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Schottky barriers on clean-cleaved silicon
Surface Science 104 (1981) L205-L209 North-Holland Publishing Company
SURFACE SCIENCE LETTERS SCHOTTKY BARRIERS
ON CLEAN-CLEAVED
SILICON
J.D. VAN OTTERLOO Department Received
of Electrical Engineering,
23 October
1980; accepted
Delft University of Technology, for publication
5 December
Delft, The Netherlands
1980
Experiments dealing with barrier height measurements of various metals on ultra high vacuum (UHV) cleaved silicon are reported. It is shown that the barrier height is affected by the presence of cleavage steps. Furthermore it is concluded that the position of the Fermi level at the interface on these rough surfaces is influenced to only a minor degree by the metal used. Therefore it is suggested that the Fermi level be pinned by cleavage-step-induced states instead of by metal-induced states according to Rowe et al.
Rowe and co-workers [l-3] have gathered new information on the fundamentals of Schottky barrier formation on covalent semiconductor surfaces. They found that on the majority of sputter-cleaned and subsequently annealed surfaces the intrinsic surface states are annihilated by the metal atoms. New metal-inducedinterface states take their place and cause the observed pinning of the Fermi level at the interface. However, it was observed [l] that on the (110) face of Ge, the intrinsic surface states were less sensitive to the metal atoms and the Fermi level was pinned by these intrinsic states. Consequently, this latter situation is consistent with the classical Bardeen [4] model, which states that the Fermi level is pinned in one invariant position under the condition that the density of surface states is sufficiently high. Similar observations were made by Eastman and Freeouf [S ] on cleaved (1 IO) surfaces of III-V compounds. They found a relation between the Fermi level and the empty intrinsic surface state band. Both results were explained by Rowe et al. [I], by introducing a specific surface relaxation of the (110) face, which makes certain surface atoms less sensitive to the absorbed metal atoms which thus retain their intrinsic surface properties. The results of Eastman and Freeouf [5] have been recently discussed by van Laar et al. [6] who argued that the band gap of cleaved GaAs, GaSb and InP is free of empty surface states provided the samples are well cleaved (no cleavage imperfections on the surface). Consequently the results of Eastman and Freeouf [S] for these surfacesmust be interpreted as a relation between the Fermi level and extrinsic surface states, while in this particular case these states are a property of the semiconductor (artificial Bardeen model). And so the question arises: to what extent do 0039-6028/81/0000-OOOO/$
02.50 0 North-Holland
Publishing
Company
L206
J.D. van Otterloo
/ Schottky
barriers on clean-cleaved
Si
cleavage steps influence the Fermi level of Schottky barriers on cleaved silicon? From the extensive investigations of Thanailakis [7,8] it is known that the barrier height of metal contacts on cleaved silicon depends only slightly on the metal used. However, in the literature a metal-dependent barrier height on cleaved silicon is sometimes reported (for alkaline metals [9] heights as low as 0.43 eV on n-type Si have been reported). But it has been pointed out [ 10,l l] that these latter results may be explained as coming from a less reliable measuring method (current-voltage measurements). The most accurate measuring method [lo] is the C-I/ method (the reciprocal of the differential capacitance squared as a function of reverse bias across the contact) and these barrier heights vary between 0.70 and 0.98 eV for simple and transition metals following Thanailakis [7,8]. In none of these works has information been presented about the density of cleavage steps. Therefore we have remeasured the barrier heights for various metals (ranging from alkaline to noble metals) on clean-cleaved silicon, while simultaneously recording the presence (or absence) of cleavage steps. This latter is simply done by comparing photomicrographs (taken through a normal optical microscope‘) of the Schottky barrier contacts. The result is labeled as “good cleavage” when the photograph has an appearance according to fig. la, mutatis mutandis a “bad cleavage” is shown in fig. lb. Obviously this is a rather crude method, because many more details can be revealed if the inclination angle with respect to the (111) face is measured [ 12,131. The samples are cleaved, according to the Gobeli and Allen [14] method, in a stream of evaporating metal atoms in UHV. Extremely flat surfaces (fig. la) result if the samples are well oriented [ 11 ,I 51 with respect to [ 1111. The surfaces were examined with Tolanski interferometer measurements and were found to exhibit remaining steps of the order of 80 _& even on the good cleavages of fig. la. The
Fig. 1. Photograph of a Schottky barrier contact on cleaved silicon: good cleavage; (b) junction area labeled as bad cleavage.
(a) junction
area labeled
as
J.D. van Otterloo / SchottkJj barriers on clean-cleaved Si Table 1 Schottky
barrier
Metal
heights
(eV) on clean-cleaved
n-Type Good
silicon at 80 K
Si cleavage
L207
p-Type Bad cleavage
Good
Si cleavage
Bad cleavage -
Ag AU Al Mg cs K Na (at 197 K) Fe
0.78 0.81 0.69 0.62 0.67 0.73 0.70 0.71
0.74 0.67 0.66 0.55 0.62 0.66 -
0.37 0.34 0.47 0.56 0.40 _ -
0.42 0.50 0.49 0.61 _ _ _ -
Ga In
0.72 0.73
0.66 -
0.45 _
0.52 0.51
larger steps in fig. lb have an average height of 1000 A; the steps run roughly toward a [1?1l] direction. On the smooth (upper) part of the cleavage face three circular Schottky barrier contacts of 0.5 mm diameter are formed. The deposited layers were found to be free of contaminations below a level of 10e4. During evaporation the substrate temperature is stabilized at 80 K in the case of alkaline metals and at 273 K for other metals. The UHV (8 X 10-l ’ Torr) system used consists of a bakeable stainless steel vessel pumped by an oil-diffusion pump (DC 70.5 oil) assembly and a titanium sublimation pump as described previously [ 11 ,151. The application of a liquid-nitrogen-cooled trap and an extra small diffusion pump between the rotation and the main diffusion pump reduced the oil residues to an undetectable level (partial pressure analysis) according to Santeler [ 161. All measurements [lo] were performed in the same UHV. The barrier heights at 80 K, resulting from Cb2- V measurements, for the various metals on both well cleaved and badly cleaved surfaces are presented in table 1. The barrier heights on n-type samples (1015-10’6/ cm”) at elevated temperatures are only slightly higher. This temperature behavior is indicative of the interface state distribution [ 11,151. The low temperature in table 1 has been selected because at higher temperatures the ohmic behavior on the p-type (2.1 X 10r6/cm3) samples dominates. The influence of cleavage imperfections on the position of the Fermi level follows from the observation that the sum of the barrier height on an n-type sample and that on a p-type sample equals the band gap (1.16 eV), provided the surface roughness looks identical. As pointed out previously [lo], the measuring inaccuracy in this sum amounts to 0.03 eV. The first conclusion is that for all metals investigated the position of the Fermi level at the interface is restricted to the lower half of the band gap, while the sec-
L208
J.D. van Otterloo
/ Schottky
barriers on clean-cleaved
Si
ond conclusion is that the influence of the cleavage steps is reflected by a movement of the Fermi level towards mid gap. And so it seems that the metal-inducedstates concept for 7 X 7 surfaces does not hold for a freshly cleaved surface because in that case one would expect a much larger variation of the barrier height for the investigated metal range. Recently a so-called “defect model” theory [17] has been proposed for Schottky barriers on cleaved III-V components. From their experiments Lindau et al. [ 171 concluded that the Fermi level is already pinned at sub-monolayer coverage, which implies that the barrier height is determined by a relatively small number of states (order lo’* cm-“). Furthermore it appeared that the barrier height was independent of the applied metal (Cs, Al, Au) which suggested the existence of “defect” states, produced by the heat of condensation of the Al atoms. They even observed a complete decomposition of the semiconductor in the case of Au on cleaved GaSb. Brielson et al. [ 181 investigated the Al-GaAs interface and concluded that the Ga and the As diffused away from the interface similar to the results of Lindau et al. [17] for Au on GaAs. They also concluded that the dipole formation has been completed at submonolayer coverage which was explained by a specific unrelaxed surface structure where Ga atoms are replaced by Al. The question now is whether the “defect” states model also holds for silicon. Some evidence for defect states is found in the fact that many metals form silicides and these interdiffusions may cause the interband states as postulated in the defect model. Even at room temperature an interdiffusion has been observed, which has been extensively pointed out for, e.g., the Au case (Green et al. [19]). However, for Ag no silicide formation could be detected [20] at room temperature. Furthermore, it is much more likely that cleavage steps cause band shifts in the defect model case than in the metal-induced states case of Rowe et al. because in the latter case the density of states is equivalent to a full monolayer. Soshea and Lucas [21] measured the difference in barrier height of Au on the smooth upper part and the rough lower part of a cleaved n-type sample. They found that the barrier height on the smooth part was 0.05 eV lower that that on the rough part, which contradicts our results in table 1. An explanation might be found in the work of Kuhlmann and Henzler [ 131 who demonstrated the existence of two types of cleavage steps depending on the inclination angle. These steps cause an opposite movement of the Fermi level on uncovered silicon surfaces. The author wishes to express his gratitude to L.J. Gerritsen for skillful assistance during the experiments and to Dr. M. Kleefstra for valuable comments.
References [l] J.E. Rowe, S.B. Christman and G. Margaritondo, [2] G. Margaritondo, J.E. Rowe and S.B. Christman,
Phys. Rev. Letters 35 (1975) Phys. Rev. B14 (1976) 5396.
1471.
J.D. van Otterloo / Schottky barriers on clean-cleaved Si [3] J.E. Rowe, G. Margaritondo [4] [5] [6] [ 71 [8] [9]
[lo] [ 1 l] [ 121 [ 131 [ 141 [ 151 [16] [17] [18] [19] [20] [21]
L209
and S.B. Christman, Phys. Rev. B15 (1977) 2195. J. Bardeen,Phys. Rev. 71 (1947) 717. D.E. Eastman and J.L. Freeouf, Phys. Rev. Letters 34 (1975) 1624. J. van Laar, A. Huyser and T.L. van Rooy, J. Vacuum Sci. Technol. 14 (1977) 894. A. Thanailakis, J. Phys. C8 (1975) 655. A. Thanailakis and A. Rasul, J. Phys. C9 (1976) 337. N. Szydlo and R. Poirier, J. Appl. Phys. 44 (1973) 1386. I.D. van Otterloo and L.J. Gerritsen, J. Appl. Phys. 49 (1978) 723. J.D. van Otterloo, Ph.D. Thesis, Delft University of Technology (1977). M. Henzler, Surface Sci. 36 (1973) 109. W. Kuhlmann and M. Henzler, Surface Sci. 69 (1977) 533. G.W. Gobeli and F.G. Allen, J. Phys. Chem. Solids 14 (1960) 23. J.D. van Otterloo and J.G. de Groot, Surface Sci. 57 (1976) 93. D.J. Santeler, J. Vacuum Sci. Technol. 8 (1971) 299. I. Lindau, P.W. Chye, CM. Garner, P. Pianetta, C.Y. Su and W.E. Spicer, J. Vacuum Sci. Technol. 15 (1978) 1322. L.J. Brillson, R.Z. Bachrach, L.S. Bauerand and J. McMenamin, Phys. Rev. Letters 42 (1979) 397. A.K. Green and E. Bauer, J. Appl. Phys. 47 (1976) 1284. G. Le Lay, M. Manneville and R. Kern, Surface Sci. 72 (1978) 405. R.W. Soshea and R.C. Lucas, Phys. Rev. Al38 (1965) 1182.
SURFACE SCIENCE LETTERS SCHOTTKY BARRIERS
ON CLEAN-CLEAVED
SILICON
J.D. VAN OTTERLOO Department Received
of Electrical Engineering,
23 October
1980; accepted
Delft University of Technology, for publication
5 December
Delft, The Netherlands
1980
Experiments dealing with barrier height measurements of various metals on ultra high vacuum (UHV) cleaved silicon are reported. It is shown that the barrier height is affected by the presence of cleavage steps. Furthermore it is concluded that the position of the Fermi level at the interface on these rough surfaces is influenced to only a minor degree by the metal used. Therefore it is suggested that the Fermi level be pinned by cleavage-step-induced states instead of by metal-induced states according to Rowe et al.
Rowe and co-workers [l-3] have gathered new information on the fundamentals of Schottky barrier formation on covalent semiconductor surfaces. They found that on the majority of sputter-cleaned and subsequently annealed surfaces the intrinsic surface states are annihilated by the metal atoms. New metal-inducedinterface states take their place and cause the observed pinning of the Fermi level at the interface. However, it was observed [l] that on the (110) face of Ge, the intrinsic surface states were less sensitive to the metal atoms and the Fermi level was pinned by these intrinsic states. Consequently, this latter situation is consistent with the classical Bardeen [4] model, which states that the Fermi level is pinned in one invariant position under the condition that the density of surface states is sufficiently high. Similar observations were made by Eastman and Freeouf [S ] on cleaved (1 IO) surfaces of III-V compounds. They found a relation between the Fermi level and the empty intrinsic surface state band. Both results were explained by Rowe et al. [I], by introducing a specific surface relaxation of the (110) face, which makes certain surface atoms less sensitive to the absorbed metal atoms which thus retain their intrinsic surface properties. The results of Eastman and Freeouf [5] have been recently discussed by van Laar et al. [6] who argued that the band gap of cleaved GaAs, GaSb and InP is free of empty surface states provided the samples are well cleaved (no cleavage imperfections on the surface). Consequently the results of Eastman and Freeouf [S] for these surfacesmust be interpreted as a relation between the Fermi level and extrinsic surface states, while in this particular case these states are a property of the semiconductor (artificial Bardeen model). And so the question arises: to what extent do 0039-6028/81/0000-OOOO/$
02.50 0 North-Holland
Publishing
Company
L206
J.D. van Otterloo
/ Schottky
barriers on clean-cleaved
Si
cleavage steps influence the Fermi level of Schottky barriers on cleaved silicon? From the extensive investigations of Thanailakis [7,8] it is known that the barrier height of metal contacts on cleaved silicon depends only slightly on the metal used. However, in the literature a metal-dependent barrier height on cleaved silicon is sometimes reported (for alkaline metals [9] heights as low as 0.43 eV on n-type Si have been reported). But it has been pointed out [ 10,l l] that these latter results may be explained as coming from a less reliable measuring method (current-voltage measurements). The most accurate measuring method [lo] is the C-I/ method (the reciprocal of the differential capacitance squared as a function of reverse bias across the contact) and these barrier heights vary between 0.70 and 0.98 eV for simple and transition metals following Thanailakis [7,8]. In none of these works has information been presented about the density of cleavage steps. Therefore we have remeasured the barrier heights for various metals (ranging from alkaline to noble metals) on clean-cleaved silicon, while simultaneously recording the presence (or absence) of cleavage steps. This latter is simply done by comparing photomicrographs (taken through a normal optical microscope‘) of the Schottky barrier contacts. The result is labeled as “good cleavage” when the photograph has an appearance according to fig. la, mutatis mutandis a “bad cleavage” is shown in fig. lb. Obviously this is a rather crude method, because many more details can be revealed if the inclination angle with respect to the (111) face is measured [ 12,131. The samples are cleaved, according to the Gobeli and Allen [14] method, in a stream of evaporating metal atoms in UHV. Extremely flat surfaces (fig. la) result if the samples are well oriented [ 11 ,I 51 with respect to [ 1111. The surfaces were examined with Tolanski interferometer measurements and were found to exhibit remaining steps of the order of 80 _& even on the good cleavages of fig. la. The
Fig. 1. Photograph of a Schottky barrier contact on cleaved silicon: good cleavage; (b) junction area labeled as bad cleavage.
(a) junction
area labeled
as
J.D. van Otterloo / SchottkJj barriers on clean-cleaved Si Table 1 Schottky
barrier
Metal
heights
(eV) on clean-cleaved
n-Type Good
silicon at 80 K
Si cleavage
L207
p-Type Bad cleavage
Good
Si cleavage
Bad cleavage -
Ag AU Al Mg cs K Na (at 197 K) Fe
0.78 0.81 0.69 0.62 0.67 0.73 0.70 0.71
0.74 0.67 0.66 0.55 0.62 0.66 -
0.37 0.34 0.47 0.56 0.40 _ -
0.42 0.50 0.49 0.61 _ _ _ -
Ga In
0.72 0.73
0.66 -
0.45 _
0.52 0.51
larger steps in fig. lb have an average height of 1000 A; the steps run roughly toward a [1?1l] direction. On the smooth (upper) part of the cleavage face three circular Schottky barrier contacts of 0.5 mm diameter are formed. The deposited layers were found to be free of contaminations below a level of 10e4. During evaporation the substrate temperature is stabilized at 80 K in the case of alkaline metals and at 273 K for other metals. The UHV (8 X 10-l ’ Torr) system used consists of a bakeable stainless steel vessel pumped by an oil-diffusion pump (DC 70.5 oil) assembly and a titanium sublimation pump as described previously [ 11 ,151. The application of a liquid-nitrogen-cooled trap and an extra small diffusion pump between the rotation and the main diffusion pump reduced the oil residues to an undetectable level (partial pressure analysis) according to Santeler [ 161. All measurements [lo] were performed in the same UHV. The barrier heights at 80 K, resulting from Cb2- V measurements, for the various metals on both well cleaved and badly cleaved surfaces are presented in table 1. The barrier heights on n-type samples (1015-10’6/ cm”) at elevated temperatures are only slightly higher. This temperature behavior is indicative of the interface state distribution [ 11,151. The low temperature in table 1 has been selected because at higher temperatures the ohmic behavior on the p-type (2.1 X 10r6/cm3) samples dominates. The influence of cleavage imperfections on the position of the Fermi level follows from the observation that the sum of the barrier height on an n-type sample and that on a p-type sample equals the band gap (1.16 eV), provided the surface roughness looks identical. As pointed out previously [lo], the measuring inaccuracy in this sum amounts to 0.03 eV. The first conclusion is that for all metals investigated the position of the Fermi level at the interface is restricted to the lower half of the band gap, while the sec-
L208
J.D. van Otterloo
/ Schottky
barriers on clean-cleaved
Si
ond conclusion is that the influence of the cleavage steps is reflected by a movement of the Fermi level towards mid gap. And so it seems that the metal-inducedstates concept for 7 X 7 surfaces does not hold for a freshly cleaved surface because in that case one would expect a much larger variation of the barrier height for the investigated metal range. Recently a so-called “defect model” theory [17] has been proposed for Schottky barriers on cleaved III-V components. From their experiments Lindau et al. [ 171 concluded that the Fermi level is already pinned at sub-monolayer coverage, which implies that the barrier height is determined by a relatively small number of states (order lo’* cm-“). Furthermore it appeared that the barrier height was independent of the applied metal (Cs, Al, Au) which suggested the existence of “defect” states, produced by the heat of condensation of the Al atoms. They even observed a complete decomposition of the semiconductor in the case of Au on cleaved GaSb. Brielson et al. [ 181 investigated the Al-GaAs interface and concluded that the Ga and the As diffused away from the interface similar to the results of Lindau et al. [17] for Au on GaAs. They also concluded that the dipole formation has been completed at submonolayer coverage which was explained by a specific unrelaxed surface structure where Ga atoms are replaced by Al. The question now is whether the “defect” states model also holds for silicon. Some evidence for defect states is found in the fact that many metals form silicides and these interdiffusions may cause the interband states as postulated in the defect model. Even at room temperature an interdiffusion has been observed, which has been extensively pointed out for, e.g., the Au case (Green et al. [19]). However, for Ag no silicide formation could be detected [20] at room temperature. Furthermore, it is much more likely that cleavage steps cause band shifts in the defect model case than in the metal-induced states case of Rowe et al. because in the latter case the density of states is equivalent to a full monolayer. Soshea and Lucas [21] measured the difference in barrier height of Au on the smooth upper part and the rough lower part of a cleaved n-type sample. They found that the barrier height on the smooth part was 0.05 eV lower that that on the rough part, which contradicts our results in table 1. An explanation might be found in the work of Kuhlmann and Henzler [ 131 who demonstrated the existence of two types of cleavage steps depending on the inclination angle. These steps cause an opposite movement of the Fermi level on uncovered silicon surfaces. The author wishes to express his gratitude to L.J. Gerritsen for skillful assistance during the experiments and to Dr. M. Kleefstra for valuable comments.
References [l] J.E. Rowe, S.B. Christman and G. Margaritondo, [2] G. Margaritondo, J.E. Rowe and S.B. Christman,
Phys. Rev. Letters 35 (1975) Phys. Rev. B14 (1976) 5396.
1471.
J.D. van Otterloo / Schottky barriers on clean-cleaved Si [3] J.E. Rowe, G. Margaritondo [4] [5] [6] [ 71 [8] [9]
[lo] [ 1 l] [ 121 [ 131 [ 141 [ 151 [16] [17] [18] [19] [20] [21]
L209
and S.B. Christman, Phys. Rev. B15 (1977) 2195. J. Bardeen,Phys. Rev. 71 (1947) 717. D.E. Eastman and J.L. Freeouf, Phys. Rev. Letters 34 (1975) 1624. J. van Laar, A. Huyser and T.L. van Rooy, J. Vacuum Sci. Technol. 14 (1977) 894. A. Thanailakis, J. Phys. C8 (1975) 655. A. Thanailakis and A. Rasul, J. Phys. C9 (1976) 337. N. Szydlo and R. Poirier, J. Appl. Phys. 44 (1973) 1386. I.D. van Otterloo and L.J. Gerritsen, J. Appl. Phys. 49 (1978) 723. J.D. van Otterloo, Ph.D. Thesis, Delft University of Technology (1977). M. Henzler, Surface Sci. 36 (1973) 109. W. Kuhlmann and M. Henzler, Surface Sci. 69 (1977) 533. G.W. Gobeli and F.G. Allen, J. Phys. Chem. Solids 14 (1960) 23. J.D. van Otterloo and J.G. de Groot, Surface Sci. 57 (1976) 93. D.J. Santeler, J. Vacuum Sci. Technol. 8 (1971) 299. I. Lindau, P.W. Chye, CM. Garner, P. Pianetta, C.Y. Su and W.E. Spicer, J. Vacuum Sci. Technol. 15 (1978) 1322. L.J. Brillson, R.Z. Bachrach, L.S. Bauerand and J. McMenamin, Phys. Rev. Letters 42 (1979) 397. A.K. Green and E. Bauer, J. Appl. Phys. 47 (1976) 1284. G. Le Lay, M. Manneville and R. Kern, Surface Sci. 72 (1978) 405. R.W. Soshea and R.C. Lucas, Phys. Rev. Al38 (1965) 1182.