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Probing the chemistry and manipulating surfaces at the atomic scale with the STM
Applied Surface Science f,AI/bl (1992)426 -436 NtJrlh.ittdland
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s u r f a c e SCiellce
Probing the chemistry and manipulating surfaces at the atornic scale with the STM Ph. Avouris and L-W. Lyo I~lll ~t,$t'tlrt'h DiciMo;z, 7~J. a~tt.w~l Resean'/i ('cruet. P.O Bor 21K ~rkroLh~+ Iteik.ht.~, N$" 10598, U.$~4 Received 2() Nt~vcmber 1991; accepted for publication nl January I'~t12
The STM can he used hnth as a probe of local surface eleclronic ~truclur¢ a~d chemistry and as a tool fl~r Ihe alomic scale manipulation iif materia!s. We demonstrate lhe use of the STM as ~1 hl(al probe by invesligatint~ Ihe spafi;d distribution of the initial stages of oxidation of Si( 111)-7 × 7 and Sit 11)111-2× I. We find Ihat the oxidation reaction is very site selecliv¢. On the St( I 11) surface. Si corner adatom sites on Ihe faulted-half of the 7 × 7 unit celi are mo,t reaclive zlLd detects play no significant role. On Ihe siinlu) surface+ on Ihe other hand, the Si direct majority sties are unruaelive while C-defecls dominate the early stages of oxidation. This apparent difference in the reactivity of tile two sul:,ces is resolved hy tunneling speclros¢opy which slaws that a common faelor, namely the density ~ff occupied states near E F. determines Ihe reactivity on both surhlces, Nexl we consider the use of STM as a manipuhtinn tool. We firsl discuss a general scheme, termed chemically-assisted field-evuporafion+ which allows Ihe aton'zie scale manipuhdion o~ materials, This sell=:me involves a combination of chemical lip-sample interactions produced by bringing the tip ~ery close to Ihe site to be affecled, and electrostatic forces produced by Ihe application of a voltage pulse. The chemical interaction ~ignifieantly reduce, the barrier for atom Iran~fer between sample and tip+ while the electrostatic hlree introduces directionality and further reduces the barrier. We illuslralu Ihe p:+wer of this approach by (a~ removing individual Si atoms or clusters uf atums from Sit I I I ) with a "d/tip and then redepusifing them at predetermined surface sites, and (h) depositing AI atoms from an AI tip to the St( I I 1) surface,
L |ntruductlon T h e e l e c t r o n i c s t r u c t u r e a n d c h e m i s t r y o f solid s u r f a c e s have b e e n s t u d i e d in t h e last f e w d e c a d e s u s i n g t e c h n i q u e s t h a t p r o v i d e i n f o r m a t i o n avera g e d o v e r a m a c r o s c o p i c a r e a o f t h e surface. Sarlktce c h e m i s t r y , h o w e v e r , is a local process. T h u s , it is i m p o r t a n t t o k n o w t h e s p a t i a l d i s t r i b u t i o n o f a r e a c t i o n w i t h a t o m i c site r e s o l u t i o n . M o r e o v e r , t o u n d e r s t a n d s u r i a e e reactivity w e n e e d t o r e l a t e t h e reactivity o f a p a r t i c u l a r site t o t h e local e l e c t r o n i c s t r u c t u r e . T h u g , ideally, w e need a surface technique which can probe both t h e spatial d i s t r i b u t i o n o f c h e m i c a l r e a c t i o a s a n d t h e local e l e c t r o n i c s t r u c t u r e w i t h single s u r f a c e cite r e s o l u t i o n . S c a n n i n g t u n n e l i n g m i c r o s c o p y ( S T M ) [1] i m a g e s t h e local density o f states ( L D O S ) o f low
b i n d i n g c n c r g y v a l e n c e e l e c t r o n s [2] a n d s c a n n i n g tunneling spectroscopy (STS) measures their ene r g i e s [3]. +~ is precisely t h e s e l o o s e l y - b o u n d elect r o n s t h a t are r e s p o n s i b l e for t h e c h e m i s t r y of m a t e r i a l s . T h u s . it w a s s o o n r e a l i z e d [4] t h a t o n e c a n t a k e a d v a n t a g e o f t h e c h a n g e s in L D O S a n d in t h e local S T S s p e c t r a t h a t t a k e p l a c e u p o n r e a c t i o n o f a p a r t i c u l a r site to o b t a i n t h e spatial distribution of the reaction and relate the obs e r v e d reactivity o f t h e d i f f e r e n t sites to t h e local e l e c t r o n i c s t r u c t u r e . T h e a p p l i c a t i o n o f this a p p r o a c h to t h e s t u d y of t h e c h e m i s t r y o f s e m i c o n d u c t o r s u r f a c e s h a s b r o u g h t n e w a n d u n i q u e ins i g h t s i n t o t h e factors t h a t d e t e r m i n e s u r f a c e reactivity. H e r e w e illustrate t h e a p p l i c a t i o n o f S T M / S T S in t h e study o f s u r f a c e c h e m i s t r y u s i n g t h e s t u d y o f t h e initial stages o f t h e Si(l 11)-7 × 7 a n d St(100)-2 × 1 s u r f a c e s as e x a m p l e s . I n p a r t i c -
0169-4332/92/$115.00 ~ 1992 E l ~ i e r Science Publishers B.V. All rights reser,'cd
Ph. At out,s, L.a< Lyo / Surfacc probiug at atomic scale wah STM
ular, we concentrate on one aspect of the problem, namely, on the factors which determine the site-selectivity of the initial stages of oxidation. In addition to being a powerful surface probe, the S T M can also be used as a tool for the modification and manipulation of materials on the atomic and nanometer scale, For this purpose one relies on imeraetions between surface atoms or adsorbates and the S T M tip. T o a large extent the magnitude of these interactions can be tuned, and recently some of them have been used to provide the unprecedented capability to manipulate, e.g.. deposit, move or desorb, and modify, e.g., dissociate, ad.zorbed atoms [5-8], clusters [9], and molecules [10,11]. In particular, applying voltage pulses has been a popular method for inducing surface modifications with the S T M [12]. In a pioneering work, Becker et al. 151 showed that atomic ~cale deposition is possible by this
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method. More recently, we have demonstrated that by combining chemical tip-sample interactions with electrical forces produced by a voltage pulse, one can break strong covalent chemical bonds between Si surface atoms, transfer individual atoms or clusters of atoms to the tip and then redeposit them at predetermined surface sites [8]. Here we will briefly explore the mechanism and implications of this process. Details on the S T M / S T S set up and experimental procedures can be foand in earlier work [4].
2. Probing surface chemistry on the atomic scale: the initial stages of oxidatlon of $1(1H)-7 × 7 and Si(100)-2 X 1 surfaces Tile oxidation of silicon is probably the most important chemical process in the area of elec-
Fig. 1. STM topograpb of Ihe unoccupied slates of u Si(I I I )-7 × 7 surface exposed to 0.2 L of 0 2 at 3or) K. Sample bias 2 V.
428
i'll. A~ ~ntris. L • IK Lb~ / Surfiwe prtlbmg at monlic stalk' with $1~1
tronic materia;s and, as such, it has attracted considerable attention, In particular, the ear|y stag¢s of oxidation have been the object of extensire studies using both conventional [13], and, more recently. STM techniques [14-17]. Here we
focus on the intriguing site selectivity of the early stages of oxidation of Si(111)-7 × 7 and Si(100}2 × I as revealed by the STM studies, and the electronic factors that determine this site selectivity.
Fig. 2. Top: ~TM lopograph tff the occupied states of a $i(100)-2 × I ~,fface. Boltom: The same urea arlur c~pt=~ure to O 2. The Ic~c~Uon of several C-defucts is indicated hy arrow~. Sample bi~, - 2 V.
Ph. At'tams, I..g( I,yo / Surfilce prabing at atomic ~cah, ,'ith STM
In fig. I we show a topograph of the unoccupied stales of the Sill 11)-7 x 7 surface after exposure to ~ 0.2 L of O 2 at 300 K. Two oxygen-induced sites are clearly seen: (a) sites that appear darker and (bl sites that appear brighter than normal unreaeted Si adatom sites. The most interesting aspect of these product sites is their spatial distribution, which is particularly site selective. We find that bright sites favor the fmdted half of the 7 × 7 unit cell over tile unfaultcd half by about a factor of eight, while corner-adatom sites arc preferred over the center sites by about a factor of four. Dark sites show the same qualitiative trends, but the selectivity ratios arc somewhat smaller. Exposure to higher 0 2 doses also shows thai reacted sites tend m cluster [17]. STS, ultraviolet photoemission spectroscopy (UPS) and electronic structure calculations have been used to identify the nature of these reaction products [15,17]. U P S studies at lower temperatures [13,17] have shown that the very first step of the reaction involves the formation of a molecular precursor, an O_~-Iikc species, which hy thermal or photochemical [18] activation gives the stable products imaged with the STM. STM images obtained soon alter O~-exposure show time dependent changes which, most likely, reflect lhe decay of the remaining population of the molecular precursor [17]. T h e observation of the precursor and the thct that it is calculated tn be strongly bound to the surface [19] and thus dissociate mostly at the site at which it was originally formed, indicate that the site selectivity of the oxidation revealed by the STM reflects a site-dependent sticking process for O , on the 7 × 7 surface. What is so special about the faulted half and corner-adatom sites of the 7 X 7 surface? STS studies [3,4] have revealed a higher L D O S near El: for these sites and this finding is confirmed by recent first principles calculations on 7 x 7 [20]. Thus we proposed that the negatively charged (O_4-1ikc) precursors are lormed by a process ("harpooning") where electrons from adatom dangling bonds tunnel into the 27r* affinity level of neutral O., molecules [17]. Incident O , molecules enter a mobile physisorbed precursor state, probe the surface and get trapped selectively at sites of high L D O S o-" occupied states near E r.
4~
This mechanism was further supported by studies which utilized ~5-doping as a means of changing the dangling bond occupation [18]. Let us now consider the corresponding oxidation reaction at the Si(1001-2 x I surface [21]. Fig. 2a shows a topograph of the occupied states of a portion of a S i ( 1 0 0 ) - 2 × l surface. The dimcr rows at the 2 × 1 and I x 2 terraces are clearly seen. There arc also many deflects of different kinds on the surface. Many of them are single or multiple dimer vacancies along with larger defects which arc usually ascribed to transition metal contamination. Another characteristic defect appears as a dlmer vacancy in which two adjacent Si atoms along the (110) direction appear to be missing. Because of its shape such a defect is usually rcterrcd to as a C-type defect; the locations of several of these defects arc marked with arrows ill fig. 2a. In fig. 2b the same area of the surface is shown after an exposure to ~ 1 L o f O , (due to the proximity of the tip to the surface during &lsing the exact exposure is r o t well defined). Several changes are observed. First, the number of dark-appearing sites has increased significantly, second there are a few new, brightlooking sites formed on top of dimcr rows (indicated by 13). Finally. we observe that exposure Io O , resulted in the buckling of extensive areas of uareacted and previously symmetric-appearing dimers. Considering the spatial distribution of the reaction-induced dark sites we find that only a couple of them appear on previously perfect dlmer sites on the terrace. The initial reaction appears to be essentially localized at C-defect sites. The locations of the same C-sites marked by arrows in fig. 2a are also marked in fig. 2b. Study of fig, 2 indicates that all C-type defects have reacted. Dimer vacancy type defects, on the other hand, show little propensity for reaction with O , . T h e ease with which incident O , molecules can find the C-defects indicates again the presence of a mobile precursor state. Thus, it appears that the initial oxidation behavior of the St(Ill0)-2 × 1 surface is completely opposite to that of the St(111)7 × 7 surface; on 7 X 7 defects do not play a significant role while on the 2 × 1 surface, defects dominate the initial reaction. In both cases, hove-
Ph. Ar'our~, L-$E Lye / SllrfacL"probing at atomic scale with STM
430
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ever, c l u s t e r i n g o f the reaction is observed a r o u n d the initial reacted sites.
T o understand the reactivity treads observed we consider the electronic structure of the different surface sites. In fig. 3 we show tunneling I - V curves obtained over a dimer site, and a C-defect [21]. We see that, unlike the adatom sites of the 7 × 7 surface which have metallic l - V s , the L D O S of dimer sites of the SJ(10O)-2 × 1 surface show a gap. The I - V over the C-defect, on the other hand, again has metallic characteristics leading to high L D O S near Ep. This behavior results fcom the fact that the two dangling bonds of a dlmer are paired to give a 1r-bond in the ease of a symmetric dimer or are paired on the up-atom of a buckled dimer. This pairing sweeps states away from E~. and creates the gap [22]. At a C-defect, on the other ]land, the two dangling bonds are far apart, there is little coupling between them and their energy remains near E F [23]. Thus, we find again that the site selectivity of the oxidation reaction is determined by the local electronic structure and, as in the c a ~ of the Si(l 11)-7 >~ 7 surface, the reactivity on the Si(10O)-2 × 1 surface is higher at sites of high L D O S of occupied states near E F. Information as that discussed above regarding the site-selectivity of chemical reactions has been largely inaccessible by spatially averaging surface science techniques. |n particular, the role of minority sites and defects in surface chemistry and catalysis has been debated for a very long time. S T M / S T S studies allow a direct investigation of
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Fig. 4. S,;hcmalic polenlial energy (PE) cuwes illustrating a ehemieaBy-assisl~d field evaporation process. The solid line PE curve represents the neutral ground stale where an atom
(A) is bonded either to the substratc (left well) or to the tip (rigla well). The dashed line corresponds ¢o an ionic state, S" A"+. Pant:is A and B ilhlnrale the field free conditions, while in C and D an electric field E is present.
the role of such sites. As we saw above, C-defects on the Si(100)-2 × 1 surface initiate the oxidation and act as nucleation centers, are probably responsible for the pinning of the Fermi level, which is known to be near the center of the gap, [23], and their reaction lead~ to permanent buckling of direct rows (see fig. 2).
3. Alumle scale manlpu|atlan of silicon with the STM: a chemicaHy-assisted field evapgrat[en process
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Fig. 3. Tunneling I V curves obtained over a dimer site of the Si{100)-2×l surface {solid line) and over a C-detect {dashed line).
Manipulation of materials on the atomic scale with the S T M involves the general capability of breaking surface chemical bonds, placing surface atoms on the S T M tip and finally re-depositing these atoms at another location. Such processes can be described by potential energy (leE) curve diagrams such as that in fig. 4a which shows the PE curve (solid line) of an atom bound to the surface of the sample (left well) or to the tip
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(right well). The objective is to be able to transfer this atom from the surface to the tip and vice versa. In doing so, we must overcome the potential energy barriers present. One way to reduce the barrier is to bring tile tip quite close to the surface so as to establish a degree of chemical bonding between tip and sample (fig. 4b). In the distance range where the apparent tunneling barrier begins to be reduced, strong chemical forces develop [24,25]. However, the preference of a particular atom for the sample surface or the tip is determined entirely by their chemical composition. T o gain control over the direction of atom transfer we employ electrostatic forces whose magnitude and direction can be controlled externally. As fig. 4u shows, at an energy higher than that of the neutral ground state curve there will be ionic excited state curves such as the one denoted with the dashed line which corresponds to the situation where a surface atom, A, has given the substxate, S, n electrons leading to a configuration S'~-A "+. When an electric field E is generated by applying a bias voltage, the neutral PE curve is only weakly perturbed through polarization effects, while the energy of the ionic curve is reduced by - n e E z . The ionic curve may cross the neutral curve but, as fig. 4c suggests, this may happen too far from the surface to have an effect. If, however, the tip is first brought close to the surface and then the strong field is generated, crossing takes place close to the surface so that only a small effective barrier Q needs to be surmounted. If the field direction is reversed, the direction of the electroslatic force is reversed. The approach of the tip to the surface is, of course, necessary if one wants to influence only a small area of the surface, a requirement for atomic scale modifications. The close proximity of tip m~.d sample leads to a significant reduction of the field strength required to initiate material transfer in the STM configuration as compared to the field required for field evaporation in the field ion microscope (FIM) [26]. This can be easily understood by reference to the image barrier model of field evaporation developed early on by Muller [27]. In this model, the barrier is due to the attractive interaction between the desorbing ion and its image in the substrate. When
431
the tip is close by, however, the new attractive interaction between the ion and its image in the tip will tend to reduce the barrier [28]. While the above classical description of the field-induced atom transfer process in the STM provides qualitative insights and probably correctly predicts various qualitative trends, it should not be trusted for quantitative predictions. This is because of the breakdown of the classical image description at short distances, the occurence of interatomic screening process which significantly reduce the net charge on the evaporating atom in the vicinity of the surface, and the onset of chemical bond formation. These complications have been examined in some detail using first principle calculations in conjunction with the problem of electronand photon-stimulated ion desorpfion [29,30]. At the very small tip-sample separations that our experiments were carried out a proper description of the atom transfer process would be chemically-assisted field-bt'aporo'.ion. Recent theoretical work [31] indeed finds that chemical interactions play a major role in reducing the barrier for atom transfer. Experimentally. we first bring the tip close to the surface over the site we want to modify. The distance scale is established by determining the electronic contact point where the apparent tunneling barrier becomes zero [8], Then the electric field is generated by applying a voltage pulse. In fig. 5 we ~ho,,' an example of surface modification. Panel A of fig. 5 shows the starting S i ( l l l ) 7 × 7 surface. The tip is then moved to ~ 3 /~ from electronic contact and a + 3 V pulse is applied, resulting in the characteristic central hill surrounded by a depressiol] structure seen in panel B. Next, the tip is placed over the hill and another + 3 V voltage pulse is applied. As a result, the entire hill is transferred to the tip. This Si cluster can now be redeposited anywhere on the surface by applying a negative voltage pulse to the sample. In panel C the cluster has been deposited to the left of the dark hole, The threshold field required for the removal of Si is a function of the tip-sample distance and at the conditions of fig. 5 was only %1 V / A ; significantly lower than the ~ 4 V / A observed with FIM [26]. The hill plus depression ~tructure re-
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Ph, Al ~nlri~. L-IK Lyo / Sttrlace prob#lg at alvmic scalcnilh STM
salts because of thc close proximity of the tip to the sample. When a voltage pulse is applied, and because of the gradient of the electric field, atoffts tend to move towards the apex of the tip where lhey pile up and form a bridge connecting the tip with the surface. This bridge formation is reflected in measurements of the tunneling current
10~ o -0.6 V ~. -.0 4 "V
I0 ° +3 V pulse
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2 gsa 2 0 Tip Displacement(~.)
Fig. b. Tunneling current versus lip di~placemenl. The tip was firs[ displaced toward the surface and after a 3 V pulse was applied tindicated by the arrow) it was retracted ~l~vil~from Ihe surface.
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Fig. 5. (At .L3eetinn of a Si( 11])-7 x 7 surf=ce. (B) A 3 V pulse applied at ~ 3 A from electronic contact has led to the formation of a strtlcture involving a central bill surrounded by a depression. (C) ~nother ~,V paise has been applied and the enlire hill ~as transfL~ed to the tip. The lip wa~ then moved to the left of the hole and the cluster was redcpnsitcd by applying a - 3 V pulse to Ihe sample.
versos distance such as those shown in fig. 6. Initially, as the tip approaches the surface, the current rises exponentially. At the point indicated by the arrow the voltage pulse is applied, and then the tip is retracted. We see that there is a hystcrisis where an apparent tip retraction of ~ 4 A is needed before the current drops again, We believe that this hysterisis is associated with the destruction of the bridge. Another interesting point revealed by fig. 6 is that after the destruction of the bridge the current drops much faster with distance tl,:m it did belorc the voltage pulse. This is a reflection of the fact that the tip has been modified by the presence of the Si cluster ~n it and the effective decay length of its wavefunction is now shorter. At - 5 A from thc contact point, the tunneling current change is ~ 102. In fact, it has been shown recently that one can use the tunneling current changes resulting from the reversible transfer of a Xe atom betwccn a nickel surface at 4 K and a tip to create an atomic switch [32]. Finer control over the affected area can be achieved by varying the tip-sample distance and the magnitude and duration of the voltage pulse. With a sharp tip, short tip-sample distance ( ~ 1
Ph. A t o , rls. L.W. L)'o / Stir[ace p *~h:~ :; :,tt,vnic ~ / , , with S T M
.~), and by applying a weak ( ~ I V) pulse single Si atoms can be manipulated. Fig. 7 shows a series of atomic scale manipulations. Panel A shows the starting surface containing a defect in the lower right (dark site) which is used as a marker. In B the field was applied while the tip was centered over the center of the upper half of
433
the unit cell; three Si atoms were removed, leaving a fourth under the apex of the tip, This fourth Si atom was in a=. unstable configuration, and as the tip was moved towards it to remove it, it migrated to the left and occupied a normal adatorq site of the 7 × 7 lattice, panel C. This atom was removed by the tip in panel D. Then
Fig, 7. A serie~ of atomic scale manipulations: (A) The tip is placed at ~ I ,~ from electronic conlact over the silo indicated by the arrow. (B) A I V pulse removes three atoms le~ving Ihc fourth under the tip, (C) Tbo first allempt to remove this alom leads to ils migrating to the left (see arrov,,) alld bonding as a center adatom. ( D ) A second pulse removes this fourth atom. (E) A new
corner-adalam is removed and in (F) it is placed back Io its originnl position.
Pit ,4t tniri~, L.H [ 1..l
another corner adatom was removed in panel E. One question that arises is: can the S T M undo a modification, e.g., can it put back a removed atom or cluster of atoms and repair the original structure? We find that. in general and especially for the clusters, the answer is no. Although we can place the a t o m / c l u s t e r ~wer the original site, it does not incorporate itself This is the resnlt of the rebondin$ at the substratc which takes place after the removal of a number of atoms, and of the optimization of the bonding bmwecn atoms in the removed cluster. The redeposited atoms must overcome a sizeable activation barrier to occupy their original sites. In the case of single atom removal, however, incorporation can be achieved as shown in panel F. [n the examples of manipulations given above, we were interested in modifying the Si ~ubstrate, For Ibis reason we chose to use a tungsten tip since W has a 2 V / , ~ higher field evaporation threshold than Si in the FIM geometry. There are
Fig. g.
cases, however, where one is interested not in modifying the substratc but in depositing foreign material on the substrate to generate nanostructures. If the material has a lower field-evaporation threshold than the substrate, 0ossibly one could accomplish this by using the material as an S T M til This has already been demonstrated for gold tips [9]. Recently, we have experimented with AI and found that AI tips are relatively stable and that atomic resolution is readily achieved. This latter observation in~acates that d-functions on the tip are not a requirement for achieving atomic resolution, In fig, 8 we show a topograph of the occupied states of Si( I l 1)-7 X 7 with an AI cluster produced by applying a - 3 V voltage pulse to the sample. Clusters currently are attracting significant attention because they represent an essentially new state of matter with prcpert,~es different from those of the individual atoms or bulk solid. The S T M can be a tool for making dusters while STS allows the in situ study
Topograph of the occupied states of Si(I I I)-7 X 7 oblained using an aluminum tip. The AI cluster in the center tff Ihc picture wa.~produc,:d by applying a - 3 V pulse 1o the sample.
Pit. At'tnlrt~', L -tg~ Lyo / Suffice probing a! area,dr scale with STM
of their valence electronic structure, hl agreement with recent calculations [33], we find that the small AI cluster of fig. 8 is not metallic but shows an ~ 2 e V gap.
4, UonelusJnns As an example o f the power of S T M and STS to probe Ideal chemistry and relate it to local electronic structure, we have used the study of the initial stages of oxidafiol, of Sill 11 )-7 × 7 and Si(100)-2 × I. W e find that these reactions are very site.selective. On the 7 × 7 surface, the top layer Si a d a t o m sites are the most reactive. A m o n g a d a t o m sites, corner sites are more reactive than c e n t e r sites and sites in the f a u h c d half o f the 7 X 7 unit cell are more reactive than those in the unfaulted half. Defects are found not to play a si~niflcant role. T h e common characteristic of the reactive sites is a high density of occupied states near E F. T h e initial stage oxidation of Si(lll0)2 × I appears to show the opposite behavior, in that the majority d i m e r sites a r e rather unreactive while defect sites, in particular C-defects. dominate the early stages of oxidation. STS, however, shows that the same requirement responsible for the high reactivity of a d a t o m sites on the 7 x 7 surface, i.e., high L D O S n e a r Et-, d e t e r m i n e s the reactivity o f the S i ( 1 0 f 0 - 2 × l surface. W h i l e dimer sites show a gap in L D O S at E~., C-defects show a metallic behavior with a high L O b S n e a r /"v. W e have also discussed how the S T M can be used as a tool for the atomic scale modification and manipulation o f covalent solids such as silicon. W c have shown that by a combination of chemical t i p - s a m p l e interactions and electrostatic forces g e n e r a t e d by a voltage pulse, individual Si a t o m s or clusters of atoms can be removed from the surface, placed on the tip and then redeposited at p r e d e t e r m i n e d locations on t h e surface. D u e to the presence of t i p - s a m p l e chemical interactions, t h e field necessary to break the S i - S i bonds is much reduced from that n e e d e d in ordinary field evaporation. W e term this process chemically-assisted field eeaporation. Finally, we note that chemically-assisted field-evaporation
~5
may take place during constant current S T M imaging of surlaces on which regions of low L D O S have been generated as a result of chemical reactions [341.
References [1] G. Binnlg, II, Rohrer, Ch. Gerber and E, Weib¢l, Phys. Rev. [+ett. 40 (lqb21 571. [2] J, Tcrstaf and D,R. ttmnann, PhySr Rev, B 31 { 19851805. [3] R.J, Ilamers. Ann, Rev. Phys. Chem. 411(1989) 531. [41 R. Wl~lkllw and Ph. A~ouris. Phys. Rev. l.en. 60 (1988) 1049: Ph. Avouris and R. Wi)]kow. Phys, Rev. B 5I) (108a) sa71. [5[ R.S. neckL,r, J.A, Gohwchcnko and I].S. Swartzclltruber,
Niaure 325 t 1987) 419, [61 D.M. Figler and IS.K, Schweizer, Nalure 344 (ItttJ()) 524. [TJ L.J. Whilm;m. J,A Slroseiu. R.A. Dragoset und RJ. Celolla. Science 1206 (l~JUl)251. [al I.-W, Lyo and Ph. Avouris. Science 253 (1991) 173. [9] ll.M. Mamin. PH. Ouether and D, nugar. Phys. Rcv. Lett. 05 tlt~oal 2418. Ibll JS. Foster. J.E. Frtlmmer and P.C. Arneu. Nalure 331 (1988) g24. [II] G. Dojardin. R.E. W~lkup alia Ph. Avouris. Science (1992) 1232. [121 C.F. Ollalt:, in: tlighlights and Pr~shects in Condensed Malter Physics. Ed. L. Esaki (Plenum. blew York. I"Jt)2). and relk~rence~therL.in. [13] LI. Hnefer. P. Morgen. W. Wurlh and E. Umbacb. Phys. Rev. 134O (1989) 11311.£1ndreferences therein. [14] H. T~lkumolo. K. MikL H, Murakomi. II, grando, M. Ona and K. Kajimur~l.J. Vac. Sci. Technol. A 8 (19901 255. [15} I-W. Lyo, Ph. Avouris. B. Schubert ~lnd R. Hoffmann. J. Phys. Chenl, 04 (it)lxa 44:1(h Ph. Avouris and I.-W, Lyu. 3urf. Sci. 242 t1991) I. [16] J.P, Pelt and R.PI. Koch, Phys. Ne~. B 42 ( 199(1137bi. [17] Ph. Avouris. 1.-W. Lyu and F. Bnzso. J, Va¢. SoL Tcchnol. B 9 ( 1991) 424. il8] F. Bozso and Ph. Avouris. Phys. Re~; 1344 tlqgl) 9129. [19] B. Schubert. Ph. Avouris and R, Hoffmann, to De published. [2(I] M. Fujita, II. Nagayosh[ and A, Ytlshimori, Surf. Sei. 242 (190~ 220. [21] Ph. Avnuris arid D.E Culdll. Ullramicroseopy 42-44 (1t~92). in pre~s. [22] R.J. ttamers. Ph. Avouris and F. Bozso, Phys. Rev. Left. 59 11987) 2071. [23] R,J. Hamers and U.K, Koehler, J. Va¢. Sci. Teehnoh A 7 ttqSql 28,84. [24] C.J. ('hen. ]. Phys. (Cond. MaLl 3 (19911 I227. 125] S. Ciraci. E. Tekman. M. Gokcedag and LP. Batra, Ultramicroscopy, to he published.
Iqt. Al'otlri.~. L -||~ I.y~ / .Yllrlit('(' prtJhill.~ Ill a,Jmil' Nt'lllt" tt'tlh .~1~,11 [26] T.T. Tsong. At<~m-Pmb~ Field hm Microscopy (Cambridge University Press, Canlbridg¢, I~)l-)()). ~lnd [¢1"¢1, cnccs Ihcr¢in. 127l E,W, Mulh:r. Phys. Rcv. 1(~2(P].~l~)61s, 128) T.T, Tsont~.Jpn. J. App]. Phys. 30 ( I~)L)I) 1~38:~. 12~q Ph. Avoud~. R. I ~ s , i . N.D. Long and I:JA'I. Newns, Phys. Rev. Lett. 5~) ( Iq871 2215; J. ('hem. Phys. ~l~ (P~8~) 238~'I.
[~0j R . E Wnlkup. Ph, A~lluris, N,D. Lang and R. Kawai.
Phys. Roy. L¢II. h3 (IIJS~l)H)72. [31} N.D. Ling. to he published. [32} D.M Eiglcr. ('.P. Lulz and W,E rudg*', f'(alure 352 ( Iq~)ll 6[~). [33] J.-Y. Yi. D.J. Oh and J. IBcrnholc, Phys. R~:v. L¢IL 67 [34] Ph. Avourin and I.-W. l.yt~. Surf. ~ci, 242 (U~ql) I.
app~O
s u r f a c e SCiellce
Probing the chemistry and manipulating surfaces at the atornic scale with the STM Ph. Avouris and L-W. Lyo I~lll ~t,$t'tlrt'h DiciMo;z, 7~J. a~tt.w~l Resean'/i ('cruet. P.O Bor 21K ~rkroLh~+ Iteik.ht.~, N$" 10598, U.$~4 Received 2() Nt~vcmber 1991; accepted for publication nl January I'~t12
The STM can he used hnth as a probe of local surface eleclronic ~truclur¢ a~d chemistry and as a tool fl~r Ihe alomic scale manipulation iif materia!s. We demonstrate lhe use of the STM as ~1 hl(al probe by invesligatint~ Ihe spafi;d distribution of the initial stages of oxidation of Si( 111)-7 × 7 and Sit 11)111-2× I. We find Ihat the oxidation reaction is very site selecliv¢. On the St( I 11) surface. Si corner adatom sites on Ihe faulted-half of the 7 × 7 unit celi are mo,t reaclive zlLd detects play no significant role. On Ihe siinlu) surface+ on Ihe other hand, the Si direct majority sties are unruaelive while C-defecls dominate the early stages of oxidation. This apparent difference in the reactivity of tile two sul:,ces is resolved hy tunneling speclros¢opy which slaws that a common faelor, namely the density ~ff occupied states near E F. determines Ihe reactivity on both surhlces, Nexl we consider the use of STM as a manipuhtinn tool. We firsl discuss a general scheme, termed chemically-assisted field-evuporafion+ which allows Ihe aton'zie scale manipuhdion o~ materials, This sell=:me involves a combination of chemical lip-sample interactions produced by bringing the tip ~ery close to Ihe site to be affecled, and electrostatic forces produced by Ihe application of a voltage pulse. The chemical interaction ~ignifieantly reduce, the barrier for atom Iran~fer between sample and tip+ while the electrostatic hlree introduces directionality and further reduces the barrier. We illuslralu Ihe p:+wer of this approach by (a~ removing individual Si atoms or clusters uf atums from Sit I I I ) with a "d/tip and then redepusifing them at predetermined surface sites, and (h) depositing AI atoms from an AI tip to the St( I I 1) surface,
L |ntruductlon T h e e l e c t r o n i c s t r u c t u r e a n d c h e m i s t r y o f solid s u r f a c e s have b e e n s t u d i e d in t h e last f e w d e c a d e s u s i n g t e c h n i q u e s t h a t p r o v i d e i n f o r m a t i o n avera g e d o v e r a m a c r o s c o p i c a r e a o f t h e surface. Sarlktce c h e m i s t r y , h o w e v e r , is a local process. T h u s , it is i m p o r t a n t t o k n o w t h e s p a t i a l d i s t r i b u t i o n o f a r e a c t i o n w i t h a t o m i c site r e s o l u t i o n . M o r e o v e r , t o u n d e r s t a n d s u r i a e e reactivity w e n e e d t o r e l a t e t h e reactivity o f a p a r t i c u l a r site t o t h e local e l e c t r o n i c s t r u c t u r e . T h u g , ideally, w e need a surface technique which can probe both t h e spatial d i s t r i b u t i o n o f c h e m i c a l r e a c t i o a s a n d t h e local e l e c t r o n i c s t r u c t u r e w i t h single s u r f a c e cite r e s o l u t i o n . S c a n n i n g t u n n e l i n g m i c r o s c o p y ( S T M ) [1] i m a g e s t h e local density o f states ( L D O S ) o f low
b i n d i n g c n c r g y v a l e n c e e l e c t r o n s [2] a n d s c a n n i n g tunneling spectroscopy (STS) measures their ene r g i e s [3]. +~ is precisely t h e s e l o o s e l y - b o u n d elect r o n s t h a t are r e s p o n s i b l e for t h e c h e m i s t r y of m a t e r i a l s . T h u s . it w a s s o o n r e a l i z e d [4] t h a t o n e c a n t a k e a d v a n t a g e o f t h e c h a n g e s in L D O S a n d in t h e local S T S s p e c t r a t h a t t a k e p l a c e u p o n r e a c t i o n o f a p a r t i c u l a r site to o b t a i n t h e spatial distribution of the reaction and relate the obs e r v e d reactivity o f t h e d i f f e r e n t sites to t h e local e l e c t r o n i c s t r u c t u r e . T h e a p p l i c a t i o n o f this a p p r o a c h to t h e s t u d y of t h e c h e m i s t r y o f s e m i c o n d u c t o r s u r f a c e s h a s b r o u g h t n e w a n d u n i q u e ins i g h t s i n t o t h e factors t h a t d e t e r m i n e s u r f a c e reactivity. H e r e w e illustrate t h e a p p l i c a t i o n o f S T M / S T S in t h e study o f s u r f a c e c h e m i s t r y u s i n g t h e s t u d y o f t h e initial stages o f t h e Si(l 11)-7 × 7 a n d St(100)-2 × 1 s u r f a c e s as e x a m p l e s . I n p a r t i c -
0169-4332/92/$115.00 ~ 1992 E l ~ i e r Science Publishers B.V. All rights reser,'cd
Ph. At out,s, L.a< Lyo / Surfacc probiug at atomic scale wah STM
ular, we concentrate on one aspect of the problem, namely, on the factors which determine the site-selectivity of the initial stages of oxidation. In addition to being a powerful surface probe, the S T M can also be used as a tool for the modification and manipulation of materials on the atomic and nanometer scale, For this purpose one relies on imeraetions between surface atoms or adsorbates and the S T M tip. T o a large extent the magnitude of these interactions can be tuned, and recently some of them have been used to provide the unprecedented capability to manipulate, e.g.. deposit, move or desorb, and modify, e.g., dissociate, ad.zorbed atoms [5-8], clusters [9], and molecules [10,11]. In particular, applying voltage pulses has been a popular method for inducing surface modifications with the S T M [12]. In a pioneering work, Becker et al. 151 showed that atomic ~cale deposition is possible by this
427
method. More recently, we have demonstrated that by combining chemical tip-sample interactions with electrical forces produced by a voltage pulse, one can break strong covalent chemical bonds between Si surface atoms, transfer individual atoms or clusters of atoms to the tip and then redeposit them at predetermined surface sites [8]. Here we will briefly explore the mechanism and implications of this process. Details on the S T M / S T S set up and experimental procedures can be foand in earlier work [4].
2. Probing surface chemistry on the atomic scale: the initial stages of oxidatlon of $1(1H)-7 × 7 and Si(100)-2 X 1 surfaces Tile oxidation of silicon is probably the most important chemical process in the area of elec-
Fig. 1. STM topograpb of Ihe unoccupied slates of u Si(I I I )-7 × 7 surface exposed to 0.2 L of 0 2 at 3or) K. Sample bias 2 V.
428
i'll. A~ ~ntris. L • IK Lb~ / Surfiwe prtlbmg at monlic stalk' with $1~1
tronic materia;s and, as such, it has attracted considerable attention, In particular, the ear|y stag¢s of oxidation have been the object of extensire studies using both conventional [13], and, more recently. STM techniques [14-17]. Here we
focus on the intriguing site selectivity of the early stages of oxidation of Si(111)-7 × 7 and Si(100}2 × I as revealed by the STM studies, and the electronic factors that determine this site selectivity.
Fig. 2. Top: ~TM lopograph tff the occupied states of a $i(100)-2 × I ~,fface. Boltom: The same urea arlur c~pt=~ure to O 2. The Ic~c~Uon of several C-defucts is indicated hy arrow~. Sample bi~, - 2 V.
Ph. At'tams, I..g( I,yo / Surfilce prabing at atomic ~cah, ,'ith STM
In fig. I we show a topograph of the unoccupied stales of the Sill 11)-7 x 7 surface after exposure to ~ 0.2 L of O 2 at 300 K. Two oxygen-induced sites are clearly seen: (a) sites that appear darker and (bl sites that appear brighter than normal unreaeted Si adatom sites. The most interesting aspect of these product sites is their spatial distribution, which is particularly site selective. We find that bright sites favor the fmdted half of the 7 × 7 unit cell over tile unfaultcd half by about a factor of eight, while corner-adatom sites arc preferred over the center sites by about a factor of four. Dark sites show the same qualitiative trends, but the selectivity ratios arc somewhat smaller. Exposure to higher 0 2 doses also shows thai reacted sites tend m cluster [17]. STS, ultraviolet photoemission spectroscopy (UPS) and electronic structure calculations have been used to identify the nature of these reaction products [15,17]. U P S studies at lower temperatures [13,17] have shown that the very first step of the reaction involves the formation of a molecular precursor, an O_~-Iikc species, which hy thermal or photochemical [18] activation gives the stable products imaged with the STM. STM images obtained soon alter O~-exposure show time dependent changes which, most likely, reflect lhe decay of the remaining population of the molecular precursor [17]. T h e observation of the precursor and the thct that it is calculated tn be strongly bound to the surface [19] and thus dissociate mostly at the site at which it was originally formed, indicate that the site selectivity of the oxidation revealed by the STM reflects a site-dependent sticking process for O , on the 7 × 7 surface. What is so special about the faulted half and corner-adatom sites of the 7 X 7 surface? STS studies [3,4] have revealed a higher L D O S near El: for these sites and this finding is confirmed by recent first principles calculations on 7 x 7 [20]. Thus we proposed that the negatively charged (O_4-1ikc) precursors are lormed by a process ("harpooning") where electrons from adatom dangling bonds tunnel into the 27r* affinity level of neutral O., molecules [17]. Incident O , molecules enter a mobile physisorbed precursor state, probe the surface and get trapped selectively at sites of high L D O S o-" occupied states near E r.
4~
This mechanism was further supported by studies which utilized ~5-doping as a means of changing the dangling bond occupation [18]. Let us now consider the corresponding oxidation reaction at the Si(1001-2 x I surface [21]. Fig. 2a shows a topograph of the occupied states of a portion of a S i ( 1 0 0 ) - 2 × l surface. The dimcr rows at the 2 × 1 and I x 2 terraces are clearly seen. There arc also many deflects of different kinds on the surface. Many of them are single or multiple dimer vacancies along with larger defects which arc usually ascribed to transition metal contamination. Another characteristic defect appears as a dlmer vacancy in which two adjacent Si atoms along the (110) direction appear to be missing. Because of its shape such a defect is usually rcterrcd to as a C-type defect; the locations of several of these defects arc marked with arrows ill fig. 2a. In fig. 2b the same area of the surface is shown after an exposure to ~ 1 L o f O , (due to the proximity of the tip to the surface during &lsing the exact exposure is r o t well defined). Several changes are observed. First, the number of dark-appearing sites has increased significantly, second there are a few new, brightlooking sites formed on top of dimcr rows (indicated by 13). Finally. we observe that exposure Io O , resulted in the buckling of extensive areas of uareacted and previously symmetric-appearing dimers. Considering the spatial distribution of the reaction-induced dark sites we find that only a couple of them appear on previously perfect dlmer sites on the terrace. The initial reaction appears to be essentially localized at C-defect sites. The locations of the same C-sites marked by arrows in fig. 2a are also marked in fig. 2b. Study of fig, 2 indicates that all C-type defects have reacted. Dimer vacancy type defects, on the other hand, show little propensity for reaction with O , . T h e ease with which incident O , molecules can find the C-defects indicates again the presence of a mobile precursor state. Thus, it appears that the initial oxidation behavior of the St(Ill0)-2 × 1 surface is completely opposite to that of the St(111)7 × 7 surface; on 7 X 7 defects do not play a significant role while on the 2 × 1 surface, defects dominate the initial reaction. In both cases, hove-
Ph. Ar'our~, L-$E Lye / SllrfacL"probing at atomic scale with STM
430
20 ,, , ~i,Ln=' lS ',. . . . . . .
ever, c l u s t e r i n g o f the reaction is observed a r o u n d the initial reacted sites.
T o understand the reactivity treads observed we consider the electronic structure of the different surface sites. In fig. 3 we show tunneling I - V curves obtained over a dimer site, and a C-defect [21]. We see that, unlike the adatom sites of the 7 × 7 surface which have metallic l - V s , the L D O S of dimer sites of the SJ(10O)-2 × 1 surface show a gap. The I - V over the C-defect, on the other hand, again has metallic characteristics leading to high L D O S near Ep. This behavior results fcom the fact that the two dangling bonds of a dlmer are paired to give a 1r-bond in the ease of a symmetric dimer or are paired on the up-atom of a buckled dimer. This pairing sweeps states away from E~. and creates the gap [22]. At a C-defect, on the other ]land, the two dangling bonds are far apart, there is little coupling between them and their energy remains near E F [23]. Thus, we find again that the site selectivity of the oxidation reaction is determined by the local electronic structure and, as in the c a ~ of the Si(l 11)-7 >~ 7 surface, the reactivity on the Si(10O)-2 × 1 surface is higher at sites of high L D O S of occupied states near E F. Information as that discussed above regarding the site-selectivity of chemical reactions has been largely inaccessible by spatially averaging surface science techniques. |n particular, the role of minority sites and defects in surface chemistry and catalysis has been debated for a very long time. S T M / S T S studies allow a direct investigation of
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o • / -lO ~
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Fig. 4. S,;hcmalic polenlial energy (PE) cuwes illustrating a ehemieaBy-assisl~d field evaporation process. The solid line PE curve represents the neutral ground stale where an atom
(A) is bonded either to the substratc (left well) or to the tip (rigla well). The dashed line corresponds ¢o an ionic state, S" A"+. Pant:is A and B ilhlnrale the field free conditions, while in C and D an electric field E is present.
the role of such sites. As we saw above, C-defects on the Si(100)-2 × 1 surface initiate the oxidation and act as nucleation centers, are probably responsible for the pinning of the Fermi level, which is known to be near the center of the gap, [23], and their reaction lead~ to permanent buckling of direct rows (see fig. 2).
3. Alumle scale manlpu|atlan of silicon with the STM: a chemicaHy-assisted field evapgrat[en process
/iI
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a.
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Fig. 3. Tunneling I V curves obtained over a dimer site of the Si{100)-2×l surface {solid line) and over a C-detect {dashed line).
Manipulation of materials on the atomic scale with the S T M involves the general capability of breaking surface chemical bonds, placing surface atoms on the S T M tip and finally re-depositing these atoms at another location. Such processes can be described by potential energy (leE) curve diagrams such as that in fig. 4a which shows the PE curve (solid line) of an atom bound to the surface of the sample (left well) or to the tip
I'h, Arottrls. L-W. Lj'o / Stir[aceprobing at atomic scale with STM
(right well). The objective is to be able to transfer this atom from the surface to the tip and vice versa. In doing so, we must overcome the potential energy barriers present. One way to reduce the barrier is to bring tile tip quite close to the surface so as to establish a degree of chemical bonding between tip and sample (fig. 4b). In the distance range where the apparent tunneling barrier begins to be reduced, strong chemical forces develop [24,25]. However, the preference of a particular atom for the sample surface or the tip is determined entirely by their chemical composition. T o gain control over the direction of atom transfer we employ electrostatic forces whose magnitude and direction can be controlled externally. As fig. 4u shows, at an energy higher than that of the neutral ground state curve there will be ionic excited state curves such as the one denoted with the dashed line which corresponds to the situation where a surface atom, A, has given the substxate, S, n electrons leading to a configuration S'~-A "+. When an electric field E is generated by applying a bias voltage, the neutral PE curve is only weakly perturbed through polarization effects, while the energy of the ionic curve is reduced by - n e E z . The ionic curve may cross the neutral curve but, as fig. 4c suggests, this may happen too far from the surface to have an effect. If, however, the tip is first brought close to the surface and then the strong field is generated, crossing takes place close to the surface so that only a small effective barrier Q needs to be surmounted. If the field direction is reversed, the direction of the electroslatic force is reversed. The approach of the tip to the surface is, of course, necessary if one wants to influence only a small area of the surface, a requirement for atomic scale modifications. The close proximity of tip m~.d sample leads to a significant reduction of the field strength required to initiate material transfer in the STM configuration as compared to the field required for field evaporation in the field ion microscope (FIM) [26]. This can be easily understood by reference to the image barrier model of field evaporation developed early on by Muller [27]. In this model, the barrier is due to the attractive interaction between the desorbing ion and its image in the substrate. When
431
the tip is close by, however, the new attractive interaction between the ion and its image in the tip will tend to reduce the barrier [28]. While the above classical description of the field-induced atom transfer process in the STM provides qualitative insights and probably correctly predicts various qualitative trends, it should not be trusted for quantitative predictions. This is because of the breakdown of the classical image description at short distances, the occurence of interatomic screening process which significantly reduce the net charge on the evaporating atom in the vicinity of the surface, and the onset of chemical bond formation. These complications have been examined in some detail using first principle calculations in conjunction with the problem of electronand photon-stimulated ion desorpfion [29,30]. At the very small tip-sample separations that our experiments were carried out a proper description of the atom transfer process would be chemically-assisted field-bt'aporo'.ion. Recent theoretical work [31] indeed finds that chemical interactions play a major role in reducing the barrier for atom transfer. Experimentally. we first bring the tip close to the surface over the site we want to modify. The distance scale is established by determining the electronic contact point where the apparent tunneling barrier becomes zero [8], Then the electric field is generated by applying a voltage pulse. In fig. 5 we ~ho,,' an example of surface modification. Panel A of fig. 5 shows the starting S i ( l l l ) 7 × 7 surface. The tip is then moved to ~ 3 /~ from electronic contact and a + 3 V pulse is applied, resulting in the characteristic central hill surrounded by a depressiol] structure seen in panel B. Next, the tip is placed over the hill and another + 3 V voltage pulse is applied. As a result, the entire hill is transferred to the tip. This Si cluster can now be redeposited anywhere on the surface by applying a negative voltage pulse to the sample. In panel C the cluster has been deposited to the left of the dark hole, The threshold field required for the removal of Si is a function of the tip-sample distance and at the conditions of fig. 5 was only %1 V / A ; significantly lower than the ~ 4 V / A observed with FIM [26]. The hill plus depression ~tructure re-
432
Ph, Al ~nlri~. L-IK Lyo / Sttrlace prob#lg at alvmic scalcnilh STM
salts because of thc close proximity of the tip to the sample. When a voltage pulse is applied, and because of the gradient of the electric field, atoffts tend to move towards the apex of the tip where lhey pile up and form a bridge connecting the tip with the surface. This bridge formation is reflected in measurements of the tunneling current
10~ o -0.6 V ~. -.0 4 "V
I0 ° +3 V pulse
tO~
0
2 gsa 2 0 Tip Displacement(~.)
Fig. b. Tunneling current versus lip di~placemenl. The tip was firs[ displaced toward the surface and after a 3 V pulse was applied tindicated by the arrow) it was retracted ~l~vil~from Ihe surface.
• . ~
~;_;N
Fig. 5. (At .L3eetinn of a Si( 11])-7 x 7 surf=ce. (B) A 3 V pulse applied at ~ 3 A from electronic contact has led to the formation of a strtlcture involving a central bill surrounded by a depression. (C) ~nother ~,V paise has been applied and the enlire hill ~as transfL~ed to the tip. The lip wa~ then moved to the left of the hole and the cluster was redcpnsitcd by applying a - 3 V pulse to Ihe sample.
versos distance such as those shown in fig. 6. Initially, as the tip approaches the surface, the current rises exponentially. At the point indicated by the arrow the voltage pulse is applied, and then the tip is retracted. We see that there is a hystcrisis where an apparent tip retraction of ~ 4 A is needed before the current drops again, We believe that this hysterisis is associated with the destruction of the bridge. Another interesting point revealed by fig. 6 is that after the destruction of the bridge the current drops much faster with distance tl,:m it did belorc the voltage pulse. This is a reflection of the fact that the tip has been modified by the presence of the Si cluster ~n it and the effective decay length of its wavefunction is now shorter. At - 5 A from thc contact point, the tunneling current change is ~ 102. In fact, it has been shown recently that one can use the tunneling current changes resulting from the reversible transfer of a Xe atom betwccn a nickel surface at 4 K and a tip to create an atomic switch [32]. Finer control over the affected area can be achieved by varying the tip-sample distance and the magnitude and duration of the voltage pulse. With a sharp tip, short tip-sample distance ( ~ 1
Ph. A t o , rls. L.W. L)'o / Stir[ace p *~h:~ :; :,tt,vnic ~ / , , with S T M
.~), and by applying a weak ( ~ I V) pulse single Si atoms can be manipulated. Fig. 7 shows a series of atomic scale manipulations. Panel A shows the starting surface containing a defect in the lower right (dark site) which is used as a marker. In B the field was applied while the tip was centered over the center of the upper half of
433
the unit cell; three Si atoms were removed, leaving a fourth under the apex of the tip, This fourth Si atom was in a=. unstable configuration, and as the tip was moved towards it to remove it, it migrated to the left and occupied a normal adatorq site of the 7 × 7 lattice, panel C. This atom was removed by the tip in panel D. Then
Fig, 7. A serie~ of atomic scale manipulations: (A) The tip is placed at ~ I ,~ from electronic conlact over the silo indicated by the arrow. (B) A I V pulse removes three atoms le~ving Ihc fourth under the tip, (C) Tbo first allempt to remove this alom leads to ils migrating to the left (see arrov,,) alld bonding as a center adatom. ( D ) A second pulse removes this fourth atom. (E) A new
corner-adalam is removed and in (F) it is placed back Io its originnl position.
Pit ,4t tniri~, L.H [ 1..l
another corner adatom was removed in panel E. One question that arises is: can the S T M undo a modification, e.g., can it put back a removed atom or cluster of atoms and repair the original structure? We find that. in general and especially for the clusters, the answer is no. Although we can place the a t o m / c l u s t e r ~wer the original site, it does not incorporate itself This is the resnlt of the rebondin$ at the substratc which takes place after the removal of a number of atoms, and of the optimization of the bonding bmwecn atoms in the removed cluster. The redeposited atoms must overcome a sizeable activation barrier to occupy their original sites. In the case of single atom removal, however, incorporation can be achieved as shown in panel F. [n the examples of manipulations given above, we were interested in modifying the Si ~ubstrate, For Ibis reason we chose to use a tungsten tip since W has a 2 V / , ~ higher field evaporation threshold than Si in the FIM geometry. There are
Fig. g.
cases, however, where one is interested not in modifying the substratc but in depositing foreign material on the substrate to generate nanostructures. If the material has a lower field-evaporation threshold than the substrate, 0ossibly one could accomplish this by using the material as an S T M til This has already been demonstrated for gold tips [9]. Recently, we have experimented with AI and found that AI tips are relatively stable and that atomic resolution is readily achieved. This latter observation in~acates that d-functions on the tip are not a requirement for achieving atomic resolution, In fig, 8 we show a topograph of the occupied states of Si( I l 1)-7 X 7 with an AI cluster produced by applying a - 3 V voltage pulse to the sample. Clusters currently are attracting significant attention because they represent an essentially new state of matter with prcpert,~es different from those of the individual atoms or bulk solid. The S T M can be a tool for making dusters while STS allows the in situ study
Topograph of the occupied states of Si(I I I)-7 X 7 oblained using an aluminum tip. The AI cluster in the center tff Ihc picture wa.~produc,:d by applying a - 3 V pulse 1o the sample.
Pit. At'tnlrt~', L -tg~ Lyo / Suffice probing a! area,dr scale with STM
of their valence electronic structure, hl agreement with recent calculations [33], we find that the small AI cluster of fig. 8 is not metallic but shows an ~ 2 e V gap.
4, UonelusJnns As an example o f the power of S T M and STS to probe Ideal chemistry and relate it to local electronic structure, we have used the study of the initial stages of oxidafiol, of Sill 11 )-7 × 7 and Si(100)-2 × I. W e find that these reactions are very site.selective. On the 7 × 7 surface, the top layer Si a d a t o m sites are the most reactive. A m o n g a d a t o m sites, corner sites are more reactive than c e n t e r sites and sites in the f a u h c d half o f the 7 X 7 unit cell are more reactive than those in the unfaulted half. Defects are found not to play a si~niflcant role. T h e common characteristic of the reactive sites is a high density of occupied states near E F. T h e initial stage oxidation of Si(lll0)2 × I appears to show the opposite behavior, in that the majority d i m e r sites a r e rather unreactive while defect sites, in particular C-defects. dominate the early stages of oxidation. STS, however, shows that the same requirement responsible for the high reactivity of a d a t o m sites on the 7 x 7 surface, i.e., high L D O S n e a r Et-, d e t e r m i n e s the reactivity o f the S i ( 1 0 f 0 - 2 × l surface. W h i l e dimer sites show a gap in L D O S at E~., C-defects show a metallic behavior with a high L O b S n e a r /"v. W e have also discussed how the S T M can be used as a tool for the atomic scale modification and manipulation o f covalent solids such as silicon. W c have shown that by a combination of chemical t i p - s a m p l e interactions and electrostatic forces g e n e r a t e d by a voltage pulse, individual Si a t o m s or clusters of atoms can be removed from the surface, placed on the tip and then redeposited at p r e d e t e r m i n e d locations on t h e surface. D u e to the presence of t i p - s a m p l e chemical interactions, t h e field necessary to break the S i - S i bonds is much reduced from that n e e d e d in ordinary field evaporation. W e term this process chemically-assisted field eeaporation. Finally, we note that chemically-assisted field-evaporation
~5
may take place during constant current S T M imaging of surlaces on which regions of low L D O S have been generated as a result of chemical reactions [341.
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