synthesis and characterization of cluster-assembled carbon ... · centered at 350 atoms/clusters!...

Synthesis and characterization of cluster-assembled carbon thin filmsP. Milani,a) M. Ferretti, and P. PiseriINFM-Dipartimento di Fisica, Universita` di Milano, via Celoria 16, 20133 Milano-Italy

C. E. Bottani, A. Ferrari, and A. Li BassiINFM-Dipartimento di Ingegneria Nucleare (CESNEF), Politecnico di Milano, via Ponzio 34/3,20133 Milano-Italy

G. Guizzetti and M. PatriniINFM-Dipartimento di Fisica ‘‘A. Volta,’’ Universita` di Pavia, via Bassi 6, 27100 Pavia-Italy

~Received 18 April 1997; accepted for publication 4 September 1997!

Nanostructured carbon thin films have been produced by deposition of supersonic cluster beams.The clusters are generated by a pulsed arc cluster ion source modified in order to achieve high fluxesand stability. Scanning electron microscopy, Raman, and optical spectroscopy show that the filmsare a low density network of nanometer-size particles. The nature of the films is essentiallygraphite-like with a large number of distorted bonds. The formation of structures based onsp3

bondings is not observed. The use of cluster beam deposition for the synthesis of nanocrystallinethin films is discussed. ©1997 American Institute of Physics.@S0021-8979~97!07123-5#

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I. INTRODUCTION

Nanostructured materials are subject of an increasingterest as a consequence of their peculiar structural, etronic, and mechanical properties.1 This field of research canstrongly benefit from the efforts devoted to the charactertion of clusters and small aggregates. The physicalchemical properties of these objects are size specific sothey can be considered as building blocks for the synthesmaterials with tailored properties controlled by changingsize and composition of the primeval clusters.2

Among many methods of production of nanostructurmaterials,3 the assembling of clusters seems to be very vsatile and promising, expecially for thin film synthesis.necessary requisite for large scale and reliable use oftechnique is the development of efficient production acharacterization methods to control cluster nucleation, desition, postdeposition coalescence etc, in order to correthe final product with the precursors.

Several authors have proposed cluster beam deposas a tool for the synthesis of nanostructured thin films:4–6

Clusters produced in the gas phase are accelerated inpersonic beam and then deposited on a substrate. Compto ion beam implantation the use of clusters offers newportunities such as chemically activated reactions, shalimplantation,6 moreover the control of cluster mass distribtion and cluster deposition energies can allow the growthfilms with a tailored nanocrystalline structure.4,5

The application of cluster beam deposition to the syntsis of nanocrystalline carbon thin films seems to be vattractive. The control of deposition energy and mass disbution should open the possibility of tuning thesp2–sp3

ratio of the film, moreover the introduction of fullerenstructures embedded in an amorphous matrix could influethe mechanical properties of the nanostructured materia7,8

Compared to the enormous amount of experimentaltheoretical results on carbon films constructed. ‘‘atom

a!Electronic mail: [email protected]

J. Appl. Phys. 82 (11), 1 December 1997 0021-8979/97/82(11)/5

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atom,’’ 9 the characterization of cluster-assembled carbmaterials is still in its infancy. Theoretical researchesnanostructured and nanoporous materials have exploredclasses of solids, based on the assembling of small caunits.10,11 On the other hand, relatively few experimental eforts have been produced towards the synthesis and the cacterization of these materials.

Recently nanocrystalline carbon films have been pduced by depositing supersonic cluster beams generatedlaser vaporization source4,12 and by collecting on a substratthe particles produced in an arc-plasma apparatus.7 In bothexperiments, the deposition conditions were not systemcally established. Moreover, recent numerical simulationscarbon cluster deposition raised some questions aboutinterpretation of the experimental results of film charactization and in particular on the formation ofsp3 networksfrom the assembling of small fullerenes.10

Here we present a characterization performed by scning electron microscopy~SEM!, Raman spectroscopy, optcal reflectance, and ellipsometry of nanocrystalline carbthin films deposited by cluster beams produced with a noplasma cluster source. The analysis of carbon films shthat, in the deposition energy range used, they have aoporous structure with an essentialsp2 character of thebonds. We have also carefully characterized the sourceformance and the cluster beam prior to deposition. We shthat this kind of source can be conveniently used fordeposition of thin films of refractory materials.

II. EXPERIMENTAL SETUP AND SAMPLE DEPOSITION

A schematic view of the deposition apparatus is shoin Fig. 1. A supersonic beam of clusters is produced bmodified version of a pulsed arc cluster ion sour~PACIS!.13 A carbon plasma is generated by an intense etrical discharge produced between two graphite rods. Tplasma is mixed with a pulse of inert buffer gas~helium witha pulse width of several hundreds of microseconds! to favorthermalization and cluster aggregation; the mixture is th

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expanded into a vacuum. In order to improve the stabiland the intensity of the source, we have introduced sevemodifications as compared to the original design.5 A detaileddescription of the source will be published elsewhere,14 herewe only underline the introduction of a chamber15 where theplasma thermalizes before the expansion, drasticallycreases the stability, the mass range, and the intensity attable, rendering this type of source very attractive for tdeposition of nanostructured thin films of refractory mateals.

The characterization of the different cluster mass distbution and charge states in the beam has been perforwith a linear time-of-flight~TOF! mass spectrometer.16 Neu-tral clusters have been ionized with the fourth harmonic~266nm! of a Nd: yttrium aluminum garnet~YAG! laser. Clusterfluxes have been characterized with a quartz microbalaand with a Faraday cup placed in the deposition chamafter the first skimmer~see Fig. 1!. In Fig. 2, we show a massspectrum of neutral clusters produced with source conditio

FIG. 1. Schematic view~not to scale! of the experimental apparatus used fothin film deposition.

FIG. 2. Mass distribution of carbon clusters used as precursors of thin filThe TOF spectrum has been obtained by ionizing the clusters in the busing the fourth harmonic~266 nm! of a Nd:YAG laser.

5794 J. Appl. Phys., Vol. 82, No. 11, 1 December 1997


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used for film depositions. The mass distribution is peakaround 500 atoms per cluster, with contributions from agggates up to about 2500 atoms per cluster.

The center of mass of the size distribution and chastate are strongly influenced by the presence of the thermization cavity. The clusters residence time depends onrameters such as the pressure reached in the cavity, itsume, and the conductance of the nozzle. The spectrumFig. 2 has been recorded with a delay timeTd between thearc discharge and the laser ionization of 1100ms. This delayis the convolution of the residence time of the cluster insource with the time necessary for the cluster to reachTOF mass spectrometer. Mass spectra have been recofor a very wide range ofTd ~several milliseconds!, showingthat the center of mass of the size distribution is influencby the residence time. For very largeTd , clusters of severathousands of carbon atoms contributed to the mass spehowever without changing substantially the position of tcenter of mass of the size distribution. The mass distributof clusters ions~cations! is substantially different from thaof neutrals: 0–1200 atoms/cluster~with mass distributioncentered at 350 atoms/clusters! for ions, against 0–3000atoms/cluster~centered at 750 atoms/cluster! for neutrals.This is probably due to a trade off between cluster growand charge neutralization: the growth of large clustersquires long residence times inside the source, but thisimplies a bigger chance to be neutralized. Our results sugthat the assumption frequently made on the correspondeof the neutral and ion cluster beam produced by the sasource must always be carefully verified.4

By growth rate measurements, performed with the quamicrobalance, the overall beam intensity has been estimto be in the range of 331014'131015 s21 sr21 particles,depending on the discharge voltage. A fraction of about 1of the total average flux is due to anions, while cationsabout 2%. Neutral cluster velocities are in the range1400–1800 m/s, depending on the exit time. The typicalnetic energy of a medium-size cluster is thus about 0.3 kThe velocity of the cluster ions is spread over the ran1200–1900 m/s, with the distribution being peaked at 17m/s.

Thin film deposition has been performed by intersectthe cluster beam with the substrate placed on a threemicromanipulator. The temperature of the substrate@Si~100!#can be controlled from 77 to 600 K. Films of thicknessfrom 100 to 500 nm over an area of several square centiters, have been grown under a vacuum of 1027 Torr withdeposition rates of 25 Å/min. Some of the deposited filhave been annealedin situ at different temperatures.

Micrographs of the films were performed with a Cambridge Stereoscan 360 scanning electron microscope withkV SEM acceleration and a 150 pA probe current. Unpolized Raman spectra were recordedex situ, at room tempera-ture in backscattering geometry, using an I.S.A. Jobin–Yvtriple-grating spectrometer with a liquid nitrogen coolecamera detection~CCD! system. The spectral resolution waabout 3 cm21; the 514.5 nm line of an Ar ion laser was useas excitation source; power on the sample was about 3 m

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Reflectance spectra at near normal incidence and at rtemperature were measured by a spectrophotometer Vamod. Cary 5E is utilized over the spectral range of 0.4–6An Al mirror, whose absolute reflectivity was previously anindependently measured, was used as reference.

Ellipsometric functions tanc and cosD were measuredbetween 1.5 and 5 eV with a spectroscopic ellipsometerpra mod. Moss ES4G was equipped with a microspot systThe angle of incidence was 65°, close to the Brewster anfor optimum sensitivity. Spectral resolution was better thameV over the whole spectral range. We recall that ellipsoetry measures the complex ratior of the parallel to perpendicular polarization reflection coefficients, i.e.,r5 r s / r p

5tanceiD. In order to determine the complex refractive idex n5n1 ik of the film, wheren is the usual refractiveindex andk is the extinction coefficient, we inverted thtanc and cosD spectra employing a three-phase opticmodel~air-film substrate!. Then andk values of silicon weretaken from literature; the film thicknessd was initially takenfrom SEM measurements and then adjusted to optimizeinversion algorithm.

III. RESULTS AND DISCUSSION

Figure 3~a! shows a low-magnification micrograph offilm deposited at room temperature. A section of the filmshown at different magnifications in Figs. 3~b! and 3~c!. Aspongelike structure is visible, formed by aggregates wdimensions ranging from several hundreds of nanomedown to 20 nm, randomly stacked. This visual inspectsuggests a porous structure with a possible self-similar onization. The softness of the material17 prevented a detailedinvestigation of the morphology of the smallest grains sinthey vibrate under the high electron current necessary totain high magnification.

Figure 4 shows the Raman spectrum of a film deposat room temperature. In the low frequency range, we obsea broad hump extending from roughly 650 to 800 cm21 andpeaked at about 740 cm21.

The region between 1000 and 1800 cm21 presents tworesolved peaks at about 1584 and 1363 cm21, superimposedto a broad structure. The two peaks can be identified as thGpeak of crystalline graphite arising from zone-centerE2g

mode, and theD peak assigned to anA1g zone-edge phononactivated by the disorder due to finite crystalline size.18,19

The Raman signal in this region can be fitted with four gasians. If all 13 fitting parameters (121background) are al-lowed to vary, the procedure is unstable and, in most cathe outcome is the overlapping of gaussians in pairs, givexcellent fits but without any physical significance. The bsolution is obtained fixing the shifts of two features at 11and 1490 cm21 and allowing all other unknowns to var~Fig. 5!. The above shifts were chosen because, whenunconstrained fit was not giving overlapping results, thproduced two small features around those positions.

The region between 2000 and 3400 cm21 is character-ized by second order Raman scattering.18 The spectrum dis-plays a faint bump around 2000 cm21 and a broad structurefeature between 2200 and 3400 cm21, with a peak around2700 cm21.

J. Appl. Phys., Vol. 82, No. 11, 1 December 1997


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ey The interpretation of the Raman features can be relato the mass distribution of the precursors and the historythe precursors after the deposition. The degree of fragmtation and coalescence of the cluster will determine the fistructure and morphology of the film. As a starting point, wcan assume that our clusters substantially retain on thestrate of their individuality, due to the low energy depositi

FIG. 3. SEM micrographs of a nanocrystalline carbon thin film.~a! Low-magnification micrograph of the surface.~b! and~c! high magnification viewof a section of the film: The granular structure is clearly visible. Grainssmall as roughly 20 nm can be seen.

5795Milani et al.


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~less than 1 eV per atom for medium size clusters!. Thissuggests that the films should be composed of a ranstacking of clusters eventually linked, as proposed in Refand 12. Although the SEM analysis confirms the granunature of the film and the presence of a ‘‘memory effect,4

the observation of structures of several tens nanometediameter, suggests that the original clusters may stronglyteract and coalesce. It should be reminded that severalmers are present in the cluster beam with different structuand reactivities.20 Only particular fullerenes~C60, C70! havea high stability; other cage structures, especially in excistates, are fragile and they can react to each other. Theexistence of different types of clusters in the beam cancount for the formation of a low density material composby primeval carbon clusters embedded in a connectingdium formed by the coalescence of small fragments andefected large clusters. Moreover, in our case, due tocluster mass range used for deposition, it is reasonablexpect a film with asp2 character, with the presence odeformed rings and only a small number ofsp3 bonds.

The predominantsp2 character of the film is confirmedby Raman spectra. The overall spectra, and in particularintensities and the shapes ofG and D bands are typical of

FIG. 4. Raman spectrum of a film deposited at room temperature. The speaks at 520 and 970 cm21 are due to the silicon substrate.

FIG. 5. Raman spectrum in the region between 1000 and 2000 cm21 whichis expected to consist in the sum of four lines. Four gaussians haveused to fit this spectrum.



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sp2 disordered carbon films.18,21The low-frequency region isalso characterized by features which can be attributed to disordered sp2 bonds, as reported for different types ofcarbon.22 The low-frequency region of sputtereda-C is char-acterized by a broad peak at 760 cm21 which has been re-lated to a peak in the phonon density of states of graphite.23

Disorder induced Raman scattering was also observed in thregion by Li and Lannin ina-C and nanocrystalline glassycarbon.22 Laser-deposited amorphous carbon shows similastructures peaked at different energies~500– 600 cm21,800 cm21!.24 Although the origin of these structures is still amatter of debate, the disorder of asp2 carbon network ismost likely responsible for the observed spectra.

A moderate annealing of the samples induces changes ithe Raman spectra. In Fig. 6, we report the evolution of theRaman spectrum for thermal treatmentin situ up to 550 K.The resulting trends, as functions of annealing temperatur~AT!, are as follows. The center ofD line shows an overallsmall decrease~from 1362 to 1354 cm21! and its full widthat half-maximum~FWHM! lessens. While the FWHM of theG band decreases as well, the peak position is nearly constant around 1585 cm21. The intensity ratioI D /I G grows

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FIG. 6. Evolution of the Raman spectrum as a function ofin situ annealingof the samples. The bottom spectrum is taken from the as-deposited samplTemperature increases from the bottom spectrum to the top one.~T5410,470, and 520 K, respectively!. Even for moderate thermal treatment a pro-gressive graphitization can be recognized.

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from 0.75 to 0.82 while FWHMG /FWHMD decreases from0.6 to 0.5. The 2700 cm21 peak becomes more definite. Wobserve a slight growth inI 1490/I T ~I T5total first order in-tensity! with AT. These trends are consistent with a progrsive graphitization and a removal of distorted bond angand structural disorder.24–26 This picture is supported by thtrend of FWHMG /FWHMD , related to disorder within graphitic bonding and variation in bond angle, which we finddecrease.

The feature around 1180 cm21 deserves particular attention since it has been suggested as related to thesp3 contentin a-C films.27 A similar structure has also been reportedfilms grown with cluster beam deposition in a lower marange compared to ours, although with a less characterbeam.4,12 Perez and co-workers have deconvoluted theman spectra in the 1000– 2000 cm21 range with three gaussians only,12 and they attributed the 1180 cm21 peak to thepresence of a microcrystalline or ‘‘amorphous’’ diamophase originated by the linking of very small cage-like cluters (C28).

12

The presence of this amorphous diamond phase in cter assembled films needs further experimental confirtions. In our samples, the presence of an amorphous diamphase is unlikely since small cagelike clusters are eventuproduced and deposited together with graphitic clusters,preventing the formation of asp3 network. It has also beenshown28 that a model with a small percentage ofsp3 cangive density of states~DOS! contributions in this spectraregion, without requiring the presence of an amorphoussp3

phase, which would also probably shift the observedG peakto much lower frequencies. Li and Lannin22 reported a dis-tinct reproducible shoulder in the tail of theD band in glassycarbon which they assigned to a peak at about 1170 cm21 inthe corresponding DOS determined by inelastic neutron stering. Moreover, the observed bump at about 2000 cm21

may be seen as a second order combination of 1180750 cm21 bands, thus indicating that these two modes ocin the same region of the sample which should be graphlike rather than diamond-like.

Recent quantum molecular dynamics simulations ofC28

deposition have shown that small fullerenes can act as buing blocks of materials characterized by hyperdiamondlconfigurations, with only a small portion ofsp3 sites.10 Vi-brational spectra ofC28 cages suggest that the Raman fetures around 600 and 1200 cm21 can originate from the presence of distortedsp2 bonds ~nonhexagonal rings! withoutinvoking diamond phases. Our experiments, although pformed with cluster beams in a wide mass range, areagreement with the calculations, showing that the Ramfeatures are essentially due to disorderedsp2 bonds.

The graphitic nature of the cluster-assembled filmsalso confirmed by optical spectroscopy. The results ofellipsometric and reflectance,R, measurements for a 100 nmthick film are shown in Figs. 7~a! and 7~b!, respectively.Films with different thicknesses display similar spectral fetures and lineshapes; the main difference is seen in the ption of the reflectance minimum, varying from 2.3 to 2.8 eWe note that the reflectance calculated by usingn and kvalues from ellipsometry agrees with the experimentalR

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spectrum. This shows two peaks at 3.4 and 4.4 eV, cosponding to theE1 and E2 interband transitions of siliconscreened by the film, which strongly reduces their intensitThe minimum at about 2.7 eV, where the film is absorbilittle, is due to interference between the film and the sustrate, and it is strongly dependent on the film thickness.

The k spectrum displays a broad peak at 4.3 eV, thedecreases rapidly at lower energies, with an abrupt slchange around 3.2 eV. In correspondence with the peak ik,n shows a dispersion-like feature, followed by a slight icrease with decreasing energy. Then andk behavior~apartfrom the absolute values! resembles that of the optical functions of the graphite, for light polarized in the basal planIn fact, the graphite in this spectral range is characterizeda strong peak ink at about 4.7 eV, due to the optical gaptheQ point of the Brillouin zone between the bondingp andantibondingp* electron bands, originated by thepz orbitalsperpendicular to the basal planes.29

It is expected that in a graphite-like film the structurand compositional disorder causes the shrinkage ofp-p* gap; moreover, a new gap at lower energy canopened, due to the localization of thep electrons, destroyingthe semimetallic behavior of graphite.30,31 These effects arejust evidenced in our optical spectra. In particular, the tailk below 3 eV has a dependence on the energyE typical ofthe Tauc model,32 for which k}(E2ET)2/E2, yielding anoptical gap ofET50.6 eV.

We note that ourn andk absolute values are much lowethan in graphite; this fact can be ascribed to the porous stture of the films, producing a density about half of graphiWe calculated the optical functions of a graphite film wi0.5 volume fraction of voids, in the framework of th

FIG. 7. ~a! Refractive index (n) and extinction coefficient (k) of a carbonthin film ~solid lines!, as measured by ellipsometry; for comparisonn andkcalculated for a graphite thin film with 0.5 volume fraction of voids~dottedlines! are reported.~b! Reflectance of a carbon thin film on a silicon sustrate.

5797Milani et al.


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Bruggeman effective-medium approximation,33 obtainingvalues ofn and k comparable with the experimental on@Fig. 7~a!, dotted lines#.

In conclusion, we have deposited and characterinanocrystalline carbon thin films from supersonic clusbeams. The films are characterized by a low-density, noporous structure based on grains with diameters of sevtens of nanometers. Raman spectra show that the filmsessentially graphitic-like with the presence of a high numof distortedsp2 bonds. The optical spectra confirm the grphitic structure and show the presence of a gap of 0.6The granularity of the films may have important consquences on the electronic and mechanical properties of tmaterials. Localization effects due to the granular structof the films are currently being investigated. Moreover,large surface area of these materials can represent anesting aspect for many technological issues.34 We have alsodemonstrated that the deposition of cluster beams produby PACIS sources can be considered as an efficient rtowards the production of nanostructured thin films.

ACKNOWLEDGMENTS

We wish to thank G. Galli for providing results prior tpublication and for many insightful discussions, and M. Scrotti and M. Bernasconi for useful comments. The financsupport of INFN and of CNR under project FILINCLUBE igratefully acknowledged.

1R. W. Siegel, Mater. Sci. Eng. B19, 37 ~1993!.2H. P. Cheng and U. Landman, Science260, 1304~1993!.3H. Gleiter, Prog. Mater. Sci.33, 223 ~1989!.4P. Melinon, V. Paillard, V. Dupuis, A. Perez, P. Jensen, A. Hoareau, JPerez, J. Tuaillon, M. Broyer, J. L. Vialle, M. Pellarin, B. Baguenard, aJ. Lerme, Int. J. Mod. Phys. B9, 339 ~1995!.

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synthesis and characterization of cluster-assembled carbon ... · centered at 350 atoms/clusters!...

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