Thin Solid Films 518 (2010) 4231–4233

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Thin Solid Films

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Luminescent properties of ZnO structures grown with a vapour transport method

Matthew Foley ⁎, Cuong Ton-That, Matthew R. PhillipsDepartment of Physics and Advanced Materials, University of Technology Sydney, P.O. Box 123, Broadway, NSW 2007, Australia

⁎ Corresponding author. Tel.: +612 9514 7574.E-mail address: Matthew.Foley@uts.edu.au (M. Foley

0040-6090/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.tsf.2009.12.083

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 June 2009Received in revised form 27 November 2009Accepted 8 December 2009Available online 16 December 2009

Keywords:Zinc oxideSurface morphologyCathodoluminescenceDefects

ZnO structures were synthesised on the sapphire (112—0) substrate by a vapour transport method in a gas

flowing furnace. The influence of the oxygen content in the gas mixture on the morphology and luminescentproperties of ZnO structures grown on a strip-like substrate was investigated, with all other growthparameters being kept nominally identical. Integrated electron microscopy and cathodoluminescenceanalysis shows gradual variations of structural and optical emission properties for ZnO structures grown onthe long substrate. Defect-related green luminescence of ZnO is found to be highly dependent on the oxygenvapour in the growth region of the furnace. Our findings demonstrate that the green luminescence isassociated with oxygen deficiency in ZnO.

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1. Introduction

As a wide band gap (3.37 eV) semiconductor with a large excitonbinding energy (60 meV), zinc oxide (ZnO) is of great interest for low-voltage and short wavelength photonic devices. Since the first reporton ZnO nanowire-based lasers in 2001 [1], various methods –

including vapour transport deposition – have been developed tosynthesise a wide range of structures, such as wires, rods, helices andnails [2–4]. Because of the rich variety of structures combined with itsversatile optical properties, these micro- and nano-structures havebeen extensively synthesised and studied with a possibility of largescale-up production. In this context, the ability to grow ZnO structuresover a large area with controlled optical properties is important indevice fabrication.

In a typical vapour transport deposition method with the employ-ment of gold as a catalyst, Zn vapour is generated from the carbo-thermal reduction reaction of ZnO in the high-temperature zoneof the furnace and transported by a carrier gas into the lower-temperature growth zone. The Zn vapour can dissolve into the catalystto form a eutectoid of Zn–Au and be subsequently oxidised by reactingwith oxygen from the ambient gas in the growth zone. The growthbehaviour of ZnO structures is thus related to various synthesisconditions such as temperature of the substrate [3,5], the startingreagents [6], material flow rate [7] and growth pressure [8]. Sample-to-sample variability often occurs as a result of variations in growthconditions or unintentional surface impurities. In this paper wedemonstrate that the morphology and the chemistry of native defects

in ZnO structures are strongly influenced by the reactant vapour in thegrowth zone. The variation in the oxygen content along the length of astrip-like substrate leads to various structures grown on the samesubstrate, thus allowing the investigation of defect-related lumines-cence in relation to the formation of native defects in ZnO. Thisapproach to investigation of native defects and optical properties ofZnO is advantageous in that all other growth parameters can be keptnominally constant. Our work elucidates the relation betweenreactant vapour conditions and the luminescent properties of grownstructures, which can offer references for growth of ZnO structureswith controlled optical properties.

2. Experimental details

The growth of ZnO structures was carried out in a horizontal tubefurnace via the chemical vapour transport method with the use of Auas catalyst, as described in detail elsewhere [9]. Prior to the growth,the tube was purged with N2 for 1 h to evacuate ambient gas in thesystem. The source material was a mixture of ZnO (N99.9% purity,obtained from Sigma-Aldrich) and carbon powder (ZnO:C=1:3 byweight), which was loaded in an alumina boat and heated in theupstream zone. A long, narrow strip of the (112

—0) sapphire, chosen

for its close lattice match with ZnO, was placed downstream in thegrowth zone at temperature 900 °C. The carrier gas used was amixture of oxygen and argon, which were introduced at the inlet atflow rates of 5 sccm and 95 sccm, respectively. This experimentalsetup allows a continuous variation in Zn:O flux ratio along the lengthof the substrate while all other growth parameters are nominallyidentical.

The morphology and optical properties of grown products wereinvestigated using a FEI QUANTA 200 scanning electron microscope(SEM) with an attached GATAN Mono-CL3 spectrometer system

4232 M. Foley et al. / Thin Solid Films 518 (2010) 4231–4233

for cathodoluminescence (CL) analysis. The SEM was operated at anacceleration voltage of 5 kV and a 0.6 nA beam current in all CL mea-surements. The CL signal was dispersed by a 1200 lines/mm grating,collected by a parabolic mirror and detectedwith a Hamamatsu R943-02 peltier cooled photomultiplier tube. All CL spectra were correctedfor the overall detection response of the CL spectroscopy system.The crystallinity of the ZnO structures was characterised by X-raydiffraction (XRD) on a Siemens D5000 X-ray diffractometer em-ploying monochromated Cu-Kα radiation on a conventional θ–2θgoniometer.

3. Results and discussion

As shown in Fig. 1, three distinct types of as-grown products wereobtained along the length of the strip-like substrate. The mostinteresting phenomenon is that the as-grown structures are distrib-uted over a distance of 10 cm from the source material. In the regionclosest to the source (within 5 mm), vertically aligned nanorods offairly uniform size (type I ZnO structure) are observed (Fig. 1a). The

Fig. 1. SEM images of type I, II and III ZnO structures (a, b and c respectively) obtainedwith increasing distance from the source. These structures were grown on a long stripof the (112

—0) sapphire. Inset of a) shows an individual nanorod with well-faceted tip.

morphology of the nanorods is similar to those previously reported forZnO nanowires grown by chemical vapour deposition on sapphire(112

—0) substrate [9]. The nanowires exhibit a hexagonal cross-section

of 100–250 nm in diameter. Type II and III ZnO structures (Fig. 1band c) formed at locations further from the source consist of a ped-estal base from which the nanorods grow. Type III ZnO structures,formed over a large area at locations greater than 2 cm from thesource, exhibit isolated island characteristics. Since all other growthconditions including the substrate temperature were kept identicalacross the substrate, the morphological variation of grown ZnO isascribed to changes in the Zn:O ratio (discussed further below).

The XRD pattern of the grown type I ZnO, shown in Fig. 2, displaysonly two peaks of strong intensity, (0002) and (0004), which areboth multiples of the [0001] ZnO growth direction. This demonstratesa preferential ZnO [0001] growth direction parallel to the sapphire[112

—0], consistent with the symmetry characteristic of ZnO growing

on the [112—0] sapphire [10]. The peak positions are consistent with

the standard values for ZnO (a=0.3249 nm and c=0.5206 nm),indicating that the grown structures are highly crystalline structure.No diffraction peaks from Zn were found in any region of the sample.These results indicate that the grown structures are of high qualityand grow predominantly along [0001].

Fig. 3 shows the CL spectra of as-grown ZnO nanocrystals shown inFig. 1. These spectra were acquired using an unchanged CL setup toensure that they have identical excitation conditions (5 kV acceler-ation voltage and 0.6 nA beam current). The CL spectra consist oftwo bands: a near band edge (NBE) emission at 3.25 eV and a strong,broad green peak at ∼2.4 eV. The green luminescence band hasbeen attributed to native defect levels within the band gap, typicallyoxygen vacancies or zinc interstitials [11,12], while the NBE emissionis known to be dominated by the radiative combination of freeexcitons [13]. As seen in Fig. 3a, the intensity of the green lumi-nescence, relative to the NBE intensity, becomes progressively moreintense as the distance from the material source increases. For typeIII ZnO structures grown at locations furthest from the source, thegreen emission is significantly stronger than that of type I and II. Itis known that the crystalline quality of ZnO structures depends onthe availability of oxygen during growth from the gas phase [14]. Theincreased intensity of the green luminescence of type III ZnO coincideswith irregularly shaped island nanostructures, which are indicative ofpoorer crystallinity compared to type I ZnO. Since all other growth

Fig. 2. XRD pattern of the grown ZnO, displaying only two peaks with strong intensity,(0002) and (0004), in addition to the sapphire (112

—0).

Fig. 3. (a) Comparison of CL spectra obtained from type I, II and III ZnO at identicalexcitation conditions (5 kV, 0.6 nA). The spectra are normalised to the NBE intensity at3.25 eV for easy comparison. (b) Effects of post-annealing treatment on the greenemission from type I ZnO, showing the luminescence being eliminated by annealing inoxygen atmosphere at 900 °C.

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conditions were kept constant, the spatial variation of oxygen contentin the vapour mixture along the length of the substrate is the likelycause of the differences in the crystalline quality and defect-relatedgreen luminescence. Because of the rapid consumption of oxygen byZn and C in the growth zone, the oxygen concentration decreasesrapidly with distance from the source. In contrast, the concentrationof Zn vapour, which is continuously generated by the carbothermalreduction of ZnO, varies insignificantly. Consequently, type III ZnO isexpected to be more deficient in oxygen compared to type I and II.To gain further insight into the defect emission from the grown ZnO,the as-grown sample was annealed under a constant flow of pureoxygen at 900 °C for 1 h. The annealed sample displays negligiblegreen emission (Fig. 3b). The disappearance of the defect-relatedemission from the grown ZnO is due to in-diffusion of oxygen, which

reduces the level of oxygen deficiency [11]. This further supportsoxygen-deficiency related defects being the primary cause of thegreen emission.

The observedmorphological evolution obtained from the variationin reactant fluxes allows us to evidence the occurrence of differentnucleation and growth processes of ZnO. According to the vapour–liquid–solid (VLS) mechanism [15], Au droplets alloy with Zn and ZnOstructures grow with the assistance of the liquid catalyst. Since ZnOvapour is a dominant factor influencing the anisotropic crystalgrowth [16], sufficiently high ZnO vapour pressure at locations closeto the source enables the droplets to become supersaturated andsubsequent axial growth by the VLS mechanism. Assuming that thenanowire growth continues until the VLS process stops, the axialgrowth rate is estimated as 3 nm/s for type I ZnO. The observedgrowth behaviour at locations further away from the source does notfit the VLS mechanism. The reduction of the oxygen content and ZnOvapour pressure appears to favour the nucleation of ZnO moleculesover the supersaturation of the metal droplets, leading to predom-inantly lateral growth. In this case, Au droplets do not becomesaturated but act as a preferential site adsorption site for ZnO due tothe strong Au–ZnO interaction [17].

4. Conclusions

We have demonstrated that the morphology and the luminescentproperties of ZnO structures depend strongly on the synthesisconditions, especially the oxygen content in the vapour mixture.Various ZnO structures were grown on a strip-like sapphire substrate,allowing a gradual variation of the Zn:O flux ratio along its lengthwhile all other growth parameters being kept nominally constant. Thestructures grown near the source show evidence of reduced defectemission, suggesting oxygen deficiency is responsible for the greenluminescence in ZnO.

Acknowledgements

The authors would like to thank G. McCredie, K. McBean and M.Berkahn for their technical support.

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