adsorption of methylchloride on si(100) from first principles

JOURNAL OF CHEMICAL PHYSICS VOLUME 119, NUMBER 2 8 JULY 2003

Adsorption of methylchloride on Si „100… from first principlesAldo H. Romeroa)

Facultad de Fisica, Pontificia Universidad, Catolica de Chile, Casilla 306, Santiago 22, Chile

Carlo SbracciaINFM (Udr Padova and DEMOCRITOS National Simulation Center, Trieste, Italy)and Dipartimento di Fisica ‘‘G. Galilei,’’ Universita` di Padova, via Marzolo 8, I-35131 Padova, Italyand Scuola Internazionale Superiore di Studi Avanzati (SISSA), Via Beirut 2/4, 34014 Trieste, Italy

Pier Luigi Silvestrellib) and Francesco AncilottoINFM (Udr Padova and DEMOCRITOS National Simulation Center, Trieste, Italy)and Dipartimento di Fisica ‘‘G. Galilei,’’ Universita` di Padova, via Marzolo 8, I-35131 Padova, Italy

~Received 25 March 2002; accepted 10 April 2003!

The chemisorption of methylchloride (CH3Cl) on Si~100! is studied from first principles. We findthat, among a number of possible adsorption configurations, the lowest-energy structure is one inwhich the methylchloride molecule is dissociated into CH3 and Cl fragments which are bound to thetwo Si atoms of the same surface dimer. Our calculations show that dissociative chemisorption ofmethylchloride on Si~100! may proceed along different reaction paths characterized by differentenergy barriers that the system must overcome: some dissociation processes are mediated by amolecular precursor state and, at least in one case, we find that the dissociation process isnonactivated, in agreement with recent experimental findings. We have also generated, for manypossible adsorption structures, theoretical scanning tunneling microscopy images which couldfacilitate the interpretation of experimental measurements. ©2003 American Institute of Physics.@DOI: 10.1063/1.1578993#

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I. INTRODUCTION

The interaction of alkyl halides with semiconductor sufaces is of great technological interest. In particular, the hreactivity of methyl halides compared to hydrocarbomakes them potential candidates for diamond and siliconbide ~SiC! film growth.1,2

It has been shown that methyl halides adsorb dissotively on Si~100! at room temperature to produce methgroups and halogens bound on the surface.1–4 In particular,adsorption of methylchloride (CH3Cl) on Si~100! has beenrecently investigated. Brown and Ho,2 using electron energyloss spectroscopy, Auger electron spectroscopy, and tempture programmed desorption, found that methylchloride dsociatively adsorbs on Si~100! with high probability belowroom temperature, to produce a methyl group and a chlorthey also discussed the possible existence of a precursorto dissociative adsorption; in addition, their observation tno chlorine is present on the surface for exposure temptures higher than 700 K is particularly interesting, becausmakes methylchloride a promising candidate for carbon figrowth. Bronikowski and Hamers3 used scanning tunnelinmicroscopy~STM!, in the temperature range of 300–450and confirmed dissociative adsorption of methylchlorideSi~100!. On the basis of counting statistics of Cl and CH3

fragments deposited on the Si~100! surface, they suggestethat, at room temperature, dissociative adsorption of met

a!Present address: Advanced Materials Department, Av. Venustiano Car2425-A, 78270 San Luis Potosi, SLP, Mexico.

b!Electronic mail: [email protected]

1080021-9606/2003/119(2)/1085/8/$20.00

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chloride on Si~100! can occur in two different ways: one iwhich both Cl and CH3 fragments bond to the surface, anthe other in which the Cl atom is abstracted from the CH3Clmolecule by the surface and the methyl group so createejected away from the surface.

More recently, Lee and Kim4 studied the adsorptionmechanism of CH3Cl on Si~100! using both experimentatechniques~Auger electron spectroscopy! and semiempiricalcalculations. The experiments showed a decrease insticking probability with increasing incident energy, indicaing that the chemisorption of methylchloride is probably mdiated by a precursor~physisorbed! state of the intact mol-ecule. Moreover, the chemisorption probability was foundRef. 4 to decrease with increasing surface temperature.is a clear indication that the dissociation process isnonacti-vated, i.e., the top of the activation barrier to chemisorptiis lower than the vacuum zero potential.

These conclusions are supported by calculations,4 forboth the optimized precursor state geometry and the poteenergy curve for the chemisorption of CH3Cl on Si~100!,which are based on the semiempirical PM3 method appto a cluster model.4 From these calculations Lee and Kimwere able to calculate the difference in activation barrheights for chemisorption and desorption from the precurstate, which turned out to be in reasonable agreementthe value estimated from the measured sticking coefficie

Although these theoretical results are interesting andparently in semiquantitative agreement with some expmental findings, the accuracy of the method used is sowhat questionable. For instance, application5 of the sametheoretical approach to the adsorption of benzene on Si~100!

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1086 J. Chem. Phys., Vol. 119, No. 2, 8 July 2003 Romero et al.

gives, as the lowest-energy configuration, a ‘‘pedestal’’ owhile more accurateab initio calculations and a number oexperimental measurements show that this configuratioenergetically highly unfavored with respect to others~see,for instance, Ref. 6, and further references quoted there!.Moreover, if the Si~100! surface is modeled using a smacluster, as done in Ref. 4, it is not possible to reproducecorrect ‘‘buckling’’ of the surface Si dimers. It is well knownhowever, that the asymmetric electron distribution resultfrom the buckling distortion of the surface dimers is a kfactor for the chemistry of dissociative molecular adsorpton the Si~100! surface: in fact buckling is essential for estalishing orbitals that can interact effectively with the incidemolecules.7

In order to overcome the limitations of previous theorical calculations and to clarify a number of experimenfindings for this system, we performed a fullab initio studyof methylchloride adsorption on Si~100!, by using the Car–Parrinello approach,8 with a slab geometry and periodiboundary conditions. We confirm that, in the lowest-enestructure, the methylchloride molecule is dissociated iCH3 and Cl fragments which are bound to the two Si atoof the same surface dimer.

However, as far as the detailed chemisorption mecnism is concerned, our results differ considerably from threported by Lee and Kim.4 In fact we find different possibledissociation paths: the starting configuration of the methchloride molecule can be one of two possible precurstates, where the molecule is almost parallel to the Si dimand either both the CH3 and Cl fragments produced by thdissociation, or only the Cl atom form new bonds with tsurface; alternatively a direct~i.e., not mediated by any precursor state! activated chemisorption may occur where tmolecule dissociates via the breaking of the CH3– Cl bondand the formation of a new Cl–Si bond, while the CH3 frag-ment is ejected from the surface. These results are in qutative agreement with the interpretation of STmeasurements,3 as discussed in the following. We find, iagreement with experiments, that one of the physisorbedcursor states can dissociate following anonactivatedreactionpath, which is different however from that proposed by Land Kim,4 and which involves adsorption of the dissociatCH3 and Cl fragments on different surface Si dimers.

II. METHOD

Calculations have been carried out within the CaParrinello approach8 in the framework of the density functional theory, using gradient corrections in the BLYimplementation;9 some calculations have been also pformed using other gradient corrected functionals~see Sec.III !. Gradient-corrected functionals have been adopted inmost recent theoretical studies of adsorption of organic mecules on Si~100! because they are typically more accurathan the local density functional in describing chemical pcesses on the Si~100! surface.10,11The calculations have beecarried out considering theG-point only of the Brillouinzone, and using norm-conserving pseudopotentials.12 Wavefunctions were expanded in plane waves with an energy


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off of 40 Ry. We have explicitly checked that, at this valuethe energy cutoff, the structural and binding properties ofsystem are well converged.

In order to test our method, we have preliminarily caculated the structural, electronic, and bonding propertiesthe free methylchloride molecule~see Table I! and find agood agreement with the experimental results.13 Methylchlo-ride is a prototypical polar molecule whose dipole momemay affect its reactivity with the Si~100! surface. Our com-puted dipole moment is in reasonable agreement withexperimental measurements. The same is true also forvibrational properties. In additional test calculations of strutural and bonding properties of other small molecules ctaining Si, C, H, and Cl atoms, namely SiCl4 , CCl4 ,ClSiH3 , and SiH2Cl2 , we obtained a level of accuracy comparable to that achieved for the methylchloride molecule

The Si~100! surface is modeled with a periodically repeated slab of five Si layers and a vacuum region of 11~test calculations have been performed using even a lasize of the vacuum region!; such a relatively large value irequired in order to make the interactions between the ladipole moment of the methylchloride molecule with its peodic images negligible. A monolayer of hydrogen atomsused to saturate the dangling bonds on the lower surfacthe slab. We have used a supercell withp(A83A8)R45°surface periodicity, corresponding to eight Si atoms/layer awith the 232 surface reconstruction, i.e., the lowest-enerreconstruction of the Si~100! surface which is compatiblewith this supercell. However, in order to check finite-sieffects, some calculations have been repeated using a lap(434) supercell with 16 atoms/layer. Other details of tmethod can be found in Refs. 6, 14, and 15.

The most likely pathways and energy barriers characizing the different dissociation processes have been obtaby means of a recently proposed variant of the popu‘‘nudged elastic band’’ method, i.e., the ‘‘climbing imagnudged elastic band’’~CI-NEB! method.16

III. RESULTS

Our calculations confirm, in agreement with the predtions of previous studies,2–4 that the stable adsorption configuration of methylchloride on Si~100! is a dissociatedstructure with a methyl group and a chlorine atom boundthe Si surface. Two such structures are possible: configtion A in Fig. 1, which is found to be the most stable on

TABLE I. Calculated structural parameters, dipole momentm, and energy ofdissociation~into CH3 and Cl fragments,DE1 , and into CH2Cl and Hfragments,DE2) of the free CH3Cl molecule, and available experimentadata~Ref. 13!.

Present calculation Experiment

d ~C–Cl! ~Å! 1.82 1.78d ~C–H! ~Å! 1.09 1.09

/H–C–H ~deg! 111.1 110.8/Cl–C–H ~deg! 107.8 ¯

m ~D! 1.98 1.89DE1 ~eV! 3.40 3.62DE2 ~eV! 4.34 ¯


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1087J. Chem. Phys., Vol. 119, No. 2, 8 July 2003 Adsorption of methylchloride on Si(100) first principles

where the two fragments are bound to the same Si surdimer, and the one~not shown in the figure! where the me-thyl group and the chlorine atom are bound to Si atomslonging to different surface dimers. The latter configuratiois higher in energy than the A structure by 0.24 eV. In prciple, however, other dissociative adsorption processespossible, which we have investigated: the resulting omized structures are shown in Fig. 1. They can be obtainfor instance, by breaking a C–H bond with the formationa CH2Cl and a H fragment~configuration B in Fig. 1!; notethat a configuration similar to the B one~with the C atomreplaced by a Si atom! has been suggested as the stable,energy structure in a recent study of the adsorption of chrosilanes on the Si~100! surface.17 Two other different struc-tures can be obtained if the detached Cl or H atom, insteabeing bound to a single Si atom of a dimer, actually bridgthe two Si atoms of a dimer which is then essentially clea~configurations C and D, respectively!. The binding energiesand structural parameters for the various configuratishown in Fig. 1 are reported in Table II. The configuratiocharacterized by the scission of the C–H bond, althoughresenting possible stable adsorption states, are clearly egetically unfavored. This is consistent with the observat~see Table I! that in the free methylchloride molecule thC–H bond is considerably stronger than the C–Cl bond.

One key question in the chemistry of organic molecuon semiconductor surfaces is the existence of precu

FIG. 1. Possible configurations for dissociative chemisorption of CH3Cl onSi~100!. White, black, dark-gray, and light-gray balls indicate H, C, Cl, aSi atoms, respectively. For clarity only a few surface Si atoms are sho

TABLE II. Binding energies and structural parameters of the configuratishown in Fig. 1.

A B C D

Ebind ~eV! 3.07 1.44 1.82 0.29d ~C–Cl! ~Å! 3.84 1.84 3.13 1.84d ~C–Si! ~Å! 1.91 1.92 1.89 1.92d ~Cl–Si! ~Å! 2.12 ¯ 2.26 ¯

d ~Si–H! ~Å! ¯ 1.48 ¯ 1.64d ~Si–Si dimer! ~Å! 2.44 2.43 4.12 3.34

Buckling ~Si–Si dimer! ~deg! 2.4 0.1 ¯ ¯


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states, i.e., physisorbed states for the intact molecule, webound to the Si~100! surface, and characterized by a reltively large mobility of the adsorbed molecule. From a lonlived precursor state the molecule can eventually find a loenergy barrier pathway to dissociative chemisorption intofinal, stable configuration.

The existence of a precursor state for adsorption of mthylchloride on Si~100! has been proposed by somauthors.2,4 In particular, Lee and Kim,4 using the semiempir-ical PM3 method, studied three adsorption configurationsthe intact methylchloride molecule on the Si~100! surface~see Fig. 3 of Ref. 4!. By comparing the potential energcurves computed as a function of the molecule–surfacetance of these physisorbed configurations with the potenenergy curve of the chemisorbed structure~corresponding tothe dissociated molecule in configuration A!, they propose,as the most likely precursor state, a configuration characized by a methylchloride molecule having its axis parallela surface Si dimer. They also conclude, on the basis osimple curve-crossing argument, that the dissociation procinitiated from this precursor state is nonactivated, i.e.,activation barrier to chemisorption,Ea , is lower than theenergy required to desorb the methylchloride molecule,Ed

~see Fig. 4 of Ref. 4!. From their calculated potential enegies curves they were able to derive a value for the diffenceEd2Ea in the activation barrier heights for chemisortion and desorption from the precursor state, which issemiquantitative agreement with the value obtained fromanalysis of the experimental sticking probabilities.

Although the approach used in Ref. 4 may give a quatative hint on the barrier for dissociation in the case of simsystems, it cannot adequately predict the behavior of coplex molecules on covalently bonded semiconductor sfaces. Here the concerted optimization of several degreefreedom is essential in the correct determination of a realireaction path, and the potential energy surface for the retion cannot be simplified via a one-dimensional curvcrossing argument like the one used in Ref. 4.

We have thus repeated the calculations reported in Reby using a different approach, which we believe to be maccurate in several respects. Our method is fullyab initiowhile that of Lee and Kim4 is semiempirical in nature~seealso the discussion in Sec. I!. Moreover, we use a slab geometry in contrast with the cluster model adopted by Lee aKim: this allows us to reproduce the characteristic ‘‘bucling’’ of the surface silicon dimer~which is absent in thecluster calculations of Ref. 4! and correctly describe the different electronic properties of the ‘‘up’’ and ‘‘down’’ Si atoms, and also to take into account possible strain effectsthe Si~100! surface due to the adsorption of the methylchride molecule.

Our results and conclusions differ considerably frothose of Lee and Kim.4 Among the three adsorption configurations proposed by Lee and Kim, as possible precurstates~see Fig. 3 of Ref. 4!, we find that only a configuration~P hereafter!, characterized by a C-Cl bond almost parallela Si dimer~see Fig. 2!, is stable, even though its adsorptioenergy of 0.1 eV~see Table III! is very small~being of theorder of magnitude of a typical value for a physisorb

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state!. The Cl–Si distance in this structure is 2.69 Å, i.considerably longer than the Cl–Si bond length~2.12 Å! inthe dissociated configuration A shown in Fig. 1, while tSi–Si bond length of the underlying Si dimer is 2.42 Å, i.only slightly larger than the value of the clean Si surfa~2.34 Å!. Note that, in this structure, the very electronegatCl atom prefers to bind to the ‘‘down’’ Si atom of a surfacdimer ~the electrophilic one! rather than to the ‘‘up’’ Si atom~the nucleophilic one!.

The properties of weakly bound molecules, such asconfiguration P, could be sensitive to the form of tgradient-corrected density functional used to compute telectronic properties~see, for instance, Ref. 18!. We havethus checked that the main conclusions described abovenot substantially altered when a different recipe for tgradient-corrected functional is adopted. The main diffence, when using the HCTH functional19 or the PBEfunctional20 instead of BLYP, is that the adsorption energyconfiguration P is increased to a value of 0.20 and 0.29respectively~this is not surprising, since it is known thathese functionals tend to give more weight to weak intermlecular interactions18,19!, while the structural parameters re

FIG. 2. Potential energy curves, plotted as a function of the distance fthe CH3Cl molecule to the Si~100! surface, relative to the physisorbed stafor the intact molecule~dashed line with open circles!, and relative to thedissociated state CH31Cl ~solid line with closed circles!. The minima of thetwo curves correspond to configurations P and A, respectively, and theof the potential energy corresponds to an infinite separation of the CH3Clmolecule from the Si surface. The distance from the surface is taken adistance between the ‘‘center’’ of the methylchloride molecule~defined byaveraging the coordinates of the C and Cl atom! and the ‘‘center’’ of theclosest silicon dimer~defined by averaging the coordinates of the twoatoms of the dimer!.

TABLE III. Adsorption energies and structural parameters of the weabound, precursor states, P and P8, shown in Figs. 3 and 4, respectively. Dahave been obtained using the BLYP functional while values obtainedthe PBE functional are in parentheses: as can be seen the PBE confitions are characterized by larger adsorption energies and shorter Cl–Stances.

P P8

Ed ~eV! 0.10 ~0.29! 0.04 ~0.27!d ~C–Cl! ~Å! 1.85 ~1.82! 1.85 ~1.83!d ~Cl–Si! ~Å! 2.69 ~2.56! 2.61 ~2.53!

d ~Si–Si dimer! ~Å! 2.42 ~2.42! 2.42 ~2.41!


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main essentially unchanged: however the other two confirations proposed in Ref. 4 as possible stable states arefound to be unstable.

Having determined a configuration~P! for a possible ad-sorption state of the intact molecule, the next relevant stesearching for a low-energy pathway connecting this statethe final, dissociative chemisorption state~configuration A inFig. 1! and trying to estimate the height of the energy barrseparating the two states. Of course this represents acomplex optimization problem since, in principle, many dferent routes are possible. As already mentioned, LeeKim4 investigated this important issue using an oversimpfied and indirect approach: they considered the behaviothe potential energy curve, calculated as a function ofmolecule–surface distance, for the selected physisorbedfiguration, and compared it to the corresponding curvethe chemisorbed structure: since the crossing point betwthe two curves~see Fig. 4 of Ref. 4! corresponds to a valueof the potential energy which is negative~assuming that thepotential energy is set to zero when the CH3Cl molecule isfar from the Si surface!, they conclude that the dissociatioprocess is nonactivated. However, no detailed descriptiothe pathway followed by the methylchloride molecule is prposed. Moreover, as our calculations show, the simple ament based on the crossing of the potential energy curvenot as cogent as the authors of Ref. 4 make it.

Starting from the physisorbed configuration P shownFig. 2, we have computed the potential energy curvesdone in Ref. 4. As can be seen from our results, reporteFig. 2, a simple curve-crossing criterion should lead toconclusion that the dissociation process is actually barrless, at variance with the results of Ref. 4 where a barEa;0.12 eV was instead found by using the same simargument.

We will show in the following that none of the abovementioned conclusions are correct, and that a realisticscription of the dissociation reaction of methylchlorideSi~100! requires a more sophisticated treatment than tbased on the above-described curve-crossing argument.apparent agreement of Lee and Kim with the experimendata, being based on an oversimplified approach, shouldbe considered fortuitous.

In order to estimate as precisely as possible the reacpathways and energy barriers characterizing the differentsociation processes which may occur at the surface we hused a recently proposed variant of the ‘‘nudged elaband’’ method,16 which has proven to be a very accurate aefficient technique to determine minimum energy pathscomplex chemical reactions.

By applying this method we indeed find a reaction paway starting from configuration P, and resulting in the dissciation of the adsorbed molecule~configuration A in Fig. 1!.The detailed path for such a reaction, together with its cresponding energy profile, is shown in Fig. 3. From Fig. 3valueEa50.51 eV for the activation energy can be extract(Ea50.45 eV adopting the PBE functional instead of tBLYP one!. By using the calculated adsorption energyEd forconfiguration P, reported in Table III, we can thus compthe differenceEd2Ea520.41 eV (20.16 eV using the PBE

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Ed2Ea50.28 eV. We recall that a positive value for thquantity implies anonactivatedprocess, whereas a negativvalue implies anactivatedprocess.

We note at this point the sensitivity of our results to tform of the gradient corrected density functional used foractual calculations. As it appears from Table III, quite diffeent values for the adsorption energies of weakly bound stare found, depending on the use of the BLYP functionalthe PBE one.20 The calculated value for the activation enerEa ~see Table IV! does also depend on the choice madethe gradient-corrected functional, although in a less dramway. In particular, for the dissociation process initiated frothe physisorbed state P described earlier, we find anegativevalue forEd2Ea , irrespective of the functional form usedWe are thus forced to conclude that the dissociation procinitiated from the physisorbed state P is anactivatedone:therefore the precursor state for a nonactivated dissociaprocess must be different from that proposed by LeeKim4 ~which is similar to our configuration P!.

Note that, always starting from configuration P, a diffeent dissociation process is possible in which, after the breing of the CH3– Cl bond, only the Cl atom forms a new bonwith a surface Si atom, while the CH3 fragment is insteadejected away from the surface. In this case~see Table IV! theenergy barrier that the system must overcome, 0.49 eVvery similar to that relative to the previous process. T

FIG. 3. Dissociative reaction path from the weakly bound precursor stato the configuration A~the BLYP functional is used!. For clarity only a fewsurface Si atoms are shown. The zero of the energy is taken as therelative to the separated subsystems@CH3Cl molecule and clean Si~100!surface#. The value 0.0 of the reaction coordinate corresponds to the presor state P while the value 1.0 corresponds to configuration A. Linesobtained using the interpolation method proposed in Ref. 16.

TABLE IV. Estimated energy barriers,Ea , for different possible dissociation processes~see the discussion in the text!. Data have been obtained usinthe BLYP functional~values obtained with the PBE functional are in paretheses!.

Initial conf. Ea ~eV! Chemisorbed fragments

P 0.51~0.45! Cl, CH3

P 0.49 ClP8 0.16 ~0.13! Cl, CH3

P8 0.55 ClV1 0.51 Cl


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Interestingly, we found another weakly bound statethe intact molecule (P8 hereafter!, shown in Fig. 4~see alsoTable III!, whose adsorption energy is even smaller than tof the P one. In this case the methylchloride molecule is salmost parallel to the Si dimers, but the CH3 group is nowlocated between two dimer rows, while the Cl atom~as inconfiguration P! is again close to the ‘‘down’’ Si atom of adimer. Two different dissociation processes initiating from8are possible, in which either~i! both the CH3 and Cl frag-ments~see Fig. 4! or ~ii ! only the Cl atom form new bondswith the surface Si atoms; the energy barriers for these pcesses~see Table IV! are 0.16 and 0.55 eV, respectively. Nothat, at variance with the similar process initiated from tstate P~see Fig. 3!, when both the Cl atom and the CH3

group stick to the surface they bind to different Si surfadimers. Although the final~fully relaxed! configuration isslightly less stable than structure A, as discussed previouour calculations indicate that it is possible to reach this cfiguration by overcoming an energy barrier which is conserably lower than that corresponding to the pathway whleads to the most stable configuration A. Interestingly, if tPBE functional~which attributes a larger adsorption enerto the P8 state! is used instead of BLYP we find that thP8-mediated chemisorbed process, with both the Cl atomthe CH3 group sticking to the surface, isnonactivated, inqualitative agreement with the experimental findingsported in Ref. 4. As an estimate of the difference in tactivation barrier height for chemisorption and desorptfrom the precursor (P8) state,Ed2Ea , we find the value0.14 eV, to be compared to the experimental estimate ofEd

2Ea50.2860.01 eV. In order to explain the remaining dicrepancy in the value ofEd2Ea we note that, although theadopted CI-NEB method is usually very efficient in findinminimum energy paths, our reported estimates should beconsidered as upper limits for the actual energy barriers ain principle, we cannot exclude that lower energy paths exMoreover, the presence of other physisorbed states, wthe molecule is weakly bonded to the surface by van

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FIG. 4. Dissociative reaction path from the weakly bound precursor stat8to a final configuration in which the CH3 and Cl fragments are bound to thSi atoms of different dimers~the solid line has been obtained using thBLYP functional, the dashed line using the PBE one!. The zero of the energyis taken as the value relative to the separated subsystems@CH3Cl moleculeand clean Si~100! surface#.


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Waals forces, cannot be excluded in principle, becausedensity functional approach used here is not able to propreproduce van der Waals interactions.

Another possible chemisorption pathway, where thetial configuration is not a~meta-! stable state, is shown inFig. 5. Here the methylchloride molecule interacts with tSi surface being almost vertical, with the Cl atom pointitoward the surface~we generically denote the initial configuration with V1, although it does not correspond to a wedefined, metastable structure!; the C–Cl bond is then brokenwhile a new Cl–Si bond is formed and the detached meradical is ejected into the vacuum~of course this descriptionis valid provided that the methylchloride molecule doessubstantially change, due to its rotational motion, its oriention during the time it interacts with the surface!. The poten-tial energy barrier separating the initial and final configution is 0.51 eV; in this case the value of the energy bardoes not vary appreciably if the Cl atom approaches‘‘up’’ Si atom of the dimer instead of the ‘‘down’’ atom. Ifthe approach to the surface follows the path shown in Figit appears very unlikely that the CH3 group can rapidly forma new bond with the surface, unless, before the dissociathe molecule actually becomes trapped into one of theprecursor states, P or P8: in this case the chemisorptiomechanisms illustrated in Figs. 3 and 4 may become eftive.

In Table IV we summarize our calculated energy barrifor the above-described dissociation processes, showingthe dissociation process mediated by the P8 state, with boththe Cl atom and the CH3 group bonded to the Si surface,largely favored.

Our results are also in qualitative agreement withSTM measurements of Bronikowski and Hamers,3 who stud-ied the decomposition of methylchloride on Si~100! in thetemperature range of 300–450 K. They found that, whmost of the Cl atoms produced by the dissociation are boto the surface, a finite fraction of methyl groups apparendo not stick to it. In particular, the Cl:CH3 ratio on the sur-face is found to be approximately 2:1. This led the authorspropose that, besides an adsorption mechanism in whichCl and CH3 fragments bond to the surface, an alternatprocess may occur in which the Cl atom is deposited on

FIG. 5. Dissociative reaction path when the CH3Cl molecule approaches thSi surface starting from~see the text! the initial configuration V1~the BLYPfunctional is used!. The zero of the energy is taken as the value relativethe initial state.


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Si~100! surface, while the methyl radical is ejected into tvacuum. In our simulations we observe that both these kiof dissociation processes are possible, although our resindicate that the actual chemisorption mechanisms are mcomplex that those proposed by Bronikowski and Hame3

and that a selected dissociation process appears to be lafavored.

Note that, from Table IV, it appears that those reactiowhere the methyl group is ejected into the vacuum are chacterized by energy barriers of the order of 0.5 eV, i.e., csiderably larger than the 0.16 eV barrier relative to the dsociation of the P8 configuration, where both the resultinCH3 and Cl fragments bind to the Si surface. However,STM experiments both types of reactions occur apparewith similar probabilities. As possible explanations for thdiscrepancy with the experiments we suggest that:~i! dy-namics effects could alter the picture proposed here beceven along the low-barrier dissociation path the excessergy at the transition state could be converted into trantional energy thus carrying the methyl fragment away frothe surface before it could be able to stick to the low enebinding site;~ii ! alternatively, ejection of the CH3 group intothe vacuum might occur along the low-barrier path if steconstraints prevent the group from attaching to a Si surfdimer, which could be already saturated by other molecufragments.

After the methylchloride dissociation on the Si~100! sur-face, the resulting Cl atoms can bind in a variety of geoetries; according to STM observations3 the most commonconfiguration is that with two Cl atoms bound to a singledimer ~the ‘‘silicon monochloride’’ dimer!; when the surfaceis heated to 150 °C, all Cl rearrange to this monochloridtype bonding configuration~the Cl atoms are very mobile athis temperature and can easily migrate to their most stabinding site!, and STM images3 reveal the beginnings omonochloride island formation. In particular, two relevakinds of features are observed3 in the STM images when thesurface is heated to 150 °C~we use here the same nomencture adopted by Bronikowski and Hamers!: the ‘‘Cl2’’ fea-tures are due to columns of Si dimers, each dimer of whappears lower or darker than the surrounding dimers anda minimum in intensity at the center of the dimer, giving it‘‘split’’ appearance similar to that observed in ordinarydimers at positive bias, or in hydrogenated Si dimers at netive bias; these features are attributed to Si dimers on whCl atoms have bound to both of the Si atoms, i.e., the ‘‘scon monochloride’’ dimers; the ‘‘CH3’’ features consist in-stead of a low protrusion sitting atop a Si dimer~exactly ontop of the position of one Si atom!, displaced to one side othe dimer; these features are much less pronounced tha‘‘Cl2’’ features. Note that, in this case, it cannot be expementally determined whether the STM actually imagesCH3 group or the ‘‘dangling bond’’ of the other silicon atomof the original Si–Si dimer~see the following discussion!.

At room temperature other features labeled ‘‘H’’~abright area next to a dark area on the same Si dimer! and‘‘C’’ ~bright and dark areas on the same side of two adjacdimers within a dimer row!, are observed, which are tentatively attributed to adsorbed individual~nondimerized! Cl


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atoms: in this case an unambiguous assignment is diffisince undimerized Cl atoms have features very similarthose due to defects of the clean Si~100! surface.3

In order to compare our results with the experimenSTM data we have generated ‘‘theoretical’’ STM imageusing the simplest approximation based on the TersoHamann theory21 ~see, for instance, Ref. 22 for details!.Since the comparison is done with STM images acquirenegative bias~i.e., considering tunneling from occupied suface states into empty tip states!, charge density iso-surfacehave been obtained by including the occupied Kohn–Shelectron orbitals within an energy range down to;1 eV be-low the highest occupied state, which corresponds to typexperimental STM bias voltages.3,23 The simulated imagesare produced by viewing these iso-surfaces at typical tsurface distances~a few angstrom above the adsorbed framents!.

In spite of the obvious limitations of the approach ushere~the tip is represented by a point source with an orbof s character and the interactions between the tip andsurface are completely neglected!, we are nonetheless ablereproduce the basic features of the experimental STMages. This has been checked for the case of two Cl atadsorbed on a single Si dimer~the ‘‘silicon monochloride’’dimer!, a situation where the interpretation of the experimetal images is rather clear:3,23 in this case the dimer on whicthe two Cl atoms are bound appears in the STM imadarker than the surrounding clean Si dimers~see, for in-stance, Fig. 2 of Ref. 3!. Our corresponding simulated imagclearly shows~see Fig. 6! a minimum in intensity at thecenter of the dimer and two slightly brighter features at bthe ends of the dimer, where the Cl atoms sit, so that, onwhole, this dimer appears darker than the surrounding oin agreement with the experimental observations. This reis explained by the fact that the STM images are typicaacquired at sample~negative! bias of 1–2 V, so that they arsensitive to electronic states whose energy lies within;1 eVbelow the top of the valence band; the electronic states

FIG. 6. Theoretical STM images for various chemisorbed configuratiinvolving the CH3 and Cl fragments. The sticks indicate the Si dimers athe fragments are bound to the central dimer.


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sociated with the strongly bound Cl–Sis bond orbitals andCl lone-pair orbitals of the ‘‘silicon monochloride’’ dimer armuch lower in energy than the weak bindingp-type orbitalof the unsaturated Si–Si dimers and thus are not samplethe tip.

Note that, at variance with the experimental results,STM images of the unsaturated Si dimers appear sligasymmetric, with the ‘‘up’’ Si atom of the buckled dimeslightly brighter than the ‘‘down’’ atom; the reason is thawhile the measured images are obtained at room tempera~or even higher temperature!, our calculations implicitly as-sume a temperature of 0 K: the Si dimers are buckled attemperatures but are expected to appear symmetric at rtemperature due to the low barriers for dimer flipping.24

Interestingly~see Fig. 6!, when a single CH3 or Cl frag-ment is bound to a Si dimer most of the contribution to tSTM image comes from the remaining dangling bond ofSi dimer rather than from the fragment itself. In our imagthe adsorbed CH3 fragment gives rise to a characteristtriangular-shape spot which, however, cannot be resolveactual STM images due to the finite experimental resoluti

We have also generated the STM images corresponto the adsorption structures shown in Fig. 1. As can be sin Fig. 7, the most stable adsorbed configuration A, produa STM image in which the center of the dimer is dark, whtwo relatively bright spots are observed in correspondencthe CH3 and Cl fragments. Note that the STM image prduced by configuration A is similar to that of the ‘‘silicomonochloride’’ dimer; therefore, in actual STM measurments, it could be difficult to distinguish the two differenstructures. In the case of configuration B the brightest feais due to the Cl atom of the CH2Cl fragment, which is sig-nificantly higher, with respect to the Si surface, than in cofiguration A. Finally, the ‘‘bridge’’ configurations C and Dproduce STM images clearly different from those of configrations A and B, so that they could be easily identified.

An increased buckling of the Si dimers upon adsorptof single Cl atoms on surface Si dimers has beconjectured3 on the basis of the increased asymmetriesserved in the STM images of the CH3Cl-dosed surface com

sFIG. 7. Theoretical STM images for the configurations shown in Fig. 1


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pared to the clean Si surface. We find, on the contrary,when a single Cl atom is adsorbed on a Si dimer the buckangle is reduced, from ;19° to ;0°; note that a similarflattening of the Si dimer occurs also in the case of the dsociated, chemisorbed configurations A and B~see Table II!.However, although the Si dimer is geometrically less buckwhen a Cl atom is adsorbed on it, its STM image appeless symmetric~see Fig. 6! than in the case of the unsaturatdimer, and this can explain why, on the basis of the obsetion of the STM images, a higher degree of buckling hbeen inferred.

IV. CONCLUSIONS

Adsorption of methylchloride on Si~100! has been investigated usingab initio simulations. The structural and enegetic properties of possible adsorption configurations hbeen studied. We confirm that, in the lowest-energy adsorstructure, the methylchloride molecule is dissociated iCH3 and Cl fragments, which are bound to the two Si atoof the same surface dimer. We have investigated in depossible dissociation processes and found different reacpaths: the methylchloride molecule can dissociate fromof two possible precursor states, P or P8, characterized byadsorption energies as large as 0.3 eV, and either bothCH3 and the Cl fragments produced by the dissociation pcess or only the Cl atom form new bonds with the Si dimealternatively, a direct, activated dissociation process is psible, resulting in the breaking of the CH3– Cl bond and theformation of a new Cl–Si bond, while the CH3 fragment isejected from the surface. We find in particular, in agreemwith recent experimental findings, anonactivatedreactionpath ~provided that the PBE functional is used! where themethylchloride molecule from the precursor state P8 dissoci-ates into CH3 and the Cl fragments bound to different surfaSi dimers. Finally, we have presented theoretical STMages, relative to different chemisorbed fragments, whcould be useful for the interpretation of experimental STimages of this system.

ACKNOWLEDGMENTS

The authors acknowledge financial support from INFthrough the PRA ‘‘1MESS,’’ and allocation of computer r


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sources from INFM ‘‘Progetto Calcolo Parallelo.’’ A.R. wapartially supported by FONDECYT, Chile under Grant N1010988. We thank M. Boero for useful discussions.

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adsorption of methylchloride on si(100) from first principles

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