A SLAB-MINDO study of half monolayer and monolayer chemisorption of chlorine on the silicon (001) surface

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Surface Science 319 (1994) 232-242

A SLAB-MIND0 study of half monolayer and monolayer chemisorption of chlorine on the silicon ( 001) surface M.W. Radny ‘, P.V. Smith * Physics Department,

The University of Newcastle, Callaghan, New South Wales, Australia, 2308

Received 21 December

1993; accepted for publication

22 July 1994

Abstract The chemisorption of chlorine onto the silicon (001) surface is studied at both 0.5 and 1.0 monolayer (ML) coverage using the periodic SLAB-MIND0 method. This enables a comprehensive picture of the progressive chemisorption of chlorine onto this surface. The dangling-bond site is determined to be the preferred chemisorption site with the optimum configurations at both 0.5 and 1.0 ML coverage being dangling-bond topologies. It is found that chlorine does not favour subsurface chemisorption nor the shared-dimer or “SiF,-like” topologies which have been shown to constitute minimum energy structures for fluorine chemisorption on the Si(OO1)2 X 1 dimerised surface. The results are compared with experiment and other theoretical calculations.

1. Introduction The study of the interaction of halogens with silicon surfaces is of considerable fundamental and technological importance. This is due primarily to the use of halogen-containing radicals in the plasma or reactive ion etching of silicon surfaces. These etching processes play an essential role in the fabrication of semiconductor devices and in the manufacture of patterned silicon substrates for the verylarge-scale-integrated (VLSI) circuits which underlie so much of modem day electronics. Experiments have shown that it is the action of atomic fluorine and chlorine in forming volatile silicon fluorides and chlorides that is actually responsible for the etching

* Corresponding author. Fax: +61 49 21 6907. 1On leave from the Institute of Experimental Physics, University of Wroclaw, p1.M. Boma 9, 50-205 Wroclaw, Poland. 0039-6028/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0039-6028(94)00481-l

of silicon surfaces [l-3] and that the use of these halogens can significantly enhance the etching rate compared with that produced by physical sputtering [4,5]. It has also been shown that chlorine and fluorine exhibit quite different reactivities on silicon, that the etching rates increase significantly in the presence of UV irradiation, and the relative abundance of the various desorbed species depends strongly on the surface structure [l-3]. Despite this large amount of experimental work, however, little is known about the actual etching mechanisms and associated surface chemistry at a fundamental level. This is because most etching technology has been driven predominantly by manufacturing needs. Such an understanding is really essential, however, for the future development and optimisation of these etching processes. The main aim of this work is to endeavour to lay the basis for such an understanding by performing detailed and systematic studies of the chemisorption of atomic fluorine and chlorine onto the silicon (001) surface.

M.W.Radny,P.K Smith/ SurjaceScience 319 (1994) 232-242

In a previous paper we have reported results for the chemisorption of atomic fluorine onto the (001) surface of silicon at both 0.5 and 1.0 ML coverage [6]. These calculations revealed several new features which, we believe, may have blent impli~tions for the subsequent etching process. Firstly, we have found that fluorine can penetrate the silicon (001) surface and occupy a highly coordinated subsurface site just below the second layer. Secondly, we have found that, in addition to the expected dangling-bond site, fluorine can reside at a shared-dimer (or bridge) site between two adjacent silicon dimer atoms. Finally, two new stable structures have been determined for the Si(~l)2 X l-2F system at monolayer coverage: a “SiF,-like” ~nfig~ation and a shareddimer-dangling-bond topology in which one fluorine occupies a dangling-bond site while the other resides at a shared-dimer site. In this paper we report the results of analogous calculations for the chemisorption of atomic chlorine onto the silicon (001) surface. Extension of this work to higher coverages, and hence to the actual etching of silicon by chlorine, will form the basis of a later publi~tion.

2. Calculational details The bulk of the calculations reported in this paper have been performed using the periodic MIND0 method [7]. This approach is a periodic reformulation of the well-known and highly successful MIND0/3 molecular orbital method developed by Dewar and co-workers f&10] and is applicable to both bulk solids and surfaces. In the latter case, one employs slabs possessing the two-dimensional periodicity of the surface, and the total energy is expressed as a summation over wavevectors lying within the first Brillouin zone (BZ) of the surface unit ceil. This BZ summation is performed using special k-point averaging techniques [ill. The parameters characterising this ~~-consistent ~mi-ernpi~~l method have been determined by fitting to a wide range of bulk electronic and lattice dynamical properties as well as to relevant molecules such as SiCI, and Si,Cl,. The actual parameter values used in this paper are identical to those employed in previous work [12,13]. For both half-monolayer and monolayer chemisorption we have employed a slab characterised by a 2 X 2

233

surface unit cell and containing 5 silicon layers with hydrogen termination on the bottom surface. In order to determine the optimum geometries within this SLAB-MIND0 method we have employed a NAG library routine 1141which determines the un~nstr~ned minima of a unction of several variables. This numerical algorithm is based on the quasi-Newton method and uses numerically computed first and second derivatives to determine the search path(s) and check the validity of the predicted minima. In the geometry optimisations reported here, the total energy of the system has been minimised with respect to all of the atomic coordinates within the three topmost surface layers. Different starting geometries have been employed to check the reliability of the determined minimum energy configurations. Ab-initio Hartree-Fock-Roothan cluster calculations using the GAMESS package [15] have also been performed in support of the SLAB-MIND0 results. 2.1. Had-monolayer covertzge In order to investigate the chemiso~tion of chlorine onto the Si(OO1)2X 1 reconstructed surface at half-monolayer coverage we have employed starting geometries in which the two chlorines per 2 X 2 surface unit cell are positioned at the various possible chemisorption sites shown in Fig. 1. Placing the chlorines at the No. 1 (dimer-bridge) site of each 2 X 1 unit cell and optimising the geometry to minimise the energy, results in the chlorines moving to the adjacent dangling-bond sites to pro-

Fig. 1. Top view of the Si(OO1)2x 1 reconstructed surface indicating various possible chemisorption sites. From darkest = top layer, via second and third layer, to lightest = fourth layer.

234

Table 1 The electronic

hi.W. Radny, P. V. Smith /Surface

charge on the chlorine and top-layer Si(OO1)2 X l-Cl

Si (second layer) Energy

7.42 3.61 * 3.61 * 4.13 - 3208.39

silicons, and the total energy values, for the 0.5 ML configurations

shown in Figs. 2-5

(0.5 ML)

Type 1 Cl Si (first layer)

Science 319 (1994) 232-242

Type 2s 7.42 3.97 3.97 4.13

7.48 3.42 * 4.57 4.23 - 3208.28

Type 2a 7.44 3.52 * 3.97 3.91

7.52 3.44 * 4.56 4.17 -3208.17

Bridge 7.44 3.70 * 3.69 3.95

7.27 3.68 * 3.67 * 4.19 - 3207.45

7.28 ** 3.97 3.98 4.19

The single asterisks denote silicon atoms which are directly bonded to a chlorine whilst the double asterisk labels the upper chlorine in the bridge-site configuration. The energy values are in eV per 2 x 2 surface unit cell.

duce the type-2s ’ dangling-bond configuration shown in Fig. 2. Both Si-Cl bonds are oriented at approximately the tetrahedral angle to their respective Si-Si diomer bond and have a length of approximately 2.1 A. Each chlorine has gained N 0.46e, derived mainly from its associated silicon dimer atom (see Table 1). The Si-Si dimer bonds have bondlengths close to the bulk nearest-neighbour separation of 2.35 A but are buckled such that the chlorine-chemisorbed silicon lies above its dimer counterpart. Both dimer configurations are also slightly different suggesting that this low-coverage phase has a 2 X 2 topology rather than 2 X 1, although this would probably be quite difficult to detect experimentally. Starting with the chlorine atoms at the same No. 2 (dangling-bond) site of each 2 X 1 surface unit cell also results in the type-2s dangling-bond configuration discussed above. Chemisorbing the two chlorine atoms on alternating No. 2 sites, on the other hand, yields the topology shown in Fig. 3. As before, the Si-Cl bonds form essentially tetrahedral angles with their neighbouring Si-Si dimer bonds and have a bondlength close to 2.1 A. Each Si-Si dimer bond is again tilted upwards towards the chlorine-chemisorbed silicon and has a bondlength very close to the bulk nearest-neighbour distance. This type-2a dan-

’ Labelling the various dangling-bond configurations as type 1, 2s and 2a follows Boland [16] and simply discriminates between the chlorine being chemisorbed onto the dangling bonds of the same dimer (type l), or on the same side (type 2~1, or opposite sides (type 2a), of adjacent dimers.

(4

Fig. 2. The 0.5 ML type-2s dangling-bond configuration for the Si(Wl)-Cl system. The shading of the Si atoms in (a) is the same as for Fig. 1.

M. W. Radny, P. V. Smith /Surface

Fig. 3. The 0.5 ML type-2a dangling-bond configuration for chlorine chemisorption on the Si(OOl) surface. The shading of the Si atoms in (a) is the same as for Fig. 1.

gling-bond configuration is predicted to be 0.05 eV per chemisorbed chlorine higher in energy than the type-2s topology of Fig. 2 (see Table 1). The chemisorbed chlorines now have charges of 0.52e and 0.44e. Placing the chlorines above the surface at the No. 3 (cave) sites and varying the atomic coordinates to minimise the total energy gives rise to the same type-2s configuration as that shown in Fig. 2. Re-

Science 319 (1994) 232-242

235

peating these calculations with either one or both of the chlorines per 2 X 2 cell initially placed well below the surface at the No. 3 sites, on the other hand, produces stable topologies in which one chlorine lies above the surface and the other remains below the surface. These latter topologies are muchhigher energy structures (by 1.33 and 1.88’ eV per chlorine), however, than the type-2s dangling-bond configuration of Fig. 2. Starting with the chlorines either above or below the surface at the No. 4 (valley-bridge) sites yields a geomftry in which one of the chlorine atoms lies 2.10 A almost directly above one of the silicon dimer atoms whilst the other is located 1.07 A above the ideal surface and close to an adjacent No. 4 site. The energy of this structure is about 0.38 eV per chlorine greater than the chlorine-chemisorbed dangling-bond configurations of Figs. 2 and 3. Commencing our optimisation procedure with the chlorines sited above the reconstructed surface at the No. 5 (pedestal) sites yields the dangling-bond topology shown in Fig. 4. In this case, both dangling-bonds in one 2 X 1 unit cell are saturated whilst those in the other 2 X 1 unit cell remain unsaturated. This dangling-bond structure is completely symmetric with each Si-Cl bond making an angle of 106.6” to the Si-Si dimer bond. The bondlengths of the Si-Cl and (saturated) Si-Si dimer bonds are 2.12 and 2.41 A, respectively. The unsaturated Si-Si dimer bonds have a bondlength of 2.14 A, identical to that predicted by the SLAB-MIND0 method for the clean Si(OO1)2 X 1 dimerised surface 1171 ‘. This type-l dangling-bond topology is 0.05 eV per chlorine lower in energy than the type-2s configuration of Fig. 2 and is the lowest energy structure that we have found for a uniform 0.5 ML coverage of chlorine on the silicon (001) surface. Each chlorine in this configuration has a net charge of 0.42e derived mainly from its corresponding silicon dimer atom (see Table 1). Starting with the two chlorines per 2 X 2 unit cell below the surface at the No. 5 sites results in both

2

It is to be noted that the SLAB-MIND0 method, like most other theoretical calculations, predicts a dimer bondlength for the SK00112 X 1 clean surface which is substantially less than both the nearest-neighbour distance (2.35 A) and that suggested by experiment ( > 2.37 A).

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M. W. Radny, P. V. Smith /Surface

chlorines moving above the surface to form the geometry shown in Fig. 5. In this case, the dimer bonds are broken and the chlorines occupy bridge-site positions between the surface silicon atoms which now lie close to their ideal positions. The energy of this bridge-site configuration is 0.47 eV per chlorine higher than the type-l dangling-bond structure. Optimising the geometry for the case where the chlorines are chemisorbed from above the surface at

Fig. 4. The minimum-energy figuration for the Si(OOl)-Cl is identical to that in Fig. 1.

0.5 ML type-l dangling-bond system. The Si atom shading

conin (a)

Science 319 (I 994) 232-242

equivalent No. 6 (shared-dimer) sites of each 2 X 1 surface unit cell yields the type-2s dangling-bond configuration of Fig. 2. Chemisorbing onto the No. 6 sites between the two Si-Si dimers in each 2 X 2 surface unit cell, on the other hand, produces either the type-l symmetric dangling-bond configuration of Fig. 4, or a 1.03 eV per chemisorbed chlorine higher energy structure in which the chlorines remain close to the No. 6 sites, 1.11 A above the ideal surface. Starting with the remaining option of chlorines on alternative No. 6 sites of each 2 X 1 surface unit cell results in the chlorines again remaining close to their original chemisorption sites, 1.09 A above the surface. The energy of this 2 X 2 structure is 1.44 eV per chlorine higher than the type-l symmetric dangling-bond configuration. From all of the above results it is clear that the preferred chemisorption site for chlorine chemisorption on the silicon (001) surface at half-monolayer coverage is the dangling-bond site. Chemisorbing chlorine from above the surface at the sites 1, 2, 3, or 5 always produces a dangling-bond configuration. The lowest energy configurations for 0.5 ML chemisorption at the No. 6 sites are also danglingbond structures. This is in sharp contrast to the results obtained from the SLAE&MINDO method for the chemisorption of atomic fluorine onto the Si(OO1) surface. In this case, chemisorption above the surface at positions 1 and 5 (and also position 3 with a small amount of activation energy), results in the fluorine penetrating the surface and occupying a highly coordinated site just below the second layer [6]. The only stable configurations which we have found for 0.5 ML coverage of chlorine on the silicon (001) surface which incorporate subsurface chlorine are those obtained by starting our geometry optimisation with some chlorine already below the surface. Chemisorbing chlorine from above the surface always gives rise to on-top configurations. In addition, the two subsurface configurations which we have obtained for 0.5 ML coverage have relatively high energies and hence are unlikely to occur. The topology which we have obtained from chemisorbing chlorine at the No. 4 site also differs significantly from that of fluorine. In this latter case, the fluorine was found to move to a position lying just above the surface, between the silicons of two adjacent dimers [6]. This shared-dimer configuration

hf. W. Radny, P. V. Smith /Surface

was determined to be slightly lower in energy than the 0.5 ML dangling-bond configuration. Chlorine chemisorption at the No. 4 sites, on the other hand, results in both chlorines per 2 X 2 surface unit cell lying well above the surface with one chlorine essentially in an on-top position and the other close to a No. 4 site. Many of the differences in the chemisorption behaviour of chlorine and fluorine on the Si(OO1) surface are a direct consequeonce of theiroquite different covalent radii (Cl: 0.99 A, F: 0.72 A) compared to that of Si (1.11 A). It is because of its small relative size that fluorine, for example, is able to occupy a highly coordinated subsurface site within the Si(OO1) reconstructed surface, whilst chlorine cannot. Similar considerations apply to the shared-dimer site and the “SiF,-like” 1.0 ML configuration to be considered later. One very interesting feature which has emerged from the above calculations is that all of the stable

(a)

k------3*91

A

d

Science 319 (1994) 232-242

237

configurations that we have determined for a uniform 0.5 ML coverage of chlorine on the Si(OO1) dimerised surface have 2 X 2 topologies rather than 2 X 1. As pointed out earlier, however, the type-2s dogleg-bond conflation of Fig. 2, which is only 0.05 eV per chemisorbed chlorine higher in energy than our lowest energy structure, only departs slightly from 2 X 1 symmetry. Unfortunately, a reliable experimental determination of the surface periodicity of the Si(OO1):Cl surface at half monolayer coverage is not yet available. 2.2. Mono2ayer couerage The above calculations indicate that the initial chemisorption of chlorine up to 0.5 ML will occur on the dangling-bond sites. Moreover, experimental work has established that the chemisorption of chlorine at monolayer coverage retains the 2 X 1 surface reconstruction. In order to study monolayer

3.77 A” 4

Fig. 5. The 0.5 ML bridge-site topology for chlorine chemisorption on the Si(OO1)surface. The shading of the Si atoms in (a) is the same as for Fig. 1.

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M. W. Radny, I? V. Smith /Surface

chemisorption of chlorine we have therefore assumed that equivalent dangling-bond (No. 2) sites in each 2 X 1 unit cell are saturated by chlorine and then considered the effect of adding the requisite two additional chlorine per 2 X 2 surface unit cell at the various chemiso~tion sites of Fig. 1. In previous work [12,13], the monolayer chemiso~tion of chlorine onto the Si(OO1) dimerised surface was investigated using a 2 X 1 surface unit cell and starting from a few selected geometries for the two chlorines within each 2 X 1 unit cell. The present calculations represent a more systematic approach as well as incorporating the additional flexibility of employing a 2 X 2 surface unit cell. Starting with the additional chlorines above the surface at the No. 1 dimer-bridge sites of each 2 X 2 surface unit cell, and optimising the geometry with respect to all of the atomic coordinates in the three topmost surface layers, results in these chlorine atoms moving to occupy the remaining unsaturated dangling bonds. The final minimum energy configuration is the 1.0 ML symmetric dangling-bond topology shown in Fig. 6. This is identical to the optimum topology for monolayer coverage of chlorine on the Si(OO1)2 X 1 surface reported previously [13]. The Si-Cl bonds are of length 2.16 A and are oriented at 104.8” to the Si-Si dimer bonds. The Si-Si dimer bondlength is 2.38 A, just marginally greater than the bulk nearest-neighbour distance of 2.35 A, whilst the Si-Si backbonds are slightly shorter at 2.30 A.

CL l

P 2.16 A*

Si

Si 2,3OA\

Fig. 6. Minimum-energy monolayer dangling-bond configuration for the Si@O1)2 X l-C1 system. The Si atom shading is identical to that in Fig. 1.

Science 319 Cl9941 232-242 Table 2 The electronic charge on the chlorine and top-layer silicons, and the total energy values, for the 1.0 ML structures shown in Figs. 6 and 7 Si(OOl)ZX l-Cl

Cl Si (first layer) Si (second layer) Energy

(1.0 ML)

Dangling bond

Bridge site

7.57 7.57 3.48 * 3.48 * 4.05 4.05 -3145.83

7.30 7.34 ** 3.61* 3.61* 4.20 4.20 - 3142.87

The single asterisks denote silicon atoms bonded directly to a chlorine whilst the double asterisk labels the upper chlorine atom in the bridge-site configuration. The energy values are in eV per 2X 1 unit cell.

Each chemisorbed chlorine has acquired a net charge of 0.57e whilst the first-layer silicon dimer atoms have lost 0.52e (see Table 2). The surface topology corresponds to a 2 X 1 surface reconstruction as the two 2 X 1 surface unit cells making up each 2 X 2 unit cell now have identical structure. We find this 2 X 1 symmetric dangling-bond geometry, which is extremely stable against both buckling and twisting of the dimers, to be the lowest energy configuration for monolayer chemiso~tion of chlorine onto the silicon (001) surface. Starting with the two additional chlorines per 2 X 2 unit cell above the surface at either the No. 2, 3 or 5 sites also results in the symmetric 2 X 1 dangling-bond configuration shown in Fig. 6. When the two extra chlorines per 2 X 2 unit cell are initially sited above the surface at the No. 4 (valleybridge) sites, however, the dangling-bond chemisorbed chlorines are found to move to positions virtually on top of their associated Si dimer atoms whilst the additional chlorines lie 1.16 A above the ideal surface, close to the No. 4 sites. This behaviour is similar to that found for 0.5 ML chemisorption from above the No. 4 sites. Since both 2 X 1 surface unit cells are identical, this 1.0 ML topology has a 2 X 1 surface periodicity. Its energy, however, is 0.45 eV per chlorine higher than for the above symmetric d~gling-bond configuration. Four different possible configurations arise for adding chlorine to our 0.5 ML dangling-bond configuration at the No. 6 sites of the 2 X 2 surface unit cell. When the additional chlorine is sited on the No. 6 sites immediately adjacent to the dangling-bond

M. W. Radny, P. V. Smith/Surface

chlorines, the latter are found to move to occupy essentially on-top sites on opposite sides of the dimers whilst the remaining chlorines move halfway towards the No. 4 sites and lie 1.16 A above the ideal surface. The resulting topology is thus identical to that obtained from adding the additional chlorine at the No. 4 sites. Simulating monolayer coverage by chemisorbing the chlorine onto the No. 6 (shared-dimer) sites on the opposite side of the dimers to the chlorinesaturated dan@ng-bonds results in these chlorines moving 0.44 A towards the No. 5 (pedestal) sites, and closer together by 0.92 A, to sit 1.69 A above the ideal surface. The dangling-bond chlorines and the Si-Si dimer bonds remain essentially unchanged. This topology, which has a 2 X 2 surface periodicity, is 0.17 eV per chlorine lower in energy than the above No. 4 site topology but 0.28 eV per chlorine higher in energy than the 1.0 ML symmetric dangling-bond configuration of Fig. 6. A third No. 6 starting geometry corresponds to the two additional chlorines being chemisorbed at the No. 6 sites lying between each pair of dimers within a given 2 X 2 surface unit cell. The optimum topology in this case is again.found to be the symmetric dangling-bond configuration of Fig. 6. The final No. 6 site scenario involves chemisorption at alternate No. 6 sites in each 2 X 1 surface unit cell. In this case the Si-Si dimer bonds and Si-Cl dangling bonds again remain essentially unaltered but the No. 6 chlorines intermediate between the dangling-bond chlorines move to within 0.47 A of their adjacent No. 4 sites, whilst the other No. 6 chlorines remain close to the? original positions. The 2 values are 0.54 and 1.11 A, respectively. This 2 X 2 structure has the highest energy of the No. 6 optimised topologies and is 1.03 eV per chemisorbed chlorine less stable than the 1.0 ML symmetric dangling-bond configuration. In previous work [13], we have also found a stable 2 X 1 bridge-site configuration for monolayer coverage of chlorine on the Si(OO112 X 1 surface. This structure is shown in Fig. 7. The chlorines lie directly on top of %e bridge sites of the ideal surface, 0.98 and 1.20 A above the silicon surface layer. The silicon substrate is essentially that of the ideal undistorted Si(OOl)l X 1 surface apart from an inward relaxation of the surface layer of 0.17 A. The

239

Science 319 (1994) 232-242

2.26 A’

2.26 A’

Fig. 7. The 1.0 ML bridge-site topology for chlorine chemisorption onto the Si@O1)2 X 1 reconstructed surface. The shading of the Si atoms is the same as that for Fig. 1.

Si-Cl bond lengths are 2.17 and 2.25 A. The upper and lower chlorines have acquired a charge of 0.34e and 0.30e respectively, with the surface silicons giving up 0.39e and the second-layer silicons gaining 0.20e. This bridge-site configuration is determined to be 0.75 eV per chemisorbed chlorine less stable than the symmetric dangling-bond configuration (see Table 2). This is consistent with the ab-initio calculations of Kruger and Pollmann [18] which predict an almost identical bridge-site topology. It is interesting to note, however, that the only evidence that we have found for the formation of this bridge-site topology in the current work is the 0.5 ML structure of Fig. 5 which resulted from chemisorption from below the surface at the No. 5 sites. Such a structure was not observed to occur for chemisorption from above the surface for any of the possible chemisorption sites considered in this paper for either 0.5 or 1.0 ML coverage. The starting configuration which yielded this local minimum in the earlier work involved chemisorbing one chlorine directly above one of the silicon dimer atoms and the other at an adjacent dimer-bridge (No. 1) site. The repulsion between the chlorines was then presumably large enough to break the Si-Si dimer bond and cause the on-top chlorine to move past the danglingbond site to the neighbouring bridge site. First saturating one of the dimer dangling bonds, as we have done in the current monolayer calculations, on the other hand, produces a sufficiently stable structure that subsequent chemisorption onto the dimer-bridge site is unable to break the Si-Si dimer bond and

240

M. W. Radny, P. V. Smith /Surface

results in the formation of the (considerably lower energy) symmetric dangling-bond configuration. This suggests that the structures that one might obtain from a typical adsorption experiment could depend quite critically not only on the total exposure but also on the rate of exposure of the surface to the adsorbate. Taken altogether, these results again indicate quite clearly that the dangling-bond site is the most energetically favoured site for chlorine chemisorption on the Si(OO1) surface and leads us to conclude that chemisorption for concentrations of up to one monolayer will take place predominantly on these sites to produce the fully saturated 1.0 ML configuration shown in Fig. 6. This is in agreement with both the ab-initio local density calculations of Kruger and Pollmann [18] and experiment [19-231. The local density of states distribution corresponding to this symmetric dangling-bond structure has also been shown [13] to be in good agreement with the LEEDS spectra of Aoto et al. [20] and the polarization-dependent angle-resolved photoemission studies of Johansson et al. [22]. The SLAB-MIND0 values of 2.16 A, 104.8”, 2.38 A and 2.30 A shown in Fig. 6 fre also in goo! agreementowith the values of 2.05 A, 105”, 2.40 A and 2.33 A predicted by Kruger and Pollmann, and the values of 2.10 A, 114.8”, 2.39 A and 2.36 A which we have obtained from a GAMESS ab-initio RHF calculation employing a Si,H,,Cl, cluster and the 6-31G* basis set. It should be noted, however, that all three of the above-calculated values for the Si-Cl bondlength are significantly larger than the NEXAFS determination of 1.95 f 0.04 A by Thornton et al. [21] and the SEXAFS value of 2.00 & 0.02 A obtained by Purdie et al. [26,27]. Our SLABMIND0 tilt angle of 14.8” and Kruger and Pollmann’s value of 15” are also considerably smaller than the recent ESDIAD determinations of 25 _+4” [22] and - 30” [23]. It is also worth mentioning that the experimental data for the Si(OOl)-Cl system appears somewhat contradictory with some work [19-231 exhibiting off-normal desorption consistent with dangling-bond chemisorption, while other research, based on the same techniques, indicates predominantly normal emission [24-281. Purdie et al. [26,27] suggest that the resolution of this problem revolves around the

Science 319 (1994) 232-242

degree of Cl exposure. They suggest that at low coverage ( < 0.5 ML), a single Cl sits essentially on top of a raised Si of a buckled dimer and hence gives rise to near-normal emission. At higher exposures, however, symmetric dimers occur with Cl atoms saturating the dangling bonds at close to the tetrahedral angle and produce the observed off-normal emission. This latter suggestion is clearly consistent with both our results, and the LDA results of Kruger and Pollmann [18], for monolayer coverage. It is also interesting to note in this context that our optimised type-2s and type-2a 0.5 ML configurations are characterised by two different buckled dimers with the corresponding Si-Cl bonds making angles with the surface normal of around 10 and 20 degrees. To investigate the low-coverage limit, we have performed a UHF geometry optimisation calculation for a single chlorine chemisorbing onto one of the dangling bonds of a Si,H,, cluster using the 6-31G* basis set. We find, contrary to the low-exposure structure proposed by Purdie et al., that the dimer is little buckled and the Si-Cl bond is oriented at about 20” to the surface normal. The precise origin of the near-normal emission which has been observed in some of the experiments is thus still somewhat unclear. It may arise, as some authors [24,25] have suggested, from bridge-site chemisorption, although our calculations find that chemisorption at these sites requires significantly more energy than alternative sites. Prediction of the symmetric dangling-bond configuration to be the optimum structure for the Cl chemisorbed Si(OO1) surface at monolayer coverage is in complete contrast to the case of fluorine chemisorption on the silicon (001) surface for which the SLAB-MIND0 method predicts the symmetric dangling-bond configuration to be unstable. In addition, no evidence has been found for either the “SiF,-like” or shared-dimer-dangling-bond configurations which were previously determined for monolayer coverage of fluorine on the silicon (001) surface [6,29]. It is worth noting in passing that an SiF, structure, in which two fluorines are bonded to a single silicon surface atom, has recently been observed in MD calculations of the Si(OO1)2 X 1-F surface [30]. Starting our SLAB-MIND0 geometry optimisation calculations with one of the dangling bonds per 2 X 1 unit cell saturated with fluorine and

M. W. Radny, P.V. Smirh/Surface

chemisorbing a further fluorine atom from above the No. 1 or No. 2 positions produces a “SiF,-like” topology, whereas performing the same calculation with chlorine gives rise to the symmetric danglingbond configuration of Fig. 6. Similarly, saturating one of the dangling bonds in each 2 X 1 unit cell and chemisorbing an additional fluorine from above the No. 5 site gives rise to a shared-dimer-dangling-bond configuration in which one of the fluorines occupies a dangling-bond site and the other lies just above the surface dimer at a bridge-site between the silicon atoms of two adjacent dimers (see Fig. 6 of Ref. [6]). Chlorine, on the other hand, again yields the symmetric dhgling-bond configuration for chemisorption above the No. 5 sites. Starting our chlorine optimisation from the “SiF,-like” topology found for fluorine chemisorption on the Si(OO1)2 X 1 surface results in the dimer bridge-site chlorine atom moving back to the unoccupied dangling-bond site to form the fully saturated symmetric dangling-bond configuration.

3. Conclusions The results of this paper indicate, in agreement with experiment, that chlorine exhibits quite different chemisorption behaviour on the Si(OO1)2 X 1 dimer reconstructed surface than fluorine. The danglingbond site is the preferred chemisorption site for chlorine concentrations of up to one monolayer. The most stable configurations for both 0.5 and 1.0 ML coverage are symmetric dangling-bond configurations. Subsurface chemisorption is unlikely to occur as the only stable configurations in which chlorine lies below the surface for concentrations up to one monolayer are those with quite high relative energies. In contrast to fluorine, the shared-dimer site does not constitute a very stable chemisorption site for chlorine, nor does chlorine form either of the 2x1 “shared-dimer-dangling-bond” or “SiF,like’ ’ monolayer minimum energy configurations found for fluorine. We are currently extending these calculations to concentrations of greater than one monolayer in an attempt to elucidate the mechanisms underlying the associated etching process. This work will form the basis of a subsequent publication.

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241

Acknowledgements We would like to thank the Australian Research Council (ARC) and our own university Research Management Committee for financial support during the course of this work. We also wish to acknowledge the Quantum Chemistry Group at North Dakota State University for providing us with the GAMESS’92 Software.

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