High coverage phases of Pb on the Si(111) surface: structures and phase transitions

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Surface Science 323 (1995) 241-257

High coverage phases of Pb on the Si(111) surface: structures and phase transitions Ing-Shouh Hwang

a,d R.E.

Martinez

b Chien Liu a J.A.

Golovchenko a,b,c,,

a Division of Applied Sciences, Harvard University, Cambridge, MA 02138, USA b Department of Physics, Harvard University, Cambridge, MA 0213& USA c Rowland Institute for Science, Cambridge, MA 02238, USA d Institute of Physics, Academia Sinica, Taipei, Taiwan, ROC Received 6 May 1994; accepted for publication 1 September 1994

Abstract We have studied the high coverage phases of Pb on a Si(111) surface using a scanning tunneling microscope (STM). For annealed samples, 1 ML Pb forms an incommensurate (IC) phase composed of alternating domains of two types of trimer regions and a quasi-1 X 1 region. The detailed morphology of the domains depends sensitively on the stress fields resulting from imperfections on the surface. We observe that the stress fields originating from nearby reconstructions transform the IC phase into a 1 X 1 structure. The transformation involves only small displacements of Pb atoms. We observe that the 1 / 3 ML vr3 phase suppresses the formation of trimers in the 1 x 1 phase, but Pb islands and Pb clusters enhance their formation. The same structural transformation from IC to 1 X 1 can occur at high temperatures. The local transformation temperature depends strongly on surface imperfections nearby. The high coverage phase for room temperature (RT) deposition, Pb/Si-7 × 7, is also resolved. Pb deposition on this phase causes interesting changes in its morphology. The 1 x 1 adatoms in Pb/Si-7 x 7 appear to relax normal to the surface relative to the surrounding unreconstructed regions. The formation of the IC phase for P b / S i ( l l l ) offers a novel system for understanding incommensurate surface reconstructions in a chemisorption system. Keywords: Adatom; Chemisorption; Lead; Scanning tunneling microscopy; Single crystal surfaces; Surface relaxation and reconstruction; Surface stress; Surface thermodynamics

1. Introduction Surface reconstructions have been an important subject in the study of condensed matter physics [1]. The minimum energy structure on a semiconductor crystal surface results from the balance between energetically favorable dangling bond reduction and u n f a v o r a b l e b o n d strains. The situation on

* Corresponding author. Fax: + 1 617 4965144.

chemisorbed surfaces is complicated by competition between adsorbate-adsorbate and adsorbate-substrate interactions, which can produce atomic arrangements of surprising complexity. Surfaces usually exhibit reconstructions that are commensurate with the substrate because of the energy cost associated with domain wall formation. However, there are a few solid surfaces that have reconstructions incommensurate with the substrate [1-5]. Previous theoretical studies of the incommensurate (IC) phase were based primarily on physisorbed systems, such as

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noble gases on graphite. Incommensuration in chemisorbed systems may stem from the stress produced by the reconstruction itself. As the size of a commensurate domain increases, the buildup of strain energy associated with the surface stress may drive the system to become incommensurate in spite of the free energy cost of domain walls. In this work, we study a displacive IC phase [6] of 1 monolayer (ML) Pb on the S i ( l l l ) surface. This IC surface is composed of alternating domains of two types of trimer regions and a quasi-1 X 1 region. The local bonding geometry and mesoscopic domain structure of this reconstruction are very sensitive to surface stress. In contrast to physisorbed high temperature IC phases which exhibit domain wall fluctuations [2-5], this system exhibits an IC structure at low temperature, which is stabilized by the creation of static domain walls whose major role we believe is stress relief rather than addition of entropy. Also in contrast to physisorbed systems, the IC phase of P b / S i ( l l l ) undergoes a reversible transformation to a commensurate phase at elevated temperatures. The transition temperature is strongly affected by the presence of surface inhomogeneities and their stress fields. Pb on Si has been considered as a model system for the study of abrupt Schottky barriers because Pb and Si have negligible mutual bulk solubility [7]. Heslinga et al. reported that the Schottky barrier height (SBH) for P b / S i ( l l l ) contacts depends on the interface structure [8]. For diodes prepared by deposition of thick Pb films at room temperature (RT) onto Si(lll)-7 X 7 they found a SBH of 0.70 eV; whereas, deposition onto an annealed 1 ML P b / S i ( l l l ) interface produced a high SBH of 0.93 eV. Determination of the interface structures prepared in these two methods may help resolve questions concerning the SBH difference. However, previous results have been very confusing and controversial because of the complicated phase diagram of Pb/Si(111). We use an STM to image surface structures on annealed Pb/Si(111) samples with coverages from 0.5 to 3 ML and on a novel two-stage deposition. The high coverage phase on annealed sample (the IC phase) and that on RT deposited samples (Pb/Si7 x 7) are both resolved. We find that the Pb-Si bonding for both high coverage phases is similar.

Much higher defect densities are seen on the latter samples, which may be responsible for their lower SBH. We also study the evolution of the IC phase from a 1 X 1 structure and find that the local transformation temperature depends sensitively on stress fields induced by nearby defects and reconstructions. The seemingly complicated phase diagram can be consistently explained. The adsorption site of Pb atoms on Si(111) surfaces is very selective and nucleation of high coverage phases is found to be highly inhomogeneous. We also observe interesting motion of the interface between a 1 x 1 and 1 / 3 ML v~- phases induced by perturbing the surface with the STM tip. In the following, we summarize previous work of Pb/Si(111) in Section 2. Our experimental procedures are presented in Section 3 and detailed results are shown in Section 4. General discussions of the IC phase and of its structural transformation to a 1 x 1 phase are presented in Section 5. Our conclusions are summarized in Section 6.

2. Review P b / S i ( l l l ) has been studied extensively using various techniques by many groups. It was first studied by Estrup and Morrison in 1964 using lowenergy electron diffraction (LEED) [9]. They proposed two ~ x v~-R30° (in short, v~) phases corresponding to 1 / 3 and 4 / 3 ML (1 ML -- 7.84 X 1014 Pb atoms/cm2), respectively, for the annealed samples. A later study by Saitoh et al. confirmed and extended these results [10]. However, Le Lay et al. [11-13] and Quentel et al. [14] reported three different v~ phases at 1/3, 2 / 3 , and 1 ML, respectively. Subsequently, Grey et al. studied the phases near 1 ML using X-ray diffraction [15,16]. For RT deposition they proposed a close-packed 8 x 8 Pb layer on the Si(111)7 X 7 unit cell, while for an annealed surface they proposed a 30 ° rotated, close-packed, incommensurate (IC) model, corresponding to a saturation coverage of ~ 1.3 ML. Weitering et al. used reflection high-energy electron diffraction (RHEED) and LEED to study this system and basically supported the models proposed by Grey et al. [17]. In addition, they suggested that the Pb sites of the IC

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phase are spatially modulated by the substrate corrugation potential. Recently, Ganz et al. employed Rutherford backscattering (RBS), thermal desorption spectroscopy, LEED, and STM to study the growth and morphology of P b / S i ( l l l ) [18,19]. For RT deposition at low coverage on S i ( l l l ) 7 X 7, Pb atoms were found to occupy sites above the rest atoms and between the Si adatoms with a preference for the faulted half of the unit cell. At ~ 0.6 ML, Pb forms an ordered overlayer based on the 7 X 7 unit cell. They proposed a model with a mixture of Pb and Si adatoms on the T 1 site inside both faulted and unfaulted halves for this Pb/Si-7 x 7 phase, in contrast with the 8 x 8 structure proposed by Grey et al. On annealed samples at 1 / 6 ML, they found a mosaic ~ - phase which consists of alternating chains of Pb and Si adatoms occupying T4 sites. At 1 / 3 ML the surface is terminated by Pb atoms in T4 sites, forming the 'standard' ~ phase. Between 1 / 3 and 1 ML a 1 X 1 Pb overlayer was found to coexist with the 1 / 3 ML ~ phase, while above 1 ML an IC phase was observed. For the IC phase, Ganz et al. adopted the 30 ° rotated close-packed model proposed by Grey et al., but they concluded that the coverage can range from 1 to ~ 1.5 ML corresponding to a range of different P b - P b spacings. They also used channeling RBS to show that the 7 X 7 stacking fault is still present on RT deposited samples and that it is removed after annealing. The studies by Ganz et al. determined the number of distinct phases and their saturation coverages and characterized most atomic structures. However, the nature of the IC phase was still not clear because individual atoms in this phase were not resolved with the STM. The IC phase was found to undergo a reversible phase transition to a 1X 1 phase at ~ 3 0 0 ° by several techniques [11-13,17,20,21]. X-ray diffraction by Grey et al. [20] and RHEED by Weitering et al. [17] revealed a diffuse scattering halo around the specular beam in the 1 X 1 phase, which led these investigators to concluded that the IC to 1 X 1 transition is a 2D melting transition. However, Le Lay et al. argued that it is an order-order solid transition because of the persistence of a sharp LEED pattern and a pronounced surface state in photoemission measurements above the transition temperature [1113]. Very recently, Hwang et al. used a high-temper-

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ature STM to study a very similar phase transition, monolayer Pb on Ge(lll)/3-v/-3 - ~ 1 x 1 at ~ 180°C [22], which was also previously described as a 2D melting by RHEED [23,24] and X-ray diffraction [16,25] studies because of the observed diffuse halos. Both the f l - ~ and 1 x 1 phases were resolved in the STM study, and the transition was shown to be order-order. The low-temperature fl-v~- phase is composed of a periodic array of Pb trimers centered on the three-fold hollow H 3 site [22], rather than a 30 ° rotated close-packed structure proposed earlier by Feidenhans'l et al. using X-ray diffraction [26]. The saturation coverage for the /3 phase is 1 ML, which was also confirmed by RBS [22]. The hightemperature phase is a simple Pb-terminated 1 X 1 structure. As we shall see, Pb also forms trimers on S i ( l l l ) surfaces at 1 ML, similar to the /3 phase of Pb/Ge(lll).

3. Experimental Our experiments were performed using a homemade STM in an ultrahigh vacuum (UHV) chamber with a base pressure of 6 × 10 -11 Tort. S i ( l l l ) samples with a resistivity of 0.05 f~ cm were chemically etched using the Shiraki procedure [27] before being introduced into the UHV chamber. The samples were cleaned in situ by flashing up to 1250°C for 10-20 s, and annealing at ~ 920°C for a few minutes before cooling down slowly to room temperature. Sample temperatures were measured with an infrared pyrometer. This cleaning procedure consistently yields high-quality S i ( l l l ) 7 x 7 reconstructions. Two Pb deposition schemes were used in this study. In the conventional 'one-stage deposition', 99.999% pure Pb was evaporated onto a clean RT S i ( l l l ) sample from an effusion cell. After annealing the sample briefly at ~ 300°C and checking the LEED pattern, it was transferred onto the STM stage for imaging. A mixture of the 1 X 1, 7 X 7, and 1 / 3 ML ~ phases was observed for samples with Pb coverages between 0.3 and 1 ML. Samples having slightly more than 1 ML of Pb show only the IC phase and scattered 3D Pb islands. These phases and their associated Pb coverages have been identified previously [18,19].

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In another deposition scheme, we evaporated additional Pb at RT onto samples prepared by the one-stage deposition that exhibited a mixture of the 1 X 1, 7 X 7, and 1 / 3 ML V~- phases. The surface was studied both before and after the second-stage deposition with the STM. This 'two-stage' procedure produced regions of P b / S i - 7 x 7, together with domains of the IC phase. The coexistence of these reconstructions, which cannot be obtained by the one-stage process, allowed us to determine the registration of Pb atoms in the IC phase, and observe interesting stress field effects. We calibrated Pb doses reported for this second deposition by evaporating Pb under identical conditions onto clean, RT S i ( l l l ) - 7 X 7 substrates. The coverage was then checked with the STM. The substrate orientation was also revealed in this manner because Pb atoms preferentially occupy the faulted half of the Si(111)-7 x 7 unit cell [18,19].

4. Results 4.1. The IC phase 4.1.1. Structures

Fig. la shows a tunneling image of a region of the IC surface. This sample was prepared by depositing 0.2 ML of Pb at RT onto an annealed surface already covered with ~ 0.8 ML of Pb. Individual Pb atoms are resolved in the IC surface. Surface Pb atoms are arranged in two types of trimers, differing in orientation by 60 °. Alternating domains of trimers with local periodicity can clearly be identified. Additionally, a quasi-1 X 1 region with Pb atoms slightly displaced from the ideal 1 X 1 registration sites can be seen. As we shall show, the trimers and the quasi-1 × 1 structure are consistently imaged on both polarities of the tunneling bias. Thus, we believe they are the real atomic structure rather than merely an electronic artifact. Fig. lb is the corresponding atomic model for the IC region outlined in Fig. la. In the IC phase, each Pb atom is bonded to the Si atom underneath. The majority of Pb atoms are displaced laterally from their ideal T 1 site to form trimers centered on either T 4 sites o r H 3 sites (see Fig. lb). Note that trimers centered on the T 4 site are

T4

;

T1

Q (~ 0

Pb adatom

[TIO]

Pb cluster (b)

o

1st layer Si atom

[112]

Fig. 1. (a) 90 .~X 110 A tunneling image of the Pb/Si(lll) IC phase on a surface prepared by two-stage deposition, taken at a sample bias of + 2.0 V and a tunneling current of 50 pA. Black parallel lines are drawn to indicate the structure of a domain boundary. (b) Atomic model for the IC structure outlined in (a).

rotated 60 ° relative to those centered on the H3osite. The P b - P b spacing inside a trimer is 3.3 + 0.3 A, as measured in STM images. The two types of trimers are aligned on (211) axes which span two domains, as marked in Fig. lb. The trimers on either side of

L-S. Hwang et al. /' Surface Science 323 (1995) 241-257

S

Fig. 2. 100 ,~x 100 ,~ tunneling image of the IC structure on a surface prepared by one-stage deposition, taken at sample bias of + 0.8 V and a tunneling current of 40 pA. the domain boundary point toward each other and are separated by a Pb dimer. Trimers adjacent to the dimer are often distorted, with the Pb atoms nearest

245

the dimer slightly displaced toward it. Regions in which boundaries intersect form a quasi-1 X 1 structure. In Fig. l b , the slight displacements in the quasi-1 x 1 region and the trimer distortion near the dimer are not drawn. The saturation coverage for the IC phase determined from the tunneling images is exactly 1 ML, with every first-layer Si atom bonded to a Pb atom. This agrees with a previous RBS measurement which showed that annealed samples exhibited the IC phase at 1 M L coverage [18,19]. The detailed features of the IC phase, such as the size and the shape of the trimer domains, are strongly influenced by the presence of defects, step edges, or other nearby reconstructions. For example, Pb clusters are always surrounded by six alternating trimer domains, resulting in a hexagon-like domain structure as shown in Fig. la. Samples prepared by the two-stage deposition often show a small concentration of Pb clusters. The IC phase prepared by onestage deposition is also composed of alternating domains of trimers and quasi-1 X 1 regions with the same boundary structure as illustrated in Fig. lb, but the trimer domains are more irregular and often elongated in a ( 2 1 1 ) direction. Fewer compact hexagonal domains are seen in the one-stage deposi-

Fig. 3. (a) 180 ,~ x 200 ,~ tunneling image of the same surface as in Fig. 1 taken at + 2.0 V sample bias and 45 pA tunneling current. (b) The same tunneling image with H3, T4 trimer domains and quasi-1 x 1 regions marked with symbols ' + ', ' - ', and '0', respectively. Two Pb clusters are indicated by arrows. Part of Pb/Si-7 X 7 regions can be seen at upper and lower right corners of the image. Crystal directions are as in Fig. 1.

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tion than the two-stage procedure. A small region of the IC structure prepared by one-stage deposition is shown in Fig. 2. We have scanned many regions of the IC phase for as long as 10 min each,in an attempt to observe fluctuations in either the size or shape of the trimer domains that might be indicative of thermal motion of the domain boundaries. We never observe such fluctuations. Fig. 3a shows a tunneling image of a 180 ,~ X 200 ,~ region of the IC phase on the same surface as shown in Fig. la. Fig. 3b is the same tunneling image with boundary atoms highlighted with dots and ' + ', ' - ', and ' 0 ' indicate each of the two types of trimers and quasi-1 x 1 regions, respectively. An analysis of many STM images of this surface, containing a total of 451 trimers, shows that (50 + 5)% of the trimers are of each orientation. Each domain contains 16 + 5 trimers. As seen clearly in Figs. 1, 2, and 3, the domain shape and relative orientation between domains of the same type of trimers are somewhat irregular and they depends sensitively on the local defects such as Pb clusters and P b / S i - 7 X 7. Because of this, a direct Fourier transform of a particular region of the IC structure (such as Fig. 3) may yield a pattern not representative of the observed LEED pattern [19]. For example, IC domains super lattice spots may be slightly displaced and rotated relative to the results characteristic of the

Fig. 4. 70 A X 70 .~ filled-state tunneling image of the IC structure, taken at - 1.15 V sample bias and 40 pA tunneling current. Crystal directions are as in Fig. 1.

Fig. 5. 220 ,~ x 230 ,~ tunneling image of a surface prepared by two-stage deposition showing a mixture of the IC, 1 x 1, and Pb/Si-7 ×7 phases, taken at a sample bias of + 2.0 V and a tunneling current of 50 pA. ' U' and ' F' indicate the unfaulted and faulted halves of Pb/Si-7 x 7. Crystal directions are as in Fig. 1.

average surface. Most of our tunneling images are either of smaller regions of the surface with high resolution revealing the detailed trimer structure (and not the long-range properties of the domains) or are of large areas of the surface without sufficient resolution to completely resolve the trimers. These are in agreement with the observed LEED pattern. A representative example of such a large area scan (at low resolution) has already been published in Fig. 10b of Ref. [19]. The trimer images shown in Figs. la, 2, and 3 are not likely to be purely electronic in nature because they can be seen in images taken at both polarities of the tunneling bias. Fig. 4 is a filled state tunneling image of a region of the IC phase on a sample prepared by the two-stage deposition. Two different types of Pb trimers and quasi-1 X i regions can also be seen at this polarity. These structures can be imaged over a large range of sample bias, from + 0 . 4 to + 2 . 2 V and from - 0 . 4 5 to - 1 . 3 V. We note that most of the tunneling images presented here were taken of unoccupied states because the tip is often not stable when imaging filled surface states. When the tip is not very sharp or the sample bias is

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higher than 1.5 V, each trimer often appears as a single protrusion, as seen in previous tunneling images of the IC phase [19]. The best condition to resolve individual atoms is tunneling at sample biases between + 0.5 and + 1 V, but occasionally we obtain a tip that resolves individual atoms at around + 2 V sample bias. For some tips, individual atoms cannot be resolved, but the triangular shape corresponding to a trimer can still be identified.

4.1.2. Stress field effects Fig. 5 is a tunneling image of another portion of the surface shown in Fig. 1. It exhibits a mixed LEED pattern containing both the IC and Pb/Si7 x 7 diffraction spots [19]. On top of the image is a sharp, straight step edge. The step edge, Pb islands, and a region of Pb/Si-7 X 7 divide the surface into three regions, labeled A, B and C. Strong X v/3-R30° local periodicity is seen in region A, but region B appears fiat. Region C also shows domains of vr3 corrugation but the effect is of smaller amplitude than in region A. A high resolution tunneling image of regions A, B, and a part of region C is

Fig. 6. 100 , ~ x 110 ,~ tunneling image taken at the same tunneling condition as in Fig. 5. Domains of Pb trimers like those shown in Fig. la can be clearly seen in region A. Region B shows an ordered 1 x 1 structure with Pb atoms occupying T 1 sites. In region C, the surface is also reconstructed into trimer domains, but with a larger Pb-Pb spacing.

Fig. 7. 100 , ~ x 100 ,~ tunneling image of region C (Fig. 5) taken at the same tunneling condition as in Fig. 5.

shown in Fig. 6. Domains of Pb trimers like those shown in Fig. la can be clearly seen in region A. Region B, which is bordered by the Pb islands and a remnant of Pb/Si-7 x 7 (triangular areas in the figure, which will be discussed later), shows an ordered 1 × 1 structure with Pb atoms occupying T t sites. Note that the Pb atom registration switches smoothly from 1 X 1 to the trimer domains. In region C, the surface is also reconstructed into trimer domains, but the Pb-Pb spacing in these trimers is larger than that in region A, making this area look more 1 X l-like (Fig. 7). These figures show that the IC phase can be considered 'soft', in the sense that the lateral positions of Pb atoms are easily perturbed, presumably because of their single Tl-site bond to first-layer Si atoms. Stress fields originating from neighboring regions of Pb/Si-7 X 7 can drive a region of the IC phase to a 1 x 1 structure, as shown in region B of Fig. 6. Region C is also influenced by the stress fields resulting from the surrounding Pb/Si-7 x 7 regions and the step edge but its large size (relative to region B) may allow Pb trimers to form with a larger Pb-Pb spacing than that in the regular IC phase. Clearly, stress fields play a crucial role in the detailed structure of the IC phase. We have been unable to explain the subtle phenomena associated

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248

.

-.

F

"e

0

Pb adatom Pb dimer

o 1st layer Si atom • 3rd layer Si atom • 4th layer Si atom

\

corner holes, so they may correspond to three Pb atoms or three Pb dimers. Note that the Pb atoms inside the 7 x 7 unit cell and those just next to Pb chains in regions B and C in Fig. 6 appear ~ 0.2 .~ lower than those further from the boundaries of regions B and C. This may be a real height difference due to surface relaxation in the 7 x 7 region relative to regions B and C, where the underlying Si layers do not reconstruct. The effect may be related to that predicted for the Si terminated 7 X 7 reconstruction studied theoretically by Brommer et al. [28].

4.2.2. Effects of Pb deposition on Si(111)7 X 7

[T1 o] 'L_~. [112]

Fig. 8. Simple model for the P b / S i - 7 x 7.

with the IC phase that we have observed based on the 30 ° rotated close-packed structure proposed by Grey et al. [15].

There are two interesting observations concerning the Pb/Si-7 X 7 regions prepared by the two-stage deposition. One observation is that the fraction of the surface covered by Pb/Si-7 X 7 is about 20% smaller than that covered by Si(lll)-7 x 7 before the second deposition. Presumably, some Si(lll)-7 X 7 regions have been removed. Region B in Fig. 6 must surely have contained both faulted and unfauled halves of Si(lll)-7 X 7 before Pb deposition. The faulted areas have been removed by the Pb deposition. The other observation is that RT deposition often leaves isolated faulted halves of the 7 X 7 cell trapped in the surface, as can be seen in Fig. 6.

4.2. Pb / Si-7 x 7 4.2.1. Structure The structure of Pb/Si-7 X 7 has not been resolved previously. Fig. 6 shows that the Pb atoms in the center of Pb/Si-7 x 7 unit cells are bonded in a 1 x 1 array of T 1 sites. Pb atoms in the faulted half of the Si(lll)-7 X 7 remnant (triangular area with vertex pointing to the right) are displaced from registry with those in region B. A model for the Pb/Si7 X 7 phase is shown in Fig. 8, which is similar to the model proposed by Ganz et al. previously [19]. However, the slight difference is based on the fact that we do not observe Si adatoms inside the 7 X 7 unit cell. Presumably they do not intermix with Pb atoms and segregate to form defects (additional discussion will be in Section 4.6). We find that about 30% of the Pb/Si-7 X 7 area is defective. The Pb chains along the 7 X 7 dimer rows are also different from the model proposed by Ganz et al. [19]. Three protrusions are often seen between two neighboring

14x14

Fig. 9. 110 ~,X 110 A tunneling image of the Pb/Si-7X7 structure, taken at a sample bias of + 2.0 V and a tunneling current of 50 pA. A unit cell of 14 X 14 can also be seen.

I.-S. Hwang et al. / Surface Science 323 (1995) 241-257

Occasionally, we observe that an unfaulted half has been transformed into a faulted half after the Pb deposition. Fig. 9 shows a region of the Pb/Si-7 x 7 phase on the right-hand side of Fig. 5. A large fraction of this region is defective (the dark regions of Fig. 9). Most of the ordered regions still preserve the 7 X 7 periodicity, but there is a region with a unit cell of 14 × 14, i.e., twice the size of 7 X 7 on each side. It also exhibits a 1 X 1 structure inside the unit cell. Presumably an unfaulted half of 7 x 7 has been transformed into a faulted half inside the faulted half of 14 x 14. Similarly, a faulted half of 7 X 7 has been transformed into an unfaulted half inside the other half of 14 X 14. This suggests that the activation energy for transforming an unfaulted half of a Si(111)-7 X 7 unit cell to a faulted half and that for removing a faulted half are significantly reduced when these regions are covered with ~ 1 ML of Pb. The latter is smaller than the former because the removal of faulted halves is observed much more often. This is consistent with the fact that no stacking fault remains after a gentle annealing on Pb/Si-7 x 7. The surface is then reconstructed into the IC phase. Clearly, the Pb/Si-7 X 7 phase is a metastable structure.

4.2.3. Schottky barrier height The P b - S i bonding inside the Pb/Si-7 x 7 unit cell is identical to that in the IC phase, except that there are stacking faults in the former phase. This agrees well with a photoemission study by Carlisle et al., in which the surface sensitive Si2p core-level spectra obtained for the IC and the Pb/Si-7 X 7 are almost identical [29]. It was shown by Heslinga et al. that Pb overlayers grown on the IC phase and on the P b / S i - 7 X 7 phase have different Schottky barrier heights [8]. They suggested that the interface structures are responsible for the difference. Since the local Pb-Si bonding is the same for these two phases, we suggest that the very high defect density in the Pb/Si-7 X 7 phase relative to that in the IC phase may be the cause for its lower SBH. 4.3. Morphology on annealed samples - One-stage deposition Annealed samples with less than 1 ML Pb coverage contain Si(111)-7 X 7 and neighboring 1 X 1 and

249

Fig. 10. 160 , ~ × 160 ,~ tunneling image of the evolution of trimers from the 1 X 1 regions, taken at a sample bias of + 2.0 V and a tunneling current of 50 pA. Each trimer appears as a protrusion in the local ¢'3 order, as indicated by an arrow. Crystal directions are as in Fig. 1.

1 / 3 ML v~- phases. By triangulating from the well understood 1 / 3 ML f3-, Pb atoms in the 1 x 1 phase are determined to occupy the T t site, just like the 1 X 1 structure seen in region B of Fig. 6. The region of i X 1 grows at the expense of the 1 / 3 ML v ~ and Si(111)-7 × 7 phases with increasing Pb coverage. Above 1 ML, where no 1 / 3 ML v~- and Si(111)7 X 7 phases are present, the surface exhibits the IC phase with scattered Pb islands, whose nature will be discussed later. Interestingly, with increasing Pb coverages the step edges in the IC phase become straight and parallel to (011) directions. We also imaged surfaces with slightly less than 1 ML of Pb, which show only the 1 X 1 LEED pattern. These surfaces mainly exhibit the 1 x 1 structure, but scattered trimer domains are found to nucleate, as shown by an arrow in Fig. 10. We note surrounding each of the nucleated trimers is a region of weakly ~ modulated 1 x 1. The v ~ modulation dies away within 3 - 4 ~ unit cells. We believe the degree of v~- modulation reflects the P b - P b spacing within the trimer. The effect is analogous to that

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L-S. Hwang et al. /Surface Science 323 (1995) 241-257

discussed in Figs. 5-7. Clearly, the IC phase evolves gradually from the 1 x 1 structure with increasing Pb coverage.

Pb islands nucleate on the IC phase above 1 ML. They are close-packed with a [111] surface. Our STM images show that the Pb-Pb spacing is ~ 3.5

Fig. 11. (a) 130 ,~ x 140 ,~ tunneling image, taken at a sample bias of + 2.0 V and a tunneling current of 55 pA. Trimers are seen to nucleate near Pb islands. (b) 60 ,~ x 60 A tunneling image of the region at the upper left-hand corner of (a). Crystal directions are as in Fig. 1.

L-S. Hwang et al. / Surface Science 323 (1995) 241-257

at the top of the Pb islands, as in bulk Pb. The edges or 'facets' of these crystallites lie along (011) substrate directions. 4.4. Effects of Pb deposition at R T deposition

two-stage

We studied how Pb deposition at RT modifies the morphology on surfaces originally prepared by onestage deposition. As we shall see, Pb atoms can travel several thousand ~ingstroms before being incorporated into surface sites and their adsorption site is highly selective. The study of Pb adsorption is crucial to the understanding of how the 1 X 1 evolves to the IC with increasing coverage on annealed samples. 4.4.1. Pb adsorption and evolution from 1 X 1 to IC On an annealed surface which originally contained scattered regions of 1 / 3 ML v~-, 1 X 1, and S i ( l l l ) - 7 X 7, we found that RT Pb de oosition first reduces the area of the 1 / 3 ML ¢3 phase and increases that of the 1 X 1 phase (Pb also condenses on the S i ( l l l ) - 7 X 7 regions, as discussed in Section 4.2). This suggests that Pb atoms are incorporated into the 1 / 3 ML v~- phase preferentially and trans-

251

form it to 1 X 1. When no 1 / 3 ML v ~ phase is left on the surface, the IC phase starts to form and further RT Pb deposition leads to nucleation of Pb islands on the IC phase and on the Pb/Si-7 X 7 phase. In order to observe how the trimers evolve from the 1 X 1 structure, we deposited only ~ 0.05 ML of Pb onto a 1 x 1 surface which contained small, isolated regions of Si(111)-7 x 7 and 1 / 3 ML v~. A tunneling image in Fig. l l a shows a region of the high coverage phase around several Pb islands. Trimers (corrugation in v~- local periodicity) start to nucleate near those Pb islands and defects. Fig. 11b is a high-resolution image around the island at the upper left comer of Fig. 11a. Two kinds of trimers can be identified. Thus Pb islands enhance nearby Pb trimer formation. We also observe that several trimers nucleate around the Pb cluster. However, no trimers form inside a region of 1 X 1 with the 1 / 3 ML vr3 phase next to it, which indicates that the 1 / 3 ML v~phase suppresses the formation of the IC phase. This is consistent with the fact that the IC phase does not form below 1 ML on annealed samples at RT. We note that RT deposition produces elongated Pb islands (often two adjacent parallel islands) aligned with a (011) substrate directions, as shown

4

11'

4

4

Fig. 12. 140 A x 140 A tunneling imagesof alternating, elongatedtrimer domains, taken at a samplebias of + 2.0 V and a tunneling current of 50 pA. Trimer orientations are revealedin the image and are indicated on the left-hand side of the images.(b) is a tunneling image of a region slightly to the right of that in (a). A reference defect is indicated by an arrow. Crystal directions are as in Fig. 1.

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in Fig. 11a. Presumably the shape of Pb islands grown at RT is determined by adatom kinetics because Pb islands nucleating on the annealed sample have a flat [111] surface and do not show elongation in any direction. We also find that RT deposition of more than 0.2 ML Pb on the IC structure prepared by the one-stage deposition increases the average domain size and changes the domain shape. An example is shown in Figs. 12a and 12b, which are two tunneling images of the IC phase prepared by depositing ~ 0.5 ML at RT onto an IC surface originally with ~ 1 ML of Pb. In Fig. 12a, elongated alternating domains ( ~ 200 ,~ in length) of the two types of trimers can be seen between two regions of defects. The trimers are not completely resolved in this image, but their orientation is revealed by their triangular shape. Fig. 12b is a tunneling image of a region slightly to the right of that in Fig. 12a. A reference defect is indicated by an arrow. The structure changes from the altemating bands of the trimer structure into hexagon-like domains with a smaller aspect ratio and a smaller average domain size on the right-hand side of Fig. 12b. Trimer domains on such a sample are often very elongated and defects seem to play an important role in this kind of domain morphology. The LEED pattern for such a sample shows a smaller spacing between the satellite spots around (1/3, 1/3) and (2/3, 2 / 3 ) than that exhibited by the IC structure before the RT deposition (see Figs. 5a and 5b in Ref. [19]). Ganz et al. interpreted this variation in LEED satellite spot spacing as resulting from a smaller, coverage-dependent Pb-Pb spacing in the IC layer [19]. However, our observations show that the IC phase has an exact coverage of 1 ML in STM images. We can also obtain the special LEED pattem like Fig. 5a in Ref. [19] on samples with coverages as low as ~ 1.2 ML. The LEED satellite spots around (1/3, 1 / 3 ) and (2/3, 2 / 3 ) are consistent with interference between v~- X v~- periodicities of the trimer domains. The variation in the spacing between the LEED satellite spots reflects the change of domain morphology on the surface. A smaller satellite spot spacing at a higher Pb coverage is consistent with our observation of a larger average domain size after extra Pb deposition. In conclusion, the shape of IC domains is strongly affected by their local environment, which in turn

depends heavily on the preparation history (e.g., one-stage vs. two-stage deposition). On average, the domain size increases with increasing coverage, but the Pb density within the IC layer remains 1 ML. The excess Pb is incorporated into clusters, islands, and other defects on the surface. These add an 'extrinsic', long-range perturbation to the surface stress field, which affects the shape and the size of trimer domains in the IC phase. 4.4.2. Diffusion of Pb atoms upon deposition We have made a number of observations which show that Pb atoms deposited on a previously annealed surface can travel a long distance at RT before being incorporated at the surface. We find that the added Pb atoms are consumed in converting increasingly small 1 / 3 ML f3- into the 1 X 1 phase. In order to do this the Pb atoms must travel over large distances of 1 x l and 7 x 7 to reach the boundaries with f3-. Similarly, the formation of the Pb/Si-7 X 7 phase develops inhomogeneously. At low additional Pb doses, most areas still exhibit Si(lll)-7 X 7 with a few substitutional Pb adatoms, but Pb/Si-7 X 7 nucleates at some small regions. The formation of Pb islands in the sample shown in Fig. l l a provides additional evidence for Pb diffusion. The deposition of only 0.05 ML leads to their nucleation in a very small region, while no other Pb islands are found within 1000 A of this region. The Pb diffusion upon deposition and the inhomogeneous incorporation into certain phases or Pb islands complicate surface coverage measurements using nonmicroscopic techniques. Using RBS measurements, Ganz et al. concluded that the saturation coverage of the IC phase can range from 1 to ~ 1.5 ML [19]. Our study shows that the IC phase has an exact coverage of 1 ML, and the excess Pb at coverages > 1 ML is incorporated into step edges, Pb islands, or clusters. 4.5. Interface motion between the 1 × I and 1 / 3 ML v ~ phases

The interface between 1 / 3 ML ~ and 1 × 1 is usually straight and along ( 0 1 i ) substrate directions. No interface movement is seen normally at RT. By pressing on the surface with the STM tip, we found it possible to induce interesting interface motion.

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Fig. 13a shows a tunneling image with 1//3 ML v~-, 1 x 1 , and a small region of 7 X 7 , which was prepared by one-stage deposition. Individual atoms in the 1 x 1 phase were not resolved and thus no corrugation appears. The interface between 1//3 ML x/ff X x/ff and 1 X 1 is along a ( 0 1 i ) direction. Upon pressing the tip gently inside the 1 × 1 region (at the location marked by an arrow in Fig. 13b), the interface moves. The interface keeps changing as seen in Fig. 13c, which was taken about an hour after the tiP pressing. It seems remarkable that the room temperature time constants for the surface to adjust itself to the new binding condition are compatible with the STM. Further motion of the interface can be achieved by pressing the tip inside the 1 X 1 region. The interface is seen to change until it becomes straight and aligned along a (011) direction. 4.6. L o w solubility of Si atoms in the high coverage phases It is known that Pb and Si do not intermix in the bulk. We find that they also do not intermix in the high coverage phases of Pb/Si(111). On annealed samples with less than 1 ML of Pb, we often observed small island terraces ( ~ 100 ,~ in diameter) that were about 3 ,~ (a Si(111) atomic step height) higher than the surrounding 1 X 1 region. These island terraces exhibit the 1//3 ML vC3 reconstruction, and were similar to features shown previously in Fig. 12 of Ref. [19]. After second-stage RT deposition, the 1 x 1 phase starts to grow on these small terraces at the expense of the 1//3 ML v ~ phase. As we have observed on large terraces, these small island terraces also transformed to the IC phase after additional Pb deposition. We gently annealed such a surface (at less than 200°C for 3 h) to desorb a small number of Pb atoms from the islands. These small

Fig. 13. 200 ,~X 210 A tunneling image of interface motions at a boundary between 1 x 1 and 1/3 ML Vr3 phases, taken at a sample bias of + 2.1 V and a tunneling current of 30 pA. The tip was gently pressed on the surface at the location indicated by an arrow in (b) and (c), causing the interface to become ragged. The interface appears to heal itself over a period of ~ 1 h. Crystal directions are as in Fig. 1. (a) Before the perturbation. (b) 1/2 h later. (c) 1 h later.

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terraces then exhibit the 1 / 3 ML ~ phase with less than 1% Si adatoms. In comparison, ~ 10% of the adatoms in the "regular" 1 / 3 ML v~- phase are Si, as seen in Fig. 13. This implies that a very low concentration of Si adatoms is present in the IC and 1 X 1 phases. We never observe 'dark atoms', or changes in the apparent height of the atoms that would indicate the presence of Si adatoms inside the 1 X 1 and IC phases. Such local height differences are seen in the 1 / 3 ML V'-J- phase. The low solubility of surface Si adatoms in the high Pb-coverage phases on S i ( l l l ) is easily explained. Si atoms tend to form tetrahedral bonds and there would be three dangling bonds left if a Si adatom were on the T 1 site, which would be energetically unfavorable. In the one-stage deposition, perhaps due to the low Si solubility in the 1 X 1 phase, excess Si adatoms from original 7 X 7 unit cells move to step edges or segregate to form an island terrace locally during annealing. Pb and Si intermix in the 1 / 3 ML ~ phase [19] probably because Pb and Si adatoms are relatively far apart and because they each form three bonds to the substrate. Also, it has been suggested that charge transfer between Si and Pb adatoms stabilizes the structure of the phase from 1 / 6 to 1 / 3 ML [30].

of any given type may induce a significant surface strain energy, and it may be more energetically favorable to form alternating small domains of trimers since the formation of walls does not cost much energy. The alternation of two types of trimer domains and the special structure across the wall (as marked in Fig. 1) in the IC phase of P b / S i ( l l l ) may play an important role in lowering the free energy. Detailed theoretical calculations on such a system will hopefully shed light on the reason for the formation of the subtle structure exhibited in this IC phase. It is also reasonable that external surface stress fields originating from defects, step edges, or other reconstructions affect the displacement of Pb atoms and thus the trimer domain morphology. The clean Si(lll)-7 x 7 has a greater tensile surface stress than does the IC phase [31]. Although the stress in Pb/Si7 X 7 is not yet known, this structure retains the dimer, corner hole, and stacking fault of clean S i ( l l l ) 7 X 7 [19], which suggests that it may also have a significant tensile stress. The presence of a neighboring Pb/Si-7 X 7 region may alter the stress field of the 1 ML IC phase, driving it to the 1 X 1 structure over distances of ~ 40 A, as shown in Fig. 6. The large tensile stress in the 1 / 3 ML ¢3- X f3reconstruction [32] may also suppress the formation of the IC phase. On the other hand as we have seen, Pb islands and Pb clusters enhance trimer formation.

5. General discussion 5.2. Phase transition IC ~ 1 × 1 5.1. Stress sensitivity and stress-induced reconstruction

The IC and 1 X 1 phases have the same Pb-Si bonding, and the former can evolve smoothly from the latter as a result of small lateral displacements. Presumably the single Pb-Si bond allows Pb atoms to move laterally and form trimers as a result of an attractive Pb-Pb interaction, as is seen in P b / G e (111) [22]. It is interesting to note that the IC surface contains T4 and H 3 trimers with equal populations. In a similar system, the /3-v~ phase of Pb on G e ( l l l ) , only H 3 trimers are seen. Perhaps the difference in trimer formation energy between H 3 and T4 sites is much smaller for P b / S i ( l l l ) than it is for P b / G e ( l l l ) . For the former system, the formation of a large domain of commensurate trimers

The Pb/Si(111) IC surface (with > 1 ML Pb) undergoes a transition to the 1 X 1 structure above ~ 300°C, as observed using LEED [11-13], RHEED [17], and surface X-ray diffraction [20]. The transition temperature drops sharply with decreasing coverage (below 1 ML). On surfaces with less than saturation Pb coverage, an X-ray diffraction study showed that the room temperature 1 X 1 phase transforms into the IC phase below - 2 0 ° C [20], which suggests that the transition temperature for IC *-, 1 X 1 drops below RT in the presence of the 1 / 3 ML x/ff phase. The 1 X 1 phase is an ordered phase in Pb/Si(111), as seen in Fig. 6. The phase transition involves only displacement of Pb atoms around the T 1 site. Photoemission measurements by Le Lay et al. [11-13] showed no change in a pronounced sur-

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face state associated with the IC phase above the transition temperature, which is consistent with our observations. A recent high-temperature STM study of a similar phase transition, 1 ML P b / G e ( l l l ) fl~ 1 X 1, showed that the i X 1 phase is also an ordered 1 × 1 Pb-terminated G e ( l l l ) surface [22]. A sharp drop in the transition temperature with decreasing Pb coverage below 1 ML was also observed in this system [16,22,23,33]. The 1 X 1 phase in these two systems was previously described as a 2D liquid by some researchers because a diffuse scattering halo was observed in X-ray diffraction and RHEED [17,23-25]. We saw no evidence of 2D melting on P b / S i ( l l l ) . Pb atoms in the 1 x 1 phase may have a large in-plane thermal vibrational amplitude because of their T 1 site binding, which may explain diffuse scattering halos [33]. Our microscopic observations provide a consistent framework for understanding the phase transition and the coverage dependence of its transition temperature. A coexisting 1 / 3 ML ~ phase drives the IC ~ 1 × 1 transition temperature below RT, while the presence of Pb islands and Pb clusters increases the transition temperature and enhances the 'condensation' of the low-temperature trimer structures from the high temperature 1 X 1 phase. This behavior is similar to that observed in the / 3 - ~ ~, 1 X 1 phase transition of 1 ML P b / G e ( l l l ) [22,33]. It may be interesting to explore the dynamics near the transition temperature using the STM. Work toward this end is currently in progress. Since the transition of 1 ML P b / S i ( l l l ) IC ,~, 1 X 1 depends sensitively on the local stress fields, it may also be possible to alter its phase transition by macroscopically stressing the sample.

5.3. Driving force for the formation of incommensurate phases in chemisorbed systems According to current understanding, the IC monolayer is visualized as an array of commensurate domains separated by domain walls [2-5]. Excess overlayer density is carried by the domain walls. The domain walls are considered as the basic fluctuating degrees of freedom and through their entropy contribute to the stabilization of the IC phase. This theory was developed to explain phenomena seen in physisorbed systems, such as noble gases on graphite.

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It cannot explain the IC phase we have observed on P b / S i ( l l l ) and the phase transition to a commensurate 1 x 1 phase at high temperatures. We observe no increase or decrease of atomic density at domain walls relative to the surface within the domains. Also, within our observation time ( > 10 min) we do not see wall fluctuations, which are predicted for honeycomb IC phase in the domain wall theory [4]. Furthermore, the IC phase of P b / S i ( l l l ) comprises two nearly degenerate states (trimers centered o n T 4 and on H3). This is surely a new and interesting system for theoretical study. For physisorbed systems, the formation of IC phases results from an adsorbate-adsorbate interaction that is stronger than the adsorbate-substrate interaction. The simple argument does not apply to chemisorbed semiconductor systems. The buildup of surface strain energy due to a large domain of a periodic structure may be the driving force for the wall formation. In most surface reconstructions, the wall formation costs a significant energy and thus the structure remains commensurate with the substrate. The competition between the strain buildup and the energy cost for wall formation determines whether a structure is commensurate or incommensurate. For the case of P b / S i ( l l l ) , the wall formation energy may be quite low since it involves only small displacements of Pb atoms. The detailed explanation for why this particular formation of alternating, small domains of trimers is more energetically favorable than formation of a single large domain of trimers of any given type remains to be discovered.

5. 4. Comparison of Pb / Si(111) and Pb / Ge(111) Pb/Ge(111) system shows a strong similarity to the annealed case of Pb/Si(111) [13,17] probably because the Pb-Si and Pb-Ge bondings are very similar. Bulk Pb and Ge also do not intermix or react. P b / S i ( l l l ) has a phase diagram very similar to that of P b / G e ( l l l ) . At ~ 1 / 6 ML, a mosaic v~- phase was found in both systems [18,19,34]. Between 1 / 6 ML and 1 / 3 ML, an alloy v~- phase with an appropriate portion of Pb was observed for both systems. The high coverage, low temperature phase on S i ( l l l ) and on G e ( l l l ) is very similar. Pb forms a periodic array of trimers centered on the H 3 site in periodicity on Ge(111); Pb/Si(111) exhibits alternat-

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ing domains of Pb trimers centered on either the T4 or H 3 site. The small differences in Pb-Si and Pb-Ge bonding and in the lattice constant of Si and Ge may be responsible for the formation of a commensurate phase for P b / / G e ( l l l ) and an incommensurate one for Pb//Si(lll). For both systems, the low-temperature phase undergoes a similar reversible phase transition at high temperatures to an ordered 1 x 1 phase, which is basically a Pb-terminated unreconstructed surface. Both transitions are displacive, i.e. only a small lateral displacement of each Pb atom away from the T 1 site is involved. The transition temperature for both cases is low in the presence of the 1 / 3 ML v~- phase and a sharp increase occurs near the completion of the first Pb overlayer. Pb deposition reduces the area of the 1//3 ML v~ phase first and then leads to nucleation of Pb islands and modifies step edges, which changes surface stress fields and cause the increase of the transition temperature. The 1 X 1 phase of both systems shows a diffuse scattering halo in X-ray diffraction as well as in RHEED. After the completion of the first Pb overlayer, Pb islands having lattice vectors aligned with those of the substrate begin to grow on both surfaces. The morphologies of both systems observed in tunneling images are also very similar, except that S i ( l l l ) - 7 X 7 is a stronger reconstruction than G e ( l l l ) c ( 2 X 8) and thus annealing is needed to remove it. We found that the interface between the 1 × 1 and 1//3 ML v~- phase is often straight and along a (01"1) direction. The 1 / 3 ML V~- phase is found to nucleate preferentially at upper step edges, while the 1 ML phase nucleates at lower step edges. The tunneling characteristics of these two systems are also similar. The trimers in the /3 phase of P b / / G e ( l l l ) and the IC phase of Pb//Si(lll) are observed over a large range of biases at both polarities. For the empty-state images at high biases or when the tip is not very sharp, the trimers appear as a single protrusion. Usually, the trimers are more easily resolved below 1 eV in empty-state images.

6. Conclusions We have studied the high coverage phases for Pb//Si(lll). For annealed samples, the IC phase was

resolved. It is composed of alternating domains of two types of trimers and a quasi-1 X 1 region. The detailed morphology of the trimer domains depends sensitively on the stress fields resulting from imperfections on the surface. The high coverage phase for RT deposition, Pb/Si-7 X 7, was also resolved and the Pb-Si bonding within the 7 X 7 unit cell is identical to that in the IC phase. A very high defect density was found in Pb/Si-7 X 7 and may be responsible for its low SBH [8]. We observed that the stress fields originating from Pb/Si-7 X 7 transform the IC phase into a 1 X 1 structure. The transformation involves only small lateral displacements of Pb atoms. We find that neighboring areas of the 1//3 ML v~- phase also suppress trimer formation from the 1 X 1 phase, but Pb islands and Pb clusters enhance the formation. The same structural transformation from IC to 1 X 1 occurs at high temperatures. The transition temperature depends strongly on the presence of surface imperfections. A coexisting 1//3 ML V~- phase lowers the transition below room temperature, while Pb islands and clusters nucleated at high coverages raise the transition temperature to ~ 300°C. Our observations provide a consistent framework for understanding the coverage-dependent transition temperature. The formation of the IC phase in Pb//Si(lll) may be driven by surface stress induced by the reconstruction itself. The balance between the buildup of surface strain energy due to the increasing domain size, the wall formation, and external stress fields may determine the shape of the IC phase in chemisorption systems. Detailed calculations on the energetics of the IC structure we have uncovered and on the critical phenomena associated with the IC ,~, 1 x 1 structural transformation should prove fruitful.

Acknowledgements We would like to thank Professor Frans Spaepen, and Professor Efthimios Kaxiras for fruitful discussions and criticism of this manuscript. This research was supported by the Materials Research Laboratory at Harvard University (contract No. NSF-DMR8920490) and the Joint Services Electronics Program (contract No. N00014-89-J-1023).

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