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Electronic structure and properties of silicon-transition metal interfaces
1185
Surface Science 152/153 (1985) 1185-1190 North-Holland, Amsterdam
ELECTRONIC STRUCTURE AND PROPERTIES OF SILICON-TRANSITION METAL INTERFACES 0. BISI *, L.W. CHIAO IBM
** and K.N. TU
T.J. Watson Research Center, Yorktown
Received
22 March
Heights, New York 10598, USA
1984
We present a theoretical investigation of the reaction occurring at the interfaces between silicon and transition metals. Using the same approach successfully applied to the study of bulk silicides, the electronic properties of different models of silicon-nickel and silicon-palladium interfaces have been studied. The models investigated include: (a) epitaxial silicon-silicide interfaces; (b) isolated transition metal interstitials near the silicon surfaces; (c) adamantane geometry structures as metastable diffusion layer compounds. The theoretical results are used as a guide in order to interpret the available experimental photoemission data of these complex interfaces.
1. Introduction A great deal of effort has recently been devoted to understanding Si-transition metal interfacial reactions and the resulting silicide formation [l-3]. We present here a theoretical investigation of the electronic structure of several different models of Ni-Si and Pd-Si interfaces. The comparison between theoretical results and experimental data will elucidate the electronic and structural properties of the near-noble metal-% reactive interfaces. The investigation of these complex models was carried out using the linear combination of atomic orbitals (LCAO) method in the Iterative Extended Htickel Theory (IEHT) approximation. The approach has been applied successfully to the calculation of the electronic structure of bulk silicides [3,4]. For details of the method as applied to interface problems we refer to ref. [5].
2. Epitaxial silicon-silicide
interfaces
The unreconstructed Si(ll1) surface constitutes a 2D array of atoms at a distance of 3.84 A. By a slight variation of the lattice constants of a NiSi,(lll) * Permanent ** Permanent USA.
address: address:
Department Department
of Physics, University of Modena, I-41100 Modena, Italy. of Physics, Georgetown University, Washington, DC 20057,
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
and Fd~Si(OOO1) single layer slab, it is possible to build up Si(lll)-Nisi2 [6J and Si(lll)-Pd,Si [7,8] epitaxial interfaces. In the case of very thin films, the lattice of Nisi, has been found to be rotated by 180” about the (111) direction (type-B orientation) [8]. Figs. la and lb show the projected densities of states (PDOS) for these interfaces. Both results indicate a chemical bond picture similar to that found in bulk silicides [4]: the interaction between Si p and Ni d states leads to a
r
,
I
F
r
I
/
/
-5
0
1
a)
-15
-10
-5 E-E, WI
0
5
-10 E-E,
5
,ew
Fig. 1. Theoretical partial densities of states for: (a) one Nisi, layer on Si(ll1) (type-B orientation), Si up (Si down) atom refers to the upper (lower) Nisi, Si atom; (b) one PdzSi layer on Si(ll1); (c) Ni interstitial in Si(lll), A’ (B’) atom lies in the (111) direction on the top (directly below) the interstitial atom: (d) as in (c) with a Pd interstitial.
0. hi
et al. / Silicon - transition
metal interfaces
1187
partial dehybridization of the Si s states near - 9 eV, the sp3 configuration of Si being changed towards s2p2. Furthermore this interaction is responsible for the presence of p-d bonding antibonding structures straddling the main d peak found at -2.2 eV (Nisi, interface) and -1.5 eV (Pd,Si interface). All the energies are referred to the Fermi energy (E,). These values are to be compared with the d band main peak energy of - 3.3 and - 2.7 eV found in bulk Nisi, and Pd,Si respectively. This shift of 1.1-1.2 eV on going from the bulk environment to the epitaxial interface boundaries appears to be a characteristic of the near-noble metal-Si systems.
3. Ni and Pd interstitials near the Si surface In order to explain the high reactivity of Si surfaces upon near-noble metal deposition, an interstitial mechanism has been proposed [9,10]. According to this model, the metal atoms would move into the interstitial voids of the Si lattice with very little activation energy and this would make the Si-Si bond less covalent and easier to break. We considered the subsurface interstitial void with adamantane geometry [5]. In this geometry the interstitial metal atom is surrounded by four and six Si atoms at 2.35 and 2.71 A, respectively. The first set of atoms lies at the vertices of a tetrahedron, the second at the vertices of an octahedron. The effect of interstitial Ni and Pd atoms immediately below the Si(ll1) surface is shown in figs. lc and Id. Comparison with the bulk silicide results [4] reveals a considerable shift of the d band center towards higher binding energies (BE). Due to the large atomic radius and the resulting strong interaction with Si neighbors, the d band of Pd interstitial is about 4.5 eV wide. The Si states are perturbed with respect to the sp3 configuration, this effect being stronger in the Pd case. We also studied the effect of a Ni atom interstitial below a Si(OO1) surface [5]: the energy of the d band was found to be essentially the same as for the Si(ll1) surface case.
4. Diffusion layer compounds A metastable diffusion layer compound built up by Ni adamantane geometry interstitials in an undistorted Si lattice has been proposed in order to interpret ion channeling and ultraviolet photoemission spectroscopy erperiments [ll]. By varying the number of adamantane sites occupied by Ni in the silicon, we vary the stoichiometry of the bulk compound from Nisi, to Nisi,. The adamantane Nisi, volume is greater than that of stable Nisi,, which has the same crystal structure as CaF,, by 1.3%.
0. Bisi ez al. / Silicon - lransriion meld
1188
interfaces
Fig. ,2 shows the Ni PDOS for stable NiSi, and for the adamantane structure compounds. For the Nisi, stoichiometry we see that the d band is shifted towards lowel- BE in the ad~ant~e structure, as we might expect from the Fact that this compound is unstable. On going towards the more %-rich phases in the adamantane structure, the Si PDOS (not shown in figure) approaches the Si bulk tetrahedral features, showing corresponding changes in the interaction of SE with the Ni atoms. The main d peak, with no bonding character, is found in the region corresponding to the bulk Si gap. With respect
t
NiS$ adamantane
structwe
NSi, adamantam!
structure
Ni contribution
-20
-15
-5
E-E, @VI Fig. 2. Theoretical Ni partial densities of states for stable NiSi, and for the adamantane structure compounds. Vertical dotted lines indicate the energies of the Ni-Si(OO1) interface features of ref.
IIll.
0. Bisi et al. / Silicon-transition
metal interfaces
to the Fermi energy, the location of this peak is found - 1.6 eV for Nisi,, Nisi,, and Nisi,, respectively. The Ni interstitial in Si(ll1) and Nisi, gives evidence of the configuration. In both models an isolated Ni atom lies in but the greater reactivity of the Si surface atoms, no configuration, shifts the main d peak by 2.3 eV.
1189
at -2.7, - 2.0, and comparison between effect of the Si atom an adamantane cage, longer in a bulk sp3
5. Discussion We plot in figs. 1 and 2 the metal contribution because we are mainly interested in a comparison with photoemission data with a dominant d photoemission cross section. The results presented are based on calculations without any free parameters: we simply varied the geometry of the investigated crystal structures, so the different locations of the metal d band reflect the different geometrical configurations. The experimental UPS data for Ni-Si [11,12] and Pd-Si [13] interfaces show a complex temperature and coverage dependence of the main spectral features. It is difficult to achieve a firm understanding of these interfaces because of the lack of any systematic study of temperature and coverage effects. With the limited amount of theoretical and experimental information presently available, only a tentative interpretation of the experimental results can be proposed. For the Ni-Si(OO1) interface, the presence of a metastable diffusion layer compound with adamantane geometry is confirmed by our results. The two vertical dotted lines of fig. 2 show the positions of the Ni-Si(OO1) interface features [ll], which may be interpreted as due to Ni atoms in adamantane inhomogeneous structures with Nisi, and Nisi, stoichiometry, or in homogeneous Nisi 4. The low coverage spectra for the Ni-Si(ll1) interface [12] show a well defined structure with the same energy location of the wide d band centered around 2 eV below E, in Nisi, (fig. 2). The model of an epitaxial interface of Nisi, on Si(ll1) shows a d band in the same position, but much narrower than the experimental spectra. The experimental shift of the Pd d band towards higher BE on going from bulk Pd,Si to a Pd-Si(ll1) interface [13] disagrees with the opposite shift found in the corresponding theoretical models (see section 2). The correct direction of the d band shift is found for the interstitial geometry. Our theoretical model of Pd interstitials over-estimates this shift because we do not account for the relaxation of Si neighbors surrounding the large Pd atom. These interstitial-interdiffusion models of reaction between near-noble metals and Si do not contradict the experimental findings of epitaxial growth for thicker overlayers. We found that Si atoms around the metal interstitials
1190
0. Bisr et al. / Silicon-transition
metal inte-faces
are in a metallic configuration. On increasing the metal coverage we increase the number of metallic clusters surrounding the interstitial atoms. We may envisage a critical coverage characterized by a phase transition to a full and ordered metallic system.
References [I] K.N. Tu and J.W. Mayer, in: Thin Films - Interdiffusion and Interactions. Eds. J.M. Poate, K.N. Tu and J.W. Mayer (Wiley, New York, 1978) p. 359. [2] S.P. Murarka, in: Silicides for VLSI Applications (Academic Press, New York, 1983). [3] C. Calandra, 0. Bisi and G. Ottaviani. Surface Sci. Rept., to be published, and references therein. [4] 0. Bisi and C. Calandra, J. Phys. C. (Solid State Phys.) 14 (1987) 5479. [5] 0. Bisi and K.N. Tu, Phys. Rev. Letters 52 (1984) 1633; 0. Bisi, L.W. Chiao and K.N. Tu, Phys. Rev. B30 (1984) 4664. [6] K.N. Tu, E.I. Alessandrini, W.K. Chu, H. Krautle and J.W. Weaver, Japan. J. Appl. Phys. Suppl. 2. Pt. 1 (1974) 669. (71 W.D. Buckey and SC. Moss, Solid State Electron. 15 (1972) 1331. [S] F. Fall, P.S. Ho and K.N. Tu, J. Appl. Phys. 52 (1981) 2.50. [9] K.N. Tu, Appl. Phys. Letters 27 (1975) 221. [IO] N.W. Cheung and J.W. Mayer, Phys. Rev. Letters 46 (1981) 671. [ll] Y.J. Chang and J.L. Erskine, Phys. Rev. B28 (1983) 5766. [12] I. Abbati, L. Braicovich, B. De Michelis, U. de1 Pennino and S. Valeri, Solid State Commun. 43 (1982) 199. [13] G.W. Rubloff, P.S. Ho, J.F. Freeouf and J.E. Lewis, Phys. Rev. 823 (1981) 4183.
Surface Science 152/153 (1985) 1185-1190 North-Holland, Amsterdam
ELECTRONIC STRUCTURE AND PROPERTIES OF SILICON-TRANSITION METAL INTERFACES 0. BISI *, L.W. CHIAO IBM
** and K.N. TU
T.J. Watson Research Center, Yorktown
Received
22 March
Heights, New York 10598, USA
1984
We present a theoretical investigation of the reaction occurring at the interfaces between silicon and transition metals. Using the same approach successfully applied to the study of bulk silicides, the electronic properties of different models of silicon-nickel and silicon-palladium interfaces have been studied. The models investigated include: (a) epitaxial silicon-silicide interfaces; (b) isolated transition metal interstitials near the silicon surfaces; (c) adamantane geometry structures as metastable diffusion layer compounds. The theoretical results are used as a guide in order to interpret the available experimental photoemission data of these complex interfaces.
1. Introduction A great deal of effort has recently been devoted to understanding Si-transition metal interfacial reactions and the resulting silicide formation [l-3]. We present here a theoretical investigation of the electronic structure of several different models of Ni-Si and Pd-Si interfaces. The comparison between theoretical results and experimental data will elucidate the electronic and structural properties of the near-noble metal-% reactive interfaces. The investigation of these complex models was carried out using the linear combination of atomic orbitals (LCAO) method in the Iterative Extended Htickel Theory (IEHT) approximation. The approach has been applied successfully to the calculation of the electronic structure of bulk silicides [3,4]. For details of the method as applied to interface problems we refer to ref. [5].
2. Epitaxial silicon-silicide
interfaces
The unreconstructed Si(ll1) surface constitutes a 2D array of atoms at a distance of 3.84 A. By a slight variation of the lattice constants of a NiSi,(lll) * Permanent ** Permanent USA.
address: address:
Department Department
of Physics, University of Modena, I-41100 Modena, Italy. of Physics, Georgetown University, Washington, DC 20057,
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
and Fd~Si(OOO1) single layer slab, it is possible to build up Si(lll)-Nisi2 [6J and Si(lll)-Pd,Si [7,8] epitaxial interfaces. In the case of very thin films, the lattice of Nisi, has been found to be rotated by 180” about the (111) direction (type-B orientation) [8]. Figs. la and lb show the projected densities of states (PDOS) for these interfaces. Both results indicate a chemical bond picture similar to that found in bulk silicides [4]: the interaction between Si p and Ni d states leads to a
r
,
I
F
r
I
/
/
-5
0
1
a)
-15
-10
-5 E-E, WI
0
5
-10 E-E,
5
,ew
Fig. 1. Theoretical partial densities of states for: (a) one Nisi, layer on Si(ll1) (type-B orientation), Si up (Si down) atom refers to the upper (lower) Nisi, Si atom; (b) one PdzSi layer on Si(ll1); (c) Ni interstitial in Si(lll), A’ (B’) atom lies in the (111) direction on the top (directly below) the interstitial atom: (d) as in (c) with a Pd interstitial.
0. hi
et al. / Silicon - transition
metal interfaces
1187
partial dehybridization of the Si s states near - 9 eV, the sp3 configuration of Si being changed towards s2p2. Furthermore this interaction is responsible for the presence of p-d bonding antibonding structures straddling the main d peak found at -2.2 eV (Nisi, interface) and -1.5 eV (Pd,Si interface). All the energies are referred to the Fermi energy (E,). These values are to be compared with the d band main peak energy of - 3.3 and - 2.7 eV found in bulk Nisi, and Pd,Si respectively. This shift of 1.1-1.2 eV on going from the bulk environment to the epitaxial interface boundaries appears to be a characteristic of the near-noble metal-Si systems.
3. Ni and Pd interstitials near the Si surface In order to explain the high reactivity of Si surfaces upon near-noble metal deposition, an interstitial mechanism has been proposed [9,10]. According to this model, the metal atoms would move into the interstitial voids of the Si lattice with very little activation energy and this would make the Si-Si bond less covalent and easier to break. We considered the subsurface interstitial void with adamantane geometry [5]. In this geometry the interstitial metal atom is surrounded by four and six Si atoms at 2.35 and 2.71 A, respectively. The first set of atoms lies at the vertices of a tetrahedron, the second at the vertices of an octahedron. The effect of interstitial Ni and Pd atoms immediately below the Si(ll1) surface is shown in figs. lc and Id. Comparison with the bulk silicide results [4] reveals a considerable shift of the d band center towards higher binding energies (BE). Due to the large atomic radius and the resulting strong interaction with Si neighbors, the d band of Pd interstitial is about 4.5 eV wide. The Si states are perturbed with respect to the sp3 configuration, this effect being stronger in the Pd case. We also studied the effect of a Ni atom interstitial below a Si(OO1) surface [5]: the energy of the d band was found to be essentially the same as for the Si(ll1) surface case.
4. Diffusion layer compounds A metastable diffusion layer compound built up by Ni adamantane geometry interstitials in an undistorted Si lattice has been proposed in order to interpret ion channeling and ultraviolet photoemission spectroscopy erperiments [ll]. By varying the number of adamantane sites occupied by Ni in the silicon, we vary the stoichiometry of the bulk compound from Nisi, to Nisi,. The adamantane Nisi, volume is greater than that of stable Nisi,, which has the same crystal structure as CaF,, by 1.3%.
0. Bisi ez al. / Silicon - lransriion meld
1188
interfaces
Fig. ,2 shows the Ni PDOS for stable NiSi, and for the adamantane structure compounds. For the Nisi, stoichiometry we see that the d band is shifted towards lowel- BE in the ad~ant~e structure, as we might expect from the Fact that this compound is unstable. On going towards the more %-rich phases in the adamantane structure, the Si PDOS (not shown in figure) approaches the Si bulk tetrahedral features, showing corresponding changes in the interaction of SE with the Ni atoms. The main d peak, with no bonding character, is found in the region corresponding to the bulk Si gap. With respect
t
NiS$ adamantane
structwe
NSi, adamantam!
structure
Ni contribution
-20
-15
-5
E-E, @VI Fig. 2. Theoretical Ni partial densities of states for stable NiSi, and for the adamantane structure compounds. Vertical dotted lines indicate the energies of the Ni-Si(OO1) interface features of ref.
IIll.
0. Bisi et al. / Silicon-transition
metal interfaces
to the Fermi energy, the location of this peak is found - 1.6 eV for Nisi,, Nisi,, and Nisi,, respectively. The Ni interstitial in Si(ll1) and Nisi, gives evidence of the configuration. In both models an isolated Ni atom lies in but the greater reactivity of the Si surface atoms, no configuration, shifts the main d peak by 2.3 eV.
1189
at -2.7, - 2.0, and comparison between effect of the Si atom an adamantane cage, longer in a bulk sp3
5. Discussion We plot in figs. 1 and 2 the metal contribution because we are mainly interested in a comparison with photoemission data with a dominant d photoemission cross section. The results presented are based on calculations without any free parameters: we simply varied the geometry of the investigated crystal structures, so the different locations of the metal d band reflect the different geometrical configurations. The experimental UPS data for Ni-Si [11,12] and Pd-Si [13] interfaces show a complex temperature and coverage dependence of the main spectral features. It is difficult to achieve a firm understanding of these interfaces because of the lack of any systematic study of temperature and coverage effects. With the limited amount of theoretical and experimental information presently available, only a tentative interpretation of the experimental results can be proposed. For the Ni-Si(OO1) interface, the presence of a metastable diffusion layer compound with adamantane geometry is confirmed by our results. The two vertical dotted lines of fig. 2 show the positions of the Ni-Si(OO1) interface features [ll], which may be interpreted as due to Ni atoms in adamantane inhomogeneous structures with Nisi, and Nisi, stoichiometry, or in homogeneous Nisi 4. The low coverage spectra for the Ni-Si(ll1) interface [12] show a well defined structure with the same energy location of the wide d band centered around 2 eV below E, in Nisi, (fig. 2). The model of an epitaxial interface of Nisi, on Si(ll1) shows a d band in the same position, but much narrower than the experimental spectra. The experimental shift of the Pd d band towards higher BE on going from bulk Pd,Si to a Pd-Si(ll1) interface [13] disagrees with the opposite shift found in the corresponding theoretical models (see section 2). The correct direction of the d band shift is found for the interstitial geometry. Our theoretical model of Pd interstitials over-estimates this shift because we do not account for the relaxation of Si neighbors surrounding the large Pd atom. These interstitial-interdiffusion models of reaction between near-noble metals and Si do not contradict the experimental findings of epitaxial growth for thicker overlayers. We found that Si atoms around the metal interstitials
1190
0. Bisr et al. / Silicon-transition
metal inte-faces
are in a metallic configuration. On increasing the metal coverage we increase the number of metallic clusters surrounding the interstitial atoms. We may envisage a critical coverage characterized by a phase transition to a full and ordered metallic system.
References [I] K.N. Tu and J.W. Mayer, in: Thin Films - Interdiffusion and Interactions. Eds. J.M. Poate, K.N. Tu and J.W. Mayer (Wiley, New York, 1978) p. 359. [2] S.P. Murarka, in: Silicides for VLSI Applications (Academic Press, New York, 1983). [3] C. Calandra, 0. Bisi and G. Ottaviani. Surface Sci. Rept., to be published, and references therein. [4] 0. Bisi and C. Calandra, J. Phys. C. (Solid State Phys.) 14 (1987) 5479. [5] 0. Bisi and K.N. Tu, Phys. Rev. Letters 52 (1984) 1633; 0. Bisi, L.W. Chiao and K.N. Tu, Phys. Rev. B30 (1984) 4664. [6] K.N. Tu, E.I. Alessandrini, W.K. Chu, H. Krautle and J.W. Weaver, Japan. J. Appl. Phys. Suppl. 2. Pt. 1 (1974) 669. (71 W.D. Buckey and SC. Moss, Solid State Electron. 15 (1972) 1331. [S] F. Fall, P.S. Ho and K.N. Tu, J. Appl. Phys. 52 (1981) 2.50. [9] K.N. Tu, Appl. Phys. Letters 27 (1975) 221. [IO] N.W. Cheung and J.W. Mayer, Phys. Rev. Letters 46 (1981) 671. [ll] Y.J. Chang and J.L. Erskine, Phys. Rev. B28 (1983) 5766. [12] I. Abbati, L. Braicovich, B. De Michelis, U. de1 Pennino and S. Valeri, Solid State Commun. 43 (1982) 199. [13] G.W. Rubloff, P.S. Ho, J.F. Freeouf and J.E. Lewis, Phys. Rev. 823 (1981) 4183.