Effects of Annealing Environments on the Solution-Grown, Aligned Aluminium-Doped Zinc Oxide ...

16 Pages·2014·6.23 MB·English

HindawiPublishingCorporation
JournalofNanomaterials
Volume2012,ArticleID189279,15pages
doi:10.1155/2012/189279
Research Article
Effects of Annealing Environments on the Solution-Grown,
Aligned Aluminium-Doped Zinc Oxide Nanorod-Array-Based
Ultraviolet Photoconductive Sensor
MohamadHafizMamat,1MohdIzzudinCheKhalin,1
NikNoorHafizahNikMohammad,2ZuraidaKhusaimi,2
NorDiyanaMdSin,1ShafinazSobihanaShariffudin,1
MusaMohamedZahidi,1andMohamadRusopMahmood1,2
1NANO-ElecTronicCentre(NET),FacultyofElectricalEngineering,UniversitiTeknologiMARA(UiTM),
40450ShahAlam,Selangor,Malaysia
2NANO-SciTechCentre(NST),InstituteofScience(IOS),UniversitiTeknologiMARA(UiTM),40450ShahAlam,Selangor,Malaysia
CorrespondenceshouldbeaddressedtoMohamadHafizMamat,hafiz [email protected]
Received8June2012;Revised24August2012;Accepted29August2012
AcademicEditor:RaymondL.D.Whitby
Copyright©2012MohamadHafizMamatetal. This is an open access article distributed under the Creative Commons
AttributionLicense,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkis
properlycited.
Wehavefabricatedmetal-semiconductor-metal-(MSM-)typeultraviolet(UV)photoconductivesensorsusingaluminium-(Al-)
dopedzincoxide(ZnO)nanorodarraysthatwereannealedindifferentenvironments:air,oxygen,oravacuum.TheAl-doped
ZnOnanorodshadanaveragediameterof60nmwithathicknessofapproximately600nmthatincludedtheseedlayer(with
thickness∼200nm).Ourresultsshowthatthevacuum-annealednanorod-array-basedUVphotoconductivesensorhasthehighest
photocurrentvalueof 2.43 × 10−4A.Thehighphotocurrentisduetothehighconcentrationofzinc(Zn)interstitialsinthe
vacuum-annealed nanorod arrays. In contrast, the oxygen-annealing process applied to the Al-doped ZnO nanorod arrays
producedhighlysensitiveUVphotoconductivesensors,inwhichthesensitivityreached55.6,duetothesurfacepropertiesofthe
oxygen-annealednanorods,whichhaveahigheraffinityforoxygenadsorptionthantheothersamplesandweretherebycapableof
reducingthesensor’sdarkcurrent.Inaddition,thesensorfabricatedusingtheoxygen-annealednanorodarrayshadthelowestrise
anddecaytimeconstants.Ourresultshowsthattheannealingenvironmentgreatlyaffectsthesurfaceconditionandpropertiesof
theAl-dopedZnOnanorodarrays,whichinfluencestheperformanceoftheUVphotoconductivesensors.
1.Introduction photocurrent and are highly sensitive to UV light [10, 12].
The use of ZnO nanorod arrays in UV photoconductive
Recently,zincoxide(ZnO)nanostructureshavebeenwidely sensorsisattractiveforpracticalapplicationsbecauseofthe
studied because of their potential for various applications, diversity of fabrication methods for these nanorods, their
includingsolarcells[1–3],lightemittingdevices[4],electron largesurfaceavailability,andexcellentphysicalandchemical
emitters [5, 6], photocatalysts [7], and sensors [8, 9]. properties. In addition, the high surface-to-volume ratio
The performance of devices containing one-dimensional of ZnO nanorod arrays produces better sensitivity to UV
(1D) ZnO nanostructures, such as nanobelts and nanorod lightcomparedtoconventionalZnOthinfilmsbecausethe
arrays, has significantly improved because of the unique photoconductivitymechanisminZnOisstronglyrelatedto
properties of these nanostructures, such as high mobility thesurfacereactionactivities[13].ZnOnanorodarrayscan
and a high surface-to-volume ratio [10, 11]. ZnO nanorod be successfullygrown on substrates using various methods,
arrays are very promising materials for use as sensing such as chemical vapour deposition (CVD) [14], metal-
elementsinultraviolet(UV)photoconductivesensorappli- organicchemicalvapourdeposition(MOCVD)[15],pulsed
cationsbecausethesearraysexhibitafastresponseandhigh laserdeposition (PLD) [16],and solution-based techniques
2 JournalofNanomaterials
[17–20]. However, solution-based techniques offer many glass substrate using spin-coating technique. The prepared
advantagesoverothersynthesismethodscapableofgrowing solution was dropped onto glass substrates while the sub-
ZnOnanorodarraysatlowtemperaturewithlowcost. strateswerespunat3,000rpmfor60s.Afterthespin-coating
Generally, fabricated ZnO nanorod arrays, especially process, the substrates were heated at 150◦C for 10min to
those prepared using solution-based techniques, contain a evaporate the solvent and remove the organic component
considerablenumberofdefects,resultinginalowresponseto fromthefilm.Thecoatingwasrepeatedfivetimestoincrease
UVlight.BecausetheUVphotoconductivityisgovernedby the film thickness to approximately 200nm. The prepared
thesurfaceconditionsofthenanorodarrays,effectivefabri- filmsthenwereannealedinairusingafurnaceat500◦Cfor
cationmethodsshouldbedevelopedtoreducethenumberof 1h.
defectsandimprovethesurfacequalityofthepreparedZnO The solution for preparing the aligned Al-doped ZnO
nanorod arrays. A few approaches have been proposed that nanorod arrays contained 0.1M zinc nitrate hexahydrate
canenhancetheperformanceoftheZnO-basedUVphoto- (Zn(NO ) ·6H O, 98%, Systerm) as a precursor, 0.1M
3 2 2
conductive sensor by improving the ZnO surface condition hexamethylenetetramine(HMT,C H N ,99%,Aldrich)as
6 12 4
andreducingthedefects.Theseapproachesincludetheuseof a stabiliser, and 0.001M aluminium nitrate nonahydrate
polymerandmetalsurfacecoatings[13,21–23]andplasma (Al(NO ) ·9H O, 98%, Analar) as a dopant, which were
3 3 2
and chemical surface treatments [24–28]. These techniques dissolved in deionised (DI) water. The solution was then
facilitate efficient photogeneration processes in the ZnO sonicatedat50◦Cfor30minusinganultrasonicwaterbath
under UV illumination, thereby contributing to improved (HwashinTechnologyPowersonic405,40kHz)beforebeing
performance of the device, such as a high and stable pho- agedandstirredfor3hatroomtemperature.
tocurrent, high sensitivity, and a fast photoresponse. How- TodepositthealignedAl-dopedZnOnanorods,approx-
ever,annealingisalsoaZnOsurfacetreatmentprocessthat imately 100mL of the prepared solution was poured into
could remove surface defects and contamination, thereby a vessel that contained the horizontal glass substrates that
improvingthesurfaceconditionofZnO[29–31].Although were coated with the seeded catalyst at the bottom. The
theannealingtemperatureiswidelyreportedtoinfluencethe sealed vessel was immersed into a water bath deposition
performanceoftheUVphotoconductivesensor[32–35],the systemat95◦Cfor50min.Afterthedepositionprocess,the
effectsofannealingunderdifferentenvironmentshaverarely sampleswereremovedfromthevesselandcleanedwithDI
beenreportedintermsoftheperformanceofZnOnanorod- water. The resulting Al-doped ZnO nanorod arrays were
array-basedUVphotoconductivesensors. heated at 150◦C for 10min to evaporate the water. Next,
Herein, we reported on the performance of a UV pho- the samples were annealed at 500◦C in an oxygen, air, or
toconductive sensor developed using thin Al-doped ZnO vacuumenvironmentfor1husingafurnace.UndopedZnO
nanorodarrayspreparedusingthesonicatedsol-gelimmer- nanorodarrayswerealsopreparedusingthesameprocedure
sion method and annealed in different environments. The and were annealed in air at 500◦C for 1h. In addition, the
nanorod arrays were prepared in a very short immersion Al-doped ZnO nanorod arrays were prepared at different
time, within 50min, using sonicated sol-gel immersion lengths of immersion time (i.e., 50, 100, and 150min) and
method,whichyieldsmallandthinnanorodarrayswithan wereannealedinairat500◦Cfor1h.Tocompletethemetal-
average diameter of 60nm and thickness of approximately semiconductor-metal-(MSM-)typesensorstructure,60nm
600nm.Weobservedthatannealingunderdifferentenviron- thick Al-metal contacts were deposited onto the prepared
ments significantly influenced the surface conditions of the nanorod arrays using a thermal evaporator at a deposition
nanorodarrays,therebyaffectingtheperformanceoftheUV pressureof4×10−4Pa.Thedistancebetweenthetwometal
photoconductivesensor. contactswas2mm.
The prepared nanorod arrays were characterised using
2.ExperimentalProcedure field-emissionscanningelectronmicroscopy(FESEM,JEOL
JSM-7600F), which was used to observe their surface mor-
Al-doped ZnO nanorod arrays were grown on a glass sub- phology and the cross-section images. The crystallinity of
stratethatwascoatedwithanAl-dopedZnOseedlayerusing thesampleswascharacterisedusingX-raydiffraction(XRD,
sonicated sol-gel immersion. The Al-doped ZnO thin film, Panalytical X(cid:4)pert PRO). The structural and selected area
whichwasusedasaseededcatalyst,waspreparedontheglass electron diffraction (SAED) of the air-annealed undoped
substrateusingsol-gelspin-coating[36].Thesolutionforthe and Al-doped ZnO nanorod arrays was investigated using
seed layer was prepared using 0.4M zinc acetate dihydrate transmission electron microscopy (TEM; JEOL JEM 3010).
(Zn(CH COO) ·2H O,99.5%purity,Merck)asaprecursor, Thetransmittanceandabsorbancepropertiesofthenanorod
3 2 2
0.4M monoethanolamine (MEA, H NCH CH OH, 99.5% arrays were characterised using ultraviolet-visible-near-
2 2 2
purity, Aldrich) as a stabiliser, 0.004M aluminium nitrate infrared (UV-vis-NIR) spectrophotometry (Perkin Elmer
nonahydrate (Al(NO ) ·9H O, 98% purity, Analar) as a Lambda750).Thephotoluminescencepropertiesofthepre-
3 3 2
doping source (to achieve approximately 1 at.% Al-doped paredsampleswerecharacterisedusingaphotoluminescence
ZnO),and2-methoxyethanolasasolvent.Thesolutionwas (PL)spectrometer(HoribaJobinYvon79DU420A-OE-325)
then heated at 80◦C and stirred using a hot plate magnetic equipped with a helium-cadmium (He-Cd) laser excitation
stirrer for 3h to yield a clear, homogeneous solution. The source at 325nm. The current-voltage (I-V) characteristics
solution was further stirred at room temperature for 24h of the UV photoconductive sensors were investigated using
for the ageing process. The seed layer was deposited onto a two-probe I-V measurement system (Keithley 2400). The
JournalofNanomaterials 3
UV photoresponse measurements of the fabricated sensor the XRD pattern of the vacuum-annealed ZnO nanorod
were conducted using a UV photocurrent measurement array presents the weakest peak intensity compared to the
system (Keithley 2400) operating at 365nm with a power other annealed samples. In addition, the XRD pattern of
densityof750μW/cm2atabiasvoltageof10V. the vacuum-annealed nanorod arrays shows the emergence
of Zn metal peaks. This condition could be attributed to
3.ResultsandDiscussion the desorption of OH groups from the ZnO nanorod array
duringannealing,whichleavesalargeamountofZnorZn
Figures 1(a)–1(d) present the FESEM images of the interstitials in the nanorod. Because of the unavailability
as-grown, oxygen-annealed, air-annealed, and vacuum- of oxygen molecules, the oxidation of Zn could not occur
annealedAl-dopedZnOnanorodarrayspreparedusingthe during the annealing process in the vacuum. As a result,
sonicated sol-gel immersion method. These images reveal the growth of ZnO was inhibited while maintaining a high
thatthenanorodswereuniformlydepositedontheglasssub- concentrationofZnorZninterstitials,therebydegradingthe
stratecoatedwiththeseedlayerafterimmersionfor50min. crystallinityoftheAl-dopedZnOnanorodarrays.
The FESEM images also reveal that the morphology of the Figure4(a) shows the transmittance spectra of the Al-
Al-doped ZnO nanorod arrays did not exhibit a significant doped ZnO nanorod arrays prepared at different annealing
change after annealing in different environments. Based on environmentsinthewavelengthrangesfrom300to1500nm.
the FESEM images, the average diameter of the Al-doped These spectra demonstrate that the nanorod arrays have
ZnOnanorodisapproximately60nm.AccordingtotheEDS goodopticaltransmittanceinthevisible(400–800nm)and
spectrumofas-grownAl-dopedZnOnanorodarraysshown near-infraredranges(800–1500nm).Thespectraalsoreveal
inFigure1(e),thepeaksofZn,Al,andOcorrespondtoan thatupontheannealingprocessintheoxygenenvironment,
atomic ratio of 49.21:0.64:50.16. Figures 2(a)–2(d) show thetransmittanceofthenanorodarraysimprovesrelativeto
the cross-sectional images of the Al-doped ZnO nanorod theas-grownsample.Notethattheaveragetransmittanceof
arrays that were annealed under different environments. the as-grown Al-doped ZnO nanorod arrays in the visible
These images indicated that the Al-doped ZnO nanorods region is approximately 72%. The average transmittance
were deposited perpendicular to the substrate with good of the oxygen-annealed Al-doped ZnO nanorod arrays in
alignment. The thicknesses of these films were estimated the visible region was estimated to be 78%. However, the
fromtheFESEMimagestobeapproximately600nm. transmittance slightly decreased after annealing in air and
Figure3 presents the XRD patterns of the as-grown, vacuum,withaveragetransmittancesof77and76%,respec-
oxygen-annealed, air-annealed, and vacuum-annealed Al- tively. We also observed that interference fringes appeared
doped ZnO nanorod arrays. The nanorod arrays present in the spectra, which indicated that the nanorod arrays
diffractionpeaksthatcanbeindexedtotheZnOhexagonal are uniformly deposited on the substrate [40]. Figure4(b)
phasewithawurtzitestructure(JointCommitteeonPowder showstheabsorbancespectraoftheAl-dopedZnOnanorod
DiffractionStandards(JCPDS)PDFnumber36-1451).The arrayspreparedusingdifferentannealingenvironments.The
XRDpatternsrevealthatthe(002)peakofthenanorodisthe absorbancespectrarevealthatallofthesamplesexhibitgood
most prevalent, which indicates that the nanorod preferen- UVabsorptionpropertiesbelow400nmwithanabsorption
tiallygrowsalongthec-axis(perpendiculartothesubstrate). edge of 380nm. The sharp absorption edge observed at
The other weak diffraction peaks in the XRD patterns may approximately 380nm corresponds to the direct transition
bearesultofimperfectnanorodalignmentonthesubstrate of electrons between the edges of the valence band and the
[37]. According to this result, the relative intensities of the conductionband.ThehighabsorptionpropertiesintheUV
XRDpeakfromthe(002)planeareoxygen-annealed>air- regionindicatethatthepreparedsamplesaresuitableforUV
annealed > vacuum-annealed > as-grown. This ordering photoconductivesensorapplications.
indicatesthatoxygenisabetterannealingenvironmentthan Figure5 shows the PL spectra of the as-grown, oxygen-
theotherannealingenvironments(i.e.,airandvacuum)for annealed, air-annealed, and vacuum-annealed Al-doped
improving the crystallinity of the Al-doped ZnO nanorod ZnO nanorod arrays. Two emission peaks were detected in
arrays.Annealinginoxygenpromotesthereactionbetween thePLmeasurement,locatedintheUVregionandthevisible
oxygen and the Zn interstitials to form crystalline ZnO. region. The UV emission is due to the recombination of
Similarly, this process also occurred during annealing in excitons,whereasthevisibleemissioniscausedbytheemis-
air. However, due to the higher concentration of oxygen sionofdefects[41–43].Theas-grownnanorodarrayshavea
molecules in the oxygen-annealing process relative to air broadandlow-intensityUVemissionpeakcentredat383nm
annealing, the oxidation rate in oxygen annealing is much andahigh-intensityvisibleemissionpeakcentredat573nm.
fasterduetothegreaterreductioninthefree-surfaceenergy. This visible peak is due to the high concentration of OH-
Therefore,thecrystallinityoftheoxygen-annealednanorod groupsonthenanorodsurface[44].Fortheoxygen-annealed
arraysisbetterthanthatoftheair-annealednanorodarrays. nanorod arrays, the intensity of the UV emission peak that
The as-grown sample presented low-intensity XRD peaks, is centred at 379nm increased relative to that of the as-
which were attributed to the high density of defects in the grown nanorod arrays. However, the visible region exhibits
nanorod arrays, particularly the presence of hydroxyl (OH) a reduced intensity and shift to a higher wavelength. This
groupsonthenanorodsurface.ThepresenceofOHgroups visible emission peak was shifted to 630nm and attributed
onthesurfaceofthenanorodiscommonlyreportedforlow- tointerstitialoxygenorexcessoxygenatthenanorodsurface
temperaturesolution-basedsyntheses[29,38,39].However, [39, 45]. The oxygen adsorption in the nanorod arrays
4 JournalofNanomaterials
(a) (b)
(c) (d)
Zn Spectrum 2
Element App. Intensity Weigh... Weigh... Atomic (%) Comp... Formula Number
conc. corrn. sigma of ions
Al K 0.03 0.5226 0.42 0.11 0.64 0.8 Al2O3 0.1
Zn L 7.81 0.8073 79.69 0.27 49.21 99.2 ZnO 7.85
O 19.88 0.26 50.16 8
Totals 100
Cation... 7.95
O
Zn
Al Zn
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Full scale 16509 cts Cursor: 3.810 (137 cts) (keV)
(e)
Figure1:Surfacemorphologyof(a)as-grown,(b)oxygen-annealed,(c)air-annealed,and(d)vacuum-annealedAl-dopedZnOnanorod
arrays.(e)EDSspectrumofas-grownAl-dopedZnOnanorodarrays.
also decreases the excitonic recombination, as reported 588nm, respectively. We suspected that this condition is
by other groups [39, 46]. However, the air-annealed and duetodifferentdefectconditions,whichaffectedthevisible
vacuum-annealednanorodarraysshowalmostidenticalUV emissionofthenanorodarrays.Fortheair-annealedsample,
emissionpeakintensityofthepeaksthatarecentredat379 the defect state may be dominated by oxygen adsorption,
and 380nm, respectively, but slightly different intensities but with lower concentration compared to the oxygen-
in the visible emission peak. The visible emission peak annealednanorodarrays,whereasforthevacuum-annealed
intensity for the air-annealed nanorod arrays was slightly sample,thezincinterstitialsandoxygenvacanciesmaybethe
higherthanthatofthevacuum-annealednanorodarrays.In prominentdefectsinthenanorodarrays.
addition, these visible emission peak centres for the air- Figure6 presents the I-V measurement spectra of the
annealed and vacuum-annealed Al-doped ZnO nanorod Al-doped ZnO nanorod arrays annealed in different envi-
arrays are located at different wavelengths of 600nm and ronments. The I-V curves indicate that all of the prepared
JournalofNanomaterials 5
(a) (b)
(c) (d)
Figure2:Cross-sectionalimagesof(a)as-grown,(b)oxygen-annealed,(c)air-annealed,and(d)vacuum-annealedAl-dopedZnOnanorod
arrays.
cps) 567...565758 (100(0)0(21)01) Zn(101) (102)Zn(102) (103) Vanacnueualmed- odtahnxoeynpgeeheadnlieg-dZhanenssnaOtmeacplnueladernr,,eofnoariotlrldo-vawnaaelnurdereaabylyeissd.th,eAeaxcnahcidiorb-riadvtneaindncgueaubltmyeod-tatshnhaemnisevpaasllepec,duetcuhtAmrealn--,
30 5 bytheas-grownsample,andfinally,bytheoxygen-annealed
×1 4.5 sample. From these results, the resistance of the samples
nsity ( 3.354 annAeira-led wtheereas-cgarlcouwlant,eodxytogenb-ean0n.8e0a,le1d.,5a9i,r-0a.n1n6e,aalendd, a0n.1d0vMacΩuumfor-
Inte 2.25 annealed Al-doped ZnO nanorod arrays, respectively. The
1.5 Oxygen- lowest resistance of the vacuum-annealed nanorod arrays
1 annealed
0.5 might be a result of the excess of free carriers due to the
As-grown
0 Zn interstitials and Al-doping. In addition, annealing in
20 25 30 35 40 45 50 55 60 65 70
vacuumalsoremovedtheadsorbedoxygenonthenanorod
2θ(◦)
surface,whichdecreasedtheresistanceofthenanorodarrays
Figure 3: XRD patterns of the as-grown, oxygen-annealed, air- [47]. However, annealing in oxygen significantly increased
annealed,andvacuum-annealedAl-dopedZnOnanorodarrays. the resistance of the nanorod arrays, which is likely due
to the large amount of oxygen adsorbed on the surface
of the nanorod arrays. During the annealing process in
Al-dopedZnOnanorodarraysformedohmiccontactswith oxygen, oxygen could also diffuse into the nanorod arrays,
the Al contacts. However, the current with respect to the whichinducesahighdensityofacceptor-likedefects(i.e.,Zn
supplied voltage shows different values for the as-grown, vacancies, oxygen antisites) [48]. This condition results in
6 JournalofNanomaterials
100
4
80
%) u.)3
nce ( 60 e (a.
mitta banc2
ans 40 bsor
Tr A
1
20
0 0
400 600 800 1000 1200 1400 400 600 800 1000 1200 1400
Wavelength (nm) Wavelength (nm)
As-grown Air-annealed As-grown Air-annealed
Oxygen-annealed Vacuum-annealed Oxygen-annealed Vacuum-annealed
(a) (b)
Figure4:(a)Transmittanceand(b)absorbancespectraoftheas-grownandannealedAl-dopedZnOnanorodarrays.
120
100
Vacuum- A)
annealed μ 80
nt ( 60
y (a.u.) annAeira-led Curre 2400
nsit Oxygen-
e
Int annealed −10 −8 −6 −4 −2−20 2 4 6 8 10
Voltage (V)
−40
−60
As-grown −80
−100
350 400 450 500 550 600 650 700 750
Wavelength (nm) −120
Figure5:RoomtemperaturePLspectraoftheas-grown,oxygen-
As-grown Air-annealed
annealed, air-annealed, and vacuum-annealed Al-doped ZnO
Oxygen-annealed Vacuum-annealed
nanorodarrays.
Figure 6: Current-voltage (I-V) plots of the as-grown, oxygen-
annealed, air-annealed, and vacuum-annealed Al-doped ZnO
nanorodarrays.
the compensation of donor-like defects (i.e., oxygen vacan-
cies, Zn interstitials) and depletion of the carrier density.
Meanwhile, annealing the nanorod arrays in air produced
thenanorodarrayswithamoderateresistance,inwhichthe UV light with an optical power density of 750μW/cm2
resistance value was between that of the vacuum-annealed at a bias voltage of 10V. Under UV illumination, the
and oxygen-annealed nanorod arrays. This phenomenon is Al-doped ZnO nanorod-array-based UV photoconductive
due to the decreased concentration of oxygen molecules sensor showed good photoresponses; the current rapidly
in air compared to the concentration of oxygen molecules increasedandthengraduallybecamesaturated.Thecurrent
availableduringtheoxygenannealingprocess,whichreduces suddenly decreased when the UV illumination was turned
the amount of adsorbed oxygen. The as-grown sample off,eventuallyreturningtoitsinitialvalue.
presented the highest resistance due to its high defect Thespectraindicatethatthephotocurrentpropertiesof
concentrationsandpoorcrystallinityproperties,asshownin the sensors provide different responses with the use of Al-
theXRDpatterns. doped ZnO nanorod arrays that were annealed in different
Figure7 depicts the time-resolved photoresponse of the environments. The spectra reveal that the UV photocon-
UV photoconductive sensor prepared using the Al-doped ductive sensor with the vacuum-annealed Al-doped ZnO
ZnO nanorod arrays that were annealed in different envi- nanorod arrays has the highest photocurrent value of
ronments.Themeasurementswereperformedusing365nm 2.43 × 10−4A. The UV photoconductive sensor that uses
JournalofNanomaterials 7
UV on UV off is low because of the low concentration of photogenerated
0.26
electronsintheconductionband.Thisconditionisaresult
0.24
of the large concentration of OH-groups on the nanorod
0.22 surface,whichhindersefficientphotogenerationduringUV
0.2
illumination. In addition, the as-grown nanorod arrays
A) 0.18 have a large number of structural defects, which is evident
m 0.16
nt ( 0.14 by the less intense XRD peaks compared to the annealed
e samples.Thisconditionreducesthemobilityofthenanorod
r
Cur 0.12 arraysandinducedtherecombinationofthephotogenerated
0.1
electrons with the defects, which decreased the number of
0.08
photogeneratedelectronsintheconductionband.
0.06
Generally, the adsorption of oxygen on the nanorod
0.04
surfacecausestheexistenceofanon-conductivebarrierora
0.02
surfacedepletionregionandthus,anupwardbandbending.
0
However, the width of this barrier in nanorod arrays may
0 500 1000 1500 2000 2500 3000
Time (s) differafterannealinginoxygen,air,andvacuum.According
to our results, the band bending of the as-grown Al-doped
As-grown Air-annealed
ZnOnanorodarraysandtheAl-dopedZnOnanorodarrays
Oxygen-annealed Vacuum-annealed
thatwereannealedinoxygen,air,andvacuumcanbeillus-
Figure7:PhotoresponsespectraoftheUVphotoconductivesen- tratedinFigure8.Inthiscase,amongtheannealedsamples,
sor using as-grown, oxygen-annealed, air-annealed, and vacuum- the oxygen-annealed Al-doped ZnO nanorod arrays have
annealed Al-doped ZnO nanorod arrays under UV illumination the most depleted surface region due to the large amount
(365nm,750μW/cm2)anda10Vbias. of adsorbed oxygen, followed by the air-annealed sample
and the vacuum-annealed sample. For this reason, the
oxygen-annealedAl-doped ZnOnanorodarrayshavelower
theair-annealednanorodarraysalsohasahighphotocurrent photocurrent properties compared to the other annealed
value(2.39×10−4A)butitisslightlylowerthanthatofthe samples.ThistheorywasalsosupportedbyourPLmeasure-
vacuum-annealed nanorod-arrays. However, the photocur- ment result, in which the UV emission was suppressed and
rentvalueoftheoxygen-annealedAl-dopedZnOnanorod- haslowerintensitycomparedtotheotherannealedsamples
array-based UV photoconductive sensor was significantly despite the oxygen-annealed nanorod arrays having the
reduced to 1.69 × 10−4A. Furthermore, the as-grown Al- highest crystallinity. Meanwhile, for the as-grown nanorod
dopedZnOnanorodarray-basedUVphotoconductivesen- arrays, the depletion region is primarily attributed to the
sor shows the lowest photocurrent value of 3.91 × 10−5A. OH-groupsandothernativedefectsofZnO,suchasoxygen
The highest photocurrent value of the vacuum-annealed vacanciesandZninterstitials.
Al-doped ZnO nanorod-array-based UV photoconductive From the photocurrent spectra, the responsivity of the
sensor could be attributed to the high concentration of Zn sensorswasestimatedusingthefollowingequation[49]:
interstitials. According to Bera et al., when ZnO nanowires
I −I
with a high concentration of Zn interstitials are irradiated R= ph dark, (1)
with UV light, the photogenerated excitons could interact P
op
with the Zn interstitials, which results in ionisation of Zn
interstitials [13]. When Zn interstitials are ionised, more where I isthephotocurrent,I isthedarkcurrent,and
ph dark
free electrons are generated in the conduction band, which P is the optical power of the UV source. In this study,
op
increases the photocurrent of the sensor. We also proposed theI valuesofthesensorsfabricatedusingtheas-grown,
dark
that the photogenerated excitons also interact with Al- oxygen-annealed, air-annealed, and vacuum-annealed Al-
dopingleveltocontributemorefreeelectronstotheconduc- doped ZnO nanorods are 6.29 × 10−6, 3.03 × 10−6, 2.26 ×
tionband.However,annealinginairandoxygenresultsinan 10−5, and 5.44 × 10−5A, respectively. From this calcula-
improvementoftheZnOstoichiometry,therebydecreasing tion, the responsivity values of the sensors composed of
the concentration of defects, including Zn interstitials. the as-grown, oxygen-annealed, air-annealed, and vacuum-
Duringtheannealingprocess,vacantsitesintheZnOlattice, annealed Al-doped ZnO nanorods are 0.73, 3.69, 4.81,
which resulted from the evaporation of OH groups from and 4.19A/W, respectively. This result indicates that the
the nanorod surface, were immediately replaced by oxygen sensor composed of air-annealed ZnO nanorod arrays had
fromtheenvironment.Becauseoftheexcessiveoxygenflow a higher responsivity than the other samples. Although the
during the oxygen annealing process, oxygen molecules are photocurrentvalueofthevacuum-annealedAl-dopedZnO
easily adsorbed onto the nanorod surface, which results nanorod-array-based UV sensor is the highest, the sensor
in the increase of the nanorod resistance. As a result, the exhibits a lower responsivity value than the air-annealed
photocurrents of the Al-doped ZnO nanorod array-based nanorod-based UV photoconductive sensor due to its high
UVphotoconductivesensorswerereducedafterannealingin darkcurrentvalue.Webelievedthatthislargedarkcurrent
air and oxygen. For the as-grown Al-doped ZnO nanorod- value of vacuum-annealed Al-doped ZnO nanorod-array-
array-based UV photoconductive sensor, the photocurrent based UV photoconductive sensor is primarily contributed
8 JournalofNanomaterials
Oxygen- Air- Vacuum-
As-grown
annealed annealed annealed
Nanorod
Conduction
Surface band
trap
state
Valence
band
Depleted
region
Figure8:Schematicbanddiagramoftheas-grown,oxygen-annealed,air-annealed,andvacuum-annealedAl-dopedZnOnanorodarrays
illustratingthebandbendingduetooxygenadsorptionanddefects.
tohighconcentrationofZninterstitialsandAl-doping.This holes recombine with the adsorbed oxygen ions on the
condition improves the conductivity of the sensor, which surface, producing oxygen molecules; this reaction also
results in significant enhancement of the dark current than eliminatesthebarriernearthenanorodsurface.Thisprocess
thatoftheothersamples. isdescribedbythefollowingequation:
The sensitivity of the sensors, which is defined as the
photocurrent-to-dark-currentratio,wascalculatedforallof O2−+h+−→O2. (4)
thedevices.Thesensitivityvaluesofthesensorscomposedof
Atthesametime,thedesorptionofadsorbedoxygenionson
the as-grown, oxygen-annealed, air-annealed, and vacuum-
the nanorod surface leaves photogenerated electrons in the
annealed Al-doped ZnO nanorod arrays are 6.2, 55.6, 10.6,
conduction band, thereby increasing the film conductivity
and 4.6, respectively. The results indicate that the oxygen-
andcontributingtothephotocurrent.WhentheUVlightis
annealedAl-dopedZnOnanorodarray-basedUVphotocon-
switched off, oxygen once again begins to adsorb onto the
ductivesensorshadthehighestsensitivity.Theimprovement
nanorodsurface,therebydecreasingtheconductivityofthe
of the sensor sensitivity by oxygen annealing is due to the
sensor.
significantreductionofthedarkcurrentvalueascompared
Usingthephotocurrentspectra,theriseanddecaytime
with the other samples. We suspect that the significant
constantsofthefabricatedsensorswerecalculatedusingthe
reductioninthedarkcurrentvalueiscorrelatedwithmore
followingequations:
adsorbed oxygen on the oxygen-annealed nanorod arrays,
which increases the resistance of the film, as shown by I =
the I-V measurement results. In addition, annealing in (cid:2) (cid:3) (cid:4)(cid:5)
t
oxygenimprovesthesurfaceconditionofthenanorodarrays, I 1−exp − :riseprocesswithUVillumination on,
whichincreasestheiraffinityfortheoxygenmolecules.The 0 τr
sensitivity of the sensor has been reported to be correlated (cid:3) (cid:4)
t
withtheoxygenadsorptionanddesorptionprocesses[12,33, I =I exp − :decayprocesswithUVillumination off,
0 τ
50]. In the dark, oxygen molecules tend to adsorb onto the d
(5)
nanorodsurfacesbycapturingfreeelectronsandproducing
adsorbedoxygenions,asshownbythefollowingequation: where I is the magnitude of the current, I is the saturated
0
O +e−−→O −, (2) photocurrent, t is the time τr is the rise time constant,
2 2 and τd is the decay time constant. The calculation results
where O2 is the oxygen molecule, e− is the free electron, indicate that the sensors constructed using the as-grown,
andO2−istheadsorbedoxygeniononthenanorodsurface. oxygen-annealed, air-annealed, and vacuum-annealed Al-
The adsorbed oxygen ions create a barrier near the surface, dopedZnOnanorodarrayshaverise(decay)timeconstants
whichgeneratesalowcurrentbeforeUVillumination.When of 38 (109), 3 (13), 10 (26), and 15 (52)s, respectively.
UV light is irradiated onto the nanorods, photogenerated Based on these results, it can be concluded that the
electron-holepairsaregeneratedatthesurfaceaccordingto oxygen-annealed Al-doped ZnO nanorod array-based UV
thefollowingequation: photoconductive sensor exhibits the fastest response. This
hv−→h++e−, (3) behaviourmaybeattributedtothesurfaceconditionofthe
oxygen-annealedAl-dopedZnOnanorodarrays,whichcan
wherehvisthephotonenergyofUVlight, h+ isthephoto- promptly adsorb and desorb oxygen during the switching
generatedholeinthevalenceband,ande− isthephotogen- on and off of the UV illumination, thereby resulting in
eratedelectronintheconductionband.Thephotogenerated sudden changes in the nanorod resistance. In addition,
JournalofNanomaterials 9
(a) (b)
(c) (d)
Figure9:Surfacemorphologyandcross-sectionalimagesofAl-dopedZnOnanorodarraysasafunctionofimmersetime:((a)-(b))100min
and((c)-(d))150min.
Table1:Thecharacteristicsoftheas-grown,oxygen-annealed,air-annealed,andvacuum-annealedAl-dopedZnOnanorod-array-based
UVphotoconductivesensors.
Resistance Darkcurrent Photocurrent Risetime Decaytime Responsivity
Sample Sensitivity
(MΩ) (A) (A) constant(s) constant(s) (A/W)
As-grown 0.80 6.29×10−6 3.91×10−5 38 109 0.73 6.2
Oxygen-annealed 1.59 3.03×10−6 1.69×10−4 3 13 3.69 55.6
Air-annealed 0.16 2.26×10−5 2.39×10−4 10 26 4.81 10.6
Vacuum-annealed 0.10 5.44×10−5 2.43×10−4 15 52 4.19 4.6
theoxygen-annealedAl-dopedZnOnanorodarrayspossess immersedfor100and150min,respectively.IntheseFESEM
better crystallinity than the other samples, which enhances images, dense nanorod arrays can be observed with high
the mobility of the photogenerated carriers in the nanorod uniformityacrossallsamples.However,thediameterofthe
arrays.Incomparison,thevacuum-annealednanorodarrays nanorods is almost constant at 60nm with further increase
show a higher rise and decay time constant due to the of immersion time up to 150min. Figures 9(b) and 9(d)
prominent electron trapping at the positively charged Zn show the cross-sectional images of air-annealed Al-doped
interstitial defect state, which has a large lifetime [51]. ZnOnanorodarrayspreparedatimmersiontimeof100and
The characteristics of the as-grown, oxygen-annealed, air- 150nm, respectively. Based on the cross-sectional images,
annealed, and vacuum-annealed Al-doped ZnO nanorod- the thickness of the film increase, with the increase of
array-based UV photoconductive sensors are presented in immersiontime.OnthebasisoftheFESEMmeasurements,
Table1. the thicknesses of the films prepared by immersion for 100
We also investigate the UV photoconductive sensors and150minareapproximately650and700nm,respectively.
performance using air-annealed Al-doped ZnO nanorod Figure10 depicts I-V curves of Al-doped ZnO nanorod
arrays that were prepared for longer immersion times of arrays prepared at different immersion times. The graphs
100 and 150min. Figures 9(a) and 9(c) show the surface reveal that the current intensity with respect to the voltage
morphologiesofair-annealedAl-dopedZnOnanorodarrays decrease when the immersion time was increased up to
10 JournalofNanomaterials
80 and 150min, respectively. The reduction in photocurrent
value for sensors using nanorod arrays prepared for longer
A) 60
μ time influenced the responsivity of the devices; devices
ent ( 40 using nanorod arrays prepared for longer immersion time
urr exhibited lowerresponsivity values.Theresponsivity values
C 20 of the sensors are 1.75 and 1.36A/W for the nanorod
arrays prepared for immersion times of 100 and 150min,
−10 −8 −6 −4 −2 2 4 6 8 10 respectively. In this case, we believed that the decrease of
−20 Voltage (V) thephotocurrentandresponsivityvaluesofthedeviceswas
affected by the increase of the resistance of the nanorod
−40
arrays after being immersed for longer times. However,
−60 the sensitivity of the devices is slightly increased using Al-
dopedZnOnanorodarraysimmersedforlongertimes.The
−80 sensitivity values of the devices are 15.5 and 16.0 using Al-
doped ZnO nanorod arrays prepared for 100 and 150min,
50 min
respectively. This sensitivity increment may be attributed
100 min
to decrease of the dark current and increase of the surface
150 min
area for the devices using Al-doped ZnO nanorod arrays
Figure 10: The I-V characteristic of air-annealed Al-doped ZnO immersedforlongertimes.
nanorodarrayspreparedfordifferentimmersiontimes. In order to understand the effects of Al doping on
ZnO nanorod array-based UV photoconductive sensor
performance, air-annealed undoped and Al-doped ZnO
UV on UV off
nanorod-array-basedUVphotoconductivesensorwerestud-
0.25 ied. Figure12(a) shows FESEM image of air-annealed
undoped ZnO nanorod arrays at magnification of 30,000x.
The image reveals that undoped ZnO nanorods have been
0.2
uniformlydepositedontheseed-layer-coatedglasssubstrate.
A)
m The diameter of the undoped ZnO nanorod is estimated
nt ( 0.15 to be 100nm, in which the size is larger than that of the
e
r air-annealed Al-doped ZnO nanorod and other samples. A
ur
C 0.1 cross-sectionalimageasshowninFigure12(b)confirmsthat
the nanorods grew perpendicular to the substrate with the
thickness estimated to be approximately 600nm. Figures
0.05
13(a)–13(d) show TEM images and SAED patterns of air-
annealed undoped and Al-doped ZnO nanorods. These
0
results indicate that both undoped and Al-doped ZnO
0 500 1000 1500 2000 2500 3000
nanorods are single crystals with wurtzite ZnO structure.
Time (s)
The TEM images also reveal that the size of the Al-
50 min doped ZnO nanorod is smaller than the undoped ZnO,
100 min which attributed to the effect of the doped Al. The Al3+
150 min
ions (0.054nm) have smaller ionic radius than Zn2+ ions
Figure 11: Photoresponse spectra of the UV photoconductive (0.074nm)anditisexpectedthatthisionicradiusdifference
sensorusingair-annealedAl-dopedZnOnanorodarraysprepared may influence the diameter of ZnO nanorods when doped
for different immersion times under UV illumination (365nm,
withAl[52,53].
750μW/cm2)anda10Vbias.
Figure14 shows the I-V properties of the undoped and
Al-doped ZnO nanorod arrays, which clearly show that
the undoped ZnO nanorod arrays exhibit lower current
150min. The resistance of the samples was calculated to be value than that of Al-doped ZnO nanorod arrays with
0.72 and 2.41MΩ for the nanorod prepared at immersion respecttothesuppliedvoltage.FromthisI-V characteristic,
timeof100and150min,respectively. the resistance of the undoped arrays was calculated to be
Figure11showsthetime-resolvedphotoresponseofthe 1.15MΩ.Itisunderstoodthatthisresistancevalueishigher
UVphotoconductivesensorpreparedusingtheair-annealed thanresistancevalueofair-annealed,vacuum-annealed,and
Al-dopedZnOnanorodarraysthatwerepreparedatlonger as-grown Al-doped ZnO nanorod arrays but show lower
immersion times of 100 and 150min. The results show valuesthanthatofoxygen-annealedAl-dopedZnOnanorod
that both photocurrent and dark current of the sensors arrays.
were decreased with the increase of immersion time. The Figure15 shows the photoresponse spectra of the air-
photocurrent/dark current values of the sensors are 8.41 annealedundopedandAl-dopedZnOnanorod-array-based
× 10−5/5.43 × 10−6 and 6.50 × 10−5/4.07 × 10−6A for UV photoconductive sensors. The spectra reveal that both
the nanorod arrays prepared for immersion times of 100 photocurrent and dark current of the undoped ZnO