raman spectroscopy of cvd carbon thin films excited by near-infrared light

Raman Spectroscopy of CVD Carbon ThinFilms Excited by Near-Infrared Light

Margit Koos, Miklos Veres, Sara Toth, and Miklos Fule

Research Institute for Solid State Physics and Optics of the Hungarian Academyof Sciences, Konkoly Thege M.u. 29-33, H-1121 Budapest, [email protected]

Abstract. The interpretation of the Raman spectra of amorphous carbon thinlms is still controversial. The concept of their decomposition into D and G peaksdoes not work in some cases, when presence of additional bands can be deducedfrom the shape of the spectra. One to investigate these extra component bands is tochange the excitation wavelength. This results in the enhancement of the Ramanscattering cross-section of the dierent structural units. Our investigations wereaimed to detect the component bands become intense by using infrared excitation(785 nm) in the Raman spectra of amorphous carbon thin lms prepared frombenzene and methane in a wide range of deposition parameters. By comparing thevisible and infrared excited Raman spectra it will be proven that the well-knownG band of the a-C:H layers consists of two components, one of which exhibitingno dispersion. It will be also shown that he infrared excitation makes distinguish-able Raman bands assigned to delocalized () electronic structure due to resonantenhancement of the scattering.

1 Introduction

1.1 The Raman Eect

When monochromatic light of frequency L scatters in a medium, scatteredlight intensities shifted from L by certain i values can be observed in thespectrum of the scattered light. The value of these shifts depends on theproperties of the scattering media and it does not vary when changing L.This phenomenon is the Raman scattering. During a Raman experimentthe scattered light is measured, and it is represented in relative wavenumbers,taking the laser wavenumber being equal to zero.

Raman scattering is an inelastic scattering of light on elementary exci-tations of the medium. The elementary excitations are usually rotational orvibrational transitions of a molecule or lattice vibrations (phonons) in a solid.The inelastic scattering is a two-phonon process, where the absorption of anincoming photon of energy EL = L with energy transfer and the creationof an S scattered photon take place simultaneously in the medium. Theenergy dierence is equal to the energy of the elementary excitation.If the energy transfer is positive (L S) the process is called Stokesscattering, the opposite case (L S) is known as anti-Stokes scattering.G. Messina, S. Santangelo (Eds.): Carbon, The Future Material for Advanced Technology Ap-plications, Topics Appl. Phys. 100, 423445 (2006) Springer-Verlag Berlin Heidelberg 2006

424 Margit Koos, Miklos Veres, Sara Toth, and Miklos Fule

The E = E0 cos (Lt) electromagnetic eld of frequency L acting on amedium induces a dipole moment in determined by the polarizability of themedium:

in = E = E0 cos (Lt). (1)

A molecule or an elementary cell consisting of N atoms has 3N degrees offreedom, 3 of which are transitional, 3 (2 for a linear system) are rotationaland 3N 6 (3N 5 for a linear system) are vibrational ones. 3N Cartesiancoordinates are needed to describe the motion of such a system. It is suitableto use internal coordinates, describing the variations of bond lengths andbond angles during vibrations. Combining these coordinates, the orthogonalvibrational normal modes of the molecule (cell) can be described. The ijcomponents of the polarizability tensor can be expressed through the normalcoordinates of the system:

ij = (ij)0 +k

(ijQk

)0

Qk +12

k,k

(2ij

QkQk

)QkQ

k + . (2)

Restricting (2) to rst-order terms, combining (1) with (2) and applyingtrigonometric identities, we get:

in,ij = (ij)0 E0 cos (Lt)

+12

k

(ijQk

)0

QkE0

{cos [(L + )t + k] + cos [(L )t k]} . (3)

From (3) it can be seen that during a scattering process a medium willemit photons at frequencies L, L+ and L corresponding to Rayleigh,anti-Stokes and Stokes scattering, respectively. While Raleigh scattering doesalways exist, Raman scattering occurs only if the polarizability of the mediumchanges during the scattering process. The rst-order terms of the polariz-ability tensor represent the components of the Raman tensor R.

Energy and momentum are conserved in the Raman process:

S = L kS = kL + kph, (4)

where kL, kS and kph are the momenta of the incoming and scattered photonsand of the phonon, respectively. According to (4), the energy loss of thephoton equals the energy of the lattice vibration. Considering that, for thevisible light the 104 cm1 magnitude of kL and kS is much less than thesize of the rst Brillouin zone of the crystals (about 1010 cm1), it can beconcluded that only zone centre phonons participate in the scattering.

Raman Spectroscopy of CVD Carbon Thin Films 425

In the classical picture described above, the mechanism of the scatteringdoes not matter. The quantum mechanical treatment of the process considersthe electronphoton and electronphonon interactions through their Hamilto-nians HeR, and Heph, ( and being the polarizations of the incomingand scattered photons, respectively). The core of the quantum mechanicaltreatment of the Raman scattering is the transition matrix element K2f,10:

K2f,10 =a,b

S,f ,i|HeR,|0 ,f ,b0,f,b|Heph|0,0,a0,0,a|HeR, |L,0,i(ELEeaii)(ESEebii)

, (5)

where |L, 0, i is the initial state characterized by the incoming photon ofEL = L energy, by the phonon in 0 state (no excited phonon) and by theelectron in ground state i. The nal state S, f , i | is characterized by thephoton of ES=S energy, by the phonon in the f state and by the electronin the ground state; a and b are the intermediate states participating in thescattering process. Eeai and E

ebi are the energy dierences of states a and i as

well as b and i, respectively; is the lifetime of the excited states. Accordingto (5) the Raman scattering is a three-step process consisting of:

(a) absorption of the incoming photon with transition of the electron into anexcited state (creation of an electronhole pair)

(b) inelastic scattering of the electron on a phonon(c) recombination of the electron and the hole with emission of the scattered

photon

The three steps take place simultaneously. The electron can be excited fromthe ground state either into a virtual state or into an existing state. In thelatter case, the scattering is resonant.

The K2f,10 matrix element is related to the Raman tensor (R) throughthe polarization vectors of the incoming (eL) and scattered (eS) light:

K2f,10 = eL R eS .

The scattered intensity is proportional to the square of the transition matrixelement:

I |K2f,10|2 .

It is dicult to calculate the intensity of the process since the transitionmatrix elements are dicult to determined.

The number of lines observed in the Raman spectra is less than the num-ber of phonons existing in the crystal. This is not only due to the degeneracyof the phonons, but it is because of the number of phonons which can partic-ipate in the scattering is limited by the selection rules. The rst restrictionis that the scattering takes place only on zone-centre phonons. Additionally,during the scattering the polarizability of the medium should change, which


requires the phonons to have certain symmetries. The simplest way to deter-mine whether a given phonon will cause a nonzero polarizability change is totake into account group theory considerations.

When the energy of the incoming or outgoing photon is close to an existingenergy transition of the medium, a signicant increase in the Raman intensitycan be observed due to resonant enhancement of the scattering. Dependingon whether the resonance is for the incoming or scattered light, one candistinguish between incoming and outgoing resonances.

In noncrystalline materials there is no elementary cell and the periodicityis lost. This leads to breakdown of the selection rules. As a result, in these ma-terials any phonon can participate in the scattering. However, the vibrationswill not be extended in the whole crystal, instead they will be localized. TheRaman spectrum is obtained by summing all of these localized vibrations [1]:

I () = (L )4

b

Cb

(1

)[1 + n (, T )] gb () , (6)

where Cb is the coupling coecient of vibrational transitions, gb is the den-sity of vibrational states, n (, T ) is the BoseEinstein distribution function.From (6) it can be seen that the Raman intensity is determined by the densityof vibrational states.

1.2 Raman Spectra of Carbon Materials

The basic electronic conguration of a stand-alone carbon atom diers fromthat during bonding. In the latter case, the electronic orbitals of carboncombine, forming hybridized orbitals: sp3, sp2 and sp hybridization states areknown for carbon atoms, which form three-dimensional, layered and chain-like structures like diamond, graphite and carbyne. In disordered carbons,atoms of dierent hybridization are mixed. Besides, hydrogen can also bepresent in the structure. Because of the large variety of atomic arrangementsin carbon materials, their properties, both crystalline and amorphous, varyin a wide range.

Among the several dierent models proposed for the a-C:H [2, 3], nowa-days the cluster model [4, 5] has found wide acceptance since that best de-scribes the electronic structure of amorphous carbons. Most of the experi-mental data obtained for a-C:H lms were successfully explained with thismodel. The cluster model is based on considerations of the Huckel approx-imation, treating the and states separately [4]. It was found that the bonding of carbon atoms favours a clustering of sp2 sites predominanatlyinto planar structures formed by sixfold rings. As a result, in the amorphouscarbon matrix there are sp2 clusters of dierent sizes surrounded by a matrixof sp3 carbon atoms. It should be noted that the cluster model was reformu-lated [6]. According to it, in clusters, the sp2 hybridized carbon atoms can be


arranged, besides into rings, into chains too [7, 8]. The energy of the statesof the cluster strongly depends on the size and on the level of conjugation ofthe cluster. The higher the cluster size, the lower the gap between the valence and conduction states. The energy of the states is much higher thanthat of ones.

Raman spectroscopy has been proved to be a very suitable method forstructural characterization of carbon materials [9, 10, 11, 12, 13, 14, 15]. Themethod is highly sensitive to changes in the bonding conguration of carbonatoms, especially to that of sp2 hybridized ones, since the states of thesp2 clusters result in narrow band gaps, which cause the Raman scatteringprocess to be resonantly enhanced.

The Raman spectrum of diamond consists of one zone-centre mode at1332 cm1 having T2g symmetry. In the spectrum of single crystal graphite,a peak can be observed at 1580 cm1 corresponding to zone-centred E2gmode, usually labelled as G (graphitic) band, assigned to CC stretchingvibrations of the atoms in hexagonal rings of the graphene sheet. Besides,an interplanar E2g stretching vibration mode is also present at 42 cm1 [16].In nonperfect or microcrystalline graphite, another band of A1g symme-try, called the D (disordered) peak [17], corresponding to breathing vibra-tions of the hexagonal rings at the grain boundaries, appears at 1350 cm1.The D band is a result of a double resonant scattering process activated bythe defects of the crystal [18]. There are other bands of small intensity ob-served in the Raman spectra of graphite [19], as well as intensive second- andhigher-order peaks above 2000 cm1 [20], but these are not relevant for theinterpretation of the spectra of amorphous carbons. Due to the specic bandstructure of graphite, where the conduction and valence bands cross at theK point but a forbidden gap exists in other points of the Brillouin zone, theRaman scattering in graphite is always resonant. On the contrary, the bandgap of diamond is around 5 eV, hence for visible excitation the intensity ofits 1332 cm1 mode is about one ftieth that of the G peak of graphite [21].

The Raman spectra of amorphous carbons consist of a broad band in the10001700 cm1 range. This composite band is usually decomposed into twopeaks centred around 1350 and 1580 cm1, which are near the position ofD and G bands of microcrystalline or nonperfect graphite. This similaritywas the origin of the labelling and rst explanation of the Raman spectraof amorphous carbons [9]. The D band of amorphous carbon materials isassigned to breathing vibrations of rings or ring-like structures; the stretchingmodes of these species give rise to the scattering in the G band region [14].However, besides the stretching vibrations of carbon atoms arranged in rings,those forming chains also contribute to the G band [13]. The parameters ofthe two bands (position, width and their intensity ratio) are used for thecharacterization of carbon-containing materials [10, 11].

The shape of the Raman spectrum is considered to depend on the sp2/sp3

ratio, clustering of the sp2 hybridized carbon atoms and arrangement of these


atoms in the clusters [15]. The position of the G band indicates the type ofarrangement of carbon atoms in the clusters: if its value is around 1580 cm1,the clusters have graphitic character, while lower positions indicate a dier-ent arrangement. The peak width is the measure of structural ordering: thenarrower the peak, the more ordered the structure. Another parameter usedfor a-C:H thin layers characterization is the intensity ratio of D and G bands.The ID/IG ratio was found to be related to the size of the sp2 clusters in theamorphous carbon structure. It is assumed that for microcrystalline graphitethe ID/IG ratio is inversely proportional to the crystallite size LA [22]:

IDIG

=c

LA. (7)

Experimental measurements performed on ta-C thin lms showed that foramorphous carbons having sp2 clusters below 2 nm size the ratio is pro-portional to the square of the cluster size LC [14, 23]:

IDIG

= cLC2. (8)

The constants c and c in (7) and (8) were found to be dependent on theexcitation energy. The ID/IG ratio is the measure of ordering of the a-C:Hstructure. The higher the intensity ratio, the higher the cluster size, thus thehigher the ordering of the structure.

Raman spectroscopic measurements with dierent excitation wavelengthscan furnish additional information on the a-C:H structure [10, 24, 25, 26, 27].The bonding sites having sp2 carbon atoms are generally arranged into clus-ters of dierent sizes, which exhibit band gap depending on cluster size. Fordierent laser energies, EL, the conditions of resonant Raman scattering willbe fullled for dierent clusters, whose band gaps are equal to that. How-ever, the vibrational frequency depends on the cluster size: the higher thesize, the lower the frequency of the vibrations [28]. Additionally, when usinglasers in the UV region, the sp3 sites can be resonantly excited, thus theirbonding conguration can also be examined. A characteristic feature of theD and G bands in the Raman spectra of amorphous carbon is their disper-sion, the shift of the peak position when changing the excitation energy. Thedispersion was also observed for the D band of graphite [29]. It was foundthat for dierent amorphous carbons (hydrogenated and nonhydrogenated)the rate of the shift varies in a wide range [15]. For a-C:H thin lms (poly-meric and diamond-like) the position of both D and G bands shifts to higherwavenumbers with the increase of EL [15, 24, 30].

It was found that in graphite the Raman scattering cross-section isstrongly enhanced for the phonons having wavevector kph equal to thewavevector k of the electronic transition excited by the incident photon [29].For diverse EL, this condition is fullled for phonons at dierent distancesfrom the K point of the zone boundary, hence the energy of the phonon par-ticipating in the scattering also varies with EL [18]. In a-C:H the dispersion


of the bands is dierently explained. Since there is no Brillouin zone, the se-lection rules have no meaning. As noted earlier, when changing the excitationenergy, dierent clusters will participate in resonant scattering. The band gapof the cluster is inversely proportional to its size. Thus, the higher the EL,the lower the mean size of the clusters involved in the resonant scattering.The lower the cluster size, the higher the frequency of its vibrations.

Up to now, systematic Raman investigations on amorphous carbon thinlms of dierent types were performed mainly with excitations in the visibleand UV region [11,12,14,15]. The aim of the increase of the probe energy wasthat the sp3 hybridized carbon sites, having higher band gap, could be excitedtoo. However, the resonant enhancement of Raman scattering from graphiticstructure or/and materials containing several sp2 bonded sites arranged innanoclusters at low-energy excitation can provide additional information onthe structure of the a-C:H lms. In the following, results of Raman spectro-scopic studies, performed with visible and infrared excitations, on a seriesof a-C:H samples prepared by radio-frequency chemical vapour deposition atdierent self-bias voltages, from benzene and methane, will be presented indetail.

2 Infrared Excited Raman Spectroscopyof Amorphous Carbon Thin Films

2.1 a-C:H Thin Films Prepared from Benzene

The a-C:H thin lm samples were deposited onto Si substrates by radio-frequency (2.54 MHz) chemical vapour deposition (CVD) method [31] frombenzene at dierent chamber pressures (820 Pa) and self-bias voltages (10700 V). The electrode with the substrate had negative self-bias potential,however, for simplicity, in the following only the magnitude of the self-biasvoltage is given without the sign. Raman spectroscopic measurementswere carried out on the samples using a Renishaw 1000 Raman spectrometerattached to a microscope. A 488 nm (2.54 eV) line of an Ar ion laser and a785 nm (1.58 eV) diode laser served as excitation sources. The 100 objectivefocused the excitation beam to a spot having diameter of 1m. Baselinecorrection on the measured spectra was performed by tting their baseline toa Gaussian function and subtracting the tted data from the experimentallymeasured ones. Raman spectra of a-C:H thin layers measured at 488 nmof probe wavelength are shown in Fig. 1. The data are normalized on theirmaxima. The change of the shape of the spectra with self-bias well reects theevolution of the amorphous carbon structure. In the spectrum of the sampleprepared at 10 V the G peak is located around 1600 cm1. With the increaseof self-bias up to 300 V, it shifts to lower wavenumbers and, above this voltage,the peak positions move into opposite direction. The intensity of the D bandalso increases with self-bias. The spectra of the samples prepared at low


Fig. 1. Raman spectra excited by 488 nm of a-C:H thin lms prepared from benzeneat pressure of 18 Pa and dierent self-bias voltages

deposition voltages have bad signal/noise ratios due to the high intensity ofthe luminescence background. The weak peak around 1000 cm1, observablein some spectra, arises from the Si substrate.

In the 10001700 cm1 wavenumber region the spectra were tted to twoGaussians. The dependence of the position and peak width of the G and Dbands on the self-bias voltage is shown in Fig. 2, where data obtained for aseries of samples prepared at 8 Pa are also provided. It can be seen that thereis a minimum in the position of the G peak around 300 V, where diamond-likelayers are formed, while the D band has the lowest location around 100200V.Above 500 V, the G peak position increases and approaches the 1580 cm1

peak of graphite. The peak width decreases, implying that some kind ofordering takes place in the a-C:H structure. In this region, the graphitizationof the structure begins, and above 600 V of self-bias, the structure can beconsidered as graphitic a-C:H.

Similar behaviour is observable for the G peak parameters at low voltages,where polymeric a-C:H layers are formed. For the samples prepared below30 V, it reaches the position of 1600 cm1, well above the G peak position


Fig. 2. Dependence of (a) the peak position and (b) the peak width on self-biasvoltage of the D and G bands of a-C:H thin lms prepared from benzene at pressuresof 8 and 18 Pa

of graphite, suggesting that the G peak in the spectra of these samples doesnot arise from graphitic domains. The band narrowing suggests the orderingof the structure.

In Fig. 3, it can be seen that the ID/IG ratio has a minimal value in thediamond-like a-C:H lms, around 300 V self-bias. As we discussed before, theD band can be attributed to scattering of breathing modes rings or ring-likestructure formed by sp2-hybridized carbon atoms, while stretching modes ofboth rings and chains formed by sp2 sites contribute to scattering in the G


Fig. 3. Dependence of intensity ratio of the D and G bands of the Raman spectraof a-C:H lm prepared from benzene at pressures of 8 and 18 Pa on self-bias voltage

band region. Hence, the ID/IG ratio indicates the ratio of the amount of (sp2)ring-like structural units to the total amount of (sp2) rings and chains. Theincrease of the ID/IG ratio is due to the presence of benzene rings or of theirsubstituted forms in the structure, below 300 V self-bias. At the same time,the formation of graphitic structure, at large deposition voltages, explainsthe increase of the ID/IG ratio in that region. This explanation is in goodaccordance with the change of the G peak position with self-bias. Above andbelow that region, the a-C:H structure becomes more ordered. On strengthof the analysis of the G band behaviour, one can conclude that this orderingin polymeric and graphitic layers has dierent origin. This is supported byRaman spectroscopic investigations performed with infrared excitation.

In several works [13, 32, 33, 34], it is noted that the use of two peaks forthe decomposition of the Raman spectra of a-C:H thin lms is inaccurate,so the tted curve diers from the experimental one. This suggests that notonly structural units, vibrating at frequencies of G and D peaks, contributeto the spectra. However, since it is dicult to determine the parameters ofthe extra peaks, the attempts made for their assignment have controversialresults [13, 15, 33]. One way to obtain additional information on these bandsis to change the excitation wavelength, since the dispersion of the peaks canbe dierent.

Earlier investigations showed that the Raman scattering spectra of theamorphous carbon materials excited in the infrared light region can giveadditional information on the bonding structure [35]. Raman spectra of thesamples measured by 785 nm excitation are shown in Fig. 4. The region


Fig. 4. Raman spectra excited by 785 nm of a-C:H thin lms prepared from benzeneat pressure of 18 Pa and dierent self-bias voltage USB

around 1000 cm1 was cut out since, due to the resonance enhancement, theRaman scattering intensity arising from the Si substrate is several orders ofmagnitude higher than that of the a-C:H lms. By comparing Figs. 1 and 4,it can be concluded that there are signicant dierences in the spectra ofthe same samples, especially at low self-bias voltages. While broad bands arecharacteristic of the 488 nm excited spectra, sharp peaks, at 1200, 1300, 1450and 1600 cm1 wavenumbers, can be found in the infrared excited ones ofthese lms. Besides the 10001600 cm1 region, a broad band can also beseen around 1800 cm1. The increase of the self-bias results in broadeningof the bands and shifting of their positions. At 120 V self-bias new bandsappear in the spectra around 1200 cm1.

The spectra were decomposed by sets of Gaussian and Lorentzian curves.It is convenient to analyse the narrow bands in the spectra of polymeric lmsseparately from broad bands.

Figure 5 compares the Raman spectrum of the a-C:H lm prepared at10 V self-bias with that of the benzene. Additionally, the spectrum of thepart of the lm detached from the substrate is also provided. It can be seen


Fig. 5. Comparison of the Raman spectra excited by 785 nm of (a) benzene,(b) detached from the substrate and (c) as-prepared a-C:H thin lm depositedfrom benzene at pressure of 18 Pa and 10 V of self-bias

that bands at 1190 and 1607 cm1 in the spectra of the a-C:H lm arisefrom the presence of benzene rings in the structure. Another benzene peakat 1007 cm1, masked by the band of Si substrate, is observable only in thespectra of the detached lm. The dierences in the band positions can beexplained considering that there are not separate benzene rings, but substi-tuted ones present in the structure. The substitution changes the symmetryof the molecule, thus forbidden Raman bands appear in the spectra too.Peaks at 1037, 1160, 1288 and 1448 cm1 are related to substituted benzene,whose presence was supported by infrared transmission measurements [36](not presented here).

It is accepted that the sp3 carbon atoms contribute to the Raman spectrawhen it is excited in the UV region. Contrarily to this, information on thebonding conguration of sp3 hybridized carbon atoms can also be deducedfrom the infrared excited spectra of the a-C:H lms prepared at low self-bias voltages. The small peak around 1382 cm1 is presumably related tosymmetric CH deformation vibrations of sp3 CH3 groups. This mode usuallyhas low Raman intensity, but not in the case of sp3 CH3 group attachedto carbon atoms having double or triple bonds or to a benzene ring. Theasymmetric pair of the vibration is around 1450 cm1, overlapped by the1448 cm1 mode of benzene. The characteristic CH mode of sp3 CH2 groupcontributes to the spectra at 1300 cm1. The evolution of these bands withself-bias voltage (Fig. 4) evidences the intact benzene rings are present in thespectra of the a-C:H lms up to 80 V.

The decomposition shows that, as a background of these narrow peaks,there are two broad bands, observable in the spectra of the 10 V sample,centred at 1378 and 1599 cm1. Their position is similar to that of the D and


G bands in the spectrum excited by 488 nm (Fig. 2). The G band position(1599 cm1) is higher than that in graphite (1580 cm1). It is close to the1607 cm1 peak of the substituted benzene. This suggests that the sp2 carbonatoms in the amorphous carbon matrix have an arrangement close to thatthey have in a benzene molecule. Presumably, the amorphous phase of thesample contains a high amount of distorted benzene rings.

As the self-bias increases, the shape of the spectra changes signicantly. Inthe spectra of the 80 and 120 V samples, in the 10001700 cm1 wavenumberrange, four broad bands are observable, centred around 1140, 1260, 1430 and1566 cm1. The close position of the 1566 cm1 peak to the G peak in the488 nm excited Raman spectrum of the sample (at 1560 cm1) suggests thatthis band is the G one, too. However, we must take into account that theD and G bands of a-C:H have dispersion. By estimating from the dispersionrates up to 785 nm of excitation, the D band of a-C:H lms has to be locatedaround 1250 cm1 and the G one is around 1430 cm1. By comparing thesevalues with the data obtained for the 120 V sample, it can be assumed thatthe 1260 and 1430 cm1 peaks of that correspond to the D and G peaks.So one gets the conclusion that there are two bands in the infrared excitedspectrum that correspond to the G band: one, at lower wavenumbers, around1430 cm1, showing dispersion, and the second, around 1566 cm1, havingno dispersion.

In Fig. 6 the dependence of the positions and widths of the broad bands,found in the 785 nm excited Raman spectra, on the self-bias voltage areshown. In the spectrum of the 10 V sample, the band showing no dispersion(G(nd) in the following) is located at 1599 cm1. The increasing self-bias causes the shift of the band position to lower wavenumbers. It reaches1570 cm1 at 120 V and does not vary signicantly above that voltage value.The band width is almost the same in the whole self-bias range. The positionof the G peak that shows dispersion (G(d) in the following) uctuatesaround 1430 cm1 and broadens with the increasing self-bias.

The position of the D band located at 1378 cm1 in the spectrum ofthe 10 V lm decreases rapidly with the increase of the self-bias, down to1230 cm1 at 200 V, to the position expected with considering the disper-sion. Above that voltage the band position does not change remarkably upto 500V and increases above that value. As it was noted earlier, the disper-sion of the bands in amorphous carbons is due to the size distribution of theclusters having dierent band gaps. If one goes into the details of the phenom-enon, which was observed also in hydrocarbon polymers built of conjugatedchains (chains consisting of sp2 carbon atoms connected with alternatingsingle and double bonds), it turns out that it is not the size that determinesthe band gap of a cluster, but the delocalization of the electrons in it. Thecluster size is only the upper limit for the delocalization length. The levelof delocalization is determined by the lengths of conjugated regions, whichdepend on the bonding conguration of the sp2 carbon atoms. Several fac-


Fig. 6. Dependence of (a) the position and (b) width on self-bias voltage of thebroad bands found in the Raman spectra excited by 785 nm of a-C:H thin lmsprepared from benzene

tors can aect on the delocalization length of a conjugated chain, includingkinks, breaks in the periodicity of the alternating bonds and attachment ofside-chains. It is known that the benzene molecule has an electronic congu-ration such that its electrons are delocalized only within the ring, but notout of it.

From the absence of the G(d) band and the downshift of position of the Dpeak up to 80 V self-bias, it can be concluded that the increasing deposition


voltage causes the increase of the extent of electron delocalization in sp2

clusters. The behaviour of the G(nd) band indicates that the atomic arrange-ment in the clusters of a-C:H matrix transforms. In the layers prepared atlow self-biases, mainly distorted benzene rings form the cluster, while, withthe increase of the voltage, the ratio of carbon atoms arranged into chainsrises, as well as the delocalization length characteristic for the clusters. Theamount of the intact benzene rings also decreases with the increasing self-biasfrom 10 V, and above 120 V they are completely destroyed.

Above 120 V self-bias, the G(d) peak broadens and its intensity increasesrelative to the G(nd) band. In the spectra of the 300 V sample, the twobands overlap, so as they can easily be treated as one. When the structurebecomes graphitic (700 V), the G(nd) band rises since there are graphiticrings present in the structure.

The decomposition showed the presence of another broad peak locatedat 1130 cm1 in the spectra of samples deposited at self-biases in the 80200 V range. The band shifts to higher wavenumbers with increasing self-bias.A similar band in the spectra of a-C:H was reported earlier and was relatedto vibrations of sp2 chains (trans-polyacethylene-like structural units) [37],sp3 carbon phase [38] and nano-crystalline diamond [13, 33]. The peak ap-pears together with the 1430 cm1 one, which was assigned to CC stretch-ing vibrations of structural units having large delocalization lengths of their electrons. A typical representative of hydrocarbon chains, built of sp2

carbon atoms, is trans-polyacethylene. It has characteristic Raman peaksaround 1450 cm1 and 1100 cm1, both showing dispersion. Presumably,the 1130cm1 band can be assigned to vibrations of sp2 chains. Model cal-culations showed that, for short polyacethylene chains, the position of thelatter band strongly depends on the chain length too, and shifts to lowerwavenumbers with increasing chain length, while there is no such eect ob-servable for the other band [39]. The increase of the peak position with self-bias shows that, as the lms become more diamond-like, the length of theconjugated chains decreases in the clusters. Above 200 V, the band cannotbe decomposed due to its overlapping with the D band.

In conclusion, it was shown that infrared-excited Raman spectroscopy isan excellent tool for characterization of dierent types of a-C:H thin lms. Itcan provide additional information on the bonding conguration of carbonatoms in the structure. The spectra recorded by near-infrared excitation weredecomposed into four peaks. It was found that the G peak has two compo-nents, one of which shows dispersion, while the position of the other does notchange with excitation wavelength. In the lower wavenumber region, besidesthe D band around 1130 cm1, another peak appears in the spectra, relatedto vibrations of carbon atoms arranged into chains.

By using the infrared-excited Raman spectroscopy, the evolution of thestructure of a-C:H layers prepared from benzene was investigated. It wasshown that, at low self-bias voltages (up to 120 V), intact (substituted) ben-


zene rings embedded into the amorphous structure. In addition, the amor-phous matrix of the lm prepared at 10 V contains structural units not dif-fering remarkably from benzene. Their content, as well as that of substitutedbenzene decreases with increasing self-bias. The sp2 carbon atoms arrangeinto chains. The small delocalization length also increases with depositionvoltage. From 120 V to 400 V, the chains dominate the sp2 clusters. Theincreasing with self-bias graphitic character will dominate the spectra of thesamples above 400 V.

Visible and infrared excited Raman spectroscopic measurements were alsoperformed on a-C:H samples prepared from methane in a wide self-bias range.In the following, the results of these investigations will be presented.

2.2 a-C:H Thin Layers Prepared From Methane

The methane lms were prepared similarly to benzene ones at chamber pres-sures of 13 Pa. The Raman spectra of the a-C:H lms excited at 488 nmare shown in Fig. 7. The shape of the spectra is similar to those presentedin Fig. 1. The only dierence is that there was not so high a luminescencebackground observable in the samples prepared at low voltages.

The spectra were analysed similarly to the benzene ones. Figure 8 showsthe variation of positions and widths of D and G bands with the self-biasvoltage. Contrarily to the layers prepared from benzene (Fig. 2), the positionsof both bands increase monotonically with the self-bias. The peak width ofthe D band increases also monotonically, while that of the G peak, aftera small increase, decreases above 60 V. The comparison with the resultsobtained for the a-C:H lms deposited from dierent source gases showsthat in the low self-bias range the sample series prepared from benzene andmethane behave dierently. In case of the benzene layers, the presence ofring-like units (distorted benzene rings) in the amorphous structure causesthe G peak to appear around 1600 cm1. On the contrary, in the 30 V sampleprepared from methane, the G peak is located at 1537 cm1, suggesting thatchain-like structural units are mainly present in the sp2 clusters. The shiftof the G band to higher wavenumbers with increasing self-bias indicates theincrease of the amount of ring-like structures in the sp2 clusters of the lm.This is supported by the change of the ID/IG ratio of the lms (Fig. 9),which also diers from that of the benzene ones. It rises monotonically withthe increasing self-bias, showing the increase of the ratio of the rings in thestructure.

The huge dierence in the bonding structure of a-C:H lms preparedfrom methane and benzene at low self-biases are also evidenced from theirRaman spectra excited at 785 nm. While the spectra of the benzene lms arecharacterized by narrow peaks (Fig. 4), those of methane layers are composedof broad bands (Fig. 10). The spectrum of the layer prepared from methaneat 30 V self-bias is similar to the spectra of the lms prepared from benzeneat 300500 V potential and evolves with the increase of the self-bias similarly


Fig. 7. Raman spectra excited by 488 nm of a-C:H thin lms prepared frommethane at dierent self-bias voltage USB

to the benzene ones above 300 V, where the sites have large delocalizationlength.

The spectra were decomposed by a set of four Gaussians. The variation ofthe positions and peak widths of the component bands with self-bias voltageare shown in Fig. 11. The positions of component bands are similar to thosefound in the benzene samples above 200 V self-bias. This implies that thesepeaks have the same origin. Hence, the infrared-excited Raman spectra ofmethane layers also have the D, G(d) and G(nd) bands (around 1230, 1430and 1570 cm1, respectively), as well as the peak assigned to vibrations of sp2

carbon atoms arranged into chains (around 1150 cm1). The position of thelatter band, located in the spectra of the 30 V sample at 1105 cm1, shifts tolower wavenumbers with the increase of the self-bias, showing the increase ofthe sp2 chain lengths of in the clusters [39], and becomes undetectable in thespectrum of the 600 V sample. In the lms prepared from benzene, the bandis positioned at higher wavenumbers, indicating the smaller chain lengths inthe clusters of those layers.


Fig. 8. Dependence of (a) the peak position and (b) the peak width on self-biasvoltage of the D and G bands of a-C:H thin lms prepared from methane

The position and width of the G(d) band do not change signicantly withthe increase of the self-bias, but its intensity decreases as the graphitic char-acter of the structure strengthens. This is accompanied by the shift of theG(nd) peak from 1560 cm1 to higher wavenumbers.

The comparison of the bonding conguration of the a-C:H thin layersprepared from methane and benzene shows that the structure of the lmsprepared under similar conditions diers signicantly at low self-biases. Inthe benzene molecule, the carbon atoms have preferential arrangement. At


Fig. 9. Dependence of intensity ratio of the D and G bands of the Raman spectraof a-C:H lm prepared from methane on self-bias voltage

Fig. 10. 785 nm excited Raman spectra of a-C:H thin lms prepared from methaneat dierent self-bias voltage USB


Fig. 11. Dependence of (a) the position and (b) width on self-bias voltage of thebroad bands found in the Raman spectra excited by 785 nm of a-C:H thin lmsprepared from methane

low ion energies, the amorphous carbon matrix mainly consists of distortedbenzene rings, so that besides intact rings, distorted benzene rings are alsopresent in the a-C:H matrix. This arrangement causes the electrons to behighly localized in the rings of clusters. As the ion energy increases, theserings are destroyed in the plasma, as well as during the bombardment ofthe surface of the growing lm, and the atoms rearrange, forming sp2 clus-ters characterized by large delocalization lengths. Clusters containing large


amount of sp2 chains develop in the self-bias region, where diamond-like a-C:H forms. With further increase of the deposition voltage, the graphiticclusters will dominate in the layer.

On the contrary, for the methane molecule consisting of only one carbonatom, there is no preferential arrangement. Therefore, during lm formation,the topology of the carbon atoms develops without initial constraints, except-ing for the bonding angles, set by the hybridization state of the carbon atom.The sp2 clusters, formed at low self-biases, are built of carbon atoms arrangedin both rings and chains and have large delocalization lengths, compared tothe benzene ones prepared under similar conditions. The increase of the de-position voltage up to the region, where diamond-like carbon deposits, causesthe lengthening of the sp2 chains. Above this self-bias, the graphitization ofthe structure starts, similarly to the benzene layers.

In conclusion, it was shown that Raman scattering excited in the near-infrared region can provide additional information about the bonding cong-uration of of a-C:H thin lms in a wide range of deposition conditions. Hence,it can be dependably used for quality control of these materials. The methodwas proved to be highly sensitive to the arrangement of the carbon atoms inthe sp2 clusters. The discrepant dispersion of the component peaks made thecomposite spectra to be decomposed more obvious, and it could give an ex-perimental basis for the assignment of the bands found in the Raman spectraof a-C:H.

References

[1] R. Shuker, R. Gamon: Phys. Rev. Letters 25, 222 (1970) 426[2] S. Craig, G. L. Harding: Thin Solid Films 96, 345 (1982) 426[3] D. R. McKenzie, R. C. McPhedran, N. Savvides, L. C. Botten: Philos. Mag.

B 48, 341 (1983) 426[4] J. Robertson: Adv. Phys. 35, 317 (1986) 426[5] J. Robertson, E. P. OReilly: Phys. Rev. B 35, 2946 (1987) 426[6] J. Robertson: Mat. Sci. Eng. R 37, 129 (2002) 426[7] T. Frauenheim, P. Blaudeck, U. Stephan, G. Jungnickel: Phys. Rev. B 48, 4823

(1993) 427[8] G. Jungnickel, T. Frauenheim, D. Proezag, P. Blaudeck, U. Stephan: Phys.

Rev. B 50, 6709 (1994) 427[9] R. Nemanich, J. Glass, G. Lucovsky, , R. Shroder: J. Vac. Sci. Technol. A 6,

1783 (1988) 427[10] M. A. Tamor, J. A. Haire, C. H. Wu, K. C. Hass: Appl. Phys. Lett. 54, 123

(1989) 427, 428[11] S. Xu, M. Hundhausen, J. Ristein, B. Yan, L. Ley: J. Non-Cryst. Solids 1127,

164166 (1993) 427, 429[12] M. A. Tamor, W. C. Vassel: J. Appl. Phys. 76, 3823 (1994) 427, 429[13] J. Schwan, S. Ulrich, V. Bathori, H. Erhardt, , S. R. P. Silva: J. Appl. Phys.

80, 440 (1996) 427, 432, 437


[14] A. C. Ferrari, J. Robertson: Phys. Rev. B 61, 14095 (2000) 427, 428, 429[15] A. C. Ferrari, J. Robertson: Phys. Rev. B 64, 75414 (2001) 427, 428, 429, 432[16] M. J. Pelletier (Ed.): Analytical Applications of Raman Spectroscopy (Black-

well Science 1999) 427[17] R. Nemanich, S. A. Solin: Phys. Rev. B 20, 392 (1979) 427[18] J. Maultzsch, S. Reich, C. Thomsen: Phys. Rev. B 70, 155403 (2004) 427,

428[19] Y. Kawashima, G. Katagiri: Phys. Rev. B 52, 10053 (1995) 427[20] P. H. Tan, C. Y. Hu: Phys. Rev. B 64, 214301 (2001) 427[21] J. Wagner, M. Ramsteiner, C. Wild, P. Koidl: Phys. Rev. B 40, 1817 (1989)

427[22] F. Tuinstra, J. L. Koenig: J. Chem. Phys. 53, 1126 (1970) 428[23] M. Chhowalla, A. C. Ferrari, J. Robertson, G. A. J. Amaratunga: Appl. Phys.

Lett. 76, 1419 (2000) 428[24] M. Ramsteiner, J. Wagner: Appl. Phys. Lett. 51, 1355 (1987) 428[25] M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani, T. Akamatsu: Solid State

Commun. 66, 1177 (1988) 428[26] M. Yoshikawa, N. Nagai, M. Matsuki, H. Fukuda, G. Katagiri, H. Ishida,

A. Ishitani, I. Nagai: Phys. Rev. B 46, 7169 (1992) 428[27] I. Pocsik, M. Koos, M. Hundhausen, L. Ley: Amorphous Carbon: State of the

Art (World Scientic, Singapore 1998) p. 224 428[28] C. Mapelli, C. Castiglioni, G. Zerbi, K. Mullen: Phys. Rev. B 60, 12710 (2000)

428[29] I. Pocsik, M. Hundhausen, M. Koos, L. Ley: J. Non-Cryst. Solids 1083, 227

230 (1998) 428[30] M. Ramsteiner, J. Wagner: Appl. Phys. Lett. 51, 1355 (1987) 428[31] M. Veres, M. Fule, S. Toth, M. Koos, I. Pocsik, J. Kokavecz, Z. Toth,

G. Radnoczi: J. Non-Cryst. Solids 351, 981 (2005) 429[32] J. Rao, K. J. Lawson, J. R. Nicholls: Surface Coatings Technol. 197, 154 (2005)

432[33] S. R. P. Silva, G. Amaratunga, E. Salje, K. Knowles: J. Mater. Res. 29, 4962

(1994) 432, 437[34] A. I. Kulak, A. V. Kondratyuk, T. I. Kulak, M. P. Samtsov, D. Meissner:

Chem. Phys. Lett. 378, 95 (2003) 432[35] I. Pocsik, M. Koos, S. H. Moustafa, J. A. Andor, O. Berkesi, M. Hundhausen:

Michrochim. Acta [Suppl.] 14, 755 (1997) 432[36] M. Veres, M. Koos, I. Pocsik: Diamond Relat. Mater. 11, 1115 (2002) 434[37] A. C. Ferrari, J. Robertson: Phys. Rev. B 63, 121405 (2001) 437[38] R. E. Shroder, R. J. Nemanich, J. T. Glass: Phys. Rev. B 41, 3738 (1990) 437[39] F. J. Owens: Physica E 25, 404 (2005) 437, 439

Index

states, 423, 426

states, 427

a-C, 428

a-C:H, 426, 428443

sp2-bonded clusters, 426429, 435,437439, 442, 443


cluster conjugation level, 427cluster size, 426429, 435

sp2-chains, 427, 431, 432, 435439, 443chain length, 439, 443

sp2-rings, 426, 427, 431, 434, 435, 437,438, 442, 443

distorted rings, 435, 437, 438, 442six-fold rings, 426

sp3/sp2 bonding ratio, 427ta-C, 428

amorphous carbon thin lms, 423

chemical vapour deposition (CVD)radio frequency CVD, 429

cluster model, 426clustering, 426429, 435, 437, 438coherence length, 428crystallite size, 428

D band, 427432, 434438, 440D/G band intensity ratio, 428, 431,

432, 438, 441diamond, 426, 427diamond-like carbon (DLC), 428, 430,

431, 437, 443dispersion, 423, 428, 432, 435437, 439,

440, 443

electron-phonon coupling, 425electron-photon coupling, 425

G band, 423, 427432, 435, 437, 438,440

dispersive G band, 435437, 439, 440non-dispersive G band, 435, 437, 439,

440graphene, 427graphite, 426428, 430, 431, 435graphitization, 430, 438, 443

Huckel approximation, 426

multi-wavelength Raman spectroscopy(MWRS), 428

normal modes, 424

phonons, 423, 425zone centre phonons, 424

polarizability, 424, 425polarizability tensor, 424

Raleigh scattering, 424Raman scattering, 423

anti-Stokes scattering, 423infrared excitation, 423, 429,

432435, 437439Raman intensity, 426Raman spectrum, 426Raman tensor, 424, 425resonant Raman scattering, 425, 428selection rules, 425breakdown of the selection rules,426

Stokes scattering, 423visible excitation, 423, 424, 427, 429,

438

IntroductionThe Raman EffectRaman Spectra of Carbon Materials

Infrared Excited Raman Spectroscopy of Amorphous Carbon Thin Filmsa-C:H Thin Films Prepared from Benzenea-C:H Thin Layers Prepared From Methane

ReferencesIndex

raman spectroscopy of cvd carbon thin films excited by near-infrared light

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