Monolayer chemisorption of HCl and Cl2 on the Si(001)2 × 1 surface

Surface Soence Letters 290 (1993) L662-L666 North-Holland

surface soence letters

Surface Science Letters

Monolayer chemisorption of HCI and C12 on the Si(001)2 × 1 surface B I Craig and P V Smith Physws Department, Unwersayof Newcastle, Callaghan, NSW 2308, Austraha Received 25 February 1993, accepted for pubhcat~on 11 March 1993

In an earher paper results were reported for monolayerchemlsorptlon of HCI, CI 2 and F 2 onto the Sl(001) surface It has now been discovered that the Cl interactions were not treated correctly m these SLAB-MINDOmolecular orbital calculatmns The mare purpose of this Letter is thus to present the corrected results for both HCI and Cl 2 chemlsorptlon Danghng bond configurations are now found to represent the lowest energy configurations for both chem~sorptlonprocesses Thts ~s m contrast to the prevtously pubhshed results m which a bridge site-topologywas determined to be the minimum energy structure for Cl 2 chemtsorpUon onto the Sx(001)2× 1 reconstructed surface These new results are shown to be m good agreement with self-consistent ab-mmo Hartree-Fock cluster calculations, with the first-pnnclples local-density calculations of Kruger and Pollmann, and with experiment

1. Introduction

2. Results

In a previous pubhcatlon [1] we have exammed the technologically important problem of the chermsorptaon behavlour of HC1, Cl 2 and F 2 o n t o the S1(001) surface using the well-known SLABM I N D O molecular orbital method In these calculations, all three species were assumed to dlssocrate before bonding to the surface and the required parameters were obtained by fitting to both bulk and molecular properties [1,2] Our surface geometry optlm~satlon code, however, was not modified to take into account the fact that 2s and 2p Slater orbltals were being used to represent chlorine, rather than the customary 3s and 3p functions As a result, the tmnLrnum energy configurations which we have reported for HCI and Cl z chemlsorptlon on the $I(001) surface have not been derived from an appropriately optlmlsed set of chlorine orbltals The purpose of this Letter is to present the amended results and to compare these w~th the pred~cUons of some recent ab-mmo R H F d u s t e r calculattons on these systems All of the calculational details are the same as outhned in the earher paper [1]

The new mlmmum energy structure for monolayer HCI chemlsorptlon onto the $1(001)2 × 1 reconstructed surface is shown m fig 1 This dangling bond topology is only shghtly different from that reported earher [1] The S1-H bondlength is unaltered at 1 53 .~ whilst the angle of the S I - H bond to the surface normal has changed marginally from 19.6° to 18 3°. The S1-SI dtmer bond again exhibits no tendency to r,v~st about the surface normal but is shghtly shorter at 2 37 ,~ The main differences lie in the larger S1-CI bondlength of 2 12 ~ (compared to 2 05 A reported previously), the change in its tdt angle from 19 0 ° to 17 7°, and the fact that the silicon atom to which the chlorine is bonded now hes below its dlmer counterpart by 0.07 ,~ The SI-SI dlmer ~s thus predicted to have the opposite buckle although the tilt angle is again qmte small at 1 70° The chemlsorbed chlorine is found to be far more electronegattve, plclang up 0 46e, compared to the previous value of 0 26e, while the hydrogen again produces little charge transfer within the surface Our ab-mlt~O self-conststent

0039-6028/93/$06 00 © 1993 - Elsevier Science Pubhshers B V All rights reserved

B I Crarg, P V Smrth / Monolayer chemlsorptwn of HCl and Cl, on the S1(001)2 X I surface

restricted Hartree-Fock (RI-IF) cluster calculations using the GAMESS molecular orbital package and an STO-3G basis set give essenttally the same topology The Si-Si, Si-Cl and Si-H bondltngths are predrcted to be 2 38, 2 13 and 149 A, respectively, and the Si-Cl and Si-H bonds to make angles of 1105” and 113 5” with respect to the Si-Si dimer bond For monolayer chemisorptton of chlorme onto the S1(001)2 x 1 reconstructed surface we determme both a danghng bond conftguration and a bridge-sue conftguration as before, only now the former is more stable by 0 75 eV per surface atom This is consistent with both the ab-mitio theoretical calculations of Kruger and Pollmann [3], and wrth experiment [4-81 The mrnmum energy “tetrahedral” dangling bond configuration is shown m fig 2 Both buckling and twistmg of the dimers IS found to Increase the total energy The SI-Si dimer bondlength has a characteristic single-bond length of 2 38 A, lust slightly longer than for HCI chemrsorption The &Cl bonds are agam predicted to be tilted outwards at an angle of 14 8” to the surface normal but are considerably longer than before at 2 16 A The bondlength of the Si-Si back-bonds is found to be 2 30 A These values are m good agreement with the correspondmg values of 2 40 A, 15”, 2 05 A and

Fig 1 The SLAB-MIND0 optlmlsed topology correspondmg to the chemlsorptlon of a monolayer of HCl onto the S@Ol)2 X 1 surface

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Fig 2 The “tetrahedral” symmetnc dlmer topology resultmg from Cl, monolayer chemlsorptlon on the S1(001)2x 1 surface

2 33 A predicted by the first-prmcrples local density calculations of Kruger and Pollmann [3]. They are also consistent with the values of 2.40 A, 23 7”, 2 13 A and 2.36 A which we have obtained via GAMESS ab-mttio RHF STO-3G cluster calculattons It is interesting to note that all of these theoretically determmed Si-Cl bondlengths are stgmftcantly larger than f,he experimental NEXAFS value of 195 f 0.04 A determmed by Thornton et al. [6] whilst all of the calculated bondangle values fall within their determmatron of a tilt angle of somewhat less than 25” Both our SLAB-MIND0 tilt angle of 14 8” and Kruger and Pollmann’s value of 15”, however, are considerably smaller than the 25 f 4” mchnation derrved from the recent ESDIAD measurements of Cheng et al. [8] As for the amended HCl SLAB-MIND0 calculattons, the chlorines are found to be much more electronegatrve than before gaming 0.57e, predommantly from the surface sthcons, although srgmficant charge transfer occurs down to the fifth layer The eqmhbrmm bridge-site confrguratton which we now determme is shown m fig 3 and is slmrlar to that reported previously The St substrate is again essentially that of the ideal undistorted clean Sr(OO1)surface although the inward relaxation of the surface layer is somewhat greater

B I (rale, P V 5mtth / Monolaw'r ~hemtsorptton o] HCl und Cl 2 on the 5t(001)2

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Fig 3 The minimum energybridge-site configurationcorresponding to the chemlsorptlonof a monolayerof CI2 onto the Sl(001)2× 1 surface

at 0 17 ,~ The chlorines he directly on top of the bridge-sites of the ideal surface as before but their heights above the sd~con surface layer of 0 98 and 1 20 ,~ are much more slmdar than the earlier values of 0 71 and 1 13 ,~ The SI-CI bondlengths are also longer than before at 2 17 and 2 25 A The chlonnes acqmre less charge m thxs bridge-site configuration than for the danghng bond conflguraUon with the lower and upper chlorines gaming 0 30e and 0 34e, respectively Slgmflcant charge redistribution occurs down to the fifth layer with the sthcon surface atoms gwlng up approximately 0.40e and the second layer silicon atoms gaming about 0 20e Kruger and Pollmann [3] have also found a bridge-site topology to yield a stable configuration for monolayer chlorine chermsorpUon on the Sl(001) surface This structure they fred to be less stable than their dangling bond topology by 0 96 eV per surface dlmer Th~s is consistent with the amended results reported here which, as stated above, now predict the dangling bond geometry to be more stable than the bridge-site topology by 1 50 eV per surface dLmer Moreover, comparison of our new brldge-s~te topology with that obtained by Kruger and Pollmann reveals excellent agreement, significantly better than that of our earlier bridge-site geometry (see fig 11 of ref [3]) Ewdenee for both a danghng bond topology and a bridge-site topology on the SI(001) surface has also been observed m the ESDIAD experiments of Cheng et al [8] They have also found, by anneahng the surface, that the br~ge CI ge-

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ometry is less stable than the mono-coordlnated S~-C1 structure As before, we have found no evidence tor ,i stable molecular-like configuration similar to that which occurs for monolayer fluorine chemlsorptlon on the Sl(001) surface [1] Starting our SLAB-MINDO optlmlsation procedure from such a molecular-like configuration results m the topology converting back to the minimum energy symmetric dlmer dangling bond geometry shown in fig 2 0 p t l m l s l n g the geometry ot a cluster with a molecular-like topology using the GAMESS RHF molecular orbital package and a STO-3G set of basis functions, also produced the corresponding dangling bond topology We beheve that this IS due to repulsion between the (charged) chlorines and the fact that the SI-CI bondlength is approximately 30% greater than the SI-F bondlength The local density of state functions corresponding to a chlorine adatom in the HC1, danghng bond C12 and bridge-site CI 2 configurations, are plotted in figs 4a-4d As before, these curves have been smoothed with a 1 3 5 3 1 five-point sampling and restricted to the valence band region ( E < E F) We observe that the HCI and dangling bond C12 distributions are quite simdar and show a main peak (which is of predommantly p,, character) and a subsidiary peak (which is mainly p. in character) separated by 1 8 and 1 7 eV, respectively This is in excellent agreement with the LEELS spectra of Aoto et al [5] which clearly exhlbtt two chlorine adsorbate peaks separated by approxamately 1 6 eV Moreover. Aoto et al assign their main peak to a transition from the Cl(p x, p~) bonding states to the Cl(p:) antibonding state and their lesser peak to a transition from the Cl(p.) bonding state to the Cl(p:) antibonding state, in agreement with our results Only the distribution corresponding to the lower ot the two bridge-site chlorines exhibits a main peak and a higher energy peak separated by around 1 6 eV and this subsidmry peak is very weak and of predominantly P~v, rather than Pz, character These results thus argue strongly for the dangling bond configuration being the preferred topology for monolayer chlorine chemisorpt~on on the SI(001) surface

B I Crmg, P V Smith / Monolayer chemtsorptton of HCl and Cl2 on the S~(001)2 x 1 surface

The local density of state distribution for the dangling bond configuration is also more consistent with the polarization-dependent angle-resolved photoem~ssion studies of Johansson et al [7] These identified three main chlorine induced features at 8.2 (SI), 6 7 (S 2) and 5.7 eV (S 3) below the Fermi energy The weak 8 2 eV peak was attributed to Sl-Cl bonding orbltals of predominantly Pz character while the two lower peaks were believed to be mainly Pxy in nature Since Johansson et al. determine the Fermi energy of this chlorine chemlsorbed system to lie 1 1 eV above the valence band maximum (VBM), these three peaks are predicted to lie 7 1, 5 6 and 4 6 eV below the top of the valence band The dan-

L665

gllng bond distribution shown m fig 4b is observed to have peaks at 6 4 eV (Pz), 4 7 eV (Pxy) and 3.7 eV (pxy, Pz) with respect to the valence band maxtmum This is clearly in excellent agreement with the relative spacing and classification of the peaks reported by Johansson et al All three peaks occur about 0 8 eV lower in energy, however, and the 3 7 eV peak is quite weak. The local density of state distributions for the two bridge-site chlorines correlate poorly with the photoemission results glwng peaks (relative to the VBM) of 8 1 eV (p~r), 7 3 eV (Pxr), 5 6 eV (p~r) and 4 0 eV (Pxy, Pz) for the lower chlorine and 7 7 eV (Pxr), 5 4 eV (pxr) and 4.1 eV (Pxr, Pz) for the upper chlorine

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Fig 4 Valence band local density of states curves a p p r o p n a t e to chlonne chemlsorpUon on the Sl(001)2 × 1 surface Curves (a) and (b) correspond to the "tetrahedral" dimer HC! and C12 configurations whilst curves (c) and (d) are for the lower and upper chlorine adatoms of the CI 2 m l m m u m energy bndge-slte topology ( s-orbltals, - p=, - . . . . . Pxy, ~ total)

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B I Cratg, P V 6mtth / Monolaver chemtsorptzon o] HCI and C72 on the St(001)2 × 1 sur]ace

3. Concluding remarks In thts Letter we have presented revised resuits for the monolayer chemlsorptlon of HC1 and C12 onto the St(001)2 × 1 reconstructed surface In each case we have assumed that the molecules first dissociate into atoms which then attach themselves to the surface slhcons As expected, the minimum energy configuration for the Sl(001)2 × 1 HC1 system is a slightly buckled dlmer with a bondlength characteristic of a SI-S1 single bond and with the hydrogen and chlorine couphng into the surface danghng bonds at close to the tetrahedral angle The optimal structure for a monolayer of chlorine on the $1(001) surface is predicted to be a symmetric &mer danghng bond ("tetrahedral") geometry in agreement with both experiment and other theoretical calculations A stable bridge-site topology is also pre&cted to occur for this system but is less energetically favourable by 1 5 eV per surface &mer No evidence was found for the molecular-hke geometry which has been determined for the St(001)2 × 1 F 2 system Ab-tmtlO R H F STO-3G cluster calculations have been shown to prowde strong confirmation of the reltabdlty of the SLABM I N D O results

Acknowledgements We are grateful to the Australian Research Council for continuing to support this project We would also hke to thank the Quantum Chemistry group at North Dakota State University for providing us with the GAMESS molecular-orbital package and Professor Pollmann for sending us a preprmt of their work

References [1] B I Craig and P V Smith, Surf Scl 262 (1992) 235 [2] RC Bmgham, MJS Dewar andDH Lo, J Am Chem Soc 97 (1975) 1285 [3] P Kruger and J Pollmann, Phys Rev B 47 (1993) 1898 [4] J E Rowe G Margarltondo and S B Chnstman, Phys Rev B 16 (1977) 1581 15] N Aoto, E Ikawa and Y Kurogl, Surf So 199 (1988) 408 [6] G Thornton, P L Wlncott, R McGrath, I T McGovern, F M Qumn, D Norman and D D Vvedensky, Surf So 211/212 (1989) 959 [7] L S O Johansson, R I G Uhrberg, R Lmdsay,P L Wlncott and G Thornton, Plays Rev B 42 (1990) 9534 [8] CC Cheng, Q Gao, WJ Choyke and JT Yates, Jr Phys Rev B 46 (1992) 12810