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Journal of Crystal Growth 310 (2008) 2977– 2980

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

� Corr

E-m

twk@ha

journal homepage: www.elsevier.com/locate/jcrysgro

Microstructural properties and atomic arrangements of GaN nanorods grownon Si (111) substrates by using hydride vapor phase epitaxy

K.H. Lee a, Y.H. Kwon b, S.Y. Ryu b, T.W. Kang b, J.H. Jung c, D.U. Lee c, T.W. Kim c,�

a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Koreab Quantum Functional Semiconductor Research Center, Dongguk University, 3-26 Chungku Pildong, Seoul 100-715, Republic of Koreac Advanced Semiconductor Research Center, Division of Electronics and Computer Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu,

Seoul 133-791, Republic of Korea

a r t i c l e i n f o

Article history:

Received 16 January 2008

Received in revised form

6 March 2008

Accepted 6 March 2008

Communicated by R.M. Biefeldresolution TEM (HRTEM) images showed that the crystallized GaN nanorods contained very few defects.

Available online 12 March 2008

PACS:

61.46.Km

61.72.Uj

68.37.Lp

61.05.Cp

Keywords:

A1. Nanostructures

A3. Vapor phase epitaxy

B1. Nanomaterials

B2. Semiconducting III–V materials

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.jcrysgro.2008.03.011

esponding author. Tel.: +82 2 2220 0354; fax:

ail addresses: sige@dongguk.edu (Y.H. Kwon),

nyang.ac.kr (T.W. Kim).

a b s t r a c t

X-ray diffraction (XRD) patterns, field-emission scanning electron microscopy (FESEM) images,

transmission electron microscopy (TEM) images, and selected area electron diffraction pattern (SADP)

images showed that one-dimensional GaN nanorods with c-axis-oriented single-crystalline wurzite

structures were grown on Si (111) substrates by using improved hydride vapor phase epitaxy. The high-

The atomic arrangements for the GaN nanorods grown on the Si (111) substrates are described on the

basis of the XRD, the TEM, the SADP, and the HRTEM results.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Wide-energy-gap semiconductor materials have becomeparticularly attractive because of their promising applicationsin optoelectronic devices operating in the visible–ultravioletregion of the spectrum, such as light emitting diodes (LEDs) andlasers, due to their high-optical gains and chemical stabilities[1–5]. LEDs fabricated utilizing one-dimensional (1-D) semicon-ductor nanostructures, such as nanowires and nanorods, haveexhibited enhanced light extraction efficiency and quantumefficiency in comparison with those composed of planar struc-tures [6–8]. GaN nanostructures with excellent optical propertieshave emerged as potential candidates for applications in promis-ing next-generation nanodevices operating at relatively lowpower consumption [9].

ll rights reserved.

+82 2 2292 4135.

Various synthesis methods, such as metal-organic chemicalvapor deposition (MOCVD) [10,11], laser ablation [12], molecularbeam epitaxy [13], the sol–gel method [14], template-inducedgrowth [15], and sublimation method [16] have been introducedto obtain higher quality 1-D GaN nanostructures. Among thevarious kinds of synthesis methods, hydride vapor phase epitaxy(HVPE) has been used to form 1-D GaN nanostructures due to itsadvantages of high growth rate and low cost. Even though someworks concerning the formation and optical properties of 1-D GaNnanostructures using HVPE have been performed [17], relativelyfew studies on the microstructural properties and the atomicarrangements of the GaN nanostructures have been carried out.Because the microstructural properties of the GaN nanostructuressignificantly affect their electrical and the optical properties,which are necessary for fabricating high-efficiency devices [18,19],studies on the microstructural properties of GaN nanostructuresare very important for enhancing nanodevices fabricated utilizingGaN nanostructures. Furthermore, GaN/Si heterostructures are ofparticular interest in the integration of optoelectronic devices dueto the large exciton binding energy of the GaN semiconductorsand the cheapness and the large size of Si substrates [20,21].

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K.H. Lee et al. / Journal of Crystal Growth 310 (2008) 2977–29802978

This paper reports the microstructural properties and theatomic arrangements of GaN nanorods grown on Si (111)substrates by using improved HVPE without a catalyst. X-raydiffraction (XRD), field emission scanning electron microscopy(FESEM), transmission electron microscopy (TEM), and selectedarea diffraction pattern (SADP) measurements were carried out inorder to characterize the microstructural properties of the GaNnanorods grown on Si (111) substrates. Possible atomic arrange-ments in the GaN nanorods formed on the Si (111) substrates arepresented on the basis of the XRD, the TEM, and the SADP results.

Fig. 1. X-ray diffraction pattern of the GaN nanorods grown on Si (111) substrates.

2. Experimental details

GaN nanorods used in this study were grown on Si (111)substrates by using improved HVPE. While the reaction chamberof the conventional HVPE system divided into two reaction zones,that of the improved HVPE system divided into three reactionzones. An additional third zone provided the nucleation for theGaN nanorods occurred on the Si substrate. The carrier concen-tration of the P-doped n-Si substrates with (0 0 1) orientationsused in this experiment was 1�1015 cm�3. The substrates weredegreased in trichloroethylene (TCE), rinsed in deionized water,etched in a mixture of HF and H2O (1:1) at room temperature for5 min, and rinsed in TCE again. After the Si wafers had beencleaned chemically, they were mounted onto a susceptor in thereaction chamber. The Ga metal source was located on the front ofthe chamber, and the Ga source was reacted with HCl and N2

carrier gases at 850 1C. The Ga metal was converted into galliumchloride (GaClx) during the reaction. The converted GaClx gas wastransported to the inner chamber and was reacted with a NH3 gasat 1100 1C. The reaction temperature between the GaClx and theNH3 gases in the second zone used in this study is higher than thatused in the previous work [22], resulting in the improvement ofthe uniformity for the GaN nanorods. Then, the mixed gaswas transported to the Si substrate, and the substrate temperaturewas decreased to 670 1C. When the mixed gases were super-saturated due to the decreased temperature, nucleation of theGaN nanorods occurred on the Si substrate. The flow rates of theHCl, the N2, and the NH3 gases were 50 sccm, 1 and 3 slm,respectively.

The XRD measurements were performed by using a Rigaku D/MAX-RC diffractometer with Cu Ka radiation. SEM measurementswere carried out using a FEI XL 30 system. TEM measurementswere performed using a JEOL JEM 3010 transmission electronmicroscope operating at 300 kV. The samples for the cross-sectional TEM measurements were prepared by cutting andpolishing with diamond paper to a thickness of approximately30mm and then argon-ion milling at liquid-nitrogen temperatureto electron transparency.

Fig. 2. (a) Cross-sectional and (b) plan-view scanning electron microscopy images

of GaN nanorods grown on a Si (111) substrate.

3. Results and discussion

Fig. 1 shows the XRD pattern of GaN nanorods grown on Si(111) substrates. The dominant (0 0 0 2) and (0 0 0 4) Ka diffrac-tion peaks corresponding to wurzite GaN nanorods, together witha peak corresponding to the Si (111) substrate, are clearlyobserved in Fig. 1. The calculated c-axis lattice parameter of theGaN nanorods is in reasonable agreement with that for GaN bulkmaterials (JCPDS ]50-0792). The fact that only the (0 0 0 2) and the(0 0 0 4) peaks of the GaN nanorods are observed indicate that theGaN nanorods formed on the Si (111) substrates are preferentiallyoriented along the [0 0 0 1] direction and that the formed GaNnanorods are very well aligned perpendicular to the Si (111)substrate. The small values of the full-width at half-maxima for

the diffraction peaks also demonstrate that the GaN nanorods arehighly crystalline.

Fig. 2 shows cross-sectional bright-field and plan-view SEMimages of GaN nanorods grown on Si (111) substrates. The cross-sectional SEM image demonstrates that the GaN nanorods with anonuniform shape are vertically well aligned to the substrate,as shown in Fig. 2(a). The diameters of the GaN nanorodsare approximately between 200 and 450 nm, and their heightsare between about 850 and 1550 nm, as shown in Fig. 1. The

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Fig. 3. Bright-field transmission electron microscopy image of the GaN nanorods

grown on Si (111) substrates. The inset indicates a selected area electron

diffraction pattern taken along the [11 2] zone axis of the Si substrate and the

[2 110] zone axis of the GaN nanorods.

Fig. 4. High-resolution transmission electron microscopy images of (a) the GaN

nanorod/Si (111) heterointerface region and (b) the tip region of the nanorod

taken along the [112] zone axis of the Si substrates and the [2 110] zone axis of

the GaN nanorods. (c) High-resolution transmission electron microscopy image of

a GaN nanorod with a different orientation relationship taken along the [110] zone

axis of the Si substrate and the [2 110] zone axis of the GaN nanorods.

K.H. Lee et al. / Journal of Crystal Growth 310 (2008) 2977–2980 2979

plan-view SEM image depicts that the surface morphologyperpendicular to the growth direction has a relatively hexagonalshape, as shown in Fig. 2(b).

Fig. 3 shows a bright-field TEM (BFTEM) image of the GaNnanorods grown on Si (111) substrates. The morphology and thesize of the GaN nanorods are in reasonable agreement with theSEM results shown in Fig. 2(a). A selected area diffraction pattern(SADP) demonstrates that the growth direction of the GaNnanorods is [0 0 0 1]. The SADP indicates that the orientationalrelationships between the Si substrate and the GaN nanorods are(0 0 0 1)GaNJ(111)Si and [0 110]GaNJ[110]Si, which are not consis-tent with the typical orientational relationships of GaN(0 0 0 1)GaNJ(111)Si and [2 110]GaNJ[110]Si [23,24]. While thelattice mismatch between the GaN nanorods and the Si (111)substrate for the known orientational relationships is approxi-mately 16.9% ( ¼ (aSi�aGaN)/aSi; aGaN ¼ 3.189 A and aSi(111) ¼3.84 A), the corresponding lattice mismatch of the observedorientational relationships is about 4.1%. The formed GaNnanorods are rotated 301 with respect to the Si (111) substratein order to reduce the strain resulting from the decrease in thelattice mismatch between the GaN layer and the Al2O3 (0 0 0 1)substrate. The streaked diffraction spots corresponding to the GaNlayer in the SADP indicate that the GaN nanorods are slightly tiltedalong the c-axial growth direction.

Fig. 4 shows high-resolution TEM (HRTEM) images of the GaNnanorods at different regions and with different orientationsgrown on Si (111) substrates. The HRTEM images clearly showthat GaN nanorods have a single-crystalline wurzite structure. TheHRTEM images demonstrate that the orientational relationshipsbetween the Si substrate and the GaN nanorods are(0 0 0 1)GaNJ(111)Si and [0 110]GaNJ[110]Si, which is in reasonableagreement with the SADP images. Almost all of GaN nanorods aregrown with the same orientational relationships, regardless of theexistence of an amorphous layer with a thickness of about 2 nm atthe heterointerface between the Si (111) substrate and the GaNnanorods. The HRTEM images show that the formation directionof the GaN nanorods is slightly tilted to the vertical direction ofthe Si (111) substrate, which is in reasonable agreement with theSADP results. Fig. 4(b) shows that the surface of the tip of the GaNnanorod consists of relatively flat (0 0 0 1) planes. Fig. 4(c) showsthat the GaN nanorods have different orientational relationshipsof (0 0 0 1)GaNJ(111)Si and [2 110]GaNJ[110]Si. The HRTEMimages show that the GaN nanorods contain very few defects,such as stacking faults.

The atomic arrangements at the heterointerface between theGaN nanorod and the Si substrate are displayed in Fig. 5. The

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Fig. 5. (a) Top-view and (b) side-view atomic arrangements for the GaN nanorods

grown on Si (111) substrates. The GaN nanorods are rotated 301 to the Si (111)

substrates.

K.H. Lee et al. / Journal of Crystal Growth 310 (2008) 2977–29802980

atomic arrangements of the GaN bottom layers are significantlyaffected by the Si substrate, regardless of the existence of anamorphous layer between the Si substrate and the GaN nanorods.The GaN nanorods are rotated 301 with respect to the Si (111)substrate, as shown in Fig. 5(a), which is in reasonable agreementwith the SADP and the HRTEM results. A side-view atomicarrangement for the GaN nanorods on the Si (111) substrate isshown in Fig. 5(b). The side-view atomic arrangement for theGaN/Si heterostructure is represented along the Si [112] directionand the GaN [2 110] direction obtained from the HRTEM resultsshown in Fig. 4(a).

4. Summary and conclusions

Well-aligned GaN nanorods were synthesized on Si (111)substrates by using an improved HVPE method without a catalyst.XRD, SEM, and TEM results showed that the GaN nanorods werepreferentially oriented along the [0 0 0 1] direction and were verywell aligned perpendicular to the Si (111) substrate. TEM results

showed that the diameters of the GaN nanorods were approxi-mately between 200 and 450 nm and that their heights wereabout 850 and 1550 nm. The SADP results indicated that theorientational relationships between the Si substrate and the GaNnanorods were (0 0 0 1)GaNJ(111)Si and [0 110]GaNJ[110]Si. TheHRTEM images showed that the GaN nanorods had a single-crystalline wurzite structure and that the nanorods contained fewdefects, such as stacking faults. Almost all of the GaN nanorodswere grown with the same crystal-orientation relationships,regardless of the existence of an amorphous layer at theheterointerface. These results can help to improve understandingof the microstructural properties and the atomic arrangements ofGaN nanorods grown on Si (111) substrates.

Acknowledgments

This work was supported by the Korea Science and EngineeringFoundation (KOSEF) grant funded by the Korea government(MOST) (No. R0A-2007-000-20044-0) and also by the KoreaScience and Engineering Foundation through the QuantumFunctional Semiconductor Research Center at Dongguk University.

References

[1] K. Domen, A. Kuramata, T. Tanahashi, Appl. Phys. Lett. 72 (1998) 1359.[2] A.T. Schremer, J.A. Smart, Y. Wang, O. Ambacher, N.C. MacDonald, J.R. Shealy,

Appl. Phys. Lett. 76 (2000) 736.[3] M.L. Reed, N.A. El-Masry, H.H. Stadelmaier, M.K. Ritums, M.J. Reed, C.A. Parker,

J.C. Roberts, S.M. Bedair, Appl. Phys. Lett. 79 (2001) 3473.[4] T.N. Oder, K.H. Kim, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 84 (2004) 466.[5] J.W. Shin, J.Y. Lee, Y.S. No, J.H. Jung, T.W. Kim, W.K. Choi, Appl. Phys. Lett. 90

(2007) 181907.[6] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv.

Mater. 15 (2003) 353.[7] H.-M. Kim, Y.-H. Cho, H. Lee, S.I. Kim, S.R. Ryu, D.Y. Kim, T.W. Kang, K.S. Chung,

Nano Lett. 4 (2004) 1059.[8] S.D. Hersee, X. Sun, X. Wang, Nano Lett. 6 (2006) 1808.[9] N. Thillosen, K. Sebald, H. Hardtdegen, R. Meijers, R. Calarco, S. Montanari,

N. Kaluza, J. Gutowski, H. Luth, Nano Lett. 6 (2006) 704.[10] T. Kuykendall, P. Pauzauskie, S. Lee, Y. Zhang, J. Goldberger, P. Yang, Nano Lett.

3 (2003) 1063.[11] G.T. Wang, A.A. Talin, D.J. Werder, J.R. Creighton, E. Lai, R.J. Anderson, I. Arslan,

Nanotechnology 17 (2006) 5773.[12] X.F. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188.[13] R.K. Debnath, R. Meijers, T. Richter, T. Stoica, R. Calarco, H. Luth, Appl. Phys.

Lett. 90 (2007) 123117.[14] Yuxin Wu, Chengshan Xue, Huizhao Zhuang, Deheng Tian, Yi’an Liu, Appl.

Surf. Sci. 253 (2006) 485.[15] J. Goldberger, R. He, Y. Zhang, S. Lee, H. Yan, H.-J. Choi, P. Yang, Nature 22

(2003) 599.[16] J.Y. Li, X.L. Chen, Z.Y. Qiao, Y.G. Cao, Y.C. Lan, J. Crystal. Growth 213 (2000) 408.[17] H.M. Kim, D.S. Kim, Y.S. Park, D.Y. Kim, T.W. Kang, K.S. Chung, Adv. Mater. 14

(2002) 991.[18] W. Han, S. Fan, Q. Li, Y. Hu, Science 277 (1997) 1287.[19] C. Diaz-Guerra, J. Piqueras, V. Popa, A. Cojocaru, I.M. Tiginyanu, Appl. Phys.

Lett. 86 (2005) 223103.[20] J.B. Schlager, N.A. Sanford, K.A. Bertness, J.M. Barker, A. Roshko, P.T. Blanchard,

Appl. Phys. Lett. 88 (2006) 213106.[21] R. Calarco, R.J. Meijer, R.K. Debnath, T. Stoica, E. Sutter, H. Luth, Nano Lett. 7

(2007) 2248.[22] H.M. Kim, W.C. Lee, T.W. Kang, K.S. Chung, C.S. Yoon, C.K. Kim, Chem. Phys.

Lett. 380 (2003) 181.[23] L. Dobosa, B. Pecz, E. Feltin, B. Beaumont, P. Gibart, Vacuum 71 (2003) 285.[24] Y.H. Kim, J.Y. Lee, S.H. Lee, J.E. Oh, H.S. Lee, Appl. Phys. A-Mater. 80 (2005)

1635.