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nCorrespondinLaboratory, SchPenang, MalaysiTel.: þ60 17417

E-mail addreqnabbdullah@gm

Please cite thiand dimension

Ceramics International ] (]]]]) ]]]–]]]www.elsevier.com/locate/ceramint

One-dimensional GaN nanostructures prepared via chemical vapordeposition: Substrate induced size and dimensionality

Q.N. Abdullaha,b,n, F.K. Yama, N.K. Hassanb, M.A. Qeeda, K. Al-Heuseenc, M. Bououdinad,e,Z. Hassana

aNano-Optoelectronics Research and Technology Laboratory, School of Physics, University Sains Malaysia, 11800 USM Penang, MalaysiabPhysics Department, Collage of Education, Tikrit University, IraqcAl-Balqa Applied University, Ajloun University College, Jordan

dNanotechnology Centre, University of Bahrain, PO Box 32038, Kingdom of BahraineDepartment of Physics, College of Science, University of Bahrain, PO Box 32038, Kingdom of Bahrain

Received 9 January 2014; received in revised form 10 February 2014; accepted 10 February 2014

Abstract

Large-scale uniform one-dimensional (1D) GaN nanostructures have been successfully synthesized via the chemical vapor deposition method.The type of substrate was found to be a crucial factor in the synthesis of GaN, it affects the shape as well as the diameter of the formed nanowires,and nanorods. It was noticed that the size and dimensionality were the most influential on structural and optical properties of the grown GaN. X-ray diffraction confirms the formation of hexagonal wurtzite structure where the lattice parameters vary with the type of substrate. Similarly,transmission electron microscopy observations reveal the formation of different morphologies: nanowires for Si-substrate; nanorods for AlN andAl2O3-Substrate. Photoluminescence spectra of GaN nanostructures showed two peaks at a near-band-edge (NBE) emission in the ultravioletregion and a broad emission around the red emission (RE). The crystallinity of product materials and defects in the crystal structure were studiedthrough Raman spectroscopy.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Defects; GaN Nanostructures; NWs; NRs; CVD

1. Introduction

Nanostructured materials have attracted great attention inrecent years because of their unique physical propertiescompared to their bulk counterpart [1–6]. GaN has becomeone of the most important functional components in thefabrication of optoelectronic devices because of its direct wideband gap of 3.4 eV at room temperature, an excitation bindingenergy of 26 meV, excellent optical properties, and highthermal stability [7–10]. Additionally, 1D GaN nanostructures

10.1016/j.ceramint.2014.02.03114 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author at. Nano-Optoelectronics Research and Technologyool of Physics, University Sains Malaysia, 11800 USM,a.0405; fax: þ60 46579150.sses: kahtanabdullah21@yahoo.com,ail.com (Q.N. Abdullah).

s article as: Q.N. Abdullah, et al., One-dimensional GaN nanoality, Ceramics International (2014), http://dx.doi.org/10.1016/j.cer

has received wide attention owing to its distinguishableperformance particularly in sensing [11,12], field emissionand photocatalysts applications [13,14]. Bulk GaN is difficultto achieve due to its extreme growth conditions, and complexprocesses. However, with the advancement of technology, GaNsubstrates with different facets are now commercially available.In order to overcome the above mentioned difficulties, varioussubstrates such as Al2O3, Si or GaAs have been utilized for thegrowth of various GaN nanostructures [4,15,16]. R. Czernetzkiet al. [17] have reported the growth of GaN bulk crystals at highhydrostatic N2 pressures of 10–20 kbar. However, their processis unworkable owing to the small dimension of such crystalsadding to that the growth process is carried out at a very highpressure as well as high temperature of 1500 1C.Numerous efforts have been devoted to the fabrication of 1D

GaN nanostructures through various methods such as chemical

structures prepared via chemical vapor deposition: Substrate induced sizeamint.2014.02.031

Fig. 1. The high resolution X-ray diffraction (HR-XRD) patterns of GaNnanostructures grown on (a) Si, (b) AlN, and (c) Al2O3 substrates.

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vapor deposition (CVD) [4,8], metallorganic (MOCVD) [18],pulsed laser deposition (PLD) [19], molecular beam epitaxy[2,7], and hydride vapor phase epitaxy (HVPE) [15]. Recently,F. Schuster et al. [20] have reported the synthesis of GaNnanowires by plasma assisted MBE. Their results show thegrown of self-assembled, epitaxial GaN nanowires on (111)single crystalline diamond without using a catalyst or bufferlayer. Similarly, Fernández-Garrido et al. [21], have reportedthe fabrication of GaN nanowires on SiC substrate using thesame technique. However, the main disadvantage of theirtechnique is the high cost, complexity of the process as well asthe growth rate is very low thus a typical growth process needsseveral days.

The CVD process is one of the most important approachesfor growing uniform GaN with a variety of morphologies suchas nanowires (NWs), nanorods (NRs), and nanotubes (NTs)[11,13,22]. The synthesis of GaN through this method offersseveral advantages over other methods, including simplicity,low cost and much better control over the size and morphologyof the grown GaN [10]. The fabrication of GaN with highcrystallinity and 1D dimensionality is the main advantage ofusing this method. Therefore, GaN is introduced as anattractive material in optoelectronic applications, in which1D nanomaterial plays an important role in achieving themain objective of such devices. Moreover, in the CVDprocess, many researchers have deposited GaN on metal-coated substrates such as gold (used as a catalyst) which iscalled vapor liquid solid (VLS) [23,24], whereas others havesynthesized GaN via a catalyst-free vapor–sold process (V–S)[25,26].

The aim of this study is to fabricate a large-scale 1D GaNusing a low-cost technique, and to find out the optimumconditions for the development of uniform 1D GaN nanos-tructures grown via V–S on various substrates such Si, AlNand Al2O3. The experimental results demonstrate that bothsize and dimensionality are most probably responsible forthe evolution of structural and optical properties of thegrown GaN.

2. Experimental procedure

Prior to deposition, the substrates were cleaned using theRadio Corporation of America (RCA) method. AlN thin filmwith 200 nm thickness was deposited on Si substrate by RFsputtering. GaN nanostructures are grown on three differentsubstrates; Si, AlN and Al2O3 by heating a GaN powder(0.25 g, purity 99.99%) loaded on a ceramic boat as sourcematerial and positioned at the centre of quartz tube. Thesubstrates were placed away from the source in the down-stream direction of the carrier gases. The tube furnacetemperature is set at 1130 1C for a period of time of 120 minunder 1 standard litre per minute flow of Ar gas (99.99%).Afterward, the nitridation was then performed under the flowof ammonia gas (100 sccm, high purity, 99.99%). Thenanostructures were grown at 1000 1C for a period of timeof 30 min on various substrates.

Please cite this article as: Q.N. Abdullah, et al., One-dimensional GaN nanoand dimensionality, Ceramics International (2014), http://dx.doi.org/10.1016/j.cer

The crystal structure of the samples characterized using highresolution XRD PANalytical X' pert Pro MRD equipped with aCu-Kα1 radiation source (λ=1.5406 Ǻ). The morphologieswere characterized using FESEM ( FEI-NOVA NANOSEM450) equipped with an energy-dispersive X-ray spectroscopy(EDX, Oxford instrument Analytical Ltd.). TEM images weretaken with Philips CM 12 transmission electron microscope.The PL and Raman backscattering measurements were per-formed at room temperature using JobinYvon HR800UV.

3. Results and discussion

3.1. Structural analysis

Fig. 1 shows typical HR-XRD patterns of GaN nanostruc-tures grown via CVD technique for various substrates: (a) Si(111), (b) AlN, and (c) Al2O3. All deposited films show threemain reflection peaks that agree very well with the standardbulk GaN having a hexagonal structure (JCPDS card no.50-0792) as previously reported for GaN nanostructures[9,13,22–24]. The diffraction patterns of the samples weresimilar. The three pronounced GaN diffraction peaks wereindexed as (100), (002), and (101), in addition to thefew detected weak peaks indexed as (102), (110), and (103).

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From the XRD data, the lattice constants have been determinedfor the three samples and listed in Table 1. These values are ingood agreement with reported values [9,23,25,26].

Table 1Lattice constants and strain of different GaN nanostructures derived from XRDmeasurements.

Substrate d (Å) c (Å) a (Å) c/a εa¼Δa/a0 εc¼Δc/c0

Si 2.5960 5.192 3.192 1.626 þ0.09 þ0.115AlN 2.5933 5.1866 3.190 1.625 þ0.031 þ0.011Al2O3 2.5910 5.1820 3.192 1.623 þ0.09 �0.077

Fig. 2. Low and high magnification FESEM images of GaN nanostructures

Please cite this article as: Q.N. Abdullah, et al., One-dimensional GaN nanoand dimensionality, Ceramics International (2014), http://dx.doi.org/10.1016/j.cer

The in-plane (along a-axis) and out-of-plane (along c-axis)strains, Ɛa and Ɛc have been calculated from the two relations:Ɛa=Δa/ao and Ɛc=Δc/co [27,28] where Δa and Δc are definedas the deviation of the calculated lattice parameters (a) and (c)from the corresponding un-strained values ao=3.189 Å andco=5.186 Å [25,27]. Positive value for Ɛa and Ɛc denotestensile strain while negative value denotes compressive strain.According to the results reported in Table 1, both values for Siand AlN-substrate are positive, which indicates that the as-grown GaN suffer from tensile strains, while tensile andcompressive strains appeared on Al2O3 (Ɛa40, Ɛco0) sub-strate. The sample deposited on AlN-substrate was found to

grown on: (a and b) Si, (c and d) AlN, and (e and f) Al2O3 substrates.

structures prepared via chemical vapor deposition: Substrate induced sizeamint.2014.02.031

Fig. 3. EDX analysis of a GaN nanostructures grown on (a) Si, (b) AlN, and(c) Al2O3 substrates.

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have the smallest strain than the remaining samples depositedon Si and Al2O3 substrates. This can be associated with thesmaller mismatching between the gown GaN and substrate,for this case AlN-substrate. It is important to highlightthat tensile and compressed strain developed in epitaxialGaN nanostructures appear during growth and could beattributed mainly to the thermal mismatch as well as latticemismatch between GaN and substrates.

3.2. Morphological observations

Low- and high-magnification FESEM images illustrating theevolution of the morphology of GaN grown on differentsubstrates: Si, AlN and Al2O3, are shown in Fig. 2. The 1DGaN nanostructures were successfully fabricated through thethermal evaporation of GaN powder via the CVD method. Theinfluence of substrate on the surface morphology of the GaNnanostructures is obviously observed. The substrate affectedthe shape and diameter of the 1D GaN nanostructures,consisting of a high density of GaN NWs twisted togetherwere grown on Si substrate, Fig. 2(a and b). Highly dense GaNNRs were achieved when substrate is changed from Si to AlN,Fig. 2(c and d), where both diameter and length have beenchanged. As shown in Fig. 2(e and f), a significant convert inthe 1D GaN nanostructures length and diameters occurredwhen changing substrate to Al2O3; straight-shaped GaN NRswere grown with length about several μm. Energy-dispersiveX-ray spectroscopy (EDX) was used to detect the compositionof nanostructured materials as shown in Fig. 3. The chemicalcompositions of all nanostructures are summarized and shownin insert of Fig. 3. The films consist of Ga, N, Si, O, and Alelements. Si and Al comes from substrates while O originatesmost probably due to surface contamination arising fromsubstrate or atmospheric air during growth process insidefurnace tube.

Further investigation of the surface morphology of GaNgrown on different substrates was carried out by TEM. Asshown in Fig. 4(a and b) TEM image displays typical GaNNWs grown on Si-substrate with diameter in the range 30–50 nm and length up to 3 mm. Fig. 4(c and d) shows GaNnanorods were obtained when replacing substrate from Si toAlN, the changing in both diameter and length observed; thediameter ranged from 70 to 140 nm and length reached to6 μm. Finally, replacing AlN substrate by Al2O3 resulted inchanging both the shape and size of the grown 1D GaNnanostructures; thin nanorods with a diameter in the rangefrom 53 nm to 73 nm and length around 4 mm observed; seeFig. 4(e and f). This demonstrated that both shape and size(diameter and length) of the grown nanostructures are con-trolled by the type of substrate.

Both FESEM and TEM results show that no metallicparticles were attached to the tips of the NWs, and NRs,which indicate that the growth of nanostructures may beguided by the V–S mechanism. According to the aboveFESEM/TEM observations, a growth mechanism is proposed.It complies with V–S mechanism, for nanostructured materialsprepared on different substrates and having various sizes

Please cite this article as: Q.N. Abdullah, et al., One-dimensional GaN nanoand dimensionality, Ceramics International (2014), http://dx.doi.org/10.1016/j.cer

(diameter and length), there are three main growth stages: (i)during the first stage, the starting source material (GaNpowder) might be decomposing into Ga and N2 [29] abovethe raised temperature limit. Therefore, Ga may combine withO2 vapor to form gallium oxide (Ga2O3) in the reaction

structures prepared via chemical vapor deposition: Substrate induced sizeamint.2014.02.031

Fig. 4. Transmission electron microscopy (TEM) images of as grown GaN nanostructures on different substrates: (a and b) Si; (c and d) AlN; and (e and f) Al2O3.

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chamber afterward. Ga vapor and as-formed Ga2O3 clusters arealso transferred downstream by the carrier gas to the substrateposition; (ii) in the second stage, during annealing process,NH3 (source of N) can act as a nitriding agent andwill decompose stepwise into NH2, NH, H2 and N2 [30] whenthe temperature reaches 1000 1C, then Ga2O3 nanostructuresare deoxidized to gaseous Ga2O by reacting with H2;(iii) during the third stage, GaN molecules are producedthrough the chemical reaction of Ga2O and NH3. As a result,GaN grains are produced as the nitridation progresses of GaNnanostructures. Therefore, GaN crystallite nuclei propagate upprogressively and various nanostructures (NWs and NRs) arethereby produced.

Please cite this article as: Q.N. Abdullah, et al., One-dimensional GaN nanoand dimensionality, Ceramics International (2014), http://dx.doi.org/10.1016/j.cer

3.3. Optical properties

Fig. 5 shows PL spectra of GaN nanostructures grown ondifferent substrates. All PL spectra showed two peaks, a smallintensity UV peak located at 382.6, 384 and 382 nm, and abroad red emission (RE) peak located at 850, 700 and 650 nmcorresponding to GaN grown on Si, AlN, and Al2O3 sub-strates, respectively. Generally, the weak UV emission rangingfrom 382 to 384 nm is the band-edge emission resulting fromthe recombination of free excitons [11,31], while, the RE maybe related to native point defects such as VGa–ON or VGa–SiGa[32]. From PL spectra, both peak intensity and position (shift)are influenced by the nature of substrate.

structures prepared via chemical vapor deposition: Substrate induced sizeamint.2014.02.031

Fig. 5. PL spectra of the GaN nanostructures deposited on (a) Si, (b) AlN and(c) Al2O3, substrates. Fig. 6. Raman spectra of the GaN nanostructures deposited on (a) Si, (b) AlN

and (c) Al2O3, substrates.

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Fig. 6 shows Raman spectra of GaN nanostructures grownon different substrates. Three dominated Raman-active pho-nons corresponding to A1(TO) around 525 cm�1, E2 (high)around 566 cm�1, and A1 (LO) around 732 cm�1 wereobserved, which agrees with the Raman selection rules forGaN structure [11,30] but their peak's position was found tovary according to the type of substrate. The Raman peak of E2

(high) mode indicates that the grown GaN nanostructurespossess a hexagonal wurtzite structure. In additional, it can beseen in Fig. 6 (b) that peak located at around 850 cm�1 isassigned to the A1 (LO) phonon mode of AlN substrate [33].The peaks situated around 255, and 416 cm�1 phonon modecorresponds to the acoustic overtone (ZB) arising from crystal-line defects and finite size effects, as well as the acousticovertone of GaN nanostructures GaN [30,34]. The position ofE2 (high) for all GaN nanostructures is slightly shifted fromstandard value. This is a clear evidence for the presence of thestrain. It is well known that GaN E2 (high) phonon peak isassociated to the residual strain inside the layer [34].

4. Conclusions

1D GaN nanostructures were successfully synthesized usinga simple cost-effective CVD without any additives. Under the

Please cite this article as: Q.N. Abdullah, et al., One-dimensional GaN nanoand dimensionality, Ceramics International (2014), http://dx.doi.org/10.1016/j.cer

same conditions, various GaN nanostructures were grown onthree different substrates; Si, AlN, and Al2O3. Detailedstructural characterizations confirmed that the formed filmscrystallize within wurtzite hexagonal GaN structure with goodcrystallinity, depending on the nature of the substrate. Mor-phological images showed the formation of 1D GaN NWs andNRs having different diameters and lengths. The differences inlattice mismatch, structural type and orientation between thesubstrates and GaN have a great influence on the morphologyof the grown GaN nanostructures. A growth mechanism wasproposed for the formation of different morphologies based onFESEM/TEM analyses. The difference in peaks position,broadening and intensity of the UV emission with changingthe substrate, led to an important modification of the defectslevel within the grown nanostructures. Raman spectra showedthat 1D GaN deposited on Si, AlN, and Al2O3 substrates wereunder low strain, revealing that the as-obtained GaN nanos-tructures possess a good crystallinity quality.

Acknowledgments

This work was conducted under research University (RU)Grant: (1001/PFIZIK/811155) and the support from UniversitiSains Malaysia is gratefully acknowledged.

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