influence of buffer layer and 6h-sic substrate polarity on the nucleation of aln grown by the...

Journal of Crystal Growth 233 (2001) 177–186

Influence of buffer layer and 6H-SiC substrate polarity on thenucleation of AlN grown by the sublimation sandwich

technique

Y. Shia, Z.Y. Xiea, L.H. Liua, B. Liua, J.H. Edgara,*, M. Kuballb

aDepartment of Chemical Engineering, Durland Hall, Kansas State University, Manhattan, KS 66506-5102, USAbH.H. Wills Physics Laboratory, University of Bristol, BS8 1TL, UK

Received 23 July 2000; accepted 13 June 2001

Communicated by A.F. Witt

Abstract

The initial nucleation stage of AlN crystal grown by the sublimation method on on-axis (0 0 0 1)Si and (0 0 0 1)C 6H-SiC as well as 3.51 off-axis (0 0 0 1)Si 6H-SiC substrates was investigated. Sublimation growth from a pure sintered AlN

source was carried out in a resistively heated furnace with a source temperature of about 18001C at a nitrogen pressureof 500Torr. Direct growth on Si-terminated as-received SiC substrates was discontinuous, marked by sparse nucleationand slow lateral growth of nuclei. Several hexagonal sub-grains were usually obtained on the substrates after a long

growth time (more than 3 h) due to the incomplete coalescence of the nuclei. In contrast, no sublimation growthoccurred on the as-received C-terminated substrates. To enhance two-dimensional (2D) growth, an AlN epitaxial layerwas first deposited on the substrates by MOCVD before sublimation growth. Continuous films could then be grown onall the substrates with AlN MOCVD buffer layers. The tensile stress of the AlN layer due to thermal expansion

coefficient mismatch between SiC and AlN caused cracking across the AlN/SiC interface into the SiC substrates duringthe cooling process limiting the maximum of the thickness and lateral size of the AlN crystals. r 2001 Elsevier ScienceB.V. All rights reserved.

Keywords: A1. Crystal morphology; A1. Nucleation; A2. Growth from vapor; A2. Seed crystals; B1. Nitrides

1. Introduction

The GaN-based semiconductors have deve-loped rapidly in blue and green light emittingdiodes and blue emitting lasers [1]. The primarysubstrates for epitaxial growth of GaN are SiC

and sapphire, which are poorly lattice andthermal expansion coefficient matched to GaN.Aluminum nitride, a wurtzite structure wide bandgap (6.2 eV) semiconductor, is a good candidatesubstrate for GaN epitaxial films due to itsrelatively small lattice constant mismatch alongthe a-axis (�3.5%), good thermal stability (melt-ing point >25001C), high resistivity and similarcoefficient of thermal expansion [2]. The use ofAlN buffer layers in MOCVD significantly im-proves the quality of GaN epitaxy [3] also

*Corresponding author. Tel.: +1-913-532-4320; fax: +1-

913-532-7372.

E-mail address: [email protected] (J.H. Edgar).

0022-0248/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 5 6 0 - 3

suggesting that AlN would be an ideal substratefor GaN epitaxy.The most successful method for producing bulk

AlN crystal is by sublimation growth (vapor phasetransport). In this technique, AlN undergoes thereaction AlN=Al+1

2N2 in a high temperaturesource zone and this reaction is driven in thereverse direction to form AlN single crystals in alower temperature zone. This technique was mostsuccessfully developed by Slack and McNelly [4,5]in the mide-1970s. Large single crystals of AlN upto 1 cm long and 0.4 cm in diameter were grown byself-seeding in the sharp tip of sealed tungstencrucibles in an rf induction furnace. Recently,Rojo et al. [6,7] have further advanced thistechnique to produce single crystals up to 15mmin diameter. Tanaka et al. [8] investigated theaxis orientation of AlN freely nucleated ongraphite crucible walls. Most relevant to thiswork, Balkas et al. [9] produced single crystalplatelets of AlN (p1mm thick) on 10� 10mm2

6H-SiC substrates in a resistively heated graphitefurnace. However, the crystals contained small(B2� 2mm2) individual hexagonal sub-grains,which was attributed to the severe degradation ofthe SiC substrates resulting in isolated stablenucleation sites.A prior investigation by the author’s group [10]

showed freely nucleated needles and platelets andexhibited much better crystal quality than thecrystals grown on 6H-SiC substrates although bythe latter method the crystal orientation and initialnucleation was usually better controlled. Up tonow, no systemic investigation of the initialnucleation stage of AlN sublimation on 6H-SiCsubstrate has been reported, although propernucleation critically effects the quality of thesubsequently grown crystals. Nor has the bufferlayer technique common to MOCVD growth beenemployed to prepare substrates for the sublima-tion growth. Herein for the first time the initialnucleation stage of AlN crystal grown by sublima-tion method on on-axis (0 0 0 1)Si and (0 0 0 1)C 6H-SiC as well as off-axis (0 0 0 1)Si 6H-SiC substrateswere investigated. The influence of the AlN bufferlayer deposited on different SiC substrates byMOCVD on the subsequent AlN sublimationgrowth were also studied.

2. Experimental procedures

2.1. Crystal growth

Sublimation growths were conducted in aresistively heated furnace using tungsten wiremesh heating elements. The seed and source wereenclosed in two concentric tungsten crucibles tocontain the Al vapor as it deteriorates the tungstenheating elements. Sintered AlN with approxi-mately 1% oxygen as the main impurity analyzedby a standard inert gas fusion method (TC-136,Leco Co.) was used as source and seed holders.The as-received (0 0 0 1) 6H-SiC silicon terminated(on-axis and 3.51 off-axis) and carbon terminatedwafers (only on-axis) were cut into about 1 cm2

substrates. All the substrates were ultrasonicallycleaned in organic solvents (TCE, acetone andmethanol), rinsed with DI water, and dried withhigh purity nitrogen gas before loaded into thefurnace. The distance between the source and thesubstrate was kept at 2mm. In order to reduce SiCdissociation from the backside of the seed, it wascovered by an AlN cap.The growth ambient was 99.99993% pure

nitrogen. Before growth, the furnace was evacu-ated to a pressure less than 10�4 Torr, then purgedby nitrogen twice to remove residual gases. Duringthe heating process the furnace was maintained at800Torr, as this higher nitrogen pressure sup-pressed both AlN sublimation and the decomposi-tion of the SiC substrate which may roughen thesubstrate surface causing unfavorable AlN nuclea-tion. As soon as the temperature reached thedesired value, the background pressure wasdecreased to 500Torr very quickly to start thesublimation growth. The growth pressure wasmaintained by an automatic throttle valve. Atthe end of the growth, the system pressure wasincreased to 800Torr again during cooling toavoid unwanted AlN recondensation.The vertical temperature profile of the growth

chamber was measured by an optical pyrometerfocused on a moving target at different furnaceoutput powers and pressure. To obtain compar-able results, the growth temperature was keptconstant for all growths by fixing the outputpower. The in situ growth temperature was

Y. Shi et al. / Journal of Crystal Growth 233 (2001) 177–186178

measured by the same optical pyrometer focusedon the top lid of the outside crucible. According tothe pre-measured temperature profile, at ourexperimental condition the source temperaturewas about 18001C and the source-substratetemperature gradient was about 101C for 2mmsource-substrate distance. Several experimentscarried at the temperature above 20001C showedthat higher growth temperatures had no significantinfluences on the nucleation and morphologyexcept the growth rate. Therefore the low tem-perature was employed to avoid the degradation ofthe furnace and the SiC substrates.

2.2. MOCVD technique

The AlN buffer layer growth was performed in avertical-type MOCVD reactor operated at lowpressure (76Torr). Before loading into the reactor,the 6H-SiC substrates were cleaned using the sameprocedure described above. Trimethylaluminum(TMA) and ammonia (NH3) were the Al and Nsources, and Pd-cell purified H2 was the carriergas. The substrates were preheated at 11001C for10min in 3 slm of H2 flow for surface cleaning.Then the AlN films were deposited at the sametemperature under the H2, NH3 and TMA flowrates of 3, 3 slm and 30 sccm, respectively. Anapproximate 2 mm thick AlN buffer layer for thesubsequent sublimation growth was obtained forthe 4 h growth time.

2.3. Characterization

The as-grown AlN crystals were first character-ized by Nomarski differential interference contrastmicroscopy (NDIC). Macrostructural featureswere observed at magnifications of 5–40X. X-raydiffraction measurements were conducted usingXDS 2000 diffractometer (Scintag Inc.) with theCu Ka1 radiation. The surface morphology of thesamples was examined by Digital InstrumentsNanoscope E AFM in contact mode. Micro-Raman spectra were obtained at UltravioletRenishaw with the 325 nm line of a HeCd laseras excitation source. The spot size and the spectralresolution were 1–2 mm and 3–4 cm�1, respectively.

3. Results

3.1. Long time growth result of AlN on(0 0 0 1)Si 6H-SiC

Initially, different growth temperature, pressureand substrate-seed distance were tried on the Si-polarity 6H-SiC substrate to optimize the growthcondition. The pressure during the heating andcooling process was first kept consistent with thegrowth pressure. The typical sublimation time wasusually about 10 h. In all experiments, crystallineAlN was deposited on the majority of the SiCsubstrates. Every AlN crystal thus formed con-tains several hexagonal sub-grains. Grain sizevaried slightly under different growth conditionswith average about 1mm2. Fig. 1 shows the typicalmorphology of AlN sublimation crystal; thissample was grown at 18001C, 500Torr for 10 hwith source–substrate distance 2mm.The same result was reported by Balkas et al. [9]

who speculated that the sub-grains were caused byseparate, isolated nucleation, due to the thermaldecomposition of the SiC substrate. To suppressthis decomposition, a slightly higher (800Torr)nitrogen pressure was employed during our initialheating to the growth temperature. The effective-ness of this procedure was tested by examining aSiC substrate subjected to only the heating andcooling steps under 800Torr nitrogen pressure,without any AlN grow. No significant SiCdecomposition was observed by optical micro-scopy. Nevertheless, AlN crystals grown by sub-limation on SiC substrates using this technique stillcontained many sub-grains. Thus, it is not SiCdecomposition, but the initial nucleation statewhich leads to the sub-grain growth mode. There-fore the initial AlN nucleation and subsequentgrowth was studied to provide insights into thesub-grain formation.

3.2. Nucleation and further growth onas-received 6H-SiC substrates

Although the surface polarity of the SiCsubstrate and the misorientation has a pronouncedimpact on the morphology of the epitaxy III-nitrides films [11–14], Balkas et al. did not specify

Y. Shi et al. / Journal of Crystal Growth 233 (2001) 177–186 179

which they employed in Ref. [9]. Therefore, threekinds of 6H-SiC wafers i.e. on-axis (0 0 0 1)Si,(0 0 0 1)C 6H-SiC and 3.51 off-axis (0 0 0 1)Si wereused as the substrates in our paper. To investigatethe initial nucleation and subsequent growth modesystematically, AlN was deposited on each kind ofsubstrate under the above-described experimentalcondition for 15, 45 and 120min, respectively.Their optical micrographs are shown in Fig. 2denoted by (1), (2) and (3) in a sequence ofincreasing growth time.The mechanism of sub-grains formation were

clearly seen from Fig. 2a (1–3) for the crystalsgrown on on-axis Si-terminated substrates. After15 and 45min growth, only individual nuclei wereformed, with only slow lateral growth of the nucleiafter 120min. This growth mode inevitably led tothe formation of hexagonal sub-grains due to theincomplete coalescence of the nuclei. The nuclea-

tion and growth modes on the 3.51 off-axis Si-terminated substrate as shown in Fig. 2b(1–3) wasvery similar with those of on-axis Si-terminatedsubstrate. Relatively large macrosteps were formedon the off-axis SiC substrates for the 15 and 45grown samples. The 120min growth sampleshowed larger grain size in Fig. 2b(3) than inFig. 2a(3) probably because the stepped featuresenhanced the coalescence of the nuclei. Becausethe AlN deposited layers were discontinuous for15min growth samples on on-axis and off-axis Si-terminated substrates, the (0 0 2) AlN peak couldnot be detected by the y–2y X-ray diffraction. Onlya very weak and broad (0 0 2) peak could bedetected for the 45min samples. In contrast, thediffraction (0 0 2) peak from the 120min growthsample was strong and sharp due to the formationof large size crystals.The growth on the on-axis C-terminated sub-

strates was strikingly different. After sublimationgrowth, the majority of the substrate surfaces werevery smooth and featureless. Some unusual areaswhere deposition formed irregular crystals arepresented in Fig. 2c (1–3). The (0 0 2) AlN peakwas not detected in the X-ray diffraction pattern ofthe as-grown surface even after 120min of growth,but it was detected from the backside of thissubstrate. Apparently, AlN cannot be depositedon the C-terminated SiC substrate; instead thealuminum and nitrogen vapor recondensed on thebackside of the substrate. A 14 h growth sampleverified this hypothesis. As seen in the opticalmicrograph (Fig. 3), the as-grown surface onlycontained sparse hexagonal AlN crystals, whichdid not cover the whole substrate. In contrast, thesublimation growth on the Si-terminated substratefor only 10 h produced a thick AlN layercomposed by several coalescence hexagons asshown in Fig. 1. After a long growth time, someregions of the C-terminated surface decomposed,so isolated AlN crystals could form.We conclude that bare C-terminated SiC wafer

cannot be used as the substrate for AlN sublima-tion growth. Sasaki et al. [11,12] observed that theSi-terminated 6H-SiC surface produced smoothand featureless GaN by MOCVD technique, whilethe C-terminated surface yielded prominent hex-agonal pyramids of GaN. The reason is generally

Fig. 1. Optical micrograph of AlN crystal grown on the on-axis

(0 0 0 1)Si 6H-SiC substrate at 18001C, 500Torr for 10 h with

source–substrate distance 2mm. The magnification is 120� .


presumed that the surface mobility of adatoms islow on the N-terminated GaN surface grown onthe C-terminated SiC substrate so step flow growthis hindered and statistical roughening proceeds[15]. Ren and Dow [16] proposed that the observeddifferences in GaN surface morphologies were dueto large local microscopic lattice mismatch differ-ences. The electrical polarity of the GaN/SiCinterface created interfacial charges which locallyaltered the lattice mismatch. For our AlN filmsprepared by MOCVD, the different polaritysubstrates did not have any significant differencesin morphology as observed by AFM, nor werethere differences in the crystal quality as measured

by X-ray rocking curve. The reason why AlNcould not be sublimated on C-terminated surfaceof SiC needs further investigation.

3.3. Nucleation and further growth on 6H-SiCsubstrates with AlN MOCVD buffer layer

Since the AlN nucleation directly on the as-received (0 0 0 1)Si 6H-SiC substrate was discontin-uous which inevitably lead to sub-grain growth,the buffer layer technique successfully employed inGaN epitaxy growth by MOCVD [3] was intro-duced in the sublimation growth. Unlike the verythin buffer layer adopted in MOCVD growth, an

Fig. 2. Optical micrographs of AlN crystals grown on the different as-received (0 0 0 1) 6H-SiC substrates with magnification 60� :

(a) on-axis Si-terminated, (b) 3.51 off-axis Si-terminated and (c) on-axis C-terminated substrates; (1–3) denote the growth time sequence

of 15, 45 and 120min.


approximate 2 mm thick AlN buffer layer wasgrown on the SiC substrates since the AlN bufferlayer may partially decompose at a high sublima-tion temperature before the nucleation begins. Aswas done for the as-received substrates withoutbuffer layer, the nucleation and subsequent growthon the different substrates with buffer layer werestudied for 15, 45, 120min growth, respectively.The subsequent sublimation growth on the three

kinds SiC substrates with AlN MOCVD bufferlayer showed very similar morphologies. Thereforeonly the optical micrographs of off-axis seriessamples are presented in Fig. 4. The AlN sublima-tion growth on the MOCVD buffer layer exhibitedcontinuous growth mode, but the density of crackscaused by the stress during cooling process

increased with the growth time. For the 15mingrowth sample, only a few cracks were observed.The crack density increased much more for the45min grown sample. After 120min growth thestress rings were clearly observed on the as-grownsurface indicating a high level of stress, and someAlN pieces even peeled off from the substrates.Despite the crack, using the buffer layer singlegrain growth was achieved by the continuousnucleation on the buffer layer.The relative intensity of the AlN (0 0 0 2) X-ray

diffraction peaks from the series of off-axissubstrates increased with the growth time(Fig. 5). The X-ray rocking curve from the AlNcrystals grown on the Si-terminated substrates hadslightly better quality than those on the C-

Fig. 3. Optical micrograph of AlN crystal grown on the on-axis (0 0 0 1)C 6H-SiC substrate at 18001C, 500Torr for 14 h with source–

substrate distance 2mm. The magnification is 120� .

Fig. 4. Optical micrographs of AlN crystals grown on the 3.51 off-axis (0 0 0 1)Si 6H-SiC substrates with magnification 60� : with AlN

MOCVD buffer layers in the growth time sequence of 15, 45 and 120min.


terminated substrates. As seen from the resultssummarized in Table 1, the AlN sublimationcrystals for 15min growth had better qualitiesthan those of the AlN MOCVD buffer layerwithout growth sample, but the qualities deterio-rated for 45min growth because of the increase inthe crack density. The X-ray rocking curve for the15min growth sample on the off-axis substratewith buffer layer is presented in Fig. 6. It has thenarrowest full-width at half-maximum value of293 arcsec.The atomic force microscopy (AFM) images of

the off-axis series samples also showed that themorphology of the AlN deposited by MOCVDimproved after the subsequent sublimation growthdue to the formation of more faceted surface. As-received off-axis (0 0 0 1)Si 6H-SiC substrates typi-cally contain polishing induced randomly orientedscratches as shown in Fig. 7(a). The surface of theAlN MOCVD buffer layer grown on the off-axissubstrate had irregularly oriented island-like fea-tures, indicating a 3D growth mode in Fig. 7(b).After 15min of subsequent sublimation growth, azig-zag stepped surface structure was producedwith smooth terraces as shown in Fig. 7(c). Stepbunching was also clearly seen on the image. Mostterrace widths and step heights varied in the rangeof 300–500 nm and 10–50 nm, respectively. There-fore, we conclude that the AlN sublimed on the

MOCVD buffer layer by the step-flow growthmode. Fig. 7(d) showed the image of subsequent45min sublimation growth sample. The zig-zagstepped surface was still observed but each terracesurface became much rougher. This is consistentwith the results of X-ray rocking curve. Accordingto the characterization of the threading dislocationstructure in GaN films by X-ray rocking curve[17], the symmetric (0 0 0 2) rocking curves are onlysensitive to the screw dislocations due to itsBurgers vectors lying parallel to [0 0 0 1], causingthe distortion of c-plane. Therefore, the moreregular and smooth the surface is, the lower thescrew dislocation density, and hence, the (0 0 0 2)peaks are narrower.

Fig. 5. y–2y X-ray diffraction of the samples grown on the 3.51off-axis (0 0 0 1)Si 6H-SiC substrates with AlN MOCVD buffer

layers for 15, 45 and 120min.

Fig. 6. Double crystal X-ray rocking curve around the (0 0 0 2)

AlN reflection from the 15min growth samples grown on the

3.51 off-axis (0 0 0 1)Si 6H-SiC substrates with AlN MOCVD

buffer layers.

Table 1

FWHMs of double crystal X-ray rocking curve of (0 0 0 2) peak

AlN films on different (0 0 0 1) 6H-SiC substrates with AlN

MOCVD buffer layer

Growth

condition

Si-terminated

on-axis

substrates

Si-terminated

3.51 off-axis

substrates

C-terminated

on-axis

substrates

No growth 511 581 560

15min growth 359 293 491

45min growth 369 367 647


4. Discussion

Although the continuous nucleation and stepflow growth mode could be achieved for the AlNsublimation growth using the MOCVD bufferlayer technique, cracks still appeared even afteronly 15min growth. For AlN growth on 6H-SiC,compressive lattice mismatch stresses are relievedafter a few nanometers of growth [18]. So the onlyremaining stress is due to the mismatch of thermalexpansion coefficients when the composite AlN/SiC structure is cooled to the room temperature.Since the thermal expansion of AlN is greater thanthat of SiC [19], the tensile stress is the inevitableresult of the AlN growth layer on SiC substrates.For the AlN sublimation directly on the as-received substrate, cracking is not a serious

problem because most stresses could be relaxedby the sub-grain boundaries. But for the singlegrain growth on the substrate with MOCVDbuffer layer, the unrelaxed residual tensile stresseswill bend the whole structure toward the AlN layerand cause serious cracking.As mentioned above, some pieces even

peeled off from the substrate for the 120mingrowth sample. Analysis of the front andback sides of these pieces by micro-Ramanindicated that the cracks occurred acrossthe AlN/SiC interface into the SiC substrates.As the Raman spectroscopy shows in Fig. 8,all the peaks of the as-grown surface canbe assigned as AlN Raman modes, the peaks ofthe peeling-surface are due to the SiC Ramanmodes.

Fig. 7. AFM images of off-axis series samples: (a) as-received off-axis wafer, (b) with AlN MOCVD buffer layer, no further growth, (c)

15minutes subsequent sublimation growth and (d) 45min subsequent sublimation growth. Z scale of AFM: 200nm.


Cracking has been a serious problem whichlimits the maximum of the thickness and lateralsize of the AlN crystals sublimated on the SiCsubstrates with AlN MOCVD buffer layers. Incontrast, a compressive thermal mismatch stress isintroduced in the AlN layer grown on the sapphiresubstrate because the thermal expansion of AlN issmaller than that of sapphire. Since the crackingthreshold for the compressive strength is generallymuch higher than that of the tensile strength forAlN ceramic, cracking may be reduced by growthon sapphire substrate. Therefore sapphire was alsotried as a substrate for AlN sublimation growthwith the MOCVD buffer layer first deposited.Unfortunately the sapphire decomposed at theAlN sublimation temperature (above 17001C).This decomposition was not suppressed even bya thick AlN MOCVD layer. Thus, sapphire is nota viable substrate for the AlN sublimation growth.

5. Conclusion

The influence of the buffer layer and 6H-SiCsubstrate polarity on the nucleation of AlN grownby the sublimation sandwich technique have been

investigated in this paper. Direct growth on Si-polarity as-received substrates was discontinuouscaused by the initial sparse nucleation and slowlateral growth of nuclei. Several hexagonal sub-grains were usually obtained on the substrates dueto the incomplete coalescence of the nuclei. Incontrast, no sublimation growth occurred on theas-received C-terminated substrates. Continuousfilms could then be grown on all the substrateswith AlN MOCVD buffer layers. The tensile stressdue to thermal expansion coefficient mismatch inthe AlN layer during cooling process causedcracking across the AlN/SiC interface into theSiC substrates which limits the maximum of thethickness and lateral size of the AlN crystals. Thebest way to overcome this problem is to use ahomoepitaxial seed for further sandwich sublima-tion technique growth of AlN.

Acknowledgements

We are grateful for the support of thisresearch from BMDO (Contract No. N00014-98-C-0407) and ONR (Contract No. N00014-99-1-0104).

Fig. 8. Raman spetra of the crystalline separated from SiC substrate: (a) as-grown surface, (b) back side of the peeling crystal.


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influence of buffer layer and 6h-sic substrate polarity on the nucleation of aln grown by the...

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