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Direct observations of the behaviour of single silicon atoms on a metal surface
L497
Surface Science 109 (1981) L497-L503 North-Holland Publishing Company
SURFACE SCIENCE LETTERS DIRECT OBSERVATIONS
OF THE BEHAVIOR OF SINGLE SILICON
ATOMS ON A METAL SURFACE
Rodrigo CASANOVA and T.T. TSONG P/zJssics Department, USA
Received
The Penns~~lvania State University, University Park, Pennsylvania 16802,
4 May 1981
The behavior of single silicon adatoms on the W (1 lo} plane has been successfully for the first time. Single atom diffusion parameters are found to be Ed = 0.70 + 0.07 Do = 3.08 X IO4 X 10’1.28 cm2/s. The field desorption behavior of Si atoms is similar of metal adatoms. Si-Si adaton-adatom interaction shows nonmonotonic distance dence, but the repulsive region around 3.2 A is much weaker than those found in metal interactions.
studied eV, and to that depenadatom
Some of the best defined experiments in modern surface science include field ion microscope [l] (FIM) studies of behavior of single atoms and simple atomic clusters on metal surfaces. Migration of single atoms with [Z] and without [2-41 a driving force can be directly observed and diffusion parameters measured. Formation and dissociation of atomic clusters can be visualized [.5,6] and atomic jumping process revealed [7]. Interatomic forces, or energies, between two atoms over large distances can be derived [8,9]. Surface induced dipole moment [ IO,1 I] and polarizability [2,12] of single surface atoms can be measured. This whole new class of experiments, because of the simplicity of the physical environments and directness of the principles involved, provides valuable data for theoretical understanding of atomic processes occurring on solid surfaces. Unfortunately, this class of experiments so far can be done only with metallic adatoms despite the fact that it has been around for more than fifteen years [3,13]. Early attempts to extend this class of experiments to nonmetallic adatoms were not successful. We report here a successful experiment with single silicon adatoms on a metal surface. In this first report, we present a quantitative determination of diffusion parameters of single Si atoms, a measurement of field desorption field, a rough estimate of the binding energy, and a preliminary observation of Si-Si interaction on a metal surface. We thus demonstrate without doubt that with some effort, studies of the behavior of single Si adatoms on metal surfaces can be done. Our choice of Si adatoms is aimed to gain fundamental understanding in siliconsilicon and in silicon-metal atomic interactions on solid surfaces, and in the atomic 003%6028/8
l/0000-0000/$02.50
0 North-Holland
L498
R. Casanova,
T.T. Tsong /Single
Si atoms on a metal surface
transport phenomena which occur during the initial stages of silicides and semiconductor-metal interface formation. Such interfaces are important in Schottky barrier and thin film formations [14] in microelectronics technology. In solid states, silicon atoms interact with each other usually by forming covalent bonds rather than metallic bonds. Very little is presently known about the behavior of single Si atoms on metal surfaces. Thus either a similarity or a difference in the Si adatom behavior on metal surfaces as compared to metal adatoms should both be viewed as new information of significance. Experimental procedures for FIM single adatom experiments can be found elsewhere [7]. We emphasize the rigorousness of our vacuum processing. It normally takes more than a week, and ion gauge readings are always -2 X 10-i’ Torr, the X-ray limit of the gauge. As the desorption field of Si adatoms is drastically reduced by traces of hydrogen, we use exclusively Vyco glass diffused helium for FIM imaging. Observations are done at 78 K. Si adatoms are deposited by resistivety heating of a stripe of high purity Si wafer, which has been degassed near the melting point in lo-” Torr for over 24 h, repeatedly. In the FIM, single Si adatoms appear very much dimmer compared to metal adatoms. While a metal adatom can be easily seen at any location of a W { 110)) a Si adatom near the center of the plane can be seen very difficult, even close to its desorption field. However, outside the central region of the plane, Si adatoms can be easily seen. The reason for this peculiar imaging property is not clear; it is perhaps because of a lack of field adsorption of image gas atom and the smaller size of Si atoms. The low image intensity of Si adatoms improves significantly the accuracy of adatom position mappings to -+-0.3 A, but inevitably causes a frequent loss of adatoms by accidental field desorption in adatom-adatom interaction studies where a few atoms are usually on a plane. Figs. la and lb show the Arrhenius plot obtained for single Si adatom migration and FIM images of one Si atom on a W{l lo}. Each data point is the result of 90 heating periods with one Si adatom on a W{llO). From the plot, we derive the activation energy of surface migration to be Ed = 0.70 +_0.07 eV, and a pre-exponential factor to be De = 3.08 X 1O-4 X 1O”.28 cm’/s. These values are comparable to metallic adatoms such as Pt or Ir on the W (110). We have also made a preliminary observation of Si adatom diffusion on the W { 112). Single atoms start to diffuse at -200 K. Ed is estimated to be -0.55 eV. No cooperative walk [5] of two Si adatoms in the adjacent atomic channels of the (112) was found. However, the Si atoms tended to form either a straight or a staggered bond near a plane edge.‘The preferred site of adsorption was the second adsorption site to a plane edge. Plane edges are reflective on both the { 110) and the (112) planes. The desorption voltage of Si adatoms is relatively high, ranging from -70% to 85% of the field evaporation voltage of the substrate W{llO} plane when the adatom positions change from the plane edges to the plane center. Fig. 2a shows such a variation from 105 desorption events and fig. 2b shows the desorption events. Assuming that the adatoms at the plane edge have an electric field the same
R. Casanova,
.05
3.5
3.6
a
T. T. Tsong / Single Si atoms
3.7 1000/T
3.0
3.9
on a metal
4.0
surface
L499
4.1
( K”)
Fig. 1. (a) An Arrhenius plot derived with one Si adatom on a W {llO}. (b) FIM image of a Si atom. Even after over-exposing the film by a factor of -5, Si atom image is very dim.
as the plane edge atoms of the substrate, the desorption field of Si adatoms is then estimated to be -4.0 V/A. Derivation of binding energy from desorption field is at the present time rather uncertain. As this quantity is of great interest [15,16], a rough estimate of its value is made using [l]
A=er$,-I+@+3.6/r0eV,
(1)
LSOO
R. Casanova,
T.T. Tsong /Single
Si atoms on a metal surface
\ i 4.61
a
O
.2
.4
R/R,
.6
1
.8
1.C
Fig. 2. (a) Desorption voltage of Si adatoms as a function of radial locations. (b) Field desorption of Si adatoms at 5.52,5.55,5.64 (two Si gone) and 5.78 kV.
where r. is the single bond atomic radius of Si atoms in A,Z the ionization energy of Si, 4 the substrate work function, Fe the desorption field in Vi.&, and A the binding energy. The last term represents image potential energy. Using 4 = 5.3 eV, rO = 1.173 8, I = 8.149 eV, and F, = 4.0 V/a, the binding energy is estimated to be 4.9 eV. This value is again comparable to metal adatoms. Perhaps as important, but certainly quantitatively valid, a use of fig. 2a is an accurate determination of the variation of electric field across the substrate plane [ 15,161. Using linear and quadratic digressions, we found I/(R) = 1/,(1 - 0.197R/Ro),
r* = 0.87 ,
(2)
or V(R)=
VJ1 - 1.71 X 10v2R/Ro - 0.174(R/Ro)‘]
,
r2 =0.91
(3)
V, is the desorption voltage of a Si adatom when it is located at the plane center. R. is the radius of the plane, and Y* is the coefficient of determination. Since desorption field is a constant, the field variation across the plane is given by b’(R) = E‘,V,./V(K) ,
(4)
where /“C is the field at the center of the plane. The most interesting finding is the completely different behavior of the Si-Si adatom--adatom interaction as compared to metal adatoms. Since when a Si adatom is located near the plane center, it is very difficult to be seen without losing the other Si adatom which may be near the plane edge, a pair distribution of Si-Si interaction is most difficult to obtain. In this first study, we report frequencies of observing clusters of various configurations. Out of 2 17 heating periods of 50 s each at 300 K with four to six Si atoms on a plane of -20 to 30 a in radius, clusters with structures shown in figs. 3 and 4 are observed. The frequencies of observation, bond directions, and approxiltiate bond lengths are listed in table 1. Four important pieces of information can be drawn. (1) In W-W, WPRe, and W-lr interactions,
a
b
Fig. 3. FIM micrographs showing five cluster configurations observed.
L502
R. Casanova,
l:ip. 4. Structures
of clusters
T. T. Tsong / Single Si atoms
shown
ori a metal
srufhce
in fig. 3.
a strong repulsive region exists in the 3 to 5 a distance range. Thus configurations (b) and (c) are never observed out of -4,000 observations [7-91. No such strong repulsive region exists in Si-Si interaction. (2) A weak repulsive region exists in Si-Si interaction around 3.2 A since configuration (b) is much less frequently observed than configurations (a) and (c). The Si-Si pair energy is thus non-monotonic in distance dependence also. (3) For Pt or Ir adatoms [17] on the W {llO}, multiatomic chain direction is always along the (111). Si multiatomic chain direction is, however, along the [liO]. (4) None of the clusters observed is stable with respect to migration. Si atom transport on the W{IIO} is therefore dominantly accomplished by single Si atom migration. In summary, we have succeeded for the first time in studying the behavior of single Si atoms on a metal surface. Although further work has to be done, in this first study we already found a similar diffusion and binding behavior of Si atoms
Table 1 Clusters observed Cluster configuration (referring to figs. 3 and 4)
z c d e
Bond direction
-[iii] lOOl1 Ill01 [1111 [I101
Bond length
-2.74 3.16 4.47 5.48 4.47
ar r + c
(A)
0.30 0.30 0.30 0.30 (two bonds)
Frequencieb observation
193 26 5 2
of
R. Casanova, T. T. Tsong / Single Si atoms on a metal surface
with the substrate as metal adatoms, interaction behavior of the silicon. The authors acknowledge
and a drastically
different
503
adatom-adatom
the support of NSF under grant DMR-7904862.
References (11 E.W. Mullet and T.T. Tsong, Field Ion Microscopy, Principles and Applications New York, 1969). (21 T.T. Tsong and C.L. Kellogg, Phys. Rev. B12 (1975) 1343. [3] G. Ehrlich and F.G. Hudda, J. Chem. Phys. 44 (1966) 1039. [4] D.W. Bassett and M.J. Parsely, Brit. J. Appl. Phys. 2 (1969) 13. [S] T.T. Tsong, J. Chem. Phys. 54 (1971) 4205; T.T. Tsong, Phys. Rev. B6 (1972) 417. [6] D.W. Bassett and M.J. Parsely, Nature (London) 221 (1969) 1046. [7] T.T. Tsong and R. Casanova, Phys. Rev. B21 (1980) 4564; B22 (1980) 4632. [S] T.T. Tsong,Phys. Rev, Letters 31 (1973) 1207. (91 R. Casanova and T.T. Tsong, Phys. Rev. B22 (1980) 5590. [lo] E.W. Plummer and T.N. Rhodin, Appl. Phys. Letters 11 (1967) 194. [ 1 l] G.L. Kellogg and T.T. Tsong, Surface Sci. 62 (1977) 343. [12] T.T. Tsong, J. Chem. Phys. 54 (1971) 4205. [13] E.W. Miiller, 2. Electrochem. 61 (1957) 43. [14] See for example, J.M. Poate and K.N. Tu, Phys. Today, May 1980. 1151 G. Ehrlich and C.F. Kirk, J. Chem. Phys. 48 (1968) 1468. [ 161 E.W. Plummer and T.N. Rhodin, J. Chem. Phys. 49 (1969) 3479. [17] D.W. Bassett, Surface Sci. 23 (1970) 240.
(Elsevier,
Surface Science 109 (1981) L497-L503 North-Holland Publishing Company
SURFACE SCIENCE LETTERS DIRECT OBSERVATIONS
OF THE BEHAVIOR OF SINGLE SILICON
ATOMS ON A METAL SURFACE
Rodrigo CASANOVA and T.T. TSONG P/zJssics Department, USA
Received
The Penns~~lvania State University, University Park, Pennsylvania 16802,
4 May 1981
The behavior of single silicon adatoms on the W (1 lo} plane has been successfully for the first time. Single atom diffusion parameters are found to be Ed = 0.70 + 0.07 Do = 3.08 X IO4 X 10’1.28 cm2/s. The field desorption behavior of Si atoms is similar of metal adatoms. Si-Si adaton-adatom interaction shows nonmonotonic distance dence, but the repulsive region around 3.2 A is much weaker than those found in metal interactions.
studied eV, and to that depenadatom
Some of the best defined experiments in modern surface science include field ion microscope [l] (FIM) studies of behavior of single atoms and simple atomic clusters on metal surfaces. Migration of single atoms with [Z] and without [2-41 a driving force can be directly observed and diffusion parameters measured. Formation and dissociation of atomic clusters can be visualized [.5,6] and atomic jumping process revealed [7]. Interatomic forces, or energies, between two atoms over large distances can be derived [8,9]. Surface induced dipole moment [ IO,1 I] and polarizability [2,12] of single surface atoms can be measured. This whole new class of experiments, because of the simplicity of the physical environments and directness of the principles involved, provides valuable data for theoretical understanding of atomic processes occurring on solid surfaces. Unfortunately, this class of experiments so far can be done only with metallic adatoms despite the fact that it has been around for more than fifteen years [3,13]. Early attempts to extend this class of experiments to nonmetallic adatoms were not successful. We report here a successful experiment with single silicon adatoms on a metal surface. In this first report, we present a quantitative determination of diffusion parameters of single Si atoms, a measurement of field desorption field, a rough estimate of the binding energy, and a preliminary observation of Si-Si interaction on a metal surface. We thus demonstrate without doubt that with some effort, studies of the behavior of single Si adatoms on metal surfaces can be done. Our choice of Si adatoms is aimed to gain fundamental understanding in siliconsilicon and in silicon-metal atomic interactions on solid surfaces, and in the atomic 003%6028/8
l/0000-0000/$02.50
0 North-Holland
L498
R. Casanova,
T.T. Tsong /Single
Si atoms on a metal surface
transport phenomena which occur during the initial stages of silicides and semiconductor-metal interface formation. Such interfaces are important in Schottky barrier and thin film formations [14] in microelectronics technology. In solid states, silicon atoms interact with each other usually by forming covalent bonds rather than metallic bonds. Very little is presently known about the behavior of single Si atoms on metal surfaces. Thus either a similarity or a difference in the Si adatom behavior on metal surfaces as compared to metal adatoms should both be viewed as new information of significance. Experimental procedures for FIM single adatom experiments can be found elsewhere [7]. We emphasize the rigorousness of our vacuum processing. It normally takes more than a week, and ion gauge readings are always -2 X 10-i’ Torr, the X-ray limit of the gauge. As the desorption field of Si adatoms is drastically reduced by traces of hydrogen, we use exclusively Vyco glass diffused helium for FIM imaging. Observations are done at 78 K. Si adatoms are deposited by resistivety heating of a stripe of high purity Si wafer, which has been degassed near the melting point in lo-” Torr for over 24 h, repeatedly. In the FIM, single Si adatoms appear very much dimmer compared to metal adatoms. While a metal adatom can be easily seen at any location of a W { 110)) a Si adatom near the center of the plane can be seen very difficult, even close to its desorption field. However, outside the central region of the plane, Si adatoms can be easily seen. The reason for this peculiar imaging property is not clear; it is perhaps because of a lack of field adsorption of image gas atom and the smaller size of Si atoms. The low image intensity of Si adatoms improves significantly the accuracy of adatom position mappings to -+-0.3 A, but inevitably causes a frequent loss of adatoms by accidental field desorption in adatom-adatom interaction studies where a few atoms are usually on a plane. Figs. la and lb show the Arrhenius plot obtained for single Si adatom migration and FIM images of one Si atom on a W{l lo}. Each data point is the result of 90 heating periods with one Si adatom on a W{llO). From the plot, we derive the activation energy of surface migration to be Ed = 0.70 +_0.07 eV, and a pre-exponential factor to be De = 3.08 X 1O-4 X 1O”.28 cm’/s. These values are comparable to metallic adatoms such as Pt or Ir on the W (110). We have also made a preliminary observation of Si adatom diffusion on the W { 112). Single atoms start to diffuse at -200 K. Ed is estimated to be -0.55 eV. No cooperative walk [5] of two Si adatoms in the adjacent atomic channels of the (112) was found. However, the Si atoms tended to form either a straight or a staggered bond near a plane edge.‘The preferred site of adsorption was the second adsorption site to a plane edge. Plane edges are reflective on both the { 110) and the (112) planes. The desorption voltage of Si adatoms is relatively high, ranging from -70% to 85% of the field evaporation voltage of the substrate W{llO} plane when the adatom positions change from the plane edges to the plane center. Fig. 2a shows such a variation from 105 desorption events and fig. 2b shows the desorption events. Assuming that the adatoms at the plane edge have an electric field the same
R. Casanova,
.05
3.5
3.6
a
T. T. Tsong / Single Si atoms
3.7 1000/T
3.0
3.9
on a metal
4.0
surface
L499
4.1
( K”)
Fig. 1. (a) An Arrhenius plot derived with one Si adatom on a W {llO}. (b) FIM image of a Si atom. Even after over-exposing the film by a factor of -5, Si atom image is very dim.
as the plane edge atoms of the substrate, the desorption field of Si adatoms is then estimated to be -4.0 V/A. Derivation of binding energy from desorption field is at the present time rather uncertain. As this quantity is of great interest [15,16], a rough estimate of its value is made using [l]
A=er$,-I+@+3.6/r0eV,
(1)
LSOO
R. Casanova,
T.T. Tsong /Single
Si atoms on a metal surface
\ i 4.61
a
O
.2
.4
R/R,
.6
1
.8
1.C
Fig. 2. (a) Desorption voltage of Si adatoms as a function of radial locations. (b) Field desorption of Si adatoms at 5.52,5.55,5.64 (two Si gone) and 5.78 kV.
where r. is the single bond atomic radius of Si atoms in A,Z the ionization energy of Si, 4 the substrate work function, Fe the desorption field in Vi.&, and A the binding energy. The last term represents image potential energy. Using 4 = 5.3 eV, rO = 1.173 8, I = 8.149 eV, and F, = 4.0 V/a, the binding energy is estimated to be 4.9 eV. This value is again comparable to metal adatoms. Perhaps as important, but certainly quantitatively valid, a use of fig. 2a is an accurate determination of the variation of electric field across the substrate plane [ 15,161. Using linear and quadratic digressions, we found I/(R) = 1/,(1 - 0.197R/Ro),
r* = 0.87 ,
(2)
or V(R)=
VJ1 - 1.71 X 10v2R/Ro - 0.174(R/Ro)‘]
,
r2 =0.91
(3)
V, is the desorption voltage of a Si adatom when it is located at the plane center. R. is the radius of the plane, and Y* is the coefficient of determination. Since desorption field is a constant, the field variation across the plane is given by b’(R) = E‘,V,./V(K) ,
(4)
where /“C is the field at the center of the plane. The most interesting finding is the completely different behavior of the Si-Si adatom--adatom interaction as compared to metal adatoms. Since when a Si adatom is located near the plane center, it is very difficult to be seen without losing the other Si adatom which may be near the plane edge, a pair distribution of Si-Si interaction is most difficult to obtain. In this first study, we report frequencies of observing clusters of various configurations. Out of 2 17 heating periods of 50 s each at 300 K with four to six Si atoms on a plane of -20 to 30 a in radius, clusters with structures shown in figs. 3 and 4 are observed. The frequencies of observation, bond directions, and approxiltiate bond lengths are listed in table 1. Four important pieces of information can be drawn. (1) In W-W, WPRe, and W-lr interactions,
a
b
Fig. 3. FIM micrographs showing five cluster configurations observed.
L502
R. Casanova,
l:ip. 4. Structures
of clusters
T. T. Tsong / Single Si atoms
shown
ori a metal
srufhce
in fig. 3.
a strong repulsive region exists in the 3 to 5 a distance range. Thus configurations (b) and (c) are never observed out of -4,000 observations [7-91. No such strong repulsive region exists in Si-Si interaction. (2) A weak repulsive region exists in Si-Si interaction around 3.2 A since configuration (b) is much less frequently observed than configurations (a) and (c). The Si-Si pair energy is thus non-monotonic in distance dependence also. (3) For Pt or Ir adatoms [17] on the W {llO}, multiatomic chain direction is always along the (111). Si multiatomic chain direction is, however, along the [liO]. (4) None of the clusters observed is stable with respect to migration. Si atom transport on the W{IIO} is therefore dominantly accomplished by single Si atom migration. In summary, we have succeeded for the first time in studying the behavior of single Si atoms on a metal surface. Although further work has to be done, in this first study we already found a similar diffusion and binding behavior of Si atoms
Table 1 Clusters observed Cluster configuration (referring to figs. 3 and 4)
z c d e
Bond direction
-[iii] lOOl1 Ill01 [1111 [I101
Bond length
-2.74 3.16 4.47 5.48 4.47
ar r + c
(A)
0.30 0.30 0.30 0.30 (two bonds)
Frequencieb observation
193 26 5 2
of
R. Casanova, T. T. Tsong / Single Si atoms on a metal surface
with the substrate as metal adatoms, interaction behavior of the silicon. The authors acknowledge
and a drastically
different
503
adatom-adatom
the support of NSF under grant DMR-7904862.
References (11 E.W. Mullet and T.T. Tsong, Field Ion Microscopy, Principles and Applications New York, 1969). (21 T.T. Tsong and C.L. Kellogg, Phys. Rev. B12 (1975) 1343. [3] G. Ehrlich and F.G. Hudda, J. Chem. Phys. 44 (1966) 1039. [4] D.W. Bassett and M.J. Parsely, Brit. J. Appl. Phys. 2 (1969) 13. [S] T.T. Tsong, J. Chem. Phys. 54 (1971) 4205; T.T. Tsong, Phys. Rev. B6 (1972) 417. [6] D.W. Bassett and M.J. Parsely, Nature (London) 221 (1969) 1046. [7] T.T. Tsong and R. Casanova, Phys. Rev. B21 (1980) 4564; B22 (1980) 4632. [S] T.T. Tsong,Phys. Rev, Letters 31 (1973) 1207. (91 R. Casanova and T.T. Tsong, Phys. Rev. B22 (1980) 5590. [lo] E.W. Plummer and T.N. Rhodin, Appl. Phys. Letters 11 (1967) 194. [ 1 l] G.L. Kellogg and T.T. Tsong, Surface Sci. 62 (1977) 343. [12] T.T. Tsong, J. Chem. Phys. 54 (1971) 4205. [13] E.W. Miiller, 2. Electrochem. 61 (1957) 43. [14] See for example, J.M. Poate and K.N. Tu, Phys. Today, May 1980. 1151 G. Ehrlich and C.F. Kirk, J. Chem. Phys. 48 (1968) 1468. [ 161 E.W. Plummer and T.N. Rhodin, J. Chem. Phys. 49 (1969) 3479. [17] D.W. Bassett, Surface Sci. 23 (1970) 240.
(Elsevier,