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doi:10.1016/j.jc

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Campinas, SP,

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Journal of Crystal Growth 308 (2007) 37–40

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GaN nano- and micro-spheres fabricated selectively on silicon

L.A.M. Bareaa,b,�, A.A.G. von Zubena, A.Z. Marqueza, N.C. Frateschia,b

aApplied Physics Department, Gleb Wataghin Physics Institute, CP 6165, State University of Campinas - UNICAMP, CEP 13083-970, Campinas, SP, BrazilbCenter for Semiconductor Components CP 6061, State University of Campinas - UNICAMP, CEP 13083-870, Campinas, SP, Brazil

Received 14 May 2007; received in revised form 18 July 2007; accepted 31 July 2007

Communicated by M. Tischler

Available online 6 August 2007

Abstract

This work presents the selective growth of three-dimensional metallic gallium nitride structures on silicon substrates by chemical beam

epitaxy (CBE) with a subsequent plasma nitridation process. The use of titanium pre-deposited stripes over silicon (1 0 0) is shown to

provide high selectivity where spherical and semi-spherical structures are obtained only over the metal. These structures have diameters

ranging from 100 nm to 3 mm depending on the growth conditions. The nitridation process was performed on an electron cyclotron

resonance (ECR) plasma system. Raman micro-spectroscopy results showed GaN formation with zinc blend crystal structure and

photoluminescence emission in the visible spectrum in a range between 350 and 650 nm.

r 2007 Elsevier B.V. All rights reserved.

PACS: 78; 78.67.�n; 78.67.Bf

Keywords: A1. Nano-structures; A3. Chemical beam epitaxy; B1. Gallium compounds; B1. Nitrides; B2. Semiconducting gallium compounds; B2.

Semiconducting III–V materials; B2. Semiconducting materials

1. Introduction

Wide band-gap materials are of great interest for lightemitting in the visible [1–3]. Several techniques for theepitaxial growth of these structures on different substrateshave been demonstrated for the generation of efficient lightemitters [4,5]. The quality of this material is very dependentupon the substrate and very little compatibility existsbetween them and the traditional III–V compounds orsilicon. Recently, it has been shown that micro-spheres ofindium and/or gallium can be obtained by chemical vapordeposition (CVD). The metallic material is held by carbonmembranes originated from group III precursors. Subse-quently, a nitridation process is performed for theformation of the wide band-gap material, InGaN [6]. Thedevelopment of these nanoscopic 3D structures maygenerate a new class of III-N materials. These materials

e front matter r 2007 Elsevier B.V. All rights reserved.

rysgro.2007.07.052

ing author. Center for Semiconductor Components CP

niversity of Campinas - UNICAMP, CEP 13083-870,

Brazil. Tel.: +55 19 3521 5357; fax: +55 19 3521 5343.

ess: barea@ifi.unicamp.br (L.A.M. Barea).

may provide great flexibility in the engineering of thevis–UV emission properties. This is of great relevanceconsidering the development of high brightness white light-emitting diodes where chromacity control is paramount.In this work, we present the results of a new way for the

fabrication of 3D gallium nitride nano- and micro-structures by a post-nitridation process. The structuresare selectively deposited on silicon substrates patternedwith titanium stripes. Thus, a potential technique for theintegration of III-N light-emitting material and silicon isinvestigated.

2. Experimental procedure

Silicon (1 0 0) substrate was cleaned with buffered HFand de-ionized H2O and N2 blow-dried. Titanium2.5–50 mm wide stripes, spaced by 250 and 0.03 mm thick,were deposited by lift-off. The silicon substrates with the Tistripes were indium-mounted on molybdenum blocks forthe growth. The 3D gallium structures were grown bychemical beam epitaxy (CBE) using triethyl-gallium(TEGa) as the gallium source and hydrogen as the carrier

ARTICLE IN PRESSL.A.M. Barea et al. / Journal of Crystal Growth 308 (2007) 37–4038

gas. No group V vapor pressure was supplied. Severalgrowth temperatures and durations were examined. Thegrowth temperature was in a range between 450 and 575 1Cand the growth duration between 7 and 30min. The 3Dgallium structures nitridation was achieved by a 15-minprocess at 20 1C in an electron cyclotron resonance (ECR)chamber. Ar and N2 (20 sccm each), 4mTorr pressure andmicrowave power of 950W were used. The characteriza-tion of these structures was realized by optical microscopy,scanning electron microscopy and atomic force micro-scopy. Photoluminescence (PL), energy-dispersive X-rayspectroscopy and Raman micro-spectroscopy were used tostudy the PL emissions, chemical composition and crystalstructure.

Fig. 2. SEM picture of metallic gallium structures grown over titanium

stripes. Growth conditions: 525 1C, 30min.

0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000

Co

un

ts

30µµm

30µm

0

0

Fig. 3. Histogram of the spheres’ diameters for a typical sample grown at

450 1C for 30min. The inset shows an atomic force microscopy image of

the sample.

3. Results

For the depositions with growth duration between 15and 30min, and substrate temperature between 450 and525 1C, gallium droplets are developed only over thetitanium stripes with 100% selectivity. The surface densityincreases with the deposition time. Above 30min growth,the structures tend to coalesce into larger structures. Thesurface density is greatly reduced for temperatures above525 1C. Fig. 1 shows a series of optical micrographsshowing the selectivity and surface density of the as-grownsamples. Fig. 2 shows a scanning electron microscopemicrograph of a sample grown at 525 1C for 30min. Mostof the structures are semi-spherical. We also observed theformation of spheres. However, we have not identified theparameters determining the shape of the structures. Thespheres and semi-spheres have diameters in a range from100 nm to 3 mm range.

A typical sample grown at 450 1C for 30min wasexamined by atomic force microscopy as shown in theinset of Fig. 3. The measured surface density was found tobe between 106 and 107 cm�2. Fig. 3 shows the histogram ofthe spheres diameters. A large peak around 220 nm indiameter is observed.

Energy-dispersive X-ray spectroscopy analysis of a 2-mmsphere (525 1C, 30min) showed the presence of gallium,

525

°C450

°C

t = 1t = 7 min.

10 µm

5 µm

Fig. 1. Series of optical micrographs showing the selec

titanium, silicon, oxygen and carbon in the chemicalcomposition. Gallium appears as the largest element inthe analysis followed by silicon and titanium. The presence

t = 30 min.5 min.

10 µm10 µm

5 µm 10 µm

tivity and surface density of the as-grown samples.

ARTICLE IN PRESSL.A.M. Barea et al. / Journal of Crystal Growth 308 (2007) 37–40 39

of oxygen and carbon is originated from atmosphericcontamination and/or from the metal-organic precursorused [6]. After ECR nitridation (20 1C, 15min), theenergy-dispersive X-ray spectroscopy analysis showed thepresence of the same elements but with a clear presence ofnitrogen.

The same samples were investigated by Raman micro-spectroscopy. Fig. 4 shows the Raman spectra before andafter the nitridation process. A large peak near 207 cm�1 isobserved after nitridation. This peak is a possible evidenceof GaN in the zinc blend crystal structure for this structurehas a transverse acoustic peak at 207 cm�1 [7–9]. Therefore,we believe that a portion of the formed GaN is indeed incrystalline form.

PL was performed with a 325 nm laser (2.54W/cm2,HeCd) at 4K. Fig. 5 shows the PL spectrum before and

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

1.01

0 200 400

No

rmalized

In

ten

sit

y

Sh

Fig. 4. Raman micro-spectroscopy measurement of structures of Ga o

0

10

20

30

40

50

60

350 400 450 5

Inte

nsit

y (a.u

.)

lamb

After Nit

Before Nitridation

Fig. 5. Photoluminescence spectra at 4K of Ga structures, GaN structures, Si

line, Si/Ti; solid circles, Ga before nitridation and solid triangles, Ga after nit

after nitridation for a sample at 450 1C and 15min. Inorder to eliminate the possibility that the emission could bedue to Si or Ti nitrides, we also included in this picture, thespectrum of a Si substrate and a Ti/Si sample thatunderwent the same nitridation process. The grown samplewith nitridation shows a clear blue–green emission centeredat 475 nm. This emission is similar to that reported bySacilotti et al. [10]. Fig. 6 shows the PL results of threesamples grown at T ¼ 450, 525 and 575 1C for 15min, afterthe same nitridation process. We have not observed adependence of the spheres’ diameters and diameterhomogeneity with growth parameters. The PL intensityreduction is in agreement with the surface density reduc-tion with growth temperature. This result reinforces thefact that the emissions are indeed related to the GaNstructures present on the sample.

600 800 1000

ift (cm-1)

Before Nitridation

After Nitridation

ver Ti before and after nitridation. Growth conditions: 525 1C, 30min.

00 550 600 650

da (nm)

GaN

Ga

Si

Si/Ti

ridation

(1 0 0) and Ti. Growth conditions: 450 1C, 15min. Solid line, Si; solid thin

ridation.

ARTICLE IN PRESS

0

20

40

60

350 450 550 650

Inte

nsit

y (a.u

.)

lambda (nm)

T = 450°C

T = 525°C

T = 575°C

Fig. 6. Photoluminescence spectra at 4K of GaN over Ti with different growth temperatures and 15min growth.

L.A.M. Barea et al. / Journal of Crystal Growth 308 (2007) 37–4040

4. Conclusion

This work presents a new way for the development of 3DGaN nano- and micro-structures on Si (1 0 0) substratesusing CBE and an ECR plasma nitridation process.Structures with a spherical shape with diameters varyingfrom 100 nm to 0.3 mm with surface density between 106

and 107 cm�2 are obtained with 100% selectivity overpatterned titanium stripes. Raman micro-spectroscopyresults showed GaN formation with zinc blend crystalstructure and PL results showed that those 3D structuresemit in the visible spectrum in a range between 350 and650 nm. The formation of the spheres is obviously aconsequence of group III species excess and its migration inthe substrate. The low-temperature nitridation process ispossible due to the high efficiency of the ECR system inionizing N2. More work is required to determine themechanism for the growth and nitridation of thesestructures. Particularly, processing new structures on newsubstrates with variable stripe widths and separations willhelp quantifying the migration mechanism. Also, transmis-sion electron microscopy will be helpful for the directobservation of the GaN structures.

Acknowledgments

This work was supported by FAPESP. The authorswould like to acknowledge Celso Ramos, Jose RobertoMialichi, Dailto Silva, Rosane Palisari and Julio CesarBertin for all experimental assistance.

References

[1] B. Levi, Phys. Today (1996) 18.

[2] N. Johnson, A. Nurmikko, S. DenBaas, Phys. Today (2000) 31.

[3] S. Nakamura, T. Mukai, M. Senoh, J. Appl. Phys. 76 (1994)

8189.

[4] S. Haffouz, P. Hageman, V. Kirilyuk, L. Macht, J. Weyher,

P. Larsen, Mater. Sci. Eng. B 97 (2003) 9.

[5] X. Duan, C. Lieber, J. Am. Chem. Soc. 122 (2000) 188.

[6] M. Sacilotti, L. Imhof, S. Burgeois, C. Dumas, J. Decobert,

P. Baldeck, I. Colombier, J. Crystal Growth 261 (2004) 253.

[7] S. Tripathy, et al., Appl. Surf. Sci. 253 (2006) 236.

[8] L. Bergman, R. Nemanich, Annu. Rev. Mater. Sci. 26 (1996) 551.

[9] J.M. Zhang, T. Ruf, M. Cardona, Phys. Rev. B 56 (1997) 14399.

[10] M. Sacilotti, L. Imhoff, S. Bourgeois, C. Dumas, J.C. Vial,

P. Baldeck, I. Colombier, F. Donatini, Jpn. J. Appl. Phys. 43

(2004) L698.