- Email: [email protected]
Si(111)
Surface Science
56X
251,/‘252 (1991)
56%572
North-Holland
LEED investigation Timothy
and analysis
of Pb/Si( 111)
N. Doust and Steven P. Tear
Received 1 October 1990; accepted for publication 16 December 1990
The adsorption process of Ph onto clean Si(ll I)-7 X 7 substrates over D wide range of deposition doses and depositiilnlannealing temperature has been studied using LEED intensity t I( V )) spectra. Two fi X &R30 o phases have been observed. For fixed substrate temperature only one of these phases is present independent of the adsorbed dose. There is an irreversible phase change between the two surfaces at - 350’ C. Using Tensor LEED the high temperature phase has been analysed and adsorption shown to occur at the T4 site for this phase with a saturation coverage of i ML.
The formation of metal-se~conductor, and in particular metal-Si(lll) interfaces, has been studied for some time since these interfaces are of both technological and fundamental interest [ 11. It is well-known that a wide variety of metals, when deposited on the clean Si(lll)-7 x 7 surface, in doses of the order of 1 ML produce a fi x fiR30 o (henceforth referred to as fi) LEED pattern [2]. Of these many systems the Pb/Si( 111) interface has received relatively little attention [3IO]. This system is both unreactive and grows in the Stranski..-Krastanov mode [4]. This combination of properties means that the interface between the two materials is two-dimensional and so is very amenable to being solved using quantitative LEED analysis. We will confirm, using LEED f(V) spectra, that there are two distinct 6 superstructures [3], that the phase change occurs at a substrate temperature of - 350 “C and is indrpendent of the amount of adsorbate on the substrate surface. We have performed an analysis of the high-temperature phase using Tensor LEED calculations and demonstrated that for this phase the Pb adatom sits in the Ta adsorption site above the second layer Si atom. All the experiments detailed were carried out using the York computer controlled LEED diffractometer [ll] operating at a base pressure of 2 x IO- “I mbar. The surface of the Si(l11) sub0WLhUZX,‘91/$03St1
( 1991 -
strate was cleaned by flashing the sample to a temperature of 1200° C for up to 2 min using electron bombardment. The sample was cooled slowly ( - 1 K s ’ ) between 1000 and 800 o C to ensure a good 7 x 7 reconstruction. The cleanliness of the sample could be checked using AES and the quality of the 7 X 7 surface using the LEED pattern. Ph was deposited for all experiments at the same rate of - 0.03 ML min ‘. The sample temperature was measured using an IR pyrometer which with neutral density filters has a temperature range of 300-3OOOOC in addition to a Pt~PtRh(lO~) thermocouple which was pressed against the rear of the sample. Experiments were performed for a wide variety of deposition times (O-140 min) and deposition/annealing temperatures (RT-380°C). It was observed that the i( V ) spectra were identical for an interface which had been formed at RT and then annealed for 30 min to those from an interface which had been formed at the same elevated temperature for the same dose. For RT deposits there was no obvious modification of the 7 x 7 LEED pattern for deposits shorter than 20 min. Further Pb deposition produced a 1 x 1 LEED pattern with a high hackground indicating a disordered surface. For deposition times greater than 30 min we were able to observe rings of spots around the integer beams
Elsevier Science Publishers B.V. (North-Holland)
T. N. Dot&, S. P. Tear / LEED
investigation
[3]. These rings are due to double diffraction where the adsorbate unit cell is incommensurate with that of the substrate. These beams are from a hexagonal structure in parallel orientation with the substrate. The background for the LEED pattern was high for this surface. For deposition times of 20 min or longer the Z(V) spectra no longer change with dose. Annealing the sample following Pb deposition changed the LEED pattern to show a fi superstructure. Two distinct types of 6 LEED pattern were observed depending on the annealing temperature. Using the system of Saitoh et al. [4] the two surfaces will be referred to as the o(a) and the fi( p) phases. We observed the I phase when the sample was heated to temperatures below 350 o C following or during Pb deposition and then cooled to RT. There is a high background in the LEED pattern for this surface. The inner ring of fractional-order beams are focused much more poorly then the integer beams and none of the fractional beams are visible until several hours after the sample heating has finished. If the sample was heated to a temperature of 350” C or above the o(p) phase was obtained. For this surface all the fractional-order beams are as well focussed as the integer beams and can be clearly seen minutes after any heating cycle has finished. Fig. 1 shows I(V) spectra for the (1, 0) beam for a 140 min RT deposit annealed to various temperatures for 30 min periods. While there are only minor changes in the spectrum for annealing temperature up to 300° C it can be seen there is a large change in the data for an annealing temperature of 350 o C. Changes at 350 o C are also seen in the spectra of the other beams. More experiments have been performed using a range of deposition times. temperatures and annealing temperatures, but we have only observed two types of I(V) curves for annealed Pb/Si(lll) surfaces. It is interesting to note that the I(V) spectra from the Pb/Si(lll) interface formed at RT with no annealing are all but identical to those which have been heated to substrate temperatures up to 300 o C though the LEED patterns are different. This indicates that the 6 surface makes only a weak contribution to the I(V) spectra of the integer beams. This combined with the poorly
569
and analysis of Pb/ Si(1 II) I
1
I
I
I
7X7 Si
+
30
55
80
105 130 155 180 Primary Energy (eV)
205
230
Fig. 1. I(V) spectra from the (1,O) beam from RT Pb deposits for a range of deposition times.
focused inner fractional beams implies a degree of disorder in the Pb layer [12]. 1(V) spectra from low coverage deposits annealed to 350-380 o C contain features from both the Si(lll)-7 X 7 and the o(p) spectra. Indeed these spectra can be reproduced by mixing the two “pure” sets of spectra. However one cannot reproduce these spectra by mixing 7 X 7 and 6(a) spectra. The LEED patterns for these low coverages were 7 x 7 in character. This suggests that the surface consists of domains of the o(p) phase which are small in comparison to the transfer width of the LEED system. Similarly the spectra from low adsorbate coverage surfaces formed at RT can only be reproduced using mixtures of 7 X 7 and a( a) spectra. Therefore for sub-saturation coverages the surface consists of domains of 7 x 7 Si(ll1) along with just one 6 phase ie. there is no coverage dependence for the Pb adsorption site as has been observed for the
We have performed calculations for four adatom adsorption models. These are the Td site with 8 = * ML and B = 1 ML, the Hj site, where the adatom sits on top of the fourth layer Si atom, with 8 = i ML and 8 = 1 ML. Using the Pendry R-factor we were able to rule out all but the Tj with 19= 4 ML model which is shown in fig. 2. Previous work has shown that the fi( j3) phase has a saturation coverage of t ML [4]. For the model calculations performed we allowed the atoms in the Pb layer and the first two Si double layers to move from their bulk positions in the pass 2 program. The LEED program required each Si double layer to be considered as one rumpled layer. This results in 6 atoms per unit cell for a 6 unit cell of the Si rumpled layer. Though there are a total of thirteen atoms within the unit cells of the three layers, from symmetry there are only three different atom types within each Si layer. We therefore have seven atomic positions to vary but since only two atom types can have any lateral movement and the direction of these movements is along a bond direction we have a total of nine parameters to define the surface. At this stage the search strategy is not fully converged on the solution. However. we believe the Tensor LEED analysis has enabled us to get close to the true structure. Improved precision on the solution will follow with further refinement of
Pb/Ge(lll) interface 1131 and as has been previously reported for this system [.5,6]. Tensor LEED [14] has been used together with the search strategy previously described [15] to investigate the G(p) phase. The Tensor LEED subroutines of Rous have been modified to work with the N atom per unit cell version of the CAVLEED program. The Tensor LEED method is ideally suited to the search strategy outlined in ref. fl5] where a base point is established in the structual parameter space (Tensor pass 1 calcuiation) followed by exploratory moves in the parameter space around this base point. These exploratory moves are carried out using the fast perturbation method in the pass 2 program and the I(V) curves calculated for the new trial structures are compared with the experimental 1( I’) curves using the Pendry R-factor [16]. Once a minimum in the R factor is obtained in these exploratory moves then a new base point is calculated using the value of the structural parameters which give the minimum R factor, and the space around this explored further. The full dyna~cal calculations in pass I used up to 9 phase shifts and up to 223 beams over an energy range of 40 to 200 eV. The Tensor LEED calculations have been done for the four beams whose I(V) curves showed the biggest change for the different phase, there were the (1, Oi (0. 1). (f, f) and (4..$) beams.
-
2.55 2.40 2 32 2 40 2 34
2.52 9.30 2.37
Fig. 2. Top and side views of the 1;; adsorption site. The adatom is labelled 1 and the arrows indicate the movement of the substrate atoms from their bulk positions for the hest Tensor LEED theorv/experiment fit. The atomic seperativns are also shown.
571
TX. Dour!, S. P. Tear / LEED investigation and analysis of Pb/ Si(ll1)
the T4 adsorption site with a saturation coverage of f ML. There are two common trends for these systems: the first is that elevated temperatures are required for this particular reconstruction though other reconstructions do occur at lower deposition/annealing temperatures, the second is that where the structure has been solved the Si substrate has gone through a major reconstruction which brings the adatom close to the sum of the covalent radii of the adsorbate and substrate species. Both the T, and the H, adsorption sites saturate the dangling bonds of the Si(ll1) surface for f ML coverage but total energy calculations
nine parameters. The values of these parameters used to obtain the best overall R-factor agreement are shown in fig. 2 and the theory curves calculated using these values are shown in fig. 3. It can be seen that except for the 3b-4b bond the calculated bond lengths are within 6% of the sum of the relevant tetrahedral Pauling covalent radii. The adatom though is 12% closer to the atom type 3a than the sum of the Pb and Si radii but this substrate atom is not tetrahedraly bonded to the adatom. Several other group-III and -IV metalSi(lll)-fi structures have been analysed by other workers [17-191 and shown to have the adatom in the
R-Factor
0.23
I
0.31
‘/
0.41
0.32
I
50 Fig. 3. Tensor
1
70
I
I
1
I
90 110 130 150 Primary Energy (eV)
1
t
170
190
LEED theory/experiment comparison for the parameters shown in fig. 2. The individual Pendty The overall R-factor for the four beams is 0.32 with an inner potential of - 5 f 1 eV.
R-factors
are shown.
572
T. N. Dourt. S. P. Tear / LEED mwstigation
have shown that for group-III-Si( 111) interfaces the T4 site has a lower energy than the H, site providing the surface has reconstructed [20,21]. This preference for the T4 site has also be shown for the Pb/Ge(lll) system [22] and LEED analysis has confirmed this result [23]. The Pb/Ge(lll) interface unlike the Pb/Si(lll) system has a higher coverage phase at the same substrate temperature. Huang et al. [23] have proposed a model for this interface in which the surface relaxation of the substrate caused by the initial occupation of the T4 site opens up the H, site. This reconstruction allows a Pb adatom to fall into the H, site and in doing so pushes the adatom previously in the T4 site towards the T, site giving a saturation coverage of : ML. As previously noted we observed only one coverage dependent surface at fixed substrate temperature for the Pb/Si(lll) system. The 4% smaller lattice constant of Si means that the substrate reconstruction on the occupancy of the T4 site does not create an open enough H, site for an adatom to be fixed at this site at the temperatures required for the o(a) phase to form. It seems clear that the high temperatures (> 350°C) required for the fi( p) phase to form result form the large substrate reconstruction involved. This reconstruction gives the top Si double layers a fi unit cell which explains the superior quality of the fractional-order beams for the t/!?(p) phase over the 6(a) phase. This implies that at temperatures below that required for the substrate reconstruction the v’??(a) surface has its fi character largely from the adsorbate only. the elevated temperature needed to form this phase is required to give the adatoms sufficient mobility to form a coherent two-dimensional layer. The authors would like to acknowledge the technical assistance of J.C. Dee and A. Gebbie and to thank the LJLCC for the use of the Cray.
and ana(vsis of Ph / Si( I I I )
References of [II See. for example, Proc. 1st Int. Conf. on the Formation Semiconductor Interfaces. Marseille-Luminy. June 1985, Eds. G. Le Lay. J. Derrien and C.A. Sebenne, Surf. Sci. 168 (1986). 121W.S. Yang. S.C. Wu and F. Jnna, Surf. Sci. 169 (1986) 383. [31 J.P. Estrup and J. Morrison, Surf. Sci. 2 (1964) 465. [41 M. Saitoh. K. Oura, K. Aaano, F. Shoji and T. Hanawa. Surf. Sci. 154 (1985) 394. Surf. Sci. 193 ISI G. Quentel. M. Gauch and A. Dogiovanni. (1988) 212. and W.S. Yang, Surf. [61 G. Le Lay. J. Peretti. M. Hanhucken Sci. 204 (1988) 57. S. Baba and A. Kinhara. Appl. Surf. Sci. I71 H. Yaguchi. 33/34 (1988) 75. M. Nielson and R.I.. Johnson. J. PI F. Grey. R. Feidenhans’l. Phys. (Paris) C7 (1989) 50 181. [91 G. Le Lay, K. Hricovini and J.E. Bonnet. Appl. Surf. Sci. 41,‘42 (1989) 25. D.P. van der Werf, T.M. DO] D.R. Heslinga, H.H. Weitering, Klapwijk and T. Hibma, Phys. Rev. Lett. 64 (1990) 1589. IllI V.E. de Carvalho. M.W. Cook. P.G. Cowell. OS. Heavens. M. Prutton and S.P. Tear. Vacuum 34 (1984) 893. F. Grey and R.L. Johnson. in: [I21 B.N. Dev. G. Materlik. Proc. ICSOS-IL Amsterdam. 19X7. Springer Series in Surface Science. Vol. 11 (Springer, Berlin. 198X) p. 340. 1131 H. Li and B.P. Tonner. Surf. Sci. 193 (1988) 10. [I41 P.J. Rous, P.B. Pendry. D.K. Saldin, K. Heinz. K. Mueller and N. Bickel. Phys. Rev. Lett. 57 (1986) 2951. Surf. Sci. 1X7 (1987) [I51 P.G. Cowell and V.E. de Carvalho. 175. 1161 J.B. Pendry. J. Phys. C 13 (1980) 937. u71 A. Kawazu and H. Sakama. Phys. Rer. B 37 ( 1YXX)7704. C. Norris. I-. Vlq and [181 K.M. Conway. J.E. MacDonald. J.F. van der Veen. Surf. Sci. 215 (1989) 555. C.S. Chang and I.S.T. Tsong. J. Vat. [I91 D.M. Cornelison. Sci. Technol. A 8 (7990) 3443. Phys. Rev. Lett. 53 (19X4) 683. WI I.E. Northrup, Phys. Re\. B P11 J.M. Nicholls. B. Reihl and J.E. Northrup, 35 (1987) 4137. [22] J.N. Carter, V.M. Dwyer and B.W. Holland, Solid State Commun. 67 (1988) 643. [23] H. Huang. C.M. Wei. H. Li and B.P. Tanner. Phys. Rev. Lett. 62 (1989) 559.
56X
251,/‘252 (1991)
56%572
North-Holland
LEED investigation Timothy
and analysis
of Pb/Si( 111)
N. Doust and Steven P. Tear
Received 1 October 1990; accepted for publication 16 December 1990
The adsorption process of Ph onto clean Si(ll I)-7 X 7 substrates over D wide range of deposition doses and depositiilnlannealing temperature has been studied using LEED intensity t I( V )) spectra. Two fi X &R30 o phases have been observed. For fixed substrate temperature only one of these phases is present independent of the adsorbed dose. There is an irreversible phase change between the two surfaces at - 350’ C. Using Tensor LEED the high temperature phase has been analysed and adsorption shown to occur at the T4 site for this phase with a saturation coverage of i ML.
The formation of metal-se~conductor, and in particular metal-Si(lll) interfaces, has been studied for some time since these interfaces are of both technological and fundamental interest [ 11. It is well-known that a wide variety of metals, when deposited on the clean Si(lll)-7 x 7 surface, in doses of the order of 1 ML produce a fi x fiR30 o (henceforth referred to as fi) LEED pattern [2]. Of these many systems the Pb/Si( 111) interface has received relatively little attention [3IO]. This system is both unreactive and grows in the Stranski..-Krastanov mode [4]. This combination of properties means that the interface between the two materials is two-dimensional and so is very amenable to being solved using quantitative LEED analysis. We will confirm, using LEED f(V) spectra, that there are two distinct 6 superstructures [3], that the phase change occurs at a substrate temperature of - 350 “C and is indrpendent of the amount of adsorbate on the substrate surface. We have performed an analysis of the high-temperature phase using Tensor LEED calculations and demonstrated that for this phase the Pb adatom sits in the Ta adsorption site above the second layer Si atom. All the experiments detailed were carried out using the York computer controlled LEED diffractometer [ll] operating at a base pressure of 2 x IO- “I mbar. The surface of the Si(l11) sub0WLhUZX,‘91/$03St1
( 1991 -
strate was cleaned by flashing the sample to a temperature of 1200° C for up to 2 min using electron bombardment. The sample was cooled slowly ( - 1 K s ’ ) between 1000 and 800 o C to ensure a good 7 x 7 reconstruction. The cleanliness of the sample could be checked using AES and the quality of the 7 X 7 surface using the LEED pattern. Ph was deposited for all experiments at the same rate of - 0.03 ML min ‘. The sample temperature was measured using an IR pyrometer which with neutral density filters has a temperature range of 300-3OOOOC in addition to a Pt~PtRh(lO~) thermocouple which was pressed against the rear of the sample. Experiments were performed for a wide variety of deposition times (O-140 min) and deposition/annealing temperatures (RT-380°C). It was observed that the i( V ) spectra were identical for an interface which had been formed at RT and then annealed for 30 min to those from an interface which had been formed at the same elevated temperature for the same dose. For RT deposits there was no obvious modification of the 7 x 7 LEED pattern for deposits shorter than 20 min. Further Pb deposition produced a 1 x 1 LEED pattern with a high hackground indicating a disordered surface. For deposition times greater than 30 min we were able to observe rings of spots around the integer beams
Elsevier Science Publishers B.V. (North-Holland)
T. N. Dot&, S. P. Tear / LEED
investigation
[3]. These rings are due to double diffraction where the adsorbate unit cell is incommensurate with that of the substrate. These beams are from a hexagonal structure in parallel orientation with the substrate. The background for the LEED pattern was high for this surface. For deposition times of 20 min or longer the Z(V) spectra no longer change with dose. Annealing the sample following Pb deposition changed the LEED pattern to show a fi superstructure. Two distinct types of 6 LEED pattern were observed depending on the annealing temperature. Using the system of Saitoh et al. [4] the two surfaces will be referred to as the o(a) and the fi( p) phases. We observed the I phase when the sample was heated to temperatures below 350 o C following or during Pb deposition and then cooled to RT. There is a high background in the LEED pattern for this surface. The inner ring of fractional-order beams are focused much more poorly then the integer beams and none of the fractional beams are visible until several hours after the sample heating has finished. If the sample was heated to a temperature of 350” C or above the o(p) phase was obtained. For this surface all the fractional-order beams are as well focussed as the integer beams and can be clearly seen minutes after any heating cycle has finished. Fig. 1 shows I(V) spectra for the (1, 0) beam for a 140 min RT deposit annealed to various temperatures for 30 min periods. While there are only minor changes in the spectrum for annealing temperature up to 300° C it can be seen there is a large change in the data for an annealing temperature of 350 o C. Changes at 350 o C are also seen in the spectra of the other beams. More experiments have been performed using a range of deposition times. temperatures and annealing temperatures, but we have only observed two types of I(V) curves for annealed Pb/Si(lll) surfaces. It is interesting to note that the I(V) spectra from the Pb/Si(lll) interface formed at RT with no annealing are all but identical to those which have been heated to substrate temperatures up to 300 o C though the LEED patterns are different. This indicates that the 6 surface makes only a weak contribution to the I(V) spectra of the integer beams. This combined with the poorly
569
and analysis of Pb/ Si(1 II) I
1
I
I
I
7X7 Si
+
30
55
80
105 130 155 180 Primary Energy (eV)
205
230
Fig. 1. I(V) spectra from the (1,O) beam from RT Pb deposits for a range of deposition times.
focused inner fractional beams implies a degree of disorder in the Pb layer [12]. 1(V) spectra from low coverage deposits annealed to 350-380 o C contain features from both the Si(lll)-7 X 7 and the o(p) spectra. Indeed these spectra can be reproduced by mixing the two “pure” sets of spectra. However one cannot reproduce these spectra by mixing 7 X 7 and 6(a) spectra. The LEED patterns for these low coverages were 7 x 7 in character. This suggests that the surface consists of domains of the o(p) phase which are small in comparison to the transfer width of the LEED system. Similarly the spectra from low adsorbate coverage surfaces formed at RT can only be reproduced using mixtures of 7 X 7 and a( a) spectra. Therefore for sub-saturation coverages the surface consists of domains of 7 x 7 Si(ll1) along with just one 6 phase ie. there is no coverage dependence for the Pb adsorption site as has been observed for the
We have performed calculations for four adatom adsorption models. These are the Td site with 8 = * ML and B = 1 ML, the Hj site, where the adatom sits on top of the fourth layer Si atom, with 8 = i ML and 8 = 1 ML. Using the Pendry R-factor we were able to rule out all but the Tj with 19= 4 ML model which is shown in fig. 2. Previous work has shown that the fi( j3) phase has a saturation coverage of t ML [4]. For the model calculations performed we allowed the atoms in the Pb layer and the first two Si double layers to move from their bulk positions in the pass 2 program. The LEED program required each Si double layer to be considered as one rumpled layer. This results in 6 atoms per unit cell for a 6 unit cell of the Si rumpled layer. Though there are a total of thirteen atoms within the unit cells of the three layers, from symmetry there are only three different atom types within each Si layer. We therefore have seven atomic positions to vary but since only two atom types can have any lateral movement and the direction of these movements is along a bond direction we have a total of nine parameters to define the surface. At this stage the search strategy is not fully converged on the solution. However. we believe the Tensor LEED analysis has enabled us to get close to the true structure. Improved precision on the solution will follow with further refinement of
Pb/Ge(lll) interface 1131 and as has been previously reported for this system [.5,6]. Tensor LEED [14] has been used together with the search strategy previously described [15] to investigate the G(p) phase. The Tensor LEED subroutines of Rous have been modified to work with the N atom per unit cell version of the CAVLEED program. The Tensor LEED method is ideally suited to the search strategy outlined in ref. fl5] where a base point is established in the structual parameter space (Tensor pass 1 calcuiation) followed by exploratory moves in the parameter space around this base point. These exploratory moves are carried out using the fast perturbation method in the pass 2 program and the I(V) curves calculated for the new trial structures are compared with the experimental 1( I’) curves using the Pendry R-factor [16]. Once a minimum in the R factor is obtained in these exploratory moves then a new base point is calculated using the value of the structural parameters which give the minimum R factor, and the space around this explored further. The full dyna~cal calculations in pass I used up to 9 phase shifts and up to 223 beams over an energy range of 40 to 200 eV. The Tensor LEED calculations have been done for the four beams whose I(V) curves showed the biggest change for the different phase, there were the (1, Oi (0. 1). (f, f) and (4..$) beams.
-
2.55 2.40 2 32 2 40 2 34
2.52 9.30 2.37
Fig. 2. Top and side views of the 1;; adsorption site. The adatom is labelled 1 and the arrows indicate the movement of the substrate atoms from their bulk positions for the hest Tensor LEED theorv/experiment fit. The atomic seperativns are also shown.
571
TX. Dour!, S. P. Tear / LEED investigation and analysis of Pb/ Si(ll1)
the T4 adsorption site with a saturation coverage of f ML. There are two common trends for these systems: the first is that elevated temperatures are required for this particular reconstruction though other reconstructions do occur at lower deposition/annealing temperatures, the second is that where the structure has been solved the Si substrate has gone through a major reconstruction which brings the adatom close to the sum of the covalent radii of the adsorbate and substrate species. Both the T, and the H, adsorption sites saturate the dangling bonds of the Si(ll1) surface for f ML coverage but total energy calculations
nine parameters. The values of these parameters used to obtain the best overall R-factor agreement are shown in fig. 2 and the theory curves calculated using these values are shown in fig. 3. It can be seen that except for the 3b-4b bond the calculated bond lengths are within 6% of the sum of the relevant tetrahedral Pauling covalent radii. The adatom though is 12% closer to the atom type 3a than the sum of the Pb and Si radii but this substrate atom is not tetrahedraly bonded to the adatom. Several other group-III and -IV metalSi(lll)-fi structures have been analysed by other workers [17-191 and shown to have the adatom in the
R-Factor
0.23
I
0.31
‘/
0.41
0.32
I
50 Fig. 3. Tensor
1
70
I
I
1
I
90 110 130 150 Primary Energy (eV)
1
t
170
190
LEED theory/experiment comparison for the parameters shown in fig. 2. The individual Pendty The overall R-factor for the four beams is 0.32 with an inner potential of - 5 f 1 eV.
R-factors
are shown.
572
T. N. Dourt. S. P. Tear / LEED mwstigation
have shown that for group-III-Si( 111) interfaces the T4 site has a lower energy than the H, site providing the surface has reconstructed [20,21]. This preference for the T4 site has also be shown for the Pb/Ge(lll) system [22] and LEED analysis has confirmed this result [23]. The Pb/Ge(lll) interface unlike the Pb/Si(lll) system has a higher coverage phase at the same substrate temperature. Huang et al. [23] have proposed a model for this interface in which the surface relaxation of the substrate caused by the initial occupation of the T4 site opens up the H, site. This reconstruction allows a Pb adatom to fall into the H, site and in doing so pushes the adatom previously in the T4 site towards the T, site giving a saturation coverage of : ML. As previously noted we observed only one coverage dependent surface at fixed substrate temperature for the Pb/Si(lll) system. The 4% smaller lattice constant of Si means that the substrate reconstruction on the occupancy of the T4 site does not create an open enough H, site for an adatom to be fixed at this site at the temperatures required for the o(a) phase to form. It seems clear that the high temperatures (> 350°C) required for the fi( p) phase to form result form the large substrate reconstruction involved. This reconstruction gives the top Si double layers a fi unit cell which explains the superior quality of the fractional-order beams for the t/!?(p) phase over the 6(a) phase. This implies that at temperatures below that required for the substrate reconstruction the v’??(a) surface has its fi character largely from the adsorbate only. the elevated temperature needed to form this phase is required to give the adatoms sufficient mobility to form a coherent two-dimensional layer. The authors would like to acknowledge the technical assistance of J.C. Dee and A. Gebbie and to thank the LJLCC for the use of the Cray.
and ana(vsis of Ph / Si( I I I )
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