Journal of Crystal Growth 370 (2013) 254–258

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Journal of Crystal Growth

0022-02

http://d

n Corr

E-m

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

Study on the effects of AlN interlayer in thick GaN grownon 3C-SiC/Si substrates

Hao Fang a,n, Yoshifumi Takaya a, Hideto Miyake a, Kazumasa Hiramatsu a,Hidetoshi Asamura b, Keisuke Kawamura b, Hidehiko Oku b

a Department of Electrical and Electronic Engineering, Mie University, 1577 Kurima-machiya, Tsu, Mie 514-8507, Japanb Air Water Inc., 2-6-40 Chikko Shinmachi, Nishi-ku, Sakai 592-8331, Japan

a r t i c l e i n f o

Available online 15 August 2012

Keywords:

A1. Defects

A1. Stress

A3. MOVPE

B1. GaN

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jcrysgro.2012.08.003

esponding author.

ail address: fanghao@opt.elec.mie-u.ac.jp (H.

a b s t r a c t

For epitaxy of GaN on 3C-SiC/Si substrates, optimization of growth temperature of AlN interlayers (ILs)

was performed. With a proper growth condition of AlN IL, crack-free 3.5 mm thick GaN layer was

realized by multiple AlN ILs on 3C-SiC/Si (1 1 1) substrate. The distribution of D0X peak position in low

temperature cathodoluminescence spectra was mapped to investigate the stress in as-grown wafer. An

increase of tensile stress was found in the top GaN layers above AlN ILs. Cross-sectional transmission

electron microscopy images confirmed that AlN ILs could induce compressive stress and reduce

threading dislocations in the GaN epilayer grown on 3C-SiC/Si. The reduction of dislocations should

account for the part of incremental tensile stress revealed by the inhomogeneous distribution of

luminescence.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Recently, the integration of GaN-based optoelectronics with well-established Si-based electronics becomes an attractive research topicknown as the optoelectronic integration of circuits (OEIC). [1] Withthis technique, high-density information processors, precision imagesensor and/or display can be realized.

III-nitrides were first deposited on Si by metal organic chemi-cal vapor phase epitaxy (MOVPE) in 1971 by Manasevit et al. [2].Since there is a large amount of tensile stress in GaN grown on Si,which is induced by the mismatch of lattice constants andcoefficients of thermal expansions (CTE) between the two materi-als, it is fairly difficult to grow high-quality and crack-free nitridefilm on Si substrate. In recent years, a series of breakthrough hadbeen made with combining SiNx interlayer (IL) and AlN IL [3], orAlxGa1�xN grading layer [4]. Using 3C-SiC intermediate layer,which is almost lattice matched with GaN, is another way toobtain high-quality GaN on Si with simpler structure [5–9]. Asthere is still a mismatch of CTE, there has been few report aboutcrack-free GaN thicker than 2 mm obtained on 3C-SiC/Si (1 1 1)substrate. Thus, it is promising to realize crack-free thick GaNlayer on 3C-SiC/Si for high performance device with a strainengineering method, such as a low temperature (LT) AlN IL.LT AlN IL has been widely employed to obtain thick, crack-free

ll rights reserved.

Fang).

GaN-layers on silicon [10,11]. Previous results indicated that AlNIL could induce dual effects into GaN epilayer, including com-pressive stress [7] and annihilation of threading dislocation [12].However, the trade-off relationship of these two effects in strainengineering on Si substrate has seldom been mentioned. Thestudy on the relationship of stress distribution and structuraldefects in GaN with AlN ILs on Si will give more convincinginformation about the role of AlN IL in strain engineering.

In the present research, we studied the effect of AlN ILs in thickGaN on 3C-SiC/Si (1 1 1) substrates in terms of strain engineeringand structural defects. Our results confirmed that part of thecompressive stress was eliminated by dislocation annihilationprocess, which occurred at the same time as stress compensation.This phenomenon is supposed to be meaningful in realizing OEICwith nitrides based devices.

2. Experiments

The GaN samples, which we studied on, were grown by low-pressure MOVPE on 3C-SiC/Si substrates. Since the thickness ofthe 3C-SiC layers were about 3 mm, there was an initial curvatureof substrates as 120 km�1, which corresponded to a tensile stressabout 0.1 GPa for the SiC intermediate layers. Trimethylgallium(TMG), trimethylaluminum (TMA), and ammonia (NH3) wereused as the precursors for Ga, Al, and N, respectively. H2 wasused as the carrier gas.

Fig. 2. FWHM of XRCs on (0 0 4) and (1 0 2) planes and RMS surface morphology

of the samples in first series.

H. Fang et al. / Journal of Crystal Growth 370 (2013) 254–258 255

To study the effect of AlN IL, two series of experiments wereperformed. The first series was carried out to optimize the growthtemperature of AlN IL with four samples. The growth structureincluded a 40 nm AlN buffer layer grown at 1200 1C and a firstGaN layer with a thickness of about 700 nm. Then, two cycles of15 nm AlN ILs and 700 nm GaN epilayers were grown on theinitial GaN subsequently. Growth temperature of the AlN ILs waschanged from 950 to 1080 1C. In the second series of experiments,at the optimized growth temperature, a sample with four AlN ILswas made to investigate the evolution of stress and structuraldefects in GaN layers. The growth temperature of AlN ILs and GaNwas set at 1050 and 1080 1C, respectively. The total thickness ofthe as-grown sample was about 3.5 mm.

After the GaN epitaxy processes, an optical microscope andatomic force micrscope (AFM) were used to characterize thesurface morphology of the as-grown samples. The crystallinequality of the as-grown samples were evaluated with the X-rayrocking curves (XRCs) on the (0 0 4) and (1 0 2) planes. Nanopho-ton RAMAN-11 Raman scattering spectroscopy system, with a532 nm laser source, was used to investigate strain status ofsamples. For the samples to optimize growth temperature, X-raydiffraction and Raman scattering spectroscopy were used to checkstress status of each sample. For the thick GaN sample with fourAlN ILs, low-temperature cross-sectional cathodoluminescence(CL) spectroscopy and scanning electron microscopy (SEM) werecarried out to analyze the distribution of stress in the GaNepilayer. Cross-sectional transmission electron microscopy(TEM) was employed to investigate the structural defects of theas-grown sample.

3. Results and discussion

3.1. Optimization of AlN ILs growth temperature

Fig. 1 shows the in plane lattice constant a and E2H of Raman

shift of GaN samples in first series experiment as a function of AlNgrowth temperature. In our experiment, the Raman spectra weremeasured in z(x, xþy)-z geometry and the laser focused on thesample surface. The pink line in Fig. 1 is the value of E2

H peak instress free GaN [13]. For the sample, AlN of which was grown at1050 1C, lattice constant a is 3.1889 A and E2

H peak is locatedat 567.7 cm�1. Therefore, when the growth temperature was1050 1C, the compressive stress induced by AlN ILs could totallycompensate the tensile stress induced by substrate.

Fig. 1. Lattice constants a and E2H of Raman shift of GaN epilayers as a function of

AlN growth temperature.

Full width at half maximum (FWHM) of XRCs and surfacemorphology of the samples in first series were shown in Fig. 2.Surface morphology of the samples were characterized by rootmean squared (RMS) surface roughness of 1�1 mm2. The samplewith AlN ILs grown at 1050 1C is also with lowest value of RMSsurface roughness and FWHM of (0 0 4) XRC. This phenomenonconfirmed that the AlN ILs could not only induce compressivestress, but also improve quality of GaN epilayer.

3.2. Effects of AlN ILs

Fig. 3 shows the surface morphology of the thick GaN layerwith four AlN ILs characterized by using (a) optical microscopeimages and (b) AFM images. The image in Fig. 3(a) reveals asmooth surface and no cracks of the film over the entire area ofthe GaN. There are some shallow pits on the surface, which areattributed to the morphology of the 3C-SiC/Si substrates. TheFWHMs of the XRCs on the GaN (0 0 4) and (1 0 2) planes are597.9 and 949.1 s for the as-grown sample. The RMS surfaceroughness measured by 5�5 mm2 of the AFM image is 0.55 nm.

To investigate the luminescent property of the GaN layer withfour AlN ILs, CL measurement was performed at 10 K and 5 keVusing a system composed of a HORIBA ISA charged-coupleddevice 3500 and a JEOL JSM-7001F field emission scanningelectron microscop (SEM). Using a scanning collection system, a50�50 point CL spectra matrix was also obtained at 10 K in thesame area. A cross-sectional SEM image of the 3.5 mm-thick GaNlayer is shown in Fig. 4(a). The structure of the four AlN ILssandwiched by five GaN layers is characterized by the imagecontrast, which resulted from the different material conductiv-ities. Fig. 4(d) shows the CL spectra of overall structure, the topand bottom GaN layers of the sample cross section. The dominantemission of CL spectra is attributed to the donor-bound-excitonrecombination (D0X) around 3.466 eV, the spectral shape ofwhich can be described by Gaussian function. The linewidths ofthe overall, top and bottom D0X emission are 13, 13 and 16 meV,respectively. Since the near-band-edge emissions are sensitive tothe biaxial stress [14], a mean residual tensile stress about0.14 GPa in the as-grown sample could be found with comparisonof the overall spectrum with the D0X line of strain-free GaN(3.470 eV) [15]. Fig. 4(c) shows the wavelength map extractedfrom the CL matrix taken in the area shown in the SEM image.Combining with Fig. 4(b) and (c), an obvious redshift of the D0Xpeak could be observed from the bottom to the top two layers.The redshift of D0X line position was about 6 meV. According to

Fig. 3. Surface morphology of thick GaN layers with and without AlN ILs characterized by optical microscope images (a) and AFM images (b).

Fig. 4. Results of low-temperature CL spectroscopy: (a) cross-sectional SEM image of the as-grown sample; (b) CL spectra of top, bottom GaN layer and overall observation

area corresponding to SEM image; (c) wavelength map of CL matrix; and (d) intensity map.

H. Fang et al. / Journal of Crystal Growth 370 (2013) 254–258256

the bottom (3.470 eV) and overall (3.466 eV) D0X line energy, thetensile strain from CTE mismatch was basically compensated byAlN ILs. This phenomenon is consistent with Raman results in the

optimization experiments of AlN IL temperature. Nevertheless,the redshift in the top layers indicates other mechanism in theAlN/GaN structure.

H. Fang et al. / Journal of Crystal Growth 370 (2013) 254–258 257

The map of CL emission intensity was also extracted from CLmatrix shown as Fig. 4(d). In Fig. 4(d), GaN and AlN layers couldbe distinguished by the weak and strong emission intensity. Thisvariation of emission intensity should be caused by differentcarrier concentration of the two kinds of materials. Moreover, anincrease of luminescent intensity from bottom to top GaN layerswas also observed in the intensity map. Since all the GaN layersare grown without doping, the increase of emission intensityshould be resulted by a higher quality and carrier concentration.With the decrease of nonradiative centers, more carriers con-tribute to the luminescence process.

Cross-sectional TEM observation revealed more details aboutthe structural defects of the sample, especially screw and edgecomponents of the threading dislocations. Fig. 5(a) and (b) showdark-field images with diffraction vectors g¼0002 and g¼11–20,respectively, which were taken along the /1 �1 0 0S zone axisunder weak-beam condition. As can be seen from the two images,both the screw and edge components of the dislocationsare eliminated by the AlN ILs. The screw and edge componentof threading dislocation density (TDD) can be evaluated byFig. 5(a) and (b), respectively. The total TDD (screw and edge)decreases from 5.0�109 cm�2 (at the bottom layer) to1.2�109 cm�2 (at the top layer). It means that nearly 80% of

Fig. 5. Dark-field TEM images taken under weak-beam condition and along

/1 �1 0 0S zone axis with diffraction vectors (a) g¼0002 and (b) g¼11–20.

Fig. 6. In-situ monitoring plots: (a) reflectivity si

TDs was blocked by the AlN ILs. Therefore, the AlN IL can not onlyinduce compressive stress into a GaN epilayer but also improvethe crystal quality by blocking dislocations. It is known that thecompressive stress in GaN induced by the AlN underlayer anni-hilates the threading dislocations effectively [16]. The latticemismatch between AlN and GaN results in a compressive stressfield in the above GaN epilayer. This compressive stress induces a3D growth mode into epitaxy process. During the 3D growth, partof threading dislocations bend to the inclined growth surface andbecome misfit dislocations.

The in-situ monitoring of experiment shows some informationof the growth surface morphology and curvature, which is usefulfor understanding of the dislocation evolution. In Fig. 6(a), afterthe growth of low temperature AlN, amplitude of reflectivity signaldamped with three-dimensional (3D) growth of GaN on AlN. Subse-quently, reflectivity signal gradually recovered to normal level as GaNcoalescing. This rough surface promotes the reduction of threadingdislocations at the initial 200–300 nm in GaN layer after AlN in 3Dgrowth. Fig. 6(b) is the curvature plot of one AlN IL and GaN cycle. Asis shown in Fig. 6(b) that curvature decreased linearly during epitaxyof GaN on AlN IL, which means compressive stress was induced intoGaN epilayer by the AlN IL.

Thus, the 6 meV redshift in the CL wavelength map should beattributed to an increase of tensile stress about 0.2 GPa [14].Dark-field TEM images indicate that mismatch between AlN andGaN was alleviated by misfit dislocations formed in 3D growth. Inhigh temperature growth process, the alleviation exhibited as arelaxation of compressive strain. After growth, cooling downprocess induced a tensile stress into GaN epilayer due to CTEmismatch. As a result, the bottom layers became strain free whilethe top layers suffered a weak tensile strain at room temperature.This phenomenon means that the reduction in threading disloca-tion density should account for the redshift, which is observed inCL wavelength map.

4. Conclusion

Optimized growth temperature of AlN IL in GaN on 3C-SiC/Sisubstrate was determined to be 1050 1C. At this temperature, AlNIL can not only compensate the tensile stress induced by CTE, butalso improve crystal quality by changing growth mode from 2D to3D. Dark-field TEM images indicate the transition of threadingdislocations into misfit dislocations. Therefore, part of the com-pressive stress induced by AlN ILs is relaxed by the misfitdislocations. Based on this analysis, redshift in growth directionobserved by the CL wavelength map is identified as an increasedtensile stress.

gnal and (b) curvature as a function of time.

H. Fang et al. / Journal of Crystal Growth 370 (2013) 254–258258

Acknowledgements

This work was partly supported by Grants-in-Aid for Japan Societyfor the Promotion of Science (JSPS) Fellows (No. 23-01357)

References

[1] N. Sawaki, T. Hikosaka, N. Koide, S. Tanaka, Y. Honda, M. Yamaguchi, Journalof Crystal Growth 311 (2009) 2807.

[2] H.M. Manasevit, F.M. Erdmann, W.J. Simpson, Journal of the ElectrochemicalSociety 118 (1971) 1864.

[3] P. Drechsel, H. Riechert, Journal of Crystal Growth 315 (2011) 211.[4] M. Haeberlen, D. Zhu, C. McAleese, M. J. Kappers and C. J., Humphreys, Journal

of Physics: Conference Series 209 (2010) 012017.[5] T. Takeuchi, H. Amano, K. Hiramatsu, N. Sawaki, I. Akasaki, Journal of Crystal

Growth 115 (1991) 634.[6] H.M. Liaw, R. Venugopal, J. Wan, R. Doyle, P.L. Fejes, M.R. Melloch, Solid-State

Electronics 44 (2000) 685.

[7] H.M. Liaw, R. Venugopal, J. Wan, M.R. Melloch, Solid-State Electronics 45(2001) 1173.

[8] J. Komiyama, Y. Abe, S. Suzuki, H. Nakanishi, Applied Physics Letters 88(2006) 091901.

[9] M. Narukawa, H. Asamura, K. Kawamura, H. Miyake, K. Hiramatsu, JapaneseJournal of Applied Physics 49 (2010) 041001.

[10] A. Dadgar, J. Blasing, A. Diez, A. Alam, M. Heuken, A. Krost, Japanese Journal ofApplied Physics 39 (2000) L1183.

[11] H.P.D. Schenk, E. Frayssinet, A. Bavard, D. Rondi, Y. Cordier, M. Kennard,Journal of Crystal Growth 314 (2011) 85.

[12] G. Pozina, N.V. Edwards, J.P. Bergman, T. Paskova, B. Monemar, M.D. Bremser,R.F. Davis, Applied Physics Letters 78 (2001) 1062.

[13] H. Harima, Journal of Physics: Condensed Matter 14 (2002) R967.[14] C. Kisielowski, J. Kruger, S. Ruvimov, T. Suski, J.W. Ager III, E. Jones,

Z. Liliental-Weber, M. Rubin, E.R. Weber, M.D. Bremser, R.F. Davis, PhysicalReview B 54 (1996) 17745.

[15] Michael A. Reshchikov, Hadis Morkoc- , Journal of Applied Physics 97 (2005)061301.

[16] N. Kuwano, T. Tsuruda, Y. Kida, H. Miyake, K. Hiramatsu, T. Shibata, PhysicaStatus Solidi C 0 (2003) 2444.