controllable hydrothermal synthesis of zno nanowires arrays on al-doped zno

4.a Transmittance spectra of the quartz glass curve 1, AZO thin film curve 2, ZnO thin film curve 3, ZnO NW arrays grown on the ZnO seed layer at 80 ◦ C for 1 h ◦... 6.TEM images of the Zn

Applied Surface Science 257 (2011) 10134– 10140

j o ur na l ho me p age :w w w e l s e v i e r c o m / l o c a t e / a p s u s c

substrate

Jin Zhanga, Wenxiu Quea,∗, Qiaoying Jiaa, Xiangdong Yeb, Yucheng Dingb

a Electronic Materials Research Laboratory, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, People’s Republic of China

b State Key Laboratory of Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, People’s Republic of China

Article history:

Received 11 December 2010

Received in revised form 7 June 2011

Accepted 10 June 2011

Available online 7 July 2011

Keywords:

Zinc oxide

Nanowires

Seed layer

Fluorination

Photoluminescence

© 2011 Elsevier B.V All rights reserved

1 Introduction

ZnOisasemiconductorwithexceptionalelectronicand

pho-tonicpropertiesaswellasgreatthermalstability andoxidation

resistance.RecentdevelopmentsandcapacitytosynthesizeZnO

nanostructures with different shapes [1–3] have led to novel

andenhancedpropertiesascomparedtoitsbulkform,andthus

enablingittohavemanyattractiveapplications.Forexample,it

canbeusedasapotentialmaterialfornanodeviceassemblyand

applicationsinblue-UVlightemitters[4]andphotodetectors[5],

fieldemissiondevices[6],anddye-sensitizedsolarcells [7],etc

Indeed,well-alignedZnOnanowire(NW)arrayshavebeenformed

onGaN,AlN,Al1−xGaxN,6H–SiC,andZnObufferlayers[8–10],but

theopticalpropertiesoftheNWsgrownonbufferlayershavebeen

scarcelyinvestigated,especiallywithrespecttoimpurityanddefect

distribution,whichcanhindertheapplicationsoftheNWarrays

Inrecentyears,anumberoftheZnOthinfilmsdopedwithvarious

metallicionshavebeenextensivelystudiedforthemanipulation

oftheiropticalandelectricalproperties,theAl-dopedZnO(AZO)

thinfilmsareattractiveduetotheirgoodconductivity,high

trans-∗ Corresponding author Tel.: +86 29 82668679; fax: +86 29 82668794.

E-mail address: wxque@mail.xjtu.edu.cn (W Que).

parencyand relativelylowcost[11,12].Inviewofthis,theuse

ofalattice-matchedandconductingbufferlayermaycircumvent theproblemandleadtopotentialintegrationwithsilicon micro-electronics[13–15].Therefore,theluminescentandelectronfield emissionpropertiesoftheZnONWarraysgrownontheAZOseed layersarealsoreportedbymanyresearchgroups[16,17] Further-more,inordertoachieveanimmensepotentialoftheZnO NW arrays,itisimportantandnecessarytohaveagoodcontrolforthe spatialarrangementsandpropertiesoftheZnONWarrays[16]

In this paper,the ZnO NWarrays weregrown onAZO seed layer,whichwasdepositedbyasol–gelprocess,bya hydrother-malmethod,andeffectsofthetemperatureandgrowthtimeof thehydrothermalprocessonthemorphologicaland photolumines-cencepropertiesoftheas-grownZnONWarrayswerealsostudied anddiscussed.Inaddition,whatwebelievetobethefirstreport

onthefabricationofthepatternedAZOseedlayeronthesilicon substratebycombiningasol–gelprocesswithanelectron-beam lithographyprocess,aswellasasurface fluorinationtechnique, whichcaneliminatetheeffectoftheelectron-beamresistonthe boundaryofthepatternedAZOseedlayer,andthustheZnONW arrayscouldbesuccessfullygrownonthepatternedregionsofthe AZOseedlayerbyemployingthehydrothermalprocess Further-more,thephotoluminescenepropertiesoftheselectively grown ZnONWarrayswerealsocharacterizedandinvestigated

0169-4332/$ – see front matter © 2011 Elsevier B.V All rights reserved.

Fig 1. Schematic representation of the patterning process of the ZnO NW arrays on silicon substrate.

2 Experimental

2.1 PreparationoftheAZOseedlayer

InordertocomparethepropertiesoftheZnONWarraysgrown

ondifferentseedlayers,theZnOandAZOseedlayerswere

pre-paredbythesol–geltechnique.Here,theAl-dopedconcentration

oftheAZOseedlayerwas1.0at.%withrespecttoZndue toits

relativelyoutstandingperformancethanotherdoping

concentra-tionsas shown in our previousreport [18] The ZnO and AZO

solswerepreparedasfollows.Inbriefly,Zn(CH3COO)2·2H2Owas

firstdissolvedina2-methoxyethanolmonoethanolamine

(MEA)-deionizedwater solutionat roomtemperature.Themolarratio

oftheMEA anddeionized watertozinc acetate wasfixedat1

and 0.5,respectively, and theconcentrationof thezinc acetate

was0.75mol/L.For theAZOsol, anappropriateamountof

alu-minumdopingwasobtainedbyaddingAlCl3·6H2Otoabovethe

as-preparedprecursorsolution.Then,thefinalsolutionwasstirred

at60◦Cfor30minuntiltoyieldaclearandhomogeneoussolution

TheZnOandAZOseedlayersweredepositedontoaquartzglass

substratebyamulti-spin-coatingprocessfor20sat3000rpm.It

shouldbementionedthatafterspin-coatingonelayer,thecoated

sampleshouldbepreheatedintheair at200◦Cfor 10minand

thustheone-layerthinfilmwithabout50nmthickcanbe

eas-ilyobtained.Finally,thesamplewiththreelayerswaspost-heated

atatemperatureof500◦Cfor1hintheair[19]

2.2 HydrothermalsynthesisoftheZnONWsontheZnOandAZO

seedlayers

VerticallyalignedZnONWarraysweregrowninaTeflon-lined

stainlesssteel autoclavebyimmersingthesubstratesdeposited

withtheZnOor AZOseedlayers intothemixedaqueous

solu-tion,whichincludesZn(NO ) (0.04mol/L)andNaOH(0.8mol/L),

at80–180◦Cfor1–3h.Theobtainedsampleswerethenwashedby thedeionizedwateranddriedintheairat80◦C[20]

2.3 FabricationofthepatternedZnONWarraysonsilicon substrate

Fig.1showsthefabricationprocessofthepatternedZnONW arrays on the silicon substrate Here, the electron-beam resist (ZEP520AfromZeonCorp.)wasfirstspin-coatedonthesilicon sub-strateandfollowedthecoatedsamplewasputinanoventoprebake

at180◦Cfor30min.Then,theprebakedsamplewasexposedfor patterningat30kVunderahigh-resolutionelectron-beam lithog-raphysystem(CABL-9000CCrestecCorp.).Subsequently,thesilicon substratewiththepatternedresistwasimmergedandrinsedin

aZMD-B(Methylisobutylketone89%andIsopropylalcohol11%) solutionfor1mintoremovetheexposedEB-resist.Tomakelow surfaceenergycoatingsonthesubstrate,thesiliconsubstratewith patternedresistwasimmergedintothesolution,whichconsists

of2.0vol.%(Heptadecafluoro-1,1,2,2-tetradecyl)trimethoxysilane (SC-1060F,fromSicongNewMaterialsCorp.),0.5vol.%aceticacid and97.5vol.%isopropylalcohol,for2handthenpickedout Fol-lowedthattheimmergedsamplewasheatedat150◦Cinanoven for1handcooleddowntoroomtemperature,theresidualresist wasthenremovedfromthesilicon substratebyrinsingit with chlorobenzene,thus,thetemplatewasobtained.Finally,theAZO solwasspin-coatedonthepatternedsiliconsubstrateandtheZnO

NWarrayswereselectivelygrownonthepatternedregionsofthe AZOseedlayerbythehydrothermalprocess

The structural properties of the ZnO NW arrays were char-acterizedbyusingaD/max-2400X-raydiffractionspectrometer (Rigaku)withCuK␣radiationandoperatedat40kVand100mA from20to70◦in2,andthescanningspeedwas15◦min−1atastep

of0.02◦.ThemorphologicalpropertiesoftheZnONWarrayswere observedby aJEOL JSM-7000Ffield-emissionscanningelectron

10136 J Zhang et al / Applied Surface Science257 (2011) 10134– 10140

Fig 2. (a) SEM image of the ZnO seed layer, (b) SEM image of the AZO seed layer.

microscopy(FE-SEM).TheUV–visabsorptionspectraand

transmit-tancespectraoftheZnONWarrayfilmswerecharacterizedbya

JASCOV-570UV/VIS/NIRspectrometerandthephotoluminescence

spectraoftheZnONWarraysweremeasuredatroomtemperature

byaFLUOROMAX-4spectrometer

3 Results and discussion

SEMimagesandXRDpatternsofboththeZnOandAZOseed

layer,whicharedepositedonthequartzglasssubstrate,areshowed

inFigs.2and3,respectively.ItcanbeobservedfromFig.2that

theAZOseedlayerhasasmallergrainsizeascomparedtothat

oftheZnOseedlayer,andthe(002)diffractionpeakoftheAZO

seedlayerinintensityishigherthanthatoftheZnOseedlayer

asseeninFig.3.ItisindicatedthattheAZOseedlayerhasabetter

crystallineorientation(002)thanthatoftheZnOseedlayer,which

coincideswiththosereportedinourpreviouswork[18].Fig.4(a)

showsthetransmittancespectraoftheZnOandAZOseedlayers

aswellasthecorrespondingZnONWarraysgrownontheseseed

layers.ResultsindicatethatalltheZnOandAZOthinfilmsexhibita

transmittanceofhigherthan85%inthevisibleregion.However,it

isworthytonotethatthetransmittanceoftheAZOthinfilmlayer

isobviouslyhigher(90%)thanthatoftheZnOthinfilmlayer,which

isprobablyrelatedtotheoptimizedcrystallineorientationofthe

(002)andtheseedlayerwithsmallergrainsize.Furthermore,the

transmittanceoftheZnONWarrays,whicharegrownontheZnO

2 Theta / degree

ZnO AZO (002)

Fig 3. XRD patterns of the ZnO and AZO seed layers.

orAZOseedlayerat80◦Cfor1h,isstillabove40%inthevisible region.Inaddition,thetransmittanceoftheZnONWarraysgrown

ontheAZOseedlayerislowerthanthatgrownonZnOseedlayer owingtoitshighlightscatteringanddecreaselighttransmittance

[18].Fig.4(b)showstheabsorptionspectraoftheobtainedsamples

ItisfoundthattheabsorptionpeakoftheAZOthinfilmlayerhasa blueshiftascomparedtothatoftheZnOthinfilmlayerduetothe Burstein–Mosseffect[21,22]

30 40 50 60 70 80 90

100

Wavelength (nm)

1

2

3

4

5

0.0 0.1 0.2 0.3 0.4 0.5

0.6

ZnO AZO

Wavelength / nm

Fig 4.(a) Transmittance spectra of the quartz glass (curve 1), AZO thin film (curve 2), ZnO thin film (curve 3), ZnO NW arrays grown on the ZnO seed layer at 80 ◦ C for 1 h

◦

Fig 5.SEM images of the ZnO NWs grown on the ZnO and AZO seed layers: (a), (b) and (c) are SEM images of the ZnO NWs grown on the ZnO seed layer at 80 ◦ C for 1 h,

2 h and 3 h, respectively, (d), (e) and (f) are SEM images of the ZnO NWs grown on the AZO seed layer at 80 ◦ C for 1 h, 2 h and 3 h, respectively, (g) and (h) are SEM images of the ZnO NWs grown on the ZnO seed layer for 1 h at 130 and 180◦C, respectively, (i) and (j) are SEM images of the ZnO NWs grown on the AZO seed layer for 1 h at 130 and

180◦C, respectively.

Fig.5showstheSEMimagesoftheZnONWarraysgrownon

theZnOandAZOseedlayerat80–180◦Cfor1–3h,respectively

TheinsetsasshowninFig.5arethecross-sectionoftheZnONWs

arrays.Fig.6shows thattheTEMimagesand theselectedarea

electrondiffraction(SAED)patternoftheZnONWswhichgrown

ontheAZOseedlayerat80◦Cfor1h.Fig.6(a)isatypical low-magnificationimageofthesynthesizedZnONW.Thediameterof thetipisslightlysmallerthanthebottom.Theatomicarrangements

10138 J Zhang et al / Applied Surface Science257 (2011) 10134– 10140

Fig 6.TEM images of the ZnO NWs grown on the AZO seed layer (a) low-magnification image, (b) high-magnification image, (c) corresponding selected area electron diffraction pattern (SAED).

oftheZnONWareseeninFig.6(b).ItclearlyshowstheZnO(002)

fringesperpendiculartothewireaxisareonaverageseparatedby

0.26nm,indicatingthecrystallineZnONWsgrowthalongtheZnO

(002)direction.Also,thediffractionpatternconfirmsthattheZnO

NWshaveasinglecrystallinegrowthalongZnO(002)asshown

inFig.6(c).Inaddition,thesimilarresultsarealsoobservedforthe

ZnONWsgrownontheZnOseedlayerat80◦Cfor1h.Thevalues

ofthelengthanddiameteroftheZnONWs,whichisobtainedfrom

Fig.5aresummarizedinFig.7.Correspondingtothedifferentseed

layers(ZnO,AZO),theZnONWsgrownontheZnOseedlayerare

labeledasZnO-NWZ,andtheZnONWsgrownontheAZOseed

layerarelabeledasZnO-NWA.ItcanbeseenfromFig.5thatallthe

ZnONWarraysobtainedunderdifferenthydrothermalconditions

areverticallyalignedontheseedlayers.Forthehydrothermal

tem-peratureat80◦Candthegrowthtimebetween1and3h,thelength

anddiameteroftheZnO-NWZandtheZnO-NWAincreasewitha

lengtheningofthegrowthtimeasshowninFig.7.However,itis

alsointerestingtonoteforthesamegrowthtimethatthelengthof

theZnO-NWAismuchlongerthatoftheZnO-NWZandthe

diam-eteroftheZnO-NWAismuchsmallerthanthatoftheZnO-NWZ

Theseresultsareprobablyrelatedtothecrystalgrainsizeofthe

seedlayer,thatistosay,thebiggerthecrystalgrainsizeis,the

shorterandwiderthegrownZnONWisasshowninFig.5(a)–(f)

Inotherwords,theaspectratiooftheZnO-NWAishigherthanthat

oftheZnO-NWZ.Moreover,thedistanceamongtheZnO-NWAis

biggerthanthatamongtheZnO-NWZ.Thatisbecausethedistance

amongtheZnONWsisalsodependentonthecrystalinterspaces

andthecrystalgrainsizeoftheseedlayer.AscanbeseeninFig.2,

thequantityofthecrystalgrainswithintheunitareaoftheAZO

seedlayerismorethanthatoftheZnOseedlayer,whichleadstoa largernumberoftheZnONWscanbegrownontheAZOseedlayer

asshowninFig.5.Furthermore,itisalsoobservedforthesame growthtime(1h)butdifferenthydrothermaltemperaturesthatthe lengthoftheZnO-NWZincreasesandthediameteroftheZnO-NWZ enlargeextremelywiththeincreaseofthehydrothermal tempera-tureascomparedtothatoftheZnO-NWA.Areasonableexplanation

isthatthesmallcrystalgrainoftheAZOseedlayerrestrictsthe cross-growthoftheZnONWs.ThelengthoftheZnO-NWAalso increaseswiththeraisingofthehydrothermaltemperaturefrom

80◦Cto130◦C,butwhenthehydrothermaltemperatureisfurther

upto180◦C,thelengthoftheZnO-NWAisshorterthanthatofthe ZnO-NWAgrownat130◦C.Apossibleexplanationisasfollows.As reportedinRefs.[23,24],thehydrothermalsynthesisoftheZnO NWsisadynamicbalanceprocessasfollows

[Zn(OH)n]n−2−→ZnO+H2O+[OH]−,(n=2,4) (1) ZnO+2[OH]−→[ZnO2]2−+H2O (2) Thus,the[Zn(OH)n]n−2−groupsdehydrateatthesurfaceofthe ZnO seedlayertoformtheZnO molecules,H2Omoleculesand [OH]−,andtheformed[OH]−dissolvestheZnOmoleculestoform [ZnO2]2− groupsatthesametime.Whilethesupersaturationof the[Zn(OH)n]n−2−groupsishighenough,thegrowthrateofthe ZnONWswillbemuchhigherthanthedissolutionrate.Whilethe hydrothermal temperatureincreasedto130◦C,the supersatura-tionandthemoleculeenergyofthe[Zn(OH)n]n−2−groupsachieve the best values, which leads to a fast growth rate of the ZnO NWs.Whilethehydrothermaltemperaturefurtherincreasesup

1 2

3

Length of the ZnO NWs / µm

80 100 120 140 160 180

0 50 100 150 200 250 300 350 1

2 3

Diameter of the ZnO NWs / nm

80 100 120 140 160 180

Fig 7. Effect of the hydrothermal growth time and temperature on the length and diameter of the ZnO NWs: (a) a relationship between the length of the ZnO NWs and the

canbeseenthatwiththeincreasethehydrothermaltemperature,

thepositionofthepeaksoccurredred-shiftandtheintensityofthe

peaksdecreasesgradually.Itisprobablyrelatedtoanincreaseof

thecrystaldefectsintheZnONWarraysduetohighergrowth

tem-perature[25–27].WhentheZnONWsgrowat80◦C,asharpand

strongUVpeakat380nm,whichcanbeassignedtotheintrinsic

excitationofZnO,dominatesthePLspectraandnootherpeaksare

observedinthespectrumcurve,indicatingthatfewcrystaldefects

existintheZnONWarraysgrownontheAZOseedlayerat80◦C

for1h Itshouldbementioned herethatthesimilarresultsare

alsoobservedfortheZnONWarraysgrownontheAZOseedlayer

at80◦Cforlongergrowthtime.However,withtheincreasethe

hydrothermaltemperatureto130◦Corabove,arelativeweakband

for 1 h at 80, 130 and 180 ◦ C, respectively.

between390and420nmcanbeclearlyobservedasshowninthe insetofFig.7,indicatingthatsomecrystaldefectsstarttooccurdue

tohighergrowthtemperature,whichmayleadtothered-shiftof thePLpeaksandthedecreaseofthePLpeaksinintensity.Thatisto say,withtheincreasethehydrothermaltemperature,the dissolu-tionrateofthe[OH]−willbeintensifiedonthesurfaceoftheZnO NWs,anditisprobablytoleadtomorecrystaldefectsintheZnO NWsduetothecorrosivenessoftheaqueoussolution

AsshowninFig.1,thesiliconsubstrateisfirstpatternedwithEB resistandEBexposal,thenthelowsurfaceenergycoatingoverthe outsideofthepatternedregionisderivedbythesurfacefluorizated processonthesiliconsubstratebyusingfluoricorganicsolvents

Fig 9.SEM images of the patterned ZnO NW arrays (a) top-view of the ZnO NW arrays, (b) line width of 1 ␮m, (c) line width of 500 nm, (d) line width of 200 nm, (e) line

10140 J Zhang et al / Applied Surface Science257 (2011) 10134– 10140

380 390 400 410 420 430 440 450

0.0

2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

1.2x106

Wavelength / nm

Wavelength / nm

Fig 10. PL spectra of the patterned ZnO NW arrays grown on the AZO seed layer at

80 ◦ C for 1 h.

Itshouldbementionedherethatthiskindofsurfacefluorination

techniqueisauniversalmethodforpatterningsol–gelthinfilms

Thus,whentheAZOsolisspin-coatedonthesiliconsubstrate,the

AZOsolcannotbedepositedonthelowsurfaceenergyregiondue

toitslow adhesion,buttheAZOsolcanbefirmlydepositedon

thosepatternedregions.Inadditionto,thisprocesscaneliminate

theeffectoftheresistontheboundaryofthepatternedAZOseed

layer.Inordertoachieveagoodphotoluminescenceproperty,the

patternedZnONWarraysareonlygrownat80◦Cfor1h.Images

ofthepatternedZnONWarraysareshowninFig.9.Itcanbeseen

thattheZnONWarrayscanbeselectivelyandsharplygrownon

thepatternedregionsoftheAZOseedlayer.Fig.9(a)showsthe

patternedZnONWarraysatalargefeaturesizearea.Fig.9(b)–(g)

showtheas-grownZnONWarraysonthepatternedregionswith

awidthof1␮m,500nm,200nm,100nm,and50nm,respectively

ItcanbeclearlyobservedfromFig.9thatverticallyalignedZnO

NWarrays canbeeasilygrownonthepatternedregions ofthe

AZOseedlayer.Theas-grownZnONWarrays showanacicular

morphology,andtheaveragelengthanddiameteroftheZnONWs

arearound1␮mand50nm,respectively.Fig.10showstheroom

temperaturePLspectrum(exciteat365nm,1nmslitwidth)ofthe

patternedZnONWarraysgrownontheAZOseedlayers.Itisfound

thatonlyasharpandstrongUVpeakat380nmdominatesthePL

spectrum.TheinsetshowsthepartialenlargedPLspectrumfrom

390to420nm,andnootherpeaksareobservedinthecurve.These

resultsindicatethattherearefewcrystaldefectstoexistinthe

patternedZnONWarraysgrownonthesol–gelderivedAZOseed

layeratthehydrothermaltemperatureof80◦Cfor1h

4 Conclusions

TheverticallyalignedZnONWarrayshavebeensuccessfully

grownontheAZOseedlayerbythehydrothermalmethod.Effects

ofthehydrothermal parametersonthemorphologicaland

pho-toluminescencepropertiesoftheZnONWarrayshavebeenalso

studied.Resultsindicatethatthefastestgrowthrateofthe

ZnO-NWAcanbeobtainedat130◦CandtheZnO-NWAhasthehigher

aspectratiothanthatoftheZnO-NWZduetotheeffectofthe

Al-dopingontheseedlayer.Furthermore,thepatternedZnO-NWA

arrayswithstrongPLemissionandfewcrystaldefectshavebeen

obtainedbycombiningthesol–gelprocesswiththeelectron-beam

lithographyprocess,aswellasthesurfacefluorinationtechnique,

whichisprobablysuitablefortheapplicationsintheluminescent

andelectronfieldemissiondevices

Acknowledgments

ThisworkwassupportedbytheMinistryofScienceand

Tech-nologyofChinathrough863-projectundergrant2009AA03Z218,

theMajorProgramoftheNationalNaturalScienceFoundationof Chinaundergrantno.90923012,andXi’anAppliedMaterials Inno-vationFund(XA-AM-200909)

Appendix A Supplementary data

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.apsusc.2011.06.163

References

[1] Z.W Pan, Z.R Dai, Z.L Wang, Nanobelts of semiconducting oxides , Science 291 (2001) 1947–1949.

[2] M.H Huang, Y.Y Wu, H Feick, N Tran, E Weber, P Yang, Catalytic growth

of zinc oxide nanowires by vapor transport , Adv Mater (Weinheim Ger.) 13 (2001) 113–116.

[3] Y Liu, Z.H Chen, Z.H Kang, I Bello, X Fan, I Shafiq, W.J Zhang, S.T Lee,

In situ self-catalytic synthesis of ZnO nanostructures in air: tetrapods, nano-tetraspikes and nanowires , J Phys Chem C 112 (2008) 9214–9218 [4] P.D Yang, H.Q Yan, S Mao, R Russo, J Johnson, R Saykally, N Morris, J Pham, R.R He, H.J Choi, Controlled growth of ZnO nanowires and their optical prop-erties , Adv Funct Mater 12 (2002) 323–331.

[5] C Soci, A Zhang, B Xiang, S.A Dayeh, D.P.R Aplin, J Park, X.Y Bao, Y.H Lo, D Wang, ZnO nanowire UV photodetectors with high internal gain , Nano Lett 7 (2007) 1003–1009.

[6] Y.K Tseng, C.J Huang, H.M Cheng, I.N Lin, K.S Liu, I.C Chen, Characteriza-tion and field-emission properties of needle-like zinc oxide nanowires grown vertically on conductive zinc oxide films , Adv Funct Mater 13 (2003) 811–814 [7] M Law, L.E Greene, J.C Johnson, R Saykally, P.D Yang, Nanowire dye-sensitized solar cells , Nat Mater 4 (2005) 455–459.

[8] X.D Wang, J.H Song, P Li, J.H Ryou, R.D Dupuis, C.J Summers, Z.L Wang, Growth of uniformly aligned ZnO nanowire heterojunction arrays on GaN, AlN, and Al 0.5 Ga 0.5 N substrates , J Am Chem Soc 127 (2005) 7920–7923 [9] H.J Fan, W Lee, R Hauschild, M Alexe, G.L Rhun, R Scholz, A Dadgar, K Nielsch,

H Kalt, A Krost, M Zacharias, U Gösele, Template-assisted large-scale ordered arrays of ZnO pillars for optical and piezoelectric applications , Small 2 (2006) 561–568.

[10] J.S Jie, G.Z Wang, Y.M Chen, X.H Han, Q.T Wang, B Xu, J.G Hou, Synthesis and optical properties of well-aligned ZnO nanorod array on an undoped ZnO film , Appl Phys Lett 86 (2005) 031909.

[11] Z.Q Xu, H Deng, Y Li, Q.H Guo, Y.R Li, Characteristics of Al-doped c-axis ori-entation ZnO thin films prepared by the sol–gel method , Mater Res Bull 41 (2006) 354–358.

[12] W Tang, D.C Cameron, Aluminium-doped zinc oxide transparent conductors deposited by the sol–gel process , Thin Solid Films 238 (1994) 83–87 [13] A Ohtani, K.S Stevens, R Beresford, Microstructure and photoluminescence

of GaN grown on Si(1 1 1) by plasma–assisted molecular beam epitaxy , Appl Phys Lett 65 (1994) 61.

[14] T Lei, T.D Moustakas, R.J Graham, Y He, S.J Berkowitz, Epitaxial growth and characterization of zinc–blende gallium nitride on (0 0 1) silicon , J Appl Phys.

71 (1992) 4933–4943.

[15] P.F Carcia, R.S McLean, M.H Reilly, G Nnues, Transparent ZnO thin-film tran-sistor fabricated by rf magnetron sputtering , Appl Phys Lett 82 (2003) 1117 [16] T.F Chung, J.A Zapien, S.T Lee, Luminescent properties of ZnO nanorod arrays grown on Al: ZnO buffer layer , J Phys Chem C 112 (2008) 820–824 [17] Z.H Chen, Y.B Tang, Y Liu, G.D Yuan, W.F Zhang, J.A Zapien, I Bello, W.J Zhang, C.S Lee, S.T Lee, ZnO nanowire arrays grown on Al:ZnO buffer lay-ers and their enhanced electron field emission, J , Appl Phys 106 (2009) 064303/1–164303/6.

[18] J Zhang, W.X Que, Preparation and characterization of sol–gel Al-doped ZnO thin films and ZnO nanowire arrays grown on Al-doped ZnO seed layer by hydrothermal method , Sol Energy Mater Sol Cells 94 (2010) 2181–2186 [19] Y.S Kim, W.P Tai, Electrical and optical properties of Al-doped ZnO thin films

by sol–gel process , Appl Surf Sci 253 (2007) 4911–4916.

[20] X.J Feng, K Shankar, O.K Varghese, M Paulose, T.J Latempa, C.A Grimes, Verti-cally aligned single crystal TiO 2 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications , Nano Lett.

11 (2008) 3781–3786.

[21] E Burstein, Anomalous optical absorption limit in InSb , Phys Rev 93 (1954) 632–633.

[22] T.S Moss, The Interpretation of the properties of indium antimonide , Proc Phys Soc Lond B 67 (1954) 775–782.

[23] K Yu, Z.G Jin, X.X Liu, J Zhao, J.Y Feng, Shape alterations of ZnO nanocrys-tal arrays fabricated from NH 3 H 2 O solutions , Appl Surf Sci 253 (2007) 4072–4078.

[24] X.L Hu, Y Masuda, T Ohji, K Kato, Micropatterning of ZnO nanoarrays by forced hydrolysis of anhydrous zinc acetate , Langmuir 24 (2008) 7614–7617 [25] H.H Guo, J.Z Zhou, Z.H Lin, ZnO nanorod light-emitting diodes fabricated by electrochemical approaches , Electrochem Commun 10 (2008) 146–150 [26] C.C Yang, S.Y Cheng, H.Y Lee, S.Y Chen, Effects of phase transformation on photoluminescence behavior of ZnO:Eu prepared in different solvents , Ceram Int 32 (2006) 37–41.

[27] F.H Zhao, W.J Lin, M.M Wu, N.S Xu, X.F Yang, Z.R Tian, Q Su, Hexagonal and