Thin Solid Films 447–448(2004) 231–238

0040-6090/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0040-6090(03)01098-8

Growth of highly-oriented diamond films on 6H–SiC(0001) and Si(111) substrates and the effect of carburization

Tae-Hoon Lee , Soo-Hyung Seo , Seung-Min Kang , Jin-Seok Park *a b c a,

Department of Electrical Engineering, Hanyang University, 1271, Sa 1-dong, Sangrok-ku, Ansan, Kyunggi-do 425-791, South Koreaa

Crystal Growth Division, Neosemi-Technical Corporation, 357-13, Dangha-dong, Seo-ku, Inchon 404-818, South Koreab

Department of Material Science and Engineering, Hanseo University, 360 Daegok-ri, Haemi-myeon, Seosan, Chungnam 356-706, South Koreac

Abstract

The growth of highly oriented diamond is performed on Si(111) and 6H–SiC(0001) substrates via two-step and three-stepprocesses using microwave plasma CVD. The two-step process involves bias-enhanced nucleation(BEN) and deposition, and thethree-step process involves carburization in addition to the two-step process. The diamond films grown on the Si(111) substrateexhibit high quality and desirable(111)-orientation under the carburization condition of 5.3=10 Pa in pressure with no bias3

applied. The mechanism for the formation of conversion layer during the carburization step is investigated on both the substratesthrough the Raman and X-ray photoelectron spectroscopy(XPS) studies. The results indicate that the carburization mainlycomposed ofb-SiC, which plays a crucial role for the formation of the conversion layer and which eventually promotes thediamond nucleation. It is also suggested that a highly-oriented and high-quality diamond film can be successfully achieved bycarburization.� 2003 Elsevier B.V. All rights reserved.

Keywords: Highly-oriented diamond; Si(111); 6H–SiC(0001); Carburization; Bias-enhanced nucleation;b-SiC layer; Raman; XPS; XRD

1. Introduction

Diamonds have been considered as a promising semi-conductor material for high-temperature and high-powerelectronic devices due to their superior properties, suchas wide energy gap, high thermal conductivity and highhole mobility w1x. To realize the electronic usage ofdiamonds, it is necessary to synthesize hetero-epitaxialdiamond films on non-diamond substrates by employingproper growth techniques.Recently, oriented growth of diamond has been report-

ed via bias-enhanced nucleation(BEN) on Si substratesusing microwave plasma chemical vapor deposition(MPCVD) w1–3x. The BEN method can significantlyincrease the nucleation density of diamonds, which iscrucial for hetero-epitaxial growth of diamond film.However, the inordinate bias voltage increases bombard-ment of the surface with positive ionsw1,4x. The ionbombardment can damage the substrate surface during

*Corresponding author. Tel.:q82 31 400 5166; fax:q82 31 4193042.

E-mail address: jinsp@hanyang.ac.kr(J.-S. Park).

the BEN step, thus causing poorer alignment of thesubsequent nucleating diamond crystalsw5,6x. In addi-tion, the hetero-epitaxial growth of a diamond on siliconsubstrates has been considered very difficult, due to therelatively large lattice mismatch(52%) of diamond tosilicon and a much higher surface energy of diamondsw7x. To overcome this, a three-step process(carburiza-tion, nucleation and deposition) has been performed forthe growth of the diamond on Si(111) substratew8x. Ingeneral, the carburization step is used to grow anepitaxial SiC conversion layer, which may promote theformation of stable diamond nucleiw4,8x. The latticemismatch (approx. 22.9%) between theb-SiC anddiamond is smaller than that between the Si and adiamond. In the same aspect, crystalline SiC,(e.g. 6H–SiC) may also be considered as a desirable substrate forthe oriented growth of diamond, since it provides amuch smaller lattice mismatch(13.4%) than theb-SiCw9x. However, there have been few studies on this issuein literature.In this research, we have systematically investigated

the effect of carburization on the growth of highly-oriented diamond films on non-diamond substrates, such

232 T.-H. Lee et al. / Thin Solid Films 447 –448 (2004) 231–238

Table 1Process conditions used to grow diamond films

Conditions Carburization Nucleation Deposition Sample ID

m-wave power(W) 900 900 700 (ii)–(vi),CH (sccm)4 5 3 0.2 (ii)*–(vi)*H (sccm)2 100 100 100Process time(min) 10 15 300Substrate bias(V) y50 y200 0 (vi), (vi)*

y30 y200 0 (v), (v)*0 y200 0 (iii ), (iii )*

Pressure(=10 Pa)3 4.0 5.3 5.3 (ii), (ii)*5.3 5.3 5.3 (iii ), (iii )*6.7 5.3 5.3 (iv), (iv)*

Time duration(min) a( ) – 30 300 (i), (i)*10 15 300 (iii ), (iii )*

– 25 – A, A*10 – – B, B*20 – – C, C*20 15 – D, D*

Note: The films considered in the experiment marked by(a) have been prepared under the bias voltage of 0 V and the pressure of 5.3=103

Pa.

as Si (111) and 6H–SiC(0001). In particular, for thefirst time the X-ray photoelectron spectroscopy(XPS)and Raman data are presented to characterize the car-burization effect and elucidate the mechanism responsi-ble for the growth of diamond films on both thesubstrates.

2. Experimental

Polycrystalline diamond films were deposited on Si(111) and 6H–SiC(0001) substrates via two-step andthree-step processes by MPCVD(2.45 GHz) usingCH and H as precursors. The Si substrates were4 2

ultrasonically cleaned with sulfuric acid with de-ionizedwater for 10 min and etched in 48% HF for 10 min toremove native oxides. The 6H–SiC substrates were alsocleaned with acetone for 10 min and in HF for 1 minto remove contaminants. The details of process condi-tions used in the carburization(included only in thethree-step process), nucleation and deposition procedu-res are listed in Table 1. To identify the conversionlayer, which may be formed at the interface between thesubstrate and the diamond during the initial stages ofcarburization and nucleation, several films(as denotedby A–D in Table 1) were prepared by excluding thedeposition step from the three process.For all the grown diamond films, the Raman spec-

troscopy(Raman microscope 1000, Renishaw) was usedto monitor a cubic diamond peak at 1332 cm and ay1

graphite-related peak in the vicinity of 1550 cm , fromy1

which the Raman quality factor(f ) was also estimatedq

w10,11x. Surface morphologies of grown films weremeasured using a field-emission scanning electronmicroscope(SEM, JSM-6330F, JEOL). The X-ray dif-fraction (XRD, Cu Ka radiation, u-2u scan,

DMAX2200 Ultima, Rigaku) spectra were also meas-ured to analyze the crystal orientation of diamond films.From the XRD spectra, the relative ratio intensity of(111)-peak to(220)-peak(denoted byI yI ) was111 220( ) ( )

estimated for the films tested. The formation of conver-sion layer was identified by using X-ray photoelectronspectroscopy(XPS, ESCA2000, VG Multilab) as wellas Raman spectroscopy. To examine the carburizationeffects in more detail, the structural properties of grownfilms were characterized in terms of the carburizationconditions used, such as working pressure and negativebias voltage.

3. Results and discussion

The effects of the carburization conditions includedin the three-step growth process on the diamond quality,surface morphology and crystal orientation of diamondfilms have been characterized. The Raman spectra andquality factor, SEM morphologies and XRD spectrawere measured from all the films grown on the Si(111)substrate, of which results are shown in Figs. 1 and 2.As denoted by(i) in Fig. 1, the diamond film

deposited without carburization(i.e. using the two-stepprocess) was observed to include the non-diamond peaknear 1550 cm with intensity comparable to that ofy1

the cubic diamond peak at 1332 cm . However, asy1

denoted by(iii ) in Fig. 1, the film grown involvingcarburization was found to possess a very sharp andintense diamond peak at 1332 cm and a noticeablyy1

depressed non-diamond peak consisting of sp hybrid-2

ized carbon configuration. As also specified in Fig. 1,the estimated Raman quality factors(f ) were 52.53%q

for the two-step diamond and 89.40% for the three-stepdiamond, respectively. Furthermore, the corresponding

233T.-H. Lee et al. / Thin Solid Films 447 –448 (2004) 231–238

Fig. 1. The Raman spectra and SEM morphologies of diamond films grown on the Si(111) substrate, in terms of the carburization conditionsused. The Raman numbers(i)–(vi) indicate the sample IDs as listed in Table 1 and the Raman quality factor(f ) for each film is also specifiedq

with the corresponding sample.

SEM morphology(as denoted by(iii )* in Fig. 1) andXRD spectra(as denoted by(iii ) in Fig. 2) showed awell-textured film surface of triangular shape and ahighly (111)-oriented diamond film, respectively. Thisindicates that the use of carburization(i.e. the three-stepgrowth) may possess advantages for obtaining a high-quality and highly-oriented diamond film on the Si(111) substrate.Next, we consider the influences of working pressure

and negative bias voltage used in the carburizationprocess. In growth of diamond films via MPCVD, thenucleation density of the diamond generally depends onplasma density, which is affected by the process varia-bles used. At a relative low pressure of 4.0=10 Pa, it3

appeared from the surface morphology(as denoted by(ii)* in Fig. 1) that the film did not exhibit a continuousform of film, in spite of high crystal quality. This wasattributed to the low plasma density at the surface of Si(111) substrate, not providing sufficient nucleation forthe growing diamond. At a relatively high pressure of6.7=10 Pa(as denoted by(iv)* in Fig. 1), the film3

was grown in the form of nearly continuous film, mainly

composed of the mixed crystals of truncated-octahedronand tilted grains, as well as large-sized voids. Increasingthe pressure may enhance the plasma density of H ions2

at the substrate surface and hence cause the increase ofetch rate for the diamond nuclei, which may support thelarge voids. In addition, the XRD intensity ratio ofI yI estimated from the corresponding XRD spec-111 220( ) ( )

tra (as denoted by(ii) and (iv) in Fig. 2) was approx-imately 18.8 and 7.9, respectively, which was smallerthan that (approx. 21.7) obtained from the film(asdenoted by(iii ) in Fig. 2) grown at an intermediatepressure of 5.3=10 Pa.3

It has been reported that an applied negative biasincreases the energy of positive ion toward the substrate,and this promotes the diamond nucleation with highdensity during the nucleation stepw3,12x. However, thedamage caused by the ion of unreasonable energy mustalso be considered. In this research, a relatively low biasvoltage between 0 V andy50 V was applied duringthe carburization step to study the bias-related carburi-zation effect. The corresponding SEM images(as denot-ed by (v)* and (vi)* in Fig. 1) and XRD patterns(as

234 T.-H. Lee et al. / Thin Solid Films 447 –448 (2004) 231–238

Fig. 2. The XRD spectra obtained from the diamond films grown on Si the(111) substrate, in terms of the carburization conditions used. TheRaman numbers(i)–(vi) indicate the sample IDs as listed in Table 1.

denoted by(v) and(vi) in Fig. 2) clearly indicated thatthe films being biased during carburization revealedrandomly-oriented diamond grains(accordingly, signif-icantly low I yI ratios of 1.8–2.0) with tilted111 220( ) ( )

triangular(111) faces. This may be due to the increaseof positive ion bombardment by negative bias, promot-ing the eroding effect at substrate surfaces. Such resultsobserved from the three-step produced films under thebiased-carburization condition were similar to thoseobserved from the two-step produced film without car-burization. In addition, the experimental results as shownin Fig. 1 and Fig. 2 may suggest that the formation ofa conversion layer by carburization prior to negative-biased nucleation is required for preventing the substratesurface from being damaged.To examine the formation mechanism of the conver-

sion layer, the Raman and XPS spectra were monitoredfor diamond films prepared under different durations ofBEN and carburization without being accompanied byfilm deposition, as already remarked by samples A–Din Table 1. Fig. 5 shows a typical Raman band measured,which consists of the second-order peak of Si substrate(denoted by I ) appeared at 971 cm and they1

Si_2nd

b-SiC characteristic peak of longitudinal optical(LO)phonons(denoted byI ) appeared at 965 cmy1

SiC_LO

w13x, and also includes the estimated Raman intensity

ratio, (i.e. I yI ). It was found that the RamanSiC_LO Si_2nd

intensity of the b-SiC LO peak was significantlyincreased by prolonging the carburization time. In par-ticular, almost two times a larger value ofI ySiC_LO

I ratio was observed in the sample D(with 20Si_2nd

min-carburization, accompanied by 15 min-nucleation),compared to the sample A(without carburization, onlywith 25 min-nucleation). This may indicate that theb-SiC conversion layer had already been formed viacarburization, performed prior to nucleation, at theinterface between the Si substrate and diamond. Fur-thermore, this result confirms that the carburizationprocess is essential for the formation of conversionlayer. In this research, the formation mechanism viacarburization was also identified through the XPS study.Fig. 4 shows the evolutions of the XPS Si2p and C1s

core levels measured for all the samples shown in Fig.3. In regards to the deconvolution of Si2p spectra, twomain components, corresponding respectively to a Si–Cbond at 100.3 eV and a Si–Si bond at 99.0 eV wasobserved, as depicted in Fig. 4aw14x. It was clearlyconfirmed that the surface of the Si substrate wascovered with a Si–C bond and in particular, the relativeintensity of Si–C bond to Si–Si bond was significantlylarger for the films prepared with carburization(i.e.samples C and D) than for the two-step produced film

235T.-H. Lee et al. / Thin Solid Films 447 –448 (2004) 231–238

Fig. 3. The variation of the relative peak ratio(namely,I yI ) estimated from the Raman spectra of the diamond filmsSiC_LO Si_2nd

grown on the Si(111) substrate, according to the time duration ofcarburization and nucleation. The capital letters A–D indicate thesample IDs, as listed in Table 1.

Fig. 4. The variation of XPS spectra for(a) Si2p and(b) C1s peaks measured from the diamond films grown on the Si(111) substrate, accordingto the time duration of carburization and nucleation. The capital letters A–D indicate the sample IDs, as listed in Table 1.

without carburization(sample A). The use of carburi-zation virtually leads to the decrease of the total nucle-ation time, and the conversion layer ofb-SiC formedduring the carburization process may effectively reducethe eroding effects due to ion bombardments and alsosuppress the dislocation of grainsw8x, eventually pro-ducing a highly-oriented diamond film. In addition, theC1s spectra were resolved into two major components,namely, a C–Si bond at 282.7 eV and a C–C bond at284.3 eV, as depicted in Fig. 4bw14x. The result indicatesthat the C–C bond dominated the substrate surface after

nucleation, and theb-SiC conversion layer promotedthe C–C bond for diamond nucleation.In this research, diamond films have also been grown

on 6H–SiC(0001) substrate, of which the lattice con-stant is known to be similar to that ofb-SiC, by usingthe identical conditions that was performed for the Si(111) substrate, as denoted by samples A*–D* in Table1. Fig. 5 shows the Raman spectra obtained from thefilms grown on the 6H–SiC(0001) substrate, alongwith the spectrum measured from the substrate itself.For the films prepared either without carburization(sample A*) or only with 10 min-carburization(sampleB*), no other peaks, but the 6H–SiC peak was observed,the same as observed in the substrate. However, byincreasing the carburization time(samples C* and D*)the transverse optical(TO) band ofb-SiC was observedon the surface 6H–SiC. Further modification in thelattice structure of the conversion layer(e.g. 4H–SiC)was also discovered by subsequently performing theBEN step(sample D*). This indicates that the modifi-cation of lattice structure in the conversion layer canalso be caused during the BEN process with carburiza-tion. However, it is conclusively suggested from Fig. 5that the carburization plays an essential role for theformation of the conversion layer consisting ofb-SiCeven on the surface of 6H–SiC substratew3x, whicheventually promotes the diamond nucleation on thesurface of 6H–SiC substrate. As shown in Fig. 6, theXPS spectra monitored from the films grown using the6H–SiC (0001) substrate also confirms that the carbu-rization promotes a C–C bond on the surface of 6H–

236 T.-H. Lee et al. / Thin Solid Films 447 –448 (2004) 231–238

Fig. 5. The variation of Raman spectra for the diamond films grown on the 6H–SiC(0001) substrate, as a function of the time duration ofcarburization and nucleation. The capital letters A*–D* indicate the sample IDs, as listed in Table 1.

Fig. 6. The variation of XPS spectra for C1s peak measured from the diamond films grown on the 6H–SiC(0001) substrate, as a function ofthe time duration of carburization and nucleation. The capital letters A*–D* indicate the sample IDs, as listed in Table 1.

237T.-H. Lee et al. / Thin Solid Films 447 –448 (2004) 231–238

Fig. 7. For the diamond film grown on the 6H–SiC(0001) substrate with carburization,(a) the Raman spectrum and SEM morphology and(b)the XRD spectrum, respectively.

SiC as it affects the films grown on the Si(111)substrate.Fig. 7 shows the structural properties of diamond film

grown with carburization on the 6H–SiC(0001) sub-strate. The Raman and SEM morphology(Fig. 7a) aswell as the XRD spectrum(Fig. 7b) indicate that adiamond film possessing a high-quality withf as highq

as 90.71% and highly(111)-preferred orientation withwell-textured facets of triangular grains can also begrown on the 6H–SiC substrate by virtue ofcarburization.

4. Conclusion

In this research, the effects of carburization, per-formed prior to nucleation in the three-step process, onthe growth of highly-oriented diamond films on boththe Si (111) and 6H–SiC(0001) substrates were sys-tematically studied, in terms of the carburization condi-tions such as process pressure, applied negative-bias andtime durations of carburization and nucleation. For thefirst time, the mechanism for the formation of the

conversion layer(mainly consisting ofb-SiC) and therole of the conversion layer for the diamond nucleationwas discussed by considering the Raman and XPSspectra. The results indicate that the carburization exertsa crucial influence on the formation of the conversionlayer and promotes the diamond nucleation for thegrowth of highly-oriented diamond films on non-dia-mond substrates. It may also be concluded that a highly-oriented and high-quality diamond film withwell-textured grains can be successfully achieved byvirtue of carburization.

Acknowledgments

This work was carried out by using the facilities ofcenter for Electronic Materials and Components(EMand C) of Hanyang University at Ansan campus.

References

w1x T.F. Chang, L. Chang, Diamond Relat. Mater. 11(2002) 509.w2x S. Yugo, T. Kanai, T. Kimura, T. Muto, Appl. Phys. Lett. 58

(10) (1991) 1036.

238 T.-H. Lee et al. / Thin Solid Films 447 –448 (2004) 231–238

w3x L. Chang, J.E. Yan, F.R. Chen, J.J. Kai, Diamond Relat. Mater.9 (2000) 283.

w4x N. Yokonaga, Y. Katsu, T. Machida, T. Inuzuka, S. Koizumi,K. Suzuki, Diamond Relat. Mater. 5(1996) 43.

w5x S. Barrat, S. Saada, J.M. Thiebaut, E.B. Grosse, DiamondRelat. Mater. 10(2001) 1637.

w6x M. Schreck, K.H. Thurer, R. Klarmann, B. Stritzker, J. Appl.Phys. 81(7) (1997) 3096.

w7x X. Jiang, C.P. Klages, R. Zachai, M. Hartweg, H.J. Fusser,Appl. Phys. Lett. 62(26) (1993) 3438.

w8x S.D. Wolter, B.R. Stoner, J.T. Glass, P.J. Ellis, D.S. Buhaenko,C.E. Jenkins, P. Southworth, Appl. Phys. Lett. 62(11) (1993)1215.

w9x A. Bauer, J. Kraublich, L. Dressler, P. Kuschmerus, J. Wolf,K. Goetz, Phys. Rev. B57(5) (1998) 2647.

w10x A. Gicquel, K. Hassouni, F. Silva, J. Achard, Cur. Appl. Phys.1 (2001) 479.

w11x F. Silva, A. Gicquel, A. Tardieu, P. Cledat, T. Chauveau,Diamond Relat. Mater. 5(1996) 338.

w12x B.X. Yang, W. Zhu, J. Ahn, H.S. Tan, J. Crystal Growth 151(1995) 319.

w13x Z.C. Feng, C.C. Tin, R. Hu, J. Williams, Thin Solid Films 266(1995) 1.

w14x M.J. Chiang, M.H. Hon, J. Crystal Growth 211(2000) 211.