 0 - 4 - 1 0 -5 cm 2 s -1 at ll00°C [15,16]). The amount of Ni, diffusing through Si bulk, is limited by Ni solubility, which is 2 × 1017-6 × 10 j7 c m - 3 in the temperature range of 1000-1300°C [ 17].
That is why in the distributions, presented in Figs. 2 and 3, Ni concentrations did not reach the values typical of the nickel disilicide. However, nickel diffusion on silicon surfaces does take place. It manifests itself by the formation of ordered surface structures and epitaxial disilicide islands. But the coefficients of nickel surface diffusion are by orders of magnitude less than those of interstitial nickel diffusion in silicon. We could not measure the temperature dependence of the Co diffusion coefficients on Si surfaces because of their small values and limited spatial resolution of the LEED and AES apparatus [18]. However, our estimates show that this coefficient is about 10 -5 cm2/s at 1200°C. It is known from literature that the coefficients of Co diffusion in silicon bulk may range 10-8-10 -4 cmZ/s at 10001300°C [ 19], and, specifically, at 1100°C this coefficient approximates 10 -5 cm2/s [20]. We can see that the Co diffusion coefficient deduced at Si surface is similar to that in Si bulk, like in the case of Ni diffusion. Thus, one can infer that the transport of Co atoms at Si surfaces is also by way of their diffusion through Si bulk. The solubility of Co in Si is 5 X 1014-3 X 1016 cm -3 in the temperature range of 1000-1300°C [17]. Thus, our finding is contrary to the existing notions that the activation energy of surface diffusion is much less than that of the diffusion through the body of solids and that the rate of surface atom migration is considerably greater than that in the bulk. 3.2. Two states o f adsorbed atoms
Our results show that there are two states of Ni and Co atoms on Si surfaces. These states correspond to different minima of the surface system free energy. The atoms in the first state are likely to be regularly distributed on surfaces and they cause the formation of various surface structures (Fig. 1). Several structures can be formed on a Si surface of given orientation, each corresponding to a certain concentration of Ni or Co atoms. The adsorbed atoms in the second state are involved in the formation of the epitaxial disilicide islands. This state of atoms is more stable. The conditions to fall in each of these states are different for Ni and Co adsorbed atoms.
B.Z. Olshanetsky/Applied Surface Science 104/105 (1996) 130-136
plane. In this case on the free of islands parts of the surface the structure with the maximum nickel content remained. Our explanation of this phenomenon is as follows. At sample cooling nickel atoms segregate to the surface. However, some time is needed for the formation of critical nuclei of the nickel disilicide phase and for their growth. During fast sample cooling (at the rates above 100°C/s) there is not enough time to do this and the outdiffused nickel atoms occupy the sites corresponding to the less deep minimum of energy. At slow sample cooling or low-temperature annealing (600-700°C) the stable phase has time to
In the case of Ni adsorption the first state of adsorbed atoms could be obtained by cooling a sample at a rate above 100°C/s. However, the Ni induced structures on Si surfaces are metastable. Slow cooling or annealing at 600-700°C of samples with adsorbed nickel causes formation of the epitaxial disilicide islands, nickel atoms diffusing to these islands from the adjacent parts of surface and leaving them free of nickel impurity. The islands can be seen using scanning electron microscope. Their density was about 5 × 10 6 c m - 2 . The first state of adsorbed atoms is associated with the shallow minima and the disilicide phase - - with the deep minimum of free energy. Disilicide islands started to grow also when Ni concentration exceeded the value, which was necessary for the formation of the surface structure with the maximum nickel content for a given surface
T'C
133
form.
We did not observe the effect of sample cooling on the structure of cobalt adsorbed Si surfaces. In the case of Co adsorption a condition which determined
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Fig. 4. Surface structures induced by Co versus Co coverage and sample temperature: (a) Si(100), SPE mode; (b) Si(100), MBE mode; (c) Si(l 11), SPE mode; (d) Si(l 11)+ MBE mode.
B.Z Olshanetsky/ Applied Surface Science 104/105 (1996) 130-136
134
the state of adsorbed atoms was the coverage value. The epitaxial disilicide islands started to appear on Si surfaces, when Co concentrations exceeded a certain value at given temperature. For Si(100) surfaces there is a certain Co concentration for any temperature, so that if a Co concentration is below this value then a uniform and ordered distribution of Co atoms is possible. If the Co concentrations exceed this value, only disilicide phase forms, and Co atoms outgo from the parts of Si surfaces adjacent to the epitaxial islands (Fig. 4a and b).
3.3. Surface structures Adsorption of Ni and Co leads to the formation of many surface structures on Si surfaces with different orientations, depending on the concentration of ad-
sorbed atoms and causes the reversible order-order transitions at varying temperatures [ 1,12,13,18,21 ]. For the reasons discussed before, the Ni induced surface structures were studied at room temperature after sample cooling at the rate above 100°C/s (Table 1). One can see that Ni causes the formation of a great number of surface structures on Si and that the concentrations of Ni, at which they start to appear and form, are low. In the case of Co adsorption [18,21] the rates of sample cooling did not influence the state of the surface. However, surface structures depended on the conditions, at which Co atoms were deposited on Si surface. Because of this, the formation of Co induced surface structures were studied under the two operating modes, which may be arbitrarily called as molecular beam epitaxy (MBE) mode and solid phase epitaxy (SPE) mode. The MBE mode means the
Table 1 Correlation between Ni concentrations and structures of Si surfaces Miller indices
Surface structure
Singular surfaces (100) 2 x 1 10 x 2 ~ 7 X 2 (111) 7X7 1~
X lv~
5×8 5 x 1 5× 1 ~2× 4 × 5 Vicinal (111 ) surfaces
1
Inclined towards [211 ]: 1d i ~i step bands and (111) terraces ,L $ 800°C I d ll i regular steps 2dl] i steps ld]l I steps Inclined towards [2]1-]: 80o°c 3d]l I steps ~ l d l l I steps 2dll I steps
Nickel concentration
Nickel concentration
CNil a
CN i b
0.5%
< 0.1% 1.0% <0.1%
0.5%
6.0%
< 0.1% 0.8% 1.4%
<0.1% 0.8% 1.4% 2.0%
Miller indices
High index surfaces (511 ) stepped 3X 3 3X 1
(211) (331)
< O. 1%
< 0.1% 2.3%
1.0%
Surface structure
2.3% 4.0%
(510)
<0.1%
(210)
2.8% (320)
a The concentrations at which the surface structures start to appear. b The concentrations at which the surface structures are completely developed.
Nickel concentration
Nickel concentration
CNi, a
CN i b
0.3% 0.5%
< 0.2% 0.5% 1.0%
7X 1
1.3%
1.8%
3x2 2 × 2 4×2 2 × 2 X
<0.2% 0.3% 1.5%
0.3% 1.0% 2.5% < 0.2% 3.0%
6 × 2 c(13 × 2) c(17 × 2) Y stepped 1x 4 1X 2
1.5% < 0.2% 0.4% 1.5% 0.3% 0.5%
1x 1 2 x 2 (320) facets (23 15 3) facets 2 X 1
< 0.2% 0.4% 1.5% 3.0% < 0.2% 0.5% 1.3% <0.2%
0.4% 1.0% 0.2%
1.0% 2.0% < 0.2% 2.0%
B.Z. Olshanetsk3'/Applied Surface Science 104/105 (1996) 130-136
formation of surface structures during the deposition process at the sample temperatures above 300°C. By the SPE mode we mean the Co deposition on the sample at room temperature with the subsequent annealing, which brings about the formation of surface structures. The duration of sample annealing at the SPE mode was determined by the time necessary for the completion of the surface reconstruction and was in the range of tens of minutes. Surface structures induced by Co versus Co coverage and sample temperature are shown in Fig. 4. In most cases the nickel and cobalt concentrations measured by AES were significantly less than those calculated with the assumption that there is at last one metal atom per unit cell of the surface structure. This feature is more pronounced at nickel adsorption. In some cases Si surface reconstructions occur at the nickel concentrations which are near or below the threshold sensitivity of an Auger spectrometer. Two plausible explanations of this phenomena can be put forward: (1) the adsorbed atoms evoking surface reconstructions do not enter in the unit meshes of the induced surface structures but they are embedded at specific sites in the surface lattice and generate strains, which lead to surface reconstruction; (2) the adsorbed atoms are located not on the surface atomic plane but at certain depth in subsurface layer, which must cause the corresponding weakening of AES signal. We believe that the second explanation is more realistic. 3.4. Phase transitions
One of the features of Si surfaces with adsorbed transition metals atoms is the possibility of reversible order-order phase transitions in surface region at temperature variations. It is accepted that the surface region of crystals may be treated as a special phase with its own thermodynamical criterion of equilibrium and its own set of the phase transitions. Order-disorder phase transitions take place on clean Si surfaces of almost all orientations at certain temperatures. The orderorder transitions at temperature variations are known on clean vicinal Si(l 11) surfaces [8,12,22,23]. In the case of transition metals adsorption on Si the phase transitions in surface structures are caused
135
by reversible variations of surface concentrations of adsorbed atoms with the temperature. In its turn, the surface concentrations of adsorbed atoms change by two reasons. First, solubility in silicon depends on temperature. Because of this, with temperature raising a certain portion of adsorbed atoms dissolves in silicon, and with the sample cooling it segregates to the surface. Second, the redistribution of the adsorbed atoms between the two possible states may take place, namely, between surface phase and the epitaxial disilicide islands. Reversible reconstructions of surface structures with the temperature are more inherent to Ni adsorbed Si surfaces of different orientations. In most cases the transitions occur between different two-dimensional structures with the unit vectors, which lie in the same crystal plane. However, the morphologic transitions are also possible, as it takes place on Si(320) and (210) surfaces, where atomically flat surfaces reversibly reconstruct into faceted one, on Si(511) and (510) surfaces, where stepped surfaces reconstruct into flat ones, and on the vicinal S i ( l l l ) surfaces, where transitions occur between regular steps of different heights. At Co adsorption we observed only the reversible reconstructions of Si(111)7 X 7 to Si(111)-1 X 1 structure at about 770°C.
4. Conclusions Our studies have revealed some peculiarities of atomic processes on Si surfaces with adsorbed transition metals. (1) The coefficients of nickel diffusion on Si surfaces are several orders of magnitude less than those in the bulk of silicon. Nickel transport along Si surfaces is provided by its diffusion through Si bulk. The estimated values of Co diffusion coefficients on Si surfaces do not exceed those in silicon known from literature. (2) Ni and Co adsorbed atoms on Si surfaces may be in two states which correspond to the different minima of free energy. (3) According to AES data Si surfaces reconstructions may take place at the Ni and Co surface concentrations which are significantly less than those calculated with the assumption that there is at last one metal atom per unit cell of the surface structure.
136
B.Z. Olshanetsky/ Applied Surface Science 104/105 (1996) 130-136
(4) Ni and Co atoms may cause reversible orderorder phase transitions on silicon surfaces at temperature variations, due to the redistribution of adsorbed atoms between the two possible states and the temperature dependence of their solubility in silicon.
Acknowledgements
The research described in this publication was made possible in part by Grant No. JCZI00 from the International Science Foundation and Russian Government and by Grant No. 95-02-05336-a from the Russian Foundation for Fundamental Investigations.
References [1] B.Z. Olshanetsky, S.M. Repinsky and A.A. Shklyaev, JETP Lett. 25 (1977) 178. [2] B.Z. Olshanetsky, S.M. Repinsky and A.A. Shklyaev, Surf. Sci. 64 (1977) 224. [3] B.Z. Olshanetsky and A.A. Shklyaev, Surf. Sci. 67 (1977) 581. [4] B.Z. Olshanetsky, S.M. Repinsky and A.A. Shklyaev, Surf. Sci. 69 (1977) 205. [5] B.Z. Olshanetsky and A.A. Shklyaev, Surf. Sci. 82 (1979) 445. [6] B.Z. Olshanetsky and V.I. Mashanov, Surf. Sci. I II (1981) 414.
[7] B.Z. Olshanetsky, V.I. Mashanov and A.1. Nikiforov, Surf. Sci. 111 (1981)429. [8] V.I. Mashanov and B.Z. Olshanetsky, JETP Lett. 36 (1982) 355. [9] T. Ichinokawa and Y. Ishikawa, Ultramicroscopy 15 (1984) 193. [10] T. Ichinokawa, H. Ampo, S. Miura and A. Tamura, Phys. Rev. B 31 (1985) 5183. [11] A.E. Dolbak, B.Z. Olshanetsky, S.I. Stenin, S.A. Teys and T.A. Gavrilova, Surf. Sci. 218 (1989) 37. [12] B.Z. Olshanetsky and S.A. Teys, Surf. Sci. 230 (1990) 184. [13] B.Z. Olshanetsky, A.E. Solovyov, A.E. Dolbak and A.A. Maslov, Surf. Sci. 306 (1994) 327. [14] B.Z. Olshanetsky and A.E. Dolbak, Phys. Low-Dim. Struct. 2 (1994) 81. [15] J.H. Aalberts and M.L. Verneijke, Appl. Phys. Lett. 1 (1962) 19. [16] M. Yoshida and K. Furusho, Jpn. J. Appl. Phys. 3 (1964) 521. [17] E.R. Weber, Appl. Phys. A 30 (1983) 1. [18] A.E. Dolbak, B.Z. Olshanetsky and A.E. Solovyov, Poverkhnost 11 (1993) 55. [19] M.K. Bakhadyrkhanov, B.I. Boltaks and G.S. Kulikov, Fiz. Tverd. Tela 12 (1970) 181 [Sov. Phys. Solid State 12 (1970)
144]. [20] H. Kitagawa and K. Hashimoto, Jpn. J. Appl. Phys. 16 (1977) 173. [21] A.E. Dolbak, A.A. Maslov and B.Z. Olshanetsky, Phys. Low-Dim. Struct. 8 (1994) 39. [22] R.J. Phaneuf and E.D. Williams, Phys. Rev. Lett. 58 (1987) 2563; Phys. Rev. B 41 (1990) 2991. [23] R.J. PhaneuL E.D. Williams and N.C. Bartelt, Phys. Rev. B 38 (1988) 1984.
ELSEVIER
Applied Surface Science 104/105 (1996) 130-136
Atomic phenomena on Si surfaces at adsorption of transition metals B.Z. Olshanetsky Institute of Semiconductor Physics, Russian Academy of Sciences, Siberian Branch, Nol,osibirsk 630090, Russian Federation
Received 28 June 1995; accepted 14 November 1995
Abstract The effect of Ni and Co adsorption on Si surfaces with different orientations has been studied by LEED and AES. It has been established that the transport of Ni and Co atoms at Si surfaces is actually by their diffusion in the bulk of Si. There are two states of adsorbed Ni and Co atoms on Si surfaces. Nickel and cobalt adsorption may cause the reversible phase transitions on silicon surfaces at varying temperatures due to the change of their solubility in silicon and the redistribution of adsorbed atoms between the two possible states. The conditions for the existence of each of these states are different for Ni and Co atoms.
1. Introduction The nature of atomic processes on crystal surfaces has been attracting much attention in surface science. At the end of the nineteen-seventies and early eighties we discovered a number o f reversible o r d e r - o r d e r phase transitions on germanium and silicon surfaces with different orientations by LEED in the Institute of Semiconductor Physics in Novosibirsk [1-8]. W e observed several types of such reversible transitions: (1) reconstruction of one surface plane structure to another ( S i ( l l 0 ) surface); (2) reconstruction of atomically fiat surface to faceted one ( G e ( l l 0 ) and Si(320) surfaces); (3) reconstruction of faceted surface to stepped one (vicinal Ge(110) surfaces); (4) reconstruction of one step-terrace configuration to another (vicinal Si and Ge(111) surfaces). Later on Ichinokawa and coauthors had shown that the phase transitions on Si(110) surfaces discovered by us were due to the presence of nickel impurity and the variations of its surface concentra-
tion with temperature [9,10]. This was one of the reasons, which prompted us to carry out a systematical study of nickel surface heterodiffusion and of the effect of nickel adsorption on the structure and phase transitions on silicon surfaces with different orientations. They were described in our papers [11-14], which contain also the references to the related studies carried out in other laboratories. As a result we have found some interesting peculiarities of nickel behaviour on silicon surfaces, which, in turn, led us on to study the adsorption of other transition metal atoms on silicon surfaces. It was reasonable to begin such studies with Co adsorption, because the properties of Co atoms are close to those of Ni atoms.
2. Experimental The silicon samples were of p-type with resistivities between 5 - 1 0 f~ cm and the dimensions of
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0169-4332(96)00133-X
B.Z. Olshanetsky/ Applied Surface Science 104/105 (1996) 130-136
20 × 5 × 0.4 mm. Samples were oriented using the Laue X-ray technique with the precision of ___1°. The experiments were performed in an UHV system with the base pressure about 1 × 10 -~° Torr. The surface cleaning procedure was carried out by heating in vacuum at 1200°C for l - 2 min. The sources of Ni and Co atoms consisted of a tantalum ribbon to which a piece of Ni or Co plate was welded. The surface structure and the surface chemical composition were studied by LEED and AES in situ. To measure the Ni and Co surface concentrations the Auger Ni LMM (848 eV) and Co LMM (775 eV) electron peaks were used. The spatial resolution of the Auger spectrometer was several tens of microns.
3. Results a n d d i s c u s s i o n
3.1. Surface diffusion The unexpected outcome of our studies of the transition metals atoms adsorption on Si surfaces was the discovery of peculiarities of their transport along Si surfaces. In order to study surface diffusion of transition metals on silicon surfaces, a metal strips several nanometers thick were deposited perpendicular to the longer side of a sample (Fig. 1). In these experiments Co was evaporated from a BeO crucible. Then samples were annealed at certain temperatures by passing an alternating current. As it is known from literature, Ni and Co layers of several nanometer thickness fully react to the disilicides at 300-350°C. At about 960°C nickel disilicide strips melted and the films became uncontinuous. The same happened with Co disilicide strips at about 1240°C.
131
¢.%
lO
1
2
3
4
5
Fig. 2. Concentration distributions C(x) of nickel on S i ( l l l ) surface after annealing at 860°C for: (1) 15 s, (2) 60 s, (3) 240 s.
After annealing the concentration distributions of metal atoms along Si surfaces were measured by AES. Surface structures were studied by LEED. The examples of Ni concentration distributions measured in our experiments [11] are shown in Figs. 2 and 3. It was commonly supposed, and we also expected, that the surface diffusion coefficients are well in excess of those in the bulk, though there is a modest number of experimental data on surface diffusion in literature at present. It is known that the process of surface diffusion may depend on the surface orientation and structure, atomic steps, concentration of adsorbed atoms and so on. However, we did not observe the relationship between the measured nickel diffusion coefficients and the features of silicon surfaces. 10
C,% AAA ~
•
LEED, AES I
2
3
2 •
o • •
Fig. 1. Schematic of experimental setup. (1) Ni strip; (2) nickel disilicide islands; (3) Ni induced surface structures; (4) Ni atoms.
•
•
•
~ •
•
o
Fig. 3. Typical nickel C(x) distributions on silicon surfaces after annealing at 1050°C for 80 s: (1) (111); (2) (110); (3) (100).
132
B.Z Olshanetsky/ Applied Surface Science 104 / 105 (1996) 130-136
If nickel concentration did not exceed a certain value for given surface orientation, the concentration distributions, which we measured along the specimen, obeyed the equation C ( x ) = C Oe r f c ( x / 2 x / ~ ) , where x is the distance from the edge of the disilicide strip (the Ni concentration on the strip is not shown), D the diffusion coefficient and t the annealing time (Fig. 2). Such distributions are typical for one-dimensional surface diffusion from the source of constant strength. At nickel concentrations exceeding certain values (different for different planes) the C ( x ) curves deviate from those which are typical for the diffusional distributions: they become either steady-state or nonmonotonous and are badly reproducible [1]. As can be seen from Fig. 3, on S i ( l l l ) surfaces it takes place with nickel concentrations above 6%, and on Si(100) and (110) surfaces it is observed with concentrations above 1% and 2.5%, respectively. These deviations are due to the formation of three-dimensional silicide islands which could be observed by scanning electron microscope. The area occupied by three-dimensional islands of nickel silicide is much lower than that occupied by uniformly distributed nickel. And this causes either a decrease or a stabilization of the Auger signal from nickel on the surface with islands. However, the surprising thing was that nickel could not be detected at the surface by Auger spectroscopy during the sample heating. Nickel concentration distributions appeared on surfaces only after the sample cooling. So we drew the conclusion that the nickel transport along silicon surfaces is not by way of surface diffusion, but by its diffusion through the bulk with subsequent segregation to the surface at sample cooling due to the decrease of nickel solubility in silicon (Fig. 1). This conclusion agrees with the absence of correlation between the nickel diffusion coefficients and the surface orientation and structure, which was mentioned before. Besides, the diffusion coefficients deduced from our data are close to those of interstitial nickel diffusion in silicon ( 1 0 - 4 - 1 0 -5 cm 2 s -1 at ll00°C [15,16]). The amount of Ni, diffusing through Si bulk, is limited by Ni solubility, which is 2 × 1017-6 × 10 j7 c m - 3 in the temperature range of 1000-1300°C [ 17].
That is why in the distributions, presented in Figs. 2 and 3, Ni concentrations did not reach the values typical of the nickel disilicide. However, nickel diffusion on silicon surfaces does take place. It manifests itself by the formation of ordered surface structures and epitaxial disilicide islands. But the coefficients of nickel surface diffusion are by orders of magnitude less than those of interstitial nickel diffusion in silicon. We could not measure the temperature dependence of the Co diffusion coefficients on Si surfaces because of their small values and limited spatial resolution of the LEED and AES apparatus [18]. However, our estimates show that this coefficient is about 10 -5 cm2/s at 1200°C. It is known from literature that the coefficients of Co diffusion in silicon bulk may range 10-8-10 -4 cmZ/s at 10001300°C [ 19], and, specifically, at 1100°C this coefficient approximates 10 -5 cm2/s [20]. We can see that the Co diffusion coefficient deduced at Si surface is similar to that in Si bulk, like in the case of Ni diffusion. Thus, one can infer that the transport of Co atoms at Si surfaces is also by way of their diffusion through Si bulk. The solubility of Co in Si is 5 X 1014-3 X 1016 cm -3 in the temperature range of 1000-1300°C [17]. Thus, our finding is contrary to the existing notions that the activation energy of surface diffusion is much less than that of the diffusion through the body of solids and that the rate of surface atom migration is considerably greater than that in the bulk. 3.2. Two states o f adsorbed atoms
Our results show that there are two states of Ni and Co atoms on Si surfaces. These states correspond to different minima of the surface system free energy. The atoms in the first state are likely to be regularly distributed on surfaces and they cause the formation of various surface structures (Fig. 1). Several structures can be formed on a Si surface of given orientation, each corresponding to a certain concentration of Ni or Co atoms. The adsorbed atoms in the second state are involved in the formation of the epitaxial disilicide islands. This state of atoms is more stable. The conditions to fall in each of these states are different for Ni and Co adsorbed atoms.
B.Z. Olshanetsky/Applied Surface Science 104/105 (1996) 130-136
plane. In this case on the free of islands parts of the surface the structure with the maximum nickel content remained. Our explanation of this phenomenon is as follows. At sample cooling nickel atoms segregate to the surface. However, some time is needed for the formation of critical nuclei of the nickel disilicide phase and for their growth. During fast sample cooling (at the rates above 100°C/s) there is not enough time to do this and the outdiffused nickel atoms occupy the sites corresponding to the less deep minimum of energy. At slow sample cooling or low-temperature annealing (600-700°C) the stable phase has time to
In the case of Ni adsorption the first state of adsorbed atoms could be obtained by cooling a sample at a rate above 100°C/s. However, the Ni induced structures on Si surfaces are metastable. Slow cooling or annealing at 600-700°C of samples with adsorbed nickel causes formation of the epitaxial disilicide islands, nickel atoms diffusing to these islands from the adjacent parts of surface and leaving them free of nickel impurity. The islands can be seen using scanning electron microscope. Their density was about 5 × 10 6 c m - 2 . The first state of adsorbed atoms is associated with the shallow minima and the disilicide phase - - with the deep minimum of free energy. Disilicide islands started to grow also when Ni concentration exceeded the value, which was necessary for the formation of the surface structure with the maximum nickel content for a given surface
T'C
133
form.
We did not observe the effect of sample cooling on the structure of cobalt adsorbed Si surfaces. In the case of Co adsorption a condition which determined
(a)
IIIIItlIIIIIIIIIIIIIIIIIIMIIIIIIIIIIIIIII ! IIIIll
1200 |
(b) 7oo
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600
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5oo
5o0 400
400 3o0
300 i
I
I
0.1
0.2
03
,
*
I
*
i
i
i
I
to O, NI.
O. ML
(c) 80
i
i
T,*C (d)
lxl
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7x7
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1 xl
600
T 2x1'
7x
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7x7
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200
•
7
400
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1x 1
i
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i
i
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i
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Fig. 4. Surface structures induced by Co versus Co coverage and sample temperature: (a) Si(100), SPE mode; (b) Si(100), MBE mode; (c) Si(l 11), SPE mode; (d) Si(l 11)+ MBE mode.
B.Z Olshanetsky/ Applied Surface Science 104/105 (1996) 130-136
134
the state of adsorbed atoms was the coverage value. The epitaxial disilicide islands started to appear on Si surfaces, when Co concentrations exceeded a certain value at given temperature. For Si(100) surfaces there is a certain Co concentration for any temperature, so that if a Co concentration is below this value then a uniform and ordered distribution of Co atoms is possible. If the Co concentrations exceed this value, only disilicide phase forms, and Co atoms outgo from the parts of Si surfaces adjacent to the epitaxial islands (Fig. 4a and b).
3.3. Surface structures Adsorption of Ni and Co leads to the formation of many surface structures on Si surfaces with different orientations, depending on the concentration of ad-
sorbed atoms and causes the reversible order-order transitions at varying temperatures [ 1,12,13,18,21 ]. For the reasons discussed before, the Ni induced surface structures were studied at room temperature after sample cooling at the rate above 100°C/s (Table 1). One can see that Ni causes the formation of a great number of surface structures on Si and that the concentrations of Ni, at which they start to appear and form, are low. In the case of Co adsorption [18,21] the rates of sample cooling did not influence the state of the surface. However, surface structures depended on the conditions, at which Co atoms were deposited on Si surface. Because of this, the formation of Co induced surface structures were studied under the two operating modes, which may be arbitrarily called as molecular beam epitaxy (MBE) mode and solid phase epitaxy (SPE) mode. The MBE mode means the
Table 1 Correlation between Ni concentrations and structures of Si surfaces Miller indices
Surface structure
Singular surfaces (100) 2 x 1 10 x 2 ~ 7 X 2 (111) 7X7 1~
X lv~
5×8 5 x 1 5× 1 ~2× 4 × 5 Vicinal (111 ) surfaces
1
Inclined towards [211 ]: 1d i ~i step bands and (111) terraces ,L $ 800°C I d ll i regular steps 2dl] i steps ld]l I steps Inclined towards [2]1-]: 80o°c 3d]l I steps ~ l d l l I steps 2dll I steps
Nickel concentration
Nickel concentration
CNil a
CN i b
0.5%
< 0.1% 1.0% <0.1%
0.5%
6.0%
< 0.1% 0.8% 1.4%
<0.1% 0.8% 1.4% 2.0%
Miller indices
High index surfaces (511 ) stepped 3X 3 3X 1
(211) (331)
< O. 1%
< 0.1% 2.3%
1.0%
Surface structure
2.3% 4.0%
(510)
<0.1%
(210)
2.8% (320)
a The concentrations at which the surface structures start to appear. b The concentrations at which the surface structures are completely developed.
Nickel concentration
Nickel concentration
CNi, a
CN i b
0.3% 0.5%
< 0.2% 0.5% 1.0%
7X 1
1.3%
1.8%
3x2 2 × 2 4×2 2 × 2 X
<0.2% 0.3% 1.5%
0.3% 1.0% 2.5% < 0.2% 3.0%
6 × 2 c(13 × 2) c(17 × 2) Y stepped 1x 4 1X 2
1.5% < 0.2% 0.4% 1.5% 0.3% 0.5%
1x 1 2 x 2 (320) facets (23 15 3) facets 2 X 1
< 0.2% 0.4% 1.5% 3.0% < 0.2% 0.5% 1.3% <0.2%
0.4% 1.0% 0.2%
1.0% 2.0% < 0.2% 2.0%
B.Z. Olshanetsk3'/Applied Surface Science 104/105 (1996) 130-136
formation of surface structures during the deposition process at the sample temperatures above 300°C. By the SPE mode we mean the Co deposition on the sample at room temperature with the subsequent annealing, which brings about the formation of surface structures. The duration of sample annealing at the SPE mode was determined by the time necessary for the completion of the surface reconstruction and was in the range of tens of minutes. Surface structures induced by Co versus Co coverage and sample temperature are shown in Fig. 4. In most cases the nickel and cobalt concentrations measured by AES were significantly less than those calculated with the assumption that there is at last one metal atom per unit cell of the surface structure. This feature is more pronounced at nickel adsorption. In some cases Si surface reconstructions occur at the nickel concentrations which are near or below the threshold sensitivity of an Auger spectrometer. Two plausible explanations of this phenomena can be put forward: (1) the adsorbed atoms evoking surface reconstructions do not enter in the unit meshes of the induced surface structures but they are embedded at specific sites in the surface lattice and generate strains, which lead to surface reconstruction; (2) the adsorbed atoms are located not on the surface atomic plane but at certain depth in subsurface layer, which must cause the corresponding weakening of AES signal. We believe that the second explanation is more realistic. 3.4. Phase transitions
One of the features of Si surfaces with adsorbed transition metals atoms is the possibility of reversible order-order phase transitions in surface region at temperature variations. It is accepted that the surface region of crystals may be treated as a special phase with its own thermodynamical criterion of equilibrium and its own set of the phase transitions. Order-disorder phase transitions take place on clean Si surfaces of almost all orientations at certain temperatures. The orderorder transitions at temperature variations are known on clean vicinal Si(l 11) surfaces [8,12,22,23]. In the case of transition metals adsorption on Si the phase transitions in surface structures are caused
135
by reversible variations of surface concentrations of adsorbed atoms with the temperature. In its turn, the surface concentrations of adsorbed atoms change by two reasons. First, solubility in silicon depends on temperature. Because of this, with temperature raising a certain portion of adsorbed atoms dissolves in silicon, and with the sample cooling it segregates to the surface. Second, the redistribution of the adsorbed atoms between the two possible states may take place, namely, between surface phase and the epitaxial disilicide islands. Reversible reconstructions of surface structures with the temperature are more inherent to Ni adsorbed Si surfaces of different orientations. In most cases the transitions occur between different two-dimensional structures with the unit vectors, which lie in the same crystal plane. However, the morphologic transitions are also possible, as it takes place on Si(320) and (210) surfaces, where atomically flat surfaces reversibly reconstruct into faceted one, on Si(511) and (510) surfaces, where stepped surfaces reconstruct into flat ones, and on the vicinal S i ( l l l ) surfaces, where transitions occur between regular steps of different heights. At Co adsorption we observed only the reversible reconstructions of Si(111)7 X 7 to Si(111)-1 X 1 structure at about 770°C.
4. Conclusions Our studies have revealed some peculiarities of atomic processes on Si surfaces with adsorbed transition metals. (1) The coefficients of nickel diffusion on Si surfaces are several orders of magnitude less than those in the bulk of silicon. Nickel transport along Si surfaces is provided by its diffusion through Si bulk. The estimated values of Co diffusion coefficients on Si surfaces do not exceed those in silicon known from literature. (2) Ni and Co adsorbed atoms on Si surfaces may be in two states which correspond to the different minima of free energy. (3) According to AES data Si surfaces reconstructions may take place at the Ni and Co surface concentrations which are significantly less than those calculated with the assumption that there is at last one metal atom per unit cell of the surface structure.
136
B.Z. Olshanetsky/ Applied Surface Science 104/105 (1996) 130-136
(4) Ni and Co atoms may cause reversible orderorder phase transitions on silicon surfaces at temperature variations, due to the redistribution of adsorbed atoms between the two possible states and the temperature dependence of their solubility in silicon.
Acknowledgements
The research described in this publication was made possible in part by Grant No. JCZI00 from the International Science Foundation and Russian Government and by Grant No. 95-02-05336-a from the Russian Foundation for Fundamental Investigations.
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