Diamond and Related Materials 12(2003) 241–245

0925-9635/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S0925-9635(02)00287-X

Studies of heteroepitaxial growth of diamond

C. Bednarski , Z. Dai , A.-P. Li , B. Golding *a,b a,b a,b a,b,

Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824-2320, USAa

Center for Sensor Materials MRSEC, Michigan State University, East Lansing, MI 48824-2320, USAb

Abstract

Large-scale heteroepitaxial growth of diamond depends critically on the development of a suitable lattice-matched substratesystem. Oxide substrates, notably MgO and SrTiO , on which thin epitaxial films of iridium serve as a nucleation layer for3

diamond have already shown considerable promise. We describe here improvements in the growth of single crystal diamond bylow-pressure microwave plasma-enhanced CVD. Oxide substrates with flat, low-index surfaces form the initial basis for theprocess. Iridium was deposited on heated substrates in a UHV electron-beam evaporation system resulting in epitaxial films,typically 150–300 nm thick, with Ir(1 0 0) parallel to the surface of all substrates as confirmed by X-ray and electronbackscattering diffraction. Following Ir deposition, the samples were transferred to a CVD reactor where a bias-enhancednucleation step induced a dense condensate that completely covered the Ir surface. Uniform nucleation densities of order 1012

cm were observed. Interrupted growth studies, carried out at intervals from seconds to minutes subsequent to terminating they2

nucleation step, revealed a rapid coalescence of grains. One hour of growth resulted in a smooth, nearly featureless,(0 0 1)diamond film. For extended growth runs, slabs of diamond were grown with thickness as great as 38mm and lateral dimensionsnear 4 mm. The crystals were transparent in visible light and cleaved on(1 1 1) planes alongN1 1 0M directions, similar to naturaldiamond. Of particular significance is the successful use of sapphire as an underlying substrate. Its high crystalline perfectionresults in epitaxial Ir films with X-ray linewidths comparable to those grown on SrTiO . However, Al O possesses superior3 2 3

interfacial stability at high temperatures in vacuum or in a hydrogen plasma with a better thermal expansivity match to diamond.Since sapphire is available as relatively inexpensive large diameter substrates, these results suggest that wafer-scale growth ofheteroepitaxial diamond should be feasible in the near future.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Single crystal growth; Heteroepitaxy; Nucleation; Microstructure

1. Introduction

Heteroepitaxial growth of advanced electronic mate-rials depends on the development of suitable, relativelylow-cost, lattice-matched substrate systems. A great dealof progress was made toward heteroepitaxy of diamondwith growth of highly-oriented crystallites of diamondon silicon, despite the existence of a large latticeparameter mismatchw1–3x. Later, a significant advancewas made with the discovery that films of Ir, grown asa buffer layer on MgO, could serve as a substrate forthe nucleation and growth of CVD diamondw4–6x. Witha lattice parameter 7% larger than diamond, Ir appearsto have the requisite long-term chemical and physical

*Corresponding author. Tel.:q1-517-355-9200x2204; fax:q1-517-432-5501.

E-mail address: golding@pa.msu.edu(B. Golding).

stability in the high-temperature environment of a hydro-gen plasma. More recently, the use of SrTiO as a3

replacement for MgO has proven useful in decreasingthe mosaic spread of the epitaxial Ir and the resultantheteroepitaxial diamondw7,8x.In parallel with heteroepitaxial growth efforts, the

conditions for achieving high diamond nucleation den-sities on various substrates have been extensively exam-ined. The bias-enhanced nucleation processw9x, in whicha negative voltage applied to the substrate results in itsbombardment by relatively low-energy positive ionsextracted from the plasma, is a key step for inducingthe formation of diamond nuclei. It is important that thenuclei adopt the underlying orientation of the substrateand that their density be maximized, so as to lead torapid coalescence of crystallites during the early stagesof growth. Although the conditions that lead to effectivenucleation are well known in principle, there is little

242 C. Bednarski et al. / Diamond and Related Materials 12 (2003) 241–245

Fig. 1. X-ray diffraction rocking curves for 300 nm epitaxial iridiumgrown on A-plane sapphire. Linewidths(in deg): sapphire, 0.04; irid-ium, 0.21.

agreement on the physical mechanisms that underpinthe process. The process is also system-specific to somedegree, depending on details of reactor geometry and amultitude of processing parameters.We describe the results of a series of investigations

of nucleation and growth of CVD diamond grown onepitaxial Ir. The oxide substrates utilized for Ir growthwere predominantly SrTiO and Al O , although MgO3 2 3

and LaAlO were also studied. Care was taken in the3

preparation and characterization of the low-index sub-strate surfaces. The nucleation process was optimized toproduce high densities, of order 10 cm across the Ir12 y2

surface, irrespective of the underlying oxide substrate.The resulting(0 0 1) single crystal diamond films exhib-ited homogeneity on a scale of millimeters.

2. Experimental methods and results

2.1. Substrates and Ir growth

All experiments were carried out on 5=5=0.5 mm3

substrates obtained from commercial sources. The(0 0 1) SrTiO plates were typically cleaned in buffered3

HF before insertion into a UHV electron-beam evapo-rator. After in situ annealing for 20–120 min at 8008C,Ir was deposited at 2.4 nm min to a thickness of 150y1

or 300 nm. No post-deposition annealing was used as ittended to degrade the(0 0 1) epitaxial Ir. We observedsurface reactions of SrTiO with stainless steel substrate3

holders. Reduction of the oxide, as evidenced by itsdarkening, led to a finite electrical conductance acrossthe substrate. Similar behavior was noted for LaAlO3

but not for MgO.We also explored Ir growth on polished(1 1 0)2̄

sapphire, or A-planea-Al O oriented within 0.2 deg2 3

of the A-direction. Substrate preparation consisted ofannealing in air at 14508C for 15 h followed by a lightsolvent cleaning. The process resulted in atomically flatsurfaces, with terraces extending for several microme-ters. Ir was deposited at 8008C under the UHVconditions noted above. The resulting Ir films wereepitaxial with orientation Ir(0 0 1)NNAl O (1 1 0) and2̄2 3

Irw1 1 0xNNAl O w1 0 0x w10x. Fig. 1 shows a X-ray1̄2 3

diffraction rocking curve for a 300 nm Ir film, confirm-ing the (0 0 1) orientation. We have found that thisorientation is stable only if deposited above a thresholdsubstrate temperature near 7008C. The X-ray linebroadening of Ir on A-plane sapphire is comparable toIr on SrTiO w7x. However, the stability of sapphire3

appears to be higher as no chemical reactions or dark-ening is observed.

2.2. Nucleation of diamond on Ir(0 0 1)

Following Ir evaporation, the samples were transferredto a CVD chamber with a base pressure of 10 Torr.y7

The system(2.45 GHz, 6 kW max) contained a quartzdome w11x to confine the plasma to a central region inclose proximity to the Mo sample holder. A tungstenbias ring was mounted on posts 32 mm above anelectrically isolated sample stage. A typical process wasinitiated by a pure hydrogen plasma, followed by theaddition of CH after 5 min, at a 2% methaneyhydrogen4

ratio. System pressure was 18 Torr, with flow rates 6and 300 sccm for CH and H , respectively, at 700 W4 2

microwave power. No nitrogen gas was added intention-ally. A bias potential ofy140 toy200 V was appliedto the substrate 17 min after starting the process. Byinterrupting the procedure at times varying from 20 to180 min we determined that uniform coverage of thesubstrate with carbon occurred approximately 60 minfrom the start of biasing. All results described here useda 60 min bias duration. The temperature during biaswas approximately 7008C, as measured by an opticalpyrometer focused on the Ir surface. As noted by others,we found the presence of a bright, secondary plasmaabove the sample to be a necessary condition forachieving efficient nucleationw12–14x.To estimate nucleation density, we interrupted the bias

process by terminating the microwave power, leading torapid quenching of the sample. Examination by SEMrevealed regions of high electron emissivity that weinterpret to be diamond crystallites at an early growthstage. Fig. 2 shows a result for an Irysapphire substrate(in reverse contrast for clarity). We observe a uniformdistribution of crystallite sizes with mean linear dimen-sions 5–10 nm and areal density of at least 5=1011

cm . This is a lower bound on nucleation density sincey2

some coalescence has already occurred. Furthermore,the distribution, although random, is not far from a

243C. Bednarski et al. / Diamond and Related Materials 12 (2003) 241–245

Fig. 2. Scanning electron micrograph of the surface of a(0 0 1)IryA-plane sapphire substrate immediately subsequent to bias quenching.The dark objects are epitaxial diamond crystallites in an early stageof growth. (Reverse contrast image.)

close-packed array. This suggests that the distributionresults from ‘interactions’ among nuclei. The liquid-likearray leads us to infer that nucleation does not need tooccur at special topographic features of the substrate.Careful examination by SEM also reveals that thecrystallite shapes, even at this early stage, are facetedwith w1 1 0x lateral faces. The uniform distribution ofcrystallite sizes across the entire surface of the substratestrongly suggests that nucleation begins synchronouslywith the cessation of bias current.

2.3. Short-time growth and coalescence of diamond onIr(0 0 1)

At a later stage of growth, the crystallites coalesce toform a continuous thin film. We have carried out studiesof coalescence by interrupting growth at intervals froma few seconds to 30 min following cessation of biasing.The CH yH ratio was decreased to 1% following4 2

biasing for the following interrupted growth experi-ments. Fig. 3 shows SEM images of diamond grown onIr(0 0 1)ySrTiO for intervals of 10 and 20 min subse-3

quent to nucleation. They show a surface with a nearlycontinuous layer of diamond that is penetrated by anetwork of dark regions composed either of voids or ofnon-diamond carbon. The dark network is not randombut has a structure readily revealed in 2-D Fouriertransforms of the images. Since appreciable coalescencehas taken place even after 10 min growth, the Fouriertransform shows that the network has square anisotropy.Furthermore, the boundaries are predominantly alignedalongw1 1 0x directions, as expected for(0 0 1)-orientedcubo-octahedral diamond crystallites. Thus, coalescencemust occur by the annihilation of these interfaces, as

observed in Fig. 3b. After 20 min, it is clear that themicrostructure has coarsened, with appreciably fewerboundaries. This process is reflected by the contractionin reciprocal space of the Fourier transformed image.Square anisotropy is still present but the average grainsize, or distance between interfaces, has now increasedto ;100 nm.For a growth time of 60 min, the film is continuous,

with thickness 0.15mm. The surface shows little evi-dence of the voids or grain boundaries apparent atshorter times. SEM images of the diamond surface areshown in Fig. 4, for a region near the center of the filmand for a region at its outermost periphery. The surfaceis quite smooth with the exception of small features thatappear as bright spots. Their average height is a fewnanometer as measured by AFM and are most noticeableon fully coalesced surfaces. We suspect that they arisefrom charging and enhanced emission from slight pro-trusions on the surface. This is most pronounced whenthe diamond is highly insulating without low impedancegrain boundaries that act as current shunts to thesubstrate.Films grown for 60 min have been extensively char-

acterized by X-ray diffraction rocking curves, azimuthalscans, and pole figures. We have also used electronbackscattering diffraction to develop pole figures overselected areas;10 mm . In all cases, the results are2

fully consistent with the development of single crystal(0 0 1) diamond, with no evidence for misoriented ortwinned regions.

2.4. Long-time growth of diamond on Ir(0 0 1)

To explore the properties of thick diamond films, wehave carried out growth for longer periods, from 36 to48 h. Growth proceeds for 90 min with a CHyH ratio4 2

of 1%. Then, the total gas pressure is raised to 28 Torrover a period of 30 min and the CHyH ratio is4 2

decreased to 0.75%. We have successfully grown(0 0 1)slabs of diamond with thickness from 25 to 38mm. Thethickest crystals are transparent in visible light andcleave alongw1 1 0x directions on(1 1 1) surfaces asobserved for natural diamond. Polarized Raman spec-troscopy shows that the crystals obey the selection rulesexpected for light wavevectors parallel tow0 0 1x andw1 1 0x directions in backscattering geometry. Ramanlinewidths are typically 5–6 cm , quite good for CVDy1

diamond, but clearly much greater than for naturaldiamond.

3. Conclusions

We have shown that significant improvements in theheteroepitaxial growth of diamond using a(0 0 1) Irbuffer layer are possible. By optimizing CVD reactor

244 C. Bednarski et al. / Diamond and Related Materials 12 (2003) 241–245

Fig. 3. Images of heteroepitaxial diamond growth on Ir(0 0 1)ySrTiO for two time intervals after nucleation:(a) 10 min; (b) 20 min. Upper3

panels: SEM micrographs. Lower panels: 2-D Fourier transforms of the binarized real-space images. The monotonic logarithmic contours aremaximum atks0. The Fourier transforms reveal the anisotropic distribution of crystallite interfaces, aligned along Irw1 1 0x directions. The Irsurface is covered with diamond after 10 min growth(a). Appreciable coalescence occurs by 20 min, as seen by the contraction of the Fouriertransform in(b).

geometry and the biasing process, nucleation densitiesof order 10 cm have been inferred from quenching12 y2

experiments. The uniform crystallite sizes provide com-pelling evidence that nucleation occurs synchronouslywith the termination of biasing. In the presence of abias current, the carbon condensate is in a highly excited,nonequilibrium state. Upon removal of the bias, thecondensate cools rapidly, initiating nucleation. The liq-uid-like distribution of crystallites shown in Fig. 2implies that correlations are induced among crystallitesat a very early stage of growth. The most likely scenariois one in which early growth is controlled by depletionof the carbon condensate deposited on the Ir surfaceduring biasing. It is thus possible that there are much

higher densities of nucleation sites initially, but compe-tition for surface carbon amplifies growth of largernuclei.A higher density of nuclei leads to coalescence of

grains at an earlier stage of the growth process, and wehave considerable evidence that continuous diamondfilm formation can occur after only a few minutes ofgrowth. The subsequent films are more highly-orientedsince spatial correlations decay exponentially with dis-tance and annihilation of interfacial defects is thereforemore probable. However, for large area films, one mustinsure that the substrates are free of surface scratches,particulates, and pinholes that may compromise overallcrystal quality.

245C. Bednarski et al. / Diamond and Related Materials 12 (2003) 241–245

Fig. 4. Scanning electron microscope images of the(0 0 1) surface of heteroepitaxial diamond on(0 0 1)Iry(0 0 1)SrTiO after 60 min growth:3

(a) substrate center;(b) substrate edge. Note the slightly incomplete coalescence at the substrate edge in(b). The bright spots correspond toenhanced electron emission from small, easily charged topographic features.

Our discovery that A-plane sapphire can be used togrow (0 0 1) diamond promises to lead to improvementsin diamond heteroepitaxy. We have observed thatSrTiO substrate quality is problematic, frequently3

exhibiting voids and macroscopic growth-induced stressbands. At high temperatures it tends to react even withrefractory metals and is readily reduced in vacuum.Sapphire, on the other hand, exhibits much greatermechanical and chemical stability at high temperaturesand is available with remarkably good crystallographicquality. We expect that this improved substrate systemwill enable the growth of wafer-scale heteroepitaxialdiamond films in the near future.

Acknowledgments

We are grateful to Ryan Kruse for assistance incalculating 2-D Fourier transforms and to Baokang Bifor assistance with electron microscopy and Ramanspectroscopy. We thank Reza Loloee and Refik Kortanfor contributions to electron-beam and X-ray diffractioncharacterization. We acknowledge use of the KeckMicrofabrication Facility and the support from theNational Science Foundation under MRSEC programgrant DMR-9809688.

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