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Ordered High-Density Si [100] Nanowire ArraysEpitaxially Grown by Bottom Imprint Method

By Zhang Zhang,* Tomohiro Shimizu, Stephan Senz,* and Ulrich Gosele

[*] Z. Zhang, Dr. T. Shimizu, Dr. S. Senz, Prof. U. GoseleMax Planck Institute of Microstructure PhysicsWeinberg 2, 06120 Halle (Germany)E-mail: zzhang@mpi-halle.de

DOI: 10.1002/adma.200802156

� 2009 WILEY-VCH Verlag Gmb

Among all 1D nanomaterials, silicon nanowires (SiNWs) inparticular attracted attention because of their potential usage innanoelectronic, optoelectronic, and thermolelectronic devices.[1–3]

Therefore, considerable effort has been devoted to developingthe controlled synthesis of SiNWs by various techniques.[4]

Vapor–liquid–solid (VLS) is one widely used method that cangrow epitaxial nanowires on single-crystal substrates with thehelp of metal catalysts.[5,6] However, using a VLS chemical vapordeposition (CVD) process, mainly three epitaxial growthdirections on bare silicon substrates were obtained, which donot include the h100i growth direction.[6] Considering conven-tional Si micro/nanoelectronics based on Si (100) wafers, it ismeaningful to realize vertically grown high-density epitaxial Si[100] nanowire arrays on Si (100) wafers. Especially for devicesusing vertical-nanowire arrays, ordering and size distribution arealso important parameters.

Anodic aluminum oxide (AAO) membranes are widely used astemplates to prepare various nanostructures.[7,8] Much effort hasbeen devoted to improving the ordering of thin AAO templateswith their adjustable pore size and thickness.[9] The methoddiscovered by Masuda et al. relies on self-ordering of pores at thebottom of porous alumina channels after a long first anodizationstep, which can give rise to a self-assembled honeycomb array ofuniformly sized parallel channels.[10] An ordered AAO templatecan be obtained in a second anodization after selective etching offthe first anodized one.[11] Lithographic methods allow thefabrication of nanowire arrays with controlled size and well-defined growth positions through patterned templates. Long-range ordered AAO templates have already been achieved byfocused ion beam (FIB) patterning of Al films deposited on siliconsubstrates,[12] although this process is limited by high costs andthe long time required for sequential writing.

In order to use the well-defined AAO templates after two-stepanodization to direct the epitaxial growth of Si nanowire arrays,the most important issue is a good interface between the grownnanowires and the substrate. Without an interface to thesubstrate, SiNWs have already been synthesized inside thechannels of AAO templates.[13,14] Au catalyst seeds were insertedinto the middle of pores of AAO before the CVD process, whichformed SiNWs inside the pores, with both ends capped by Au.[14]

This method allows poor control over the crystallographic growthdirection without epitaxial growth, and the SiNWs released fromthe template have been difficult to integrate into vertical-array

devices. Lombardi et al. transferred an AAO membrane onto a Si(111) substrate as mask of Au catalyst seeds. Subsequently, theysuccessfully prepared vertically grown epitaxial Si [111] nanowirearrays with high density and well-defined size distribution.[15]

However, this approach is not possible for Si [100] nanowires. Torealize the [100] growth direction, our previous work successfullyutilized directly anodized AAO templates to grow epitaxial Si [100]nanowires vertically on Si (100) substrates. We successfullyresolved the task of direct contacting between Au and silicon atthe bottom of the pores with the help of HF-assisted electrolessdeposition.[16] However, for the direct deposition of an Al layer ona silicon wafer, a high-enough thickness for the long-time firstanodization is not easy to achieve, and is important for theordering. The following anodization plus chemical etching resultin unwanted alteration of the Si-surface morphology.[17] There-fore, up to now, epitaxial growth of vertical high-density Si [100]nanowire arrays in AAO templates with well-ordered pores and anarrow size distribution has not been achieved.

In this paper, we report a novel approach, which we call bottomimprint (BI) method, for growing high-density epitaxial Si [100]nanowire arrays on Si (100) substrates with well-defined orderingas well as a narrow size-distribution. In comparison with directlyanodized AAO templates on silicon substrates, thin AAOmembranes using long-time first anodization are more con-venient for our purpose. Highly ordered AAOmembranes can beproduced by the well-developed two-step anodization of high-purity Al foils.[10,11] Furthermore, with the pore-opened thin filmstructure transferred onto the prepared silicon substrate, we canavoid any chemical contamination and surface-morphologymodification. The central idea of our approach is to bond thinpore-through AAO membranes directly onto Si (100) substratescovered by pre-deposited Au layers, forming conformal contact.Au is a soft metal compared with alumina, both Young’s modulusand hardness decrease as the temperature and indentation loadincrease.[18] Under proper temperature and load, it is possible toimprint the harder bottom structure of the AAO template into theAu layer to form homogeneously separated nanoparticle arrays.Finally, in a UHV-CVD process, SiNWs catalyzed by the VLSmechanism grow epitaxially inside the pores with the sameordering and size as the AAO template.

The thin (<500 nm) AAO membranes consist of a brittleceramic film, and are difficult to handle after removal of thealuminum beneath the barrier layer. Herein, we adopt amodification of a method that was developed for imprints.[19]

After the second anodization, a diluted polystyrene (PS) isspin-coated on the surface of AAO to fill the pores, which formspolymer pillars filling the channels. The PS layer plays animportant role in the handling of the fragile AAOmembrane, andit also prevents pore widening when selectively etching off thebarrier layer. For imprint purposes, the ultrathin AAOmembrane

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Figure 2. a) Cross-section SEM image of the bonded AAO template on thesubstrate before imprinting. PS was removed with CHCl3, the dashed-lineindicates the top side of the AAO template, and the perpendicular onesshow one channel. b) Top-view SEM image of bonded sample afterimprinting, after part of the AAO template was removed using an adhesivetape.

must be integrally elastic enough to enable conformal contactwith the substrate, which means that it must adapt elasticallywithout leaving voids created by the natural roughness of thesubstrate and itself. The polymer material used can play such arole to increase the conformally contacting area. After beingtransferred onto the substrate, the sealed pore structure becomesmore important for protecting the silicon surface againstoxidation in the imprint procedure with increased temperature,as we have already noticed the importance of a direct contactbetween Au and Si surface.[16,20] When the PS is removed, thesilicon surface beneath the patterned catalyst seeds will not beoxidized, a precondition to the high-quality epitaxial growth ofSiNWs.[16]

The BI method used to synthesize ordered vertical Si [100]nanowire arrays is schematically depicted in Figure 1. First, apore-through well-defined AAO template with a PS protectivelayer filled in is bonded onto the desired substrate (Fig. 1a), whichis a H-terminated Si (100) substrate covered by a Au layer throughUHV e-beam evaporation. After imprinting, the Au layer ispatterned by the hexagonally ordered porous structure of thebottom side of the AAO template. Because of the big hardnessdifference between Al2O3 and Au, the elastic metal is extrudedinto each pore bottom, forming Au nanoparticle arrays (Fig. 1b).The nanopores are filled by PS and thus air cannot enter,preventing the oxidization of the Si surface beneath the Au layerduring imprint with heating. Subsequently, the PS is removedand the sample is inserted into the UHV system. The Aunanoparticles at the bottom of the pores act as catalysts duringVLS growth and crack the silane gas, which finally forms theSi-nanowire arrays inside the pores with Au tips (Fig. 1c). Forbetter observation and application, the AAO can be removed withchemical selective etching, producing bare Si-nanowire arraysdirectly on the Si substrate with the same ordering and sizedistribution as the AAO template (Fig. 1d).

Figure 2a shows a cross-sectional scanning electron micro-scopy (SEM) image of the bonded AAO template on the substratebefore imprinting. The PS has been removed to enhance thecontrast of the pore structure of AAO tightly bonded on the Si.From the distinct contrast difference, a homogeneous Au layer

Figure 1. Schematics of the BI method for synthesis of Si nanowire arraysusing Au as catalyst: a) bonding the PS-filled AAO template onto the Si(100) substrate with Au layer; b) patterning of the Au layer into theseparated nanoparticle array by imprinting from the bottom structure ofthe AAO template; c) growth of SiNWs inside the template with Au ascatalyst; d) removal of the AAO template by chemical selective etching.

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40 nm thick is identified as sandwiched between the AAOtemplate and Si (100) substrate. No voids or remaining barrierlayers were observed in between of these two bonding surfaces,which would greatly affect the homogeneity of the Si nanowirearrays. In a previous experiment,[20] using a predeposited Au layerbeneath the directly anodized AAO template, it was shown that, ifthere was a connection or any channel between the neighboringAu seeds, Au preferred to move around forming bigger particleswhen the temperature was above the Au–Si eutectic point(363 8C). After imprinting and removing of PS inside the pores,part of the AAO was detached by an adhesive tape to reveal thebottommorphology (Fig. 2b). The remaining AAO on the top canbe seen on the down-left part of Figure 2b. The bright contrast inthe upper-right region corresponds to the Au particles formedinside the bottom of AAO channels, and the dark one is thesurface of the Si substrate. It is clearly observed that the Au layer isimprinted into the open pores of the bottom structure of the AAOtemplate, and Au particles are separated from each othercompletely. Caused by the force used during lift-off of theadhesive tape, milky defects can be seen on the edge area, andseveral of the Au particles are missing.

The Si (100) substrate with bottom-imprinted hexagonallyordered Au seeds inside the AAO template is transferred into thegrowth chamber of the UHV-CVD system. During the VLSgrowth of SiNWs, the Au particles sitting at the bottom of poresact as catalysts. When the silane gas enters into the pores andcracks on the Au surface, the Si atoms diffuse through the

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supersaturated Au–Si liquid eutectic, and freeze out at the otherinterface on the silicon substrate. For a steady growth process, theAu–Si liquid droplet moves upward from the Si surface along theperpendicular channel. The SEM images of the grown Sinanowire arrays within and after removal of the template arepresented in Figure 3. The top-view image (Fig. 3a) shows that theAu heads almost grow out from the pores of the AAO template,and have good ordering and 100% filling factor in one singledomain area of the AAO template. Thismeans that for the specificAAO membrane used, the density of SiNWs is above the level of1� 1010 wires per square centimeter. The black contrast in someof the pores is mainly caused by the shape of the Au tips on top ofnanowires, which deviate from the typical semispheres formed infree-space conditions.[21] After removing of the template, we findsome of the Au tips have cone shapes. The result is a noncircularimage of the top, observed by top-view SEM with bright contrast.The growth temperature should be as low as possible to avoid theparasitic deposition of silicon on the AAO surface, because thealumina can also dissociate the silane gas at high temperature inour experiments. There is no obvious diameter shrinkage of thepore opening, and only some small white grain-shaped particleson the top surface of AAO were visible (shown by arrows inFig. 3a) using 400 8C as growth temperature. Figure 3b shows theSi-nanowire arrays after selective etching of the AAO template.High-density arrays of SiNWswith Au tips on the top are vertically

Figure 3. a) Top-view SEM image of Si-nanowire arrays after CVD processwithin the AAO template. The black arrows point to the parasitically grownSi. b) Side-view SEM image with 508 tilt after selective etching of the AAOtemplate using phosphoric acid.

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grown on the Si substrate. We can distinguish one domain withgood ordering from the linear alignment of SiNWs on theupper-left part of this SEM image.

The vertically ordered alignment of the nanowires is achievedby effectively imprinting the Au layer into separateAu-nanoparticle arrays. The conformal anchoring of the bottomside of AAO on the Si substrate prevents the agglomeration ofbigger Au particles during the CVD process, which causes thehigh quality of the as-grown SiNWs inside the channels. Fromtop-view images of the as-grown Si-nanowire arrays after removalof the AAO template, we can directly observe the size distribution.Figure 4 shows the top-view SEM image of the Si nanowire arraysafter selective etching of the AAO. We calculated the diame-ter-size distribution of the SiNWs in one single ordered domain.The upper-right area of Figure 4a contains around 200 SiNWsarranged in hexagonal ordering, and was used to plot thediameter distribution of Figure 4b. The average diameter of thecounted 180 SiNWs is 39 nm, while keeping the 100 nm wirespacing distance unchanged, which is the exact copy of the 40 nm

Figure 4. a) Top-view SEM image of Si-nanowire arrays with Au tops on Si(100) substrate after selective etching of the AAO template. b) Sizedistribution of 180 SiNWs selected by the rectangular black dashed-linebox in a).

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average pore size AAO template. We approximated the distribu-tion by a Gauss fit and obtained a full-width at half maximum(FWHM) of 9%. The narrow size distribution and the unchangeddiameter from the AAO template is one of the advantages ofusing the BI method. The only remaining question is whetherthis method can produce epitaxial growth of Si [100] nanowirearrays on Si (100) substrates.

Figure 5a shows a cross-sectional transmission electronmicroscopy (TEM) image of the Si nanowire arrays after removalof the AAO template (see also Supporting Information). Thebottom part is the Si (100) substrate, and the horizontal dashedline indicates the surface of the silicon substrate. The Au particleswere originally connected with the silicon surface before the VLSgrowth. Then the Au eutectic supersaturates, and the Si movesupwards along the channel. As long as the Au tips are containedentirely within the pores, their shape is similar to the shape

Figure 5. a) Cross-sectional TEM image of the Si nanowire arrays afterremoval of the AAO template. The horizontal dashed line indicates thesurface position of the Si substrate, and the two parallel ones mark theshape of one single nanowire. b) High-resolution TEM image of the regioncircled in a white box shown in a). The insets from 1 to 3 show the FFTs ofthe three corresponding square regions.

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observed in prior work.[22] From the contrast difference betweenSi and the SiO2, which was deposited as a protection layer duringTEM sample preparation, the two parallel dashed lines indicateone single Si nanowire with the Au tip. The bottom whiterectangle includes the interface between this wire and thesubstrate, which is further examined by high-resolution TEM(Fig. 5b). We chose three areas from different parts to do the FastFourier Transformation (FFT). Area 1 is from the lower part ofthe Si nanowire grown. Area 2 refers to the interface between thewire and the substrate, including the substrate and the bottom ofthe wire grown on it. And Area 3 only includes the Si substrate. Ifthe FFTs of the three areas are exactly the same, the epitaxialgrowth can be convincingly confirmed. The images of three FFTsof the selected three square regions sequentially from 1 to 3 areshown in the lower part of Figure 5b. The spots in each insertshow not only the typical pattern for single-crystal Si viewed alongthe [011] zone axis, but also the same size and direction. Theseresults confirm that this nanowire is grown epitaxially on the Si(100) substrate, and the growth direction is parallel to the [100]direction of the Si substrate. The other nanowires observed inFigure 5a were all proven to be epitaxially grown with the [100]direction using the same method. Additionally, from Figure 5aand b, the diameter enlargement in the bottom part of thenanowires, as described in previous work, no longer exists.[16] Forthis AAO template, the barrier layer was etched away withprotective PS inside the pores, and the pore diameter did notchange in the chemical solution. During imprint, the Au isplastically deformed and extruded through the AAO pores, but atsome positions a small amount of Au was trapped beneath thebottom surface of AAO (Fig. 5a). In our TEM characterizations,no obvious Au particles were observed in other regions of theSiNWs; the Au was sitting on top of the wires. Additionally, thebottom part of the AAO contacting the substrate was slightlydeformed. As shown in Figure 5b, the left-side wall of thenanowire is straight above the interface, however, the right side isbent inwards, causing diameter shrinkage. Thus there is asmaller diameter in the bottom part of the nanowire, whereas20 nm above the interface it grows with constant diameter insidethe template. The epitaxial growth of the Si nanowire continuesduring the whole VLS process with the Au tip moving up in thetemplate.

In this work, we have demonstrated that a novel BI method canbe successfully used to grow ordered high-density epitaxial Si[100] nanowire arrays on Si substrates. Separated Au-nanoparticlearrays are imprinted directly on the Si (100) substrates, withtransferring of the ordering and dimensions of the bottomstructure of AAO templates. The protective PS layer in theimprinting process works effectively to ensure direct contactbetween the extruded Au particles and the Si substrate at thebottom of each pore. SiNWs can grow epitaxially along thevertically standing channels with the help of UHV-CVD. Goodordering, narrow size distribution, and high-density Si [100]nanowire arrays directly on Si (100) substrates can be achievedafter selective etching of the template. The newmethod describedhere can be easily extended using different catalyst element layersdeposited directly on single-crystal substrates with desiredorientation before imprint. Smaller diameters and higherdensities of nanowires may be achieved using properly preparedtemplates.

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Experimental

Free-standing thin AAO membranes were prepared by a two-stepanodization of surface polished aluminum [11]. In brief, Al foil (99.999%,Goodfellow) after electrochemical polishing was anodized under aconstant voltage of 40 V in 0.3 M oxalic acid at 5 8C. First, anodizationwas conducted for 24 h. After removal of the first anodized AAO film withan acid mixture (1.8wt% chromic acid and 6wt% phosphoric acid) at 60 8Cfor 12 h, a 300 s second anodization at the same conditions was performed,which corresponded to a thickness around 250 nm and pore diameter of40 nm. Later, 1 wt% PS/CHCl3 solution was spin-coated on the top of theAAO membrane at 2500 rpm for 90 s, and heated to 100 8C for 1 h toevaporate the CHCl3. The aluminum substrate was removed with an acidmixture of CuCl2 andHCl prior to selective etching off the remaining barrierlayer in 5wt% H3PO4 at 30 8C for 30min. The surface of the bottom sidewas rinsed with deionized water for several times, then the thin AAOmembrane was carefully transferred onto the desired substrate, a highpressure Ar gas stream was applied perpendicular to the top side of AAO.This process enables not only the removal of any voids but also theconformal contact between the two surfaces.

The imprint was performed using a hot-plate manual lab press/PW 40TEMPRESS (P/O/WEBER) under a pressure of 1 kN cm�2 at 250 8C for30min. A single-crystalline sapphire wafer was used as counterpart todistribute the load homogenously onto the top side of the AAOmembrane.PS inside the AAO channels could be completely removed later byimmersing the sample into CHCl3 for a whole day, the sample was thentransferred into the UHV-CVD system. Growth of SiNWs wascarried out at a temperature of 400 8C and a total pressure of 1.0 Torr(1 Torr¼ 133.32 Pa) that contained a gas mixture of 5% silane and 95% Ar.The CVD process continued for 20min. The was then sample cooled downand taken out for further characterization. Selective etching of the AAOtemplate was performed using 10% phosphoric acid at 45 8C for 2 h. Afield-emission scanning electronmicroscope (JEOL-6701F) was used for allsamples. TEM cross-section specimens were prepared in a focused ionbeam system (Nova Nanolab 600, FEI), where SiO2 and Pt were usedsequentially as protection layers, being deposited on the top of the Sinanowire arrays. The transmission electron microscope used was a JEOL4010.

Acknowledgements

We acknowledge partial support by the European NODE project 015783and by the BMBF project DIP-K 6.1. We thank Dr. W. Erfurth and Dr. H.Blumtritt for their contribution to the FIB manipulation, and S. Swatek for

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TEM sample preparation. Supporting Information is available online fromWiley InterScience or from the author.

Received: July 25, 2008

Revised: November 7, 2008

Published online: May 4, 2009

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