Evolution of the faceting, morphology and aspect ratio of gallium oxidenanowires grown by vapor–solid deposition

Ian D. Hosein, Manu Hegde, Peter D. Jones, Vadim Chirmanov, Pavle V. Radovanovic n

Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1

a r t i c l e i n f o

Article history:Received 15 January 2014Received in revised form19 March 2014Accepted 22 March 2014Communicated by: K. DeppertAvailable online 29 March 2014

Keywords:A1. Crystal morphologyA1. NanostructuresA1. Growth from vaporA3.Growth modelsB2. Semiconducting gallium compoundsB2. Phosphors

a b s t r a c t

Gallium oxide nanostructures with high aspect ratio and variable faceting were synthesized by thechemical vapor deposition method via vapor–solid growth mechanism. Systematic investigation of thegrowth conditions revealed that these nanowires can be produced under the conditions of hightemperature and low precursor flow. The nanowires crystalize as the β-phase Ga2O3, which has themonoclinic crystal structure. Preferred growth was along the [0 1 0] direction, as corroborated withlattice-resolved imaging and crystal plane models. The high degree of faceting is discussed in terms ofthe evolution of the nanowire cross section morphology, based on the growth rate of the facetboundaries relative to the nanowire surface planes. The obtained nanowires show intense blue emission,characterized by a broad-band photoluminescence spectrumwith a maximum at 430 nm and long decaytime. This emission arises from the defect-related donor-acceptor pair recombination mechanism,and depends on the nanostructure dimensionality and morphology. The possible influence of controllednanowire faceting on the observed optical properties is also discussed. Owing to their morphology-dependent optical properties, these nanowires are promising building blocks for electronic andoptoelectronic structures and devices.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Gallium oxide can be classified as a transparent conductingoxide (TCO) with the widest transparency range (band gap energyis ca. 4.9 eV), high chemical stability, n-type conductivity [1,2], andstrong blue photoluminescence (PL) [3]. It crystalizes in fiveknown crystalline modifications (α, β, γ, δ and ε phases), of whichβ-Ga2O3 is the most stable polymorph [4,5]. The β-phase Ga2O3

adopts the monoclinic crystal structure with two distinct galliumsites (six- and four-coordinated) and three oxygen sites (three-and four-coordinated). Native point defects are known to formreadily in this complex oxide lattice and are responsible for a rangeof important physical properties. Specifically, oxygen vacancieshave been proposed to lead to the formation of shallow donorstates which are considered to be the source of n-type electricalconductivity [1,2]. Furthermore, the existence of deep acceptorstates associated with gallium vacancy or gallium–oxygen vacancypairs have also been reported in Ga2O3 [3,6]. The recombination ofan electron trapped on a donor with a hole trapped on an acceptor,a process known as donor–acceptor pair (DAP) recombination,produces an intense blue PL [3,6]. Owing to a variety of its

properties, β-Ga2O3 has found applications in gas sensors [7] andas a luminescent phosphor [3]. Furthermore, it has been consid-ered for applications in photodetectors [8], as a deep-ultraviolettransparent conducting oxide [9,10], and as an insulating barrierfor spin-dependent tunnelling junctions [11]. The catalytic proper-ties of β-Ga2O3 have also been demonstrated [12].

Nanostructured β-Ga2O3 has been grown in nanowire, nano-belt, nanoribbon, and nanosheet morphologies, using differentgas-phase growth techniques, which can readily produce nanos-tructures having smooth surfaces [13–26]. Quasi one-dimensional(1D) nanostructures are particularly attractive building blocks fornanoelectronics, nanophotonics, and bio-medical applications[26]. Specifically, high refractive index (�2) and wide transpar-ency in the visible-to-UV range make Ga2O3 an attractive materialfor waveguiding applications [27,28]. It has been suggested thatnative defects responsible for conductivity and PL of Ga2O3 form inthe near-surface region [6]. Controlling the surface structure andmorphology of Ga2O3 nanowires (NWs) could therefore be criticalfor controlling functional properties of this multifunctional mate-rial. However, to our knowledge, there has been no systematicinvestigation of the development and manipulation of Ga2O3 NWfaceting.

Majority of the reported syntheses of β-Ga2O3 NWs involvemetal nanocluster-catalyzed growth [13–23,25,29,30], presumablyenabled by the vapor–liquid–solid (VLS) growth mechanism [31].

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Journal of Crystal Growth

http://dx.doi.org/10.1016/j.jcrysgro.2014.03.0370022-0248/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ1 519 888 4567x38144.E-mail address: pavler@uwaterloo.ca (P.V. Radovanovic).

Journal of Crystal Growth 396 (2014) 24–32

On the other hand, vapor–solid (VS) growth mechanism [32] mayprovide the advantage of producing single crystal, high-aspectratio nanostructures with unique morphologies. Crystal facets thatemerge on the surface of a nanostructure [33] during growth mayexhibit different physical and chemical properties. The facets maydiffer in electron density, affinity for the adsorption of differentgas analytes, and catalytic activity [34,35]. Highly faceted nanos-tructures can also lead to interesting optical properties [36,37].More importantly, possible control of Ga2O3 NW faceting by VSgrowth mechanism may yield single crystalline nanostructureswith controlled orientation of the crystal lattice and native defectformation, resulting in highly direction-dependent electronic and

optical properties. However, a limited number of studies havefocussed on VS-grown gallium oxide structures [14,24,38–47],producing elongated features having simple rectangular andquasi-hexagonal cross-sections, mostly at the micrometer scale.

Here, we report the synthesis and characterization of singlecrystalline, quasi-1D Ga2O3 nanostructures synthesized through VSgrowth. By adjusting the reaction parameters we achieved complexfaceting of these nanostructures and explored the morphologydependencies on the synthetic parameters. The resulting morphol-ogies were rationalized in the context of the VS crystallizationprocess. The formation of native defects and the corresponding PLproperties of these unique nanostructures were also demonstrated.

2. Experiment

2.1. Growth procedure

Gallium oxide nanostructures were grown on silicon substratesin a horizontal hot-wall three-zone tube furnace using a 2-in.diameter quartz gas flow tube. The growth substrates (1 cm�1 cm)were washed with deionized water and ethanol, and dried withnitrogen flow prior to synthesis. In a typical growth procedure,�50 mg of gallium metal (Strem Chemicals Inc.) on a micasubstrate (Ted Pella Inc.), and a growth substrate were placed ona quartz boat with a separation distance of 0.5–4.5 cm, and loadedinto the center of the furnace. The growth substrate was positioneddownstream from the gallium precursor. After purging the furnacetube with argon gas for 30 min (400 sccm), the furnace was heatedto the desired reaction temperature (700–1050 1C). Upon reachingthe final temperature, oxygen gas (1.5–6 sccm) mixed with theargon carrier gas (100–400 sccm) was introduced into the tube,and the growth was allowed to proceed at a constant temperature.The oxygen flow was then terminated and the sample allowed tonaturally cool to room temperature under argon flow (400 sccm).A white powder deposit was observed on the substrates.

2.2. Characterization and measurements

The gallium oxide nanostructures were characterized by X-raypowder diffraction (XRD) (INEL diffractometer, Cu Kα1 radiation),field-emission scanning electron microscopy (SEM, LEO 1530)equipped with an energy dispersive X-ray spectroscopy (EDX)detector, transmission electron microscopy (TEM) and selectedarea electron diffraction (SAED) (JEOL, 2010F), and micro-Ramanspectroscopy (Renishaw, 785 nm laser excitation). Photolumines-cence imaging was carried out using a 250 nm �50 mWexcitationfrom the frequency-tripled Millenia-pumped Tsunami (Spectra-Physics), and a CCD camera connected to a BH-2 Olympus micro-scope. The photoluminescence spectra (250 nm excitation) ofsamples on the growth substrate were collected at room tempera-ture with a Varian Cary Eclipse fluorescence spectrophotometer.Time-resolved PL measurements were collected with the sameinstrument by using a Xenon flash lamp as the excitation source(230 nm) and detecting the emission at the maximum of theGa2O3 DAP band. The delay time and gate time were set to 3 and1 μs, respectively. The delayed spectra were collected from 300 to800 nm with the delay time of 0.1 ms and the gate time of 5 ms.

3. Results and discussion

3.1. Morphology

The white products of the syntheses generally consisted exclu-sively of elongated nanostructures (NWs) having variable aspect

Fig. 1. Faceted Ga2O3 nanowires. (a) SEM image of a collection of nanowiressynthesized by chemical vapor deposition via vapor–solid growth mechanism.(b)–(e) SEM images of selected individual nanowires demonstrating an increaseddegree of faceting. Scale bars represent 1 μm for (a) and (b), and 500 nm for (c)–(e).(f) and (g) Thickness (f) and length (g) distribution histograms for the nanowiresshown in (a). (h) Typical EDX spectrum of as-synthesized gallium oxide nanos-tructures. The silicon peak is from the growth substrate.

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ratio (Fig. 1a). The nanostructures had different arrangements offacets on the side walls, ranging from NWs with simple rectan-gular or hexagonal cross section (Fig. 1b) to those which appear tohave evolved into highly faceted morphologies (Fig. 1e). Majorityof the NWs were generally categorized as being between thesetwo extremes (Fig. 1c and d). The NW thickness was in the range of100–200 nm with an average thickness of 120747 nm (Fig. 1f),the lengths were around 1.270.5 μm (Fig. 1g), and their surfaceswere evidently smooth. The NW thickness distribution is compar-able to those reported for β-Ga2O3 NWs grown by Au-catalyzedVLS mechanism [29,30]. EDX analysis (Fig. 1h) confirmed that NWswere consisting of only gallium (Lα peak at �1.11 keV) and oxygen(Kα peak at �0.53 keV). The silicon peak at �1.74 keV (Kα peak) isassociated with the growth substrate. No other peaks weredetected, indicating high purity of the obtained samples.

The morphology dependence on the reaction conditions wasexplored, specifically, with respect to the growth temperature,oxygen precursor concentration, and carrier gas flow. Fig. 2 showsSEM images of the reaction products synthesized under differentconditions. Fig. 2a–c demonstrates the dependence of the nanos-tructure morphology on the growth temperature, while keepingother parameters identical. As the reaction temperature increases(from 800 to 1000 1C) the resulting nanostructures transformfrom short, poorly-defined nanocrystallites into straight, longnanowires with smooth surfaces and high aspect ratios. Similarobservations were made for increasing carrier gas flow rate(Fig. 2d–f), although to a somewhat lesser extent. By increasingthe flow rate by a factor of 4 (from 100 to 400 sccm), the nano-structures adopt a high aspect ratio while retaining a multi-facetedmorphology. However, the oxygen flow rate has exactly theopposite effect on the NW formation and growth. Decreasingoxygen flow rate by the same factor (from 6 to 1.5 sccm, Fig. 2g–i) results in very similar changes to those observed in Fig. 2d–f.

From the images in Fig. 2, we conclude that NWs with both highaspect ratio and faceted morphology can be obtained only underconditions of high temperature (4950 1C), low oxygen precursorconcentration (r1.5 sccm), and high carrier gas flow (400 sccm).All other conditions produced structures with faceted surfaces butlow aspect ratios, while no growth occurred at temperaturesbelow 700 1C. Furthermore, growth parameters that producedhigh aspect ratio faceted nanostructures were independent ofthe separation distance between the precursor source and growthsubstrate; only the yield of nanostructures decreased with anincreased distance between substrate and the gallium source inthe reaction tube.

3.2. Structural analysis

Gallium oxide nanostructures grown by VS and VLS mechan-isms commonly crystallize in the thermodynamically stableβ-Ga2O3 phase, which belongs to the monoclinic crystal system(α¼γ¼901, βa901; space group: C2/m) [4,5]. The unit cellcontains four formula units, with eight Ga atoms and twelve Oatoms distributed into two Ga and three O non-equivalent sites(Fig. 3a). The Ga sites are surrounded by O sites in distortedtetrahedral, Ga(1), or octahedral, Ga(2), coordination. The oxygenions are located at three different crystallographic sites: two aretrigonally coordinated, O(1) and O(3), and one is quasi-tetrahedrally coordinated, O(2) [1,48]. The powder XRD patternof these NWs (Fig. 3b) shows sharp peaks that correspond to bulkβ-Ga2O3. The diffraction peaks are in good agreement withprevious reports for crystalline β-Ga2O3 nanostructures[7,8,13,15,16,21–23,49]. The assignment of the peaks was per-formed by matching them to those of a simulated powder XRDpattern for β-Ga2O3 generated by the PowderCell 2.3 XRD

Fig. 2. SEM images of nanostructure morphologies for different growth parameters. (a)–(c) 800 (a), 900 (b), and 1000 1C (c) growth temperature, with 100 sccm carrier gas,1.5 sccm O2, and 10 min growth time. (d)–(f) 100 (d), 200 (e), and 400 sccm (f) carrier gas flow, at 1000 1C growth temperature, 1.5 sccm O2, and 10 min growth time. (g)–(i) 6(g), 3 (h), and 1.5 sccm (i) O2, at 950 1C growth temperature, with 400 sccm carrier gas flow and 10 min growth time. Anisotropic morphologies are evident above 950 1C, forlow O2 flow, and high carrier gas flow (i.e., for samples shown in (c), (f), and (i)). Scale bars are 1 μm for (a), (b), (f), and (g); all others are 2 μm.

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calculation software, using experimentally determined latticeparameters from the literature [50].

The faceted morphology of the NWs was examined in detail byTEM. Fig. 4a shows a TEM image of a faceted NW, similar to thoseshown in Fig. 1. The SAED pattern (Fig. 4a, inset) shows cleardiffraction spots, indicating the single-crystalline nature of thesemulti-faceted NWs. The streaking present in the pattern isattributed to extended crystal lattice defects, such as stackingfaults and micro-twins [13]. SAED from selected regions with nostreaking together with lattice-resolved TEM imaging were usedfor detailed crystal structure analysis (Fig. 4b and c). The analysisof the SAED pattern indicates that nanostructure growth directionis along the [0 1 0] direction (i.e., along the b axis of the crystalstructure), and the lattice spacing perpendicular to the growthdirection corresponds to the {1 0 0} planes. The calculated latticespacings from the Bragg spots labeled 0 2 0, 1 1 0 (or 110), and2 0 0 in Fig. 4b are 0.153, 0.297, and 0.590 nm, respectively. Themeasured angle of 104.31 agrees well with the expected anglebetween the [1 0 0] and [1 1 0] directions. The crystal lattice of allNWs examined shared the common characteristic of the [0 1 0]direction oriented along the NW growth axis. Direct lattice planemeasurements of 0.149, 0.296, and 0.595 nm (Fig. 4c) agreereasonably well with those determined from the SAED pattern,and also correspond to Miller indices (h k l) of 0 2 0, 1 1 0 (or 110),and 2 0 0, respectively. The unit cell sides formed by latticeconstants a and b are indicated on the HRTEM image in Fig. 4c,and their measured values are 12.01 and 2.976 Å, respectively, ingood agreement with previously reported values [4,5,48].

To further examine the lattice planes and orientation of NWsidentified from SAED and the lattice-resolved TEM measurements,we compared the high-resolution TEM images to the

corresponding lattice plane models of monoclinic β-Ga2O3 [4]oriented in the same direction as the experimental zone axis(Fig. 5). Fig. 5a shows a lattice-resolved TEM image of a represen-tative NW perpendicular to the NW growth direction. The atomic

Fig. 3. Structure of facetted Ga2O3 nanowire samples. (a) Unit cell, indicating Ga(smaller spheres) and O (larger spheres) non-equivalent sites. Ga(1) and Ga(2) havedistorted tetrahedral and octahedral coordination, respectively. O(1) and O(3) aretrigonally coordinated, and O(2) is tetrahedrally coordinated. (b) Powder XRDpattern showing peaks corresponding to β-monoclinic Ga2O3. The broad peakcentred at �501 is attributed to the aluminum sample holder.

Fig. 4. TEM analysis of faceted nanostructures. (a) Low magnification TEM image ofa representative nanowire. Inset shows the corresponding SAED pattern. The arrowindicates the growth direction, which corresponds to [0 1 0]. Scale bar, 0.5 μm.(b) SAED along the [0 0 1] zone axis, on the corner formed by the side wall and thetip. The reciprocal lattice has a unit cell angle β¼104.31. Interplanar spacingsdetermined from the diffraction spots match the corresponding labelled directions.Inset shows the nanowire region analyzed. Scale bar, 20 nm. (c) Lattice-resolvedTEM image of the corner of the nanowire. Lattice spacing measurements paralleland perpendicular to the growth direction indicate [0 1 0] and [1 0 0] crystallinedirections, respectively, in agreement with the SAED. The unit cell face parallel tothe lattice plane is indicated, with a¼12.01 Å and b¼2.98 Å, magnified by a factorof 2 for clarity. Scale bar, 5 nm.

I.D. Hosein et al. / Journal of Crystal Growth 396 (2014) 24–32 27

arrangement and symmetry of the gallium atoms are well corre-lated with the model crystal oriented along the [0 0 1] zone axis(Fig. 5b, black circles). Similar analysis along the [1 0 0] direction(i.e., [1 0 0] zone axis) also shows a well-correlated atomicarrangement and symmetry (Fig. 5c and d). These results suggestthat a preferred NW lattice orientation exists, with the [0 1 0]direction parallel to the NW growth direction.

Raman spectrum of a typical Ga2O3 NW product is shown inFig. 6 (black trace). The Raman-active modes of β-Ga2O3 canbe grouped into three regions: (i) stretching and bending ofGaO4 tetrahedra (770–500 cm�1), (ii) deformation of Ga2O6 octa-hedra (480–310 cm�1), and (iii) libration and translation of

the tetrahedra-octahedra chains (below 200 cm�1) [51,52]. Allobserved Raman peaks agree with those of β-Ga2O3 commercialpowder (Fig. 6, red trace), and are consistent with the previousexperimental and theoretical studies [13,17,41,49,53]. Based onthe comparison with theoretical results [51], all peaks observed forthe NW sample can be unambiguously assigned to Ag modesymmetry, with a possible exception of the peaks at 480 and659 cm�1, which may be attributed to either Ag or Bg modesymmetry. We note that all peaks for Ga2O3 NWs exhibit smallblue shifts of ca. 5 cm�1 relative to the bulk. The blue shifts of theRaman mode frequencies of low dimensional materials have beenattributed to a quantum confinement effect or internal strains inthe crystal lattice at the nanoscale. Given the dimensions of NWsin this work (Figs. 1 and 2), it is unlikely that quantum size effect isresponsible for the observed spectral shift. Instead, we believe thatmulti-faceted NWs experience compressive strain, which leads tothe blue shift in the characteristic phonon frequencies [51].Interestingly, both blue and red shifts of the Raman frequenciesfor β-Ga2O3 NWs have been reported for different NW growthdirections. Specifically, a red shift of the phonon frequencies hasbeen reported for NWs with [401] growth direction, while theblue shift has been observed for NWs grown in [1 1 0] direction.This difference has been attributed to the growth direction-dependent internal strains in NWs, where red shifts were asso-ciated with tensile, and blue shifts with compressive strains. Onthe other hand, the spectral positions of the Raman peaks for NWsgrowing in [0 0 1] direction remain unchanged relative to bulkGa2O3. The results of this work suggest that NW growth along theb axis (i.e. [0 1 0]) leads to an increase in the compressive strain.Furthermore, it cannot be excluded that a change in the faceting ofβ-Ga2O3 NWs could also result in a change in the internal NWstrain. The ability to manipulate NW faceting and growth directionby changing the reaction conditions, as demonstrated in this work,could allow for detailed mapping of the faceting-strain correlationin the future.

3.3. Morphology evolution

The morphology of crystals grown from the vapor depends onthe growth kinetics of different crystal facets [54]. The classicalgrowth theories for flat crystal surfaces assume that discontinu-ities on the surface, such as kinks and ledges, act as sites for theincorporation of new atoms (Fig. 7a) [55–57]. These sites cause theirreversible adsorption of atoms and thus facilitate growth. As aresult, the growth of a crystal surface occurs through the advance-ment of these steps, and the growing crystal boundary moves inthe direction perpendicular to the surface. On the other hand,atoms that adsorb on smooth surfaces are more likely to bedesorbed back into the vapor phase before diffusing over thesurface to an adsorption site at a discontinuity, especially at highertemperatures. High index crystal planes will have a higher densityof these extended defect sites, and thus will grow faster than lowindex planes which are more atomically smooth (i.e., containfewer discontinuities). Ideally, higher index planes (i.e., highersurface energy facets) should disappear in the growth evolutionwith the lowest energy surfaces remaining as determined by theWulff plot [57,58]. Therefore, the formation of high aspect ratio,faceted structures based on growth rate differences at the surfaceis highly dependent on the underlying crystal structure, althoughother mechanisms such as defect-induced growth may also beinvolved [54]. These principles can be used to rationally tune thenanostructure morphology. Specifically, in this study, tempera-tures above 950 1C produced only thin, long NWs with simplecross section morphologies, most likely due to the ease ofdesorption of growth adatoms from the smooth side walls corre-sponding to the most stable facets. A decrease in the growth

Fig. 5. Atomic lattice modelling. (a) Close-up image of Ga2O3 nanowire latticealong the [0 0 1] zone axis direction. (b) Corresponding atomic model plane alongthe [0 0 1] direction, showing atomic positions of gallium atoms (blue) and oxygenatoms (red). The identical lattice symmetry and atomic positions in (a) and(b) confirm the zone axis and the assigned nanowire facet orientation. (c) Closeup image of the atomic lattice imaged along the [1 0 0] zone axis direction.(d) Corresponding atomic model plane along the [1 0 0] direction confirms thelattice indexing. The circles in all panels indicate gallium atom arrangement motifs.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

Fig. 6. Raman spectrum of as-synthesized Ga2O3 nanowires on the growthsubstrate (top, black). Spectrum of a commercial powder (bottom, red) is shownfor comparison. All bulk Raman modes are present in the spectrum of the nanowiresample, and the corresponding Raman shifts are indicated in the graph. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

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temperature produces thicker nanostructures with more complexfaceted surfaces, indicating a higher probability of growth speciesincorporation onto the side walls. At lower temperatures, theaspect ratio of the NWs is generally reduced. The control ofanisotropy is also highly dependent on other growth conditions,including the concentration and flow rate of the NW precursors inthe gas phase, as required to achieve low supersaturation condi-tions. At the single nanostructure level, the control of theseparameters allows the local precursor concentration to be higherthan the equilibrium concentration for some surfaces, but equal toor lower than that for others [54], leading to advancement ofspecific NW facets. Hence, under conditions of high temperature,low precursor concentration, and high carrier gas flow, growth isconcentrated at the ends of the nanostructure, resulting in NWswith high aspect ratios and low degree of faceting. Whereas, lowtemperature, high precursor concentration, and low carrier gasflow conditions lead to moderate to high supersaturation, whichsupports bulk crystal growth (i.e., generally isotropic growth withrandomly faceted surfaces). Finely tuning the growth parametersleads to suitable conditions for the formation of NWs with highaspect ratios and side wall faceting to occur.

Notably, the pointed tip observed for typical multi-faceted NWs(Fig. 4) indicates that growth along the [0 1 0] direction occurs onhigh index crystal surfaces most likely with high growth rates. It ispossible that growth at the tip occurs by (0 1 0) terrace planesgrowing laterally until they reach the side walls of the nanos-tructure, perpendicular to the NW long axis, as shown in Fig. 7b.New (0 1 0) planes would continuously grow at the outer mostedge of the pointed tip and, consequently, the high energy surfacesremain, allowing the NW growth to persist in this direction. Thiswould explain the high energy tip surfaces, and account for theadvancement of the growth along the [0 1 0] direction concur-rently with an increase in the NW thickness (and a decrease of theNW aspect ratio) under moderate-to-high supersaturation condi-tions, as demonstrated in Fig. 2. The [0 1 0] NW growth direction isalso consistent with free surface energies of β-Ga2O3, as thesurface energies of the primary facets of the unit cell decrease inthe order (0 1 0)4(0 0 1)4(1 0 0) [50]. Hence, the (0 1 0) latticeplane should grow the fastest, resulting in the NW long axis alongthis direction.

The morphology of the NW cross-section may initially bedictated by the underlying symmetry of the monoclinic β-Ga2O3

Fig. 7. Possible evolution of nanowire faceting. (a) Schematic diagram of the nanowire growth at the tip of the nanowire. The crystal grows by incorporation of atoms at theledges (red line) which move towards to the side walls over time. (b) High-resolution TEM image of the corresponding ledges located on the nanostructure tip. (c) Schematicsof the proposed evolution of the facets on the side walls of the nanowires (from left to right). Diffusion of precursor concentrated at the corners (purple arrows) focusesgrowth at these locations, which eventually evolve into facets as well. The close proximity of these regions to one another may cause them to merge, after which themorphology appears like several anisotropic structures fused together. Scale bars are 1 μm, 200 nm, 200 nm, and 200 nm, from left to right, respectively. (For interpretationof the references to color in this figure legend, the reader is referred to the web version of this article.)

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lattice along the (1 0 0) and (0 0 1) planes, resulting in theexposure of the low energy lattice planes, and a quasi-hexagonalNW morphology (Fig. 7c, left). As the facet boundaries along thewalls extend further into the surrounding precursor vapor, theywill consume more precursors and, consequently, induce a netdiffusion from the adatoms adsorbed on the side walls to theNW edges. Hence, the NW boundaries will advance faster at theseedges, which will inevitably begin to grow their own facets,leading to the change in the cross section morphology and theformation of multi-faceted structures. Furthermore, it wasobserved that NWs with higher degrees of faceting were onaverage shorter than “less-faceted” NWs (Figs. 1 and 2). Onepossible explanation for this observation is that the greater surfacearea of the side walls will consume more precursor under thefavorable conditions (vide supra), creating a net flux of precursortowards the surface and rendering the tips deficient of precursor.Thus, the growth may initiate from the tips, leading to the NWmorphology, until sufficient side wall surface area and facetingexists. Hence, a reasonable assumption is that the sequence ofimages in Fig. 1b–e represents snapshots of particle morphologyevolution over time. Possible facet development based on NWmorphologies observed in SEM images is schematically shown inFig. 7c. As nucleation may occur at different times during thegrowth, the samples consistently show nanostructures with dif-ferent degrees of evolved facets, yet all generally having highaspect ratios.

3.4. Optical properties

The intense blue PL upon the excitation of the NW sampleswith 250 nm laser beam was observed both visually and by opticalmicroscopy (Fig. 8a). A spectrum corresponding to the observedluminescence is shown in Fig. 8b (solid line). The emissionspectrum is dominated by a broad band with the maximumintensity at approximately 430 nm, in agreement with the spectrapreviously reported for different gallium oxide nanostructures

[16,23]. This emission has been associated with the DAP recombi-nation mechanism, where the donor species are proposed tobe oxygen vacancies, and the acceptors gallium vacancies orgallium–oxygen vacancy pairs [3,15]. A lower intensity transition,observed as a shoulder at ca. 340 nm, is attributed to the exciton-like transition (e.g., recombination of a self-trapped exciton) [3].The excitation spectrum is also shown in Fig. 8b (dashed line) andis in agreement with the band gap absorption of bulk β-Ga2O3 [3].Similar features have been observed in other studies of Ga2O3

NWs. In some cases, appreciable red [30,59] or even green [25]emissions have also been reported. These emissions have beenassociated with recombination processes involving external impu-rities, such as nitrogen [30,59], suggesting the importance ofcontrolling the point defect formation via synthesis conditions.

The lifetime of the DAP emission involves electron transferfrom a donor to an acceptor site (often referred to as tunneltransfer), as the rate determining step. Consequently, PL of Ga2O3

experiences an afterglow following the excitation. Fig. 8c showsthe delayed PL spectra of the same Ga2O3 NW sample collected 0.1,0.5, 1 and 3 ms upon the sample excitation. Broad PL spectra areobserved several ms after excitation, confirming the DAP origin ofthe observed emission. The delayed emission bands are red-shifted relative to the steady-state spectrum in Fig. 8b, owing tothe recombination involving larger separation between donorsand acceptors at longer delay times [60,61]. The long lifetime ofGa2O3 NW DAP emission is confirmed by time-resolved PLmeasurements. Fig. 8d (black line) shows the PL time-decay ofthe NW sample from Fig. 8a and b. The functional form of thisdecay curve is neither exponential nor hyperbolic, which is alsoconsistent with the DAP nature of the PL of Ga2O3. The survey ofthe available literature indicates that PL of the NW samples in thiswork has one of the longest lifetimes reported for any Ga2O3

nanostructures. β-Ga2O3 commonly exhibits oxygen deficiencythat is generally associated with oxygen vacancies [3,62,63], whichare inherent from the high temperature and/or reducing condi-tions of the syntheses [60,61]. The formation of these native

Fig. 8. Photoluminescence of the Ga2O3 nanowires under 250 nm excitation. (a) Optical microscopy image of the nanowires on the growth substrate. Inset: a photograph ofthe visually observable blue emission. Spot size is approximately 2 mm in diameter. (b) Photoluminescence excitation (dashed line, λem¼430 nm) and emission (solid line,λex¼250 nm) spectra of Ga2O3 nanowires. (c) Delayed PL spectra of the same sample (λex¼250 nm). Delay times between excitation and PL data collection are shown in thegraph. (d) Time-resolved PL data of the same sample (black line), collected at 430 nm. The data are compared with the analogous measurements for 6 nm diameter colloidalGa2O3 nanocrystals.

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defects is energetically favorable in the near-surface region, mak-ing the resulting blue emission a surface-related property [64].The ability to modify dimensionality, morphology, and faceting ofGa2O3 nanostructures may therefore allow for tuning of their PLproperties via native defect interactions. This phenomenon isdemonstrated in Fig. 8d, which compares the PL decay dynamicsof Ga2O3 NWs to that of colloidal nanocrystals having an averagediameter of ca. 6 nm [60,65]. The photoluminescence of NWsexhibits a significantly longer average lifetime, owing to a muchlarger range of defect separations.

4. Conclusion

In summary, high aspect ratio nanostructures with facettedmorphology were produced by vapor–solid growth method. Thenanostructures grew as pure monoclinic β-Ga2O3 NWs, with thelong axis in the direction parallel to the crystal b axis (i.e., [0 1 0]direction). The crystal orientation and lattice arrangement wereconsistent with crystal plane models for the monoclinic phase.High aspect ratio and faceted morphologies were observed underhigh temperature, low precursor concentration, and high carriergas flow conditions, as expected for the vapor–solid growthprocess. These findings may be applied to other crystal systems,particularly complex metal oxides, to explore shape tuningand faceted NW growth. These NWs display intense blue photo-luminescence under UV laser excitation, which is associated withnative defect formation, and is dependent on the synthesisconditions, nanostructure size, morphology and surface area. Thecontrol of the dimensionality, morphology and faceting of NWsdemonstrated in this work could be relevant for various applica-tions including photonics, optoelectronics, sensing, and catalysis.

Acknowledgements

This work was supported by the Natural Sciences and Engi-neering Research Council of Canada (Discovery grant 341919-2010), Ontario Ministry of Research and Innovation (EarlyResearcher Award to P.V.R., ER10-07-104), Canada Foundation forInnovation (CFI-LOF 204782), and Canada Research Chairs pro-gram (P.V.R.). The electron microscopy research described in thispaper was performed at the Canadian Centre for Electron Micro-scopy at McMaster University, which is supported by NSERC andother government agencies.

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