Communications

DOI: 10.1002/cvde.200604244

Oriented Bicrystalline GaN Nanowire Arrayssuitable for Field Emission Applications**

By Zhuo Chen, Chuanbao Cao,* and Hesun Zhu

Oriented, bicrystalline, GaN nanowire arrays are synthe-sized by a catalytic carbon thermal reduction process. Thebicrystalline structure is composed of (001) planes and thetwin angle is 124°. Field emission measurements show thatthe oriented, bicrystalline, GaN nanowire arrays produce acurrent density of 10 lA cm–2 at a lower turn-on fields of8 V lm–1. It suggests that oriented, bicrystalline, GaNnanowires capped with catalyst particle arrays can serve asa promising candidate for field emission emitters.

One–dimensional (1D), nanostructured semiconductorsas building blocks are considered to have great prospectsfor novel nanotechnological applications.[1] Among various1D semiconductor nanostructures, highly oriented nano-wires have received much attention because they can beeasily integrated into future nanodevices.[2–5] Comparedwith artificial assembly techniques, self–organized growthprovides an effective and low-cost approach for realizationof nanostructured materials with high alignment. There-fore, considerable efforts have been devoted to fabricatevarious highly oriented nanostructures by a self–organizedapproach.[6,7]

Gallium nitride (GaN) (as a wide band gap (3.4 eV)semiconductor with a high melting point, large carrier mo-bility, and high electrical breakdown field) is of intense in-terest for ultraviolet or blue light emitters and high–tem-perature, high–power electronic device applications.[8,9] Sofar, various methods have been reported for the prepara-tion of well-aligned, GaN 1D, nanostructured arrays suchas metal–organic (MO)CVD,[6] hydride vapor phase epi-taxy (HVPE),[10] template–assisted organometallic vaporphase epitaxy (OMVPE),[11] and thermal evaporation pro-cess.[12] Although much progress has been made in the syn-thesis of highly oriented GaN 1D nanostructures, exploringsimple and economical methods of synthesized verticallyaligned GaN nanostructures remains a challenge. In addi-tion, twin structures appearing in nanometer-scale materi-

als are of special interest because they not only have impor-tant effects on the physical (particularly, electronic andmechanical) properties,[13] but also could serve as templatesfor novel device structures.[14] Twinning is frequently ob-served in face–centered cubic structured metallic nanocrys-tals.[15,16] However, there are few reports about twinnedGaN 1D nanomaterials.[17–19] In this work, we report a cata-lytic carbon thermal reduction process for the fabricationof oriented GaN nanowire arrays. Interestingly, the GaNnanowires are of peculiar bicrystalline structure. Fieldemission properties for the oriented GaN nanowirescapped with catalyst particle arrays are also investigated.The mechanism of the formation of the twinned GaNnanowire arrays is discussed.

Oriented, bicrystalline, GaN nanowire arrays weregrown on silicon substrates coated with a ZnO buffer layer(500–600 nm) by a catalytic-carbon, thermal reductionmethod under an atmosphere of NH3. Figure 1a shows theX-ray diffraction (XRD) pattern of the as–synthesizedproducts. The peaks in the XRD pattern could be indexedto hexagonal wurtzite GaN with cell constants comparableto the reported data (Joint Committee on Powder Diffrac-tion Standards, JCPDS Card No. 76–0703). Figures 1b and1c show scanning electron microscope (SEM) images of theproducts. From the side–view SEM images (Figs. 1b, 1c), itcan be seen that large scale, vertically or slantingly alignedGaN nanowires were grown at high density on the siliconsubstrate. In Figure 1c, it is found that the GaN nanowireshave length of 5–20 lm and diameter of about 200 nm.Meanwhile there exist two reentrant edges in the middle ofthe GaN nanowires (as highlighted by the arrows inFig. 1c). In addition, it can be found that the GaN nano-wires were almost capped with catalyst particles, indicatingthat a vapor–liquid–solid (VLS) growth process[20] is in-volved during the formation of the GaN nanowires.

Further structural characterization of these GaN nano-wires was investigated by selected area electron diffraction(SAED), transmission electron microscopy (TEM), andhigh–resolution TEM (HRTEM). Figure 2a shows a repre-sentative TEM image of a GaN nanowire with bicrystallinestructure. From Figure 2a, it can be seen that the nanowireconsists of segments A and B. A and B form a twin struc-ture and there exists a long and narrow twin boundaryalong the middle of the GaN nanowire. The twin boundaryis denoted by T. The SAED patterns taken from differentareas (A, B) of the nanowire show nearly the same patterns(Figs. 2b, 2c), and can be indexed to be the [010] axis. Thissuggests that the segments A and B are single crystal with awurtzite structure. As shown in Figure 2d, the SAED pat-tern taken from T has two sets of spots with the [010] axis,which indicates the peculiar bicrystalline structure. The

Chem. Vap. Deposition 2007, 13, 527–532 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 527

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[*] Prof. C. B. Cao, H. S. Zhu, Dr. Z. ChenResearch Center of Materials Science, Beijing Institute of TechnologyBeijing 100081 (People’s Republic of China)E-mail: cbcao@bit.edu.cn

[**] This work was supported by the National Science Foundation of Chinaunder Grant No. 20471007.

twin plane between A and B is �101� �

. Although the orienta-tions of A and B are nearly the same, there exist some de-fects between them. From Figure 2d, the angle along the�101� axes in the A and B is about 2.7°. A HRTEM imagefor the GaN nanowire is presented in Figure 2e. The twinstructure for the GaN nanowire is also well–reflected in theHRTEM, Figure 2e, in which there exist two sets of lattice

images with lattice fringes of about 0.51 nm, correspondingto the (001) fringes. This reveals the good crystallinity ofwurtzite–type GaN. The angle between the (001) planes inA and B is about 124°. From Figure 2e, it can be found thatthe nanowires contain a twofold rotational twin axis alongthe growth direction, suggesting that they are bicrystalline.In addition, this kind of twin structure has a broad twinningboundary of about 20 nm, which is important for under-standing twin formation. The chemical composition of theGaN nanowires was further investigated by X–ray energydispersive spectrometer (EDS) analysis, as shown in Fig-ure 2f. It confirms that the nanowire is only made of Gaand N elements. The Cu peaks originate from the coppergrid. The catalyst particle on the nanowire’s tip is mainlymade up of Ag by EDS analysis, suggesting that Ag as acatalyst is suitable for preparation of GaN nanostructures.To further find out whether the twinning phenomenon isgeneral in this study, twenty–four of individual GaN nano-wires have been examined by SAED, TEM, and HRTEM.Although the angle between the crystal axes of two twindomains varies from 0° to 5°, eighty percent of the exam-ined GaN nanowires are of the bicrystalline structure, andtheir twin angles of about 124° are consistent. In addition,twenty percent of that have single–crystal structure, asshown in Figure 3. Figure 3a shows a low magnificationTEM image of a single–crystal GaN nanowire. The corre-sponding SAED pattern indicates the single–crystal wurt-zite nature in the GaN nanowire in Figure 3b. At a highermagnification, the HRTEM image shown in Figure 3c givesa lattice fringe of about 0.51 nm, corresponding to the d001

space of the bulk hexagonal GaN. Both the SAED patternand the HRTEM image confirm that the single–crystalGaN nanowire grew along the c-axis. These observed re-sults by SAED, TEM, and HRTEM indicate that the bi-crystal structure in the present study is general.

On the basis of the above morphology and compositionanalysis for the GaN nanowires, the formation mechanismcould be attributed to VLS growth process because catalystparticles were found on their tips. Under our experimentalconditions, the formation process of the GaN nanowirescan be described as follows: at the first stage, the startingmaterial, Ga2O3 powder, will be reduced gradually bygraphite and decompose to produce Ga2O vapor as thetemperature increases. The reduced process can be de-scribed by the following reactions, given in Equations 1 and2, where Ga2O is a volatile metal oxide.[21,22]

Ga2O3 + 2C → Ga2O + 2CO (1)

Ga2O3 + 2CO → Ga2O + 2CO2 (2)

The formed Ga2O vapor will be transported to the sur-face of the substrate by the Ar carrier gas. Meanwhile, theGa2O vapor will also react with CO simultaneously at a de-sired temperature, as shown in Equation 3.[22]

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528 www.cvd-journal.de © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2007, 13, 527–532

Fig. 1. a) XRD pattern taken from the as–synthesized GaN nanowire arrays.b) Low magnification, and c) high magnification side–view SEM images forthe products.

Ga2O + CO → 2Ga + CO2 (3)

The Ga clusters may be then formed. At the same time,AgNO3 will also decompose into some Ag clusters asshown in the reaction given by Equation 4.[23]

2AgNO3 → 2Ag + 2NO2 + O2 (4)

These Ag clusters will also be transported by the Ar gasto the surface of the substrate, where they will deposit on

the substrate with some Ga clusterstogether in the form of small liquiddroplets. These small liquid Ga–Agdroplets not only serve as an energet-ically favored site for the absorptionof growth species, but also can avoidthe decomposition of the sputteredZnO layer under the droplets afterthe carrier gas is switched from Ar toNH3.[24]

In the second stage, when the tem-perature reaches the preset value, theNH3 gas will be introduced instead ofAr. Both the Ga and Ga2O will reactwith NH3 with the assistance of theAg catalyst, and lead to the continu-ous growth of the GaN nanowires.An interesting question is why thespecifically bicrystalline structure ap-pearing in the GaN nanowires isformed. There are two possibilitiesabout the formation of the twin nano-wires. The first possibility is that thetwin nanowires form by the conjoin-ing of two individuals during growth.The second one is that the twin for-mation occurs during nucleation.Considering that the probability oftwin formation occurring duringgrowth is far lower than that duringnucleation, the first possibility can beexcluded. However, it remains to beclarified why the twinning nucleuscan be formed, which is not ad-dressed in the literature.[19] Accord-ing to theories of crystal growth,[25]

the twin is in a higher energy statethan a single crystal. Therefore, theformation of twin, by an amountequal to the interface energy, re-quires a higher driving force, whichwill overcome this excess energy. Ifnucleation takes place under a higherdriving force condition than that re-quired for the nucleation of a singlecrystal, the probability of formationof a twinned nucleus increases. From

an energy standpoint, the probability of formation of agrowth twin will be highest in the nucleation stage under ahigh driving force condition, i.e., at high supersaturationcondition, and the probability will diminish sharply in thegrowth stage when the driving force is lowered. In the pres-ent experiment, it can be estimated that the supersatura-tion is higher in the initial nucleation stage than the subse-quent growth stage on account of the starting materialsbeing consumed gradually. On the other hand, there exists

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Chem. Vap. Deposition 2007, 13, 527–532 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cvd-journal.de 529

Fig. 2. a) Low magnification TEM image of the bicrystalline GaN nanowire composed of two componentsA, B. b) SAED patterns taken from A. c) SAED patterns taken from B. d) SAED patterns taken from T.e) The HRTEM image taken from T. f) The EDS spectrum recorded from the nanowire.

a “selection rule” for the “allowed” bicrystal configura-tions, i.e., asymmetrically twinned nucleation would lead toa monocrystal after extended growth and only symmetricaltwins would survive to the observed state, as reported inthe literature.[19] In addition, the experimental observationsfrom the SEM image in Figure 1c show that there are tworeentrant edges in the middle of the GaN nanowires. Thereentrant edge is also an energetically more favorable sitefor two-dimensional nucleation than is a flat face. These re-sult in the twinned crystal taking an elongated form alongthis direction, i.e., the formation of the peculiar bicrystal-line nanowire. Additionally, considering that the latticemismatch between the (001) planes of GaN and ZnO isvery small ∼1.9 %, it is reasonable to conclude that a ZnObuffer layer on the silicon wafer results in the orientedgrowth of the GaN nanowires.

Field emission measurements of the aligned bicrystallineGaN nanowires were conducted in a vacuum chamberwith a pressure of 1.2 × 10–6 Pa at room temperature. Arod–like stainless steel probe (1 mm in diameter, 0.78 mm2

in area) was used as an anode. The aligned GaN nanowireswith Ag particles on their tips (Ag/GaN) were used asthe cathode. The spacing between these two electrodeswas 100 lm in our experiment. The curve of emission cur-rent density versus applied field (J–E) is depicted inFigure 4a. By definition, the turn–on field and the thresh-old field are the electronic fields required to generate anemission current density of 10 lA cm–2 and 10 mA cm–2,respectively.[26] The turn–on fields of 8 V lm–1 and thethreshold fields of 16.8 V lm–1 were obtained for thealigned Ag/GaN nanowires. The sample prepared by oursimple strategy can easily reach an emission current densityof 10 mA cm–2, a basic emission requirement for a flatpanel display.[27,28] The present reported turn–on field is

higher than that of 5.1 V lm–1 for P doping GaN nano-wires[29] and 7.5 V lm–1 for needle-like bicrystalline GaNnanowires,[17] but lower than that of 8.5 V lm–1 forthin GaN nanowires[30] and 12 V lm–1 for GaN nano-wires.[31]

The graph of Figure 4b shows the Fowler–Nordheim(FN) plot of the field emission of the aligned Ag/GaNnanowires. The FN plot exhibits linear dependence at highfields, which reveals that the emission current is reallycaused by the quantum tunneling effect. According to theFN theory,[32] the field enhancement factor b can be calcu-lated according to Equation 5.

b � �6�83 × 109�3�2�S (5)

� is the work function (in eV) and S is the slope of theln�J�E2� � 1�E plot. It should be noted that most of thenanowires have catalyst particles Ag on their tips. It is wellknown that GaN without intentional doping is always ann–type semiconductor, and �Ag (4.26 eV)[33] is larger than�GaN (2.8 eV),[34] which results in a Schottky barrier being

Communications

530 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2007, 13, 527–532

Fig. 3. a) Low magnification TEM image of the single–crystal GaN nano-wire. b) Corresponding SAED pattern. c) HRTEM lattice image.

Fig. 4. a) Field emission J–E curve. b) the corresponding FN plot.

formed at the Ag/GaN interface. In this situation, an easierelectron flow crosses the Ag/GaN interface under an ap-plied forward bias. The high electric field then narrows thepotential barrier at the metal Ag/vacuum interface suffi-ciently for the electrons to have a significant probability oftunneling from the Ag/GaN nanowire into the vacuum.Therefore, the work function of the metal Ag should beused to calculate the field enhancement factor, b. Using theslope of ln�J�E2� � 1�E plot, b can be calculated to beabout 513, which is a strong dependence on the geometryof the materials.[35] The FE stability of the orientedAg/GaN nanowire arrays was investigated at a fixed ap-plied electric field (8.5 V lm–1). Figure 5 shows the resultof emission current density versus time for a period 2 h un-der a pressure of 1.2 × 10–6 Pa. The average current density

and the standard deviation are calculated to be 15.72 and0.81 lA cm–2, respectively. The ratio of them is as small as5.1 %, which proves the high stability of the orientedAg/GaN nanowire arrays. For practical field emission ap-plications, it will require fabricating large areas of emitterswith low cost and high reproducibility. The present self–as-sembly approach is simpler and cheaper than the pre-viously reported methods for fabricating the microscaleGaN pyramids.[36,37] However, further study is necessary oncontrolled growth of the GaN nanowire arrays.

In conclusion, oriented, bicrystalline, GaN nanowire ar-rays have been synthesized under Ag as a catalyst condi-tion by a simple carbon thermal reduction process. SAEDand HRTEM observations indicate that the bicrystallinestructure is composed of (001) planes and the twin angleis 124°. Field emission measurements show that the orient-ed, bicrystalline, GaN nanowires capped with catalyst parti-cle arrays have a turn-on field of 8 V lm–1 at 10 lA cm–2

and a threshold field of 16.8 V lm–1 at 10 mA cm–2. Thefluctuation of the FE current is as small as 5.1 % for 2 h.

This means that oriented, bicrystalline, GaN nanowire ar-rays have potential applications in field emission nano-devices.

Experimental

Oriented, GaN nanowire arrays with bicrystalline structure were synthe-sized by a catalytic carbon thermal reduction process under an atmosphereof ammonia. Ga2O3 and graphite powder were mixed in a mole ratio of 2:1.The mass of the mixture was about 2 g. A silver nitrate ethanol solution(10 mL, 0.01 M) was dropped into the mixture. After drying at 150 °C, thepowders were loaded into an alumina boat, and then the boat was posi-tioned in the center of the alumina tube. A silicon substrate coated with aZnO buffer layer (500–600 nm) by magnetron sputtering was placed down-stream of the tube. The tube was put in a horizontal tube furnace. Afterpurging with Ar gas for 5 min, the furnace was heated to 1150 °C under Argas, and then kept at that temperature for 1 h under NH3 at 100 mL min–1.After the furnace was cooled to room temperature under the Ar gas flow,the silicon wafer was covered with a layer of light yellow products. The as–synthesized products were examined using XRD (Philips X’pert Pro diffrac-tometer) with Cu Ka radiation, k= 0.15418 nm, SEM (Hitachi S–3500N),and HRTEM (Tecnai F30) equipped with EDS. Field emission propertiesfor the as–synthesized products were investigated in a vacuum chamber witha pressure of 1.2 × 10–6 Pa at room temperature.

Received: October 26, 2006Revised: June 16, 2007

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Fig. 5. Field emission current stability of the oriented Ag/GaN nanowire ar-rays.

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