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2016 Semicond. Sci. Technol. 31 025005

(http://iopscience.iop.org/0268-1242/31/2/025005)

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The mechanisms of platinum-catalyzedsilicon nanowire growth

N Hibst1, P Knittel2, J Biskupek3, C Kranz2, B Mizaikoff2 and S Strehle1

1 Institute of Electron Devices and Circuits, Ulm University, Albert-Einstein-Allee 45, D-89081 Ulm,Germany2 Institute of Analytical and Bioanalytical Chemistry, Ulm University, Albert-Einstein-Allee 11, D-89081Ulm, Germany3Central Facility of Electron Microscopy, Ulm University, Albert-Einstein-Allee 11, D-89081 Ulm,Germany

E-mail: steffen.strehle@uni-ulm.de

Received 3 November 2015Accepted for publication 13 November 2015Published 14 December 2015

AbstractPlatinum (Pt) has beenknown as a catalyst material for vapor-liquid-solid (VLS) synthesis sincethe mid 1960s with the potential to grow electronic grade silicon nanowires (SiNWs). In contrastto gold-based growth, Pt-catalyzed SiNW synthesis has rarely been studied, most likely due tohigher synthesis temperatures and the formation of multiple Pt silicide phases. Here we presentthe growth of SiNWs from a Pt catalyst deposited by a focused ion or electron beam, whichopens new strategies for the assembly of Pt-catalyzed SiNW-based devices, as well as SiNWgrowth from Pt nanoparticles and thinfilms. We show that single-crystalline SiNWs exhibiteither the well-known catalyst tip or a polycrystalline silicon tip so far not reported. The local Ptconcentration was found to be one key parameter triggering the growth mode. The proposedgrowth model for both types of SiNWs is based on a solid-state silicide-mediated crystallizationrather than VLS. The discussion of the growth modes is supported by a variation of severalgrowth parameters and SiNW synthesis using the substrate materials silicon nitride, single-crystalline silicon, fused silica, and sapphire.

S Online supplementary data available from stacks.iop.org/SST/31/025005/mmedia

Keywords: silicon nanowires, beam-deposited platinum, silicide-mediated crystallization

(Some figures may appear in colour only in the online journal)

1. Introduction

Silicon nanowires (SiNWs) are one-dimensional semi-conductors, which show great potential as building-blocksfor the design of advanced nanoscale devices [1–5]. Thebottom-up growth of SiNWs commonly uses metal catalystsin vapor-liquid-solid (VLS) [6] or vapor-solid-solid (VSS)[7] synthesis yielding SiNWs with a catalyst tip. From thevariety of possible catalysts, [7, 8] platinum (Pt) representsa promising alternative to the typically used gold (Au).Platinum appears superior for electronic grade SiNWsynthesis as it is expected to form fewernear-midgap trapsin Si compared to Au [9–13]. Besides these considerations,it was recently demonstrated that SiNWs are readily grown

from beam-deposited Pt [14]. Localized catalyst depositiononto complex 3D substrates by established focused ionbeam-induced deposition (IBID) or electron beam-induceddeposition (EBID) allows a precise control of the SiNWgrowth site enabling new and versatile concepts foradvanced SiNW-based device assembly.

The catalytic function of Pt for SiNW synthesis hasbeenknown since the discovery of the VLS mechanism [6]but the number of studies, which have been published sincethen, is still rather limited. This may be related to the Pt-Siphase diagram comprising several Pt silicide phases and to thesignificantly higher eutectic temperatures compared to theAu-Si system [15, 16]. So far, solely VLS and VSS-SiNWgrowth using Pt as a catalyst were described despite differing

Semiconductor Science and Technology

Semicond. Sci. Technol. 31 (2016) 025005 (7pp) doi:10.1088/0268-1242/31/2/025005

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growth conditions. Besides Pt deposited by IBID or EBID, Ptthin films [12, 17, 18], Pt colloids [13] or Pt particles [6, 19],and galvanically deposited Pt [20] were utilized for SiNWsynthesis at temperatures between 500 and 1200 °C usingeither monosilane (SiH4) or silicon tetrachloride (SiCl4) asprecursor. Several procedures stress a separate annealing stepprior to growth aspiring catalyst dewetting or silicide for-mation. The growth of SiNWs without the typical catalyst tipwas just recently reported with Pt deposits as thin as 2 nm[14]. However, neither the origin of this silicide growth modenor the underlying mechanisms have beendisclosed so far.Here we present a detailed discussion on growth character-istics of Pt-catalyzed silane-based SiNW synthesis proposinga consistent growth model for the Pt-Si system. Under-standing the fundamental growth mechanisms should facil-itate further research studies as well as the rational control ofPt-catalyzed SiNW synthesis.

2. Methods

As substrate material, (100)-Si with native oxide layer wasused. The Pt catalyst was prepared in three different ways tostudy the effects emerging from the catalyst form: thin Ptfilms (thickness 5 nm) deposited by electron beam evapora-tion, ligand-free Pt nanoparticles (diameter 10 nm) suspendedin water, as well as IBID and EBID Pt using gaseous(methylcyclopentadienyl)trimethyl platinum as precursor.Due to the organometallic nature of the precursor, a compo-site material of Pt and amorphous carbon rather than pure Pt isdeposited. Furthermore, Ga ions are implanted in case ofIBID. Nevertheless, SiNW synthesis appears unaffected aspreviously reported [14].

Cross-sectional lamellae for transmission electronmicroscopy (TEM) were prepared by microtome cutting usinga sacrificial 500 nm thick amorphous Si3N4 interlayerbetween the (100)-Si substrate and the Pt catalyst. AfterSiNW growth, the whole sample was embedded in resinfollowed by the substrate removal in liquid nitrogen prior tolamellae cutting. The cut lamellae were transferred to TEMcopper grids for imaging and energy-dispersive x-ray

spectroscopy (EDX). Additional samples for single SiNWTEM imaging were prepared from suspensions obtained byultrasonication of the growth substrate in ethanol and directtransfer to TEM copper grids.

The SiNW growth was carried out in a quartz tube fur-nace at temperatures between 700 and 800 °C and totalpressures between 38 and 225 mbar, respectively. The SiH4

(2% in helium) flux was either 4 or 30 sccm, respectively. Theflow of hydrogen was kept constant at 270 sccm. To exploreSiNW p-type doping, 3 to 20 sccm of B2H6 (100 ppm inhelium) were additionally admixed in some experiments. Thegrowth process was initiated by ramping the system fromroom temperature to 800 °C under vacuum (pressure around0.1 mbar) within approximately 80 min, followed by a cool-ing down to the respective growth temperature. After thegrowth temperature wasset, all gases were opened at onceand kept constant at their particular flow rates throughout thegrowth process. Finally, the gases were turned off and thechamber was purged with argon but kept evacuated whilecooling down to room temperature.

3. Experimental results and discussion

3.1. Catalyst formation at the initial growth stages

During initial heating, here to 800 °C in vacuum, Pt diffuseson the substrate surface and forms silicide layers or Pt dro-plets as shown in figures 1(a) and (b), respectively. Silicidelayers are formed only if oxide-free silicon substrates are usedallowing a silicide phase formation with the substrate[21, 22]. However, SiNWs could not be grown from thesesilicides embedded in the substrate (figure 1(a)). If substratessuch as silicon oxide (figure 1(b)), silicon-nitride, or silicon-free sapphire are used, silicide formation is inhibited and purePt droplets were observed for all used Pt catalyst forms. Theresulting droplet size and spacing as well as the diameterdistribution (see supplementary information figure S1) isgoverned by the initial local Pt concentration, Pt surfacediffusion, the thermo-migration of Pt nanoparticles, andOstwald ripening [21, 23–25]. The onset of the SiNW growth

Figure 1. SEM images depicting the behavior of a dewetted 5 nm Pt thin film at different stages: (a) silicide formation (bright areas) on a(111)-Si substrate prior to SiH4 inlet (60° tilt angle), (b) thermal Pt droplet formation with an average density of 66 μm−2 on a (100)-Sisubstrate covered by a native oxide prior to growth, and (c) silicide network formation on the surface from (b) after 30 s of SiH4 exposure.The inset of (c) represents a surface image under 60° tilt angle providing an improved view of the Pt silicide network topography. Theworking pressure was 100 mbar using a 2% SiH4 in He flux of 30 and 270 sccm of H2 at 700 °C, respectively.

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Semicond. Sci. Technol. 31 (2016) 025005 N Hibst et al

process, initiated by the inlet of gaseous SiH4, induces the Ptdroplets toform a Pt silicide phase at temperatures between250 and 400 °C [26]. It is expectedthat first the Pt2Si phase isformed, followed by a PtSi phase as soon as all available Pt isconsumed [15, 27–29]. As some oxygen is present, which canbe commonly assumed for systems operating above ultra-highvacuum, several Pt silicide phases may coexist [27]. Never-theless, the catalyst phase is denominated as PtSi in the fol-lowing according to [12] and [15]. The Pt droplets,originating from the dewetted 5 nm Pt thin film, are sig-nificantly changed due to incipient Pt silicide formationwithin the first 30 s of the growth process (figure 1(c)).Although first nuclei of SiNWs may be assumed, the initiallyseparated Pt droplets coalesce forming a rough PtSi networkat the surface (figure 1(c) inset).

3.2. Pt-catalyzed SiNW growth modes

Distinct SiNW growth was observed after 60 s of growthyielding SiNWs with already up to 4 μm in length(figure 2(a)). However, only a few SiNWs grow in a straightmanner away from the surface and not all of them seem tohave a catalyst tip. A significant numberof SiNWs grow-laterally across the surface in an undulated manner, suggest-ing unstable growth conditions or the growth ofpolycrystalline SiNWs. Also, the shape of the Pt silicidecatalyst seems to vary (figure 2(a) inset) indicating differentcrystallographic orientations [30] between Pt silicide phaseand SiNW as observed before for Pt [12], Pd [31], and Ni [32]silicides. The predominant silicide morphology and orienta-tion is most likely affected by the initial Pt droplet formationconditions [30] such as annealing in hydrogen [12] or undervacuum. The substrate as an effective silicon source was ruledout by growth experiments without SiH4 maintaining all otherparameters. Regardless of the growth time, no SiNW growthwas observed. Moreover, in the presence of SiH4, usingsilicon-free sapphire as a substrate, SiNWs growth was

observed as equivalentto silicon-based substrates. Theimportance of Pt for the SiNW growth becomes evident iflocalized Pt spots, here deposited by EBID, are used. Here,SiNW growth occurs solely confined to formerly Pt coveredlocations as shown in figure 2(b).

After extending the growth time to 1 h, two distinct typesof single-crystalline SiNWs were observed as indicatedexemplary in figure 2(b). The first type of single-crystallineSiNWs has top diameters of 80 to 100 nm and exhibits a Ptsilicide catalyst at the tip (figure 3(a)). Due to post-growthanalysis, it still remains unclearwhether the catalyst droplet isliquid or solid during growth. According to the phase dia-gram, the lowest melting point derived from the bulk phasediagram exists for a Pt-rich eutectic phase with 23 at.% Si inPt melting at 830 °C [15]. However, this phase is not expectedto precipitate Si. To precipitate a pure Si phase, a PtSi phaseappears to be mandatory. The melting points of PtSi and itseutectic phase with Si are 1229and 979 °C, respectively [15]and will not differ significantly taking the nanoscale dimen-sion of the droplets into account [33]. Therefore, a VSS-modeappears reasonable for this type of SiNW growth at 700 °Cdenoted as VSS-SiNW in the following.

The VSS-SiNWs are tapered with a diameter decreaseover length of approximately 15 nm μm. The tapering can beprimarily linked to Si deposition by thermal SiH4 decom-position at the SiNW sidewalls. Dissolution of the catalyst inthe SiNW should be ruled out as Pt could not be detectedwithin the SiNW using TEM/EDX. However, some SiNWsexhibit an additional polycrystalline segment at their top withcrooked and tapered morphology as exemplary shown infigure 3(b). The tapering of these polycrystalline regions ismore distinct compared to the single-crystalline part due tothe incorporation of Pt into the Si. Therefore, the final tipdiameter is significantly smaller than the top diameter of thesingle-crystalline segment. The growth of these polycrystal-line regions might be triggered by the drop of both

Figure 2. (a) SEM images of SiNWs after 60 s of the growth. The inset of (a) shows the surface under 60° tilt angle. The arrows indicate theexistence of different silicide catalyst morphologies. (b) SEM image of SiNWs grown for 1 h from a single Pt dot deposited by EBID. Twosingle-crystalline types of SiNWs were observed as indicated by the arrows. The result is representative for other Pt deposition forms anddemonstrates furthermore that SiNW growth is confined to areas exhibiting Pt deposits or Pt silicides.

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Semicond. Sci. Technol. 31 (2016) 025005 N Hibst et al

temperature and SiH4 partial pressure inside the chamber atthe end of the growth sequence. Consistent with [29] and[34], an approximately 3.5 nm thick amorphous SiO2 layer atthe catalyst surface indicates the silicide nature of the catalyst(figure 3(a), marked by an arrow).

Besides PtSi catalyzed VSS-SiNWs, a second and so farundescribed type of single-crystalline SiNWs exists. This typeexhibits larger diameters between 100 nm and 1 μm, is sig-nificantly shorter, and lacks a silicide catalyst tip. The SiNWtip is instead decorated with a blunt polycrystalline segmentof variable length from about 100 nm up to 1 μm (figure 3(c)).TEM/EDX could not detect any Pt in this kind of SiNW,neither at the tip of the nanowire nor at the interface betweenthe poly- and single-crystalline part (EDX data available insupplementary information figure S2). Similar SiNW growthcharacteristics were reported for TiSi2 islands but lackedagrowth model [35]. Although no silicide or Pt seems to bepresent in the SiNW, significant amounts of Pt silicidesremain beneath the SiNWs at the substrate surface buried by apolycrystalline Si layer or embedded in it as shown infigure 3(d). The growth of single-crystalline SiNWs from asilicon deposit in the presence of a Pt silicide phase showssimilarities with Pd [36] and Ni [32] silicide-mediated crys-tallization (SMC) utilized to transform amorphous siliconfilms into polycrystalline films. In the following, this secondtype of SiNW is therefore denoted as SMC-SiNWs to dis-tinguish them from the VSS-SiNWs, although the VSS-SiNWmode can also be explained by SMC [31].

The proposed growth model for VSS and SMC-SiNWs isbased on the assumption that the initial PtSi crystals have aspecific orientation. The growth process comprises (i) gaseousSiH4 transport and adsorption onto the catalyst or in itsvicinity, (ii) decomposition of SiH4 releasing free Si, (iii)diffusion of Si towards or across the PtSi catalyst and finally,(iv) precipitation of a SiNW. The differences in surface freeenergy between the PtSi/substrate, PtSi/amorphous-Si, andPtSi/crystalline-Si interfaces can be considered as the overalldriving forces for directed and opposing solid-state diffusionof Si and Pt [32].

In analogy to NiSi2-SMC, the lowest surface free energyis assumed to exist for the PtSi/crystalline-Si interface.Therefore, free or amorphous Si will be consumed by the PtSicatalyst, an orthorhombic crystal [37], while simultaneouslycrystalline diamond cubic Si is precipitated. Based on theavailable lattice sites, different growth velocities and direc-tions, as well as Si crystal orientations have to be assumed asobserved for the crystallization of amorphous Si films assistedby NiSi2 [32]. To nucleate a single-crystalline SiNW exhi-biting the PtSi catalyst at the tip, stable PtSi droplets mustexist on the surface, which precipitate crystalline Si towardsthe substrate while the catalyst moves continuously awayfrom the surface fed by the SiH4. Other Pt silicide crystalorientations would result in lateral silicon crystallization orgrowth of heavily kinked SiNWs (cf figure 2(a)). TerminatedVSS-SiNW growth due to unstable growth conditions ismostlikely the reason for the observed PtSi inclusions in thepolycrystalline Si overgrowth (cf figure 3(d)). The growth ofVSS-SiNW using an SMC model is supported by the needle-shaped crystal formation exhibiting a NiSi2 tip, which wasobserved for SMC of amorphous silicon films [32]. However,the growth of SMC-SiNWs having polycrystalline tips is notdescribed yet in connection with SMC to the best of ourknowledge. The formation of silicide networks during theinitial growth stages requires that Pt diffuses from the dropletsinto thermally deposited silicon on the substrate surface. Sicrystals are precipitated but the catalyst stays at the substratesurface, which could be simply due to its crystallographicorientation or inhomogeneities of the silicide network. Therole of oxygen must be further investigated in this context.Repeated precipitation of Si crystals at the silicide top surfacecan trigger needle-shaped crystals [32], forming the poly-crystalline tip of SMC-SiNWs. As described for Pd silicidecatalyzed SiNWs [31] and PtSi droplets on a silicon surface[37, 38], the silicide/silicon interface is highly dynamic andcan readily reshape to lower the surface free energy or toreach a favored silicide/silicon interface state. Once a pre-ferred orientation is precipitated, the SiNW will elongate assingle-crystal. Notably, in addition, some SMC-SiNWs

Figure 3. TEM images of the two types of single-crystalline SiNWs: (a) VSS-SiNW with a silicide catalyst at the tip, (b) VSS-SiNWexhibiting a polycrystalline, highly tapered tip extension with catalyst tip, and (c) SMC-SiNW with a blunt, polycrystalline tip without a PtSicatalyst at the tip. (d) TEM cross-section showing a PtSi layer formed between a silicon nitride substrate surface and the polycrystalline Siovergrowth (1 h growth time). The amorphous silicon nitride substrate layer was required for the TEM sample preparation. Also, a PtSiinclusion is clearly visible. (e) TEM image of an SMC-SiNW with a polycrystalline intermediate section.

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exhibit a polycrystalline segment within the nanowire(figure 3(e)), which can be attributed to growth instabilitiescausing a temporal loss of the preferred crystal orientation. Itstill remains unclear if SMC-SiNWs are nucleated from asingle silicide crystal or if they are fed collectively from thesurrounding matrix. In order to illustrate the growth of VSS-and SMC-SiNWs as well as terminated SiNW growth, asimplified schematic is given in figure 4.

3.3. Influence of local Pt concentration, temperature, SiH4

partial pressure, and doping on the growth modes

The local Pt concentration prior to silicide formation or sili-con crystallization appears fundamental for both SMC-SiNWand VSS-SiNW growth. At higher Pt concentrations, faster

nucleation of SCM-SiNWs can be assumed and stable PtSidroplet formation should become favorable, as required forVSS-SiNW synthesis. Using IBID PtC, a distinct thicknessgradient was generated in the form of a chessboard-like pat-tern as previously reported [14]. The deposited patterns havea Pt content of about 35–45 at.% [39]. If the height increa-sedfrom about 2 to 3 nm, exclusively SMC-SiNWs withhomogeneous but short length was observed (figure 5(a)). Atincreased heights of about 4to 6 nm, a co-appearance of bothSMC- and VSS-SiNWs was observed as expected(figure 5(b)). Notably, the thickness difference of only about2 nm is affecting the length of the SiNW drastically. A furtherincreased height (gradient from 6to 13 nm) reflecting anoverall higher amount of Pt further promotes the VSS-SiNWgrowth but is still accompanied by SMC-SiNW growth asshown in figure 5(c).

Besides the Pt content, the temperature and the SiH4

partial pressure could potentially influence the effectivenessof the growth mechanisms and alter the SiH4 decomposition,the silicide phase formation, and the SiNW precipitation.Changing the growth temperature from 700to 800 °C in stepsof 20 K revealed that the overall density of SiNWs is highestaround 720 °C resulting in the longest VSS-SiNWs with anaverage length of 15 μm after 1 h (figure 6(a)). Withincreasing temperature, VSS-SiNW growth appears to becontinuously hampered so that at 780 °C almost exclusivelySMC-SiNWs are present (figure 6(b)). This indicates thatthermal SiH4 decomposition rate is another important para-meter. Therefore, at 800 °C no SiNW nucleation is observed(figure 6(c)).

Reducing the SiH4 partial pressure at 720 °C from0.14 mbar down to about 0.01 mbar by about one order ofmagnitude reduces the absolute number of SiNWs as well astheir length by about one order of magnitude (figure 6(d)). Anincrease of the SiH4 partial pressure from 1.1·10−2 mbar bya factor of two (figure 6(e)) or six (figure 6(f)) leads toincreased SiNW growth rates but hardly affectsthe ratio ofVSS- and SMC-SiNWs.

The control over both the SiNW morphology and theirelectronic properties is a prerequisite for the application ofSiNWs in electronic devices. The admixture of phosphine for

Figure 4. Schematic illustration depicting the proposed SMC-growthmechanism for VSS- and SMC-SiNWs: (a) Pt droplet formation onthe surface, (b) Pt silicide formation and thermally induced Sideposition on the substrate due to SiH4 inlet, (c) silicide dependentSi crystal precipitation leading to VSS-SiNW (left), SMC-SiNWswith a polycrystalline tip (center) and unstable SiNW growth (right),and (d) single-crystalline elongation of VSS- as well as SMC-SiNWsand termination of unstable SiNW growth in the Si overgrowth.

Figure 5. (a) to (c): SiNWs grown from a chessboard-like PtC pattern exhibiting a thickness gradient. The PtC square with the lowestthickness is at the top right and the highest at the bottom left corner ranging (a) from 2 to 3 nm, for (b) from 4 to 6 nm, and for (c) from 6to13 nm. All images are recorded under 52° tilt angle and all scale bars represent 10 μm. The average height of the PtC deposits was measuredby atomic force microscopy.

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Semicond. Sci. Technol. 31 (2016) 025005 N Hibst et al

n-type doping of Si completely inhibited the SiNW growth.This may be related to an accumulation of phosphorus at thePtSi/Si interface [40] suppressing the nucleation of SiNWs. Asimilar effect is not reported for boron [41]. Boron-dopedSiNWsynthesis, using a silicon/boron gas phase ratio of400:3, was realized without any observable impact onto thegrowth mechanisms or the morphologies in comparison tointrinsic SiNWs. The detailed investigation of the electronictransport properties of boron-doped VSS- and SMC-SiNWswill be addressed in futurestudies.

4. Conclusion

The Pt-catalyzed growth of single-crystalline SiNWs wasinvestigated. Consistent withprevious studies, single-crys-talline SiNWs were grown exhibiting the well-known catalysttip. The growth mode for these SiNWs appears to be VSSrather than VLS. Besides the VSS-SiNWs, a so far unreportedtype of single-crystalline SiNWs was studied having a poly-crystalline tip instead of a Pt silicide. Inanalogy to thecrystallization of amorphous Si thin films by silicides such asNi2Si, a silicide-mediated crystallization growth mode, drivenby directed solid-state diffusion, was proposed also com-prising the VSS-SiNW growth. The influence of substratematerial as well as the type of the Pt catalyst depositionincluding thin films, nanoparticles, and beam-deposited Ptwas studied. Except for pure Si substrates, no significantinfluence on the overall SiNW growth characteristics wasobserved. It was demonstrated that higher local Pt con-centrations promote the nucleation of VSS-SiNWs, which

was attributed to the formation of stable PtSi droplets ratherthan silicide networks spreading across the substrate surface.The derived growth characteristics should inspire more fre-quent functional implementation of Pt-catalyzed SiNWs infuture.

Acknowledgments

We gratefullyacknowledge the Focused Ion Beam CenterUUlm, which is supported by the FEI Company (Eindhoven,Netherlands), the German Science Foundation (INST40/385-F1UG), and the Struktur- und Innovationsfonds Baden-Württemberg. In addition, we thank Eberhard Schmid fromthe Central Facility for Electron Microscopy at Ulm Uni-versity for the lamellae preparation, Dr Philipp Wagener fromthe University of Duisburg-Essen for providing the platinumnanoparticles, and Dr Sebastian Gutsch from the IMTEKFreiburg for fruitful discussion. The financial support by theGerman Federal Ministry of Education and Research (BMBF:NanoMatFutur FKZ 13N12545) is gratefully acknowledged.

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