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A scanning tunneling microscopy study of PH3 adsorption on Si(1 1 1)-7 × 7 surfaces, P-segregation and thermal desorption
Surface Science 601 (2007) 1768–1774 www.elsevier.com/locate/susc
A scanning tunneling microscopy study of PH3 adsorption on Si(1 1 1)-7 · 7 surfaces, P-segregation and thermal desorption Jeong-Young Ji, T.-C. Shen
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Department of Physics, Utah State University, Logan, UT 84322, United States Received 19 August 2006; accepted for publication 9 February 2007 Available online 16 February 2007
Abstract PH3 adsorption on Si(1 1 1)-7 · 7 was studied after various exposures between 0.3 and 60 L at room temperature by means of scanning tunneling microscopy (STM). PH3-, PH2-, H-reacted, and unreacted adatoms can be identified by analyzing empty-state STM images at different sample biases. PHx-reacted rest-atoms can be observed in empty-state STM images if neighboring adatoms are hydrogen terminated. Most of the PH3 adsorbs dissociatively on the surface, generating H- and PH2-adsorbed rest-atom and adatom sites. Danglingbonds at rest-atom sites are more reactive than adatom sites and the faulted half of the 7 · 7 unit cell is more reactive than the unfaulted half. Center adatoms are overwhelmingly preferred over corner adatoms for PH2 adsorption. The saturation P coverage is 0.18 ML. Annealingpof ffiffiffi PH3-reacted 7 · 7 surfaces at 900 K generates disordered, partially P-covered surfaces, but dosing PH3 at 900 K forms P/ Si(1 1 1)-6 3 surfaces. Si deposition at 510 K leaves disorderedpclusters on the surface, which cannot be reordered by annealing up to ffiffiffi 800 K. However, annealing above 900 K recreates P/Si(1 1 1)-6 3 surfaces. Surface morphologies formed by sequential rapid thermal annealing are also presented. 2007 Elsevier B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Silicon; Phosphine; Phosphorous; Adsorption; Surface segregation; Desorption
1. Introduction Phosphine’s reaction with Si has been studied for surface science and technological reasons for about 40 years. In the last decade many precise surface science studies have been reported on this system. Recently phosphorous d-layers [1,2] and nano-wires [3,4] were fabricated in silicon by PH3 deposition on Si(0 0 1) surfaces at room temperature followed by low-temperature Si homoepitaxy. It is appealing to find out if similar embedded phosphorous nanostructures can be fabricated on the Si(1 1 1) surface. It was noted very early on that the 7 · 7 low-energy electron diffraction (LEED) pattern remains after room temperature (RT) exposure of PH3 on the Si(1 1 1)-7 · 7 surface until the adsorbed adlayer was heated to 770–
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Corresponding author. Tel.: +1 435 797 7852; fax: +1 435 797 2492. E-mail address: [email protected] (T.-C. Shen).
0039-6028/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.02.007
790 K [5]. A careful study by Wallace et al. revealed that at 120 K PH3 adsorbs on Si(1 1 1)-7 · 7 with a unity sticking coefficient up to a surface coverage of 0.20 ML (1 ML = 7.8 · 1014 cm2) and continues to a saturation coverage of 0.24 ML with a reducing sticking coefficient [6]. Temperature programmed desorption data suggested that only a small fraction of a monolayer of PH3 desorbs as PH3 (g). In addition, PH3 (g) was observed only at PH3 exposures greater than 0.19 ML. By considering the number of available dangling-bonds (DBs), these authors thus postulated that PH3 could be partially dissociated to PHx(a) + H(a) resulting in blockage of the available DB sites after initial uptake [6]. The dissociative adsorption scenario was established by high resolution electron energy loss spectroscopy of PH3 adsorption on 7 · 7 surfaces at 80 K [7]. Chen et al. found that both PH3 and PH2 species coexist at PH3 coverages above 0.19 ML [7]. For PH3 coverages below that, the predominant surface species is PH2 with very little or no PH3 [7]. As for the surface adsorption
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sites, an ultraviolet photoemission spectroscopy (UPS) study of PH3 adsorption at 100 K indicated that the restatom DB state is fully quenched while the adatom DB state remains virtually unaffected [8]. Both the dissociative nature of the PH3 and the site reactivity on the Si(1 1 1)-7 · 7 are similar to the reaction with NH3 [9]. At elevated temperatures, more reaction channels become available between PH3 and Si(1 1 1)-7 · 7. Heating the samples with PH3 adlayers adsorbed at low temperatures causes loosely bonded molecular PH3 to desorb first, peaking at 180 K [6]. Between 120 and 300 K, Auger electron spectroscopy shows gradual formation of Si–H bonds at the expense of P–H bonds [6]. Thermal dissociation of the strongly bonded PH2 species occurs between 450 and 500 K, followed by H2 desorption at 740 K and P2 desorption near 1000 K [7]. LEED patterns, however, change from 7 · 7 to 1 · 1 after annealing between 800 and 1000 K and recover to 7 · 7 after P2 desorption [8]. Exposing 7 · 7 surfaces to P at elevated temperatures generated a variety of LEED patterns in the literature. Based on ion-scattering spectroscopy and LEED data Bozso and Avouris proposed that P substitutes the outermost layer of a Si(1 1 1)-1 · 1 surface by PH3 exposures at 900 K [8]. They also pointed out that strong tensile stress for the P-terminated Si(1 1 1)-1 pffiffi·ffi 1 surface is expected. Earlier reports also observed 6 3 LEED pattern after PH3 exposure at T > 800 K [5,10,11]. This structure was settled clearly by the STM images obtained by Vitali et al., who found that after evaporating P2 from InP onto the 7 · 7 surfaces between 770 K and 920 K, somewhat irregularshaped hexagonal domains of 1 · 1 structure tessellate the whole surface [12]. They proposed that P atoms substitute the top Si atoms of the first unreconstructed double layer with 6% contracted spacing with respect to the Si–Si substrate. The P-terminated surface was shown to be inert against reactive gases [8,12]. Although a great body of knowledge has been obtained by previous works, atom-resolved images of PH3 adsorption and surface morphologies after PH3 exposure at various sample temperatures could further our understanding of the system. In this paper, we present our analysis of the empty-state STM images of PH3 adsorption on Si(1 1 1)-7 · 7 at RT after various PH3 exposures. The spatial distribution of the surface species clearly distinguishes the reactivity of various surface DBs. The bias-dependent apparent heights of different species and statistical analysis enable us to identify PH3-, PH2-, H-reacted, and bare DB sites. As a result, we can quantitatively determine P coverage from STM data. We also compare surfaces exposed to PH3 at RT followed by a 900 K annealing with those exposed to PH3 at 900 K. To p experiment with P d-layer forffiffiffi mation on the P/Si(1 1 1)-6 3 surfaces, we deposited Si over-layers at 510 K. We find P segregates readily to the surface after a short annealing at 900 K,psimilar to ffiffiffi As and Sb during Si homoepitaxy [13], and the 6 3 surface is recovered. Finally, we use rapid thermal pffiffiffiannealing (RTA) to investigate P desorption from the 6 3 surfaces.
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2. Experiment Samples were cut from P-doped Si(1 1 1) wafers with resistivity 0.1 X cm. After a standard RCA cleaning the samples were introduced into the ultra high vacuum (UHV) chamber. The 7 · 7 reconstruction of the surface was prepared by 6 h degassing at 920 K followed by a 1450 K flash for 15 s. The pressure was kept at less than 1 · 109 Torr during the flash, and the sample temperatures were measured by a pyrometer. PH3 exposure was controlled by a leak valve. The dosing tube was 2.5 cm away from the sample. During PH3 exposures, the ion gauge was turned off. The chamber pressure, usually set between 2 · 108 and 2 · 107 Torr, was measured by the ion pump current, which was calibrated by the ion gauge prior to dosing. After PH3 exposure, all the STM images were taken at RT with a tunneling current of 0.2 nA and various sample biases. Although we took both filled- and emptystate images, we use mainly empty-state images for adsorbate analysis, because the apparent height in empty-state images is uniform for all the bare 7 · 7 adatoms while the edge adatoms appear higher in the filled-state images [14]. 3. Results and discussion 3.1. Dissociation of PH3 on Si(1 1 1)-7 · 7 at room temperature After exposing Si(1 1 1)-7 · 7 to 10-Langmuirs (L) of PH3 at RT, empty-state STM images reveal that not all adatoms are at the same apparent height anymore. We can delineate four species on the surface by the images at different sample biases. At low sample bias (0.5 V), as depicted in Fig. 1a, species U and B appear higher than species A, and species H appears lower than A. As the bias increases to 1.0 V, species A appears about the same apparent height as species U and B, while H remains low (Fig. 1b). Further increasing the bias to 1.5 V, we see three levels of apparent height. As shown in Fig. 1c, species A and B are the highest protrusions, followed by species U and H. The threshold values for the ‘low’ and ‘high’ biases vary by tip condition, but the relative apparent heights for these species remain unchanged. Similar ordering of the apparent heights of the four species can also be observed in the filled-state images presented in Fig. 1d–f, albeit with less contrast. From our empty-state images of partially reacted Si(1 1 1)-7 · 7 with hydrogen, we conclude that a bare Si adatom appears higher than a H-reacted adatom, and the apparent height difference reduces as sample bias increases. The H-reacted sites appear dark at a sample bias of 0.5 V but become visible at 1.5 V. Thus, we assign the species U to unreacted DBs and H to H-reacted sites. The brighter protrusions (A and B) in Fig. 1c are assumed to be PHx (x = 2, 3) reacted sites. The PH species are not expected on the surface after PH3 exposure at RT, because PH2 dissociation was observed at 450–500 K [7]. To identify
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Fig. 1. STM images of 10-L PH3 dosed Si(1 1 1)-7 · 7. The depicted rhombus in each image represents the same unit cell and helps to identify the adatom locations. H represents a hydrogen-reacted site, U for an unreacted site, A and B for PH2- and PH3-reacted sites, respectively. The faulted (F) and unfaulted (U) half of a unit cell is labeled in (e).
species A and B, we find that at 0.5 V, the majority species A have lower apparent height than DB sites (U) while minority species B are as bright as DB sites (Fig. 1a). Since the predominant species is PH2 even for PH3 exposure at 80 K and PH3 desorption can be detected up to 550 K [7], we associate species A and B with PH2 and PH3, respectively. 3.2. The relative reactivity of the dangling-bond states on Si(1 1 1)-7 · 7 From the UPS study of PH3 adsorption on 7 · 7 [8], atom-resolved tunneling spectroscopy of NH3 adsorption on 7 · 7 [9], and a theoretical calculation of the binding energies of PH3 on Si adatoms and rest-atoms [15], it is expected that PH3 adsorbs preferentially on rest-atom sites to adatom sites. On a bare 7 · 7 surface each rest-atom is surrounded by three adatoms and is one atomic layer lower than the adatoms; hence they are usually invisible in STM. However, after PH3 exposure, we find that if the rest-atom is neighbored by at least two H-reacted adatoms (based on the adsorbate-species assignment described in the last section), then it becomes visible, as marked in Fig. 2. Comparing with the apparent height difference between the PHx- and H-reacted adatoms, we conclude that all rest-atoms in such an environment are PHx-reacted. It is likely that if PH2 bonds to a rest-atom, the dissociated H will bond to one of the neighboring adatoms. Note that the dissociative adsorption of PH3 on Si(1 1 1)-7 · 7 is apparently different from that of H2S where H bonds exclusively to the rest-atom sites [16].
Fig. 2. Part of Fig. 1c where PHx-reacted rest-atoms are marked by arrows. Half unit cells are depicted to help identify the locations of restatom sites. PHx-reacted rest-atoms are observed when nearby adatoms are H-reacted (darker in this image).
There are four inequivalent adatom sites in a 7 · 7 unit cell: faulted/unfaulted center sites and faulted/unfaulted corner sites. (The corner adatoms are the six adatoms next to the four corner holes in a 7 · 7 unit cell, and the remaining six adatoms are called center adatoms.) To elucidate
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of PH3-reacted with the surface is dissociated, and 98% and 80% of the dissociation occurs at the center adatom sites in the faulted and unfaulted half, respectively. 3.3. P-coverage of PH3 exposure to Si(1 1 1)-7 · 7 at room temperature The averaged population of the PH3, PH2, H and DB at faulted and unfaulted adatom sites per unit cell after each exposure is presented in Fig. 3. We note that the adatom DBs in the unfaulted half are about 1.5 times more than that in the faulted half at low coverages but the ratio is reduced to 1 after a 60-L exposure, suggesting that about an equal number of adatom DBs are reacted with PHx or H in each half of the unit cell. Detailed analysis, however, reveals a complex PH3 reaction kinetics involving the DBs at the adatoms sites and rest-atom sites in the two halves of the 7 · 7 unit cell. Between 0.3- and 60-L exposure, the H population in the faulted half is almost twice as prevalent as in the unfaulted half and both change little with dose. This behavior suggests that PH2 adsorption at rest-atom sites saturates very early on in both halves of the unit cell. If we assume that the dissociation products, PH2 and H, will bond to an adatom–rest-atom pair, the rest-atom sites in the faulted half could already be 94% occupied after a 0.3-L exposure but the unfaulted half is only 56% occupied. Hence, the unfaulted half has more room for further dissociative adsorption of PH3 than the faulted half, as is clearly shown in Fig. 3. The adatoms in the unfaulted half continue to react with PH3 and drive the dissociated H to bond with a neighboring rest-atom. The population of PH2 adsorbed adatom in the unfaulted side reaches 2.0 per unit cell after a 60-L exposure, about twice as many as that in the faulted half. If there are no available DBs nearby, the adsorbed PH3 cannot dissociate. Based on this assumption, we can understand why the PH3-reacted adatoms are 0.33 in the faulted half but only 5
'
Faulted
4
Adatom sites / unit cell
their different reactivities with PH3, we have analyzed our empty-state STM images of 0.3, 2, 10, and 60-L PH3 exposures. After a 0.3-L exposure, we find that about 2.8 adatom DBs in the faulted half are unreacted, compared to 4.3 in the unfaulted half. Hence, it appears that the faulted half of the Si-7 · 7 unit cell is more reactive toward PH3 than the unfaulted half, which is consistent with the behavior of many other adsorbates reported in the literature [16]. Prolonged exposure, however, steadily reduces the asymmetry of the reacted sites. After a 60-L exposure, the ratio of DB sites in the faulted to unfaulted half is 0.94. We attribute the higher reaction rate in the unfaulted half at higher coverages to more available DB sites, which will become clear after the discussion of adsorption kinetics at higher PHx coverage in Section 3.3. After a 0.3-L exposure, the average numbers of PH2-reacted adatoms in the faulted and unfaulted sides are about even, with 0.7 per half unit cell. However, this does not necessarily imply that the adatoms in the two sides have equal reactivity toward PH3. If we assume that any PH2 adsorption will create a Si–H bond, either at a neighboring adatom site or a rest-atom site in the same unit cell, we have to conclude that the large number of H-reacted adatom sites (2.1) in the faulted half is not from the PH2 adsorption at the adatom sites but from the PH2 adsorption at the rest-atom site. In contrast, only 1 H-reacted adatom site is found on the unfaulted half of a unit cell. Based on this analysis, we conclude that after a 0.3-L exposure, 2.1 PH2 adsorbs on faulted rest-atom sites, but only 1 PH2 adsorbs on unfaulted rest-atom sites. Thus, in the initial phase of PH3 adsorption, about 69% of the restatom DBs in the faulted half are bonded with PH2, compared to 32% in the unfaulted half. If we exclude the adatom DBs being occupied by the dissociated H from the PH2 adsorption at rest-atom sites and consider all the PH2- and PH3-reacted adatom sites, the adatom reaction rate toward PH3 in the faulted half is higher than the unfaulted half, 27% vs. 16%, respectively. Thus, we conclude that rest-atoms are more reactive toward PH3 than adatoms and the faulted half is more reactive than the unfaulted half. The reactivity difference between the corner and center adatom sites can also be understood from the adatom– rest-atom pair adsorption scenario. Although we do not observe a significant difference in the PH3 population between these two adatom sites, presumably because PH3 adsorption on the adatoms does not involve rest-atoms, PH2-reacted sites are found predominantly at the center adatom sites. At low coverages, the ratio of PH2-reacted center to corner sites is particularly large, 50 for the faulted half and 4 for the unfaulted half. Even after a 60-L exposure, the PH2-occupied center to corner ratio is still 9 for the faulted half and 8 for the unfaulted half. Furthermore, we note that after a 60-L exposure, the ratio of PH3/PH2 is 2 at corner sites in both halves, but is 1/4 and 1/8 at center sites in the faulted and unfaulted half, respectively. These observations suggest that initially 88%
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H PH2
PH3 DB
3 2 1 0 5
Unfaulted
4 3 2 1 0 4
6 8
2
1
4
6 8
2
4
6
10
PH3 Exposure (L) Fig. 3. Averaged population of adatom-reacted surface species per unit cell (determined from STM images) as a function of PH3 exposure.
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0.08 in the unfaulted half after a 0.3-L exposure. Because of the early saturation of the rest-atom sites in the faulted half, a PH3 molecule bonds with an adatom in the faulted half has much less chance to dissociate than that in the unfaulted half. The ratio of the faulted PH3 sites to unfaulted PH3 sites diminishes gradually to 0.6 after a 60-L exposure when the rest-atom sites are almost fully occupied in either half of the unit cell. To estimate the total P coverage by STM data, we have to include the estimated PHx population at rest-atom sites. After the required amount of PH2 and H at the rest-atom sites to balance their observed counterparts at the adatom sites, the remaining rest-atom sites could be either occupied by undissociated PH3 or remain as DBs. In addition, the upper bound of P coverage should include one PH3 adsorption at the corner hole per unit cell. By these assumptions, we estimate the total P coverage to be 4.9–7.2 per unit cell after a 0.3-L exposure and the coverage increases to 7.6–8.7 per unit cell after a 60-L exposure. Dividing the P atom per unit cell by 49 DBs, we obtain a PHx-saturation coverage of 0.16–0.18 ML. Based on our estimates, at saturation 2/3 of the rest-atoms are reacted with PH2 and 1/3 with H, and near 40% of the adatom reacted with PHx and 25% with H. Note that our estimated saturation coverage at RT is lower than the 0.24 ML at 120 K reported in Ref. [6], where the amount of PH3 flux intercepted by the Si(1 1 1) surface was used to obtain the surface coverage. 3.4. Annealing of PH3/Si(1 1 1)-7 · 7 and PH3 adsorption at 900 K Fig. 4 shows an empty-state image of Si(1 1 1)-7 · 7 after a 0.3-L PH3 exposure at room temperature followed by a 900 K annealing for 60 s. The 7 · 7 structure is mostly dis-
Fig. 4. Si(1 1 1)-7 · 7 after a 0.3-L PH3 exposure at room temperature followed by annealing at 900 K for 60 s. Two corner-holes are marked by arrows. Scale bar: 5 nm; sample bias: 2.0 V.
rupted as one can see local patches of 2 · 2 structure with a few corner holes. Since PH2 dissociates before 500 K, H desorbs before 780 K and P2 will not desorb until 1010 K [6], it is likely that the depressions are P-terminated sites and the protrusions are Si DBs. However, by counting the protrusions in Fig. 4, we find that the existing DBs are only 59% of the original adatoms prior to annealing. Apparently, many adatoms have been removed from the terrace during annealing. In fact, Fourier transformed infrared spectroscopy [17] and core-level photoemission spectroscopy [18] of annealing PH3 adlayers on Si(0 0 1)2 · 1 between 550 and 650 K suggest P will displace surface Si atoms to form Si–P heterodimers. If a similar reaction occurs on the 7 · 7 surface, we can speculate that annealing at 900 K will promote P atoms to displace the 7 · 7 adatoms and bond directly to the rest-atoms. The displaced adatoms could either diffuse to the step edge, or with sufficient adatoms on a terrace, they could form two-dimensional (2D) islands. We did not observe any island formation for annealing the RT saturated surfaces, perhaps because even at saturation coverage there were not enough displaced Si atoms on the terraces to form 2D islands. However, after a 1-L PH3 exposure at 900 K we did observe irregular 2D islands on large terraces (Fig. 5a). The existence of the denuded zone near the upper step edges suggests that the islands are Si adatoms ejected by P [19]. This could be understood by the fact that at 900 K, PH3 dissociates completely upon adsorption, but H desorbs instantly allowing more P–Si bond formation and Si ejection. Close-up images reveal that surfaces after a 1-L PH3 exposure are still disordered, similar to the surface in Fig. 4.
Fig. 5. (a) Si(1 1 1) surface after a 1-L PH3 exposure at 900 K. Scale bar: pffiffiffi 250 nm; sample bias: 2 V. (b) P/Si(1 1 1)-6 3 surface after a 30-L PH3 exposure at 870 K. Scale bar: 30 nm; bias: 2 V.
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With sufficient PH3 exposure (e.g., 30 L) at 870 K, the Si(1 1 1) is completely terminated by a layer of P atoms, aspffiffiffidepicted in Fig. 5b. The surface becomes pffiffiffi ð6 3 6 3ÞR30 reconstructed, identical to that formed by P2 exposure of Si(1 1 1) at 770–920 K [12]. The tensile stress introduced by P–Si bonds creates hexagonal domains on the surface. The displaced 7 · 7 adatoms form triangular 2D islands pointing in both ½ 1 1 2 and ½1 1 2 directions. Due to the P-termination and surface segregation (see next section), we expect that PH3 adsorption will be self-terminating after a coverage near 1 ML at 900 K. 3.5. Surface segregationpand ffiffiffi thermal desorption of P from P/Si(1 1 1)-6 3 surfaces Depositing pffiffiffi 1 nm of silicon at 510 K onto the P/Si(1 1 1)-6 3 surface results in a surface covered with small 3D clusters. In comparison, we find registered but short H-terminated Si dimer rows on the Si(0 0 1)-2 · 1 surface after silicon deposition at similar conditions [20]. Apparently, H segregates more easily than P. Subsequent annealing of the Si-clustered surface up to 800 K for 3 min does not change the surface morphology. pffiffiffi However, after annealing at 900 K for 3 min, the 6 3 structure reappears and the hexagonal domains congregate into a continuous network (Fig. 6).pffiffiffi P-desorption from the 6 3 surfaces apparently starts after a RTA at 950 K for 1 s. Fig. 7a shows the newly formed silicon DBs as bright protrusions. One should note the DB denuded zone near the upper step-edge and some consolidation of the top-layer 2D hexagonal islands. Continued RTA to 990 K creates an a/1 · 1, or adatom-covered 1 · 1 surface [21]. Similar surface morphology was
pffiffiffi Fig. 7. P/Si(1 1 1)-6 3 surface after rapid thermal annealing at: (a) 950 K, (b) 990 K, and (c) 1100 K. Scale bar: 30 nm; sample bias: 2.0 V.
also observed by P2 adsorption on 7 · 7 at 970 K [12]. Only a few large 2D islands remain on the terrace with sides along the underlying 1 · 1 lattice (Fig. 7b). These islands are dispersed to step edges after a subsequent RTA to 1000 K. It is interesting to note that no 7 · 7 structures were found after RTA up to 1070 K, near the 7 · 7 M 1 · 1 phase transition. The 7 · 7 domains appear to nucleate from the upper step edge after a RTA at 1100 K, as shown in Fig. 7c, consistent with an earlier high-temperature STM observation by Miki et al. [22]. 4. Conclusions pffiffiffi Fig. 6. Reappearance of the 6 3 surface after 1-nm Si deposition on the surface of Fig. 5b at 510 K followed by annealing at 900 K for 3 min. Scale bar: 50 nm; sample bias: 2 V. Inset: close-up image of the surface.
In conclusion, we have used the apparent height of atomic protrusions in empty-state STM images to identify
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surface species from PH3 exposures on Si(1 1 1)-7 · 7 at room temperature. The PH2-reacted adatom site appears as a small and dim protrusion at low biases but a bright protrusion at high biases. A PH3-reacted adatom site appears similar to an unreacted adatom site (DB) at low biases and a bright protrusion at high biases. The hydrogen-reacted adatom site appears as a dark spot at both biases. Based on these assignments and the assumption that the PH2 and H are dissociative products of PH3 adsorption, we conclude that dangling-bonds at faulted rest-atom sites are most reactive towards PH3 followed by unfaulted rest-atom sites, faulted and unfaulted adatoms sites. We do not observe a significant difference for PH3 adsorption between the corner and center adatom sites, but there is an overwhelming preference over the center adatom sties for PH2 adsorption. After an initial uptake of PH3, predominantly at the faulted rest-atom sites, the unfaulted-adatoms take the lead. Between 10- and 60-L exposures the increase of PH2- and PH3-reacted unfaulted adatom sites outnumber the faulted adatom sites by a factor of 3.6 and 2.9, respectively. From our observation and analysis, the rest-atoms cease to adsorb PH3 after the initial uptake, but continue to adsorb H until a complete saturation after 60 L of exposure. In comparison, about 36% of adatom DBs are still unreacted after a 60-L exposure. Assuming the corner holes are PH3-reacted, we estimate the saturation P coverage to be about 0.18 ML at room temperature exposure, corresponding to a reaction rate of 0.46 PH3/DB. At such coverage, 77% of the DBs on the Si(1 1 1)-7 · 7 are reacted with either PH3, PH2 or H. It is interesting to note that PH3 exposure leads to a saturated P/Si(1 1 1)-7 · 7 surface, but even at room temperature depositing P by P2 vapor makes the 7 · 7 structure unstable beyond a critical P coverage of 0.1 ML [12]. As for the adsorption kinetics, PH3 could adsorb at a DB site non-dissociatively or two DB sites dissociatively depending on the availability of an adatom–rest-atom pair. Based on the work of Vitali et al. [12], only adatom pairs are involved in the P2 adsorption on Si(1 1 1)-7 · 7. At room temperature P2 adsorption on Si cannot be observed in the empty-state [12], but a rich variety of the empty-state images of Si-x bonds can be distinguished in the PH3 adsorption. However, at temperatures beyond the H desorption temperature from the silicon surface (800 K), the reactions of PH3 and P2 with Si should be quite similar. Indeed we find that annealing a PH3 saturated 7 · 7 surface to 900 K results in a disordered surface similar to P2 deposition at 970 K; presumably some of the Si adatoms on the top double-layer are replaced by P atoms. The
pffiffiffi P/Si(1 1 1)-6 3 structure can be created with sufficient pffiffiffi PH3 exposure at 900 K. The P-termination on the 6 3 surfaces is so robust that Si deposition at lower temperatures (e.g., 510 K) pffiffiffi can only form 3D clusters on the surface. P/Si(1 1 1)-6 3 surfaces can be recovered by annealing the clustered surfaces to 900 K. Apparently P atoms segregate to the surface to terminate the newly formed 1 · 1 surface. Therefore, it is unlikely to create P d-layers in the h1 1 1i direction unless another surfactant is used [23]. Acknowledgement This work was supported by the National Science Foundation under Grant Number CCF-0404208. References [1] T.-C. Shen, J.-Y. Ji, M.A. Zudov, R.-R. Du, J.S. Kline, J.R. Tucker, Appl. Phys. Lett. 80 (2002) 1580. [2] L. Oberbeck, N.J. Curson, M.Y. Simmons, R. Brenner, A.R. Hamilton, S.R. Schofield, R.G. Clark, Appl. Phys. Lett. 81 (2002) 3197. [3] T.-C. Shen, J.S. Kline, T. Schenkel, S.J. Robinson, J.-Y. Ji, C. Yang, R.R. Du, J.R. Tucker, J. Vac. Sci. Technol. B 22 (2004) 3182. [4] F.J. Ruess, L. Oberbeck, M.Y. Simmons, K.E.J. Goh, A.R. Hamilton, T. Hallam, S.R. Schofield, N.J. Curson, R.G. Clark, Nanoletter 4 (2004) 1969. [5] A.J. van Bommel, J.E. Crombeen, Surf. Sci. 36 (1973) 773. [6] R.M. Wallace, P.A. Taylor, W.J. Choyke, J.T. Yates Jr., J. Appl. Phys. 68 (1990) 3669; P.A. Taylor, R.M. Wallace, W.J. Choyke, J.T. Yates Jr., Surf. Sci. 238 (1990) 1. [7] P.J. Chen, M.L. Colaianni, R.M. Wallace, J.T. Yates Jr., Surf. Sci. 244 (1991) 177. [8] F. Bozso, Ph. Avouris, Phys. Rev. B 43 (1991) R1847. [9] R. Wolkow, Ph. Avouris, Phys. Rev. Lett. 60 (1988) 1049; Ph. Avouris, R. Wolkow, Phys. Rev. B 39 (1989) 5091. [10] A.J. van Bommel, F. Meyer, Surf. Sci. 8 (1967) 381. [11] J.J. Lander, J. Morrison, J. Chem. Phys. 37 (1962) 729. [12] L. Vitali, M.G. Ramsey, F.P. Netzer, Phys. Rev. B 57 (1998) 15376. [13] M. Horn-von Hoegen, J. Falta, M. Copel, R.M. Tromp, Appl. Phys. Lett. 66 (1995) 487. [14] Z. Zhang, M.A. Kulakov, B. Bullemer, Surf. Sci. 375 (1997) 195. [15] P.-L. Cao, L.-Q. Lee, J.-J. Dai, R.-H. Zhou, J. Phys.: Condens. Matter 6 (1994) 6103. [16] M.A. Razaei, B.C. Stipe, W. Ho, J. Phys. Chem. B 102 (1998) 10941. [17] J. Shan, Y. Wang, R.J. Hamers, J. Phys. Chem. 100 (1996) 4961. [18] D.-S. Lin, T.-S. Ku, T.-J. Sheu, Surf. Sci. 424 (1999) 7. [19] Y.W. Mo, M.G. Lagally, Surf. Sci. 248 (1991) 313. [20] Jeong-Young Ji, T.-C. Shen, Phys. Rev. B 70 (2004) 115309. [21] R.M. Feenstra, M.A. Lutz, Surf. Sci. 243 (1991) 151. [22] K. Miki, Y. Morita, H. Tokumoto, T. Sato, M. Iwatsuki, M. Suzuki, T. Fukuda, Ultramicroscopy. 42–44 (1992) 851. [23] O.D. Dubon, P.G. Evans, J.F. Chervinsky, M.J. Aziz, F. Spaepen, J.A. Golovchenko, M.F. Chisholm, D.A. Muller, Appl. Phys. Lett. 78 (2001) 1505.
A scanning tunneling microscopy study of PH3 adsorption on Si(1 1 1)-7 · 7 surfaces, P-segregation and thermal desorption Jeong-Young Ji, T.-C. Shen
*
Department of Physics, Utah State University, Logan, UT 84322, United States Received 19 August 2006; accepted for publication 9 February 2007 Available online 16 February 2007
Abstract PH3 adsorption on Si(1 1 1)-7 · 7 was studied after various exposures between 0.3 and 60 L at room temperature by means of scanning tunneling microscopy (STM). PH3-, PH2-, H-reacted, and unreacted adatoms can be identified by analyzing empty-state STM images at different sample biases. PHx-reacted rest-atoms can be observed in empty-state STM images if neighboring adatoms are hydrogen terminated. Most of the PH3 adsorbs dissociatively on the surface, generating H- and PH2-adsorbed rest-atom and adatom sites. Danglingbonds at rest-atom sites are more reactive than adatom sites and the faulted half of the 7 · 7 unit cell is more reactive than the unfaulted half. Center adatoms are overwhelmingly preferred over corner adatoms for PH2 adsorption. The saturation P coverage is 0.18 ML. Annealingpof ffiffiffi PH3-reacted 7 · 7 surfaces at 900 K generates disordered, partially P-covered surfaces, but dosing PH3 at 900 K forms P/ Si(1 1 1)-6 3 surfaces. Si deposition at 510 K leaves disorderedpclusters on the surface, which cannot be reordered by annealing up to ffiffiffi 800 K. However, annealing above 900 K recreates P/Si(1 1 1)-6 3 surfaces. Surface morphologies formed by sequential rapid thermal annealing are also presented. 2007 Elsevier B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Silicon; Phosphine; Phosphorous; Adsorption; Surface segregation; Desorption
1. Introduction Phosphine’s reaction with Si has been studied for surface science and technological reasons for about 40 years. In the last decade many precise surface science studies have been reported on this system. Recently phosphorous d-layers [1,2] and nano-wires [3,4] were fabricated in silicon by PH3 deposition on Si(0 0 1) surfaces at room temperature followed by low-temperature Si homoepitaxy. It is appealing to find out if similar embedded phosphorous nanostructures can be fabricated on the Si(1 1 1) surface. It was noted very early on that the 7 · 7 low-energy electron diffraction (LEED) pattern remains after room temperature (RT) exposure of PH3 on the Si(1 1 1)-7 · 7 surface until the adsorbed adlayer was heated to 770–
*
Corresponding author. Tel.: +1 435 797 7852; fax: +1 435 797 2492. E-mail address: [email protected] (T.-C. Shen).
0039-6028/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.02.007
790 K [5]. A careful study by Wallace et al. revealed that at 120 K PH3 adsorbs on Si(1 1 1)-7 · 7 with a unity sticking coefficient up to a surface coverage of 0.20 ML (1 ML = 7.8 · 1014 cm2) and continues to a saturation coverage of 0.24 ML with a reducing sticking coefficient [6]. Temperature programmed desorption data suggested that only a small fraction of a monolayer of PH3 desorbs as PH3 (g). In addition, PH3 (g) was observed only at PH3 exposures greater than 0.19 ML. By considering the number of available dangling-bonds (DBs), these authors thus postulated that PH3 could be partially dissociated to PHx(a) + H(a) resulting in blockage of the available DB sites after initial uptake [6]. The dissociative adsorption scenario was established by high resolution electron energy loss spectroscopy of PH3 adsorption on 7 · 7 surfaces at 80 K [7]. Chen et al. found that both PH3 and PH2 species coexist at PH3 coverages above 0.19 ML [7]. For PH3 coverages below that, the predominant surface species is PH2 with very little or no PH3 [7]. As for the surface adsorption
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sites, an ultraviolet photoemission spectroscopy (UPS) study of PH3 adsorption at 100 K indicated that the restatom DB state is fully quenched while the adatom DB state remains virtually unaffected [8]. Both the dissociative nature of the PH3 and the site reactivity on the Si(1 1 1)-7 · 7 are similar to the reaction with NH3 [9]. At elevated temperatures, more reaction channels become available between PH3 and Si(1 1 1)-7 · 7. Heating the samples with PH3 adlayers adsorbed at low temperatures causes loosely bonded molecular PH3 to desorb first, peaking at 180 K [6]. Between 120 and 300 K, Auger electron spectroscopy shows gradual formation of Si–H bonds at the expense of P–H bonds [6]. Thermal dissociation of the strongly bonded PH2 species occurs between 450 and 500 K, followed by H2 desorption at 740 K and P2 desorption near 1000 K [7]. LEED patterns, however, change from 7 · 7 to 1 · 1 after annealing between 800 and 1000 K and recover to 7 · 7 after P2 desorption [8]. Exposing 7 · 7 surfaces to P at elevated temperatures generated a variety of LEED patterns in the literature. Based on ion-scattering spectroscopy and LEED data Bozso and Avouris proposed that P substitutes the outermost layer of a Si(1 1 1)-1 · 1 surface by PH3 exposures at 900 K [8]. They also pointed out that strong tensile stress for the P-terminated Si(1 1 1)-1 pffiffi·ffi 1 surface is expected. Earlier reports also observed 6 3 LEED pattern after PH3 exposure at T > 800 K [5,10,11]. This structure was settled clearly by the STM images obtained by Vitali et al., who found that after evaporating P2 from InP onto the 7 · 7 surfaces between 770 K and 920 K, somewhat irregularshaped hexagonal domains of 1 · 1 structure tessellate the whole surface [12]. They proposed that P atoms substitute the top Si atoms of the first unreconstructed double layer with 6% contracted spacing with respect to the Si–Si substrate. The P-terminated surface was shown to be inert against reactive gases [8,12]. Although a great body of knowledge has been obtained by previous works, atom-resolved images of PH3 adsorption and surface morphologies after PH3 exposure at various sample temperatures could further our understanding of the system. In this paper, we present our analysis of the empty-state STM images of PH3 adsorption on Si(1 1 1)-7 · 7 at RT after various PH3 exposures. The spatial distribution of the surface species clearly distinguishes the reactivity of various surface DBs. The bias-dependent apparent heights of different species and statistical analysis enable us to identify PH3-, PH2-, H-reacted, and bare DB sites. As a result, we can quantitatively determine P coverage from STM data. We also compare surfaces exposed to PH3 at RT followed by a 900 K annealing with those exposed to PH3 at 900 K. To p experiment with P d-layer forffiffiffi mation on the P/Si(1 1 1)-6 3 surfaces, we deposited Si over-layers at 510 K. We find P segregates readily to the surface after a short annealing at 900 K,psimilar to ffiffiffi As and Sb during Si homoepitaxy [13], and the 6 3 surface is recovered. Finally, we use rapid thermal pffiffiffiannealing (RTA) to investigate P desorption from the 6 3 surfaces.
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2. Experiment Samples were cut from P-doped Si(1 1 1) wafers with resistivity 0.1 X cm. After a standard RCA cleaning the samples were introduced into the ultra high vacuum (UHV) chamber. The 7 · 7 reconstruction of the surface was prepared by 6 h degassing at 920 K followed by a 1450 K flash for 15 s. The pressure was kept at less than 1 · 109 Torr during the flash, and the sample temperatures were measured by a pyrometer. PH3 exposure was controlled by a leak valve. The dosing tube was 2.5 cm away from the sample. During PH3 exposures, the ion gauge was turned off. The chamber pressure, usually set between 2 · 108 and 2 · 107 Torr, was measured by the ion pump current, which was calibrated by the ion gauge prior to dosing. After PH3 exposure, all the STM images were taken at RT with a tunneling current of 0.2 nA and various sample biases. Although we took both filled- and emptystate images, we use mainly empty-state images for adsorbate analysis, because the apparent height in empty-state images is uniform for all the bare 7 · 7 adatoms while the edge adatoms appear higher in the filled-state images [14]. 3. Results and discussion 3.1. Dissociation of PH3 on Si(1 1 1)-7 · 7 at room temperature After exposing Si(1 1 1)-7 · 7 to 10-Langmuirs (L) of PH3 at RT, empty-state STM images reveal that not all adatoms are at the same apparent height anymore. We can delineate four species on the surface by the images at different sample biases. At low sample bias (0.5 V), as depicted in Fig. 1a, species U and B appear higher than species A, and species H appears lower than A. As the bias increases to 1.0 V, species A appears about the same apparent height as species U and B, while H remains low (Fig. 1b). Further increasing the bias to 1.5 V, we see three levels of apparent height. As shown in Fig. 1c, species A and B are the highest protrusions, followed by species U and H. The threshold values for the ‘low’ and ‘high’ biases vary by tip condition, but the relative apparent heights for these species remain unchanged. Similar ordering of the apparent heights of the four species can also be observed in the filled-state images presented in Fig. 1d–f, albeit with less contrast. From our empty-state images of partially reacted Si(1 1 1)-7 · 7 with hydrogen, we conclude that a bare Si adatom appears higher than a H-reacted adatom, and the apparent height difference reduces as sample bias increases. The H-reacted sites appear dark at a sample bias of 0.5 V but become visible at 1.5 V. Thus, we assign the species U to unreacted DBs and H to H-reacted sites. The brighter protrusions (A and B) in Fig. 1c are assumed to be PHx (x = 2, 3) reacted sites. The PH species are not expected on the surface after PH3 exposure at RT, because PH2 dissociation was observed at 450–500 K [7]. To identify
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Fig. 1. STM images of 10-L PH3 dosed Si(1 1 1)-7 · 7. The depicted rhombus in each image represents the same unit cell and helps to identify the adatom locations. H represents a hydrogen-reacted site, U for an unreacted site, A and B for PH2- and PH3-reacted sites, respectively. The faulted (F) and unfaulted (U) half of a unit cell is labeled in (e).
species A and B, we find that at 0.5 V, the majority species A have lower apparent height than DB sites (U) while minority species B are as bright as DB sites (Fig. 1a). Since the predominant species is PH2 even for PH3 exposure at 80 K and PH3 desorption can be detected up to 550 K [7], we associate species A and B with PH2 and PH3, respectively. 3.2. The relative reactivity of the dangling-bond states on Si(1 1 1)-7 · 7 From the UPS study of PH3 adsorption on 7 · 7 [8], atom-resolved tunneling spectroscopy of NH3 adsorption on 7 · 7 [9], and a theoretical calculation of the binding energies of PH3 on Si adatoms and rest-atoms [15], it is expected that PH3 adsorbs preferentially on rest-atom sites to adatom sites. On a bare 7 · 7 surface each rest-atom is surrounded by three adatoms and is one atomic layer lower than the adatoms; hence they are usually invisible in STM. However, after PH3 exposure, we find that if the rest-atom is neighbored by at least two H-reacted adatoms (based on the adsorbate-species assignment described in the last section), then it becomes visible, as marked in Fig. 2. Comparing with the apparent height difference between the PHx- and H-reacted adatoms, we conclude that all rest-atoms in such an environment are PHx-reacted. It is likely that if PH2 bonds to a rest-atom, the dissociated H will bond to one of the neighboring adatoms. Note that the dissociative adsorption of PH3 on Si(1 1 1)-7 · 7 is apparently different from that of H2S where H bonds exclusively to the rest-atom sites [16].
Fig. 2. Part of Fig. 1c where PHx-reacted rest-atoms are marked by arrows. Half unit cells are depicted to help identify the locations of restatom sites. PHx-reacted rest-atoms are observed when nearby adatoms are H-reacted (darker in this image).
There are four inequivalent adatom sites in a 7 · 7 unit cell: faulted/unfaulted center sites and faulted/unfaulted corner sites. (The corner adatoms are the six adatoms next to the four corner holes in a 7 · 7 unit cell, and the remaining six adatoms are called center adatoms.) To elucidate
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of PH3-reacted with the surface is dissociated, and 98% and 80% of the dissociation occurs at the center adatom sites in the faulted and unfaulted half, respectively. 3.3. P-coverage of PH3 exposure to Si(1 1 1)-7 · 7 at room temperature The averaged population of the PH3, PH2, H and DB at faulted and unfaulted adatom sites per unit cell after each exposure is presented in Fig. 3. We note that the adatom DBs in the unfaulted half are about 1.5 times more than that in the faulted half at low coverages but the ratio is reduced to 1 after a 60-L exposure, suggesting that about an equal number of adatom DBs are reacted with PHx or H in each half of the unit cell. Detailed analysis, however, reveals a complex PH3 reaction kinetics involving the DBs at the adatoms sites and rest-atom sites in the two halves of the 7 · 7 unit cell. Between 0.3- and 60-L exposure, the H population in the faulted half is almost twice as prevalent as in the unfaulted half and both change little with dose. This behavior suggests that PH2 adsorption at rest-atom sites saturates very early on in both halves of the unit cell. If we assume that the dissociation products, PH2 and H, will bond to an adatom–rest-atom pair, the rest-atom sites in the faulted half could already be 94% occupied after a 0.3-L exposure but the unfaulted half is only 56% occupied. Hence, the unfaulted half has more room for further dissociative adsorption of PH3 than the faulted half, as is clearly shown in Fig. 3. The adatoms in the unfaulted half continue to react with PH3 and drive the dissociated H to bond with a neighboring rest-atom. The population of PH2 adsorbed adatom in the unfaulted side reaches 2.0 per unit cell after a 60-L exposure, about twice as many as that in the faulted half. If there are no available DBs nearby, the adsorbed PH3 cannot dissociate. Based on this assumption, we can understand why the PH3-reacted adatoms are 0.33 in the faulted half but only 5
'
Faulted
4
Adatom sites / unit cell
their different reactivities with PH3, we have analyzed our empty-state STM images of 0.3, 2, 10, and 60-L PH3 exposures. After a 0.3-L exposure, we find that about 2.8 adatom DBs in the faulted half are unreacted, compared to 4.3 in the unfaulted half. Hence, it appears that the faulted half of the Si-7 · 7 unit cell is more reactive toward PH3 than the unfaulted half, which is consistent with the behavior of many other adsorbates reported in the literature [16]. Prolonged exposure, however, steadily reduces the asymmetry of the reacted sites. After a 60-L exposure, the ratio of DB sites in the faulted to unfaulted half is 0.94. We attribute the higher reaction rate in the unfaulted half at higher coverages to more available DB sites, which will become clear after the discussion of adsorption kinetics at higher PHx coverage in Section 3.3. After a 0.3-L exposure, the average numbers of PH2-reacted adatoms in the faulted and unfaulted sides are about even, with 0.7 per half unit cell. However, this does not necessarily imply that the adatoms in the two sides have equal reactivity toward PH3. If we assume that any PH2 adsorption will create a Si–H bond, either at a neighboring adatom site or a rest-atom site in the same unit cell, we have to conclude that the large number of H-reacted adatom sites (2.1) in the faulted half is not from the PH2 adsorption at the adatom sites but from the PH2 adsorption at the rest-atom site. In contrast, only 1 H-reacted adatom site is found on the unfaulted half of a unit cell. Based on this analysis, we conclude that after a 0.3-L exposure, 2.1 PH2 adsorbs on faulted rest-atom sites, but only 1 PH2 adsorbs on unfaulted rest-atom sites. Thus, in the initial phase of PH3 adsorption, about 69% of the restatom DBs in the faulted half are bonded with PH2, compared to 32% in the unfaulted half. If we exclude the adatom DBs being occupied by the dissociated H from the PH2 adsorption at rest-atom sites and consider all the PH2- and PH3-reacted adatom sites, the adatom reaction rate toward PH3 in the faulted half is higher than the unfaulted half, 27% vs. 16%, respectively. Thus, we conclude that rest-atoms are more reactive toward PH3 than adatoms and the faulted half is more reactive than the unfaulted half. The reactivity difference between the corner and center adatom sites can also be understood from the adatom– rest-atom pair adsorption scenario. Although we do not observe a significant difference in the PH3 population between these two adatom sites, presumably because PH3 adsorption on the adatoms does not involve rest-atoms, PH2-reacted sites are found predominantly at the center adatom sites. At low coverages, the ratio of PH2-reacted center to corner sites is particularly large, 50 for the faulted half and 4 for the unfaulted half. Even after a 60-L exposure, the PH2-occupied center to corner ratio is still 9 for the faulted half and 8 for the unfaulted half. Furthermore, we note that after a 60-L exposure, the ratio of PH3/PH2 is 2 at corner sites in both halves, but is 1/4 and 1/8 at center sites in the faulted and unfaulted half, respectively. These observations suggest that initially 88%
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H PH2
PH3 DB
3 2 1 0 5
Unfaulted
4 3 2 1 0 4
6 8
2
1
4
6 8
2
4
6
10
PH3 Exposure (L) Fig. 3. Averaged population of adatom-reacted surface species per unit cell (determined from STM images) as a function of PH3 exposure.
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0.08 in the unfaulted half after a 0.3-L exposure. Because of the early saturation of the rest-atom sites in the faulted half, a PH3 molecule bonds with an adatom in the faulted half has much less chance to dissociate than that in the unfaulted half. The ratio of the faulted PH3 sites to unfaulted PH3 sites diminishes gradually to 0.6 after a 60-L exposure when the rest-atom sites are almost fully occupied in either half of the unit cell. To estimate the total P coverage by STM data, we have to include the estimated PHx population at rest-atom sites. After the required amount of PH2 and H at the rest-atom sites to balance their observed counterparts at the adatom sites, the remaining rest-atom sites could be either occupied by undissociated PH3 or remain as DBs. In addition, the upper bound of P coverage should include one PH3 adsorption at the corner hole per unit cell. By these assumptions, we estimate the total P coverage to be 4.9–7.2 per unit cell after a 0.3-L exposure and the coverage increases to 7.6–8.7 per unit cell after a 60-L exposure. Dividing the P atom per unit cell by 49 DBs, we obtain a PHx-saturation coverage of 0.16–0.18 ML. Based on our estimates, at saturation 2/3 of the rest-atoms are reacted with PH2 and 1/3 with H, and near 40% of the adatom reacted with PHx and 25% with H. Note that our estimated saturation coverage at RT is lower than the 0.24 ML at 120 K reported in Ref. [6], where the amount of PH3 flux intercepted by the Si(1 1 1) surface was used to obtain the surface coverage. 3.4. Annealing of PH3/Si(1 1 1)-7 · 7 and PH3 adsorption at 900 K Fig. 4 shows an empty-state image of Si(1 1 1)-7 · 7 after a 0.3-L PH3 exposure at room temperature followed by a 900 K annealing for 60 s. The 7 · 7 structure is mostly dis-
Fig. 4. Si(1 1 1)-7 · 7 after a 0.3-L PH3 exposure at room temperature followed by annealing at 900 K for 60 s. Two corner-holes are marked by arrows. Scale bar: 5 nm; sample bias: 2.0 V.
rupted as one can see local patches of 2 · 2 structure with a few corner holes. Since PH2 dissociates before 500 K, H desorbs before 780 K and P2 will not desorb until 1010 K [6], it is likely that the depressions are P-terminated sites and the protrusions are Si DBs. However, by counting the protrusions in Fig. 4, we find that the existing DBs are only 59% of the original adatoms prior to annealing. Apparently, many adatoms have been removed from the terrace during annealing. In fact, Fourier transformed infrared spectroscopy [17] and core-level photoemission spectroscopy [18] of annealing PH3 adlayers on Si(0 0 1)2 · 1 between 550 and 650 K suggest P will displace surface Si atoms to form Si–P heterodimers. If a similar reaction occurs on the 7 · 7 surface, we can speculate that annealing at 900 K will promote P atoms to displace the 7 · 7 adatoms and bond directly to the rest-atoms. The displaced adatoms could either diffuse to the step edge, or with sufficient adatoms on a terrace, they could form two-dimensional (2D) islands. We did not observe any island formation for annealing the RT saturated surfaces, perhaps because even at saturation coverage there were not enough displaced Si atoms on the terraces to form 2D islands. However, after a 1-L PH3 exposure at 900 K we did observe irregular 2D islands on large terraces (Fig. 5a). The existence of the denuded zone near the upper step edges suggests that the islands are Si adatoms ejected by P [19]. This could be understood by the fact that at 900 K, PH3 dissociates completely upon adsorption, but H desorbs instantly allowing more P–Si bond formation and Si ejection. Close-up images reveal that surfaces after a 1-L PH3 exposure are still disordered, similar to the surface in Fig. 4.
Fig. 5. (a) Si(1 1 1) surface after a 1-L PH3 exposure at 900 K. Scale bar: pffiffiffi 250 nm; sample bias: 2 V. (b) P/Si(1 1 1)-6 3 surface after a 30-L PH3 exposure at 870 K. Scale bar: 30 nm; bias: 2 V.
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With sufficient PH3 exposure (e.g., 30 L) at 870 K, the Si(1 1 1) is completely terminated by a layer of P atoms, aspffiffiffidepicted in Fig. 5b. The surface becomes pffiffiffi ð6 3 6 3ÞR30 reconstructed, identical to that formed by P2 exposure of Si(1 1 1) at 770–920 K [12]. The tensile stress introduced by P–Si bonds creates hexagonal domains on the surface. The displaced 7 · 7 adatoms form triangular 2D islands pointing in both ½ 1 1 2 and ½1 1 2 directions. Due to the P-termination and surface segregation (see next section), we expect that PH3 adsorption will be self-terminating after a coverage near 1 ML at 900 K. 3.5. Surface segregationpand ffiffiffi thermal desorption of P from P/Si(1 1 1)-6 3 surfaces Depositing pffiffiffi 1 nm of silicon at 510 K onto the P/Si(1 1 1)-6 3 surface results in a surface covered with small 3D clusters. In comparison, we find registered but short H-terminated Si dimer rows on the Si(0 0 1)-2 · 1 surface after silicon deposition at similar conditions [20]. Apparently, H segregates more easily than P. Subsequent annealing of the Si-clustered surface up to 800 K for 3 min does not change the surface morphology. pffiffiffi However, after annealing at 900 K for 3 min, the 6 3 structure reappears and the hexagonal domains congregate into a continuous network (Fig. 6).pffiffiffi P-desorption from the 6 3 surfaces apparently starts after a RTA at 950 K for 1 s. Fig. 7a shows the newly formed silicon DBs as bright protrusions. One should note the DB denuded zone near the upper step-edge and some consolidation of the top-layer 2D hexagonal islands. Continued RTA to 990 K creates an a/1 · 1, or adatom-covered 1 · 1 surface [21]. Similar surface morphology was
pffiffiffi Fig. 7. P/Si(1 1 1)-6 3 surface after rapid thermal annealing at: (a) 950 K, (b) 990 K, and (c) 1100 K. Scale bar: 30 nm; sample bias: 2.0 V.
also observed by P2 adsorption on 7 · 7 at 970 K [12]. Only a few large 2D islands remain on the terrace with sides along the underlying 1 · 1 lattice (Fig. 7b). These islands are dispersed to step edges after a subsequent RTA to 1000 K. It is interesting to note that no 7 · 7 structures were found after RTA up to 1070 K, near the 7 · 7 M 1 · 1 phase transition. The 7 · 7 domains appear to nucleate from the upper step edge after a RTA at 1100 K, as shown in Fig. 7c, consistent with an earlier high-temperature STM observation by Miki et al. [22]. 4. Conclusions pffiffiffi Fig. 6. Reappearance of the 6 3 surface after 1-nm Si deposition on the surface of Fig. 5b at 510 K followed by annealing at 900 K for 3 min. Scale bar: 50 nm; sample bias: 2 V. Inset: close-up image of the surface.
In conclusion, we have used the apparent height of atomic protrusions in empty-state STM images to identify
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surface species from PH3 exposures on Si(1 1 1)-7 · 7 at room temperature. The PH2-reacted adatom site appears as a small and dim protrusion at low biases but a bright protrusion at high biases. A PH3-reacted adatom site appears similar to an unreacted adatom site (DB) at low biases and a bright protrusion at high biases. The hydrogen-reacted adatom site appears as a dark spot at both biases. Based on these assignments and the assumption that the PH2 and H are dissociative products of PH3 adsorption, we conclude that dangling-bonds at faulted rest-atom sites are most reactive towards PH3 followed by unfaulted rest-atom sites, faulted and unfaulted adatoms sites. We do not observe a significant difference for PH3 adsorption between the corner and center adatom sites, but there is an overwhelming preference over the center adatom sties for PH2 adsorption. After an initial uptake of PH3, predominantly at the faulted rest-atom sites, the unfaulted-adatoms take the lead. Between 10- and 60-L exposures the increase of PH2- and PH3-reacted unfaulted adatom sites outnumber the faulted adatom sites by a factor of 3.6 and 2.9, respectively. From our observation and analysis, the rest-atoms cease to adsorb PH3 after the initial uptake, but continue to adsorb H until a complete saturation after 60 L of exposure. In comparison, about 36% of adatom DBs are still unreacted after a 60-L exposure. Assuming the corner holes are PH3-reacted, we estimate the saturation P coverage to be about 0.18 ML at room temperature exposure, corresponding to a reaction rate of 0.46 PH3/DB. At such coverage, 77% of the DBs on the Si(1 1 1)-7 · 7 are reacted with either PH3, PH2 or H. It is interesting to note that PH3 exposure leads to a saturated P/Si(1 1 1)-7 · 7 surface, but even at room temperature depositing P by P2 vapor makes the 7 · 7 structure unstable beyond a critical P coverage of 0.1 ML [12]. As for the adsorption kinetics, PH3 could adsorb at a DB site non-dissociatively or two DB sites dissociatively depending on the availability of an adatom–rest-atom pair. Based on the work of Vitali et al. [12], only adatom pairs are involved in the P2 adsorption on Si(1 1 1)-7 · 7. At room temperature P2 adsorption on Si cannot be observed in the empty-state [12], but a rich variety of the empty-state images of Si-x bonds can be distinguished in the PH3 adsorption. However, at temperatures beyond the H desorption temperature from the silicon surface (800 K), the reactions of PH3 and P2 with Si should be quite similar. Indeed we find that annealing a PH3 saturated 7 · 7 surface to 900 K results in a disordered surface similar to P2 deposition at 970 K; presumably some of the Si adatoms on the top double-layer are replaced by P atoms. The
pffiffiffi P/Si(1 1 1)-6 3 structure can be created with sufficient pffiffiffi PH3 exposure at 900 K. The P-termination on the 6 3 surfaces is so robust that Si deposition at lower temperatures (e.g., 510 K) pffiffiffi can only form 3D clusters on the surface. P/Si(1 1 1)-6 3 surfaces can be recovered by annealing the clustered surfaces to 900 K. Apparently P atoms segregate to the surface to terminate the newly formed 1 · 1 surface. Therefore, it is unlikely to create P d-layers in the h1 1 1i direction unless another surfactant is used [23]. Acknowledgement This work was supported by the National Science Foundation under Grant Number CCF-0404208. References [1] T.-C. Shen, J.-Y. Ji, M.A. Zudov, R.-R. Du, J.S. Kline, J.R. Tucker, Appl. Phys. Lett. 80 (2002) 1580. [2] L. Oberbeck, N.J. Curson, M.Y. Simmons, R. Brenner, A.R. Hamilton, S.R. Schofield, R.G. Clark, Appl. Phys. Lett. 81 (2002) 3197. [3] T.-C. Shen, J.S. Kline, T. Schenkel, S.J. Robinson, J.-Y. Ji, C. Yang, R.R. Du, J.R. Tucker, J. Vac. Sci. Technol. B 22 (2004) 3182. [4] F.J. Ruess, L. Oberbeck, M.Y. Simmons, K.E.J. Goh, A.R. Hamilton, T. Hallam, S.R. Schofield, N.J. Curson, R.G. Clark, Nanoletter 4 (2004) 1969. [5] A.J. van Bommel, J.E. Crombeen, Surf. Sci. 36 (1973) 773. [6] R.M. Wallace, P.A. Taylor, W.J. Choyke, J.T. Yates Jr., J. Appl. Phys. 68 (1990) 3669; P.A. Taylor, R.M. Wallace, W.J. Choyke, J.T. Yates Jr., Surf. Sci. 238 (1990) 1. [7] P.J. Chen, M.L. Colaianni, R.M. Wallace, J.T. Yates Jr., Surf. Sci. 244 (1991) 177. [8] F. Bozso, Ph. Avouris, Phys. Rev. B 43 (1991) R1847. [9] R. Wolkow, Ph. Avouris, Phys. Rev. Lett. 60 (1988) 1049; Ph. Avouris, R. Wolkow, Phys. Rev. B 39 (1989) 5091. [10] A.J. van Bommel, F. Meyer, Surf. Sci. 8 (1967) 381. [11] J.J. Lander, J. Morrison, J. Chem. Phys. 37 (1962) 729. [12] L. Vitali, M.G. Ramsey, F.P. Netzer, Phys. Rev. B 57 (1998) 15376. [13] M. Horn-von Hoegen, J. Falta, M. Copel, R.M. Tromp, Appl. Phys. Lett. 66 (1995) 487. [14] Z. Zhang, M.A. Kulakov, B. Bullemer, Surf. Sci. 375 (1997) 195. [15] P.-L. Cao, L.-Q. Lee, J.-J. Dai, R.-H. Zhou, J. Phys.: Condens. Matter 6 (1994) 6103. [16] M.A. Razaei, B.C. Stipe, W. Ho, J. Phys. Chem. B 102 (1998) 10941. [17] J. Shan, Y. Wang, R.J. Hamers, J. Phys. Chem. 100 (1996) 4961. [18] D.-S. Lin, T.-S. Ku, T.-J. Sheu, Surf. Sci. 424 (1999) 7. [19] Y.W. Mo, M.G. Lagally, Surf. Sci. 248 (1991) 313. [20] Jeong-Young Ji, T.-C. Shen, Phys. Rev. B 70 (2004) 115309. [21] R.M. Feenstra, M.A. Lutz, Surf. Sci. 243 (1991) 151. [22] K. Miki, Y. Morita, H. Tokumoto, T. Sato, M. Iwatsuki, M. Suzuki, T. Fukuda, Ultramicroscopy. 42–44 (1992) 851. [23] O.D. Dubon, P.G. Evans, J.F. Chervinsky, M.J. Aziz, F. Spaepen, J.A. Golovchenko, M.F. Chisholm, D.A. Muller, Appl. Phys. Lett. 78 (2001) 1505.