A Raman-active vibration mode at 418 cm-1 is observed in wurtzite gallium nitride (GaN) nanowires synthesized by different growth methods. In particular, Raman scattering measurements of a number of GaN nanowires systematically prepared by nitriding Β- Ga2 O3 nanowires at different temperatures show an interesting evolution of the mode, revealing that it is most likely the vibration mode of N-rich octahedral Ga- N6 bonds. This idea is further supported by the high-resolution transmission electron microscopic observation. © 2007 American Institute of Physics.published_or_final_versio

Lee, ST

Ning, JQ

Shan, YY

Xu, SJ

Yu, DP

Applied Physics Letters

A Raman-active vibration mode at 418 cm(-1) is observed in wurtzite gallium nitride (GaN) nanowires synthesized by different growth methods. In particular, Raman scattering measurements of a number of GaN nanowires systematically prepared by nitriding beta-Ga2O3 nanowires at different temperatures show an interesting evolution of the mode, revealing that it is most likely the vibration mode of N-rich octahedral Ga-N-6 bonds. This idea is further supported by the high-resolution transmission electron microscopic observation.Physics, AppliedSCI(E)EI0ARTICLE10null9

Ning, J. Q.

Xu, S. J.

Yu, D. P.

Shan, Y. Y.

Lee, S. T.

Institutional Repository of Peking University

418 cm(-1) Raman scattering from gallium nitride nanowires: Is it a   vibration mode of N-rich Ga-N bond configuration?

HKU Scholars Hub

Title 418 cm-1 Raman scattering from gallium nitride nanowires: Is ita vibration mode of N-rich Ga-N bond configuration?Author(s) Ning, JQ; Xu, SJ; Yu, DP; Shan, YY; Lee, STCitation Applied Physics Letters, 2007, v. 91 n. 10Issued Date 2007URL http://hdl.handle.net/10722/80609Rights Applied Physics Letters. Copyright © American Institute ofPhysics.418 cm−1 Raman scattering from gallium nitride nanowires: Is it a vibrationmode of N-rich Ga–N bond configuration?J. Q. Ning and S. J. XuaDepartment of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, ChinaD. P. YuElectron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, China and StateKey Laboratory for Artificial Microstructures and Mesoscopic Physics, School of Physics, PekingUniversity, Beijing 100871, ChinaY. Y. Shan and S. T. LeeDepartment of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong, ChinaReceived 15 May 2007; accepted 15 August 2007; published online 6 September 2007A Raman-active vibration mode at 418 cm−1 is observed in wurtzite gallium nitride GaNnanowires synthesized by different growth methods. In particular, Raman scattering measurementsof a number of GaN nanowires systematically prepared by nitriding -Ga2O3 nanowires at differenttemperatures show an interesting evolution of the mode, revealing that it is most likely the vibrationmode of N-rich octahedral Ga–N6 bonds. This idea is further supported by the high-resolutiontransmission electron microscopic observation. © 2007 American Institute of Physics.DOI: 10.1063/1.2780081In the past decade, the GaN-based optoelectronic indus-try has grown rapidly. More recently, various forms of GaNnanomaterials such as nanorods and nanowires have attracteda great deal of attention as these nanosized GaN materials areexpected to exhibit superior optical and electrical propertiesthat may greatly extend applications of GaN-based materials.For example, GaN nanorods and nanowires have been usedto fabricate light-emitting diodes1 and field-effect transis-tors.2 Further exploitation of devices based on GaN nanoma-terials demands a better understanding of the properties ofsuch nanomaterials. Raman scattering is a powerful tool tocharacterize the structural, compositional, and lattice vibra-tional properties of a material. The application of this pow-erful technique to GaN nanomaterials has led to a recent ob-servation of a 418 cm−1 vibration mode.3–5 In fact, a carefulliterature search shows that this Raman peak had been re-ported earlier in significantly disordered GaN epilayers.6,7However, its microstructural origin is still a debated issue.This letter, which is motivated by the noticeable appearanceof this Raman mode, shows that it most likely originatesfrom abundant regional N-rich Ga–N bond configurationsthat exist in various GaN nanostructures and some stronglydefected GaN films.We start our discussion from the typical Raman spectrameasured from two kinds of GaN nanostructures synthesizedby different methods, as shown in Fig. 1. The Raman spectrawere measured at room temperature under the backscatteringgeometric configuration using WITec-Alpha confocal micro-Raman system. The excitation light was the 514.5 nm line ofan Ar+ laser with 30 mW output power. During the measure-ments, the excitation light was focused onto the sample sur-face with a 60 Olympus objective. In Fig. 1, spectrum awas recorded from a GaN nanowire sample prepared on a Sisubstrate by chemical vapor deposition CVD,8 while spec-trum b was from another GaN nanowire sample obtainedby nitriding Ga2O3 nanowires in NH3 at 950 °C for 30 min.In addition to the characteristic Raman peaks at 564 and720 cm−1 of wurtzite GaN, one peak at 418 cm−1 can be wellseen in both the spectra. Besides this peak, another additionalpeak at 252 cm−1 is observed. Other peaks marked by aster-isk are from the Si substrate. It is more interesting to notethat spectrum b in Fig. 1 exactly resembles the Ramanspectrum of the GaN hollow spheres obtained by Sun andLi.5 These consistent observations of the 418 cm−1 peak inthe GaN nanostructures and in the strongly defected filmsprepared by different groups with a variety of techniquessuggest that the Raman structure is likely to be a character-istic vibration mode of some unknown Ga–N bond configu-ration which exists abundantly in various forms of nano- anddefected GaN structures.Usually, Raman spectroscopic features of a material canbe interpreted in terms of the vibrational density of statesVDOS which is a measure of the vibrational excitations ofthe material. The VDOS of GaN has been well studied.9–11The structures at 564 and 720 cm−1 can be readily identifiedsince they just correspond to the TO and LO modes in theVDOS, respectively. However, for the peak at 418 cm−1, nocorresponding structure exists in the theoretical VDOS ofaAuthor to whom correspondence should be addressed; electronic mail:sjxu@hkucc.hku.hkFIG. 1. Room-temperature Raman spectra of the two kinds of GaN nano-wires: a grown by CVD and b obtained by nitriding Ga2O3 nanowires inNH3 at 950 °C for 30 min. The peaks marked by asterisk are from the Sisubstrate.APPLIED PHYSICS LETTERS 91, 103117 20070003-6951/2007/9110/103117/3/$23.00 © 2007 American Institute of Physics91, 103117-1Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsGaN because it lies in the phonon band gap between theacoustic and optical bands of GaN. The peak, therefore, can-not be explained even with disorder-activated Raman scatter-ing and indeed it is not observed in either experimentallymeasured or theoretically calculated Raman spectra of amor-phous GaN.9,12Figure 2 shows the Raman spectra of Ga2O3 nanowiresnitrided at 750, 800, and 950 °C, respectively, under identi-cal ammonia gas flow for 30 min. The typical Raman spec-trum of pure -Ga2O3 nanowires is also plotted for refer-ence. In the reference spectrum, the sharp peaks stem fromthe different vibrational modes of -Ga2O3 except for the520 cm−1 peak of the Si substrate marked by an asterisk.From Fig. 2, one can clearly observe the evolution of thefour GaN-related Raman peaks at 252, 418, 564, and720 cm−1 indicated by vertical arrows. These peaks start toshow up in the Raman spectrum of the Ga2O3 nanowiresnitrided at 750 °C and gradually become dominant with in-creasing nitridation temperature. It is particularly interestingto note that the 418 cm−1 peak evolves from the Raman peakof -Ga2O3 at 416 cm−1. This provides an important clueabout the microstructural origin of the 418 cm−1 peak.Figure 3a shows a representative high-resolution trans-mission electron microscopy HRTEM image of a Ga2O3wire nitrided at 750 °C. The horizontal fringes parallel to thewire axial direction are found to dominate in the image andhave a spacing of about 2.81 Å which corresponds to the dspacing of the -Ga2O3 002 lattice planes. These facts in-dicate that the majority of the wire is still -Ga2O3 althoughit was nitrided at 750 °C. However, these horizontal fringesdo not coherently prolong throughout the entire wire and arebroken by many local blurred regions. These regions are be-lieved to be the disordered grains where nitrogen atoms arealready incorporated into the lattice of -Ga2O3. Such agrain is indicated by a rectangle in Fig. 3a. Besides theblurred regions, the lattice fringes in some local regions areclearly seen to be tilted at angles of 60° and 120° with re-spect to the 100 direction. We would like to point out thatno corresponding lattice planes of the -Ga2O3 phase pos-sess such tilting directions or spacing between the tiltingfringes. Furthermore, strong distortion of the horizontalfringes can be seen in these regions. For example, in such aregion outlined by the elliptical line in Fig. 3a, the obliquefringes become prominent and the horizontal fringes are sig-nificantly distorted. All these structural features can be wellinterpreted by the incorporation of nitrogen into the localregions. The inset in Fig. 3a shows the selected area elec-tron diffraction SAED pattern of such a region. Interest-ingly, the hexagonal diffraction pattern can be seen althoughit is dimmed by the strong disorders in the region. It can beviewed as further supporting evidence of nitrogen incorpora-tion into local Ga2O3 lattice regions. We thus conclude thatthe 750 °C nitridation in our case has led to the N incorpo-ration and formation of local Ga–N bonds inside -Ga2O3lattice. Increasingly blurred regions with irregularly orientedfringes are thus expected in HRTEM images of the Ga2O3nanowires nitrided at higher temperatures. Indeed, such ex-pectation is verified in the two HRTEM images of the Ga2O3nanowires nitrided at 800 °C, as shown in Figs. 3b and3c. This concurs with the observation of N incorporationmade by one of us using x-ray photoelectron spectroscopyand photoluminescence measurements.13 In the local inclu-sions embedded into -Ga2O3, the Ga–N bond configurationmay be significantly different from the normal configurationin the GaN wurtzite phase. Two kinds of Ga–N bond con-figurations are postulated: one is GaN-like and the otherGa2O3-like. GaN-like bonds exhibit vibrational propertieswhich can be interpreted using the VDOS of GaN. ForGa2O3-like Ga–N bonds, however, different vibrational prop-erties such as the observed 418 and 252 cm−1 peaks are to beexpected. It is known that the wurtzite-GaN lattice can bereferred to as an arrangement of corner-sharing Ga–N4 tet-rahedra, whereas -Ga2O3 consists of mixed edge-sharingGa–O6 octahedra and corner-sharing Ga–O4 tetrahedra.14The unique mixed bond configurations of -Ga2O3 crystalthus give rise to more vibration modes than the pure wurtzitephase, as shown in the bottom Raman spectrum in Fig. 2. Forinstance, the 416 cm−1 peak is an intrinsic vibration modeexactly associated with the Ga–O6 octahedra in the -Ga2O3FIG. 2. Raman spectra of Ga2O3 nanowires nitrided at different tempera-tures. The fresh Raman peaks indicated by the vertical arrows are due to theformation of GaN compound. The 520 cm−1 peak marked by asterisk in thebottom spectrum is from the substrate Si on which the Ga2O3 nanowireswere grown.FIG. 3. a HRTEM image of a wire treated at 750 °C. The inset is selected-area electron diffraction SAED patterns showing hexagonal symmetry. band c are the HRTEM images of different wires nitrided at 800 °C,respectively.103117-2 Ning et al. Appl. Phys. Lett. 91, 103117 2007Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionslattice.15 During the nitridation of -Ga2O3 nanowires, Natoms are expected to replace O atoms either in Ga–O6 or inGa–O4 bond configurations. The latter replacement will re-sult in the formation of normal Ga–N4 bonds while theformer may lead to the formation of local Ga–N6 octahedralstructures. Of course, it should be pointed out that localGa–Nx structures with 6x4 might exist due to incom-plete replacement chemical reaction during the nitridationprocess. These resultant N-rich local structures should bringabout some fresh peaks in the Raman spectrum. As shown inFig. 2, the close relationship between the 418 cm−1 peak ofthe GaN nanowires and the 416 cm−1 peak from the Ga–O6octahedral structure of -Ga2O3 gives strong support for theexistence of local Ga–N6 octahedral structures. As the nitri-dation temperature is increased, more Ga–O6 octahedralstructures transform into the Ga–N6 bond structures, result-ing in an increase of the 418 cm−1 peak in the Raman spec-trum, as observed in Fig. 2. However, when the nitridationtemperature is high enough, unstable Ga–N6 bonds willchange into stable Ga–N4 bonds. This accounts for the re-duction of 418 cm−1 Raman peak intensity from the samplesnitrided at 950 °C and above. Such an argument is supportedby additional experimental spectra as presented below.Figure 4 depicts three Raman spectra of Ga2O3 nano-wires nitrided at higher temperatures. The top and middleones are the Raman spectra from the samples nitrided inammonia at 1100 °C for 20 and 30 min, respectively, whilethe bottom is from a sample nitrided at 1150 °C for 20 min.The Raman signals from Ga2O3 components are no longerobserved in these spectra, indicating that almost completenitridation of Ga2O3 has occurred under these conditions. Incomparison with the top and bottom spectra, the 418 cm−1peak for the middle spectrum that corresponds to the longernitridation time is greatly enhanced. This implies that in-creasing nitridation time favors the formation of the unstableGa–N6 bonds giving the 418 cm−1 peak. However, highertreatment temperature results in the transformation of the un-stable Ga–N6 bond structure to the stable Ga–N4 phase, asindicated by the bottom spectrum in Fig. 4. Here, the peak at565 cm−1, which corresponds to the E2 high mode of hex-agonal crystalline ZnO, becomes sharper and more domi-nant, indicating a higher quality hexagonal phase of ZnO.4,16From the discussions given above and the structural na-ture of octahedral structure, the formation of Ga–N6 bondstructures requires two conditions, namely, a nitrogen richcondition and a local loosened lattice environment. In fact,all the GaN nanostructures reported in the literature showingthe 418 cm−1 Raman peak were obtained in N2 or NH3 am-biance providing an excellent N-rich environment.3–5,8,17Even in the case of ion-implanted GaN,18 the N-rich localdefected structures with the loosened lattice spacing wereformed as Ga atoms are more easily displaced than N atoms.In our case, the original -Ga2O3 lattice provides an idealenvironment for the formation of local Ga–N6 octahedralstructures during the nitridation due to the existence ofGa–O6 octahedra, thus explaining the remarkable vibrationmode at 418 cm−1 in the GaN nanowires as obtained by ni-triding -Ga2O3 nanowires. It is worth mentioning that areversed phase transition of GaN from tetrahedrally bondedstructure to an octahedrally bonded structure under highpressure has been realized.19,20 We also note that in an earliertheoretical work done by Sun et al.,21 it was pointed out that“nonbonding” lone pairs exist on the nitride surface and canlower the Raman frequencies of vibration modes. The ex-perimental results presented in the present study seem con-sistent with their theoretical prediction although there is noquantitative estimation of the Raman frequency shifts avail-able.In conclusion, we have shown that the vibrational modeat 418 cm−1 frequently observed in GaN nanostructures mostlikely originates from octahedrally bonded GaN structures.This conclusion is strongly supported by the observed evo-lution of the 418 cm−1 mode under postnitridation conditionsof -Ga2O3 nanowires grown on Si substrates.The authors wish to thank X. M. Cai, A. B. Djurišić, andM. H. Xie for supplying one kind of GaN nanowires grownwith CVD. The authors also gratefully thank C. D. Beling forhis critical reading of this letter. This work was partiallysupported by the University Development Fund and the Fac-ulty Development Fund of the University of Hong Kong.1H. M. Kim, Y. H. Choo, H. Lee, S. I. Kim, S. R. Ryu, D. Y. Kim, T. W.Kang, and K. S. Chung, Nano Lett. 4, 1059 2004.2Y. Huang, X. Duan, Y. Cui, and C. M. Lieber, Nano Lett. 2, 101 2002.3J. L. Coffer, T. W. Zerda, R. Appel, R. L. Wells, and J. F. Janik, Chem.Mater. 11, 20 1999.4H. D. Li, S. L. Zhang, H. B. Yang, G. T. Zou, Y. Y. Yang, K. T. Yue, X. H.Wu, and Y. Yan, J. Appl. Phys. 91, 4562 2002.5Xiaoming Sun and Yadong Li, Angew. Chem. 116, 3915 2004.6H. Siegle, G. Kaczmarczyk, L. Filippidis, A. P. Litvinchuk, A. Hoffmann,and C. Thomsen, Phys. Rev. B 55, 7000 1997.7V. Yu. Davydov, Yu. E. Kitaev, I. N. Goncharuk, A. N. Smirnov, J. Graul,O. Semchinova, D. Uffmann, M. B. Smirnov, A. P. Mirgorodsky, and R.A. Evarestov, Phys. Rev. B 58, 12899 1998.8X. M. Cai, A. B. Djurišić, and M. H. Xie, Thin Solid Films 515, 9842006.9W. Pollard, J. Non-Cryst. Solids 283, 203 2001.10K. Parlinski and Y. Kawazoe, Phys. Rev. B 60, 15511 1999.11G. Kaczmarczyk, A. Kaschner, A. Hoffmann, and C. Thomsen, Phys. Rev.B 61, 5353 2000.12H. J. Trodahl, F. Budde, B. J. Ruck, S. Granville, A. Koo, and A. Bittar, J.Appl. Phys. 97, 084309 2005.13Y. 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 2004.14S. Geller, J. Chem. Phys. 33, 676 1960.15D. Dohy, G. Lucazeau, and A. Revcolevschi, J. Solid State Chem. 45, 1801982.16D. G. Zhao, S. J. Xu, M. H. Xie, S. Y. Tong, and H. Yang, Appl. Phys.Lett. 83, 677 2003.17C. Vinegoni, M. Cazzanelli, A. Trivelli, G. Mariotto, J. Castro, J. G.Lunney, and J. Levy, Surf. Coat. Technol. 124, 272 2000.18W. Limmer, W. Ritter, and R. Sauer, Appl. Phys. Lett. 72, 2589 1998.19M. Ueno, M. Yoshida, A. Onodera, O. Shimomura, and K. Takemura, AIPConf. Proc. 309, 557 1994.20S. Limpijumnong and W. R. L. Lambrecht, Phys. Rev. Lett. 86, 91 2001.21C. Q. Sun, B. K. Tay, S. P. Lau, X. W. Sun, X. T. Zeng, S. Liu, H. L. Bai,H. Liu, Z. H. Liu, and E. Y. Jiang, J. Appl. Phys. 90, 2615 2001.FIG. 4. Raman spectra measured from Ga2O3 nanowires nitrided in NH3under different conditions: the top at 1100 °C for 20 min, the middle at1100 °C for 30 min, and the bottom at 1150 °C for 20 min.103117-3 Ning et al. Appl. Phys. Lett. 91, 103117 2007Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

418 cm-1 Raman scattering from gallium nitride nanowires: Is it a vibration mode of N-rich Ga-N bond configuration?

Crossref

http://hub.hku.hk/bitstream/10722/80609/1/content.pdf

418 cm-1 Raman scattering from gallium nitride nanowires: Is it a vibration mode of N-rich Ga-N bond configuration?

Abstract

Similar works

Full text

Available Versions

Institutional Repository of Peking University

HKU Scholars Hub

Crossref