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Silicon motion during antimony deposition on Si(111)
surface science ELSEVIER
Surface Science 345 (1996) 222-234
Silicon motion during antimony deposition on Si(111) Robert G. Ryland, Shigehiko Hasegawa 1, Ellen D. Williams * Department of Physics, University of Maryland, College Park, MD 20742-4111, USA Received 17 May 1995; accepted for publication 7 August 1995
Abstract We report on a scanning tunneling microscopy study of mass transport and the resulting morphological changes during Sb deposition on Si( 111 ). Formation of silicon islands is found to take place during the initial stages of Sb deposition, while additional Sb causes the formation of pits. Both observations are explained in terms of the changes in Si surface density caused by the various Sb-induced reconstructions. The islands preferentially nucleate away from step edges, leaving a denuded zone. Measuring the denuded zone width shows a decrease in the diffusion length of Si with increasing Sb coverage.
Keywords: Antimony; Low index single crystal surfaces; Scanning tunneling microscopy; Silicon; Surface diffusion; Surface structure
1. Introduction Technological interest in surfactant-mediated epitaxial growth has led to an increased interest in the adsorption of Group V metals such as Sb and As on Si [ 1-4]. A monolayer of such an adsorbed surfactant species can alter the growth mode on the substrate. Although several models for this behavior have been proposed, the detailed mechanisms involved are not yet well understood [ 1,2, 5]. It has been shown that adsorbed As atoms are substituted for Si atoms of the first S i ( l l l ) layer, leading to a simple (1 x 1) structure [1,6,7]. In contrast, low energy electron diffraction (LEED) studies have shown that Sb-terminated S i ( l l l ) surfaces exhibit a large variety of reconstructions
* Corresponding author. Fax: 301 314 9465; E-mail: [email protected]. 1 Present address: ISIR, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567, Japan. 0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0039-6028 ( 95 ) 00864-0
depending on Sb coverage and substrate temperature [8]. A recent scanning tunneling microscopy study has suggested that Sb atoms are substituted for Si atoms at low coverages [9]. Such replacement will cause mass transport due to the change of surface Si atom density, which can lead to interface roughening as the displaced atoms are accommodated on the surface. On normally prepared surfaces, step separations of 100 nm or less are common, thus it may be difficult to observe such mass transport directly since the steps act as good sources or sinks for displaced atoms. Using a novel technique for generating wide S i ( l l l ) terraces, we have observed direct evidence of mass transport originating from changes in the surface atom density during the formation of Sb-induced S i ( l l l ) reconstructions. This mass transport results in approximately 25% of the surface being covered with bi-layer deep pits or bi-layer height islands, a roughness which persists even after the Sb is subsequently removed. We also observe the
1~ G. Ryland et al./Surface Science 345 (1996) 222-234
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effective diffusion length for the displaced Si atoms decrease very quickly with increasing Sb coverage,
defined as the number of sites on an ideal S i ( l l l ) surface; 7.85 x 1014 cm -2.
2. Experimental
3. Results
All experiments were performed in a UHV vacuum chamber with a background pressure below 5 x 10-11 Torr. The chamber was equipped with a scanning tunneling microscope (STM), Sb evaporation apparatus, and commercial LEEDAES optics. The STM has home-made hardware, semi-commercial electronics, and home-made control software. The STM uses a tube-type piezoelectric actuator capable of scanning areas greater than 4/~m x 4/~m. The Si samples were cut from nominally fiat S i ( l l l ) wafers. The wafers were phosphorus doped (n-type) with a resistivity of 2-10 ~.cm. The samples were thermally cleaned by flashing to 1250°C for several minutes. Such a thermal cleaning cycle removes any previously deposited Sb leaving a clean ( 7 x 7 ) surface, thereby allowing the use of a single sample for many experiments. By using a small sample size (3 mm × 15 mm), the background pressure during the thermal cleaning was generally kept below 3 x 10- lO Torr. The sample was heated by passing a DC current though the sample in the step bunching direction [ 10,11 ]. This provided very large terraces between the step bunches, allowing the observation of effects that do not occur on terraces of more typical widths [ 12]. The Sb was sublimated from 99.9999% pure elemental Sb pieces in a Ta boat. The low sublimation temperature of Sb allowed a background pressure below 5 x 10 -11 Torr during deposition. Except as otherwise noted, the temperature of the sample during Sb deposition was kept at 600°C as measured by an infrared pyrometer. This is well above the desorption temperature for a second monolayer of Sb on Si but low enough so that the first monolayer desorbs very slowly [13]. After deposition the samples were quickly cooled to room temperature by removing the dc current used to heat them. The deposition rate was estimated to be 0.05 ML/min from the deposition time required for obtaining a well defined ( x ~ x x/J) L E E D pattern with no traces of (7 x 7). 1 M L is
A series of 2 #m x 2 #m images showing the evolution of the surface with increasing Sb coverage are presented in Figs. l a - l c . The most prominent feature in these images is the presence of islands on the terraces with denuded zones [14] at both steps bounding the terrace. The total area covered by these islands increases as the Sb coverage increases. Initially, large islands form in the middle of the terraces. At higher Sb coverages we continue to see large islands but we also see the formation of smaller islands. At the same time the width of the denuded zone next to the steps and the islands decreases. Atomically resolved images of these islands show that they are approximately 0.3 nm in height and have the same surface structure as the terraces, as will be shown below. Pit formation can be seen in Fig. ld and Fig. le after a deposition time of 10 and 20 min, respectively. These pits begin to nucleate only after the deposition of ~0.40 M L of Sb and then grow as the coverage increases. The pits seem to form on the large islands and terraces with equal probability. A series of scans taken at progressively higher resolutions show that the somewhat darker regions of Fig. ld are covered with the d-(7 x 7) reconstruction which will be discussed below. After a full monolayer of Sb has been deposited on the surface, the pits are much smaller than the largest islands with a typical pit diameter of ~ 10 nm. These pits are also 0.3 nm deep and have the same surface structure as the terraces. Atomically resolved images for various Sb coverages are given in Figs. 2a-2g. At 0.075 ML of Sb (Fig. 2a) the basic (7 x 7) pattern is clearly visible, but the " is a difference in contrast between "adatoms" in the (7 x 7) unit cell. A few "adatoms" are notably brighter than the rest. In corresponding images taken with the tip biased to - 2 . 0 V, the ratio of bright "adatoms" to dim "adatoms" is inverted. This is in agreement with a previous STM study in which it was concluded that the bright "adatoms" correspond to Sb adatoms which have
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Fig. 1. A series of STM images showing the evolution of the Si(lll) surface as the coverage of Sb increases. (a) 0.05 ML of Sb (2 ~m x 2 gm). (b) 0.08 ML of Sb (2 #m x 2 #m). (c) 0.15 ML of Sb (2 pm x 2 #m). (d) 0.5 ML of Sb (400 nm x 400 nm). (e) 1.0 ML of Sb (400 nm x 400 nm). displaced Si atoms within the (7 x 7) DAS structure [ 9 ] . This conclusion is strengthened by the observation that the n u m b e r of bright spots increases as a function of coverage. Fig. 2b shows the surface
at a coverage of 0.15 ML. The u n d e r l y i n g (7 x 7) structure is still a p p a r e n t b u t the r a n d o m l y located Sb atoms are progressively disordering the surface. The arrow in Fig. 2b points to a small region of
R.G. Ryland et al./Surface Science 345 (1996) 222-234
(e) Fig. 1 (continuedl.
(x~ x x/3) periodicity. This appears to correspond to the 1/3 ML monomer (v/3 x x/3) described by Elswijk et al. [9]. A diffuse ( 7 x 7 ) LEED pattern has been observed in this range of coverages by Park et al. [8]. Our LEED observations showed a very high background in the LEED pattern, but our apparatus was not suitable for measuring the diffraction spot profiles. In theory, since the atomic replacement observed in the STM images does not affect the domain size of the underlying (7 x 7) periodicity, it should have no effect on the width of the profile for the seventh-order beams. Thus, the surface is more properly described as disordered as opposed to diffuse, and for the purposes of this paper shall be denoted as d-(7 x 7). This does not contradict the Park paper, however, and we believe that within a single domain our surfaces are the same. In their study a full monolayer of Sb was deposited and lower coverages were reached by desorbing Sb at various temperatures. It is reasonable to assume that the process of going from a higher coverage reconstruction to a (7 x 7) reconstruction involves nucleating (7 x 7) regions. This could lead to small domains of (7 x 7), especially in light of the reduced silicon diffusion length
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discussed below, which would result in a truly diffuse LEED pattern. As the Sb coverage is increased further we begin to see areas of a different (,,/-3 x ,v/3) reconstruction form. Initially these (x/3x x/~) regions nucleate preferentially at step and island edges and at domain boundaries in the existing (7 x 7) pattern but quickly begin nucleating on the terraces as well. At a coverage of 0.375 ML (Fig. 2c) the surface is covered by approximately equal areas of (,f3 x ~/3) and d-(7 x 7) reconstructions. The arrow in Fig. 2c points out one of the many small patches of (2 x 1) reconstruction visible on this surface. The (v/3 x V~) regions are composed of Sb trimers as described in the literature [9,15-17]. Although not routinely achievable, occasional tip conditions allowed resolution capable of distinguishing the individual Sb atoms that make up the trimers. One such image, taken at a Sb coverage of 0.5 ML, is presented in Fig. 2d. In addition to the Sb trimers, a single strand of the (2 x 1) reconstruction is visible. The zigzag pattern of Sb atoms in the (2 x 1) strand is consistent with the model proposed by Elswijk et al. Several instances of "reversed" trimers can be seen in Fig. 2d 2. If the majority of the trimers are centered over the T 4 sites [16,17] then the relative position of the reversed trimers indicates that they are centered over the H 3 sites. The second arrow points out a location where three trimers (two "reversed") are arranged such that the atomic positions are very close to that of a (2 x 1) strand. This may be an intermediate step in the conversion from the 1 ML trimer (xf3 x xf3) to the 1 ME three-phase (2 x 1) reconstruction. In Fig. 2e the coverage is 0.5 ML and the surface is predominately covered by the (v/3 x v/3) reconstruction. The pits have begun to form in the remaining areas of d-(7 x 7). Although a tip effect gives Fig. 2f an odd contrast, the image shows that after 20 rain of deposition the surface is completely covered with the (x~ x x/3) reconstruction. The (2 x 1) reconstruction is almost completely absent in images from both of these samples. Aside from the small patches noted earlier, we 2Observation of reversed trimers is mentioned in a footnote of Ref. [16].
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R.G. Ryland et al./Surface Science 345 (1996) 222-234
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Fig. 2. Atomically resolved images of the Si surface at various Sb coverages. (a) 0.075 ML of Sb (20 nm x 20 nm, Vtlp= +2.5 V, I t = 0.75 nA). (b) 0.15 ML of Sb (20 n m × 20 nm, Vtip= + 3 V, I t =0.85 nA). (c) 0.375 ML of Sb (40 nm x 40 n m , Vtip = - 2 . 1 V , / t = 0 . 7 nA). (d) 0.5 ML of Sb (10 nm x 10 nm, Vtip= -- 1.25 V, It=0.22 nA). Arrows point out a single (2 × 1) chain and a curious arrangement of trimers. (e) 0.5 ML of Sb (40 nm × 4 0 n m , V t i p - - 2 V, It=0.4 hA). (f) 1.0 ML of Sb (20nm x 20 nm, Vtip~ - l V, I t - 0 . 3 hA). (g) (2 x 1) reconstruction obtained by depositing Sb on the clean (7 x 7) surface at room temperature followed by annealing at 600°C (20 nm x 20 nm, Vtip= +2.7 V, It =0.6 nA).
R.G. Ryland et al./Surface Science 345 (1996) 222-234
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(f)
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Ig) Fig. 2 (continued). observed very little of the (2 × 1) phase during Sb deposition. LEED observations show the surface going from (7 x 7) to d-(7 × 7) to (x~ x x/3) as the Sb coverage increases. Annealing the sample at 600°C after the deposition, however, leads to a (2 x2) LEED pattern just as reported by Park
et al. [8]. Fig. 2g shows an STM image of the surface after 10 min of deposition at room temperature and a 10 min anneal at 600°C. The surface is completely covered by the three-phase (2× 1) reconstruction, which would give a (2 × 2) LEED pattern. (The deposition rate calibration of 0.05
R.G. Ryland et aL /Surface Science 345 (1996) 222-234
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(a)
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Fig. 3. (a) An STM image showing both an island and the underlying terrace at 0.125 ML of Sb coverage (40 nm × 40 nm, Vtip= +2.8 V, It=0.9 nA). (b) a 4 #m x 4 #m image showing that the islands remain after the Sb is desorbed by annealing at 700°C.
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Fig. 4. Plot of the area covered by islands (O, left-hand axis) as a function of the Sb coverage as determined by deposition time. The coverage of islands is estimated from micron-size STM images such as those in Figs. l ~ l c . The expected saturation value for the area covered by islands is shown by the dashed line. Sb coverage (A, right-hand axis) as determined by counting atoms in atomically resolved STM images is also presented.
ML per min was determined for a substrate at 600°C and is not applicable at room temperature, but the (2 x 1) reconstruction indicates that the Sb
coverage is slightly less than 1 ML [8].) This is another instance in which the surface structure obtained for a given coverage of Sb depends on
R.G. Ryland et al./Surface Science 345 (1996) 222-234
the manner in which that coverage was reached, in this case either by initially depositing the desired coverage or by depositing a full monolayer and then desorbing Sb. Fig. 3a is an atomically resolved image showing both an island and the underlying terrace at 0.125 M L of Sb. It clearly shows that the island has the same surface structure as the terrace. In fact a careful examination of the image shows that it has the d-(7 × 7) reconstruction mentioned previously. This leads to the conclusion that these islands are in fact silicon islands. The only way the surface structure could be the same on both the islands and the terrace is if they are Sb-terminated Si islands or if both the islands and the terrace have multiple layers of Sb. Since a second layer of Sb will desorb at the temperature of deposition [-13] and less than a full monolayer of Sb was deposited, we can rule out the second possibility. This conclusion is confirmed by the observation that the islands remain on the surface even after all the Sb has been desorbed from the surface by heating to 700°C for 5 min as shown in Fig. 3b. Atomically resolved images of the annealed surface show a (7 × 7) pattern with a large number of atomic vacancies. The measured height of the islands is ~0.3 nm, which would indicate the Si islands are of bi-layer height.
4. Discussion The formation of the islands can be explained in terms of the changes in the silicon surface density. During the formation of the d-(7×7) reconstruction, Sb atoms replace the Si "adatoms" in the dimer adatom stacking fault (DAS) (7 x 7) structure. This leaves an excess of Si atoms on the surface. In the absence of steps to act as sinks for the displaced Si atoms, the displaced silicon has no choice but to form islands. As the coverage of Sb increases, the amount of displaced Si also increases, so that the total surface area covered by islands increases. A simple model of the (7 × 7) to d-(7 × 7) conversion would have each Sb atom replace one Si atom until all 12 "adatoms" in the (7 × 7) unit cell have been replaced. If there is a one-to-one ratio of
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deposited Sb atoms to displaced Si atoms, then we would expect the total area covered by islands to increase linearly with the Sb coverage. The island area would then saturate when all 12 adatoms have been replaced. At this point 12/49 = 0.245 ML of Si has been displaced. Since the islands formed are of bi-layer height, they should then cover 0.245/2 = 12% of the surface. Fig. 4 depicts a plot of the area of the surface covered by islands as a function of the Sb coverage. The island area is given as a percentage of the terrace area and was measured in the center of the terraces in an effort to avoid the effects of the denuded zones. The solid line indicates the prediction of the simple model given above. Clearly the prediction fits the measured data well at low coverages and agrees qualitatively at higher coverages. Two notable quantitative deviations can be seen in Fig. 4. The first is that the island area seems to increase non-linearly and is larger than expected in the 0.1-0.25 M L range. The second is that the saturation value for the island coverage is slightly higher than predicted. The first deviation would seem to indicate that at coverages greater than 0.1 ML Sb atoms are displacing more than one Si atom on average. In a closer examination of atomic resolution images, it becomes apparent that this is exactly what is happening. Fig. 5 shows a close up of the surface at a Sb coverage of 0.15 ML. The 20 protrusions within three of the (7 x 7) unit cells have been highlighted. Although distinguishing Sb atoms from Si atoms becomes difficult as the coverage of Sb exceeds one or two atoms per (7 × 7) unit cell, three of the protrusions are somewhat dimmer than the rest and are therefore most probably associated with Si atoms. This would indicate that 17 Sb atoms have displaced (3 unit cells x 12 adatoms per unit cell - 3)=33 Si atoms. This is fairly characteristic of the surface as a whole and indicates that in this coverage range Sb atoms are in fact displacing multiple Si atoms, thus creating the deviation from linearity in the beginning of Fig. 4. The one-to-one displacement of Si atoms by Sb can be qualitatively explained by counting the Si dangling bonds. A Sb atom that displaces a Si atom in the (7 × 7) DAS structure will be bonded
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R.G. Ryland et al./Surface Science 345 (1996) 222-234
Fig. 5. Three (7 x 7) unit cells are outlined and the adatoms are highlighted (Vtip= + 3 V, It =0.85 nA). Only 20 of the 36 adatom sites are occupied. The brighter atoms are Sb and the dimmer atoms are the remaining Si atoms.
to three underlying Si atoms. Since an Sb atom has 5 valence electrons compared to the 4 of a Si atom, this leaves two valence electrons left over, which will form a lone-pair electron state extending out from the surface. This explains why the Sb atoms appear bright when tunneling from filled states. It is to be expected that this lone-pair electron state has a lower energy than the dangling bond that the displaced Si atom had. The driving force for the displacement of multiple Si atoms is not clear, however. The observed island area saturates at ~15%, indicating ~ 0.30 M L of silicon has been displaced. This would correspond to the 12 adatoms plus an average of 3 additional atoms per (7 x 7) unit cell being displaced by Sb atoms. It is possible that these additional Si atoms come from the displacement of the second layer of Si atoms (the rest atoms) by Sb atoms. Such displacement would alter the binding configuration for the adatom layer and could account for the displacement of multiple Si adatoms described above. No direct confirmation of this is available, however, and the
scatter of the measured island coverages does not allow a definitive conclusion. The scatter in the measured island coverages has several sources. One source of scatter is errors in determining the Sb coverage. The coverage in the horizontal axis of Fig. 4 was determined from the time of deposition, with the current through the evaporator held constant. This allows variations in the temperature of the evaporator source to translate directly into error in the Sb coverage. As pointed out previously, however, the individual Sb atoms are distinguishable from the Si atoms in atomically resolved images at low coverage. This allows the determination of Sb coverage by simply counting the number of Sb atoms in a given number of (7 x 7) unit cells. Sb coverage as determined in this manner is also shown in Fig. 4. The Sb coverage as determined by counting is in reasonably good agreement with the estimates derived from the deposition time. In any event, errors in the determination of Sb coverage should not cause scatter in the measured island coverage after it has saturated, as this error appears in the horizontal axis in Fig. 4. To a large degree, the scatter in the measured island coverages is due to the denuded zone. In Fig. la it is apparent that the denuded zone for the island nucleation is comparable to half the terrace width. Accounting for the denuded zone in determining the fractional area covered by the islands becomes very difficult in this case. While finding the total area covered by the islands is easy, determining the area of the surface for which displaced atoms are incorporated into the islands rather than the steps is somewhat problematic. Underestimating the size of this "watershed" area will lead to an overestimation of the fractional area covered by the islands and vice versa. Even in the higher coverage images where the denuded zone is much smaller than the terrace width, the surface has gone through intermediate stages with a large denuded zone, so that the problem remains. As has already been noted, the diffusion length for the displaced Si atoms is comparatively large at low coverages of Sb, resulting in large islands and large denuded zones. As the coverage of Sb increases, the diffusion length for Si rapidly decreases, giving rise to smaller islands with smaller
R.G. Ryland et aL /Surface Science 345 (1996) 222-234
denuded zones in addition to the large islands formed at low coverages. Although the critical terrace width for the formation of the large islands is very large, the smaller islands also form on very small terraces within the step bunches. The width of the denuded zone and the average inter-island separation is given in Fig. 6. At the lowest coverage observed, the denuded zone width is ~ 3 5 0 n m , which is comparable to the ~ 150 nm measured for Si homoepitaxy at 600°C I-2,3]. This is consistent with the model in which a Si atom is displaced and left free to diffuse on the surface. At a coverage of 0.20 ML islands are observed nucleating near and even at the steps indicating a greatly decreased diffusion length. This is consistent with the results of Voigtl~inder and Zinner where it was found that a full monolayer of deposited Sb reduces the diffusion length for Si homoepitaxy by almost two orders of magnitude [2].
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There are several possible reasons for the observed decrease in diffusion length. It would not be at all unreasonable for the surface diffusion coefficient to actually be decreasing. The clean Si surface assumes the (7 x 7) reconstruction because the (7 x 7) reconstruction requires the least surface free energy and is therefore the least reactive to Si. Any small displacement from this would then be expected to be somewhat more reactive to Si, providing a deeper potential well for the Si atom and leading to a smaller diffusion coefficient. In addition, in instances where an Sb atom has displaced more than one Si atom from the (7 x 7) pattern, a comparatively deep potential well could be expected in the vacant site, also leading to a low diffusion constant. For the same reasons, the size of the critical nucleus for island formation could decrease with increasing Sb coverage, which would also decrease the effective diffusion length
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[18]. There is the additional possibility that the presence of Sb bound to the step edge would change the "sticking coefficient" for adatoms attaching to the step, which could reduce the denuded zone and island size without a corresponding change in the diffusion coefficient. The processes involved in the formation of pits can be understood in terms of the changing Si surface densities associated with the various reconstructions. Several groups have proposed models for the structure of the Sb-induced (2 x 1) and ( f i x x/3) reconstructions [9,15-17]. These models all have the Sb atoms placed on the (1 x 1) bulk-terminated silicon surface so that the Si surface density is the same as that for the bulkterminated surface, which is different from that for either the clean (7 x 7) or the Sb-induced d-(7 x 7) reconstructions. The bulk-terminated surface has a surface density of 98 atoms in the top bi-layer of a (7 x 7) unit cell. The clean (7 x 7) DAS reconstruction has 102 atoms in the same area [19]. At saturation the d - ( 7 x T ) reconstruction contains 1 0 2 - 1 2 = 9 0 atoms in the same area. So the process of going from the saturated d-(7 x 7) reconstruction to the (x/-3 x x/3) reconstruction requires an additional 8 atoms per (7 x 7) unit cell. This corresponds to 8/49=0.16 ML of silicon. In the absence of nearby steps or islands to act as sources for Si atoms, the surface forms pits to provide the required Si atoms. This would predict that the percentage of the surface covered by pits should reach a maximum of 8% at 1.0 ML of Sb coverage. This is remarkably close to the 8.2% actually observed. If we perform the same calculation using the saturated d - ( 7 x 7 ) silicon surface density derived from the observed island coverage (i.e. with 15 rather than 12 displaced Si atoms per unit cell yielding a Si surface density of 87 atoms per (7 x 7) unit cell) we arrive at a prediction of 11% pit coverage. The accuracy of the first prediction may indicate that the unexpectedly high saturated island coverage is in fact due to the difficulties in accounting for the large denuded zone rather than the displacement of rest atoms. One might still question why the pits must form, since the islands cover a greater percentage of the surface than the pits do and therefore contain more than enough silicon to fill them in. This is because the diffusion length for Si adatoms is much smaller
at this Sb coverage than it was during the formation of the islands [2]. The process of pit formation is as follows: The (x/3×x/3) reconstruction nucleates and then encroaches on the d-(7×7) regions ( Fig. 2c) until the surface is mostly (,,/-3 x x/3) with small patches of d-(7 × 7). While this is happening the silicon density within the remaining d-(7 × 7) regions is going down because they are providing the additional silicon required for the d - ( 7 x 7 ) t o (x/3xx/3)conversion. When the remaining d-(7 x 7) areas can provide no more Si atoms, pits begin to form in the shrinking d-(7 × 7) regions (Fig. 2e). The fact that the pits provide Si atoms to areas away from the pits implies that Si atoms must have a reasonable probability of spontaneously detaching from the pit edges, leading to a finite density of mobile Si adatoms. These free adatoms can then diffuse over the intervening d-(7 x 7) to be incorporated under the expanding (x/3 x x/J) areas. Examination of Fig. le shows that there is a denuded zone of ~ 20 nm around the islands where pits are not formed. Relating this denuded zone to the diffusion length of silicon is somewhat complicated by the fact that, as discussed above, the pits do not nucleate in the normal sense of the term. The location of the pits is to a large degree determined by where the (x/3xx~J) nucleates. Additionally, one would expect the diffusion length for Si to be different depending on whether it is diffusing over the (x~ x x~) or the d-(7 x 7) reconstruction. These complications show themselves in the fact that the distances between the pits is much smaller than the pit denuded zone width. Inter-pit distances of less than 5 nm are common. The pit denuded zone width and inter-pit distances still provides some information about the diffusion length. Since pit formation would be suppressed if silicon were readily available from an already existing pit, the inter-pit distance can be taken as an upper limit on the diffusion length. The larger pit denuded zone can be partially explained by the tendency for the (x/3 x x/3) reconstruction to initially nucleate preferentially at island edges. But this does not explain how the pit denuded zone can be larger than the upper limit determined above. That requires that the diffusion length over the d-(7 x 7) surface be longer than the diffusion length over the (,,/3x x/-3). When the (x/3x ,f3)
R. G. Ryland et aL/Surface Science 345 (1996) 222~34
g~,,... Fig. 7. Surface prepared by depositing Sb at room temperature for 10 min and then annealing to 600°C for 10 min. The islands cover ~ 7% of the surface. (400 nm x 400 nm).
first starts spreading out from the islands, it is being fed by silicon diffusing over the d-(7x 7) surface. Later, after the (x/3 x x/3) has begun nucleating on the terrace, pockets of d-(7 x 7) become separated by the (,f3 x ,fJ), and pits can then form in each. With this model for the process, the interpit distance can be taken as an upper limit for the diffusion length over the (x/3 x ~ ) surface, and the pit denuded zone can be taken as a lower bound for the diffusion length over the d-(7 x 7). The existence and even the coverages of both the islands and pits are well described in terms of the changes in the silicon surface density that accompanies the changes in the surface reconstructions. As a test of this understanding we looked at the case of a room temperature deposition followed by annealing. During room temperature deposition, the Sb atoms may not have the required activation energy to exchange with the Si adatoms. The resulting surface would then be the "clean" (7 x 7) buried under a layer of Sb. When the sample is subsequently heated, it may convert directly to the (x/-Jxx/3), largely bypassing the d - ( 7 x 7 ) reconstruction. As described earlier, an annealed trimer (,,/3 x xfJ) surface will convert to the (2 x 1)
233
reconstruction, but since both the (x/3 x x/3) and the (2 x 1) have the same silicon surface density, this does not affect the mass transport. Going from the clean (7 x 7) with a silicon surface density of 102 atoms per (7 x 7) unit cell to the (x/3 x x/3) or (2 x 1) with 98 atoms per unit cell leaves an excess o f ( 1 0 2 - 98)/49 = 0.08 M L of silicon on the surface. This would predict islands covering 4% of the surface with no pits. Fig. 7 shows the surface after the room temperature deposition of ~ 1 ML of Sb followed by a 10 min anneal at 600°C (described above). This can be directly compared to Fig. le where 1 ML of Sb was deposited at 600°C. The area covered by the islands in Fig. 7 is ~7%, in reasonable agreement with the predicted value of 4%. It is suspected that the difference between the predicted and measured values is due to atomic vacancies beneath the Sb overlayer, but this cannot be verified. In conclusion, we have presented STM results demonstrating directly that the formation of Sb-induced structures leads to an imbalance of surface Si atom density. The formation of Si islands originates from an excess of Si atoms caused by the transition from the clean (7 x 7) to the disordered (7 x7)-Sb. The formation of pits occurs during the transition from the d - ( 7 x 7 ) to the (x/3 x x/3) reconstruction. Pits do not form if the d-(7 x 7) structure is skipped in the transition from the clean ( 7 x 7 ) t o the (x/-3 x x/-3)and/or ( 2 x 1). The deposited Sb reduces the diffusion length for Si on the surface, which allows islands to form on even very small terraces and prevents the islands from dissolving to fill in the pits. This leads to an interface roughness of ~0.6 nm in height that is not completely reversed as the Sb is removed. Subsequent processes will occur on a rough substrate, which is likely to lead to a rough interface during growth. The formation of atomically flat interfaces therefore requires devising surfactant and deposition procedures which minimize the formation of intermediate structures that have large density differences.
Acknowledgement This work has been supported by the Office of Naval Research.
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R. G Ryland et al./Surface Science 345 (1996) 222-234
References [1] M. Copel, M.C. Reuter, E. Kaxiras and R.M. Tromp, Phys. Rev. Lett. 63 (1989) 632. [2] B. Voigtl~tnder and A. Zinner, Surf. Sci. 292 (1993) L775.B. Voigtlfinder, A. Zinner, T. Weber and H. P. Bonzel, Phys. Rev. B 51 (1995) 7583. [3] H. Nakahara and M. Ichikawa, Surf. Sci. 298 (1993) 440. [4] M. Horn-von Hoegen, M. Copel, J.C. Tsang, M.C. Reuter and R.M. Tromp, Phys. Rev. B 50 (1994) 10811. I-5] I. Markov, Phys. Rev. B 50 (1994) 11271. [6] M.A. Olmstead, R.D. Bringans, R.I.G. Uhrberg and R.Z. Bachrach, Phys. Rev. B 34 (1986) 6041. [7] R.S. Becker, B.S. Swartzentruber, J.S. Vickers, M.S. Hybertsen and S.G.Louie, Phys. Rev. Lett. 60 (1988) 116. [8] C.-Y. Park, T. Abukawa, T. Kinoshita, Y. Enta and S. Kono, Jpn. J. Appl. Phys. 27 (1988) 147. [9] H.B. Elswijk, D. Dijkkamp and E.J. van Loenen, Phys. Rev. B 44 (1991) 3802.
[ 10] Y. Homma, R. J. Mcclelland and H. Hibino, Jpn. J. Appl. Phys. 29 (1990) L2254. [11] Y.-N. Yang and E.D. Williams (1995) submitted. [12] Y.-N. Yang and E.D. Williams, Phys. Rev. Lett. 72 (1994) 1862. [13] R.A. Metzger and F.G. Allen, Surf. Sci. 137 (1984) 397. [ 14] J. Villian, A. Pimpinelli and D. Wolf, Comments Condens. Mater. Phys. 16 (1992) 1. [15] T.A. Abukawa, C.-Y. Park and S. Kono, Surf. Sci. 201 (1988) L513. [16] P. Martensson, G. Meyer, N.M. Amer, E. Kaxiras and K.C. Pandey, Phys. Rev. B 42 (1990) 7230. [17] S. Nakatani, A. Saito, Y. Kuwahara, T. Takahashi, M. Aono and S. Kikuta, Jpn. J. Appl. Phys. 31 (1992) L426. [18] J. A. Venables, Surf. Sci. 299/300 (1994) 798. [ 19] K. Takayanagi, Y. Tanishiro, S. Takahashi and M. Takahashi, Surf. Sci. 164 (1985) 367.
Surface Science 345 (1996) 222-234
Silicon motion during antimony deposition on Si(111) Robert G. Ryland, Shigehiko Hasegawa 1, Ellen D. Williams * Department of Physics, University of Maryland, College Park, MD 20742-4111, USA Received 17 May 1995; accepted for publication 7 August 1995
Abstract We report on a scanning tunneling microscopy study of mass transport and the resulting morphological changes during Sb deposition on Si( 111 ). Formation of silicon islands is found to take place during the initial stages of Sb deposition, while additional Sb causes the formation of pits. Both observations are explained in terms of the changes in Si surface density caused by the various Sb-induced reconstructions. The islands preferentially nucleate away from step edges, leaving a denuded zone. Measuring the denuded zone width shows a decrease in the diffusion length of Si with increasing Sb coverage.
Keywords: Antimony; Low index single crystal surfaces; Scanning tunneling microscopy; Silicon; Surface diffusion; Surface structure
1. Introduction Technological interest in surfactant-mediated epitaxial growth has led to an increased interest in the adsorption of Group V metals such as Sb and As on Si [ 1-4]. A monolayer of such an adsorbed surfactant species can alter the growth mode on the substrate. Although several models for this behavior have been proposed, the detailed mechanisms involved are not yet well understood [ 1,2, 5]. It has been shown that adsorbed As atoms are substituted for Si atoms of the first S i ( l l l ) layer, leading to a simple (1 x 1) structure [1,6,7]. In contrast, low energy electron diffraction (LEED) studies have shown that Sb-terminated S i ( l l l ) surfaces exhibit a large variety of reconstructions
* Corresponding author. Fax: 301 314 9465; E-mail: [email protected]. 1 Present address: ISIR, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567, Japan. 0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0039-6028 ( 95 ) 00864-0
depending on Sb coverage and substrate temperature [8]. A recent scanning tunneling microscopy study has suggested that Sb atoms are substituted for Si atoms at low coverages [9]. Such replacement will cause mass transport due to the change of surface Si atom density, which can lead to interface roughening as the displaced atoms are accommodated on the surface. On normally prepared surfaces, step separations of 100 nm or less are common, thus it may be difficult to observe such mass transport directly since the steps act as good sources or sinks for displaced atoms. Using a novel technique for generating wide S i ( l l l ) terraces, we have observed direct evidence of mass transport originating from changes in the surface atom density during the formation of Sb-induced S i ( l l l ) reconstructions. This mass transport results in approximately 25% of the surface being covered with bi-layer deep pits or bi-layer height islands, a roughness which persists even after the Sb is subsequently removed. We also observe the
1~ G. Ryland et al./Surface Science 345 (1996) 222-234
223
effective diffusion length for the displaced Si atoms decrease very quickly with increasing Sb coverage,
defined as the number of sites on an ideal S i ( l l l ) surface; 7.85 x 1014 cm -2.
2. Experimental
3. Results
All experiments were performed in a UHV vacuum chamber with a background pressure below 5 x 10-11 Torr. The chamber was equipped with a scanning tunneling microscope (STM), Sb evaporation apparatus, and commercial LEEDAES optics. The STM has home-made hardware, semi-commercial electronics, and home-made control software. The STM uses a tube-type piezoelectric actuator capable of scanning areas greater than 4/~m x 4/~m. The Si samples were cut from nominally fiat S i ( l l l ) wafers. The wafers were phosphorus doped (n-type) with a resistivity of 2-10 ~.cm. The samples were thermally cleaned by flashing to 1250°C for several minutes. Such a thermal cleaning cycle removes any previously deposited Sb leaving a clean ( 7 x 7 ) surface, thereby allowing the use of a single sample for many experiments. By using a small sample size (3 mm × 15 mm), the background pressure during the thermal cleaning was generally kept below 3 x 10- lO Torr. The sample was heated by passing a DC current though the sample in the step bunching direction [ 10,11 ]. This provided very large terraces between the step bunches, allowing the observation of effects that do not occur on terraces of more typical widths [ 12]. The Sb was sublimated from 99.9999% pure elemental Sb pieces in a Ta boat. The low sublimation temperature of Sb allowed a background pressure below 5 x 10 -11 Torr during deposition. Except as otherwise noted, the temperature of the sample during Sb deposition was kept at 600°C as measured by an infrared pyrometer. This is well above the desorption temperature for a second monolayer of Sb on Si but low enough so that the first monolayer desorbs very slowly [13]. After deposition the samples were quickly cooled to room temperature by removing the dc current used to heat them. The deposition rate was estimated to be 0.05 ML/min from the deposition time required for obtaining a well defined ( x ~ x x/J) L E E D pattern with no traces of (7 x 7). 1 M L is
A series of 2 #m x 2 #m images showing the evolution of the surface with increasing Sb coverage are presented in Figs. l a - l c . The most prominent feature in these images is the presence of islands on the terraces with denuded zones [14] at both steps bounding the terrace. The total area covered by these islands increases as the Sb coverage increases. Initially, large islands form in the middle of the terraces. At higher Sb coverages we continue to see large islands but we also see the formation of smaller islands. At the same time the width of the denuded zone next to the steps and the islands decreases. Atomically resolved images of these islands show that they are approximately 0.3 nm in height and have the same surface structure as the terraces, as will be shown below. Pit formation can be seen in Fig. ld and Fig. le after a deposition time of 10 and 20 min, respectively. These pits begin to nucleate only after the deposition of ~0.40 M L of Sb and then grow as the coverage increases. The pits seem to form on the large islands and terraces with equal probability. A series of scans taken at progressively higher resolutions show that the somewhat darker regions of Fig. ld are covered with the d-(7 x 7) reconstruction which will be discussed below. After a full monolayer of Sb has been deposited on the surface, the pits are much smaller than the largest islands with a typical pit diameter of ~ 10 nm. These pits are also 0.3 nm deep and have the same surface structure as the terraces. Atomically resolved images for various Sb coverages are given in Figs. 2a-2g. At 0.075 ML of Sb (Fig. 2a) the basic (7 x 7) pattern is clearly visible, but the " is a difference in contrast between "adatoms" in the (7 x 7) unit cell. A few "adatoms" are notably brighter than the rest. In corresponding images taken with the tip biased to - 2 . 0 V, the ratio of bright "adatoms" to dim "adatoms" is inverted. This is in agreement with a previous STM study in which it was concluded that the bright "adatoms" correspond to Sb adatoms which have
224
R. G. Ryland et al,/Surface Science 345 (1996) 222-234
(a)
(b)
(c)
(d)
Fig. 1. A series of STM images showing the evolution of the Si(lll) surface as the coverage of Sb increases. (a) 0.05 ML of Sb (2 ~m x 2 gm). (b) 0.08 ML of Sb (2 #m x 2 #m). (c) 0.15 ML of Sb (2 pm x 2 #m). (d) 0.5 ML of Sb (400 nm x 400 nm). (e) 1.0 ML of Sb (400 nm x 400 nm). displaced Si atoms within the (7 x 7) DAS structure [ 9 ] . This conclusion is strengthened by the observation that the n u m b e r of bright spots increases as a function of coverage. Fig. 2b shows the surface
at a coverage of 0.15 ML. The u n d e r l y i n g (7 x 7) structure is still a p p a r e n t b u t the r a n d o m l y located Sb atoms are progressively disordering the surface. The arrow in Fig. 2b points to a small region of
R.G. Ryland et al./Surface Science 345 (1996) 222-234
(e) Fig. 1 (continuedl.
(x~ x x/3) periodicity. This appears to correspond to the 1/3 ML monomer (v/3 x x/3) described by Elswijk et al. [9]. A diffuse ( 7 x 7 ) LEED pattern has been observed in this range of coverages by Park et al. [8]. Our LEED observations showed a very high background in the LEED pattern, but our apparatus was not suitable for measuring the diffraction spot profiles. In theory, since the atomic replacement observed in the STM images does not affect the domain size of the underlying (7 x 7) periodicity, it should have no effect on the width of the profile for the seventh-order beams. Thus, the surface is more properly described as disordered as opposed to diffuse, and for the purposes of this paper shall be denoted as d-(7 x 7). This does not contradict the Park paper, however, and we believe that within a single domain our surfaces are the same. In their study a full monolayer of Sb was deposited and lower coverages were reached by desorbing Sb at various temperatures. It is reasonable to assume that the process of going from a higher coverage reconstruction to a (7 x 7) reconstruction involves nucleating (7 x 7) regions. This could lead to small domains of (7 x 7), especially in light of the reduced silicon diffusion length
225
discussed below, which would result in a truly diffuse LEED pattern. As the Sb coverage is increased further we begin to see areas of a different (,,/-3 x ,v/3) reconstruction form. Initially these (x/3x x/~) regions nucleate preferentially at step and island edges and at domain boundaries in the existing (7 x 7) pattern but quickly begin nucleating on the terraces as well. At a coverage of 0.375 ML (Fig. 2c) the surface is covered by approximately equal areas of (,f3 x ~/3) and d-(7 x 7) reconstructions. The arrow in Fig. 2c points out one of the many small patches of (2 x 1) reconstruction visible on this surface. The (v/3 x V~) regions are composed of Sb trimers as described in the literature [9,15-17]. Although not routinely achievable, occasional tip conditions allowed resolution capable of distinguishing the individual Sb atoms that make up the trimers. One such image, taken at a Sb coverage of 0.5 ML, is presented in Fig. 2d. In addition to the Sb trimers, a single strand of the (2 x 1) reconstruction is visible. The zigzag pattern of Sb atoms in the (2 x 1) strand is consistent with the model proposed by Elswijk et al. Several instances of "reversed" trimers can be seen in Fig. 2d 2. If the majority of the trimers are centered over the T 4 sites [16,17] then the relative position of the reversed trimers indicates that they are centered over the H 3 sites. The second arrow points out a location where three trimers (two "reversed") are arranged such that the atomic positions are very close to that of a (2 x 1) strand. This may be an intermediate step in the conversion from the 1 ML trimer (xf3 x xf3) to the 1 ME three-phase (2 x 1) reconstruction. In Fig. 2e the coverage is 0.5 ML and the surface is predominately covered by the (v/3 x v/3) reconstruction. The pits have begun to form in the remaining areas of d-(7 x 7). Although a tip effect gives Fig. 2f an odd contrast, the image shows that after 20 rain of deposition the surface is completely covered with the (x~ x x/3) reconstruction. The (2 x 1) reconstruction is almost completely absent in images from both of these samples. Aside from the small patches noted earlier, we 2Observation of reversed trimers is mentioned in a footnote of Ref. [16].
226
R.G. Ryland et al./Surface Science 345 (1996) 222-234
(a)
(b)
(c)
(d)
Fig. 2. Atomically resolved images of the Si surface at various Sb coverages. (a) 0.075 ML of Sb (20 nm x 20 nm, Vtlp= +2.5 V, I t = 0.75 nA). (b) 0.15 ML of Sb (20 n m × 20 nm, Vtip= + 3 V, I t =0.85 nA). (c) 0.375 ML of Sb (40 nm x 40 n m , Vtip = - 2 . 1 V , / t = 0 . 7 nA). (d) 0.5 ML of Sb (10 nm x 10 nm, Vtip= -- 1.25 V, It=0.22 nA). Arrows point out a single (2 × 1) chain and a curious arrangement of trimers. (e) 0.5 ML of Sb (40 nm × 4 0 n m , V t i p - - 2 V, It=0.4 hA). (f) 1.0 ML of Sb (20nm x 20 nm, Vtip~ - l V, I t - 0 . 3 hA). (g) (2 x 1) reconstruction obtained by depositing Sb on the clean (7 x 7) surface at room temperature followed by annealing at 600°C (20 nm x 20 nm, Vtip= +2.7 V, It =0.6 nA).
R.G. Ryland et al./Surface Science 345 (1996) 222-234
227
(f)
(e)
Ig) Fig. 2 (continued). observed very little of the (2 × 1) phase during Sb deposition. LEED observations show the surface going from (7 x 7) to d-(7 × 7) to (x~ x x/3) as the Sb coverage increases. Annealing the sample at 600°C after the deposition, however, leads to a (2 x2) LEED pattern just as reported by Park
et al. [8]. Fig. 2g shows an STM image of the surface after 10 min of deposition at room temperature and a 10 min anneal at 600°C. The surface is completely covered by the three-phase (2× 1) reconstruction, which would give a (2 × 2) LEED pattern. (The deposition rate calibration of 0.05
R.G. Ryland et aL /Surface Science 345 (1996) 222-234
228
(a)
(b)
Fig. 3. (a) An STM image showing both an island and the underlying terrace at 0.125 ML of Sb coverage (40 nm × 40 nm, Vtip= +2.8 V, It=0.9 nA). (b) a 4 #m x 4 #m image showing that the islands remain after the Sb is desorbed by annealing at 700°C.
,
0.5
0.4
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0.6
0.8
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Fig. 4. Plot of the area covered by islands (O, left-hand axis) as a function of the Sb coverage as determined by deposition time. The coverage of islands is estimated from micron-size STM images such as those in Figs. l ~ l c . The expected saturation value for the area covered by islands is shown by the dashed line. Sb coverage (A, right-hand axis) as determined by counting atoms in atomically resolved STM images is also presented.
ML per min was determined for a substrate at 600°C and is not applicable at room temperature, but the (2 x 1) reconstruction indicates that the Sb
coverage is slightly less than 1 ML [8].) This is another instance in which the surface structure obtained for a given coverage of Sb depends on
R.G. Ryland et al./Surface Science 345 (1996) 222-234
the manner in which that coverage was reached, in this case either by initially depositing the desired coverage or by depositing a full monolayer and then desorbing Sb. Fig. 3a is an atomically resolved image showing both an island and the underlying terrace at 0.125 M L of Sb. It clearly shows that the island has the same surface structure as the terrace. In fact a careful examination of the image shows that it has the d-(7 × 7) reconstruction mentioned previously. This leads to the conclusion that these islands are in fact silicon islands. The only way the surface structure could be the same on both the islands and the terrace is if they are Sb-terminated Si islands or if both the islands and the terrace have multiple layers of Sb. Since a second layer of Sb will desorb at the temperature of deposition [-13] and less than a full monolayer of Sb was deposited, we can rule out the second possibility. This conclusion is confirmed by the observation that the islands remain on the surface even after all the Sb has been desorbed from the surface by heating to 700°C for 5 min as shown in Fig. 3b. Atomically resolved images of the annealed surface show a (7 × 7) pattern with a large number of atomic vacancies. The measured height of the islands is ~0.3 nm, which would indicate the Si islands are of bi-layer height.
4. Discussion The formation of the islands can be explained in terms of the changes in the silicon surface density. During the formation of the d-(7×7) reconstruction, Sb atoms replace the Si "adatoms" in the dimer adatom stacking fault (DAS) (7 x 7) structure. This leaves an excess of Si atoms on the surface. In the absence of steps to act as sinks for the displaced Si atoms, the displaced silicon has no choice but to form islands. As the coverage of Sb increases, the amount of displaced Si also increases, so that the total surface area covered by islands increases. A simple model of the (7 × 7) to d-(7 × 7) conversion would have each Sb atom replace one Si atom until all 12 "adatoms" in the (7 × 7) unit cell have been replaced. If there is a one-to-one ratio of
229
deposited Sb atoms to displaced Si atoms, then we would expect the total area covered by islands to increase linearly with the Sb coverage. The island area would then saturate when all 12 adatoms have been replaced. At this point 12/49 = 0.245 ML of Si has been displaced. Since the islands formed are of bi-layer height, they should then cover 0.245/2 = 12% of the surface. Fig. 4 depicts a plot of the area of the surface covered by islands as a function of the Sb coverage. The island area is given as a percentage of the terrace area and was measured in the center of the terraces in an effort to avoid the effects of the denuded zones. The solid line indicates the prediction of the simple model given above. Clearly the prediction fits the measured data well at low coverages and agrees qualitatively at higher coverages. Two notable quantitative deviations can be seen in Fig. 4. The first is that the island area seems to increase non-linearly and is larger than expected in the 0.1-0.25 M L range. The second is that the saturation value for the island coverage is slightly higher than predicted. The first deviation would seem to indicate that at coverages greater than 0.1 ML Sb atoms are displacing more than one Si atom on average. In a closer examination of atomic resolution images, it becomes apparent that this is exactly what is happening. Fig. 5 shows a close up of the surface at a Sb coverage of 0.15 ML. The 20 protrusions within three of the (7 x 7) unit cells have been highlighted. Although distinguishing Sb atoms from Si atoms becomes difficult as the coverage of Sb exceeds one or two atoms per (7 × 7) unit cell, three of the protrusions are somewhat dimmer than the rest and are therefore most probably associated with Si atoms. This would indicate that 17 Sb atoms have displaced (3 unit cells x 12 adatoms per unit cell - 3)=33 Si atoms. This is fairly characteristic of the surface as a whole and indicates that in this coverage range Sb atoms are in fact displacing multiple Si atoms, thus creating the deviation from linearity in the beginning of Fig. 4. The one-to-one displacement of Si atoms by Sb can be qualitatively explained by counting the Si dangling bonds. A Sb atom that displaces a Si atom in the (7 × 7) DAS structure will be bonded
230
R.G. Ryland et al./Surface Science 345 (1996) 222-234
Fig. 5. Three (7 x 7) unit cells are outlined and the adatoms are highlighted (Vtip= + 3 V, It =0.85 nA). Only 20 of the 36 adatom sites are occupied. The brighter atoms are Sb and the dimmer atoms are the remaining Si atoms.
to three underlying Si atoms. Since an Sb atom has 5 valence electrons compared to the 4 of a Si atom, this leaves two valence electrons left over, which will form a lone-pair electron state extending out from the surface. This explains why the Sb atoms appear bright when tunneling from filled states. It is to be expected that this lone-pair electron state has a lower energy than the dangling bond that the displaced Si atom had. The driving force for the displacement of multiple Si atoms is not clear, however. The observed island area saturates at ~15%, indicating ~ 0.30 M L of silicon has been displaced. This would correspond to the 12 adatoms plus an average of 3 additional atoms per (7 x 7) unit cell being displaced by Sb atoms. It is possible that these additional Si atoms come from the displacement of the second layer of Si atoms (the rest atoms) by Sb atoms. Such displacement would alter the binding configuration for the adatom layer and could account for the displacement of multiple Si adatoms described above. No direct confirmation of this is available, however, and the
scatter of the measured island coverages does not allow a definitive conclusion. The scatter in the measured island coverages has several sources. One source of scatter is errors in determining the Sb coverage. The coverage in the horizontal axis of Fig. 4 was determined from the time of deposition, with the current through the evaporator held constant. This allows variations in the temperature of the evaporator source to translate directly into error in the Sb coverage. As pointed out previously, however, the individual Sb atoms are distinguishable from the Si atoms in atomically resolved images at low coverage. This allows the determination of Sb coverage by simply counting the number of Sb atoms in a given number of (7 x 7) unit cells. Sb coverage as determined in this manner is also shown in Fig. 4. The Sb coverage as determined by counting is in reasonably good agreement with the estimates derived from the deposition time. In any event, errors in the determination of Sb coverage should not cause scatter in the measured island coverage after it has saturated, as this error appears in the horizontal axis in Fig. 4. To a large degree, the scatter in the measured island coverages is due to the denuded zone. In Fig. la it is apparent that the denuded zone for the island nucleation is comparable to half the terrace width. Accounting for the denuded zone in determining the fractional area covered by the islands becomes very difficult in this case. While finding the total area covered by the islands is easy, determining the area of the surface for which displaced atoms are incorporated into the islands rather than the steps is somewhat problematic. Underestimating the size of this "watershed" area will lead to an overestimation of the fractional area covered by the islands and vice versa. Even in the higher coverage images where the denuded zone is much smaller than the terrace width, the surface has gone through intermediate stages with a large denuded zone, so that the problem remains. As has already been noted, the diffusion length for the displaced Si atoms is comparatively large at low coverages of Sb, resulting in large islands and large denuded zones. As the coverage of Sb increases, the diffusion length for Si rapidly decreases, giving rise to smaller islands with smaller
R.G. Ryland et aL /Surface Science 345 (1996) 222-234
denuded zones in addition to the large islands formed at low coverages. Although the critical terrace width for the formation of the large islands is very large, the smaller islands also form on very small terraces within the step bunches. The width of the denuded zone and the average inter-island separation is given in Fig. 6. At the lowest coverage observed, the denuded zone width is ~ 3 5 0 n m , which is comparable to the ~ 150 nm measured for Si homoepitaxy at 600°C I-2,3]. This is consistent with the model in which a Si atom is displaced and left free to diffuse on the surface. At a coverage of 0.20 ML islands are observed nucleating near and even at the steps indicating a greatly decreased diffusion length. This is consistent with the results of Voigtl~inder and Zinner where it was found that a full monolayer of deposited Sb reduces the diffusion length for Si homoepitaxy by almost two orders of magnitude [2].
231
There are several possible reasons for the observed decrease in diffusion length. It would not be at all unreasonable for the surface diffusion coefficient to actually be decreasing. The clean Si surface assumes the (7 x 7) reconstruction because the (7 x 7) reconstruction requires the least surface free energy and is therefore the least reactive to Si. Any small displacement from this would then be expected to be somewhat more reactive to Si, providing a deeper potential well for the Si atom and leading to a smaller diffusion coefficient. In addition, in instances where an Sb atom has displaced more than one Si atom from the (7 x 7) pattern, a comparatively deep potential well could be expected in the vacant site, also leading to a low diffusion constant. For the same reasons, the size of the critical nucleus for island formation could decrease with increasing Sb coverage, which would also decrease the effective diffusion length
Denuded zone width and Inter-island distance 5000 \ \\
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Sb c o v e r a g e (ML) Fig. 6. The denuded zone width (O) and the average distance between islands (&) are plotted against the Sb coverage. At coverages higher than 0.15 ML, islands can be seen nucleating on the step edge. The denuded zone width was determined by measuring the distance between the step edge and several of the closest islands which results in a slight underestimation of the denuded zone width. Standard deviations were generally on the order of 15%, but for each coverage measurements were taken on a single sample.
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[18]. There is the additional possibility that the presence of Sb bound to the step edge would change the "sticking coefficient" for adatoms attaching to the step, which could reduce the denuded zone and island size without a corresponding change in the diffusion coefficient. The processes involved in the formation of pits can be understood in terms of the changing Si surface densities associated with the various reconstructions. Several groups have proposed models for the structure of the Sb-induced (2 x 1) and ( f i x x/3) reconstructions [9,15-17]. These models all have the Sb atoms placed on the (1 x 1) bulk-terminated silicon surface so that the Si surface density is the same as that for the bulkterminated surface, which is different from that for either the clean (7 x 7) or the Sb-induced d-(7 x 7) reconstructions. The bulk-terminated surface has a surface density of 98 atoms in the top bi-layer of a (7 x 7) unit cell. The clean (7 x 7) DAS reconstruction has 102 atoms in the same area [19]. At saturation the d - ( 7 x T ) reconstruction contains 1 0 2 - 1 2 = 9 0 atoms in the same area. So the process of going from the saturated d-(7 x 7) reconstruction to the (x/-3 x x/3) reconstruction requires an additional 8 atoms per (7 x 7) unit cell. This corresponds to 8/49=0.16 ML of silicon. In the absence of nearby steps or islands to act as sources for Si atoms, the surface forms pits to provide the required Si atoms. This would predict that the percentage of the surface covered by pits should reach a maximum of 8% at 1.0 ML of Sb coverage. This is remarkably close to the 8.2% actually observed. If we perform the same calculation using the saturated d - ( 7 x 7 ) silicon surface density derived from the observed island coverage (i.e. with 15 rather than 12 displaced Si atoms per unit cell yielding a Si surface density of 87 atoms per (7 x 7) unit cell) we arrive at a prediction of 11% pit coverage. The accuracy of the first prediction may indicate that the unexpectedly high saturated island coverage is in fact due to the difficulties in accounting for the large denuded zone rather than the displacement of rest atoms. One might still question why the pits must form, since the islands cover a greater percentage of the surface than the pits do and therefore contain more than enough silicon to fill them in. This is because the diffusion length for Si adatoms is much smaller
at this Sb coverage than it was during the formation of the islands [2]. The process of pit formation is as follows: The (x/3×x/3) reconstruction nucleates and then encroaches on the d-(7×7) regions ( Fig. 2c) until the surface is mostly (,,/-3 x x/3) with small patches of d-(7 × 7). While this is happening the silicon density within the remaining d-(7 × 7) regions is going down because they are providing the additional silicon required for the d - ( 7 x 7 ) t o (x/3xx/3)conversion. When the remaining d-(7 x 7) areas can provide no more Si atoms, pits begin to form in the shrinking d-(7 × 7) regions (Fig. 2e). The fact that the pits provide Si atoms to areas away from the pits implies that Si atoms must have a reasonable probability of spontaneously detaching from the pit edges, leading to a finite density of mobile Si adatoms. These free adatoms can then diffuse over the intervening d-(7 x 7) to be incorporated under the expanding (x/3 x x/J) areas. Examination of Fig. le shows that there is a denuded zone of ~ 20 nm around the islands where pits are not formed. Relating this denuded zone to the diffusion length of silicon is somewhat complicated by the fact that, as discussed above, the pits do not nucleate in the normal sense of the term. The location of the pits is to a large degree determined by where the (x/3xx~J) nucleates. Additionally, one would expect the diffusion length for Si to be different depending on whether it is diffusing over the (x~ x x~) or the d-(7 x 7) reconstruction. These complications show themselves in the fact that the distances between the pits is much smaller than the pit denuded zone width. Inter-pit distances of less than 5 nm are common. The pit denuded zone width and inter-pit distances still provides some information about the diffusion length. Since pit formation would be suppressed if silicon were readily available from an already existing pit, the inter-pit distance can be taken as an upper limit on the diffusion length. The larger pit denuded zone can be partially explained by the tendency for the (x/3 x x/3) reconstruction to initially nucleate preferentially at island edges. But this does not explain how the pit denuded zone can be larger than the upper limit determined above. That requires that the diffusion length over the d-(7 x 7) surface be longer than the diffusion length over the (,,/3x x/-3). When the (x/3x ,f3)
R. G. Ryland et aL/Surface Science 345 (1996) 222~34
g~,,... Fig. 7. Surface prepared by depositing Sb at room temperature for 10 min and then annealing to 600°C for 10 min. The islands cover ~ 7% of the surface. (400 nm x 400 nm).
first starts spreading out from the islands, it is being fed by silicon diffusing over the d-(7x 7) surface. Later, after the (x/3 x x/3) has begun nucleating on the terrace, pockets of d-(7 x 7) become separated by the (,f3 x ,fJ), and pits can then form in each. With this model for the process, the interpit distance can be taken as an upper limit for the diffusion length over the (x/3 x ~ ) surface, and the pit denuded zone can be taken as a lower bound for the diffusion length over the d-(7 x 7). The existence and even the coverages of both the islands and pits are well described in terms of the changes in the silicon surface density that accompanies the changes in the surface reconstructions. As a test of this understanding we looked at the case of a room temperature deposition followed by annealing. During room temperature deposition, the Sb atoms may not have the required activation energy to exchange with the Si adatoms. The resulting surface would then be the "clean" (7 x 7) buried under a layer of Sb. When the sample is subsequently heated, it may convert directly to the (x/-Jxx/3), largely bypassing the d - ( 7 x 7 ) reconstruction. As described earlier, an annealed trimer (,,/3 x xfJ) surface will convert to the (2 x 1)
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reconstruction, but since both the (x/3 x x/3) and the (2 x 1) have the same silicon surface density, this does not affect the mass transport. Going from the clean (7 x 7) with a silicon surface density of 102 atoms per (7 x 7) unit cell to the (x/3 x x/3) or (2 x 1) with 98 atoms per unit cell leaves an excess o f ( 1 0 2 - 98)/49 = 0.08 M L of silicon on the surface. This would predict islands covering 4% of the surface with no pits. Fig. 7 shows the surface after the room temperature deposition of ~ 1 ML of Sb followed by a 10 min anneal at 600°C (described above). This can be directly compared to Fig. le where 1 ML of Sb was deposited at 600°C. The area covered by the islands in Fig. 7 is ~7%, in reasonable agreement with the predicted value of 4%. It is suspected that the difference between the predicted and measured values is due to atomic vacancies beneath the Sb overlayer, but this cannot be verified. In conclusion, we have presented STM results demonstrating directly that the formation of Sb-induced structures leads to an imbalance of surface Si atom density. The formation of Si islands originates from an excess of Si atoms caused by the transition from the clean (7 x 7) to the disordered (7 x7)-Sb. The formation of pits occurs during the transition from the d - ( 7 x 7 ) to the (x/3 x x/3) reconstruction. Pits do not form if the d-(7 x 7) structure is skipped in the transition from the clean ( 7 x 7 ) t o the (x/-3 x x/-3)and/or ( 2 x 1). The deposited Sb reduces the diffusion length for Si on the surface, which allows islands to form on even very small terraces and prevents the islands from dissolving to fill in the pits. This leads to an interface roughness of ~0.6 nm in height that is not completely reversed as the Sb is removed. Subsequent processes will occur on a rough substrate, which is likely to lead to a rough interface during growth. The formation of atomically flat interfaces therefore requires devising surfactant and deposition procedures which minimize the formation of intermediate structures that have large density differences.
Acknowledgement This work has been supported by the Office of Naval Research.
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