Superlattices and Microstructures 44 (2008) 378–384www.elsevier.com/locate/superlattices

Tuning reactive epitaxy of silicides with surface steps:Silicide quantum dot arrays on Si(111)

Laura Fernandeza,∗, Mathias Lofflerb, Javier Cordona,Enrique Ortegaa,c,d

a Departamento de Fısica Aplicada I, Universidad del Paıs Vasco, Plaza de Onate 2, E-20018 San Sebastian, Spainb Lehrstuhl fur Festkorperphysik, Universitat Erlangen, Staudtstrasse 7, 91058 Erlangen, Germany

c DIPC, Manuel de Lardizabal 3, E-20018 San Sebastian, Spaind Unidad de Fısica de Materiales, Centro Mixto CSIC-UPV/EHU, Manuel de Lardizabal 3,

E-20018 San Sebastian, Spain

Available online 24 March 2008

Abstract

We have studied the nucleation and growth of cobalt silicides on vicinal silicon surfaces. The latterexhibit one-dimensional arrays of steps with narrow terrace widths ranging from 20 A to 40 A (miscut anglebetween 9◦ and 4.6◦, respectively). Submonolayer amounts of Co were deposited by thermal evaporationon such vicinal Si(111) surfaces by reactive thermal deposition, and the resulting silicide structures wereanalysed by scanning tunnelling microscopy. The presence of narrow terraces in the system notably altersthe usual nucleation process of cobalt silicide on flat Si(111). It is observed that by varying the Co coverageand vicinal angle, the size and amount of the hexagonal silicide clusters can be controlled producing adense array of quantum dots. This behaviour contrasts to that observed on flat substrates, where nanoclusterstructures are notably scarce.c© 2008 Elsevier Ltd. All rights reserved.

Keywords: Cobalt silicide; Reactive epitaxy; Stepped surfaces

1. Introduction

Self-organized growth is a straightforward alternative to expensive and complicatednanolithographic techniques in the preparation of ordered arrays of nanoscale units (sizes in

∗ Corresponding author. Tel.: +34 943018348.E-mail address: wabfegol@sc.ehu.es (L. Fernandez).

0749-6036/$ - see front matter c© 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.spmi.2008.02.006

L. Fernandez et al. / Superlattices and Microstructures 44 (2008) 378–384 379

the 1–100 nm range). Such fine objects have become fascinating for basic research as wellas for device applications. Vicinal surfaces with one-dimensional (1D) arrays of steps areideal to tailor such self-assembled epitaxial nanostructures. In fact, stepped surfaces have beenfrequently utilized as templates for the growth of atom chains and stripes, which may exhibit1D metallic or magnetic behaviour [1,2]. Additionally, other remarkable works based on thegrowth of semiconductor nanodots of Ge or InGaAs should be considered [3,4]. Furthermore,vicinal surfaces allow one to obtain single-domains and sharp nano-object size distributions, evenduring complex nanostructuration processes, such as the reactive epitaxy of rare earth silicidewires [5–7]. In this context, not so much experimental work has being carried out about theepitaxy of transition metal (TM) silicides, such as CoSi2 and NiSi, which exhibit resistivitiescomparable to metals. The good conductivity and thermal stability of TM silicides has motivatedtheir technological interest as low-resistivity interconnects and contacts in MOSFET devicefabrication [8].

In this work we revisit the reactive epitaxy of Co silicides using vicinal Si(111) surfaces assubstrates with the aim to prepare a dense array of CoSi2 islands with conducting properties.Several studies realized on flat Si(111) have shown the existence of a rich phase diagram witha variety of surface structures and stoichiometries, depending on the amount of evaporated Co.The ring cluster (RC) phase is the first surface structure that appears for very low Co coverage(∼0.1 ML). A RC consists of a single Co atom (although the same structure can be observedfor several TMs) surrounded by six Si adatoms, of which three are bridge bonded with the Coatom and fully coordinated. The other three Si adatoms are the “cap atoms”, each having asingle dangling bond. Increasing the Co coverage above 0.12/0.14 ML the RCs saturate thesurface forming the characteristic

√7 ×√

7-Co superstructure. Further Co deposition leads toother phases, namely the

√13 ×

√13-Co surface structure, with a higher Co content than the

RCs [9]. Both phases RCs and√

13×√

13-Co can be considered as precursors for the formationof hexagonal CoSi2 islands with flat tops, which is the most stable phase with the highest Cocontent. All these silicide phases coexist during the epitaxy on Si(111). Using scanning tunnellingmicroscopy (STM) we analyse the changes induced by a stepped Si substrate on the nucleationand growth of CoSi2 islands, grown by reactive thermal deposition (RTD). This procedure, whichenhances adatom diffusion, is known to be optimum to achieve homogeneous silicide films andenergetically stable morphologies, such as multilayered islands of CoSi2 [10,11]. The use ofvicinal silicon surfaces as templates to nucleate CoSi2 islands was already tested by the use ofsolid phase epitaxy [12]. It has been observed in this case that terraces in the 250–400 A rangehave an adverse effect on the atom diffusion, avoiding the formation of flat-top silicide islands.In contrast, here we demonstrate that the growth of flat top CoSi2 islands with controllable sizeand density can be carried out by RTD of Co and using vicinal Si(111) surfaces with narrowterraces in the 20–40 A range.

2. Experimental

Experiments have been carried out in situ in ultra-high vacuum (UHV) system with a basepressure of 7 × 10−11 mbar. The analysis chamber is equipped with an Omicron variabletemperature (VT)-STM and low-energy electron diffraction (LEED) optics. The silicon surfaceswere prepared by degassing the sample at 800 K for several hours in the vacuum chamberfollowed by heating a few times to around 1500 K up to approximately 15 s each, in order toprevent the pressure to reach values higher than 6 × 10−10 mbar during the flash. The sampleswere then rapidly cooled to 1220 K, and then slowly down to 1000 K. A final annealing was

380 L. Fernandez et al. / Superlattices and Microstructures 44 (2008) 378–384

Fig. 1. Topographic STM image of 0.3 ML of Co on flat Si(111) surface. (a) CoSi2 islands (white areas)inhomogeneously distributed along the whole sample, preferentially at the vicinity of steps (Empty-states, Vtip:+1.3 V),(b) single CoSi2 island exhibiting a hexagonal shape (Filled-states, Vtip: −1 V).

carried out at 1000 K for 10 min with the aim of improving the (7× 7)-Si surface reconstructionon the terraces. Before Co evaporation, LEED and STM measurements were routinely carriedout to check the surface cleanliness and reconstruction. Flat and vicinal Si(111) surfaces withtwo different miscut angles of 9◦ and 4.6◦ were used as substrates, revealing a terrace width of20 A and 40 A, respectively. Only slight variations of the terrace width were detected across thesurface. Co was evaporated in submonolayer ranges by RTD at substrate temperatures around800 K. Subsequently, the resulting cobalt silicide structures were analysed in detail by STM.

3. Results and discussion

The STM images of Co evaporated on flat Si(111) (0.3 ML) reveals the rich surface structureof the system, as shown in Figs. 1 and 2. The presence of CoSi2 islands, inhomogeneouslydistributed along the whole sample, is generally detected in the vicinity of steps. They exhibit flattops, heights between 1.7 and 3.8 nm, and diameters varying between 30 and 50 nm (see Fig. 1).√

7×√

7-Co and√

13×√

13-Co surface structures are found over extended areas of the sample,surrounding the large CoSi2 islands [Fig. 2(a)]. Away from CoSi2 islands, RCs are present asisolated units on the clean Si(111)-7× 7 as well as combined forming domains that tend to havea triangular shape [Fig. 2(b)]. In these domains the RCs are located on 1 × 1 surface sites, butonly few of them are occupied. RCs can be identified by STM due to their characteristic size andshape: rings with three lobes (corresponding to the three cap adatoms) in empty state images andwith a single lobe (corresponding to the Co atom) in the filled state image, as shown in Fig. 2.When the sample is thermally treated, both phases

√7 ×√

7-Co and√

13 ×√

13-Co dissolvepartially, increasing the CoSi2 islands in size. Therefore, the

√7 ×√

7-Co and√

13 ×√

13-Cophases can be considered as precursors of CoSi2 islands. In general, the lateral growth of islandsat the expense of precursor phases is energetically convenient for the system, because it resultsin the decrease of the surface energy. Thus, it is expected that after postannealing treatment theislands will undergo Ostwald-ripening in order to reduce the total energy, whereby the large

L. Fernandez et al. / Superlattices and Microstructures 44 (2008) 378–384 381

Fig. 2. Cobalt silicide surface structures found on flat Si(111) surface after 0.3 ML deposition of Co: (a) filled-stateSTM topography image (Vtip:−1 V) that discloses

√13×√

13-Co,√

7×√

7-Co and 7× 7-Si+RCs, (b) RCs arrangedin triangular domains on the 7 × 7 silicon reconstruction, and (c) Empty-states STM topography image (Vtip: +1.3 V)reveals the morphology of the RCs. The three lobes corresponding to the three cap atoms are observed. Moreover, thelobes of the other three silicon atoms located in lower positions are also detected. The zoomed area reveals more in detailthe RC architecture.

islands of CoSi2 grow at the expense of small islands or clusters (here, RCs and√

13×√

13-Cophases).

As shown in Fig. 3, the situation observed on the flat Si(111) surface changes notably bythe introduction of steps into the system. The presence of steps increases the amount of siliconadatoms that are able to get diffused and to react with Co [13,14]. Consequently, this leadsto a more efficient CoSi2 islands epitaxy. On both vicinal surfaces, the same amount of Cowas evaporated (0.4 ML). However, depending on the vicinal surface, the island growth and itsmorphology are different. For the surface with 4.6◦ miscut angle (Fig. 3), the RTD method allowsthe stabilization of coherently grown (dislocation-free) islands of CoSi2, forming a very dense

382 L. Fernandez et al. / Superlattices and Microstructures 44 (2008) 378–384

Fig. 3. Empty-state STM image (Vtip:+2.1 V) of CoSi2 islands after deposition of 0.4 ML of Co on v-Si(111) with 4.6◦

miscut. The CoSi2 islands form a dense array of nanodots over the whole sample. The inset discloses the CoSi2 islandsgrown on the steps with a higher magnification. Their hexagonal and triangular shape is revealed.

array equally distributed along the whole sample. The islands exhibit hexagonal or triangularshapes with atomically flat tops that may display both (2× 2) or (1× 1) surface reconstructions,as on flat Si(111) surfaces (see Fig. 4). Typically, the islands are 2 nm high, and have diameters ofapproximately 15 nm. The rest of the surface is completely covered by RCs with a

√7×√

7-Cosuperstructure. In addition to the

√7 ×√

7-Co phase, the surface structure√

13 ×√

13-Co isalso observed in some reduced areas located on the terraces.

The vicinal surface with 9◦ miscut angle exhibits a different morphology shown in Fig. 5.CoSi2 islands grow beyond several step edges, exhibiting an elongated form with an average sizeof 20 nm parallel to the steps, and 50 nm in the perpendicular direction. This can be explainedby the terrace width of the substrate, about two times smaller than in the former case, whichcould fall below the minimum size for a coherently strained CoSi2 island. In exchange, growingover the steps allows internal cluster dislocations, which largely reduce the elastic strain energy.Therefore, the lateral growth across many steps is preferred, since along the terraces CoSi2islands are subject to elastic energy constraints. Note, however, that despite this complex growthmechanism, islands still show the characteristic tops with a (1× 1)-superstructure.

4. Conclusions

In the present work we have shown that dense CoSi2 quantum dot arrays can be grown in acontrolled way. The use of vicinal Si(111) surfaces with a defined miscut angle (4.6◦) is criticalfor the stabilization of such CoSi2 islands. The dots are crystalline and grow homogeneouslystrained, being each single dot confined within the terrace between contiguous steps. Thisresult contrasts notably with the situation on flat silicon surfaces, where the morphology anddistribution of CoSi2 islands is notably scarce. On the other hand, it is demonstrated that for eachvicinal surface the same deposition conditions lead to different morphology and distribution of

L. Fernandez et al. / Superlattices and Microstructures 44 (2008) 378–384 383

Fig. 4. Empty-state STM image (Vtip: +2.1 V) representing the tunnelling current of CoSi2 islands on v-Si(111) with4.6◦ miscut angle. The zoomed area reveals the (1×1) and (2×2) surface reconstructions on the flat top of two differentislands. The CoSi2 islands are surrounded by RCs that saturate completely the surface with a

√7×√

7-Co superstructure.

Fig. 5. Empty-state STM image (Vtip: +2.1 V) representing the tunnelling current of CoSi2 islands after depositionof 0.4 ML of Co on v-Si(111) with 9◦ miscut: (a) the CoSi2 islands show a hexagonal shape with a preferred growthperpendicular to the steps; (b) CoSi2 islands exhibit a flat top with exclusive (1× 1) surface reconstruction.

the islands. Consequently, a fine tuning of the nanodot array by using the appropriate vicinalsurface and growth conditions could be expected.

Acknowledgments

This work was financed by Eusko Jaurlaritza (Nanotron Project), Euskal HerrikoUnibertsitatea and the Spanish Ministerio de Educacion y Ciencia (MAT-2002-11975-E,FIS2004-06490-C03-03).

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