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REVIEWOpenAccess Areviewonelectronicandopticalpropertiesof siliconnanowireanditsdifferentgrowth techniques MehedhiHasan 1 ,MdFazlulHuq 2* andZahidHasanMahmood 3 Abstract ElectronicandopticalpropertiesofSiliconNanowire(SiNW)obtainedfromtheoreticalstudiesandexperimental approacheshavebeenreviewed.ThediameterdependencyofbandgapandeffectivemassofSiNWforvarious terminationshavebeenpresented.OpticalabsorptionofSiNWandnanoconehasbeencomparedfordifferent angleofincidences.SiNWshowsgreaterabsorptionwithlargerangeofwavelengthandhigherrangeofangle ofincidence.ReflectanceofSiNWislessthan5%overmajorityofthespectrumfromtheUVtonearIRregion. Thereafter,abriefdescriptionofthedifferentgrowthtechniquesofSiNWisgiven.Theadvantagesand disadvantagesofthedifferentcatalystmaterialsforSiNWgrowtharediscussedatlength.Furthermore,three thermodynamicaspectsofSiNWgrowthviathevapor – liquid – solidmechanismarepresentedanddiscussed. Keywords: SiliconNanowires(SiNWs),Bandgap,Opticalabsorption,Reflectance,ChemicalVapourDeposition(CVD), Introduction GrowthofSiwhiskerswasfirstreportedbyWagnerand Ellisasearlyas1964(Wagner&Ellis1964).Later GivargizovelucidatedthegrowthmechanismofSiwhiskers in1975(Givargizov1975).Thefirstreportoncarbon nanotubesbyIijimain1991focusedaworldwideexponen- tialincreaseofresearchintothecarbonbasedandsilicon- basednanomaterialsespeciallycarbonnanotubesand SiNWs(Iijima1991).Subsequently,therewereextensive investigationscarriedoutonthe synthesis,physicalproper- ties,devicefabricationandapplicationsofSiNWs.Figure1 showsahistogramofthenumberofsilicon “ whisker ” and “ nanowire ” publications(Schmidt&Wittemann2009). DuetotheirnovelpropertiesSiliconnanowires(SiNWs) havegrabbedsuchgreatattentions.Itistheiruniqueelec- trical,opticalandmechanicalpropertiesthatmakethem basednanodevicesarecompatiblewiththecurrent Si-basedmicroelectronicsin dustry,andalreadyanumber ofnanodevicesbasedonSiNWsasbuildingblockshave beendemonstrated(Cui&Lieber2001). Thenanoscalediameterputstheradialdimensionof nanowiresatorbelowthecharacteristiclengthscaleof variousinterestingandfundamentalsolidstatephenom- ena:theexcitonBohrradius,wavelengthoflight,pho- nonmeanfreepath,criticalsizeofmagneticdomains, excitondiffusionlength,etc.(Alivisatos1996;Lawetal. 2004).Asaresult,manyphysicalpropertiesofsemicon- ductorsaresignificantlyalteredwithintheconfinement ofthenanowiresurfaces.Inaddition,theirlargesurface- to-volumeratioallowsfordistinctstructuralandchem- icalbehavioraswellasgreaterchemicalreactivity.This two-dimensionalconfinementendowsnanowireswith uniquepropertieswhichstrayfromthoseoftheircorre- spondingbulkmaterial.Second,thelargeaspectratioof nanowiresintimatestheirtechnologicalapplication.The oneunconstraineddimensioncandirecttheconduction ofquantumparticlessuchaselectrons,phonons,and photons.Thiscontrolovervariousformsofenergy transportrecommendsnanowiresasidealmaterialsfrom Moreover,thelengthofnanowiresisnormallysufficient tointerfacewithtop-downfabricationprocesses,suchas *Correspondence: fhuq.apece.du@gmail.com 2 DepartmentofInformationandCommunicationTechnology,Mawlana BhashaniScienceandTechnologyUniversity,Santash,Tangail1902, Bangladesh Fulllistofauthorinformationisavailableattheendofthearticle a SpringerOpen Journal ©2013Hasanetal.;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons AttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproduction inanymedium,providedtheoriginalworkisproperlycited. Hasan etal.SpringerPlus 2013, 2 :151 http://www.springerplus.com/content/2/1/151 photolithography.Asaresult,nanowiresprovideacon- venientplatformthroughwhichresearchersmaystudy confinedtransportphenomena. Review PropertiesofSiliconNanowires ElectronicProperties ThesmallsizesofSiNWsmaketheirelectronicand electricalpropertiesstronglydependentongrowthdirec- tion,size,morphologyandsurfacereconstruction.A wellknownexampleisthesizedependenceoftheelec- tronicbandgapwidthofSiNWsirrespectiveofwiredir- ection.Asthewirediameterdecreases,thebandgapof thenanowirewidensanddeviatesfromthatofbulksili- congradually.Moreover,theorientationsofthewire axisandthesurfacehaveagreateffectontheelectronic propertiesofSiNWs(Aijiang2007). MichaelNolanetal.theoreticallyinvestigatedtheband gapmodificationforsmall-diameter(~1nm)silicon nanowiresresultingfromtheuseofdifferentspeciesfor surfaceterminationbydensityfunctionaltheorycalcula- tions.Becauseofquantumconfinement,small-diameter wiresexhibitadirectbandgapthatincreasesasthewire diameternarrows,irrespectiveofsurfacetermination. Figure2showsbandgapasafunctionofnanowirediam- eterforvarioussurfaceterminations(Nolanetal.2007). Sacconietal.alsoinvestigatedtheelectronicproperties ofsiliconnanowireswithseveraldifferentapproacheslike theEmpiricalTight-Binding(ETB)model,theLinear CombinationofBulkBands(LCBB)modelandNon- EquilibriumGreenFunction(NEGF)model.Theyconsid- eredbothhydrogenatedandSiO 2 terminatedsilicon surfacesinthesemodels.WhenthediameterofSiNWis reducedfrom3.2to1.6nm,thebandgapofhydrogenated nanowireincreasesfrom1.56to2.44eV.Ontheother hand,inthecaseofSiO 2 /SiNWstructure,thisincreaseis smaller,sincethebandgapgoesfrom1.50eVfora3.2nm cell,to1.88eVfora1.6nmcell.Thisbehaviorcanbe expectedduetoalowerconfinementdeterminedbySiO 2 surroundingSiNW,withrespecttothecaseofsimple hydrogentermination. Effectivemassesforconductionandvalencebands havealsobeencalculated.TheeffectofanincreasingSi thicknessonahydrogenterminatedwireisthatofredu- cingtheconductionmass,from0.47m o to0.31m o , whichis55%greaterthanthevalueoftransversemass inbulksilicon.TheeffectontheSiO 2 -confinedwireis similar;byincreasingSithickness,effectivemassde- creasesfrom0.36m o to0.29m o (Sacconietal.2007). TheeffectofwirethicknessofSiNWonconductionval- leysplitting,holebandsplitting,effectivemassesand transmissionhasbeenreportedusingsp 3 d 5 s * modelby YunZhengetal.Theyconcludedthatintheconduction band,valleysplittingreducestheaveragedmobilitymass alongtheaxisofthewire,butquantumconfinementin- creasesthetransversemassoftheconductionbandedge. Forthewirethicknessrangethattheyhaveconsidered, theeffectivemassattheconductionbandedgeisatleast 35%heavierthanthatoftransversemassofbulkSi. Quantumconfinementhasthelargesteffectontheeffect- ivemassesinthevalenceband.Theeffectivemassatthe valencebandedgeisatleastsixtimesheavierthanthatof thebulk.Theeffectivemassofthenexthighestbandis evenheavier.Smallenergysplittingalsooccursatthecon- ductionbandminimum.Forwiresgreaterthan1.54nm thick,thefourbulkvalleyswhichcomposetheconduction bandminimumaresplitintothreeenergies.Thecenter energyistwofolddegenerateroughlyevenlysplitbetween thelowestandhighestenergy.Thesingle-bandmodelper- formsreasonablywellatcalculatingtheeffectiveband edgesforthe1.54nmwire(Zhengetal.2005). YiCuietal.showsthatBoronandphosphoruscanbe usedtochangetheconductivityofSiNWsovermanyor- dersofmagnitudeandthattheconductivityofthe dopedSiNWsrespondoppositelytopositive(negative) V gforboronandphosphorusdopants.Indeed,the V g dependenceprovidesstrongproofforp-type(holes) dopingwithboronandn-type(electrons)dopingwith phosphorusintheSiNWs(Cuietal.2000). OpticalProperties BulkSihasanindirectbandgap,withthevalenceband maximumatthe
pointandtheconductionminimumat about85%alongthe
toXdirection,andaphononisre- quiredtoconservethemomentuminanyelectronictran- sition.Remarkably,however,SiNWsgrownalongmostof thecrystallographicorientationshaveadirectbandgap, meaningthatthemaximumofthevalencebandandthe minimumoftheconductionbandoccuratthesamepoint ink-space.Thispropertyhasallowedtoenvisagetheuse Figure1 Histogramofsilicon “ whisker ” and “ nanowire ” publications.Source:ISIWebofKnowledge(SM). Hasan etal.SpringerPlus 2013, 2 :151 Page2of9 http://www.springerplus.com/content/2/1/151 ofSiNWsasopticallyactivematerialsforphotonicsappli- cations(Canham1990;Guichardetal.2006). Thepossibilityofcontrollingthebandgapwidthis tremendouslyattractiveforoptoelectronicsapplications: notonlySiNWscanhaveadirectbandgap,whichper seincreasestheopticalefficiency,butitswidthcanin principlebetuned.Itisnotdifficulttoimagine,however, thatcontrollingthewirediameterwithtoleranceswithin 1 – 3nmisamorethanchallengingtask.Asimplerroute tobandgaptuningiscontrollingthechemicalcompos- itionandthecoveragedensityofthewiresurface.Halo- genssuchasCl,Br,andIcanbeusedassurface passivationagentsinsteadofHand,whilenotaltering thesemiconductingcharacterofthewires,theyresult inasignificantshrinkingofbandgap(Leuetal.2006). Thestrongestreductionofthebandgapisprovidedby I,followedbyBrandCl,intheoppositeorderofthe bondingstrengthofthesespeciesandSiNWs.Interest- ingly,thesurfacecoverageisafurtherdegreeoffree- domandonecanspanallthebandgapvaluesbetween aH-andhalogen-passivatedwirebyvaryingtheH: halogenratio.Also,increasingthehalogensurfacecon- centrationthebandedgestates,concentratedinthe wirecoreinpresenceofH-passivation,progressively spreadtothesurface. AnalogousresultshavebeenreportedforOHandNH2 (Aradietal.2007;Nolanetal.2007).Itshouldbenoted thatthepassivatingspeciesdonotcontributesignificantly tothestatesclosetothebandedges,sothatthereduction ofthegapisnotcausedbytheintroductionofadditional bands.Itrathercomesfromthehybridizationofthe valencebandstateswiththefrontierorbitalsofthediffer- entpassivatingfunctionalgroupsthatcauseasignificant bandgapreductionrelativetoH-passivatedwires. Theseresultsindicatethatthebandgapwidthin SiNWscanbetailorednotonlybycontrollingthewire diameter,butalsobyanappropriatechoiceofthesur- facetermination. Thebroadbandopticalabsorptionpropertiesofsili- connanowire(SiNW)filmshavebeenmeasuredand foundtobehigherthanthatofsolidthinfilmsofSiof equivalentthickness.Theobservedbehaviorisad- equatelyexplainedbylightscatteringandlighttrapping thoughsomeoftheobservedabsorptionisduetoa highdensityofsurfacestatesinthenanowiresfilms,as evidencedbythepartialreductioninhighresidualsub- bandgapabsorptionafterhydrogenpassivation.There- flectanceofthesolidfilmshowstypicalbehaviorwhat expectedforsilicon,whereasthereflectanceofthe nanowirefilmislessthan5%overthemajorityofthe spectrumfromtheUVtothenearIRandbeginstoin- creaseat~700nmtoavaluesof~41%attheSiband edge(1100nm),similartothesolidfilmsample.Itis clearthatthenanowiresimpartasignificantreduction ofthereflectancecomparedtothesolidfilm.Figure3 showscomparativereflectanceofsolidsiliconandSi nanowires(Tsakalakosetal.2007). FabricationofSi:Hnanowires(NWs)andnanocones (NCs),usinganeasilyscalableandIC-compatibleprocess Figure2 Bandgapasafunctionofthe[100]siliconnanowirediameterforvarioussurfaceterminations. ( a )DFTcalculationswithinGGA- PBE.( b )Resultsfromadensity-functionaltightbinding(DFTB)parameterization. Figure3 Totalreflectancedatafromintegratedsphere measurementsforan11
mthicksolidSithinfilmand nanowirefilmonglasssubstrate(Tsakalakosetal.2007). Hasan etal.SpringerPlus 2013, 2 :151 Page3of9 http://www.springerplus.com/content/2/1/151 hasbeenreportedbyJiaZhu etal .Theyhaveshownthat Si:Hnanostructuresdisplaygreatlyenhancedabsorption overalargerangeofwavelengthsandanglesofincidence duetosuppressedreflection.Morethan90%oflightis absorbedatanglesofincidenceupto60°fora-Si:HNC arrays,whichissignificantlybetterthanNWarrays(70%) andthinfilms(45%).Inaddition,theabsorptionofNC arraysis88%atthebandgapedgeofa-Si:H,whichis muchhigherthanNWarrays(70%)andthinfilms(53%). Figure4summarizedtheresultsofdifferentstructures fordifferentcondition(Zhuetal.2009). MethodsofFabricationofNanowires Manytechniques,includingbothtop-downandbottom-up approaches,havebeendevelopedandappliedforthesyn- thesisofNanowires.Vapor – Liquid – Solid(VLS)Mechanism, ChemicalVaporDeposition(CVD),EvaporationofSiO, MolecularBeamEpitaxy(MBE),LaserAblationandElectro- lessmetaldepositionanddissolution(EMD)havebeen discussedhere. Vapor – Liquid – Solid(VLS)Mechanism TheVLSmechanism,firstproposedbyWagnerandEllis (Wagner&Ellis1964)inthemid-1960s,isthekey mechanismforsilicon-wiregrowth.TheirproposedVLS mechanismisbasedontwoobservations:thatthe additionofcertainmetalimpuritiesisanessentialpre- requisiteforgrowthofsiliconnanowiresinexperiments andthatsmallglobulesoftheimpurityarelocatedatthe tipofthewireduringgrowth.Fromthis,Wagnerand Ellisdeducedthattheglobuleatthewiretipmustbein- volvedinthegrowthofthesiliconwiresbyacting “ asa preferredsinkforthearrivingSiatomsor,perhapsmore likely,asacatalystforthechemicalprocessinvolved ” Figure4 Valueofabsorptiononsampleswitha-Si:Hthinfilm,NWarraysandNCarrays(a)Measured,(b)Calculatedoveralarge rangeofwavelengthsatnormalincidence;(c)Measured(d)Simulatedfordifferentangleofincidence(atwavelength
=488nm) (Zhuetal.2009). Hasan etal.SpringerPlus 2013, 2 :151 Page4of9 http://www.springerplus.com/content/2/1/151 (Wagner&Ellis1964).WhenAu,forexample,isde- positedonsiliconsubstrateandthissubstrateisthen heatedtotemperaturesaboveabout363°C,smallli- quidAu – Sialloydropletswillformonthesubstrate surface.Exposingsuchasubstratetoagaseoussilicon precursor,suchassilicontetrachloride(SiCl 4 )orsi- lane(SiH 4 )precursormoleculeswillcrackonthesur- faceoftheAu – Sialloydroplets,whereuponSiis incorporatedintothedroplet.Thesiliconsupplyfrom thegasphasecausesthedroplettobecomesupersa- turatedwithSiuntilsiliconfreezesoutatthesilicon/ dropletinterface.Thecontinuationofthisprocessthen leadstothegrowthofawirewiththealloydroplet ridingatopthegrowingwire(Wagner&Ellis1964) (Figure5). ChemicalVaporDeposition(CVD) InCVD,avolatilegaseoussiliconprecursor,suchassi- lane(SiH 4 )orsilicontetrachloride(SiCl 4 ),servesasthe siliconsource.Itistransportedtothedepositionsurface atwhichtheprecursorreacts,andiscrackedintoits constituentsasdepictedinFigure6. Originally,CVDwasdevisedforthedepositionof high-purityfilms.Contaminationssuchasgoldparticles, however,werefoundtocauseanisotropicgrowthofsili- con,thatis,thegrowthofsiliconwires.CVDallowsepi- taxialgrowthofSiNWs,withthegrowthvelocityvarying fromabout10 -2 to10 3 nmmin -1 ,(Kodambakaetal. 2006;Nebol ’ sinetal.2005)dependingontemperature andtypeofSiprecursorused.Furthermore,CVDoffers broadpossibilityofmodifyingthepropertiesofthesili- conwiresinacontrolledfashion.Avarietyofderivatives ofCVDmethodsexist.Thesecanbeclassifiedbypa- rameterssuchasthebaseandoperationpressureorthe treatmentoftheprecursor.Sincesiliconisknownto oxidizeeasilyifexposedtooxygenatelevatedtempera- tures,itiscrucialtoreducetheoxygenbackground pressureinordertobeabletoepitaxiallygrowuniform siliconnanowires.Inparticular,whenoxygen-sensitive catalystmaterialsareused,itturnsouttobeusefulto combinecatalystdepositionandnanowiregrowthinone system,sothatgrowthexperimentscanbeperformed withoutbreakingthevacuuminbetween(Wangetal. 2006).Inanycase,itisusefultolowerthebasepressure oftheCVDreactordowntohighorevenultrahighvac- uum,whichreducesunwantedcontaminationanden- ablesgrowthatloweredtemperatures(Akhtaretal. 2008).Thepressuresduringgrowthmainlydepends uponthegaseoussiliconprecursoranditscracking probabilityatthecatalystsurface.Growthwithdisilane, Si 2 H 6 ,forexample,can — butmustnot — becarriedout atextremelylowpartialpressuresofaround10 -6 mbar (1bar=10 5 Pa).Theselowgrowthpressuresallowthe combinationofCVDwithtransmissionelectronmicros- copy(TEM),enablinginsituobservationofthenanowire growth(Hofmannetal.2008).Incontrasttothat,silane partialpressuresrequiredforwiregrowthareaboutfive ordersofmagnitudehigher.Bymodifyingtheprecursor beforereactingwiththesamplesurface,thetemperature budgetofthesubstratecanbelowered.Incaseswhere thethermalloadiscriticalorwhereahighsupersaturation ofthedropletisnecessary,nanowiregrowthcanbeen- hancedusingplasma-enhancedCVD(PECVD)(Hofmann etal.2003;Sharmaetal.2004;Iacopietal.2007).Another advantageofCVDasabottom-upsynthesismethodisits variabilityconcerningtheintendedwiresize.Wirediame- tersrangefrombelow10nm(Cuietal.2001)uptosev- eralhundredmicrometers(Wagner&Ellis1964).Since surfacediffusiononlyplaysaminorroleinCVD,the Figure5 SchematicoftheVLSgrowthmechanism(a)Catalytic liquidalloy(b,c)Successivegrowthofnanowire. Figure6 Schematicsofexperimentalsetupfornanowire growthusingCVDmethod. Hasan etal.SpringerPlus 2013, 2 :151 Page5of9 http://www.springerplus.com/content/2/1/151 lengthofthewirescanalsobetunedaccordinglybysim- plyextendingordecreasingthegrowthtime.Thus,to summarize,alargerangeoflengthanddiameterconfigu- rationscanbefabricated(Parketal.2008).WithCVD, notonlythewiresizebutalsoitspropertiescanbe modified. EvaporationofSiO Acost-effectivemethodtoproducesiliconnanowireson alargescaleistoevaporatesolidsiliconmonoxide,SiO (showninFigure7). Atwo-zonetubefurnaceconnectedtoaninertgas supplyandsmallamountofSiOgranulatearethebasic ingredientsforthesynthesisofsiliconnanowires.Cru- cialforgrowthisatemperaturegradientfromabout 1350to900°Calongthetubeofthefurnace.SiOisevap- oratedatthehotterendofthetube,flowswiththegas streamtothecoolerpart,whereitundergoesadispro- portionationreactionintoSiandSiO 2 ,therebyforming thenanowires(Panetal.2001).Inprinciple,twodiffer- entgrowthmethodsarepossible:growthwithandwith- outmetalcatalyst.Growthassistedbythepresenceof ametalcatalystisrelativelyrapid(Guetal.2000).Con- sistentwiththeconceptofVLSgrowth,thediameters aredeterminedbythesizeofthecatalystparticle,al- thoughtheinterplaybetweenthenanowireandthecata- lystdropletseemstobemorecomplexcomparedto normalCVDgrowth.Asaconsequenceofthedispro- portionationreaction,thediameterratiobetweencrys- tallinecoreandamorphousshellremainsapproximately constant(Kolbetal.2004).Thesecondgrowthmode, metal-catalyst-freegrowth,hasbeenoriginallyproposed forgrowthvialaserablation(Wangetal.1998),whereit wasobservedthatnanowirescanbecatalyzedbysilicon dioxide(Wangetal.1999).Remarkableaboutthis oxide-assistedgrowth(OAG)isthatSiO 2 -containingtar- getsclearlyraisetheyieldofthefinalamountofsilicon nanowirescomparedtopuresilicontargetsormixedsil- icon – metaltargets(Wangetal.1998).Bycarryingout thegrowthprocessoverseveralhours,onecanobtain millimeter-longcrystallinesiliconnanowireswithvary- ingdiametersfromabout5nmto100nm,coveredby anamorphousshellofuptoseveral10nm(Shietal. 2000;Zhangetal.2000;Shietal.2005). MolecularBeamEpitaxy(MBE) InMBE,asolidhigh-puritysiliconsourceisheateduntil Sistartstoevaporate.Figure8schematicallydepictsan MBEsetup. Adirectionalgaseousbeamofsiliconatomsisaimedat thesubstrate,onwhichtheatomsadsorbandcrystallize. Toreducecontamination,thebasepressureofanMBE systemisusuallykeptatultrahighvacuum,allowingto monitorthegrowthusingReflectionHigh-EnergyElec- tronDiffraction(RHEED)(Werneretal.2006)orother surfacesensitiveexaminationmethods.SimilartoCVD, MBEwasinitiallydesignedforepitaxiallayer-by-layerde- positiononly.Yet,metalcontaminationwasalsofoundto causesilicon-wiregrowthinthiscase.Differingfrom CVD,noprecursorgasiscrackedatthesurfaceoftheli- quidmetal – siliconalloy.Therefore,thelattercannotbe treatedasaclassicalcatalystanymore.InMBE,twosilicon fluxesgovernwiregrowth.First,thedirectfluxofsilicon fromthesiliconsource;andsecond,thefluxofdiffusing Figure7 Schematicsofexperimentalsetupforsiliconnanowire growthbyevaporationofSiO. Figure8 Schematicsofexperimentalsetupforsiliconnanowire growthbyMBE. Hasan etal.SpringerPlus 2013, 2 :151 Page6of9 http://www.springerplus.com/content/2/1/151 siliconadatomsfromthesiliconsubstratesurface.The nanowiresproducedbyMBE — usuallygrownonSi(111) substrates — areepitaxialand11ᅰoriented.MBEoffers excellentcontrollabilityintermsoftheincomingflux,such thatdopedwires(Kanungoetal.2008)orheterostructures (Zakharovetal.2006)canbegrownbyswitchingbetween evaporationsources.OnedisadvantageofMBE,however,is thatthemethodislimitedwithrespecttotheminimally possibleSi-nanowirediameter.Onlynanowireswithdiame- tersgreaterthanabout40nmcanbeobtained(Shietal. 2005;Schubertetal.2004)whichseemstobeaconse- quenceoftheGibbs – Thomsoneffectandthefactthatonly smallSisupersaturationsa reachievablebyMBE.Another disadvantageofMBEisthelownanowiregrowthvelocity ofjustafewnanometersperminute(Schubertetal.2004). LaserAblation Thesiliconnanowiresproducedbylaserablationdifferin manyaspectsfromtheMBEgrownwhiskers.Onecan easilyobtainlargequantitiesofultrathinnanowireswith highaspectratios(Zhangetal.1998;Zhouetal.1998).As schematicallydisplayedinFigure9,ahigh-powerpulsed laserablatesmaterialfromamixedSi – catalysttarget, whichisplacedinatubefurnaceheldathightempera- turesandpurgedwithaninertgas. Thesiliconmaterialablatedfromthetargetcoolsbycol- lidingwithinert-gasmolecules,andtheatomscondenseto liquidnanodropletswiththesamecompositionasthetar- get(Morales&Lieber1998).Thus,thesenanoparticles containbothSiandthecatalys tmaterial.Accordingtothe VLSmechanism,siliconnanowiresstarttogrowoncethe catalystgetssupersaturatedwithsiliconandproceedsas longasthecatalystnanoparticlesremainliquid.Theadvan- tagesoflaser-ablatednanowireproductionaremanifold. First,thereisnoneedforasubstrate.Second,thecompos- itionoftheresultingnanowirescanbevariedbychanging thecompositionofthelasertarget.Byadding,forexample, SiO 2 tothetarget,single-crystallinesiliconnanowireswith variedamorphousSiO x shellthicknessescanbeobtainedin asingleprocessingstep(yangetal.2004)withsilicon-core diametersaslowas5nmandvaryingshellthicknessesof about10nm.Duetothehighgrowthtemperatures,catalyst metalssuchasFe,possessinga higheutectictemperature, canbeused.Theresultingnan owiregrowthvelocitiesare typicallyoftheorderofmicrometersperminute(Zhang etal.1998;Morales&Lieber1998).Theradiiofthe nanowiresnotonlydependonthetypeofmetalcatalysts usedbutalsoonthegasestha tarestreamedthroughthe furnace,suchasH 2 ,He,orN 2 (Zhangetal.1999). Electrolessmetaldepositionanddissolution Electrolessdeposition,anon-galvanictypeofdeposition methodthatinvolvesseveralsimultaneousreactionsinan aqueoussolution,whichoccurwithouttheuseofexternal electricalpower.Themostcommonelectrolessdeposition isnickel,silverorgoldparticledeposition.Firststepofsil- iconnanowiresynthesisusingthisprocessistodeposit metalparticlelikeAu,AgorCuonsiliconsubstrate. Thesenoblemetalswouldattractelectronsfromthesili- conandfacilitateSioxidation.Figure10showstheoxida- tionofsiliconsurfaceunderdepositedmetal. Figure9 Schematicsofexperimentalsetupforsiliconnanowire growthbyLaserAblationmethod. Figure10 Oxidationofsiliconsurfaceunderdepositedmetal. Figure11 Formationofsiliconnanowiresbyelectroless metaldeposition. Hasan etal.SpringerPlus 2013, 2 :151 Page7of9 http://www.springerplus.com/content/2/1/151 Generally,theetchingofacleanedSiwaferproceedsveryslowlyinaqueousHF/Fe(NOsolutionatlowtemperature.However,SietchingoccursrapidlywhenSisubstratescoveredwithAg/Au-nanoparticlefilmsareimmersedinHF/Fe(NOsolutionatroomtemperature.FormationofSiNWarraysduetothefurthersinkingoftheAgparticles,andlongitudinalandlateraldissolutionofbulkSi(showninFigure11)(Pengetal.2006).ConclusionsInsummary,wehaveseenthattherearesignificantchangesinelectronicandopticalpropertiesofsiliconnanowiresthanthoseofbulk.Thechangeofsomepropertiesdependsonthesizeandshapeofthenanostructures.Itsuggeststhat,carefulproductionofnanowiresofdesiredsizeandshapewouldmakeitpossibletomanipulatepropertieslikebandgap,effectivemassandopticalabsorption.Italsohasbeenseenthatsiliconnanowirescanbeproducedbydiffer-entgrowthmethods.AuandAgarethemostpopularcata-lystmaterialforsiliconnanowiresynthesis.CompetinginterestTheauthordeclaresthattheyhavenocompetinginterest.Allauthorshaveequalcontributiononthisworkandallauthorsreadandapprovedthefinalmanuscript.AuthordetailsDepartmentofElectricalandElectronicEngineering,ShahjalalUniversityofScienceandTechnology,Kumargaon,Sylhet3114,Bangladesh.ofInformationandCommunicationTechnology,MawlanaBhashaniScienceandTechnologyUniversity,Santash,Tangail1902,Bangladesh.DepartmentofappliedPhysicsElectronicsandCommunicationEngineering,UniversityofDhaka,Dhaka1000,Bangladesh.Received:15September2012Accepted:1April2013Published:10April2013AijiangLU(2007)TheoreticalStudyofElectronicandElectricalPropertiesofSiliconNanowires.Dissertation,CityUniversityofHongKong,HongKongAkhtarS,UsamiK,TsuchiyaY,MizutaH,OdaS(2008)VaporSolidGrowthofSmall-andUniform-DiameterSiliconNanowiresatLowTemperaturefrom.ApplPhysExpress1:014003AlivisatosAP(1996)SemiconductorClusters,Nanocrystals,andQuantumDots.Science271(5251):933AradiB,RamosLE,De´akP,K¨ohlerT,BechstedtF,ZhangRQ,FrauenheimT(2007)Theoreticalstudyofthechemicalgaptuninginsiliconnanowires.PhysRevB76(3):17,035305CanhamLT(1990)Siliconquantumwirearrayfabricationbyelectrochemicalandchemicaldissolutionofwafers.ApplPhysLett57(10):1046CuiY,DuanX,HuJ,LieberCM(2000)DopingandElectricalTransportinSiliconNanowires.JPhysChemB104(22):5213CuiY,LauhonLJ,GudiksenMS,WangJ,LieberCM(2001)Diameter-controlledsynthesisofsingle-crystalsiliconnanowires.ApplPhysLett78:2214CuiY,LieberCM(2001)AssembledusingSiliconNanowireBuildingBlocksFunctionalNanoscaleElectronicDevices.Science291:851GivargizovEI(1975)FundamentalAspectsofVLSGrowth.J.CrystGrowthGuichardAR,BarsicDN,SharmaS,KaminsTI,BrongersmaML(2006)Tunablelightemissionfromquantum-confinedexcitonsinTiSi2-catalyzedsiliconnanowires.NanoLett6(9):2140GuQ,DangH,CaoJ,ZhaoJ,FanS(2000)Siliconnanowiresgrownoniron-patternedsiliconsubstrates.ApplPhysLett76:3020HofmannS,DucatiC,NeillRJ,PiscanecS,FerrariAC,GengJ,Dunin-BorkowskiRE,RobertsonJ(2003)Goldcatalyzedgrowthofsiliconnanowiresbyplasmaenhancedchemicalvapordeposition.JApplPhys94(9):6005HofmannS,SharmaR,WirthCT,Cervantes-SodiF,DucatiC,KasamaT,Dunin-BorkowskiRE,DruckerJ,BennettP,RobertsonJ(2008)Ledge-flow-controlledcatalystinterfacedynamicsduringSinanowiregrowth.NatMater7:372IacopiF,VereeckenPM,SchaekersM,CaymaxM,MoelansN,BlanpainB,RichardO,DetavernierC,GriffithsH(2007)Plasma-enhancedchemicalvapourdepositiongrowthofSinanowireswithlowmeltingpointmetalcatalysts:aneffectivealternativetoAu-mediatedgrowth.Nanotechnology18:17,505307IijimaS(1991)HelicalMicrotubulesofGraphiticCarbon.Nature354:56KanungoPD,ZakharovN,BauerJ,BreitensteinO,WernerP,GoeseleU(2008)Controlledinsituborondopingofshortsiliconnanowiresgrownbymolecularbeamepitaxy.ApplPhysLett92:263107KodambakaS,TersoffJ,ReuterMC,RossFM(2006)Diameter-IndependentKineticsintheVaporliquid-SolidGrowthofSiNanowires.PhysRevLett96KolbFM,HofmeisterH,ScholzR,ZachariasM,Go¨seleU,MaDD,LeeST(2004)AnalysisofsiliconnanowiresgrownbycombiningSiOevaporationwiththeVLSmechanism.JElectrochemSoc151:G472LawM,GoldbergerJ,YangPD(2004)SemiconductorNanowiresandNanotubesAnnu.ReVMaterRes34:83LeuPW,ShanB,ChoK(2006)Surfacechemicalcontroloftheelectronicstructureofsiliconnanowires:Densityfunctionalcalculations.PhysRevB73MoralesAM,LieberCM(1998)ALaserAblationMethodfortheSynthesisofCrystallineSemiconductorNanowires.Science279(5348):208NolanM,OCallaghanS,FagasG,GreerJC,FrauenheimT(2007)Siliconnanowirebandgapmodification.NanoLett7(1):34sinVA,ShchetininAA,DolgachevAA,KorneevaVV(2005)EffectoftheNatureoftheMetalSolventontheVaporliquid-SolidGrowthRateofSiliconWhiskersInorg.Mater41(12):1256PanZW,DaiZR,XuL,LeeST,WangZL(2001)Temperaturecontrolledgrowthofsilicon-basednanostructuresbythermalevaporationofSiOpowders.JPhysChemB105:2507ParkWI,ZhengG,JiangX,TianB,LieberCM(2008)ControlledSynthesisofMillimeter-LongSiliconNanowireswithUniformElectronicProperties.NanoLett8(9):3004PengKQ,HuJJ,YanYJ,WuY,FangH,XuY,LeeST,ZhuJ(2006)FabricationofSingle-CrystallineSiliconNanowiresbyScratchingaSiliconSurfacewithCatalyticMetalParticles.AdvFunctMater16(3):387SacconiF,PerssonMP,PovolotskyiM,LatessaL,PecchiaA,GagliardiA,BalintA,FrqunheimT,CarloAD(2007)Electronicandtransport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SpringerPlus 2013 2 :151. Submit your manuscript to a journal and bene“ t from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the “ eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Hasan etal.SpringerPlus 2013, 2 :151 Page9of9 http://www.springerplus.com/content/2/1/151