GaN nanowire and Ga 2 O 3 nanowire and nanoribbon growth from ion implanted ironcatalystJason L. Johnson, Yongho Choi, and Ant Ural Citation: Journal of Vacuum Science & Technology B 26, 1841 (2008); doi: 10.1116/1.2993175 View online: http://dx.doi.org/10.1116/1.2993175 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/26/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Plasma-assisted molecular beam epitaxy growth of GaN nanowires using indium-enhanced diffusion Appl. Phys. Lett. 93, 183109 (2008); 10.1063/1.3013840 Room temperature hydrogen detection using Pd-coated GaN nanowires Appl. Phys. Lett. 93, 072109 (2008); 10.1063/1.2975173 Improvement in aligned GaN nanowire growth using submonolayer Ni catalyst films Appl. Phys. Lett. 93, 043119 (2008); 10.1063/1.2965798 Phase separations of single-crystal nanowires grown by self-catalytic chemical vapor deposition method J. Chem. Phys. 126, 064704 (2007); 10.1063/1.2432111 Self-catalytic growth of single-phase AlGaN alloy nanowires by chemical vapor deposition Appl. Phys. Lett. 89, 193105 (2006); 10.1063/1.2364272

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GaN nanowire and Ga2O3 nanowire and nanoribbon growth from ionimplanted iron catalyst

Jason L. Johnson, Yongho Choi, and Ant Urala�

Department of Electrical and Computer Engineering, University of Florida, Gainesville, Florida 32611

�Received 18 June 2008; accepted 8 September 2008; published 3 November 2008�

The authors experimentally demonstrate a simple and efficient approach for nucleating the catalyticchemical vapor deposition �CVD� growth of GaN nanowires, Ga2O3 nanowires, and Ga2O3

nanoribbons by using ion implantation of Fe+ into thermally grown SiO2 layers and subsequentannealing to form the catalyst nanoparticles. This work shows that ion implantation can be used asa versatile method to create catalyst nanoparticles for wide band gap nanowire/nanoribbon growth.They also demonstrate that ion implanted catalyst nanoparticles prepared under identical conditionscan be used to grow different types of nanowires/nanoribbons containing Ga by simply changing thegas types and flow rates during CVD growth. Furthermore, they systematically characterize thestructural properties of the as-grown nanomaterials, and find that the distance between the Ga sourceand the substrate, growth temperature, growth time, and flow rates are all critical parameters fornanowire growth. They explain the growth of single-crystal wurtzite GaN and monoclinic �-Ga2O3

nanowires by the vapor-liquid-solid �VLS� growth model, whereas the growth of monoclinic�-Ga2O3 nanoribbons by a combination of the VLS and vapor-solid �VS� mechanisms. This workopens up the possibility of controlling the origin of wide band gap nanowires/nanoribbons at thenanometer scale using the technique of catalyst ion implantation through a lithographically definedmask, of integrating nanowires/nanoribbons into nonplanar three-dimensional device structures, andof growing different Ga-based wide band gap semiconductor nanostructures on the same substrateby simply changing the gas types and flow rates. © 2008 American Vacuum Society.�DOI: 10.1116/1.2993175�

I. INTRODUCTIONNanowires and nanoribbons have attracted significant re-

search attention in recent years due to their unique structural,electronic, optical, and mechanical properties.1,2 These prop-erties make nanowires and nanoribbons promising buildingblocks for potential applications such as transistors, lasers,photodetectors, and chemical and biological sensors.

GaN, which is a III-V nitride wide band gap semiconduc-tor with a direct band gap of Eg�3.4 eV, has applications inblue light emitting diodes �LEDs�, blue laser diodes, photo-detectors, high speed field effect transistors �FETs�, and hightemperature and high power electronics.3,4 Monoclinic gal-lium oxide ��-Ga2O3� is also a wide band gap semiconductor�Eg�4.9 eV�, which has potential applications in optoelec-tronic devices and high temperature gas sensors.5,6

One-dimensional �1D� nanowires of both GaN and�-Ga2O3 have been grown in recent years using a variety oftechniques including laser ablation,7,8 arc discharge,9,10

template-assisted growth,11 thermal evaporation,12 metal-organic chemical vapor deposition,13,14 molecular beamepitaxy,15 and chemical vapor deposition �CVD�.16–19 Fur-thermore, nanobelts and nanoribbons of �-Ga2O3 have beensynthesized in recent years using a number of techniquesincluding thermal evaporation1,20 and CVD.21

For most of the synthesis using CVD, a growth modelbased on the vapor-liquid-solid �VLS� growth mechanism22

has been used to explain the observed results. An essentialcomponent of the VLS growth process is the nanoscale cata-lyst particles required to nucleate the growth of nanowires.For example, for GaN nanowire growth, nanoparticles ofmetals such as In,18 Ni,18,23 Co,18 Au,24 or Fe,18,25 have beenused as catalyst. In particular, Fe has been shown to be agood catalyst for GaN nanowire growth since it dissolvesboth Ga and N and does not form a more stable compoundthan GaN.8,25 For Ga2O3 nanowire growth, Au5,17 has com-monly been used as catalyst.

Metal catalyst nanoparticles are typically formed by de-positing a solution of catalyst powder or monodispersednanoparticles18,26 or by annealing e-beam evaporated or sput-tered thin films.24 An alternative catalyst deposition tech-nique, which has not been explored as much, is to use ionimplantation and subsequent annealing to create catalystnanoparticles.27–31 In order to control the origin of nanowiresduring growth, the catalyst is typically patterned by lithogra-phy into small “islands.”32 Nanoscale catalyst particlesformed by depositing solutions of powders are typically dif-ficult to pattern into very small dimensions and those formedby depositing thin films are difficult to pattern into nonplanarthree-dimensional �3D� device structures, such as the side-walls of high aspect ratio trenches.29 Ion-implanted catalyst,on the other hand, is much easier to pattern into very smallfeatures and over high aspect ratio topography compared toother types of catalyst, and it offers extremely accurate con-trol of the number of atoms introduced into the substrate �thedose of the implant�. Since ion implantation is a well-

a�Author to whom correspondence should be addressed; electronic mail:antural@ece.ufl.edu

1841 1841J. Vac. Sci. Technol. B 26„6…, Nov/Dec 2008 1071-1023/2008/26„6…/1841/7/$23.00 ©2008 American Vacuum Society

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established technique in silicon technology, which is veryreproducible and easily scalable, it could offer significanttechnological advantages as a method to form catalyst nano-particles for nanowire growth. A few previous experimentshave shown that ion implantation and subsequent annealingcan create catalyst nanoparticles for nucleating the growth ofmultiwalled carbon nanotubes,27–29 single-walled carbonnanotubes,30 amorphous silica nanowires,33 and siliconnanowires.31 However, using ion implanted catalyst for thegrowth of wide band gap nanowires and nanoribbons has notbeen demonstrated previously.

In this article, we experimentally demonstrate a simpleand efficient approach for nucleating the growth of GaNnanowires, Ga2O3 nanowires, and Ga2O3 nanoribbons by ionimplantation of Fe+ ions directly into thermally grown SiO2

layers and subsequent annealing to form catalyst nanopar-ticles. This work experimentally shows that ion implantationcan be used as a versatile method to create catalyst nanopar-ticles for wide band gap nanowire/nanoribbon growth. Wealso show that catalyst nanoparticles prepared under identicalconditions can be used to grow different types of Ga-basednanowires/nanoribbons by simply changing the gas type andflow rates during CVD growth. This could enable the inte-gration of different Ga-based wide band gap semiconductornanostructures on the same substrate. Furthermore, we sys-tematically characterize the as-grown nanomaterials, discussthe effect of growth parameters on the nanostructures grown,and explain the growth results in terms of simple physicalmodels based on the VLS and the vapor-solid �VS� growthmechanisms. This work opens up the possibility of control-ling the origin of wide band gap nanowires/nanoribbons atthe nanometer scale through the technique of catalyst ionimplantation for fabricating nonplanar 3D device structures.

II. EXPERIMENTAL PROCEDURE

For our experiment, a 500 nm thick SiO2 layer was ther-mally grown on silicon �100� substrates. Fe+ ions were im-planted into these substrates at an energy of 60 keV with adose of 1015 or 1016 cm−2. The implantation depth profilecalculated by SRIM simulations34 for the 1016 cm−2 dose isshown in Fig. 1�a�, where the projected range Rp is 50 nm,and the peak concentration Cp is ~1.5�1019 cm−3.

The as-implanted samples were then placed into a 1 in.atmospheric quartz tube furnace and annealed at 900 °C un-der 300 sccm Ar and 200 sccm H2 flow for 30 min to formiron catalyst nanoparticles on the oxide surface. After thisannealing step, the gallium metal source �5N purity, AlfaAesar� was poured into a quartz boat and placed in the tubefurnace, as shown in Fig. 1�b�. Next, the ion implanted andannealed substrate was placed �3 cm downstream of thegallium metal source. The substrate was heated up to 850 °Cand annealed at 850 °C for 15 min under 500 sccm Ar flow.After this step, the growth was performed at 850 °C for�2 h for all cases, but the gas flow rates were different foreach of the three nanomaterials grown: For GaN nanowires,15 sccm NH3 and 300 sccm H2 were flown, for Ga2O3 nano-wires, 20 sccm of N2 was flown, and for Ga2O3 nanoribbons,

10 sccm of H2 was flown. The effect of the different gasesused on the nanowire/nanoribbon growth will be discussed indetail later.

The as-grown samples were characterized by a JEOLJSM-6335F field emission gun scanning electron microscope�SEM�, a JEOL 2010F high resolution transmission electronmicroscope �HRTEM� operating at 200 kV equipped withselected area electron diffraction �SAED� and energy disper-sive x-ray spectroscopy �EDS� capability, a Phillips MRDX’Pert x-ray diffraction �XRD� spectrometer with Cu K� ra-diation, and a Digital Instruments Nanoscope III atomicforce microscope �AFM� operated in the tapping mode. Forthe HRTEM characterization, the as-grown nanowires/nanoribbons were dispersed in 2-propanol, the solution wassonicated, and a few drops of the resulting suspension wasdeposited on commercially available Cu TEM grids coatedwith a holey carbon film.

III. RESULTS AND DISCUSSION

Figures 2�a� and 2�b� show the AFM images of the SiO2

substrate after the ion implantation of iron and subsequent900 °C, 30 min annealing step mentioned above for the 1015

and 1016 cm−2 dose implants, respectively. We have ob-served that, for all three nanostructures grown �GaN nano-wires, Ga2O3 nanowires, and Ga2O3 nanoribbons�, under thesame anneal and growth conditions, the higher implant dosesubstrates �1016 cm−2� result in a higher nanowire/nanoribbon density compared to the lower dose ones�1015 cm−2�. This could be explained by the higher densityof larger catalyst nanoparticles formed on the higher implantdose substrate, as evident from Fig. 2. The flux of Fe atoms

FIG. 1. �a� Depth profile of Fe atoms ion implanted into thermally grownSiO2 at an energy of 60 keV and a dose of 1016 cm−2 calculated by SRIM

simulations, giving a projected range of Rp=50 nm and a peak concentra-tion of Cp�1.5�1019 cm−3, as labeled in the figure. �b� The schematic ofthe chemical vapor deposition �CVD� setup used for GaN and Ga2O3 nano-wire and nanoribbon growth, indicating the Ga source and the ion implantedsubstrate.

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diffusing to the oxide surface is proportional to the dose ofthe implant; as a consequence, a higher dose results in largernanoparticles. It is also worth noting that the AFM images inFig. 2 were taken after the 30 min anneal step at 900 °C.During the 2 h growth step at 850 °C, the implanted Featoms continue to outdiffuse toward the surface, and as aresult, the density and size of catalyst nanoparticles continueto evolve as the growth proceeds. This is in contrast tosolution-based or thin-film catalyst, where the catalyst“dose” on the surface does not change during growth.

Next, we present the growth results of GaN nanowires,Ga2O3 nanowires, and Ga2O3 nanoribbons from these cata-lyst nanoparticles for the higher implant dose substrate�1016 cm−2�, which was found to result in a higher density ofnanostructures.

A. GaN nanowires

Figure 3�a� shows an SEM image of GaN nanowiresgrown from the 1016 cm−2 dose ion implanted iron catalyst.The diameters of the as-grown nanowires range from 15–60nm and their lengths are between 1 and 20 �m based onSEM and TEM analyses. XRD measurements were per-formed on bulk samples to determine overall crystal struc-ture and phase purity of the as-grown GaN nanowires. Figure3�b� shows the XRD pattern of the as-grown GaN nanowireswhere the diffraction peaks labeled by their Miller indicesare indexed to hexagonal wurtzite GaN with lattice constantsa=0.3186 nm and c=0.5178 nm. We have also character-ized the detailed structure and composition of individualGaN nanowires by HRTEM, SAED, and EDS, as shown in

Figs. 3�c� and 3�d�, providing further experimental evidencethat the as-grown nanowires are single-crystal hexagonalwurtzite GaN.

FIG. 2. AFM images of Fe catalyst nanoparticles formed after annealing theas-implanted SiO2 substrates for 30 min at 900 °C under Ar and H2 flow.The ion implantation energy is 60 keV and the implant doses are �a� 1015

and �b� 1016 cm−2.

FIG. 3. �a� SEM image and �b� XRD pattern of the as-grown GaN nanowires�from the 1016 cm−2 dose ion implanted Fe catalyst� where the diffractionpeaks labeled by their Miller indices are indexed to hexagonal wurtzite GaNwith lattice constants a=0.3186 nm and c=0.5178 nm. �c� HRTEM imageshowing the selected area electron diffraction �SAED� pattern in the insetand �d� energy dispersive X-ray spectroscopy �EDS� spectrum of an indi-vidual GaN nanowire, providing further experimental evidence that the as-grown nanowires are single-crystal hexagonal wurtzite GaN.

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The effect of the various CVD parameters on the growthof GaN nanowires from ion implanted Fe catalyst was alsoinvestigated. The overall reaction of gallium metal vapor andammonia to form GaN nanowires can be expressed as

2Ga + 2NH3 → 2GaN + 3H2. �1�

First, by varying the distance between the Ga source and thesubstrate �see Fig. 1�b��, we observed that the density ofnanowires is reduced as the distance increases above 3 cm.Furthermore, we found that increasing the growth time in-creases the average GaN nanowire length. Increasing thegrowth temperature above 900 °C was found to reduce thedensity of GaN nanowires, most likely due to the evapora-tion of too much Ga from the source material at higher tem-peratures. In addition, having either a high ��30 sccm� orlow ��5 sccm� NH3 flow rate was observed to significantlyreduce the density and length of as-grown GaN nanowires.These findings indicate that too much or too little supply ofGa or NH3 impede GaN nanowire growth.

The presence of H2 coflow was found to be a crucialfactor for GaN nanowire growth, as well. Any residual oxy-gen in the CVD chamber easily oxidizes the nanowires,yielding Ga2O3 nanowires instead of GaN. The hydrogencoflow prevents the oxidation of GaN nanowires, as well asreducing the decomposition rate of ammonia.

As mentioned previously in the experimental procedure,we have performed a 15 min anneal in Ar atmosphere afterthe furnace temperature has reached the growth temperatureof 850 °C. We found that this anneal step increases the GaNnanowire length and density. This could be due to the factthat Ga begins to evaporate from the source material duringthis anneal step, providing a sufficient supply of Ga vapor assoon as the NH3 is introduced into the chamber in the growthstep.

The growth of the GaN nanowires can be explained by theVLS growth model22 as illustrated in Fig. 4. First, the SiO2

substrate is implanted with Fe+. Then, during the subsequentanneal step, the as-implanted Fe atoms outdiffuse to the sur-face and aggregate to form catalyst nanoparticles on the SiO2

surface. The catalyst nanoparticles from which nanowiregrowth has not yet nucleated could also get larger by a sur-face diffusion and Ostwald ripening process. During theCVD growth, liquid alloy droplets containing Fe, Ga, and Nforms from these catalyst nanoparticles. When these liquidalloy droplets become supersaturated in Ga and N, solid GaNnanowires nucleate and precipitate out of these droplets.

B. Ga2O3 nanowires

Figure 5�a� shows a SEM image of �-Ga2O3 nanowiresgrown from ion implanted iron catalyst. The diameters of theas-grown nanowires range from 15 to 60 nm and theirlengths are 1 to 20 µm based on SEM and TEM analyses. Asmentioned previously, the catalyst and growth conditions areidentical to the growth of GaN nanowires, except only 20sccm N2 is flown during growth instead of NH3 and H2.Figure 5�b� shows the XRD pattern of the as-grown Ga2O3

nanowires where the diffraction peaks labeled by their Millerindices are indexed to monoclinic �-Ga2O3 with lattice con-stants a=1.2227 nm, b=0.30389 nm, c=0.58079 nm, and�=103.82°. The detailed structure and composition of indi-vidual Ga2O3 nanowires have also been characterized byHRTEM and SAED, as shown in Fig. 5�c�, providing furtherexperimental evidence that the as-grown nanowires aresingle-crystal monoclinic �-Ga2O3. A very thin amorphouslayer on the surface of the nanowire is visible in Fig. 5�c�.

Flowing �20 sccm of N2 allows the residual oxygenpresent in the CVD chamber to react with the gallium vaporin the presence of the ion implanted Fe catalyst, formingGa2O3 nanowires, which can be described by the followingoverall reaction:

4Ga + 3O2 → 2Ga2O3. �2�

The growth of the Ga2O3 nanowires can also be explained bythe VLS growth model illustrated in Fig. 4, except in thiscase the liquid alloy droplets contain Fe, Ga, and O.

C. Ga2O3 nanoribbons

Figure 6�a� shows a SEM image of �-Ga2O3 nanoribbonsgrown from the ion implanted iron catalyst. The width of theas-grown nanoribbons range from 20 to 500 nm and theirlengths are 1 to 30 µm based on SEM and TEM analyses.The inset of Fig. 6�b� shows the AFM image of an individualnanoribbon. Based on cross-sectional AFM analysis, thethicknesses of the nanoribbons are in the range of 20–150nm. As mentioned previously, the catalyst and growth con-ditions are identical to the growth of GaN and Ga2O3 nano-wires, except only 10 sccm H2 is flown during growth in-stead of NH3 /H2 or N2. The main panel of Fig. 6�b� showsthe XRD pattern of the as-grown Ga2O3 nanoribbons wherethe diffraction peaks labeled by their Miller indices are in-dexed to monoclinic �-Ga2O3, as in the case of the Ga2O3

nanowires. Figures 6�c� and 6�d� show the HRTEM, SAED,and EDS data of an individual nanoribbon, providing furtherexperimental evidence that the as-grown nanoribbons aresingle-crystal monoclinic �-Ga2O3.

FIG. 4. Schematic illustration of the vapor-liquid-solid �VLS� growthmechanism for GaN nanowires. First, Fe atoms are ion implanted into theSiO2 substrate. During the subsequent anneal step, the as-implanted Fe at-oms outdiffuse to the surface and aggregate to form catalyst nanoparticleson the SiO2 surface. Next, during the CVD growth, liquid alloy dropletscontaining Fe, Ga, and N form from these catalyst nanoparticles. Finally,when these liquid alloy droplets become supersaturated in Ga and N, solidGaN nanowires nucleate and precipitate out of these droplets.

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In some of the previous work, �-Ga2O3 nanoribbons havebeen grown without using any catalyst.1,20 The VS growthmodel has been proposed as the growth mechanism in thosecases due to the absence of any catalyst nanoparticles.20,35 Inour case, due to the presence of ion implanted Fe catalyst, wepropose the following growth model, which is a combinationof the VLS and VS models, as illustrated in Fig. 7�a�. First,

�-Ga2O3 nanowires are nucleated out of the Fe catalystnanoparticles by a VLS mechanism, then nanoribbons grow

FIG. 5. �a� SEM image and �b� XRD pattern of the as-grown Ga2O3 nano-wires �from the 1016 cm−2 dose ion implanted Fe catalyst� where the dif-fraction peaks labeled by their Miller indices are indexed to monoclinic�-Ga2O3 with lattice constants a=1.2227 nm, b=0.30389 nm, c=0.58079 nm, and �=103.82°. �c� HRTEM image of an individual Ga2O3

nanowire, showing the SAED pattern in the inset, providing further experi-mental evidence that the as-grown nanowires are single-crystal monoclinic�-Ga2O3. A very thin amorphous layer on the surface of the nanowire isvisible in the HRTEM image.

FIG. 6. �a� SEM image and �b� XRD pattern of the as-grown Ga2O3 nanor-ibbons �from the 1016 cm−2 dose ion implanted Fe catalyst� where the dif-fraction peaks labeled by their Miller indices are indexed to monoclinic�-Ga2O3. The inset of �b� shows the AFM image of an individual nanorib-bon. By cross-sectional AFM analysis, the thicknesses of the nanoribbonswere measured to be in the range of 20–150 nm. �c� HRTEM image showingthe SAED pattern in the inset and �d� EDS spectrum of an individual Ga2O3

nanoribbon, providing further experimental evidence that the as-grown na-noribbons are single-crystal monoclinic �-Ga2O3.

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from the nucleated nanowires by a VS mechanism. The flowof a small amount of H2 is found to be critical for nanoribbongrowth. When N2 is flown, only nanowires are observed.This can be explained by the fact that H2 reacts with thenucleated �-Ga2O3 nanowires by the following reaction:

Ga2O3 + 2H2 → Ga2O + 2H2O. �3�

Ga2O vapor then reacts back with H2 and H2O to form Gaand Ga2O3, which then initiates the VS growth ofnanoribbons.36 In addition, no Ga2O3 nanoribbon growth wasobserved for high H2 flow rates ��100 sccm� since exces-sive H2 depletes the residual oxygen source in the chamber.Evidence for the proposed VLS-VS growth mechanism hasbeen observed in some TEM images, as shown in Fig. 7�b�,where a nanowire core is visible in the center region of theas-grown �-Ga2O3 nanoribbon. Furthermore, the majority ofGa2O3 nanoribbons were found to be longer than GaN andGa2O3 nanowires for equivalent growth times and tempera-tures, possibly due to an enhanced growth rate for the VSgrowth process compared to the VLS process.

IV. CONCLUSIONS

In conclusion, we have experimentally demonstrated thecatalytic CVD growth of GaN nanowires and �-Ga2O3 nano-wires and nanoribbons from Fe catalyst nanoparticles formedby ion implantation into thermally grown SiO2 layers andsubsequent annealing. This work provides experimental evi-dence that ion implantation can be used as a versatile methodto create catalyst nanoparticles for wide band gap nanowire/nanoribbon growth. We have also shown that Fe catalystnanoparticles prepared under identical conditions can beused to grow different Ga-based nanostructures by simplychanging the gas type and flow rates during CVD growth.Furthermore, we have systematically characterized the struc-tural properties of the as-grown nanomaterials. We havefound that the distance between the Ga source and the sub-strate, growth temperature, growth time, and gas flow ratesare all critical parameters for nanowire growth. Furthermore,the growth of GaN and �-Ga2O3 nanowires can be explained

by the catalytic VLS growth model, whereas the growth of�-Ga2O3 nanoribbons can be explained by a combination ofthe VLS and VS mechanisms.

This work opens up the possibility of controlling the ori-gin of wide band gap nanowires/nanoribbons at the nano-meter scale using the technique of catalyst ion implantationthrough a lithographically defined mask, of integratingnanowires/nanoribbons into nonplanar 3D device structures,and of growing different Ga-based wide band gap semicon-ductor nanostructures on the same substrate by simplychanging the source gas types and flow rates.

ACKNOWLEDGMENTS

The authors would like to thank Kerry Siebein for herassistance with TEM characterization and analysis. Fe ionimplantation was performed at Core Systems, Sunnyvale,CA. The AFM, SEM, XRD, TEM, SAED, and EDS charac-terizations were done at the Major Analytical Instrumenta-tion Center �MAIC� at the University of Florida. This workwas funded in part by the University of Florida ResearchOpportunity Fund. Jason L. Johnson acknowledges supportfrom the National Science Foundation �NSF� South East Al-liance for Graduate Education and the Professoriate�SEAGAP� award and the National Consortium for GraduateDegrees for Minorities in Engineering and Science �GEM�Fellowship.

1Z. W. Pan, Z. R. Dai, and Z. L. Wang, Science 291, 1947 �2001�.2Y. Huang, X. Duan, Y. Cui, and C. M. Lieber, Nano Lett. 2, 101 �2002�.3S. Nakamura, S. Pearton, and G. Fasol, The Blue Laser Diode: The Com-plete Story �Springer-Verlag, Berlin, 2000�.

4H. Morkoc, Nitride Semiconductors and Devices �Springer, Heidelberg,1999�.

5C. H. Liang, G. W. Meng, G. Z. Wang, Y. W. Wang, L. D. Zhang, and S.Y. Zhang, Appl. Phys. Lett. 78, 3202 �2001�.

6U. Hoefer, J. Frank, and M. Fleischer, Sens. Actuators B 78, 6 �2001�.7J. Q. Hu, Q. Li, X. M. Meng, C. S. Lee, and S. T. Lee, J. Phys. Chem. B

106, 9536 �2002�.8X. Duan and C. M. Lieber, J. Am. Chem. Soc. 122, 188 �2000�.9W. Han, P. Redlich, F. Ernst, and M. Ruhle, Appl. Phys. Lett. 76, 652�2000�.

10Y. C. Choi et al., Adv. Mater. �Weinheim, Ger.� 12, 746 �2000�.11G. S. Cheng, L. D. Zhang, Y. Zhu, G. T. Fei, L. Li, C. M. Mo, and Y. Q.

Mao, Appl. Phys. Lett. 75, 2455 �1999�.12E. A. Stach, P. J. Pauzauskie, T. Kuykendall, J. Goldberger, R. He, and P.

Yang, Nano Lett. 3, 867 �2003�.13G. T. Wang, A. A. Talin, D. J. Werder, J. R. Creighton, E. Lai, R. J.

Anderson, and I. Arslan, Nanotechnology 17, 5773 �2006�.14K.-W. Chang and J.-J. Wu, Adv. Mater. �Weinheim, Ger.� 16, 545 �2004�.15R. Calarco, R. J. Meijers, R. K. Debnath, T. Stoica, E. Sutter, and H.

Luth, Nano Lett. 7, 2248 �2007�.16M. He et al., Appl. Phys. Lett. 77, 3731 �2000�.17Y. P. Song, H. Z. Zhang, C. Lin, Y. W. Zhu, G. H. Li, F. H. Yang, and D.

P. Yu, Phys. Rev. B 69, 075304 �2004�.18C.-C. Chen et al., J. Am. Chem. Soc. 123, 2791 �2001�.19J. Li, L. An, C. Lu, and J. Liu, Nano Lett. 6, 148 �2006�.20Z. R. Dai, Z. W. Pan, and Z. L. Wang, J. Phys. Chem. B 106, 902 �2002�.21X. Xiang, C.-B. Cao, and H.-S. Zhu, J. Cryst. Growth 279, 122 �2005�.22R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 �1964�.23J.-R. Kim, H. M. So, J. W. Park, J.-J. Kim, J. Kim, C. J. Lee, and S. C.

Lyu, Appl. Phys. Lett. 80, 3548 �2002�.24S. Han, W. Jin, T. Tang, C. Li, D. Zhang, X. Liu, J. Han, and C. Zhou, J.

Mater. Res. 18, 245 �2003�.25W.-Q. Han and A. Zettl, Appl. Phys. Lett. 80, 303 �2002�.26Y. Cui, L. J. Lauhon, M. S. Gudiksen, J. Wang, and C. M. Lieber, Appl.

FIG. 7. �a� Schematic illustration of the proposed nanoribbon growth model,which is a combination of the VLS and vapor-solid �VS� models. First,Ga2O3 nanowires are nucleated out of the Fe catalyst nanoparticles by aVLS mechanism similar to Fig. 4, then nanoribbons grow from the nucle-ated nanowires by a VS mechanism. �b� TEM image of an individual Ga2O3

nanoribbon, where a nanowire core is visible in the center region, providingevidence for the proposed VLS-VS growth mechanism.

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Phys. Lett. 78, 2214 �2001�.27J. M. Mao, L. F. Sun, L. X. Qian, Z. W. Pan, B. H. Chang, W. Y. Zhou, G.

Wang, and S. S. Xie, Appl. Phys. Lett. 72, 3297 �1998�.28X.-z. Ding, L. Huang, X. T. Zeng, S. P. Lau, B. K. Tay, W. Y. Cheung,

and S. P. Wong, Carbon 42, 3030 �2004�.29A. R. Adhikari, M. B. Huang, D. Wu, K. Dovidenko, B. Q. Wei, R. Vajtai,

and P. M. Ajayan, Appl. Phys. Lett. 86, 053104 �2005�.30Y. Choi, J. Sippel-Oakley, and A. Ural, Appl. Phys. Lett. 89, 153130

�2006�.

31S. Christiansen, R. Schneider, R. Scholz, U. Gosele, T. Stelzner, G.Andra, E. Wendler, and W. Wesch, J. Appl. Phys. 100, 084323 �2006�.

32J. Kong, H. T. Soh, A. M. Cassell, C. F. Quate, and H. Dai, Nature�London� 395, 878 �1998�.

33D. K. Sood, P. K. Sekhar, and S. Bhansali, Appl. Phys. Lett. 88, 143110�2006�.

34J. F. Ziegler and J. P. Biersack �www.srim.org�.35S. S. Brenner and G. W. Sears, Acta Metall. 4, 268 �1956�.36D. P. Butt, Y. Park, and T. N. Taylor, J. Nucl. Mater. 264, 71 �1999�.

1847 Johnson, Choi, and Ural: GaN nanowire and Ga2O3 nanowire and nanoribbon 1847

JVST B - Microelectronics and Nanometer Structures

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