Initial Nucleation Study and New Techniquefor Sublimation Growth of AlN on SiC Substrate

Y. Shi (a), B. Liu1) (a), L. Liu (a), J. H. Edgar (a), H. M. Meyer III (b),

E. A. Payzant (c), L. R. Walker (c), N. D. Evans (c), J.G. Swadener (c),J. Chaudhuri (d), and Joy Chaudhuri (d)

(a) Kansas State University, Department of Chemical Engineering, Manhattan,KS 66506, USA

(b) Oak Ridge National Laboratory, Engineering Technology Division, Oak Ridge,TN 37831, USA

(c) Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge,TN 37831, USA

(d) Wichita State University, Mechanical Engineering Department, Wichita,KS 67260, USA

(Received June 21, 2001; accepted August 4, 2001)

Subject classification: 61.10.Nz; 68.37.Hk; 68.37.Ps; 81.05.Ea; 81.15.Gh; S7.14

Single crystal platelets of AlN were successfully grown on 6H-SiC(0001) by a novel technique de-signed to suppress SiC decomposition, promote two-dimensional growth, and eliminate cracking inthe AlN. X-ray diffractometry and synchrotron white beam X-ray topography demonstrate that thefinal AlN single crystal is of high structural quality.

Introduction Despite the rapid progress made in group-III nitride based semiconduc-tor film growth [1], better substrates are still needed for high quality epitaxial growth.AlN is attractive as a substrate for group-III nitride epitaxy with GaN due to its smallmismatch in both lattice constant (�2.5% along the a-axis) and thermal expansion coef-ficient, good thermal stability (melting point >2500 �C), and high resistivity [2]. To date,bulk AlN single crystals have been limited to diameters below 15 mm [3]. From thestudy by Balkas et al. [4] using 6H-SiC as seeds for bulk AlN crystal growth, threeproblems were evident: SiC decomposes at high crystal growth temperatures, individualAlN grains form instead of a single crystal, and the AlN cracks due to differences inthe thermal expansion coefficients of AlN and SiC.

A novel technique for AlN single crystal growth on 6H-SiC(0001) was developed [5, 6]to remedy these problems. This technique consists of depositing an AlN seeding layer bymetalorganic chemical vapor deposition (MOCVD) at 1100 �C, then depositing an(AlN)x(SiC)1––x alloy film by sublimation from a mixture of AlN–SiC powders at 1800 �C,and finally depositing pure AlN by sublimation from a pure AlN source, also at 1800 �C.

In this paper, details about this technique are provided. First, the initial stages ofAlN sublimation growth on bare 6H-SiC were investigated by SEM. Second, the effectof AlN MOCVD buffer layer and composition variation in the region of(AlN)x(SiC)1––x alloys was studied by SEM and SAM. Third, the surface morphologiesand crystal qualities of samples in different steps were characterized by AFM and

1) Corresponding author; Tel: 1-785-532-4325; Fax: 1-785-532-7372; e-mail: beiliu@ksu.edu

phys. stat. sol. (a) 188, No. 2, 757–762 (2001)

# WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0031-8965/01/18811-0757 $ 17.50þ.50/0

XRD. Finally, X-ray diffractometry and synchrotron white beam X-ray topography de-monstrate that the final AlN single crystal has high structural quality.

Experimental Procedures Sublimation growth experiments were conducted in a resis-tively heated furnace using tungsten wire mesh heating elements. The furnace and thegrowth process were described in detail in Ref. [5]. The thickness of long time growth sam-ples was measured with a micrometer, with an accuracy of about 1mm. The average growthrate was about 5 mm/h at 1800 �C and 500 Torr. Using an (AlN)x(SiC)1––x alloy single crystalas the seed, pure AlN was sublimated to form a high quality AlN single crystal for 100 h.

The AlN buffer layer was grown by low pressure (76 Torr) MOCVD. Trimethylalu-minum (TMA) and ammonia (NH3) were the Al and N sources, and Pd-cell purifiedH2 was the carrier gas. The substrates were preheated at 1100 �C for 10 min in 3 slm ofH2 for surface cleaning. Then the AlN films were deposited at the same temperatureunder H2, NH3 and TMA flow rates of 3 slm, 3 slm and 0.6 sccm, respectively. A 2 mmthick AlN buffer layer was obtained in 4 h.

The samples were characterized using a Hitachi S-4700 scanning electron microscope(SEM) with electron backscattered diffraction (EBSD), a Philips XL30FEG SEM withenergy dispersive X-ray spectrometry (EDS), a Physical Electronics PHI680 scanningAuger microprobe (SAM), a Park autoprobe M5 atomic force microscope (AFM) and aScintag PTS four axis X-ray diffractometer (XRD) with Cr-Ka radiation (25 kV � 8 mA).Reflection topography was undertaken at Stanford Synchrotron Radiation Laboratory(SSRL) using Laue diffraction technique and white beam radiation.

Results The initial AlN growth on bare 6H-SiC was studied using high magnificationSEM. Figure 1 shows the sample grown at 1900 �C under 500 Torr N2 for 15 min. Someareas of SiC substrate were covered by large AlN crystals, the other areas were coveredby small hillocks of SiC. AlN nucleated only on the SiC hillocks and the AlN nuclei ap-peared rotated 30� with respect to the SiC hillocks. The lattice mismatch on the basalplane between AlN and 6H-SiC is less than 1% at room temperature and 1.3% at 2200 K.This close match in the basal planes is expected to facilitate the formation of nearly per-fect junctions, i.e. the relationship between the lattices was expected to be parallel, with[0002]AlN || [0006]SiC and [1120]AlN || [1120]SiC [7]. However, Kikuchi patterns observed inEBSD studies showed that [0002]AlN || [0006]SiC but [1120]AlN and [1120]SiC differ by a 30o

rotation. Therefore, this result confirms that there was misorientation between the AlNcrystals and the SiC hillocks in the initial stageof crystal growth. Further studies will be un-dertaken to explain the reason for this unex-pected rotation. The decomposition of 6H-SiCwas discussed in detail in Ref. [8].

Such problems are avoided by first deposit-ing an AlN layer by MOCVD. Previous studies[5, 6] showed that an AlN MOCVD buffer

758 Y. Shi et al.: Initial Nucleation Study and Sublimation Growth of AlN on SiC

Fig. 1. SEM image of AlN crystal grown on 6H-SiC(0001) substrate at 1900 �C, 500 Torr and15 min with magnification 2000� and tilted 15�

layer leads to continuous, single grain growthmode. In this study, the SEM image of the crosssection of alloy crystal in Fig. 2 clearly shows theAlN MOCVD buffer layer with a thickness of1 mm. The composition of the substrate and thebuffer layer were pure SiC and pure AlN respec-tively, but the alloy layer had the same composi-tion, approximately (AlN)0.8(SiC)0.2, regardless

of the ratio of AlN to SiC powder in the source. Thus, the AlN MOCVD buffer layer notonly protected the SiC substrate, but also acted as seeding layer for (AlN)0.8(SiC)0.2 alloygrowth.

The AFM images of the samples in the subsequent growth process (i.e., from the as-received SiC substrate, SiC with AlN MOCVD buffer layer, (AlN)0.8(SiC)0.2 alloys tofinal pure AlN crystal) showed that the surface roughness of the obtained AlN is com-parable with the as-received SiC substrate. As-received (0001)Si 6H-SiC substrates typi-cally contain randomly oriented polishing scratches as shown in Fig. 3a2). The surfaceof the AlN MOCVD buffer layer grown on the substrate indicating a 2D growth modein Fig. 3b. After subsequent (AlN)0.8(SiC)0.2 alloy sublimation growth, a very rough and

phys. stat. sol. (a) 188, No. 2 (2001) 759

2) Colour figure is published online (www.physica-status-solidi.com).

alloy crystal region

MOCVD buffer layer

SiC substrate

Fig. 2. SEM image of cross section of alloy crystalwith 1 mm AlN MOCVD buffer layer

(a) (b)

100 nm

100 nm

100 nm

100 nm

RMS 0.882 nm RMS 1.751 nm

RMS 2.507 nm RMS 0.621 nm

(c) (d)

Fig. 3 (colour). AFM images of a) as-received (0001)Si 6H-SiC substrate; b) SiC with AlNMOCVD buffer layer; c) (AlN)0.8(SiC)0.2 alloy; d) final pure AlN crystal

irregular surface structure was produced as shown in Fig. 3c. Figure 3d shows the imageof the final pure AlN crystal, which had a smoother and more regular surface.

The conventional q–2q diffraction patterns of symmetric [0002] and asymmetric[1122] peaks were measured for the AlN single crystal. The lattice parameters deter-mined from these two peaks were a ¼ 3:113 �A and c ¼ 4:996 �A in agreement with therepoted lattice parmeters of AlN [9]. The rocking curves FWHM (full width at halfmaximum) and peak positions are reported in Table 1. For comparison, data obtainedfor the SiC substrate, SiC substrate with AlN MOCVD buffer layer and theðAlNÞ0:8ðSiCÞ0:2 alloy film are also presented in the same table. The AlN single crystalhas [0002] symmetric and [11�222] asymmetric FWHM as low as 238 and 461 arcsec, respec-tively. Under the same X-ray analysis condition, the [0002] and [1122] peaks rockingcurve FWHM for the SiC substrate were 489 and 562 arcsec, suggesting the AlN singlecrystal was of comparable quality to the original substrate. The alloy crystal had muchwider FWHM values of rocking curve for both symmetric and asymmetric peaks (1141and 1454 arcsec, respectively) compared to either the as-received SiC substrate and thepure AlN single crystal. This is not unexpected because of the atomic occupancy disorder

in the alloy and composition variation along the growthaxis as measured by SAM. Apparently, pure AlN subli-mation growth on the alloy seed improved the crystalquality significantly. The disparity between the FWHMof the symmetric and asymmetric peaks is commonlyobserved for nitride growth [10–12] and has been attrib-

760 Y. Shi et al.: Initial Nucleation Study and Sublimation Growth of AlN on SiC

Ta b l e 1Results of X-ray rocking curve of symmetric [0002] asymmetric [11�222] peaks and latticeparameters of SiC substrate, SiC substrate with AlN MOCVD buffer layer, (AlN)0.8(SiC)0.2

alloy and pure AlN crystal

sample [0002] [11�222� a d value

2q (�) FWHM(arcsec)

2q (�) FWHM(arcsec)

(�A) (�A)

SiC substrate*) 53.75 489 120.93 562 3.080 5.065SiC substrate withAlN MOCVDbuffer layer

53.88 478 120.89 632 3.091 5.047

(AlN)0.8(SiC)0.2

alloy54.32 1141 120.32 1454 3.104 5.016

pure AlN 54.56 238 120.12 461 3.113 4.996

*) [0006] and [11�226] for SiC

Fig. 4. Synchrotron white beam X-ray (SWBX) topographyfor the final pure AlN crystal in reflection. g ¼ [1�1103]. (A)Dislocation network structure, (B) cluster of dislocation.White region indicate absence of dislocations with Burgersvector [1�1100] type

uted to the presence in the nitride films of significantly more dislocations of edge typerather than screw type [12, 13].

Figure 4 shows the synchrotron white beam X-ray (SWBX) reflection topograph forthe final pure AlN crystal in refelction. Since individual dislocations can be identified insome areas, dislocation density is low in those areas. Estimated dislocation density inthe low dislocation density area is about 5.1 � 104 cm––2.

Conclusions Seeded bulk growth of crack-free AlN single crystal was achieved on SiCsubstrate by (i) depositing an AlN buffer layer on the SiC substrate by MOCVD at1100 �C, (ii) forming an (AlN)0.8(SiC)0.2 alloy film by condensing vapors sublimated (attemperatures above 1700 �C) from a source mixture of AlN–SiC powders, followed by(iii) condensing vapors sublimated from a pure AlN source. This method solves threeproblems previously associated with using SiC as seed crystals for AlN growth. First, thelow temperature deposited AlN layer promotes layer-by-layer growth forming a continu-ous single grain. Second, the intermediate properties of the (AlN)0.8(SiC)0.2 alloy reducescracking in the growing crystal. Third, the presence of silicon and carbon in the sourcematerial reduces or eliminates the decomposition of the SiC substrate during sublimationcrystal growth, enabling longer duration crystal growth. By employing this technique, ahigh quality AlN single crystal was produced, as determined by X-ray diffraction. Suchcrystals are ideal for seeding AlN crystal growth at high temperature and hence highgrowth rates, to produce substrates for AlxGa1––xN based electronics.

Acknowledgements We are grateful for the support of this research from BMDO(Contract No: N00014-98-C-0407), ONR (Contract No. N00014-99-1-0104), and the As-sistant Secretary for Energy Efficiency and Renewable Energy, Office of TransportationTechnologies, as part of the High Temperature Materials Laboratory User Program,Oak Ridge National Laboratory. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract number DE-AC05-00OR22725. The Oak Ridge National Laboratory SHaRE Collaborative Research Cen-ter was sponsored by the Division of Materials Sciences and Engineering, U.S. Depart-ment of Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC, andthrough the SHaRE Program under contract DE-AC05-76OR00033 with Oak RidgeAssociated Universities. J. Chaudhuri and Joy Chaudhuri acknowledge the financialsupport by NSF/EPSCoR (grant number EPS-9977776) and portions of the work wasdone by Dr. Z. Rek at Stanford Synchrotron Radiation Laboratory, operated by theU.S. Department of Energy, Office of Basic Energy Sciences.

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762 Y. Shi et al.: Initial Nucleation Study and Sublimation Growth of AlN on SiC