This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 96, 053105 (2010) and may be found at https://doi.org/10.1063/1.3294327.A deep understanding of the recombination dynamics of ZnO nanowires (NWs) is a natural step for a precise design of on-demand nanostructures based on this material system. In this work we investigate the influence of finite-size on the recombination dynamics of the neutral bound exciton around 3.365 eV for ZnO NWs with different diameters. We demonstrate that the lifetime of this excitonic transition decreases with increasing the surface-to-volume ratio due to a surface induced recombination process. Furthermore, we have observed two broad transitions around 3.341 and 3.314 eV, which were identified as surface states by studying the dependence of their life time and intensitiy with the NWs dimensions.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Cornet, Albert

Güell, Frank

Hoffmann, Axel

Morante, Joan Ramon

Reparaz, Juan Sebastián

Wagner, Markus R.

Applied Physics Letters

English

DepositOnce

Appl. Phys. Lett. 96, 053105 (2010); https://doi.org/10.1063/1.3294327 96, 053105© 2010 American Institute of Physics.Size-dependent recombination dynamics inZnO nanowiresCite as: Appl. Phys. Lett. 96, 053105 (2010); https://doi.org/10.1063/1.3294327Submitted: 17 November 2009 . Accepted: 27 December 2009 . Published Online: 01 February 2010J. S. Reparaz, F. Güell, M. R. Wagner, A. Hoffmann, A. Cornet, and J. R. MoranteARTICLES YOU MAY BE INTERESTED INA comprehensive review of ZnO materials and devicesJournal of Applied Physics 98, 041301 (2005); https://doi.org/10.1063/1.1992666Spatial mapping of exciton lifetimes in single ZnO nanowiresAPL Materials 1, 012103 (2013); https://doi.org/10.1063/1.4808441Time-resolved photoluminescence of the size-controlled ZnO nanorodsApplied Physics Letters 83, 4157 (2003); https://doi.org/10.1063/1.1627472Size-dependent recombination dynamics in ZnO nanowiresJ. S. Reparaz,1,a F. Güell,2 M. R. Wagner,1 A. Hoffmann,1 A. Cornet,3 andJ. R. Morante2,41Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany2M-2E, IN2UB, Departament d’Electrònica, Universitat de Barcelona, C/Martí i Franquès 1,Catalunya, 08028 Barcelona, Spain3MIND, IN2UB, Departament d’Electrònica, Universitat de Barcelona, C/Martí i Franquès 1, Catalunya,08028 Barcelona, Spain4Institut de la Recerca de l’Energia de Catalunya (IREC), C/Josep Pla 2, 08019 Barcelona,Catalunya, SpainReceived 17 November 2009; accepted 27 December 2009; published online 1 February 2010A deep understanding of the recombination dynamics of ZnO nanowires NWs is a natural step fora precise design of on-demand nanostructures based on this material system. In this work weinvestigate the influence of finite-size on the recombination dynamics of the neutral bound excitonaround 3.365 eV for ZnO NWs with different diameters. We demonstrate that the lifetime of thisexcitonic transition decreases with increasing the surface-to-volume ratio due to a surface inducedrecombination process. Furthermore, we have observed two broad transitions around 3.341 and3.314 eV, which were identified as surface states by studying the dependence of their life time andintensitiy with the NWs dimensions. © 2010 American Institute of Physics.doi:10.1063/1.3294327Over the last five decades the fundamental properties ofZnO have been widely studied by many authors.1 Its largeband gap of 3.3 eV, large exciton binding energy of about60 meV, and its structural compatibility with GaN are somethe many advantages that make ZnO interesting concerningdevice applications. A renewed interest in this material sys-tem has emerged in the last years due to great improvementson its growth techniques. Its applications into piezotronicshave been recently discussed since they represent the ulti-mate step for an efficient integration into novel devices. Inaddition, much effort has been put in fabricating low dimen-sional structures2 such as nanodots, nanocrystals, tetrapods,nanorods, and nanowires NWs due to the advantages of-fered by quantum confinement. Nevertheless, despite thegreat variety of nanostructures produced up to date, scalingZnO or any material into the nanoscale present some com-plications as some of its physical properties depend on thedimensionality and, thus, need to be revisited. Recently, themechanical properties of ZnO were found to be different tothose of bulk material.3 The importance of surface states aspossible recombination channels in NWs was alsodiscussed.4 A complete revision of the optical, electrical, andmechanical properties is mandatory when the surface-to-volume ratio is not negligible. The recombination dynamicsin ZnO NWs have been recently studied by many authors.4–8Different recombination times with single and biexponentialdecays have been reported for different nanostructured sys-tems. This situation is somehow dissapointing since it mightlead to different interpretations of the possible recombinationchannels in these nanostructures. Although direct comparisonbetween all these data is not possible since the lifetime de-pends on many different scattering processes which mightdiffer for different growth techniques, the knowledge of itsdependence with the NWs size is desirable. In a recentletter,8 the lifetime of the free exciton was investigated forNWs with different length but equal diameter. The authorshave observed that the lifetime increases as the length of theNWs increases. Nevertheless, the influence of the surface onthe NWs recombination processes might be stronger for thesmaller NW diameter due to the increasing surface-to-volume ratio. It is our main purpose to investigate the influ-ence of the diameter on the surface states of the NWs, aswell as on its excitonic recombination processes.In this letter, we present a systematic study of recombi-nation dynamics in ZnO NWs with different sizes. We dem-onstrate that the lifetime of the main excitonic transitionaround 3.365 eV decreases for the smaller NW diameters dueto surface induced recombination. This result was also con-firmed within each sample by the observation of fluctuationsin the lifetime of the 3.365 eV exciton. In addition, we haveobserved two broad transitions around 3.341 S1 and 3.314eV S2 whose intensity increases for the smaller NWs diam-eters. Comparing their intensities and recombination times tothose of the main excitonic recombination around 3.365 eVwe conclude that S1 and S2 might originate from surfacestates.ZnO NWs were grown by a Au catalyzed vapor-liquid-solid process on SiO2 /Si substrates.9 Three samples with dif-ferent mean wire diameter of d=70, 110, 170 nm named asA, B, and C, respectively were grown from different Aucatalyst. For samples A and B sputtered Au was used ascatalyst with nominal thicknesses of 10 and 20 nm, respec-tively. In the case of sample C, Au colloids were used ascatalyst. The NWs were grown from a ZnO powder mixedwith graphite powder and the synthesis was carried out in ahorizontal quartz tube. The furnace was heated at 900 °Cand Ar was used as carrier gas. Structural characterizationwas performed by field emission scanning electron micro-scope FESEM with a Hitachi H-4100FE. The high struc-tural quality of the NWs was studied in a previous work10using high resolution transmission electron microscopy. Thestructural analysis shows the high crystal quality of the ZnOaElectronic mail:jsreparaz@gmail.com.APPLIED PHYSICS LETTERS 96, 053105 20100003-6951/2010/965/053105/3/$30.00 © 2010 American Institute of Physics96, 053105-1NWs, and reveals the 0001 as principal growth direction.In addition, the NWs surface roughness is estimated to beof the order of one atomic layer. The optical properties ofthese nanostructures were investigated using a micro-photoluminescence PL setup. A frequency-doubled Ti:sap-phire laser set at 3.496 eV 355 nm was focused onto thesamples using a 50 microscope objective specially de-signed for UV operation. The emitted PL was collected bythe same microscope objective and recorded spectrally re-solved by a liquid nitrogen cooled charge-coupled devicecamera. The objective was mounted on a XY piezoelectricstage which allows steps of 100 nm. Nevertheless, the reso-lution was limited by the numerical aperture NA=0.75 of theobjective, and can be estimated from the radius of the firstAiry disk as d=1.22 /2NA290 nm, where =355 nm isthe wavelength of the incident laser light. For the time-resolved measurements we have used the time correlatedsingle-photon counting technique with a time resolution ofabout 20 ps.In Fig. 1 we show FESEM images for the samples A, B,and C. The NWs do not grow vertically aligned due to thelattice mismatch between the ZnO and the amorphousSiO2 /Si substrate. We have chosen this random orientationof the NWs to obtain a lower effective wire density, which incombination with a high spatial resolution micro-PL setupallows to reduce the inhomogeneous broadening due to sizedistribution. This configuration is extremely convenient inorder to study the size-dependence of the recombination dy-namics since depending on the sample’s NWs density, fromone to a few wires are present within laser spot. We point outthat this represents an important advantage of our experi-mental conditions compared to previous ones,2,4,5 since thehorizontal spatial orientation of the NWs which is origi-nated in the lattice mismatch with the SiO2 layer reduces theNWs density that is probed within the laser spot. This cannotbe achieved in the case of vertically aligned NWs, which aregenerally grown over ZnO substrates, producing a highereffective NWs density.The low temperature PL spectra for samples A, B, and Cis shown in Fig. 2. The free exciton line is observed forsample C around 3.375 eV, while for samples A and B it ishardly observed since it might be obscured by the strongerbound excitons. The transitions observed around 3.365 eVare assigned to their corresponding bound excitons in bulkZnO.11 For the three samples a common line is observedaround 3.365 eV. The origin of this transition is somehowcontroversial with two following possible candidates: i adonor bound exciton recombination11 DX, ii a surfacelocalized exciton.12 It is not our purpose to discuss in detailthe origin of this transition but instead to use it as a probe toinvestigate size-dependent effects taking into advantage thefact that it is observed in the three samples. Nevertheless, wehave some reasons to believe that this transition originatesfrom a bound exciton recombination as we will show fromthe time-resolved PL TRPL spectra and temperature depen-dence. Finally, we show in the inset to Fig. 2 two broadbands around 3.341 and 3.314 eV labeled as S1 and S2, re-spectively. The position of S1 does not depend on the wirediameter while its intensity increases with increasing thesurface-to-volume ratio. On the contrary, S2 shifts to lowerenergies with decreasing the wire diameter while its intensityincreases. The fact that the intensity ratios IS1 / IDX andIS2 / IDX increase with decreasing the NW diameter is an in-dicator that these bands might originate from surface statesin the NWs. In fact, the S2 band was recently assigned to afree-to-bound transition e ,A0 related to the intersection ofbasal plane stacking faults with the surface of the material.13In Fig. 3 we show the TRPL spectra of the DX at 3.365eV and 4 K for the three samples. In order to ensure compa-rable lifetimes all the experimental conditions were un-FIG. 1. FESEM images of three samples with ZnO NWs of different diam-eters of a 70 nm, b 110 nm, and c 170 nm. Samples A and B weregrown with sputtered Au as catalyst while sample C was grown using ancolloidal Au nanoparticles as catalyst.3.30 3.31 3.32 3.33 3.34 3.35 3.36 3.37 3.38S1S2DX Sample Bd = 110 nmSample Ad = 70 nmS1Sample Cd = 170 nmFXASX ?Intensity(arb.units)Energy (eV)ZnO nanowires4 K - 0.5 mWS23.30 3.31 3.32 3.33 3.34x40x30Energy (eV)FIG. 2. Color online PL spectra at 4 K for samples A, B, and C. Thevertical dashed line shows the position of the DX=3.365 eV neutral-boundexciton recombination. The inset shows the detail of the low energy regionwhere two surface related transitions labeled as S1 and S2 are observed.0.0 0.5 1.0 1.5 2.0Decayrate(ns-1 )Diameter, Length (nm)(B) d=110 nm & τ=98 ps(A) d=70 nm & τ=67ps(C) d=170 nm & τ=200 psZnO nanowires4 K - 0.5 mWEDX= 3.365 eVIntensity(arb.units)Time (ns)0 150 300 450 6000.0000.0050.0100.015NWs lengthRef. [8]NWs diameter,present workFIG. 3. Color online Low-temperature TRPL spectra at 3.365 eV forsamples A, B, and C. A monoexponential lifetime is observed for thesamples. The inset shows the scattering rates in full symbols and also theresults from Ref. 8 for comparison.053105-2 Reparaz et al. Appl. Phys. Lett. 96, 053105 2010changed. A monoexponential decay is observed for all thesamples with lifetimes of =67, 98, and 200 ps for samplesA, B, and C, respectively. We believe that this systematicincrease in the lifetime with increasing the wire diameter isrelated to a surface assisted recombination process, i.e., thelifetime of the DX exciton is influenced by the surfaceproximity. Consequently, for the smaller NW diametersd=70 nm the shorter recombination times are achieved=67 ps. In addition, we show in the inset to Fig. 3 thescattering rates as function of the NWs diameter togetherwith the results from Ref. 8 which relates the scattering ratesof the free exciton at ambient temperature to the length of theNWs. Although direct comparison between the absolute val-ues would not be correct since they correspond to differentexcitonic transitions, it is still possible to compare theirslopes. A difference of a factor of 10 between them showsthat the scattering rate varies faster with reducing the diam-eter of the NWs compared to their length. This result indi-cates the predominant influence of the surface in the recom-bination mechanisms of the excitons. The lifetime of S1 andS2 was also studied for samples A and B. For S1 we haveobtained a monoexponential dependence with time constantsof A=76 ps and B=109 ps at 4 K. On the contrary, a biex-ponential decay was observed for S2 with A1,2=179 and 702ps and B1,2=116 and 551 ps also at 4 K. The lifetimes of S2in the NWs are compatible with those observed in ZnOnanocrystals13 with the long component being a signature ofa free-to-bound transition as discussed in Ref. 14.The temperature dependence of the decay time of DX isshown in Fig. 4. For sample A, a quadratic dependence of thelifetime with temperature is observed from the fit in dashedline. This increase in the lifetime is understood in terms ofthermally induced exciton redistribution in reciprocalspace.15 For sample B, this quadratic dependence is obscuredby fluctuations which are beyond the experimental error.This is explained taking into account that with varying thetemperature, the position in the sample is slightly changedso that contributions from wires with different diameters arepossible. From the fluctuations observed in Fig. 4 we canestimate a diameter distribution of 705 nm, and11030 nm for samples A and B, respectively. In addition,the fact that we observe these fluctuations within eachsample is another indicator of the dependence of the lifetimewith size of the NWs. For sample C, almost no variation isobserved over the whole temperature range. Although alarger lifetime is observed for this sample, which is in accor-dance with its larger wire diameter, a strict comparison isdifficult due to the different Au catalyst used in the growthmechanism. This may lead to a different bound excitonicstructure due to the inclusion of different impurities in thegrowth process, which could also contribute to different ex-citon scattering rates. Nevertheless, this is not the case forsamples A and B since they were grown in the same condi-tions. The only difference between these samples is thethickness of the sputtered Au catalyst. Consequently, directcomparison between their optical properties is fair since asimilar impurity distribution is expected.In summary, comparing ZnO NWs of different sizes wewere able to show that the recombination time of the DX=3.365 eV donor bound exciton decreases for the smallerNW diameters. A similar effect was also observed withineach sample studying the thermal fluctuations of the recom-bination time of the DX, which was originated by contribu-tion of different NWs due slight variations in the position ofthe laser spot with temperature. In addition, two transitionslabeled as S1=3.341 eV and S2=3.314 eV were observedfor all the samples. Studying their intensities and recombina-tion times we were able to show that these may arise fromsurface localized states. These results are of great interest fora precise design of ZnO-based nanostructures, since theyrepresent a step toward a deep understanding of size-dependent recombination dynamics in this system.This work was supported by the CICyT National ProjectMOSEN: MAT2007-66741-C02-01, the NanoSci-ERA Euro-pean Project NAWACS: NAN2006-28568-E, the CON-SOLIDER INGENIO 2009 CDS 00050 MULTICAT, and byDFG within SFB787.1Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V.Avrutin, S.-J. Cho, and H. Morkoç, J. Appl. Phys. 98, 041301 2005.2A. B. Djurišić and Y. H. Leung, Small 2, 944 2006.3B. Wen, J. E. Sader, and J. J. Boland, Phys. Rev. Lett. 101, 1755022008.4L. Wischmeier, T. Voss, I. Rückmann, J. Gutowski, A. C. Mofor, A.Bakin, and A. Waag, Phys. Rev. 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Appl.Phys. 107, 024311 2010.14M. Schirra, R. Schneider, A. Reiser, G. M. Prinz, M. Feneberg, J.Biskupek, U. Kaiser, C. E. Krill, K. Thonke, and R. Sauer, Phys. Rev. B77, 125215 2008.15X. H. Zhang, S. J. Chua, A. M. Yong, H. Y. Yang, S. P. Lau, S. F. Yu, X.W. Sun, L. Miao, M. Tanemura, and S. Tanemura, Appl. Phys. Lett. 90,013107 2007.0 10 20 30 40 50 6060120180240Sample CSample Bd = 170 nmd = 110 nmd = 70 nmLifeTime(ps)Temperature (K)ZnO nanowiresPower = 0.5 mWSample AFIG. 4. Color online Temperature dependence of the lifetime of DX forsamples A, B, and C. The curve in dashed line is a quadratic fit to datapoints for sample A. The lifetime decreases as increasing the surface-to-volume ratio of the NWs.053105-3 Reparaz et al. Appl. Phys. Lett. 96, 053105 2010

Size-dependent recombination dynamics in ZnO nanowires

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Size-dependent recombination dynamics in ZnO nanowires

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