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Nucleation and growth of epitaxial silicon on Si(001) and Si(111) surfaces by scanning tunneling microscopy
10
Ultramicroscopv 31 (1989) 11) 19 North-Holland, Amsterdam
N U C L E A T I O N AND G R O W T H OF E P I T A X I A L S I L I C O N ON Si(001) AND S i ( l l l ) S U R F A C E S BY S C A N N I N G T U N N E L I N G M I C R O S C O P Y R.J. HAMERS. U.K. K O H L E R * and J.E. D E M U T H I B M Thomas J. Watson Research Center. P.O. Bo.x 2l& )orktown Heights. New }ork 1059& USA
Received 21 February 1989: at Editorial Office 3 April 1989: presented at Workshop January 1989
The epitaxial growth of silicon on Si(111)-(7 × 7) and Si(001 )-(2 × 1) substrates at temperatures between 300 and 70tl K is studied using scanning tunneling microscopy. On Si(lll )-(7 ×7), the epitaxial islands arc triangular and exhibit (7 × 7)-like reconstructions even at low coverage. STM images show that multilayer growth initiates at boundaries between different (7 × 7) domains and between (5 × 5) and (7 × 7) phases. On Si(001 ), the epitaxial islands are highly anisotropic, forming long narrow rows only a few dimers wide. Multilayer growth initiates at (2 × 1 ) anti-phase boundaries. A model is pr~*posed for the structure at these anti-phase boundaries.
1. Introduction
The epitaxial growth of silicon on Si substrates has been widely studied due to its great technological importance. While molecular beam epitaxy (MBE) is generally carried out at high temperatures, there is also a great deal of interest in understanding the mechanisms of growth at lower temperatures. In most previous studies, epitaxial growth is studied using Rutherford backscattering (RBS) or one of the diffraction techniques such as low-energy electron diffraction (LEED) or reflection high-energy electron diffraction (RHEED). One of the most interesting observations of the diffraction studies has been the observation of oscillations in the diffracted beam intensities as a function of coverage on both S i ( l l l ) and Si(001) surfaces [1]. These oscillations arise because of interference between electrons scattered from different atomic planes, when the distance between steps on the surface is smaller than the electron coherence length. One curious result of these studies is that epitaxial growth on S i ( l l l ) occurs in a layer-by-layer mode for the second (and ad* Present address: Institut ftir Festk6rperforschung, Universitiit Hannover, Appelstrasse 2, D-3000 Hannover, Fed. Rep. of Germany.
ditional epitaxial layers, but not for the first epitaxial layer. At low temperatures, multi-layer growth occurs on both S i ( l l l ) and Si(100) even at comparatively low coverage, indicating that the gas-phase Si atoms preferentially nucleate on the epitaxial islands rather than on the initial substrate surface. These results suggest that defects or possibly the reconstruction of the initial substrate may play a role in low-temperature epitaxy. Scanning tunneling microscopy is unique in its ability to provide information about the real-space nature of epitaxial growth at the atomic level. Using STM, it is possible to directly identify the roles of surface reconstruction [2] and defects in nucleation and growth phenomena as well as to measure the size and shape distributions of the islands. Using STM~ we have recently studied the in-situ epitaxial growth of Si on S i ( l l l ) [3] and Si(001) [4] surface.
2. Experiment
All experiments were performed in an ultrahigh vacuum chamber with a base pressure of -- 1 × 10 10 Torr as described previously [3]. The samples were commercial silicon wafers (Sb-doped, 6 m,g cm) oriented to within 0.5 ° of the (001) or
0304-3991/89/$03.50 :c Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R.J. Hamers et al. / Nucleation and growth of epitaxial Si on Si(O01) and S i ( l l l ) by STM
11
(111) plane. Samples were directly introduced to U H V via a load-lock system and were then outgassed by heating to = 1000 K for 8-12 h at a pressure of less than 2 × 10 -1° Torr. After outgassing, they were annealed to 1425 K, cooled rapidly to 1175 K, then cooled slowly to = 900 K, and finally cooled to room temperature at a rate limited by radiative and conductive heat transfer from the sample to the surroundings. All heating was done on a transfer assembly isolated from the STM in order to minimize thermal drift. Epitaxial growth was accomplished using a small evaporator consisting of a resistively heated Si wafer and could be performed with the sample held at a fixed temperature. The amount of Si deposited was determined from the epitaxial islands themselves.
3. Epitaxial growth on Si(tll)-(7 x 7) Starting with a clean Si(111)-(7 x 7) substrate, the structure of the epitaxial layers depends strongly on the sample temperature. Fig. 1 shows the epitaxial structures obtained by deposition of Si onto a room-temperature substrate (fig. la) and at temperatures of 525 K (fig. lb) and 625 K (fig. lc). At room temperature, the silicon adsorbs into small amorphous islands, with equal probability of nucleating in the faulted and unfaulted halves of the (7 × 7) unit cells. Here, the low temperature results in a low mobility for the impinging Si atoms, so that many small nuclei are formed. Deposition at 525 K (fig. lb) produces a somewhat smaller number of islands, which begin to show signs of structural order, as the islands develop flat tops and adatom-like structures are visible on some of the islands. At 625 K (fig. lc), however, the epitaxial islands show a very high degree of local order. Surprisingly, the structures adopted by the epitaxial islands strongly resemble the (7 × 7) reconstruction of the underlying substrate, even when they consist of only 20-30 atoms. For example, most of the islands in fig. lc show a " c o m e r hole" immediately surrounded by adatoms in a (2 × 2)-like arrangement, just as in the DAS model for Si(lll)-(7 × 7).
Fig. 1. Epitaxial growth of Si onto Si(lll)-(7 x 7) at different substrate temperatures: (a) 300 K; (b) 525 K; (c) 625 K.
At higher surface temperatures (650 K), as in fig. 2, the islands are generally larger and have nearly triangular shapes, with the step edges corresponding to [112] planes and passing through the c o m e r holes of the (7 x 7) reconstruction on the epitaxial island. The island edges thus adopt essentially the same structure as step edges on thermally annealed Si(ll 1)-(7 x 7) surfaces [5]. In some cases, however, the epitaxial islands adopt other (7 x 7)-like structures such as (5 x 5), (9 x 9) or (3 X 3), which have a dimer-adatom-stacking fault structure similar to Si(111)-(7 X 7). On extremely small islands, a (2 x 2) structure is also sometimes observed [2].
12
R.J. Hamer,s et aL / Nucleation and growth oj epitaxial Si on Si(O01) and Sit I l 1) t~l, S T M
Fig. 2. STM image of epitaxial layer formed at substrate temperature of 625 K, demonstrating the nearly triangular shape of the islands, with step edges coinciding with the corner hole locations.
STM images also reveal that the nucleation probability is considerably higher near defects on the surface. For example, fig. 3 shows a large-area STM scan obtained by depositing - - 1 / 4 m o n o layer Si o n t o a S i ( l l l ) - ( 7 × 7) substrate held at 575 K (and subsequently cooled for STM imaging). Although the island density appears to be fairly uniform in most regions, there is a noticeably enhanced nucleation probability along a line defined by the arrows at the top and b o t t o m edges. The origin of this enhanced nucleation probability
becomes apparent u p o n closer inspection of the image, which shows that this line defined by the arrows coincides with a b o u n d a r y between two different domains of the (7 × 7) reconstruction. At such a a boundary, there is likely a higher n u m b e r of unsaturated "dangling b o n d s " which act as nucleation sites for the epitaxial layer. Such enhanced nucleation at (7 x 7) d o m a i n boundaries is also observed on the epitaxial islands and leads to multilayer growth, as shown in fig. 4. Here, two epitaxial layers are observed atop
R.J. Hamers et al. / Nucleation and growth of epitaxial Si on Si(O01) and Si(111) by S T M
]3
10 nm Fig. 3. Large-scale STM image of epitaxial structures formed on a Si(111)-(7 x 7) substrate. A boundary between two domains of the (7 × 7) reconstruction lies between the arrows and results in a higher nucleation probabifity.
the original substrate. The first epitaxial layer is triangular with [112] step edges, and contains both (7 x 7) and (5 × 5) regions (with unit cells outlined). The second epitaxial layer is only slightly smaller than the first layer and consists entirely of the (5 × 5) reconstruction, again with a unit cell outlined. From the positions of the corner holes of the (5 x 5) and (7 x 7) reconstructions in the first epi-layer and the known crystallographic directions (visible from the substrate layer) it is easy to see that there m u s t be a boundary between (7 × 7) and (5 × 5) phases in the first epi-layer under the second epi-layer. We also observe similar enhanced nucleation on islands when two domains of the (7 × 7) reconstruction meet. At boundaries between (5 x 5) and (7 x 7) phases, as also between different (7 x 7) domains, it is difficult to eliminate the "dangling bonds" at the surface. These remaining dangling bonds obviously serve
as enhanced sites for nucleation of further epitaxial layers. This enhanced nucleation at (7 × 7) domain boundaries and (7 × 7)-(5 × 5) phase boundaries provides an explanation for the absence of LEED oscillations during the growth of the first epi-layer, with persistent LEED oscillations for further layers, as we discussed in a previous publication [3]. Using low-energy electron microscopy, Telieps and Bauer [6] showed that upon cooling after high-temperature annealing_, the (7 × 7) reconstruction nucleates at [112] step edges. STM images confirm this and show that step edges are usually straight and pass directly through the comer holes of the (7 × 7) reconstruction. These two criteria can be simultaneously satisfied in several different ways. Since the (7 × 7) reconstruction may nucleate simultaneously (and independently) at several different locations, we ex-
14
R.J. Harners et al. / Nucleation and growth of epitaxial Si on Si(O01) and Si( l I 1) hv S T M
Fig. 4. STM image showing multi-layer growth at low coverage. The starting substrate, first epi-layer, and second epi-layer are marked "'0", " l ' , and "2", respectively. Unit cells of (5 x 5) and (7 × 7) on the first epi-layer, and a (5 x 5) unit cell on the second epi-layer are outlined.
pect that seven translationally-inequivalent domains of (7 X 7) may coexist on each same terrace on the surfaces prepared by high-temperature annealing. On the epitaxial layers, however, (7 x 7) domain boundaries are formed when two separate islands grow together. Since the individual islands presumably nucleate independently on the terraces (rather than at step edges), the (7 × 7) superstructures can differ by any of 49 translation vectors. As a result, boundaries between different (7 × 7) domains are expected to be much more common on the epitaxial layers than on the starting substrate. The number of domain boundaries
on the epitaxial layers, however, is relatively constant.
4. Epitaxial growth on Si(001) As on Si(lll), deposition of Si onto room-temperature Si(001) leads to amorphous islands. At sample temperatures of 575 K, however, the films grow epitaxially as shown in fig. 5. This image also contains a single-height step passing diagonally through the image, with the upper terrace at lower right. On these well oriented samples at relatively low temperatures, the diffusion length of
R.J. Harners et al. / Nucleation and growth of epitaxial Si on Si(O01) and S i ( l l l ) by S T M
Fig. 5. Initial stage of epitaxial growth on Si(001) surface at 575 K.
the adsorbing Si atoms is much smaller than the distance between atomic steps. As a result, nucleation is observed primarily on flat terraces, forming isolated islands of epitaxial Si. One of the most striking features of these islands is their strongly anisotropic growth, forming long, narrow islands parallel to the direction of the rows of dimers. In some cases, the epitaxial islands are only a singledimer-row wide, reaching aspect ratios of 20 : 1 or more. Fig. 6 shows the epitaxial structures at a higher coverage ( - 0 . 5 mE) where some islands have grown together. Even at this higher coverage, the islands still show a highly anisotropic shape. The two inequivalent (orthogonal) step directions
15
are marked " A " and " B " in this figure, consistent with the notation used by Chadi [7]. One important question is whether this strongly anisotropic growth is determined by kinetics or thermodynamics. Since the formation of a step increases the total energy of the system, at true thermodynamic equilibrium the growth should occur at step edges. The existence of three-dimensional islands then immediately indicates that the system is not in global thermodynamic equilibrium. However, the size and shape distribution of each island is determined both by mass transport of Si atoms between islands (via surface diffusion) as well as diffusion of atoms within each individual island. For well separated islands, diffusion of Si atoms between different islands may be negligible, while diffusion of atoms within any given island m a y be sufficiently rapid to establish a local thermodynamic equilibrium for each island. The STM images show that the preferred growth direction is parallel to the dimer rows of the epitaxial island, or equivalently, perpendicular to the dimer rows of the underlying substrate. If the growth is determined primarily by diffusion of material between islands, this means that Si atoms would have to diffuse faster along the high-corrugation direction (perpendicular to the dimer rows of the substrate) than in the low-corrugation direction. An alternative explanation is that diffusion of atoms within each island is fast while diffusion between different islands is slow. This could result if, for instance, the activation barrier for diffusion at a step edge is lower than on a flat terrace, allowing atoms to diffuse around the island perimeter. The atoms within each island could then quickly rearrange themselves to achieve a shape which represents a local energy minimum, even though the entire system is not in true thermodynamic equilibrium. If each island is assumed to be in a local energy minimum, then the aspect ratio of epitaxial islands can be compared with the energy required to form the orthogonal steps on Si(001), assuming that the corners of the islands do not strongly influence the total energy. If we assume that the step energy per atom for a type " A " step edge (as indicated in
16
R.J. Hamers et a L / Nucleation and growth of epitaxial Si on Si(O01) and Si(l 11) t~v S T M
Fig. 6. (a) Epitaxial structures observed at higher coverage, including initial stages of multilayer growth. The two different step directions for the islands are labeled "A" and "B". Arrowheads indicate regions of multilayer growth. (b) Enlarged image of a region where multilayer growth is observed in two locations. In both cases, the nucleation site of the second epitaxial layer corresponds to the location of an anti-phase boundary in the first epitaxial layer.
fig. 6) is E a a n d the step energy per a t o m for a type B step edge is Eb, then for a rectangular island the total n u m b e r of atoms is given by N = n a n b a n d the total step energy is E = n a E a + n b E b. M i n i m i z i n g the total energy at c o n s t a n t area results in n a / n b = E a / E b. Thus, the aspect ratio of the islands provides a measure of the
relative step energies. W e observe that n~, >> n h, i n d i c a t i n g that the energy of a type " A " step is significantly lower t h a n that of a type " B " step edge. This c o n c l u s i o n is s u p p o r t e d b y total energy calculations by C h a d i [7] for type " A " a n d one of the two possible c o n f i g u r a t i o n s for a type " B " step edge. As described in a previous p u b l i c a t i o n
R.J. Harners et al. / Nucleation and growth of epitaxial Si on Si(O01) and S i ( l l l ) by STM
[8] STM images reveal that two configurations for a type " B " step edge can occur, which are observed with roughly equal probability. Chadi's calculations predict that the step formation energies for the type " A " step edge is =0.01 e V / a t o m while that for a type " B " step edge is = 0.15 eV, from which an aspect ratio of = 15 : 1 would be expected. This is quite consistent with our measurements at low coverage. Thus, it appears that the island shapes are probably determined primarily by thermodynamics rather than kinetics even at these low temperatures, due to the rather large energy associated with forming type " B " step edges.
5. Multilayer growth In some cases, growth of a second epitaxial layer is observed even at very low coverage. This indicates that, for some reason, the nucleation occurs preferentially on the epitaxial layers rather than on the original starting substrate. Multilayer growth is seen in both fig. 5 and fig. 6, at positions marked by the arrows. In many (but not all) cases, we find that multilayer growth is associated with the presence of (2 × 1) anti-phase boundaries in the first epitaxial layer, similar to what we observed earlier for epitaxial growth on Si(111)-(7 × 7). For example, in the center of fig. 6, a second epitaxial layer (labeled "2") has grown atop the first epitaxial island (labeled " I A " and "IB"), rather then the starting substrate (labeled "0"). An enlargement of this region is shown in the inset and shows two multilayer structures, both of which occur at the positions at antiphase boundaries in the first epitaxial layer. The relative phases of the (2 × 1) reconstructions in the first epi-layer on both sides of the second epi-layer can be seen from the line drawn in inset of fig. 6. This line is atop the dimer rows of the first epi-layer in the " l a " region, but coincides with the trough between the dimer rows on the " l b " region. This demonstrates that the (2 x 1) reconstruction in regions " l a " and " l b " have different phases, and that there was a (2 × 1) antiphase boundary in the first epi-layer before growth of the second epilayer. Even though the growth of the second epi-
17
layer (" 2") removes the (2 x 1) reconstruction from the first epi-layer ("1") under it, anti-phase boundary provides a very active nucleation site for the Si atoms impinging from the gas phase. Although it is possible for these (2 x 1) antiphase boundaries to also occur on the original starting substrate, they have not been observed in any previous studies of Si(001). The absence of (2 × 1) antiphase boundaries on flat terraces presumably arises from the high temperatures used in previous studies, which allows for greater diffusion and long-range order. At low temperatures and low coverage, however, these anti-phase boundaries occur frequently because the individual islands nucleate into the two equivalent (2 x 1) domains randomly; when they grow together and intersect, there is then a 50% probability of forming an anti-phase boundary. What is the structure of these (2 x 1) antiphase boundaries? In our studies, we find that virtually all the anti-phase boundaries have a unique structure. Fig. 7 shows a high-resolution STM topograph ( - 1 . 2 V sample bias) of a representative boundary. Measurement of the atomic separations shows that the boundary itself is rather extended. This distance between the first "normal" dimers on each side of the antiphase boundary is 5 lattice constants (a 0 = 3.85 A), most of which appears to be at the height of the original substrate. However, at a distance of 7.7 ,~ from one domain edge (and 11.5 A from the other), another row of atoms appears; along the row, the individual atoms are observed but a weak 7.7 A periodicity is retained. This suggests that any dimerization of these atoms is rather weak. Fig. 8 shows a proposed atomic structure at this antiphase boundary. To a first approximation, this may be considered to be two single-atom steps ($1 and $2) separated by 5a0, with a single row of dimers (D) added. In earlier STM measurements [8] we showed that this $1 step termination, in which the atoms forming the lower part of the step edge participate in dimer bonding, is a common, stable step configuration. The single column of dimers " D " and the second-layer atoms exposed midway between $1 and D are also expected to dimerize to some extent. At the column of " D " atoms, however, the images at negative
R.J. Hamers el al. / Nucleation and growth 01 epitaxial Si on Si(O01) and Si( l l 1) t~v S T M
18
Fig. 7. Anti-phase boundary on a Si(001) surface.
bias (which generally show a clear (2 × 1) periodicity on extended terraces) instead show only a strong (1 × 1) periodicity with a weak (2 × 1 ) corn-
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Fig. 8. Proposed atomic model for the anti-phase boundary. The largest circles (filled with dots) represent atoms in the top atomic layer; smaller circles represent atoms in sequentially deeper layers.
p o n e n t on this row of dimers. This indicates a somewhat different electronic structure for these dimers c o m p a r e d with the dimers on extended terraces. Since it is widely recognized that the formation of dimers involves significant displacements of atoms in underlying layers, it is reasonable to expect that complete dimerization of the second-layer atoms midway between D and $2 and dimerization of the single row of dimers D cannot both occur without inducing significant lattice strain. This strain energy may play a role in the e n h a n c e d nucleation observed at these boundaries. Further work will be required to fully understand how the unique atomic structure at these boundaries promotes nucleation of further epitaxial layers. 6. Conclusions In summary, we have used scanning tunneling microscopy to study the initial stages of low-ten>
R.J. Hamers et al. / Nucleation and growth of epitaxial Si on Si(O01) and Si(l l l) by S T M
perature M B E on Si(111) and Si(001). The results illustrate the i m p o r t a n c e of d o m a i n - a n d phaseb o u n d a r i e s as n u c l e a t i o n sites for epitaxial layers a n d also their role in p r o m o t i n g multilayer growth a low temperatures. The STM results have also provided new i n f o r m a t i o n a b o u t the shapes of the epitaxial islands. We find that the island shapes, even at low temperatures, appear to be d e t e r m i n e d by m i n i m i z i n g the energy of each island. This suggests that surface diffusion within each island is fast compared to that between islands, allowing each i n d i v i d u a l island to come to its own "local e q u i l i b r i u m " shape.
19
References [1] T. Sakamoto, N.J. Kawai, T. Nakagawa, K. Ohta and T. Kojima, J. Appl. Phys. 47 (1985) 617. [2] U.K. K/Shler, J.E. Demuth and R.J. Hamers, Phys. Rev. Letters 60 (1988) 2499. [3] U.K. K~Shler, J.E. Demuth and R.J. Hamers, J. Vacuum Sci. Technol. A, in press. [4] U.K. K~hler, J.E. Demuth and R.J. Hamers, in preparation. [5] R.S. Becker, T. Klitsner and J.S. Vickers, Phys. Rev. B38 (1988) 3537. [6] W. Telieps and E. Bauer, Surface Sci. 162 (1985) 163. [7] J. Chadi, Phys. Rev. Letters 59 (1987) 1691: J. Chadi, Uhramicroscopy 31 (1989) 1. [8] R.J. Hamers, R.M. Tromp and J.E. Demuth, Phys. Rev. B34 (1987) 5343.
Ultramicroscopv 31 (1989) 11) 19 North-Holland, Amsterdam
N U C L E A T I O N AND G R O W T H OF E P I T A X I A L S I L I C O N ON Si(001) AND S i ( l l l ) S U R F A C E S BY S C A N N I N G T U N N E L I N G M I C R O S C O P Y R.J. HAMERS. U.K. K O H L E R * and J.E. D E M U T H I B M Thomas J. Watson Research Center. P.O. Bo.x 2l& )orktown Heights. New }ork 1059& USA
Received 21 February 1989: at Editorial Office 3 April 1989: presented at Workshop January 1989
The epitaxial growth of silicon on Si(111)-(7 × 7) and Si(001 )-(2 × 1) substrates at temperatures between 300 and 70tl K is studied using scanning tunneling microscopy. On Si(lll )-(7 ×7), the epitaxial islands arc triangular and exhibit (7 × 7)-like reconstructions even at low coverage. STM images show that multilayer growth initiates at boundaries between different (7 × 7) domains and between (5 × 5) and (7 × 7) phases. On Si(001 ), the epitaxial islands are highly anisotropic, forming long narrow rows only a few dimers wide. Multilayer growth initiates at (2 × 1 ) anti-phase boundaries. A model is pr~*posed for the structure at these anti-phase boundaries.
1. Introduction
The epitaxial growth of silicon on Si substrates has been widely studied due to its great technological importance. While molecular beam epitaxy (MBE) is generally carried out at high temperatures, there is also a great deal of interest in understanding the mechanisms of growth at lower temperatures. In most previous studies, epitaxial growth is studied using Rutherford backscattering (RBS) or one of the diffraction techniques such as low-energy electron diffraction (LEED) or reflection high-energy electron diffraction (RHEED). One of the most interesting observations of the diffraction studies has been the observation of oscillations in the diffracted beam intensities as a function of coverage on both S i ( l l l ) and Si(001) surfaces [1]. These oscillations arise because of interference between electrons scattered from different atomic planes, when the distance between steps on the surface is smaller than the electron coherence length. One curious result of these studies is that epitaxial growth on S i ( l l l ) occurs in a layer-by-layer mode for the second (and ad* Present address: Institut ftir Festk6rperforschung, Universitiit Hannover, Appelstrasse 2, D-3000 Hannover, Fed. Rep. of Germany.
ditional epitaxial layers, but not for the first epitaxial layer. At low temperatures, multi-layer growth occurs on both S i ( l l l ) and Si(100) even at comparatively low coverage, indicating that the gas-phase Si atoms preferentially nucleate on the epitaxial islands rather than on the initial substrate surface. These results suggest that defects or possibly the reconstruction of the initial substrate may play a role in low-temperature epitaxy. Scanning tunneling microscopy is unique in its ability to provide information about the real-space nature of epitaxial growth at the atomic level. Using STM, it is possible to directly identify the roles of surface reconstruction [2] and defects in nucleation and growth phenomena as well as to measure the size and shape distributions of the islands. Using STM~ we have recently studied the in-situ epitaxial growth of Si on S i ( l l l ) [3] and Si(001) [4] surface.
2. Experiment
All experiments were performed in an ultrahigh vacuum chamber with a base pressure of -- 1 × 10 10 Torr as described previously [3]. The samples were commercial silicon wafers (Sb-doped, 6 m,g cm) oriented to within 0.5 ° of the (001) or
0304-3991/89/$03.50 :c Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R.J. Hamers et al. / Nucleation and growth of epitaxial Si on Si(O01) and S i ( l l l ) by STM
11
(111) plane. Samples were directly introduced to U H V via a load-lock system and were then outgassed by heating to = 1000 K for 8-12 h at a pressure of less than 2 × 10 -1° Torr. After outgassing, they were annealed to 1425 K, cooled rapidly to 1175 K, then cooled slowly to = 900 K, and finally cooled to room temperature at a rate limited by radiative and conductive heat transfer from the sample to the surroundings. All heating was done on a transfer assembly isolated from the STM in order to minimize thermal drift. Epitaxial growth was accomplished using a small evaporator consisting of a resistively heated Si wafer and could be performed with the sample held at a fixed temperature. The amount of Si deposited was determined from the epitaxial islands themselves.
3. Epitaxial growth on Si(tll)-(7 x 7) Starting with a clean Si(111)-(7 x 7) substrate, the structure of the epitaxial layers depends strongly on the sample temperature. Fig. 1 shows the epitaxial structures obtained by deposition of Si onto a room-temperature substrate (fig. la) and at temperatures of 525 K (fig. lb) and 625 K (fig. lc). At room temperature, the silicon adsorbs into small amorphous islands, with equal probability of nucleating in the faulted and unfaulted halves of the (7 × 7) unit cells. Here, the low temperature results in a low mobility for the impinging Si atoms, so that many small nuclei are formed. Deposition at 525 K (fig. lb) produces a somewhat smaller number of islands, which begin to show signs of structural order, as the islands develop flat tops and adatom-like structures are visible on some of the islands. At 625 K (fig. lc), however, the epitaxial islands show a very high degree of local order. Surprisingly, the structures adopted by the epitaxial islands strongly resemble the (7 × 7) reconstruction of the underlying substrate, even when they consist of only 20-30 atoms. For example, most of the islands in fig. lc show a " c o m e r hole" immediately surrounded by adatoms in a (2 × 2)-like arrangement, just as in the DAS model for Si(lll)-(7 × 7).
Fig. 1. Epitaxial growth of Si onto Si(lll)-(7 x 7) at different substrate temperatures: (a) 300 K; (b) 525 K; (c) 625 K.
At higher surface temperatures (650 K), as in fig. 2, the islands are generally larger and have nearly triangular shapes, with the step edges corresponding to [112] planes and passing through the c o m e r holes of the (7 x 7) reconstruction on the epitaxial island. The island edges thus adopt essentially the same structure as step edges on thermally annealed Si(ll 1)-(7 x 7) surfaces [5]. In some cases, however, the epitaxial islands adopt other (7 x 7)-like structures such as (5 x 5), (9 x 9) or (3 X 3), which have a dimer-adatom-stacking fault structure similar to Si(111)-(7 X 7). On extremely small islands, a (2 x 2) structure is also sometimes observed [2].
12
R.J. Hamer,s et aL / Nucleation and growth oj epitaxial Si on Si(O01) and Sit I l 1) t~l, S T M
Fig. 2. STM image of epitaxial layer formed at substrate temperature of 625 K, demonstrating the nearly triangular shape of the islands, with step edges coinciding with the corner hole locations.
STM images also reveal that the nucleation probability is considerably higher near defects on the surface. For example, fig. 3 shows a large-area STM scan obtained by depositing - - 1 / 4 m o n o layer Si o n t o a S i ( l l l ) - ( 7 × 7) substrate held at 575 K (and subsequently cooled for STM imaging). Although the island density appears to be fairly uniform in most regions, there is a noticeably enhanced nucleation probability along a line defined by the arrows at the top and b o t t o m edges. The origin of this enhanced nucleation probability
becomes apparent u p o n closer inspection of the image, which shows that this line defined by the arrows coincides with a b o u n d a r y between two different domains of the (7 × 7) reconstruction. At such a a boundary, there is likely a higher n u m b e r of unsaturated "dangling b o n d s " which act as nucleation sites for the epitaxial layer. Such enhanced nucleation at (7 x 7) d o m a i n boundaries is also observed on the epitaxial islands and leads to multilayer growth, as shown in fig. 4. Here, two epitaxial layers are observed atop
R.J. Hamers et al. / Nucleation and growth of epitaxial Si on Si(O01) and Si(111) by S T M
]3
10 nm Fig. 3. Large-scale STM image of epitaxial structures formed on a Si(111)-(7 x 7) substrate. A boundary between two domains of the (7 × 7) reconstruction lies between the arrows and results in a higher nucleation probabifity.
the original substrate. The first epitaxial layer is triangular with [112] step edges, and contains both (7 x 7) and (5 × 5) regions (with unit cells outlined). The second epitaxial layer is only slightly smaller than the first layer and consists entirely of the (5 × 5) reconstruction, again with a unit cell outlined. From the positions of the corner holes of the (5 x 5) and (7 x 7) reconstructions in the first epi-layer and the known crystallographic directions (visible from the substrate layer) it is easy to see that there m u s t be a boundary between (7 × 7) and (5 × 5) phases in the first epi-layer under the second epi-layer. We also observe similar enhanced nucleation on islands when two domains of the (7 × 7) reconstruction meet. At boundaries between (5 x 5) and (7 x 7) phases, as also between different (7 x 7) domains, it is difficult to eliminate the "dangling bonds" at the surface. These remaining dangling bonds obviously serve
as enhanced sites for nucleation of further epitaxial layers. This enhanced nucleation at (7 × 7) domain boundaries and (7 × 7)-(5 × 5) phase boundaries provides an explanation for the absence of LEED oscillations during the growth of the first epi-layer, with persistent LEED oscillations for further layers, as we discussed in a previous publication [3]. Using low-energy electron microscopy, Telieps and Bauer [6] showed that upon cooling after high-temperature annealing_, the (7 × 7) reconstruction nucleates at [112] step edges. STM images confirm this and show that step edges are usually straight and pass directly through the comer holes of the (7 × 7) reconstruction. These two criteria can be simultaneously satisfied in several different ways. Since the (7 × 7) reconstruction may nucleate simultaneously (and independently) at several different locations, we ex-
14
R.J. Harners et al. / Nucleation and growth of epitaxial Si on Si(O01) and Si( l I 1) hv S T M
Fig. 4. STM image showing multi-layer growth at low coverage. The starting substrate, first epi-layer, and second epi-layer are marked "'0", " l ' , and "2", respectively. Unit cells of (5 x 5) and (7 × 7) on the first epi-layer, and a (5 x 5) unit cell on the second epi-layer are outlined.
pect that seven translationally-inequivalent domains of (7 X 7) may coexist on each same terrace on the surfaces prepared by high-temperature annealing. On the epitaxial layers, however, (7 x 7) domain boundaries are formed when two separate islands grow together. Since the individual islands presumably nucleate independently on the terraces (rather than at step edges), the (7 × 7) superstructures can differ by any of 49 translation vectors. As a result, boundaries between different (7 × 7) domains are expected to be much more common on the epitaxial layers than on the starting substrate. The number of domain boundaries
on the epitaxial layers, however, is relatively constant.
4. Epitaxial growth on Si(001) As on Si(lll), deposition of Si onto room-temperature Si(001) leads to amorphous islands. At sample temperatures of 575 K, however, the films grow epitaxially as shown in fig. 5. This image also contains a single-height step passing diagonally through the image, with the upper terrace at lower right. On these well oriented samples at relatively low temperatures, the diffusion length of
R.J. Harners et al. / Nucleation and growth of epitaxial Si on Si(O01) and S i ( l l l ) by S T M
Fig. 5. Initial stage of epitaxial growth on Si(001) surface at 575 K.
the adsorbing Si atoms is much smaller than the distance between atomic steps. As a result, nucleation is observed primarily on flat terraces, forming isolated islands of epitaxial Si. One of the most striking features of these islands is their strongly anisotropic growth, forming long, narrow islands parallel to the direction of the rows of dimers. In some cases, the epitaxial islands are only a singledimer-row wide, reaching aspect ratios of 20 : 1 or more. Fig. 6 shows the epitaxial structures at a higher coverage ( - 0 . 5 mE) where some islands have grown together. Even at this higher coverage, the islands still show a highly anisotropic shape. The two inequivalent (orthogonal) step directions
15
are marked " A " and " B " in this figure, consistent with the notation used by Chadi [7]. One important question is whether this strongly anisotropic growth is determined by kinetics or thermodynamics. Since the formation of a step increases the total energy of the system, at true thermodynamic equilibrium the growth should occur at step edges. The existence of three-dimensional islands then immediately indicates that the system is not in global thermodynamic equilibrium. However, the size and shape distribution of each island is determined both by mass transport of Si atoms between islands (via surface diffusion) as well as diffusion of atoms within each individual island. For well separated islands, diffusion of Si atoms between different islands may be negligible, while diffusion of atoms within any given island m a y be sufficiently rapid to establish a local thermodynamic equilibrium for each island. The STM images show that the preferred growth direction is parallel to the dimer rows of the epitaxial island, or equivalently, perpendicular to the dimer rows of the underlying substrate. If the growth is determined primarily by diffusion of material between islands, this means that Si atoms would have to diffuse faster along the high-corrugation direction (perpendicular to the dimer rows of the substrate) than in the low-corrugation direction. An alternative explanation is that diffusion of atoms within each island is fast while diffusion between different islands is slow. This could result if, for instance, the activation barrier for diffusion at a step edge is lower than on a flat terrace, allowing atoms to diffuse around the island perimeter. The atoms within each island could then quickly rearrange themselves to achieve a shape which represents a local energy minimum, even though the entire system is not in true thermodynamic equilibrium. If each island is assumed to be in a local energy minimum, then the aspect ratio of epitaxial islands can be compared with the energy required to form the orthogonal steps on Si(001), assuming that the corners of the islands do not strongly influence the total energy. If we assume that the step energy per atom for a type " A " step edge (as indicated in
16
R.J. Hamers et a L / Nucleation and growth of epitaxial Si on Si(O01) and Si(l 11) t~v S T M
Fig. 6. (a) Epitaxial structures observed at higher coverage, including initial stages of multilayer growth. The two different step directions for the islands are labeled "A" and "B". Arrowheads indicate regions of multilayer growth. (b) Enlarged image of a region where multilayer growth is observed in two locations. In both cases, the nucleation site of the second epitaxial layer corresponds to the location of an anti-phase boundary in the first epitaxial layer.
fig. 6) is E a a n d the step energy per a t o m for a type B step edge is Eb, then for a rectangular island the total n u m b e r of atoms is given by N = n a n b a n d the total step energy is E = n a E a + n b E b. M i n i m i z i n g the total energy at c o n s t a n t area results in n a / n b = E a / E b. Thus, the aspect ratio of the islands provides a measure of the
relative step energies. W e observe that n~, >> n h, i n d i c a t i n g that the energy of a type " A " step is significantly lower t h a n that of a type " B " step edge. This c o n c l u s i o n is s u p p o r t e d b y total energy calculations by C h a d i [7] for type " A " a n d one of the two possible c o n f i g u r a t i o n s for a type " B " step edge. As described in a previous p u b l i c a t i o n
R.J. Harners et al. / Nucleation and growth of epitaxial Si on Si(O01) and S i ( l l l ) by STM
[8] STM images reveal that two configurations for a type " B " step edge can occur, which are observed with roughly equal probability. Chadi's calculations predict that the step formation energies for the type " A " step edge is =0.01 e V / a t o m while that for a type " B " step edge is = 0.15 eV, from which an aspect ratio of = 15 : 1 would be expected. This is quite consistent with our measurements at low coverage. Thus, it appears that the island shapes are probably determined primarily by thermodynamics rather than kinetics even at these low temperatures, due to the rather large energy associated with forming type " B " step edges.
5. Multilayer growth In some cases, growth of a second epitaxial layer is observed even at very low coverage. This indicates that, for some reason, the nucleation occurs preferentially on the epitaxial layers rather than on the original starting substrate. Multilayer growth is seen in both fig. 5 and fig. 6, at positions marked by the arrows. In many (but not all) cases, we find that multilayer growth is associated with the presence of (2 × 1) anti-phase boundaries in the first epitaxial layer, similar to what we observed earlier for epitaxial growth on Si(111)-(7 × 7). For example, in the center of fig. 6, a second epitaxial layer (labeled "2") has grown atop the first epitaxial island (labeled " I A " and "IB"), rather then the starting substrate (labeled "0"). An enlargement of this region is shown in the inset and shows two multilayer structures, both of which occur at the positions at antiphase boundaries in the first epitaxial layer. The relative phases of the (2 × 1) reconstructions in the first epi-layer on both sides of the second epi-layer can be seen from the line drawn in inset of fig. 6. This line is atop the dimer rows of the first epi-layer in the " l a " region, but coincides with the trough between the dimer rows on the " l b " region. This demonstrates that the (2 x 1) reconstruction in regions " l a " and " l b " have different phases, and that there was a (2 × 1) antiphase boundary in the first epi-layer before growth of the second epilayer. Even though the growth of the second epi-
17
layer (" 2") removes the (2 x 1) reconstruction from the first epi-layer ("1") under it, anti-phase boundary provides a very active nucleation site for the Si atoms impinging from the gas phase. Although it is possible for these (2 x 1) antiphase boundaries to also occur on the original starting substrate, they have not been observed in any previous studies of Si(001). The absence of (2 × 1) antiphase boundaries on flat terraces presumably arises from the high temperatures used in previous studies, which allows for greater diffusion and long-range order. At low temperatures and low coverage, however, these anti-phase boundaries occur frequently because the individual islands nucleate into the two equivalent (2 x 1) domains randomly; when they grow together and intersect, there is then a 50% probability of forming an anti-phase boundary. What is the structure of these (2 x 1) antiphase boundaries? In our studies, we find that virtually all the anti-phase boundaries have a unique structure. Fig. 7 shows a high-resolution STM topograph ( - 1 . 2 V sample bias) of a representative boundary. Measurement of the atomic separations shows that the boundary itself is rather extended. This distance between the first "normal" dimers on each side of the antiphase boundary is 5 lattice constants (a 0 = 3.85 A), most of which appears to be at the height of the original substrate. However, at a distance of 7.7 ,~ from one domain edge (and 11.5 A from the other), another row of atoms appears; along the row, the individual atoms are observed but a weak 7.7 A periodicity is retained. This suggests that any dimerization of these atoms is rather weak. Fig. 8 shows a proposed atomic structure at this antiphase boundary. To a first approximation, this may be considered to be two single-atom steps ($1 and $2) separated by 5a0, with a single row of dimers (D) added. In earlier STM measurements [8] we showed that this $1 step termination, in which the atoms forming the lower part of the step edge participate in dimer bonding, is a common, stable step configuration. The single column of dimers " D " and the second-layer atoms exposed midway between $1 and D are also expected to dimerize to some extent. At the column of " D " atoms, however, the images at negative
R.J. Hamers el al. / Nucleation and growth 01 epitaxial Si on Si(O01) and Si( l l 1) t~v S T M
18
Fig. 7. Anti-phase boundary on a Si(001) surface.
bias (which generally show a clear (2 × 1) periodicity on extended terraces) instead show only a strong (1 × 1) periodicity with a weak (2 × 1 ) corn-
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Fig. 8. Proposed atomic model for the anti-phase boundary. The largest circles (filled with dots) represent atoms in the top atomic layer; smaller circles represent atoms in sequentially deeper layers.
p o n e n t on this row of dimers. This indicates a somewhat different electronic structure for these dimers c o m p a r e d with the dimers on extended terraces. Since it is widely recognized that the formation of dimers involves significant displacements of atoms in underlying layers, it is reasonable to expect that complete dimerization of the second-layer atoms midway between D and $2 and dimerization of the single row of dimers D cannot both occur without inducing significant lattice strain. This strain energy may play a role in the e n h a n c e d nucleation observed at these boundaries. Further work will be required to fully understand how the unique atomic structure at these boundaries promotes nucleation of further epitaxial layers. 6. Conclusions In summary, we have used scanning tunneling microscopy to study the initial stages of low-ten>
R.J. Hamers et al. / Nucleation and growth of epitaxial Si on Si(O01) and Si(l l l) by S T M
perature M B E on Si(111) and Si(001). The results illustrate the i m p o r t a n c e of d o m a i n - a n d phaseb o u n d a r i e s as n u c l e a t i o n sites for epitaxial layers a n d also their role in p r o m o t i n g multilayer growth a low temperatures. The STM results have also provided new i n f o r m a t i o n a b o u t the shapes of the epitaxial islands. We find that the island shapes, even at low temperatures, appear to be d e t e r m i n e d by m i n i m i z i n g the energy of each island. This suggests that surface diffusion within each island is fast compared to that between islands, allowing each i n d i v i d u a l island to come to its own "local e q u i l i b r i u m " shape.
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References [1] T. Sakamoto, N.J. Kawai, T. Nakagawa, K. Ohta and T. Kojima, J. Appl. Phys. 47 (1985) 617. [2] U.K. K/Shler, J.E. Demuth and R.J. Hamers, Phys. Rev. Letters 60 (1988) 2499. [3] U.K. K~Shler, J.E. Demuth and R.J. Hamers, J. Vacuum Sci. Technol. A, in press. [4] U.K. K~hler, J.E. Demuth and R.J. Hamers, in preparation. [5] R.S. Becker, T. Klitsner and J.S. Vickers, Phys. Rev. B38 (1988) 3537. [6] W. Telieps and E. Bauer, Surface Sci. 162 (1985) 163. [7] J. Chadi, Phys. Rev. Letters 59 (1987) 1691: J. Chadi, Uhramicroscopy 31 (1989) 1. [8] R.J. Hamers, R.M. Tromp and J.E. Demuth, Phys. Rev. B34 (1987) 5343.