Ž .Thin Solid Films 374 2000 268]273

Effect of bias-enhancement in diamond nucleation and growthon nickel

Y. HayashiU, N. Shiraokawa, S. Nishino

Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan

Received 28 January 2000; accepted 31 March 2000

Abstract

Diamond nucleation and growth mechanisms on nickel substrates were investigated by ellipsometric monitoring, X-rayphotoelectron spectroscopy, scanning electron microscopy, and Raman spectroscopy. It was found that carbonization, diamondnucleation, and graphite growth stages exist and that the change in growth phase from diamond to graphite is earlier for aprocess with biasing than without biasing due to the higher density of carbon and hydrocarbon radicals. It was also found thatlonger biasing treatment in high methane concentration preceding a growth process without biasing enhances the growth ofhexagonal diamond on graphite. Q 2000 Elsevier Science S.A. All rights reserved.

Keywords: CVD diamond; Bias-enhanced nucleation; Nickel catalyst; Ellipsometry; Hexagonal diamond

1. Introduction

Diamond films have been synthesized by manyŽ .chemical vapor deposition CVD methods, e.g. hot-

Ž .filament HF CVD and microwave-plasma CVD, sincethe development of such methods approximately 20

w xyears ago 1,2 . However, the surface reaction mecha-nisms during the stage of diamond nucleation have notbeen well clarified. It was found that negative-bias

w xapplied to substrates enhances diamond nucleation 3w xand epitaxial growth 4 . The process is called bias-Ž .enhanced nucleation BEN , and the cause of these

effects is still not understood well. In order to analyzethe nucleation stage, methods of atomic analysis andin-process monitoring are indispensable. Ellipsometryis expected to be a useful technique for surface moni-

w xtoring in diamond nucleation stages 5,6 .Nickel is an interesting material for experiments on

diamond nucleation because it has characteristic

U Corresponding author.

properties of lattice constants close to those of dia-mond and high reactivity and solubility with carbon.Due to these properties, the heteroepitaxial growth of

w xdiamond was realized on the material 7 . Furthermore,it was reported that hexagonal diamond, in this case 2Hdiamond or lonsdaleite, had been grown on a nickelsubstrate, although the nucleation mechanism was not

w xclarified 8 .In this study, diamond films were prepared on nickel

substrates, and the bias-enhancement effect was inves-tigated by an ellipsometric monitoring method, X-ray

Ž .photoelectron spectroscopy XPS , scanning electronŽ .microscopy SEM , and Raman spectroscopy.

2. Experimental

The experimental system was the same as one pre-w xsented before 6 . Briefly, three straight tungsten wires

of 0.2 mm in diameter were stretched between twoelectrodes of an a.c. transformer. One of the elec-trodes, as well as the chamber wall, was grounded. A19-A electric current was given to the wires that act as

0040-6090r00r$ - see front matter Q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 0 0 1 1 6 2 - 7

( )Y. Hayashi et al. r Thin Solid Films 374 2000 268]273 269

hot-filaments. A substrate holder, into which the nega-tive bias voltage could be applied, was placed 8 mmunder the wires.

Polycrystalline nickel sheets of 10=10=0.1 mm3

were used as substrates for diamond growth. They werefirst ultrasonically cleaned in acetone and then treatedwith HF CVD during application of bias voltage ofy250 to y300 V into a substrate holder. The pre-treated specimens were processed in the followinggrowth of diamond without biasing.

The synthesis conditions were as follows: synthesisgas, 1 or 5% CH rH for the pretreatment and the4 2growth; flow rate, 50 sccm; gas pressure, 4 kPa; sub-strate temperature, approximately 9008C; time for thepretreatment and the growth, 0]60 min and 120 min,respectively.

In-process ellipsometric monitoring was performed

with a rotating-analyzer ellipsometer as was done inw xthe previous experiment 6 except for the wavelength

of 550 nm and the incidence angle of 788. Treatedspecimens for each stage of the evolution of ellipso-metric parameters were analyzed by XPS, SEM andRaman spectroscopy in the atmosphere.

3. Results

The pretreatment process is called plasma and hot-Ž .filament CVD P-HF CVD because luminescence by

plasma generation was observed 1]2 mm above thesubstrate. The plasma should be generated by a d.c.electric field with the help of electron emission fromhot-filaments. The luminous region corresponds to thenegative glow region, and the dark region above thesubstrate corresponds to the cathode fall region.

Ž . Ž . Ž .Fig. 1. Trajectories of ellipsometric parameters: a without biasing in 1% CH rH ; b without biasing in 5% CH rH ; c with biasing after B4 2 4 2Ž .stage in 1% CH rH ; d by simulation for refractive index of substrate of 1.7]3.5i and that of interface layer of 0.5]3.0i with a thickness of 104 2

Ž .nm. Points show data obtained every 5 s. In d , the solid, broken and dotted lines show the growth of the interface layer, diamond particles andgraphite particles, respectively, and closed lozenges show the points of growth having 100 nm in particle radius.

( )Y. Hayashi et al. r Thin Solid Films 374 2000 268]273270

Therefore, a high electric field, by which positive ionsand electrons are accelerated to and from the sub-strate, respectively, was induced above the substrates.This process is called BEN.

The trajectory of the ellipsometric parameters, Cand D, during the early stages of a HF CVD process in1% CH rH without biasing is shown in Fig. 1a. Two4 2significant change points, which are marked by B andC, can be seen in the figure. The starting point and theconverging point are marked with A and D, respec-tively. Specimens treated till each stage were analyzedby XPS, and the C and Ni spectra are shown in1s 2p3r2Fig. 2. Two peaks, the higher peak being approximately283.5 eV and the lower peak being approximately 284.5eV, are seen in the C spectra for specimen B. The1speak at approximately 284.5 eV, which is also seen forspecimens of C and D, shows the C]C bond, while thatat approximately 283.5 eV shows the bond of a carboncompound. The determination of whether the C]Cbond originates from diamond or from graphite isdifficult to make from only the result of XPS becausethe energy difference between the two peaks is small.The peak of the Ni]Ni bond at approximately 852.8 eVis seen for a nickel substrate in the Ni spectra, and2p3r2it shifts to a position of higher energy of approximately853.1 eV for the stages later than B. The peak shift isexpected to correspond with the formation of a nickelcompound. An increase in C]C bond peak height anda decrease in peak height of the nickel compound arealso seen as the process progresses. From these results,it is concluded that the Ni]C bond was formed betweenthe A and B stages and that some carbon material wasdeposited on and covered the nickel substrates afterthe B stage.

Fig. 1b,c shows the trajectories of the ellipsometricparameters in a process without biasing in 5% CH rH4 2and a process with biasing after the B stage in 1%

CH rH , respectively. It seems that the process4 2between the B and C stages was shortened in bothcases.

In order to investigate the processes with and with-out biasing, the evolutions were simulated by calcula-tion. From the fitting of the trajectory between the Aand B stages on the assumption of the growth of ahomogeneous interface layer, the refractive indices forthe nickel substrate and the interface layer were de-termined to be 1.7]3.5 and 3.5]3.0i, respectively, andthe thickness of the layer was 10 nm. On the assump-tion of the change in the growth mode from homoge-neous to nucleation at the stage of B, simulations were

w xperformed for the hemispherical nucleation growth 5of graphite and diamond with the nuclei density of109rcm3. Since the refractive indices for these materi-als at the temperature of 9008C are not known, theirvalues at room temperature were used in the calcula-

w xtion: 2.7]1.3i for graphite and 2.4 for diamond 9,10 .The results of the simulations are shown in Fig. 1d. At9008C, the trajectories should indicate similar curvefeatures because it can be supposed that the refractiveindex of diamond is real while that of graphite iscomplex. By the correspondence between the experi-mental trajectory and the simulated one, it is suggestedthat the evolution of the parameters in the experimentshown in Fig. 1a is due to the nucleation growth ofdiamond up to the C stage. This figure shows change tothe graphite growth after the C stage converging at thefinal point of graphite growth. The trajectories of theparameters in the BEN treatment shown in Fig. 1b,cshow shortcuts to the D stage.

SEM photos are shown in Fig. 3 for the specimens atthe B and C stages and the D stage after 30 mintreatment. The surface at the B stage looks somewhatroughened, while that at the C stage looks considerablyroughened with small particles scattered at a density on

Ž .Fig. 2. XPS spectra of the C and Ni regions of treated specimens for stages A nickel substrate , B, C and D indicated in Fig. 1a.1s 2p3r2

( )Y. Hayashi et al. r Thin Solid Films 374 2000 268]273 271

Fig. 3. SEM images of treated specimens for stages B, C and Dindicated in Fig. 1a.

the order of 109rcm2. The photo taken at the D stageshows larger particles covering the entire surface. Theseresults support the assumptions in the above simula-tions of ellipsometry.

Typical Raman spectra for the specimens after BENtreatment in 5% CH rH and the growth without4 2biasing in 1% CH rH for 2 h are shown in Fig. 4.4 2While the peak is seen at approximately 1334 cmy1 forthe specimen treated for 20 min BEN, it is seen atapproximately 1322 cmy1 for 30 and 60 min BEN. Itwas reported that the former peak indicates the inclu-sion of cubic diamond, while the latter indicates that of

w xhexagonal diamond 8,11 . Relative peak heights at1322, 1334 and 1360 cmy1 to that at 1580 cmy1 wereaveraged for several runs of the experiment and are

Fig. 4. Raman spectra of the specimens for different BEN timespreceding the growth without biasing for 2 h.

plotted against BEN time in Fig. 5. The figure confirmsthat longer BEN treatment increases the inclusion ofhexagonal diamond and decreases that of the amor-phous phase, which is indicated by the peak at 1360cmy1. However, the tendency of the inclusion of cubicdiamond against BEN time is not clear in this result.

4. Discussion

From the results of ellipsometric monitoring, XPSanalysis and SEM observation, a model of diamondnucleation and growth on a nickel substrate is de-

Ž .scribed as follows Fig. 6 . When a plasma is generated,the surface of nickel is first carbonized and slightlyroughened, and then diamond nucleation starts on thesurface of the intermediate layer with a thickness of 10nm. However graphite growth becomes greater than

Ž .Fig. 5. Relative peak heights at 1322 hexagonal diamond , 1334Ž . Ž . y1cubic diamond and 1360 amorphous carbon cm to that at 1580cmy1 plotted against BEN time.

( )Y. Hayashi et al. r Thin Solid Films 374 2000 268]273272

Ž .Fig. 6. Model of diamond nucleation and graphite growth: a diffu-Ž .sion of carbon-related radicals in substrate surface; b growth of

Ž . Ž .interface layer; c growth of diamond nuclei; d out-diffusion ofcarbon-related radicals from substrate surface and growth of graphiteon diamond.

diamond growth before covering the entire surface ofthe carbonized substrate. The change in growth phaseis earlier for a process with biasing than without bias-ing and for higher methane concentration.

Since nickel is a material of high reactivity andsolubility with carbon, the nucleation mechanism on itshould be different from that on a material like silicon.As in diamond formation by a high-pressure and high-temperature process, where nickel plays the role ofsolvent-catalyst, in the CVD process, a nickel substratecontains much carbon near the surface, and Ni]Cstates suppress graphite formation. Thus, diamond nu-cleation is initially activated, but the formation rate ofgraphite may exceed that of diamond after a while. It isspeculated that there is a critical density of carbon andhydrocarbon radicals above a substrate surface for dia-mond and graphite formation: with the increase indensity, the formation of diamond particles and that ofgraphite particles occur in this order. In the carboniza-tion process, carbon and hydrocarbon radicals are ab-sorbed into a nickel substrate, and then the density ofthe radicals above the surface is lower. However, oncethe solubility in the substrate is saturated and the solidphase-gas phase equilibrium is attained, diamond nu-cleation occurs due to the increase in gas phase densityof the radicals. If most of the carbon and hydrocarbonradicals are consumed for migration to and depositionon diamond nuclei, the solid phase-gas phase stateenters a non-equilibrium again and the density of theradicals above a substrate further increases due to thedesorption from the substrate. Although Ni]C states

suppress graphite formation, graphite can grow on ma-terials such as diamond that contain C]C bonds only.This explains the diamond nucleation and the subse-quent graphite formation processes.

When negative d.c. bias is induced to a substrateduring the process, a plasma is generated and thebombardment of carbon and hydrocarbon ions occurs.Consequently, the density of carbon in solid phase aswell as in gas phase increases, and the graphite forma-tion process is promoted.

A model of diamond nucleation by hydrogenation atthe edges of graphite material was proposed by Lam-

w x � 4brecht et al. 12 , who showed that the 111 projectionof the diamond bond length is only 2% larger than the

� 4corresponding distance of the 0001 projection ingraphite and that the epitaxial growth of diamond ongraphite is possibly initiated by hydrogenation. Theyalso mentioned in the paper that the random occur-rence of stacking faults is common, particularly inrelatively defective graphite. Such stacking faults, i.e.twin boundaries, tend to form hexagonal diamond, inwhich ‘boat’ configurations of the carbon ring exist,instead of cubic diamond of the ‘chair’ configurationsw x13 . Although graphite is preferentially etched by hy-drogen atoms, the deposition of graphite overwhelmsthe etching due to the higher graphite formation, asoccurs in a process with biasing. Furthermore, in aprocess biasing with many stresses are induced by ionbombardment onto films that contain many defects. Asa result, longer biasing treatment at high methaneconcentration enhances the growth of hexagonal dia-mond on graphite.

5. Summary

Diamond nucleation and growth mechanisms onnickel substrates were investigated by ellipsometricmonitoring and XPS, SEM and Raman spectroscopy.Carbonization, diamond nucleation and graphite growthstages were distinguished by ellipsometry. The changein growth phase from diamond to graphite is earlier fora process with biasing than without biasing due to thehigher density of carbon and hydrocarbon radicals.Longer biasing treatment at high methane concentra-tion preceding a growth process without biasing en-hances the growth of hexagonal diamond on graphite.It was found that well-aligned carbon nanotubes in-stead of diamond or graphite were grown on nickelsubstrates under the same BEN conditions except forhigher methane concentrations diluted in hydrogen and

w xlower substrate temperature 14 . In conclusion, thenucleation and growth forms of carbon are sensitivelychanged according to the density of carbon and hydro-carbon radicals in the gas phase.

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