PHYSICAL REVIEW B 15 FEBRUARY 1997-IVOLUME 55, NUMBER 7

Room-temperature growth of submonolayers of silicon on Si„001… studiedwith scanning tunneling microscopy

J. van Wingerden, A. van Dam, M. J. Haye, P. M. L. O. Scholte, and F. TuinstraDepartment of Applied Physics, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

~Received 11 June 1996; revised manuscript received 21 October 1996!

Room-temperature deposited submonolayers of silicon on Si~001! are investigated using STM. The observedstructures and the mechanisms leading to their formation are discussed. Isolated ad-dimers in different geom-etries are described and a kinetic model for their formation is deduced. It is shown how further growth occursvia the formation of 3-atom clusters, which act as nucleation centers for the formation of two types of linearstructures. One of the line types is formed in the@110# direction, and has been observed before. The other is inthe @310# direction. At a coverage of nearly 0.2 ML a kind of random network consisting of segments of thetwo types of atomic lines is formed. Above 0.2 ML coverage these lines are converted into epitaxial dimerrows. A pathway for this conversion is proposed on the basis of experimental observations.@S0163-1829~97!11107-9#

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I. INTRODUCTION

The homoepitaxial growth on Si~001! has been used asmodel system for studying epitaxial growth for quite a lotime. Eaglesham, Gossmann, and Cerullo1 have shown thatthere exists a maximum layer thickness for epitaxial growwhich is reduced if the temperature is lowered. They emated the epitaxial thickness for room-temperature groof silicon on Si~001! to be 10–30 Å. This indicates thatshould be possible to grow a few monolayers of silicon etaxially on Si~001! at room-temperature. The room temperture growth of silicon on Si~001! has been studied withnumber of techniques such as TEM,2 Rutherfordbackscattering/channeling,3,4 and low-energy electrondiffraction.5–7 These results led to some controversy aswhether the first layers of silicon on Si~001! grow epitaxiallyat room temperature or not.

The observation techniques mentioned before are gloin the sense that they only allow for the assessment ofepitaxial quality of atomic layers as an average propertythose layers. For a microscopic analysis of the processestrolling epitaxial growth scanning tunneling microscop~STM! is better suited. Although some attention has bepaid to room-temperature growth of submonolayers,8,9 STMstudies have mainly been focused on surfaces prepareelevated temperatures, where ad-dimers are mobile.10,11

At room temperature the growth mode is significantly dferent from that at elevated temperatures. Because ad-diare immobile at room temperature all structures are necesily formed from the addition of single adatoms. Adatoms adiffusing too fast to observe them with STM at rootemperature.12,13How these adatoms aggregate to form cluters of two, three, or more atoms is still not very well undstood. STM experiments at 160 K have nicely demonstrathe formation of ad-dimers from adatoms,14 although at thattemperature the diffusion of adatoms along the rows isactivated. Therefore, it is still unknown what processes ocin case the adatom motion is fully activated.

While there is still discussion about the early stages

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growth, even less is known about the more complicastructures and processes at higher coverages. So far, anonepitaxial structures have been observed.9,15 In thesestructures the dimer bonds and positions of the dimer atoare different from those in a epitaxially well ordered complete overlayer. The energies of ad-dimers and various ostructures have been calculated16–18 in order to explore pos-sible pathways for the evolution of the structures.

The experimental data available to date have not enaa complete analysis of all types of clusters and the varibinding sites for adatoms at these clusters. A thorough ansis of this problem is needed to understand the comppicture of room-temperature growth of silicon on Si~001!.Microscopic knowledge of the growth processes is neededetermine the pathways to the formation of epitaxial layeThe transition rates of the critical steps in these pathwdetermine the epitaxial quality.

In this paper we discuss the evolution of the structuduring room temperature growth of a submonolayer. Thetails of the experimental techniques are outlined in Sec.The increasing complexity of the structures for increascoverage is shown in the STM images in Fig. 1. First we wdiscuss the features observed at low doses. In these estages of the growth nearly all atoms are incorporatedtwo-atom clusters, i.e., dimers. We discuss the obserbinding sites for ad-dimers and present a model for thformation. The dominating influence of kinetics incorporatin this model also sheds new light on the recent discussio19

about the stability of the different ad-dimer bonding geoetries. Then we describe the formation of the 3-atom cluswhich is the next step in the growth process. As we wshow all different ad-dimer geometries observed on the sface can be converted to this cluster by the addition of oadatom. Subsequently, the formation of larger structuresthe mechanisms causing their increasing complexity arecussed. The interaction between dimers causes them toup in linear structures. This is the dominant process contling the island morphology. Finally, we show that these lear structures can be transformed into epitaxial structuExperimental evidence for the pathway involved in this tra

4723 © 1997 The American Physical Society

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4724 55J. VAN WINGERDENet al.

sition is also discussed. The analysis of the complete groprocess has been based on a large number of images obtfrom samples with various coverages. The analysis covnearly all structures observed in the experiments. All otstructures that have been observed did not seem to beevant for the growth process, as they only occurred occasally.

II. EXPERIMENT

Experiments are performed in a UHV system with a bapressure of 5310211 Torr. A commercial Beetle type STM20

with a Pt-Ir tip is used for measuring the constant currSTM images. Movies, made by capturing images at 1time intervals, show the surface dynamics at room tempture. For all dynamic processes observed with the STMneed to keep in mind that they may be influenced byelectric field. Nevertheless, they give useful indicationspossible pathways with a reasonable transition rate aslightly above room temperature, as was shown in a prevstudy.21 Silicon ~001! surfaces are prepared by flashing1250 °C, yielding the (231) reconstructed surface witmonolayer height steps. Deposition at room temperaturperformed at least 4 h after flashing the sample. As waitinfor a longer period does not change the results, the samtemperature is close enough to room temperature. Silicodeposited from a commercial miniature electron-beevaporator.

III. ISOLATED AD-DIMERS

The smallest immobile islands created during rootemperature growth on Si~001! are ad-dimers. For denotinthe different binding geometries of the most probable swe use the notation shown in Fig. 2. We have been ablidentify these dimer configurations in our STM images. N

FIG. 1. Empty-state STM images with coverages of 0.012 M~a!, 0.05 ML ~b!, 0.18 ML ~c!, and 0.20 ML~d!. The layers havebeen deposited in 10 min~a!,~b!,~d! or 15 min ~c!. Tunneling con-ditions: 0.4 nA,21.3 V ~a!–~c!; 1.0 nA,21.5 V ~d!.

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different dimer configurations have been observed in thexperiments. TheA andB dimers, which reside on top of thdimer rows, are well known and easily identified by the drection of their elongation@see Figs. 3~a!, ~b!#. The appear-ance of theC andD dimers in STM images requires speciattention as they have not been described previously.

We have never observed isolatedD dimers. This is inaccordance with the predictions based onab initiocalculations.16 To show what an isolatedD dimer might looklike, we present in Figs. 3~c!, ~d! clusters that containDdimers. Nearly allD dimers we observe have a neighborinB dimer ~see, e.g., dimerD3). This corresponds to theDBstructure required for the formation of an epitaxial overlayDimers D1 and D2 form a cluster together and show thappearance ofD dimers without a neighboringB dimer.

Three examples of the structure that we have identifiedtheC dimer are shown in Figs. 3~a,b!. While they are clearlyvisible in the empty state image@Fig. 3~b!#, their appearancein the filled-state image@Fig. 3~a!# is even fainter than that othe substrate dimers. In comparison with theD dimersD1andD2 @Figs. 3~c!, ~d!#, the structuresC1-C3 in Figs. 3~a!,

FIG. 2. Four different binding geometries (A-D) for ad-dimers.

FIG. 3. Filled-state image~a! showing anA dimer, aB dimer,and threeC dimers (C1-C3) together with empty-state image~b!,where theA dimer has rotated back to theB orientation. Filled-~c!and empty-state~d! images where threeD dimers are indicated asD1-D3. D3 looks different because there is aB dimer on top of theneighboring dimer row. Tunneling conditions: 0.4 nA~a!–~d! and1.3 V ~a!, ~c!; 21.3 V ~b!; 21.15 V ~d!.

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~b! have an aspect ratio much closer to unity. Neverthelwe observe that their long axis is directed perpendiculathe dimer rows. Similar differences are observed betwB andA dimers. This supports the identification of the struture as a dimer located in between the dimer rows withdimer bond parallel to the substrate dimer bonds, i.e., aC dimer. In the filled-state image@Fig. 3~a!# theC dimers arecharacterized by a surrounding ‘‘circle’’ of visible substraatoms. The interaction between dimers along the subsdimer rows causes an anticorrelated buckling which is vible over a range of about six dimers at both sides ofstructure. In the filled-state images theC dimers appear indifferent forms. MostC dimers do not exhibit a circle ovisible substrate atoms as perfect as that shown in Fig. 4~a!.Instead, we often observe imperfect circles, where onetwo of the atoms forming the circle seem to be missing@Fig.4~c!#. The empty-state image in Fig. 4~d!, however, showsthat they are still there. The exact origin of these imperftions is not yet clear.

From the STM images the abundances of the structuhave been determined for different coverages@see Fig. 5~a!#.For low coverages~i.e., less than one dimer per substradimer row! the numbers of isolatedA, B, and C dimersincrease linearly with coverage. At a coverage of about 0ML @see Fig. 1~a!# the formation of multiple-dimer clusterreduces the rate of increase of the number of isolateddimers.A and B dimers have been observed to transfointo each other at room temperature.21,15,19Hence, it is justi-fied to assume that these populations are in thermodynaequilibrium and, therefore, we will often address them asA/B population. From the ratioB:A of 10:1 the energy dif-ference of the sites can be estimated. This yields a 60-mlower energy for theB site compared to theA site. This neardegeneracy ofA andB dimers is in accordance with theoretical calculations.16–19

Of all isolated ad-dimers 67% are in theC position @seeFig. 5~b!#. If we assume that theC dimer population is inthermodynamic equilibrium with theA/B population we findan energy difference of only 20 meV between theC andA/B sites. This contradictsab initio calculations17,18that pre-dict an energy difference larger than 100 meV. There ahowever, a number of indications that the abundance ofC dimers is kinetically determined. First of all, we did nobserve with our STM any switching of isolated ad-dimebetween theC position and theA/B positions. Secondly, the

FIG. 4. Filled- ~a!, ~c! and empty-state~b!, ~d! images ofCdimers with perfect~a!, ~b! and imperfect~c!, ~d! buckling like acircle of adatoms. Tunneling conditions: 1.5 nA, 1.3 V~a!; 1.5 nA,21.3 V ~b!; 1.1 nA, 1.3 V~c!; 1.1 nA,21.3 V~d!.

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percentage of isolated ad-dimers in theC positions matchesthe exact value of 2/3 remarkably well. This strongly sugests the following, kinetically determined origin for the ditribution. According toab initio calculations12,13the adatomswill diffuse on top of the dimer rows, making frequent sidtrips to the more stable sites at either side of the rows. Dfusion across the dimer rows is not activated at room teperature. Therefore, the adatoms remain on the dimer rowwhich they are deposited. We now assume that two adatseparated by more than one dimer row cannot form a diand that the type of dimer formed only depends on the ron which the second adatom is deposited. Then, dimersformed either from two adatoms on neighboring dimer roor from two adatoms on the same row. This results inchance of 2:1 that the second adatom is deposited on a neboring row instead of on the same row. If the adatoms mon the same dimer rowA/B dimers are formed. If they meeon neighboring rows aC dimer is formed. This causes 2/3 othe dimers to be formed in theC position as observed experimentally.

This model for the dimer formation may seem to contdict the observations of Wolkow,14 who described the exclusive formation ofA/B dimers from adatoms on neighborindimer rows at a temperature of 160 K. The apparent intertion between adatoms on different rows was attributedstrain effects. However, these subtle strain effects willwashed out at room temperature. Consequently, at roomperature other interactions dominate the formation of dime

As discussed before the abundances ofA/B andC dimersare kinetically determined. Therefore, room-temperatureposited submonolayers cannot reveal any information abthe energy differences of these sites. Annealing experimmay be performed to determine the thermodynamic stab

FIG. 5. ~a! Density of isolatedA, B, andC dimers in arbitraryunits as a function of coverage. Straight lines indicate the linincrease with coverage for the lower three coverages.~b! The per-centage of isolated dimers in theC position as a function of coverage. The line corresponds to a ratio of 2/3.

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of the isolated ad-dimers. For interpreting such experimeit has to be realized that theC dimers may still be frozen intheir positions while the diffusion ofA/B dimers is alreadyactivated and causes the formation of clusters. Anneaexperiments19 have been reported to result in an increasethe number of dimers in between the substrate dimer roThis observation is, however, probably due to the formatof tetramers, which are easily misinterpreted as isoladimers.21

IV. THREE-ATOM CLUSTERS

As ad-dimers are immobile at room temperature, stanuclei can only grow by the addition of adatoms. The fistep in this growth process is the formation of a three-atcluster by adding one adatom to an ad-dimer. We onlyserved one type of stable three-atom cluster, an exampwhich is shown in Figs. 6~a!, ~b!. In this cluster an atom isbound next to aC dimer at site 1 or 2 in the schematdrawing in Fig. 6~c!. In this manner all three atoms are psitioned on one line perpendicular to the substrate dimrows. Our observation that this is the only stable three-acluster is corroborated by the work of Yamasaki, Uda, aTerakura,18 who did not find other stable three-atom clustein their calculations. In filled-state images theC dimer of thethree-atom structure is now also visible as a bright spocontrast to its appearance as an isolated ad-dimer. Thetom of this structure is invisible in the filled-state imageThis adatom is indicated with an asterisk in the empty-simage, Fig. 6~b!.

The most straightforward mechanism for the formationa three-atom cluster is the sticking of an adatom to aCdimer. This adatom can arrive at either side of aC dimer viadiffusion along the dimer rows next to theC dimer. Experi-mental evidence for this pathway is the fact that we obserseveralC dimers in filled-state images turning bright. Bcause we do not have empty-state images of these trations, the addition of the extra atom has not been obserdirectly. The observed increase in brightness of theC dimer,

FIG. 6. Filled- ~a! and empty state~b! images of a three-atomcluster with a schematic drawing~c! of a C dimer with the twopossible binding sites~1,2! for an adatom to form a stable threeatom cluster. Tunneling conditions: 0.3 nA, 1.3 V~a!; 0.3 nA,21.3 V ~b!.

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however, is exactly the visible effect in filled-state imagesaC dimer catches an adatom, as can be seen from the cparison of theC dimer appearances in Figs. 4~a! and 6~a!.

A more surprising observation is the existence of a secpathway for the formation of the three-atom cluster. Here,A/B dimer is converted into a three-atom cluster. In this cathe transition is more complex because the dimer bond ofA/B dimer is broken during the transition. The three imagin Fig. 7 showing this conversion are obtained from an STmovie. In Fig. 7~a! we observe threeB dimers on one dimerrow. One of theseB dimers (B1) catches an extra adatomand forms a three-atom cluster as shown in Fig. 7~b!. Thatthis structure is indeed a three-atom structure is confirmedFig. 7~c!. In contrast to theA dimer, the three-atom structurexhibits one elongated spot and one spot that is similathose of theA dimer. The elongated spot originates from tC dimer and its midpoint is visibly shifted towards thmiddle of the trough between the substrate dimer rows.

Both conversion processes starting fromA/B or C dimersoccur with more or less equal probability. Forming threatom clusters exclusively from eitherC or A/B dimerswould change the ratio of their abundances of 2:1. Focoverage of about 0.012 ML, where 15% of the atoms hformed a three-atom cluster, the abundance of isolateddimers in theC position is still 2/3. Using only theC dimerfor forming three-atom clusters would have caused a smbut significant decrease of theC dimer abundance~from67% to 63%!. The absence of any change in the ratio of tabundances indicates roughly equal efficiencies for formthree-atom clusters by sticking adatoms to eitherC or A/Bdimers. This also suggests thatA/B dimers cannot be transformed into three-atom clusters by atoms diffusing onsame dimer row. In that case the capture area for atocausing the transformation only comprises the two neighbing dimer rows for bothA/B andC dimers. This assumption

FIG. 7. Empty-state images from a movie and schematic drings demonstrating the formation of a three-atom structure fromB dimer and an invisible extra adatom before~a! and after forma-tion ~b!. Subsequent rotation of dimerB2 to theA configuration~c!enables comparison of an atomically resolvedA dimer with a three-atom structure.@In ~b! dimerB3 from ~a! and ~c! is in theA con-figuration.# Tunneling conditions: 0.4 nA,21.7 V.

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is confirmed byab initio calculations,18 as these calculationdid not yield any evidence for interactions between adatoandA/B dimers on the same dimer row.

V. DILUTED DIMER ROWS

The three-atom clusters described in the preceding secact as templates for forming larger nonepitaxial structurThey are extended by sticking adatoms in line with the atoof the cluster at either end of the structure. If adatomsbound at both sides of aC dimer @sites 1 and 2 in Fig. 6~c!#,a four-atom cluster is formed. In a previous study21 we havedemonstrated that this structure, which we called a ‘‘crstructure,’’ is formed from two dimers on neighboring sustrate dimer rows. In our present study we also observedcross structure. Because the submonolayers in this studygrown at room temperature, where ad-dimers are immobwe conclude that the cross structure can also be built up aby atom. The general form of the structure obtained bytending the three-atom cluster with an arbitrary numberatoms is a line of atoms along the@110# direction, i.e., per-pendicular to the substrate dimer rows. This@110# line con-sists of a number ofC dimers in neighboring troughs between the substrate dimer rows. Either end of the@110# linecan consist of aC dimer or an adatom. These structures habeen described before9,21,15and are denoted as diluted dimrows. The process governing their formation is analogouthe surface polymerization reaction described for AlSi~100!.22

A puzzling question is the difference in brightness btween isolatedC dimers andC dimers in these diluted dimerows. We propose two explanations, based on the geomeof the two structures obtained fromab initio calculations.17

First, the dimer bond for theC dimer in the row is muchshorter than that of the isolated dimer. We expect the resing increased overlap of the atomic orbitals to lead tohigher density of states above the dimer, and hencebrighter appearance in the STM images. Second, the dimthe row is buckled, whereas the isolated one is symmeNaturally, the upwards buckled atom appears higher tthat of a symmetric dimer. This effect can even be enhanby a charge transfer from the downwards buckled atom toupwards buckled atom. The dimer appears symmetric inimage due to temperature activated switching.

VI. DIMER ROWS ALONG †310‡ DIRECTIONS

Apart from the known diluted dimer rows we observedlinear structure that also consists ofC dimers. The structureis formed along@310# and its symmetry related directions.is very common at doses above 0.05 ML. A specialV-shapedexample consisting of two connected linear segmentsshown in Figs. 8~a!, ~b!.

The formation of the rows along the@310# directions canbe explained if we assume a precursor state in whichadatom is bound near aC dimer in one of the four metastablbinding sites~1-4! in Fig. 8~c!. These sites may originatfrom the strain induced by the presence of theC dimer. Theinteraction of adatoms bound at these sites with other difing adatoms will lead to the formation of new dimers. Anagous to the formation of isolated ad-dimers anA/B dimer

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can be formed with an atom diffusing on the same dimrow. An atom diffusing on the neighboring dimer row wiyield a newC dimer and extend the row along the@310#direction.

We have no direct evidence for the existence of the pcursor state, but we have observed ‘‘clusters’’ of precurstates, an example of which is shown in Fig. 9. We expthat the adatom binding sites of the precursor state arestable at room temperature, but that they have neverthean increased occupancy during the growth. The combinaof two precursor states shown in Fig. 9 does become stabroom temperature.

TheC dimers in the@310# chains are only faintly visiblein the filled-state images, as in the case of isolatedC dimers.Therefore, the neighboringC dimers in these lines do noseem to influence theC dimer bonds as strongly as in thcase of the diluted dimer rows.

We can now describe the growth up to doses of aboutML. At very low doses (, 0.05 ML! we get the structuresdescribed in Secs. III and IV. When growth proceeds furthadatoms stick to existing structures, forming one of the linstructures described in Secs. V and VI. Other structuresrelatively rare. Because extension of the linear structurescurs either in the@110# or in the @310# direction, we end upwith a kind of random network of atomic line segments

FIG. 8. A V-shaped row ofC dimers in the@310# directions;filled state~a! and empty state~b!. The four metastable binding site~1!–~4! for adatoms involved in the formation of@310# lines areschematically depicted in~c!. Tunneling conditions: 0.4 nA, 1.3 V~a!; 0.4 nA,21.3 V ~b!.

FIG. 9. Filled-~a! and empty-state~b! images with a schematicdrawing@inset in~a!# of a cluster of two three-atom precursor stateTunneling conditions: 0.4 nA, 1.3 V~a!; 0.4 nA,21.3 V ~b!.

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these directions, with relatively open areas in between th@see Fig. 1~c!#. At this stage of the growth deposited atomare rapidly captured by one of the numerous~metastable!binding sites of the network.

VII. TRANSITION TO EPITAXIAL STRUCTURES

After deposition of nearly 0.2 ML the majority of thdeposited material still occupies nonepitaxial binding sitsince most of the ad-dimers in the network are in theCposition. Further growth now causes a conversion of thejority of nonepitaxial diluted dimer rows into epitaxial dimerows. At 0.18-ML coverage@see Fig. 1~c!# already a fewdiluted dimer rows are observed to be partially converinto epitaxial dimer rows. A pronounced increase in tabundance of epitaxial structures is obvious from the coparison of Fig. 1~c! and Fig. 1~d!, which originate from dif-ferent experiments. The abundance of epitaxial structuwith at least 4 atoms increases from 38 to 50% whiledose differs only 0.02 ML. The fact that a substantial coversion to epitaxial structures does not take place at anlier stage of growth indicates that there exists a considerbarrier for this transition.

We observed the following characteristics of the partiaconverted diluted dimer rows:~1! apart from a few exceptions, where diluted dimer rows contain a single epitaxiaoriented dimer, the smallest epitaxial segments always ctain two dimers;~2! the conversion occurs exclusively at thends of the diluted dimer rows;~3! epitaxial segments withthe outer dimer being aB or D dimer are both observed. Aexample of one ending with aD dimer is presented in Fig10~a!.

A pathway accounting for the above-mentioned expemental observations is depicted in Fig. 11. It initiatesforming aB dimer from an atom of the outerC dimer withthe adatom at the end of the diluted dimer row. This prochas been observed with STM previously.21 Repeating theprocess at the end of the remaining diluted dimer row thcreates a modifiedBB structure, which we previously denoted as a ‘‘twin.’’21 Finally, a collapse of this twin structurleads to an epitaxialBD segment. This conversion patyields a sequence ending with:C, adatom,B, D.

The conversion path is fairly complex and the major qution is whether the kinetic barriers for the steps involved

FIG. 10. Diluted dimer row ending withBD epitaxial segment~a! and one ending with a twin~b!. The straight lines in~a! areplotted to indicate the edges of a substrate dimer row, which cadeduced from the position of the vacancy in this row at the lowright end. Tunneling conditions: 0.4 nA,21.3 V.

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the conversion process are not too large. At present wenot have a sequence of consecutive images showing the cplete conversion process. However, we did observe allindividual steps.

The formation of aB dimer at the end of a diluted dimerow according to the first step in the conversion path@from~a! to ~b! in Fig. 11# has been observed earlier,21 but in theseobservations the transition activity was clearly enhancedthe presence of the electric field. The proposed converspath requires the consecutive occurrence of two of thtransitions to form a twin structure at the end of the dilutdimer row. This may reduce the conversion rate. Howevthe fact that we have observed a few diluted dimer roending with a twin structure@see Fig. 10~b!# proves that thefirst two steps of the conversion path@see Figs. 11~a!–11~c!#do exist. Furthermore, we have observed that about 4%the atoms form twin structures at 0.18 ML coverage. Tdemonstrates that under growth conditions the conversiotwin structures does occur at a reasonable rate. These tcan only be formed from a diluted dimer row, i.e. in a prcess similar to the first two steps of the conversion pawhich is concluded from the following reasoning. Growthperformed at room temperature and, therefore, occurs exsively by the addition of single adatoms. This means thatprecursor for the twin structure with four atoms should bethree-atom structure. Because we only observed three-astructures with aC dimer, the most likely pathway@see Figs.11~e!–11~g!# is the formation of a cross structure~a diluteddimer row with four atoms!, which is then converted to atwin structure. The latter process is identical to the first pof the conversion to an epitaxial segment.

Once a twin structure has been formed, an epitaxial sment is formed@compare Figs. 11~c!, ~d!# by a collapse to aBD structure as shown in the STM images in Fig. 12. Ttwo B dimers of one of the twins in Fig. 12~a! are indicatedwith arrows. After the collapse@see Fig. 12~b!# the distanceof the dimers has been halved, in accordance with themation of theBD epitaxial segment. Similar epitaxial structures with two dimers are frequently observed on rootemperature-grown submonolayers. This shows the existeof the transition under growth conditions. The transitionthe twin structure to theBD cluster has recently been studiewith ab initiomethods.18 The transition mechanism explorein these calculations has an activation energy of 1.6 eV. T

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FIG. 11. Pathway for the conversion of diluted dimer row en~a! into epitaxial segments~d!. A three-atom cluster~e! is convertedinto a cross structure~f! by the addition of an extra adatom. Thstructure can transform into a twin structure~g!.

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activation energy renders the transition mechanism pracally forbidden at room-temperature, while we do obseBD clusters after room-temperature growth. Apparenthere should exist an alternative conversion mechanisma lower activation barrier.

The proposed conversion path accounts for the formaof epitaxial segments ending with aD dimer, which is ob-served to be the case for at least 50% of the observedments. It does not explain the form of the other epitaxsegments, which end with aB dimer. These segments mabe formed along the path proposed by Brocks and Kell17

This conversion path initiates with the arrival of an adatoand always results in a segment ending with aB dimer.

The conversion path for the formation of epitaxial sements is expected to be very sensitive to the local geomeLocal modifications of the strain distribution induced bneighboring structures may facilitate the occurrence ofcomplex transitions to epitaxial structures at higher covages. This explains the rapid increase of the abundancepitaxial structures at about 0.2 ML. Further investigatioare needed to quantify these local strain effects.

We often observe single adatoms bound at the endepitaxial segments but they are not a clue for the converprocess as they do not necessarily originate from the consion process. This is demonstrated in the transition thatcurred between Figs. 12~b! and 12~c!. Here, an adatombound nearby has moved and is finally bound alongsideD dimer of the epitaxial structure. Epitaxial segments bwith and without an extra adatom have been observed.the transition in Fig. 12 shows, the extra adatoms can ornate from nearby structures. Furthermore, atoms deposafterwards may also bind at these sites.

VIII. CONCLUSIONS

The evolution of structures during room-temperatudeposition of submonolayers of silicon on Si~001! has beenpresented. The data are obtained by a careful analysis ofeatures observed in numerous STM images of the Si~001!surface after deposition of different coverages. All observfeatures have been described except for the defects andstructures that occur in such small numbers that they doseem to be of importance for the growth process. The e

FIG. 12. Sequence of transitions from an STM movie, showthe formation of an epitaxial structure~b! from a twin ~a!. Dimersof the twin and the epitaxial structure are indicated with arrowThe extra adatom~indicated with an asterisk! from a neighboringstructure is attached to the epitaxial segment in~c!. Tunneling con-ditions: 0.4 nA,21.6 V.

ti-e,th

n

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er-ofs

ofnr-c-

ehsi-ed

all

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lution of the relevant structures during growth is schemacally depicted in Fig. 13. The growth starts with adatomforming ad-dimers. We found that adatoms diffusing on tsame dimer row formA or B dimers, while adatoms onneighboring dimer rows yieldC dimers. Stable three-atomclusters with all atoms aligned along one row perpendicuto the substrate dimer rows can be formed if adatoms sticA, B, orC dimers. These stable three-atom clusters formprecursor state for the lines of dimers along the@110# direc-tion ~diluted dimer rows!. Apart from the diluted dimer rowslines of dimers can also be formed along the@310# and itssymmetry related directions. The three-atom precursor sfor these lines is probably the one depicted in Fig. 13.adatom binding site, however, is not as stable as that inthree-atom cluster which has all atoms in one line. Both@110# dimer lines and the@310# lines are compatible witheach other in the sense that structures consisting of cnected segments of both line types can be formed. Finalltransition to epitaxial structures occurs by the formationepitaxial segments in diluted dimer rows. Based on STobservations a possible pathway for this conversion procis proposed. The shortest diluted dimer row that can be cverted to an epitaxial structure consists of aC dimer withtwo atoms~cross structure!. Its conversion into an epitaxiastructure occurs via the twin~modifiedBB cluster! as shownin Fig. 13.

Thus we have shown that epitaxial structures are formduring deposition of submonolayers of silicon on Si~001!.For growing thicker layers the probability for adatomsstep down at the step edges further influences the epitaquality. The room-temperature activation of that processquires a separate study.

ACKNOWLEDGMENTS

We thank G. Brocks for helpful discussions. Furthermothe technical support of C.D. Laman, R.F. Staakman, andVogelaar is gratefully acknowledged.

g

.

FIG. 13. Overview of the evolution of the relevant structurduring room temperature growth of silicon on Si~001!.

e

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.

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4730 55J. VAN WINGERDENet al.

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