Diamond & Related Materials 18 (2009) 734–739

Contents lists available at ScienceDirect

Diamond & Related Materials

j ourna l homepage: www.e lsev ie r.com/ locate /d iamond

Seeding of polymer substrates for nanocrystalline diamond film growth

A. Kromka a,⁎, O. Babchenko a, H. Kozak a, K. Hruska a, B. Rezek a, M. Ledinsky a, J. Potmesil a,M. Michalka b, M. Vanecek a

a Institute of Physics, Academy of Sciences of the Czech Republic, 162 53 Praha, Czech Republicb International Laser Centre, Ilkovicova 3, 812 19 Bratislava, Slovak Republic

⁎ Corresponding author.E-mail address: kromka@fzu.cz (A. Kromka).

0925-9635/$ – see front matter © 2009 Published by Edoi:10.1016/j.diamond.2009.01.023

a b s t r a c t

a r t i c l e i n f o

Available online 21 January 2009

Keywords:

Nanocrystalline diamond (NSiO2/Si substrates patternenanoparticles in polymer, b

Nanocrystalline diamondChemical vapor depositionPolymerScanning electron microscopy

y spin coating of UDD drop and by ultrasonic treatment in solution of UDD.For all samples, the deposited NCD film is strongly related to the primary polymer deposited over the Si/SiO2

substrate. For a certain concentration of UDD, seeding by polymer composite results in formation of fullyclosed layer. The drop of UDD requires using of spin coating process and by repeating of this procedure, a

CD) thin films and structures are grown by microwave plasma CVD technique ond by polymer. The substrates are seeded by ultra-dispersed diamond (UDD)

NCD layer with clear 3D geometry is achieved. Ultrasonic treatment leads in “implanting” of UDD into thepolymer bulk. The primary polymer stripes are damaged during this procedure. In addition, 3D porous likelayer is observed after the CVD growth.Optimizing the seeding procedure and the dimension of the primary polymer allow a direct growth of self-standing air bridges suitable for MEMS/NEMS applications.

© 2009 Published by Elsevier B.V.

1. Introduction

Growth of nanocrystalline diamond (NCD) has been widelystudied because of inexpensive fabrication compared to singlecrystalline diamond while showing most of its excellent propertiessuch us smooth surface, optical transparency in wide range, excellentmechanical properties, bio-compatibility, high affinity for covalentbondingwith specific organicmolecules and chemical inertness [1–3].Low cost insulating (nominally undoped) and semiconducting(boron-doped) nanocrystalline diamond films can be deposited onlarge areas [4] which opens new fields for their bio-electronic and bio-sensoric applications [5–9].

The CVD growth of continuous nanocrystalline diamond thin filmsat low thickness on non-diamond substrates is still a technologicalchallenge because of low nucleation yield. Substrate surface featureslike surface defects (scratching lines), tendency to form carbide layer,resistance to the carbon bulk diffusion, surface energy, etc. are all thefactors positively influencing the nucleation yield [10]. Therefore,prior to the CVD diamond growth (pre) treatment processes likemechanical abrading, ultrasonic seeding or ion bombardment wereinvolved to initialize the substrate surface [11]. Presently, the mostprogressive and universal technique seems to be ultrasonic seedingwhich achieves very high nucleation densities up to 1012 cm−2 [12–13]. So high nucleation densities have been achieved in so called “new

lsevier B.V.

nucleation process” (NNP) introduced in 1999 by Rotter et al. [14], andfurther modified and studied by Butler group [15].

However, all above described techniques are applicable only to“mechanically and temperature stable” substrates. One possiblesolution to overcome this limitation is applying a liquid suspensionof diamond particles onto the soft substrate. Water based ink withdiamond particles in size of 50 or 250 nm has been used todemonstrate a local seeding on glass, silicon, copper, and fused quartzby a standard ink jet technology [16]. The dip technique in carboncontaining solution was shown as very simple and low cost techniquecomparing to any plasma technique [17]. This technique has beenwellsuitable for substrates of complex 3D geometries or even to seed“inner” surfaces like holes or cavities. However, only growth ofamorphous carbon films has been presented. Using a polymer asnucleation layer has been also studied by several groups. In the earlystudy of Jingsheng et al. it was shown that spin coating of poly(phenylcarbyne) on mirror polished or on scratched silicon substratesenhanced the nucleation density [18]. Growth of isolated diamondcrystals in size ranging from nanometers up to micrometers waspresented. However, applying other polymers (phenolic resin, poly-ethylene, pump oil, etc.) as precursors did not resulted in diamondgrowth at low substrate temperatures in their work. Our previouswork has shown that the seeding by a polymer-based compositeresulted in growth of fully closed diamond films at low substratetemperature [19]. The film formation had a tendency to be dominatedby a lateral growth and this seeding technique was shown as wellsuitable for seeding of mechanically soft and/or unstable substrates(polymers, ultrathin metals, etc.).

Fig. 1. Processing of Si/SiO2 substrate and formation of the primary polymer stripes.

735A. Kromka et al. / Diamond & Related Materials 18 (2009) 734–739

In this paper, we present growth of NCD thin films on Si/SiO2

substrates patterned by the primary polymer stripes. The influence ofseeding pre-treatment and the origin of substrate material on the filmgrowth are studied. The NCD growth is found to be enhanced by thepresence of primary polymer stripes used as the substrate. In addition,these polymer stripes can be consumed (etched) during the earlystages of NCD growth and self-assembled cavity can be formed withpotential uses in MEMS/NEMS applications. The diamond character ofall deposited films is confirmed by Raman measurements.

2. Experimental

Before the CVD growth experiment, the Si/SiO2 substrates (Si:550 µm, SiO2: 1.4 µm) were cleaned in isopropanol and dried by anitrogen gun. Then, they were spin-coated with an OFPR photoresist,which was dried and photolithographically treated to realize linear“stripes” 30 µm in width, 1.5 µm in height and in distance of 30 µmstripe-to-stripe (Fig. 1). To distinguish between the seeding polymer(as below described) and these polymer stripes, they are labeled as“primary polymer stripes”.

The Si/SiO2 substrates covered with the OFPR photoresist stripeswere seeded (i.e. “nucleated”) in 4 differentways. Sample #1 (labeled as“low UDD”), was spin-coated with PVA polymer. The PVA polymerwas dissolved in DI water. The polymer suspension consisted of mixedPVA (4 wt.%) with UDD (6.7 wt.%) in ratio 1:1. Sample #2 (labeledas “high UDD”) was spin-coated with composite PVA (2.5 wt.%) andUDD (6.7 wt.%). Sample 3 (labeled as “drop of UDD”) was nucleated byspin coating of liquid drop of UDD (6.7 wt.%) on the substrate, thesubstrate was dried for 30 min at 100 °C. As a liquid was used DI water.This “drop of UDD” procedure was repeated for 3-times. Sample #4(labeled as “ultrasonic in UDD”) was nucleated by ultrasonic treatment

Fig. 2. A schematic drawing o

of substrate in liquid (DIwater) suspensionofUDD(6.7wt.%) for 30min.The schematic drawing of used seeding procedures is shown in Fig. 2.

All processed substrates were loaded into the CVD chamber togrow nanocrystalline diamond thin films and/or their structures. TheCVD deposition was performed in microwave plasma-enhanced CVDreactor using a cavity resonator (Aixtron P6). Only briefly, the total gaspressure was 30 mbar, 1% of methane diluted in hydrogen and thesubstrate temperature was 410 °C (growth time 20 h) or 850 °C(growth time 2 h). Detailed description of deposition parameters canbe found in our previous works [20,21].

Surface morphology of deposited samples was investigated byscanning electron microscopy (SEM, Raith GmbH, e_LiNE writer).Surface topography of deposited samples and height of grownstructures were investigated by AFM measurements in a tappingmode (AFM Microscope Dimension 3100, Veeco). Silicon AFMcantilevers were used with a typical tip radius of 10 nm and resonancefrequency of 230 kHz. The diamond character of grown structures wasconfirmed by Raman spectroscopy (Renishaw inVia Reflex Ramanmicroscopewith an excitationwavelength 325 nm). Themicro-Ramanspectra were collected in a matrix of microscopic spots with a lateralresolution of approx. 2 µm for an objective of 40×.

3. Results and discussions

Surface morphology of diamond layer grown on Si/SiO2 substratespatterned with the primary polymer stripes is shown in Fig. 3. All thesesamples were grown at identical process conditions at substratetemperature of 410 °C, but seeded (nucleated) by various procedures(low UDD, high UDD, drop of UDD and ultrasonic in UDD). Sample seededas low UDD shows two basic morphological types, first type exhibits aformation of fully closed film (marked in white rectangular, 30 µm in

f 4 seeding procedures.

Fig. 3. SEM morphology (a–c) and AFM topography (d) of grown samples. Scale bar of SEM images 30 µm, 400 nm, and 200 nm.

736 A. Kromka et al. / Diamond & Related Materials 18 (2009) 734–739

width) and the second one shows openings in the film. Detailed SEMscan confirms this observation (Fig. 3, low UDD) whereas grains inclustered like arrangement are preferably found on the “on stripe area”.Isolated crystal are formed on the “off stripe area” and some of them areclustered. On both areas, mainly crystals in size approx. 200 nm areformed. The Z-axis height of the formed stripes was 130 nm, ascalculated from the AFM height histogram (Fig. 3, low UDD).

When the concentration of UDD in polymer increases, theformation of fully closed and hierarchically structured film is observed(Fig. 3, high UDD). The “on stripe area” reveals a three-dimensionallike structure. Character of the high UDD film changes substantiallyand two different types of structure are observed — one represents a“fine grained” fully closed layer located at the background and thesecond shows a formation of isolated grains sitting (growing) on thisfine grained layer. The size of isolated grains is up to 200 nm. The finegrained layer replicates the morphology of the starting substratecharacter, i.e. linear stripes repeat with periodicity of 30 µm. Theheight of stripes revealed a step height of 300 nm.

The sample drop of UDD exhibits a similar surfacemorphology as thesample high UDD. In this case, higher contrast between the “on” and “offstripe area” is observedover large area SEMscan. Once again, the 3Dfilm

structure consists of two types, i.e. from a fined grained layer withisolated grains on it. Next, sample drop of UDD shows also a clearerthree-dimensional structure, as observed on the primary stripe-edge.The height of formed diamond stripes is 300 nm, as calculated from theAFM height histogram.

Sample nucleated by ultrasonic in UDD exhibits also fully closedlayer, but the character slightly differs to the previous cases. Thesurface morphology on the “off stripe area” seems to be formed fromwell packed individual grains, nearly forming a closed layer (Fig. 3,ultrasonic in UDD). The isolated grains form cauliflower like clusters“on stripe area”. The geometry of the primary polymer stripes seemsto bemore damaged than in previous cases. The formed clusters reveala complex 3D geometry, which can indicate a porous character of thelayer. The size of the grains seems to be slightly lower than 200 nm.The height of irregularity on the sample ultrasonic in UDD achieves thehighest values up to 750 nm (Fig. 3, ultrasonic in UDD). Still, atendency in keeping the periodicity of primary polymer stripegeometry is observed.

Fig. 4 shows two representative Raman spectra of the sample ul-trasonic in UDD. It must be noted that Raman spectra were nearlysimilar for all samples grown at low temperature. The Raman spectrum

Fig. 4. Representative Raman spectra of the sample ultrasonic in UDD collected from“on” and “off” stripe area.

Fig. 5. Cross-section view of continuous NCD film formed in cavity-like channelgeometry (a), SEM image taken at angle 30°, and a characteristic Raman spectrum (b).

737A. Kromka et al. / Diamond & Related Materials 18 (2009) 734–739

of the sample ultrasonic in UDD was collected from “on” (full square)and “off primary polymer stripe area” (opened circle). For both scanareas, the Raman measurement exhibits a dominant peak centered at1332 cm−1 (optical phonon in diamond) which confirms a diamondcharacter of the formed layer [22]. In addition, two broad bandscentered at 1350 and 1590 cm−1 (known as D andG band) are found inthe spectra [23]. The Raman spectrum from the “on stripe area” alsoreveals a presence of small band centered at 1180 cm−1 which canindicate sign of nanocrystalline diamond character [24].

The CVD growth at high substrate temperature differs from thegrowth at low temperature (410 °C). First, broadening of the primarypolymer stripes and a losing of the 3D geometry was observed forthe high temperature growth. Second, the grown layer was almostcompact and fully closed expect of linear gaps (1 µm inwidth) formedin a repeating array. Third, it was not observed the development oftwo-component like layer, i.e. no clear sign of the formed fine grainedfilm with isolated grains sitting on it. Surface morphology of sampledrop of UDD grown at high substrate temperature is shown in Fig. 5a.For this measurement, the sample was broken and measured in across-section at angle of 30°. In some regions, this sample exhibits athree dimensional growth and a self-standing air bridge like channel isobserved. The formed film bridge is ~600 nm inwidth and its height isup to 400 nm.

The Raman spectrum of the grown film is shown on Fig. 5b. Thespectrum exhibits two sharp peaks — one dominant peak centered at1332 cm−1 (diamond) and the second sharp peak is centered at520.7 cm−1 (silicon). Next, D and G bands are observed as broad bandscentered at 1350 and 1570 cm−1. The band centered at 1160 cm−1 isassigned to nanocrystalline diamond. The spectrumwas identical overthe whole scanned regions while the high temperature growthresulted in formation of nearly fully closed layer.

Growth of diamond films can be achieved only on the substrateswhere the seeding density is high enough. In addition, this seedingdensity can be enhanced up to 1011 cm−2 by using a polymer, as it hasbeen previously observed by Jingsheng et al. [18]. The increasednucleation density was assigned to the sp3-bonded carbon comingfrom the polymer and to the local transport of carbon from polymer todiamond particles. They suggested that sp3-bonded carbon withdangling bonds can be formed during the CVD growth at certaintemperatures [25]. It was proposed that atomic hydrogen coming fromplasma plays an important role in stabilizing the dangling bonds ofsp3-bonded amorphous carbon matrix and this matrix can beconverted to the crystalline diamond phase. It is noticeable that thepresence of atomic hydrogen is not always a crucial issue. Bianconiet al. have shown that using a thermal treatment without the presence

of atomic hydrogen can convert polymer into sp3-bonded carbonmatrix, or even into diamond crystals [26,27].

Based on previously published works and our observation weconclude that the primary polymer stripes substantially act as acarbon source which enhances the diamond nucleation and accel-erates homogenous CVD growth. This was well observed for samplelow UDD. The fully closed layer was only found over the primarypolymer stripes. The height of isolated crystals was ~130 nm and thus,the film thickness estimates slightly higher “on stripe”. Implementingof primary polymer stripes indicates similarities with the NNP process(see works Rotter and Butler), where formation of an amorphous-carbon (a-C) layer before the CVD diamond growth is a crucial issue.This a-C layer is an additional source of carbon atoms. During the CVDgrowth, some carbon atoms diffuse from the a-C layer to the diamondnanograins where they “feed” their enlargement and continuedgrowth. The density of isolated diamond grains is high enough to bedetected by Raman measurements, only the intensity from “off stripearea”was observed lower. Situation slightly differs for the sample highUDD. In this case, the seeding polymer results in growth of fully closeddiamond layer on “off stripe area”. The density of isolated diamondgrains seems to be higher for this area than for the “on stripe area”.Diamond layer formed on the primary polymer exhibits 3D-like

738 A. Kromka et al. / Diamond & Related Materials 18 (2009) 734–739

structure (similar to the sample ultrasonic in UDD). Possible explana-tion for this is that in the first stage of CVD growth, diamond (seeding)nanograins can randomly diffuse in the primary polymer stripes andat certain process time their diffusion stops and the growth starts to bedominant. On the other hand, consumption and/or etching of theprimary polymer stripes in atomic hydrogen is running in parallel tothis process. In this sense, the sample drop of UDD represents anequilibrium where fine grained layer is formed and the density ofisolated grains is similar for the “on” and “off stripe area”. We suggestthat the repeated drop of UDD process, where drop of UDD is spincoated on the sample and then the sample is heated up and dried forthree times, results in formation of specific “capping-like” layer inwhich diamond nanograins are mixed with carbon from polymerlayer. This capping layer can be compared to the super-seeded layerwhich is homogenously distributed over “on” and “off stripe area”. Theinitialization of the CVD growth leads in fast closing of the formeddiamond layer keeping the patterned geometry of the used primarypolymer. Samples high UDD and drop of UDD show approx. the sameheight of 300 nm.

The sample ultrasonic in UDD indicates the highest damage of theprimary polymer edges due to the mechanical striking of polymer bydiamond nanoparticles. We suggest that some amount of polymer isre-deposited from the “on” to the “off stripe area” and some seedingdiamond nanograins are “softly” implanted into the polymer matrixduring the ultrasonic process. The CVD growth initializes a geome-trical fixing of these nanograins and start up their enlargement. Anetching process of remaining polymer components is running inparallel to this process which results in formation of combined 3Dshapes exhibiting a porous like structure. It must be noticed that ourprevious study on ultrasonic treatment of silicon and/or glasssubstrate resulted in homogenous covering of substrate by UDDnanograins. Flat and fully closed NCD filmwas formed at low thickness[28]. Thus, the porous like character of the formed film is assigned tothe presence of the primary polymer stripes. This sample exhibits thehighest height of formed structures up to 750 nmwhich is assigned tothe fact that some diamond grains were accumulated at the edge ofprimary polymer stripe whereas side wall was seeded. Such aperpendicularly seeded layer forms a “continuous” side structurewhose height is lower than the height of the primary polymer stripe.

Unfortunately, cross-section SEMmeasurements did not show any3D like structures as observed in top view SEM patterns or in AFMimages. This could be most probably assigned to the breakingprocedure during which samples were pressed and any 3D structureswere destroyed.

It is well known that growth rate of NCD film is much higher forhigh temperature range than for low one. Based on this, we realizedthe CVD growth at 850 °C with aim quickly to close NCD film and formself-assembled cavities. For all samples deposited at high tempera-tures, the character of primary polymer stripes disappears and openedlines (i.e. linear groves, average width of 1 µm) were observed forsome regions. We propose that due to the high temperature processthe primary polymer lines play minor role in the NCD growth whilethey are etched away very quickly. Then, the growth is mostlycontrolled by the presence of UDD powder coming from the seedingprocedure. Sample drop of UDD at high temperature exhibited also agrowth region where NCD film formed self-standing air bridges overthe substrate in the submicrometer range. For formation of suchbridges a specific condition has to be fulfilled — the NCD growthand the etching of primary polymer stripe with hydrogen are inequilibriumwhile the “quickly” formed layer is thick enough to form aself-standing bridge which can further continue the growth. It isbelieved that growth of micrometer-sizes air bridges will be possibleby a proper optimizing the seeding procedure and of growthparameters. Then, this result significantly can simplify the growth ofdiamond channels for MEMS/NEMS applications where sacrificiallayer, like nickel or cooper, has to be used [29,30].

4. Conclusions

We studied the influence of seeding procedure on the growth ofnanocrystalline diamond thin films deposited on Si/SiO2 substratespatterned by the primary polymer stripes. Using a polymer compositeas the seeding layer resulted in fully closed film formation for a certainconcentration of UDD. At lowUDD, the NCD layer was fully closed onlyon the region of the primary polymer stripes. Applying of the polymercomposite as the seeding procedure is well suitable for seeding of softsubstrates over large areas.

The spin coating of UDD drop on substrate, cyclically repeated3 times, resulted in growth of diamond layer with the sharpeststructures formed at the edges of the primary polymer stripe. Thistechnique is soft and well working for small areas.

The ultrasonic seeding procedure damaged the primary polymerstripe deposited on Si/SiO2 substrate. The redeposition of the primarypolymer and “implantation” of UDD grains into these polymer stripesresulted in formation of complex porous like structures which wasgeometrically related to the primary polymer stripes.

For all seeding processes, the primary polymer stripes played animportant role for enhancing diamond nucleation and/or growth atlow substrate temperature. The primary polymer stripes behaved asthe additional source of carbon. The experimental results indicatedsimilarities with the new nucleation process (Rotter, Butler), wherethe solid state a-C layer was used as the carbon source. It can beconcluded that proper combining of the primary polymer with UDDseeding technique can result in very high-seeding density over anysubstrate material. Formation of self-assembled air bridges (channels)was achieved at high temperature range. The optimal processconditions for controllable and reproducible formation of such bridgesstill require deeper studies.

Acknowledgements

Thisworkwas supported by the national grants No. AV0Z10200521,KAN400100652. KAN400100701, and 1M06002 and by the EC projectDRIVE, No. MRTN-CT-2004-512224.

References

[1] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211.[2] O.A. Shenderova, D.M. Gruen, Ultrananocrystalline Diamond: Synthesis, Properties,

and Applications, William Andrew Publishing, 2006 ISBN: 0-8155-1524-1.[3] J. Philip, P. Hess, T. Feygelson, J.E. Butler, S. Chattopadhyay, K.H. Chen, L.C. Chen,

J. Appl. Phys. 93 (2003) 2164.[4] www.thindiamond.com, www.rhobest.com.[5] H. Kawarada, Y. Araki, T. Sakai, T. Ogawa, H. Umezawa, Phys. Status Solidi, A 185 (79)

(2001).[6] W. Yang, O. Auciello, J.E. Butler, W. Cai, J.A. Carlisle, J.E. Gerbi, D.M. Gruen, Nat.

Mater. 1 (2002) 253.[7] J.A. Carlisle, Nat. Mater. 3 (2004) 668.[8] A. Härtl, E. Schmich, J.A. Garrido, J. Hernando, S.C.R. Catharino, S. Walter, P. Faulner,

A. Kromka, D. Steinmüller-Nethl, M. Stutzmann, Nat. Mater. 3 (2004) 736.[9] B. Rezek, H. Watanabe, C.E. Nebel, Appl. Phys. Lett. 88 (2006) 042110.[10] B.V. Spitzyn, L.L. Bouilov, B.V. Derjaguin, J. Cryst. Growth 52 (1981) 219.[11] H. Li, D.S. Dandy, Diamond Chemical Vapor Deposition–Nucleation and Early

Growth Stages, NOYES PUBLICATION, New Jersey, 1995.[12] O.A. Williams, O. Douhéret, M. Daenen, K. Haenen, E. Osawa, M. Takahashi, Chem.

Phys. Lett. 445 (2007) 255.[13] A. Kromka et al., to be published in Chem. Vap. Depos.[14] S. Rotter, in: M. Yoshikawa, Y. Koga, Y. Tzeng, P. Klages, K. Miyoshi (Eds.), Proceedings

of the Applied Diamond Conference/Frontier Carbon Technologies—ADC/FCT '99,1999, MYU, K.K, Tokyo, 1999, p. 25.

[15] A.V. Sumant, P.U.P.A. Gilbert, D.S. Grierson, A.R. Konicek, M. Abrecht, J.E. Butler,T. Feygelson, S.S. Rotter, R.W. Carpick, Diamond Relat. Mater. 16 (2007) 718.

[16] N.A. Fox, M.J. Youh, J.W. Steeds, W.N. Wang, J. Appl. Phys. 87 (2000) 8187.[17] S.C. Ray, G. Fanchini, A. Tagliaferro, B. Bose, D. Dasgupta, J. Appl. Phys. 94 (2003)

870.[18] C. Jingsheng, W. Xuejun, Z. Zhihao, Y. Fengyuan, Thin Solid Films 346 (1999) 120.[19] A. Kromka et al.: Using polymer composite for seeding growth of nanocrystalline

diamond films, presented at the workshop “Surface and bulk defects in CVDdiamond films, XIII”, February 25-27 February, 2008, Hasselt, Belgium.

[20] S. Potocky, A. Kromka, J. Potmesil, Z. Remes, Z. Polackova, M. Vanecek, Phys. StatusSolidi, A 203 (2006) 3011.

739A. Kromka et al. / Diamond & Related Materials 18 (2009) 734–739

[21] S. Potocky, A. Kromka, J. Potmesil, Z. Remes, V. Vorlicek, M. Vanecek, M. Michalka,Diamond Relat. Mater. 16 (2007) 744.

[22] S. Bühlmann, E. Blank, R. Haubner, B. Lux, Diamond Relat. Mater. 8 (1999) 194.[23] S.L. Heidger, Mater. Res. Soc. Symp. Proc. 383 (1995) 319.[24] H. Kuzmany, R. Pfeiffer, N. Salk, B. Günther, Carbon 42 (2004) 911.[25] Z. Sun, Y. Sun, Q.H. Yang, X.J. Wang, Z.H. Zheng, Surf. Coat. Technol. 79 (1996) 108.[26] P.A. Bianconi, S.J. Joray, B.L. Aldrich, J. Sumranjit, D.J. Duffy, D.P. Long, J.L. Lazorcik,

L. Raboin, J.K. Kearns, S.L. Smulligan, J.M. Babyak, J. Am. Chem. Soc. 126 (10)(2004) 3191.

[27] Z. Suna, X. Shia, B.K. Taya, X. Wanga, Y. Sunb, Thin Solid Films 308–309 (1997) 159.[28] A. Kromka, to be published in Diamond Relat. Mat.[29] T. Inoue, H. Tashibana, K. Kumagai, K. Miyata, K. Nishimura, K. Kobashi, A. Nakaue,

J. Appl. Phys. 67 (1990) 7330.[30] R. Müller, P. Schmid, A. Munding, R. Gronmaier, E. Kohn, Diamond Relat. Mater. 13

(2004) 780.