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Journal of Crystal Growth 286 (2006) 235–239

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N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN compositebuffer layer and Al0.2Ga0.8N/GaN superlattice

Dong-Wook Kim, Cheul-Ro Lee�

Research Center of Advanced Materials Development (RCAMD), School of Advanced Materials Engineering, Engineering College,

Chonbuk National University, Chonju 561-756, Chonbuk, Republic of Korea

Received 13 July 2005; received in revised form 11 October 2005; accepted 19 October 2005

Available online 6 December 2005

Communicated by M. Schieber

Abstract

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are

compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-

free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In

order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of

Al0.2Ga0.8N/AlN and superlattice of Al0.2Ga0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-

crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1)

heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85� 109 and 1.32� 109 cm�2 for n-GaN

and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was

performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of

edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm

related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.

r 2005 Elsevier B.V. All rights reserved.

Keywords: A1. Composite buffer layer; A1. N-doping; A3. MOCVD; A3. Superlattice; B1. GaN/Si(1 1 1); B1. Si(1 1 1)

1. Introduction

GaN and related group III-nitride materials haveattracted extensive attention because of their wide-poten-tial application to optoelectronic and micro electronicdevices [1–5]. GaN forms continuous solid solution withAlN so that band gap engineering is possible from 3.4 to6.2 eV, which corresponds to the spectral region from blueto ultraviolet. Moreover, GaN materials can be easily usedto conduct high power and high frequency operation due totheir high electron saturation velocity, high critical field,and high stability. One of the key aspects is the growth of athick high-quality epitaxy. The GaN layer in the AlGaN/GaN heterostructure has to be thick enough in order forthe AlGaN and GaN interface to avoid the high defective

e front matter r 2005 Elsevier B.V. All rights reserved.

rysgro.2005.10.104

ing author. Tel.: +8263 270 2304; fax: +82 63 270 2305.

ess: crlee7@chonbuk.ac.kr (C.-R. Lee).

region near the GaN and substrate interface caused bylattice mismatch between GaN and substrate material.Recently, the quality of GaN epitaxial layer has been foundto be very sensitive to initial surface coating on Si substrateand is being improved by exploring AlN nucleation layer[6–8]. Also, conductive property by doping is essential forthose applications. But, so far, there has been relatively fewstudies on the growth characteristics and doping propertiesof the GaN that was grown on Si(1 1 1) substrate [9–12].In this study, the growth and characteristics of the Si-

doped and undoped GaN epitaxy grown by metalorganicchemical vapor deposition (MOCVD) on Si(1 1 1) substratewas compared and discussed. In order to achieve high-quality and crack-free GaN epitaxy on Si(1 1 1), we haveconducted specially designed multiple buffer layers; acomposite buffer layer (CBL) consisting of AlN seed layerand Al0.2Ga0.8N interlayer and 20 pair Al0.2Ga0.8N/GaNsuperlattices to reduce tensile stress, which extend from the

ARTICLE IN PRESSD.-W. Kim, C.-R. Lee / Journal of Crystal Growth 286 (2006) 235–239236

large difference of lattice mismatches and thermal expan-sion coefficients between GaN film and Si(1 1 1) substrate.The employment of these optimized multiple buffermethods make it possible to perform n-doping to high-quality GaN epitaxy.

Scanning electron microscopy (SEM) and atomic forcemicroscopy (AFM) were used to evaluate surface mor-phology and thickness, also double-crystal X-ray diffrac-tometry (DCXRD), photoluminescence point analysis, andPL mapping were implemented to determine the crystal-linity, optical property, and uniformity of the GaN epitaxy,respectively.

2. Experimental procedure

The AlGaN/GaN heterostructures investigated herewere grown using 6-pockets multi-wafer loading systemlow-pressure MOCVD; MARVEL 260-A performingvertical gas flow with optimized substrate rotation rate.2-in. one-side polished Si(1 1 1) substrates were used for theepitaxy growth. Conventional cleaning procedures with thechemical solutions and thermal cleaning to eliminate nativecontamination for 10min at 1100 1C were carried outbefore growth. The pressure was kept at 100 Torr during allgrowth procedure. For the growth of AlxGa1x�xNmaterials, trimethylaluminum (TMA), trimethylgallium(TMG) and ammonia (NH3) were employed for the MO-

Fig. 1. Layer structure of undoped GaN and n-GaN:Si on Si(1 1 1)

heteroepitaxy grown with superlattices and composite buffer layer.

Fig. 2. Plan-view SEM images of n-GaN:Si(1 1 1) surface. The e

source of Al, Ga, and N, respectively, with H2 as the carriergas. Also, monosilane (SiH4) was used for Si as an n-typedopant with a flow rate of 9.0 nmol/min. As speciallydesigned, multiple buffer layers were introduced ontoSi(1 1 1). 100-nm thick Al0.2Ga0.8N intermediate wasdeposited at 1040 1C followed by 10-nm thick AlNnucleation layer at same temperature as a composite bufferlayer. 20-pair-short period superlattice consisted of Al0.2-Ga0.8N/GaN were deposited at 1040 1C with 2-nm thick-ness that were kept equally. The aluminum percentage inthe barrier was fixed at 20% with a control in the MOCVDsystem. Then, an unintentionally doped (undoped) and Si-doped GaN epitaxy were deposited with a 1200 nmthickness as shown in Fig. 1.

3. Experimental results

Fig. 2 shows the plan-view lower and higher magnifica-tion SEM images of n-GaN:Si/Si(1 1 1) surface that revealshigh quality mirror-like surface over the entire 2-in. wafer.In spite of n-doped sample at higher magnification, no pitsor cracks were observed on the film with the 1200 nmthickness of the n-GaN:Si/Si(1 1 1) heteroepitaxy. Thismeans the growth of CBL and superlattices between n-GaN:Si and Si avoids the abrupt change in the latticemismatch and thermal expansion coefficient, therefore, thestrains induced by the large lattice and thermal mismatchbetween the n-GaN:Si epilayer and Si(1 1 1) substrate werereduced efficiently.AFM has been used to characterize the surface

morphology of the sample and the threading dislocationas shown in Fig. 3. Root mean square (RMS) value was1.427 and 1.847 nm and for n-GaN/Si(1 1 1) and undopedGaN/Si(1 1 1) measured on 4� 4 mm2 area, respectively.The roughness of the GaN layer is considerably improvedwith the use of CBL and superlattice, compared to ourprevious report [11]. The average of the threadingdislocation density was measured to 1.32� 109 and3.85� 109 cm�2 for undoped GaN/Si(1 1 1) and n-GaN:-Si/Si(1 1 1). This measurement was achieved by countingdark points relatively. This value was nearly comparable tothat obtained for GaN on sapphire. These results explain

ntire area shows mirror-like surface with no pits and cracks.

ARTICLE IN PRESS

4.00

4.00

3.00

3.00

2.00

2.00

1.00

1.000

4.00

3.00

2.00

1.00

004.003.002.001.000

µM

µMµM

µM

2 23

311

Threadingdislocation

(a)

(b)

Fig. 3. AFM image of a 1200nm undoped GaN/Si(1 1 1) (left) and n-GaN:Si/Si(1 1 1) (right) epilayer; (a) phase images of the samples, threading

dislocations were observed as a dark point on the surface. (b) 3-D morphologies of the undoped GaN/Si(1 1 1) (left) and n-GaN:Si/Si(1 1 1) (right) epitaxy.

(b)

(b) n-GaN: Si/Si (111)

PL I

nten

sity

Wavelength (nm)

(a)

(a) u-GaN/Si (111)

350 400 450 500 550 600 650 700

12000

10000

8000

6000

4000

2000

0

Fig. 5. PL point analysis of (a) undoped GaN/Si(1 1 1) and (b) n-GaN:Si/

Si(1 1 1). For n-GaN/Si(1 1 1) sample, yellow luminescence was observed at

580 nm.

n-GaN: Si/Si (111)

FWHM: 823 arcsec

FWHM: 745 arcsec

u-GaN/Si (111)

GaN (0002)

Inte

nsity

(ar

b. u

nits

)

θ (degree)16.5 17.517.0 18.0

Fig. 4. DCXRD rocking curves of undoped GaN/Si(1 1 1) and n-GaN:Si/

Si(1 1 1) heteroepitaxy.

D.-W. Kim, C.-R. Lee / Journal of Crystal Growth 286 (2006) 235–239 237

that CBL and superlattice in the samples play a key role inminimizing the tensile strain in the upper n-GaN/Si(1 1 1)and undoped GaN/Si(1 1 1) layers. The results also presentevidence of a flat 3-D image.

The FWHM values of the double-crystal X-ray rockingcurve (DCXRC) for the undoped GaN and n-GaN:Si onSi(1 1 1) with superlattice and CBL were 745 and 823 arc-sec, as shown in Fig. 4. FWHM value of the n-GaN:Si/Si(1 1 1) epitaxy is higher than the undoped sample. It hasbeen thought that Si incorporation decreases the crystal-

linity, which resulted from the difference of the atomic sizebetween Ga and Si atom. The crystal quality of the GaNepilayer is expected to be high with increasing thickness,conversely, cracks will be observed on the thick GaN layersurface due to thermal strain when the Si substrate iscooled down from the growth temperature to roomtemperature.Fig. 5 shows photoluminescence point analysis of the (a)

undoped GaN/Si(1 1 1) and (b) n-GaN:Si/Si(1 1 1) withstrong band edge emission peak observed at 365 nm in both

ARTICLE IN PRESSD.-W. Kim, C.-R. Lee / Journal of Crystal Growth 286 (2006) 235–239238

samples by 325 nm He-Cd laser source. No defect peak wasshown in undoped GaN/Si(1 1 1) through the 700 nmemission; however, the doped sample had a defect peakaround 580 nm corresponding yellow-luminescence. Thus,it can be supposed that Si incorporation causes the latticedistortion resulted from the difference of atomic latticeconstant and high dislocation density.

Fig. 6 shows the PL mapping analysis that was carriedout for evaluating the optical property distribution of GaNon Si(1 1 1) with CBL and superlattices using 325 nm He-Cd laser under room temperature. 367.1 and 367.0 nmaverage band edge emission peaks corresponding to 3.377and 3.378 eV were observed for undoped GaN/Si(1 1 1) andn-GaN:Si/Si(1 1 1) with both FWHMs of 70meV, respec-tively. The band edge emission values of the undoped GaNand n-GaN:Si on Si(1 1 1) were shifted about 20meV to aslightly lower energy, which indicates that a tensile stressremained in the GaN epilayer due to the large difference of

Peak Lambda

nmAvge.=367.1 nm 375.0

373.0371.0369.0367.0365.0363.0361.0359.0

Avge : 367.1Median : 367.0Std Dev : 0.086%

(0.314)In-Spec: 100.0% Below: 0.0% Above: 0.0%

FWHM

Avge.=7.6 nmnm10.8

10.0

9.2

8.4

7.6

6.8

6.0

5.2

4.4

Avge : 7.6Median : 7.6Std Dev : 2.925%

(0.222)In-Spec: 100.0% Below: 0.0% Above: 0.0%

(a)

(b)

Fig. 6. PL mapping images of the undoped GaN/Si(1 1 1) (left) and n-GaN:Si/

edge emission peak, (b) FWHM of the PL spectra.

the thermal expansion coefficient and lattice mismatchbetween GaN and Si(1 1 1) substrate.

4. Conclusions

In summary, we reported the growth and characteriza-tion of n-GaN:Si/Si(1 1 1) and undoped GaN/Si(1 1 1)epitaxy with the introduction of an optimized multiplebuffer layer; composite buffer layer(CBL) and super-lattices. Although the n-type doping procedure could makecracks, which behave like a non-radiation scattering centeras an obstacle of electron movement, our multiple bufferlayers have constrained the Si incorporation-induced stressproblems in n-doped GaN epitaxy effectively. Becauseof residual stress in the epitaxy, the band edge emissionpeaks of the undoped GaN and n-GaN:Si/Si(1 1 1) wereshifted about 20meV to a lower energy compared to theepitaxy grown on sapphire(0 0 0 1). Consequently, these

Peak Lambda

nmAvge.=367.0 nm 371.0

370.0369.0368.0367.0366.0365.0364.0363.0

Avge : 367.0Median : 367.0Std Dev : 0.096%

(0.354)In-Spec: 100.0% Below: 0.0% Above: 0.0%

FWHM

nmAvge.=8.6 nm 16.3

14.312.310.38.36.34.32.30.3

Avge : 8.6Median : 8.3Std Dev : 7.999%

(0.688)In-Spec: 100.0% Below: 0.0% Above: 0.0%

Si(1 1 1) (right) with CBL and superlattices on Si(1 1 1) substrate : (a) band

ARTICLE IN PRESSD.-W. Kim, C.-R. Lee / Journal of Crystal Growth 286 (2006) 235–239 239

high-quality undoped GaN and n-GaN:Si/Si(1 1 1) hetero-epitaxy make it a good candidate for application tooptoelectronic and microelectronic devices.

Acknowledgments

This paper was supported by Grant no. R01-2003-000-10075-0 from the Basic Research Program of the KoreaScience and Engineering Foundation.

References

[1] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237.

[2] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997.

[3] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki,

J. Crystal Growth 98 (1989) 209.

[4] S. Nakamura, J. Crystal Growth 170 (1997) 11.

[5] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994)

1687.

[6] Y. Nakada, I. Aksenov, H. Okumura, Appl. Phys. Lett. 73 (1999).

[7] R.F. Davis, T. Gehrke, K.J. Limthicum, T.S. Zheleva, E.A. Preble,

P. Rajagopal, et al., J. Crystal Growth 225 (2001) 134–140.

[8] Y. Hiroyama, M. Tamura, Jpn. J. Appl. Phys. 37 (1998) L630.

[9] J. Sanchez-Paramo, J.M. Calleja, M.A. Sanchez-Garsıa, Appl. Phys.

Lett. 78 (2001) 4124.

[10] C.-R. Lee, J. Crystal Growth 246 (2002) 25.

[11] S.-H. Jang, S.-J. Lee, I.-S. Seo, H.-K. Ahn, O.-Y. Lee, J.-Y. Leem,

C.-R. Lee, J. Crystal Growth 241 (2002) 289.

[12] C. Stampfl, J. Neugebauer, C.G. Van de Walle, Mater. Sci. Eng. B 29

(1999) 253.