Research Article Structure of Carbonic Layer in Ohmic Contacts:...

Hindawi Publishing CorporationISRN Physical ChemistryVolume 2013 Article ID 487485 11 pageshttpdxdoiorg1011552013487485

Research ArticleStructure of Carbonic Layer in Ohmic Contacts Comparison ofSilicon CarbideCarbon and CarbonSilicide Interfaces

PaweB Borowicz12 Adrian Kuchuk3 Zbigniew Adamus4 MichaB Borysiewicz5

Marek Ekielski5 Eliana KamiNska5 Anna Piotrowska5 and Mariusz Latek1

1 Department of Characterisation of Nanoelectronic Structures Institute of Electron Technology Al Lotnikow 324602-668 Warsaw Poland

2Department of Photochemistry and Spectroscopy Institute of Physical Chemistry Polish Academy of SciencesKasprzaka 4452 01-224 Warsaw Poland

3Diagnostic Center V Lashkaryov Institute of Semiconductor Physics NAS of Ukraine Pr Nauky 45 03028 Kiev Ukraine4 Laboratory of Growth and Physics of Low Dimensional Crystals Institute of Physics Polish Academy of SciencesAl Lotnikow 3246 02-668 Warsaw Poland

5Department of Micro- and Nanotechnology of Wide Bandgap Semiconductors Institute of Electron TechnologyAl Lotnikow 3246 02-668 Warsaw Poland

Correspondence should be addressed to Paweł Borowicz borowiczitewawpl

Received 28 October 2012 Accepted 3 December 2012

Academic Editors J G Han and S Yang

Copyright copy 2013 Paweł Borowicz et alThis is an open access article distributed under the Creative CommonsAttribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The structure of carbonic layer in three samples composed of 4H polytype of silicon carbide and the following sequence of layerscarbonnickelsiliconnickelsilicon was investigated with Raman spectroscopy Different thermal treatment of the samples ledto differences in the structure of carbonic layer Raman measurements were performed with visible excitation focused on twointerfaces silicon carbidecarbon and carbonsilicide The results showed differences in the structure across carbon film althoughits thickness corresponds to 810 graphene layers

1 Introduction

Silicon carbide (SiC) is known as an excellentmaterial for fab-rication high-power high-frequency and high-temperatureelectronic devices due to its properties like good thermalconductivity high critical electric field and simplemethod ofdielectric layer fabrication [1 2] Formation of ohmic contactswith low specific resistance is an important aspect in applica-tion of silicon carbide [3]This fabrication is realized by depo-sition of metallic layer on the substrate surface followed bythermal treatment at high temperature [4 5] Nickel is prob-ably themost popularmetal used in technology of ohmic con-tact formation because the contacts formed with its applica-tion have specific contact resistance equal to sim10minus6Ω cm2 [4]The interaction of carbonic structures with the SiC substratewas investigated with X-ray photoelectron spectroscopy andRaman scattering [6]The properties of the ohmic contact aredetermined by concentration of graphitic nanoflakes formed

during the annealing procedure The initial structure of car-bon film has no impact on properties of the formed contact[6 7] In previous work structural and electrical propertiesof different Ni- and NiSi-based contacts to silicon carbidewere investigated [8] Reported structural data obtainedby various experimental techniques like X-ray diffraction(XRD) Rutherford Backscattering Spectrometry (RBS) andSecondary Ion Mass Spectrometry (SIMS) showed thatoptimal metallization sequence for manufacturing ohmiccontacts is NiSiNiSi The thicknesses of each layer shouldprovide optimal conditions for creation of Ni

2

Si silicidesThis paper focuses the attention on structural properties

on different sides of thin carbon film introduced between SiCsubstrate and nickelsiliconnickelsilicon sequence of layerSince thermal treatment at high temperature results in SiCdecomposition new carbon atoms appear at the interfacebetween silicon carbide substrate and deposited carboniclayer Decomposition of silicon carbide leads up to creation

2 ISRN Physical Chemistry

of carbon structures with preferred ABC stocking order[9] Annealing above 800∘C results in structural changes incarbon structure andmigration of carbon atoms towards freesilicide surface

The main topic discussed in this work is the comparisonof the structure observed for carbon layer at different sidesSiCC interface and CNi

2

Si interface As an experimentaltechnique Raman spectroscopy was chosen Due to largepenetration depth of visible light into silicide layer and siliconcarbide substrate it was possible to observe Raman spectraexcited from both sides of carbon film introduced betweenNiSiNiSi sequence and SiC substrate

2 Experimental

21 Samples The preparation of the samples was alreadydescribed in the paper concerning visible and ultravioletRaman study of carbon properties in ohmic contacts [10]

The samples will be called hereafter nsc1 2 (temperatureof the second annealing step 800∘C) nsc1 3 (temperature ofthe second annealing step 950∘C) and nsc1 1 (temperature ofthe second annealing step 1000∘C)

22 Apparatus and Experimental Conditions Raman scat-tering was measured with MonoVista 2750i micro-Ramanconfocal spectrometer (Spectroscopy and Imaging GmbHGermany)Themicroscopic part was based onOlympusBX51microscopeThe images frommicroscopewere recordedwithTM 2040GE imaging camera (JAI Japan) Motorized stage(Marzhauser GmbH Germany) allowed sample positioningwith accuracy equal to 100 nm in 119909 119910 and 119911 directions Thepositioning mechanism worked in feedback loop in order tostabilize the position of the stage Spectral part of the Mono-Vista was based on Princeton Instruments spectral devicesspectrograph SpectraPro 2750i (focal length 750mm) andspectral nitrogen-cooled CCD camera LN2048 times 512BIUVAR Spec-10 System with maximum efficiency at 250 nm

As an excitation light the Ar+ laser INNOVA 90C(Coherent USA) line (120582 = 488 nm) was used The power oflaser light on the sample was below 1mW in order to avoidthermal effects caused by focused laser light The gratingwith 1800 linesmm and blazed for visible spectral range wasused to measure Raman spectra Two types of excitationswere used through the silicide layer hereafter called also topexcitation and through SiC substrate hereafter called alsobottom excitation Although confocal configuration stronglyreduced signal generated outside the focal plane bottomexcitation introduces strong component of two-phonon scat-tering from 4H-SiC Huge difference between thickness ofSiC substrate and carbon film (sim1mm versus 3 nm) causedlarge signal from 4H-SiC substrate placed outside focal planeIn order to separate Raman scattering generated by carbonlayer from two-phonon SiC background two measurementswere necessary to get ldquopurerdquo spectrum of carbon layer Thefirst measurement was taken from the area covered withcarbon layer and the second one outside of this area Theldquopurerdquo carbon spectrumwas calculated by subtraction of two-phonon SiC spectrum measured outside of the covered areafrom the spectrum collected from area covered with carbon

and silicide layers In order to obtain good signal-to-noiseratio long irradiation time about 1 hour was applied formeasurement of single spectrum

23 Spectra Analysis The spectra were mathematically pro-cessed before analysis and interpretationMathematical treat-ment included offset removal baseline correction smooth-ing and normalization Measured spectra were normalizedto unityThe procedure used to calculate pure carbon spectrafrom bottom excitation measurements was similar to theprocedure used in the study of carbon inclusions at SiCSiO

2

interface [11]

3 Results

Raman spectra measured with visible excitation (120582exc =488 nm) and irradiation through the silicide layer are pre-sented in

Figure 1(a)mdashdata obtained for nsc1 2Figure 1(b)mdashfor nsc1 3Figure 1(c)mdashnsc1 1

The main plot of each panel shows the experimental datatogether with fitted Gaussian profilesThe upper inset in eachpanel compares experimental points with fitted function (thesumofGaussian profiles used to reconstruct the experimentaldata) The lower inset presents autocorrelation functionwhich attests to quality of fitting procedure The maximaof Gaussian profiles fitted to experimental data are given inthe main plot of each panel Random distribution of pointsaround zero level in autocorrelation function bears testimonyto the quality of fitting procedure Qualitative descriptionof the spectra measured for top excitation was alreadypresented in the paper comparing visible and deep-ultravioletRaman investigation of these samples and it will be notrepeated here in details [10]Themain difference between theresults presented here and in [10] is the number of profilesnecessary for reconstruction 119863 and 119866 bands in the case ofsamples nsc1 3 and nsc1 1 In [10] one Lorentzian componentwas sufficient to reconstruct each band whereas here twoGaussian profiles are necessary The detailed description ofthe results obtained from mathematical analysis performedin this work will be reported in the part Discussion

Subtracted spectra obtained from bottom excitation arepresented in Figure 2 The data in Figure 2 are presented inthe same way as in Figure 1 It means that the main plot ofeach panel shows experimental points and Gaussian profilesused in fitted function upper inset compares the whole fittedfunction with experimental data and lower inset presents theautocorrelation function The maxima of Gaussian profilesare given in the main plot of each panel In the case ofeach sample different numbers of Gaussian profiles arenecessary to reconstruct the course of experimental pointsFour profiles are necessary to reproduce the shape of thesubtracted spectrum in the case of nsc1 2 for nsc1 3 as manyas seven profiles are necessary and for nsc1 1 sixThe detaileddiscussion of the mathematical analysis will be presented innext the part of this paper In this chapter the description of

ISRN Physical Chemistry 3

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1

1603

1533

1356

002040608

1

1200 1400 1600 1800

1300 1400 1500 1600 1700 1800

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua 0

00501

015

Analysis of Raman spectrum measured for nsc1 2

Raman shift (cmminus1)

minus015minus01minus005


(a)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1

1300 1400 1500 1600 1700 1800Raman shift (cmminus1)

1355

1363

1604

1591

1582

1622


15001300 1400 1600 18001700Raman shift (cmminus1)

0002004006

002040608

1

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua


(b)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


1355

1361

1587

1582

1620

Experimental pointsFitted Gaussian functions



0

002004

0

0204

0608

1

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

minus002minus004

(c)

Figure 1 Analysis of Raman spectra measured for different samples with excitation through silicide layer (120582exc = 488 nm) (a) presents thedata obtained for nsc1 2 (b) for nsc1 3 and (c) for nsc1 1Themain plot in each panel shows experimental points together with fitted GaussianprofilesThemaxima positions are given in the plot Upper inset compares experimental points with fitted function and the lower inset showsthe quality of fitting procedure by means of autocorrelation function

the spectra is limited to qualitative discussion of its main fea-tures Two strong bands are present in the spectrum of eachsampleThemaximumof the first one is placed between about1520 cmminus1 and about 1540 cmminus1 Themaximum of the secondband appears between 1580 cmminus1 and 1610 cmminus1 Maximumposition of the first band is correlatedwith amorphous carbon(a-C) [12 13] The position of the second observed band istypical for 119866 band reported for different types of graphite

Maximum placed slightly above 1600 cmminus1 and obtainedhere for nsc1 2 and nsc1 3 subtracted spectra is a typicaltrace of nanocrystalline graphite [12 13] The maximumshifted to lower values of Raman shift and placed between1580 cmminus1 and 1590 cmminus1 in the case of nsc1 1 is reported inthe literature for graphite in crystalline form [12 13] To sumup for the first look Raman spectra of carbon layer observedfrom the side of silicon carbide substrate can be assigned


1525

1602

1394

0

Ram

an in

tens

ity (a

u)

002

004

006

008

01



0002004006008

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua 0

002001

minus002minus001


1707

(a)

Ram

an in

tens

ity (a

u)

0

001

002

003

004


17951694

1610

1566

1537

1397

1319


0

003

002

001

004

Wei

ghte

dre

sidua

0

003002001

Ram

anin

tens

ity (a

u)


minus001

minus003minus002

(b)

Ram

an in

tens

ity (a

u)

0

001

002

003

004

005

006Analysis of Raman spectrum measured for nsc1 1

1792171016

44

1580

1517

1378


0

003002001

004005

Wei

ghte

dre

sidua

0001

Ram

anin

tens

ity (a

u)


Experimental dataFitted profiles

minus001

(c)

Figure 2 Analysis of subtracted Raman spectra obtained from visible excitation through the silicon carbide substrate (a) presents the dataobtained for nsc1 2 (b) for nsc1 3 and (c) for nsc1 1 The convention used in Figure 2 is the same as in the case of Figure 1

to the mixture of a-C with different forms of graphite Thetype of graphite component seems to be dependent on thetemperature applied in the second step of thermal treatment

The spectra measured for 2119863 band are presented inFigure 3(a) showing data obtained for nsc1 3 Figure 3(b) andfor nsc1 1 In the case of nsc1 2 sample the signal in this rangeof Raman shift was too small to distinguish unequivocally2119863 band [10] The convention used in Figure 3 is the sameas in Figures 1 and 2 The main plot of each panel presentsexperimental points together with fitted Gaussian profiles

The maxima of the profiles are given in the plot Upperinset compares experimental points with fitted function andlower inset presents autocorrelation function as a certificateof fitting quality The qualitative description of the spectrawas presented earlier [10] and it will be not repeated herein detail Briefly 2119863 band observed for nsc1 3 has slightlylarger FWHM than the band recorded for nsc1 1 The bandobserved for nsc1 1 has the red side slightly shifted towardshigher frequencies The detailed discussion of mathematicalanalysis will be presented in the next part


0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

01

Ram

an in

tens

ity (a

u)

0

02

04

06

08


2704

2725

2660

2767

2716

2500 2600 2700 2800 2900Raman shift (cmminus1)


minus01

(a)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

004



2642

2715

2720

2752

2700


minus004

(b)

Figure 3 Analysis of 2119863 band measured with top excitation for nsc1 3 (a) and nsc1 1 (b) No unequivocal signal that can be assigned to 2119863band was observed in this range of Raman shift for nsc1 2 The convention used in Figure 3 is the same as in Figures 1 and 2

4 Discussion

The list of Raman bands found with application of math-ematical analysis in spectra excited through the silicidelayer is presented in Table 1 Each band is characterized bymaximumposition half width FWHM and relative intensityThe last column of the table contains proposed assignmentof the band The carbon layer in samples from nsc1 119899(119899 =1 2 3) series has no uniform ideal crystal structure but iscomposed of graphite domains In such a case measuredRaman spectra contain the information about structureswith crystallographic parameters scattered around the mainvalues Reliable information about the intensity of the maincrystalline structure is derived from the height of the fittedprofile and not from the area under the curve [12] Becauseof this the intensities given in this work are determined fromthe heights of fitted Gaussian functions In the case of eachsample presented in Table 1 intensities are normalized to theheight of the profile reproducing 119866 band For samples nsc1 1and nsc1 3 for which two Gaussian functions are necessaryfor reconstruction of119866 band the profile with smaller FWHMis taken as reference

ThreeGaussian components are necessary for reconstruc-tion of Raman spectrum measured for nsc1 2 The first andthird ones reproduce119863 and119866 bandsTheirmaxima positionsand FWHMs are close to those which were reported forthe model based on Lorentzian functions [10] The maximapositions and FWHM values are typical for nanocrystallinegraphite The intensity ratio 119868(119863)119868(119866) is here slightly largerin comparison to the value reported for Lorentzian-basedmodel [10] In particular the values of 119868(119863)119868(119866) intensityratio presented here and in [10] are equal to 129 and116 respectively The second Gaussian component of fittedfunction has the maximum at 1533 cmminus1 large FWHM equalto about 360 cmminus1 and the intensity much lower than119866 band(016) The maximum of this component corresponds withthe data reported for a-C Also the large value of FWHM istypical for this kind of carbon structure [13] For examplefused 119863 and 119866 bands observed for carbon films depositedby supersonic cluster beam spread form about 1000 cmminus1 toabout 1650 cmminus1 [14]

The reconstruction of 119863 band observed for samplesnsc1 3 and nsc1 1 requires two Gaussian functions for eachsample In the case of each sample the pair of Gaussian


Table 1 Summary of results obtained in the case of top excitationThe range of Raman shift spreads from about 1300 cmminus1 to about 1880 cmminus1 It corresponds to the position of119863 and 119866 bands

nsc1 2 nsc1 3 nsc1 1Proposed assignmentMax

(cmminus1)FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

13553 129 13545 334 131 13554 293 068 119863 band632 13627 761 069 13614 684 059 119863 band

15333 3599 016 mdash mdash mdash mdash mdash mdash 119886-C15819 1132 070 15857 775 050 119866 band

16029 729 100 15912 403 100 15868 262 100 119866 band

16222 228 036 16198 324 028 Combination of C=Cand NiGIC vibrations

mdash mdash mdash 17840 897 006 mdash mdash mdash2oTOmode orbenzene-relatedvibrations

functions has similar properties One component has themaximum placed at about 1355 cmminus1 and FWHM equalto about 30 cmminus1 The relative intensity of this componentdecreases about two times from nsc1 3 to nsc1 1 in particularit is equal to 129 and 068 for nsc1 3 and nsc1 1 respectivelyThe decrease of normalized intensity between nsc1 3 andnsc1 1 shows the increase of graphitization degree betweenthe samples The other type of Gaussian function has themaxima shifted towards larger values of Raman shift Thepositions are equal to about 1363 cmminus1 and 1361 cmminus1 fornsc1 3 and nsc1 1 respectively The FWHM values obtainedfor this type of Gaussian functions are about twice largerthan the corresponding parameters obtained for previouslydescribed ldquonarrowrdquo profiles The values are equal to about76 cmminus1 and 68 cmminus1 for nsc1 3 and nsc1 1 respectively Thenormalized intensities are in the case of these ldquobroadrdquo profilessimilar and they are equal to 069 and 059 for nsc1 3 andnsc1 1 respectively The requirement to use two Gaussianfunction for 119863 band reconstruction can be associated withdifference in the structure of carbon layer along its thick-ness Comparison of Figures 1 and 2 shows differences incrystalline structure of carbon layer observed for top andfor bottom excitations It means that the crystalline structureof the layer should change across this layer The ldquonarrowrdquoGaussian component in Raman spectra recorded for samplesnsc1 3 and nsc1 1 describes this part of the layer which hasregular graphite structure The ldquobroadrdquo profile correspondsto structures which have the crystalline parameters scatteredaround the mean values This distribution of crystallineparameters is reflected in relatively large FWHM equalto about 70 cmminus1 The hypothesis that ldquobroadrdquo Gaussianfunction in 119863 band reproduces this part of Raman spectrumwhich is generated by structures with scattered structuralparameters is supported by the reconstruction of 119866 bandIn the case of samples nsc1 3 and nsc1 1 three Gaussianfunctions are necessary for proper reconstruction of 119866 bandThe third profile has the maximum at about 1622 cmminus1 and1620 cmminus1 for nsc1 3 and nsc1 1 respectively The FWHMvalues of this band are equal to about 23 cmminus1 and 32 cmminus1for nsc1 3 and nsc1 1 respectively Normalized intensities of

these Gaussian functions are equal to 036 and 028 for nsc1 3and nsc1 1 respectively This band originates mainly fromstructure with double C=C bonds in the case of nsc1 3 andfrom nickel-graphite intercalated compounds (NiGICs) inthe case of nsc1 1 Details of this assignment were alreadydiscussed [10] Two remaining Gaussian functions form thepair consisting of ldquonarrowrdquo and ldquobroadrdquo components similarto the pair of functions reconstructing119863 bandThe ldquonarrowrdquocomponents have the maxima placed at about 1591 cmminus1and 1587 cmminus1 for nsc1 3 and nsc1 1 respectively The corre-sponding values of FWHM are equal to about 40 cmminus1 and26 cmminus1The ldquobroadrdquoGaussian components have themaximaplaced at about 1582 cmminus1 and 1586 cmminus1 and correspondingFWHM values are equal to about 113 cmminus1 and 78 cmminus1 fornsc1 3 and nsc1 1 respectively The normalized intensities ofldquobroadrdquo components are equal to 070 and 050 for nsc1 3and nsc1 1 respectively The FWHM observed in the caseof both components (ldquonarrowrdquo and ldquobroadrdquo) is smaller fornsc1 1 in comparison with nsc1 3 This decrease suggestslarger homogeneity of carbon layer in the case of nsc1 1 Bothcomponents can be interpreted as 119866 band in particular

(i) ldquonarrowrdquo component can be associated with regulargraphite structure

(ii) ldquobroadrdquo component is generated by structures withcrystallographic parameters scattered around meanvalues

Themaxima positions obtained for ldquonarrowrdquo componentshave larger values in comparison with maxima of ldquobroadrdquostructures contributing to 119866 band The maximum of 119866 bandreported for ABA stacking order is shifted towards largerfrequencies in comparison with the maximum of this bandin the case of ABC stacking order [15] One can expect thedominance of ABA stacking order in ldquonarrowrdquo componentand ABC stacking order in ldquobroadrdquo one [10]The carbon layerwith ABC stacking order appears in the vicinity of SiCCinterface due to interaction between silicon carbide substrateand carbon layer [9]Therefore the structures contributing tothe ldquobroadrdquo components of 119863 and 119866 bands should be placed


near SiC substrate and ldquonarrowrdquo components of 119863 and 119866bands should be generated in the part of carbon layer placedin the vicinity of silicide layer

The last band given in Table 1 has the maximum at about1784 cmminus1 FWHM equal to about 90 cmminus1 and very weakintensity equal to about 6 of the intensity obtained forthe ldquonarrowrdquo component of 119866 band It was found in thespectrum of nsc1 3 only The band is placed in the rangeof Raman shift where out-of-plane vibrational modes ofgraphite are reported in the literature [16] In particular thebest correlation of the maximum band is with the positionof so-called 119872 band [17] This 119872 band was assigned to theovertone of out-of-plane Transverse Optical (2oTO) mode[18] The other reported in the literature carbon band placedin the vicinity of 1784 cmminus1 is assigned to benzene-relatedvibrations [19 20] It has the maximum at about 1740 cmminus1The correlation of the band centred around 1784 cmminus1 withthe combination of in-plane Transverse Acoustic (iTA) andLongitudinal Optical (LO) modes iTALO band (maximum atabout 1860 cmminus1) [21] or band reported for cumulene CC1199041199011vibrations (maximumat about 1980 cmminus1) [14] ismuchworse

The parameters of bands found in subtracted spectraof nsc1 1 nsc1 2 and nsc1 3 samples (bottom excitation) aregiven in Table 2 As in Table 1 each band is characterizedby maximum position FWHM and normalized intensityAs a reference the intensities of bands assigned to a-C werechosen because this type of band was found in subtractedspectra of all samples The last column of Table 2 con-tains proposed assignment The main common featureobserved for all samples was assigned to a-C [22] Themaxima of this band are placed at sim1525 cmminus1 sim1537 cmminus1and sim1518 cmminus1 for nsc1 2 nsc1 3 and nsc1 1 respectivelyCorresponding FWHMs are equal to about 59 cmminus1 66 cmminus1and 39 cmminus1 The a-C layer placed at the interface of SiC wasalready observed with cross-sectional transmission electronmicroscopy in the case of thermal decomposition of 4H-SiC [23] The a-C layer observed for all samples in presentstudy appears probably due to 20 in-plane lattice mismatchbetween 4H-SiC and graphite combined with 60 80 in-plane expansion mismatch reported for silicon carbide andgraphite at SiCgraphite interface [23]

The bands placed between 1370 cmminus1 and 1400 cmminus1 inparticular

(i) for nsc1 2maximum at sim1394 cmminus1(ii) for nsc1 3maximum at sim1397 cmminus1(iii) for nsc1 1maximum at sim1378 cmminus1

are assigned to type of vibration responsible 119863 band Typicalposition of the119863 bandmaximum is equal to about 1350 cmminus1[12 24ndash26] In the case of structures like carbon nanotubesthe maximum position of 119863 band is expected in the rangeof Raman shift between 1250 cmminus1 and 1450 cmminus1 [27] Incarbon films the position of119863 band maximum was reportedas equal to about 1380 cmminus1 [19] The band with maximumat about 1319 cmminus1 corresponds to the data reported forcubic diamond [28] This kind of carbon structure is formedform graphite which has ABC stacking order Such a type of

graphite is preferentially formed in the process of thermaldecomposition of silicon carbide [9] This kind of diamondcan be formed under low pressure in the presence of nickelwhich plays a role of catalyst in formation of cubic diamond[28] The activation barrier of diamond formation formgraphite is slightly lower for cubic form of diamond thanfor hexagonal type [28] The correlation of the maximum1319 cmminus1 with band reported for carbon nanotubes is muchworse [29] The changes of Raman spectra recorded forbottom excitation for samples nsc1 2 nsc1 3 and nsc1 1 in therange below 1400 cmminus1 can be summarized in the followingway Thermal treatment at 800∘C applied to nsc1 2 sampleresults in formation of structures generating 119863 band withmaximum at 1394 cmminus1 The maximum of the band is shiftedtowards higher frequencies in comparison with typical posi-tion of 119863 band equal to about 1350 cmminus1 The reason forthis shift is probably the interaction with nitrogen used fordoping of the substrate [10] Such kind of shift resultingfrom interaction with nitrogen was already reported forcarbon film deposited on silicon substrate [30 31] Increaseof the temperature in second step of thermal treatmentof the samples up to 950∘C [10] results in formation ofcubic diamond-like structures observed as a weak band withmaximumat 1319 cmminus1 innsc1 3 subtractedRaman spectrumFurther increase of the annealing temperature up to 1000∘Ccauses the graphitization of the sample The trace of thisgraphitization is observed for nsc1 1 119863 band obtained forsubtracted spectrum is shifted towards standard position incomparison with data recorded for nsc1 2 and nsc1 3 samples

The other strong band observed in the spectrum ofnsc1 2 obtained for bottom excitation has the maximum atabout 1602 cmminus1 and FWHM equal to about 57 cmminus1 Thesevalues are almost the same as parameters describing 119866 bandobserved for the same sample in the case of top excitation andare typical for spectra observed in the case of nanocrystallinegraphite [13 32 33] The maximum of 119866 band is shifted inthe case of nsc1 3 towards higher frequencies and is placedat about 1610 cmminus1 This kind of shift suggests increase of theconcentration of the double C=C bonds The maximum ofRaman band associated with vibration of double C=C bondsis placed at about 1620 cmminus1 [34] Small band with maximumplaced at about 1566 cmminus1 in subtracted spectrum of nsc1 3is well correlated with data reported for carbon nanotubesand known as 119866minus band [27] In the case of nsc1 1 samplethe second important band has themaximumplaced at about1580 cmminus1 which is in agreement with the position of119866 bandreported for graphite [12 24] The band with maximum at1644 cmminus1 obtained for nsc1 1 is assigned to C=C stretchingvibrationsThe typical reported value 1620 cmminus1 correspond-ing to double bond C=C vibration can be modified by theenvironment An example of such behaviour is delivered byRaman study of liquid hydrocarbons-botryococcenes In thistype of compounds three bands 1640 cmminus1 1647 cmminus1 and1670 cmminus1 were assigned to C=C stretching vibrations [35]In summary with increase the temperature of second stepof annealing process 119866 band moves to the typical positionreported for graphite In each sample large content of C=Cvibration is observed In the case of nsc1 3 sample also the


Table 2 Summary of results obtained in the case of bottom excitation The range of Raman shift which was taken into account spreads from1300 cmminus1 to 1880 cmminus1

nsc1 2 nsc1 3 nsc1 1AssignmentMax


NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

13192 321 022 Cubic diamond-likestructures

13935 966 023 13970 880 020 13782 1040 019 119863-band15253 588 100 15371 664 100 15175 392 100 119886-C

15662 93 020 119866-band carbonnanotubes

15800 330 079 119866-band graphite

16017 567 060119866-bandnanocrystallinegraphite

16103 477 117

Combination ofG-band and 1198631015840-band(C=C vibrationalmodes)

16444 331 013 119863

1015840-band (C=Cvibrational modes)

17066 622 019 16938 738 038 17098 255 045Benzene-relatedvibrations or 2oTOmode

17952 424 014 17922 1592 017 iTALO or 2oTOmode

traces of carbon nanotubes were observed in Raman spectraexcited through 4-SiC substrate

Thebands placed between about 1700 cmminus1 and 1900 cmminus1can be classified as one group In the case of the sample nsc1 2only one band of this type can be recognized in this rangeof Raman shift It is placed around 1707 cmminus1 For nsc1 3and nsc1 1 two bands appear in Raman spectra in the range1700 cmminus11900 cmminus1 The maxima positions of the bands areplaced at 1694 cmminus1 and 1795 cmminus1 for nsc1 3 For nsc1 1 thecorresponding values are equal to 1710 cmminus1 and 1792 cmminus1The bands placed near 1700 cmminus1 have the best correlationwith the119872 band assigned to the infrared active 2oTOmode[18] This mode has two components 119872minus and 119872+ The 119872band is of great interest in the case of structural investiga-tion of carbon nanotubes [29] The 119872minus band is dispersivewhereas the position of119872+ maximum does not change withexcitation wavelength [17] The maxima positions of119872 bandcomponents depend on the type of carbon structure Forexample in the case singlewall carbon nanotubes (SWCNTs)the maxima of 119872minus and 119872+ bands are placed in ranges1732 cmminus11744 cmminus1 and 1755 cmminus11766 cmminus1 respectively[18] The maxima positions depend on the tube diame-ter and they moved towards higher frequencies with theincrease of this diameter For ldquoplanarrdquo forms of carbon119872

minus and 119872+ are shifted above 1750 cmminus1 [17] For examplein the case of highly ordered pyrolytic graphite (HOPG)the maxima are placed at 1754 cmminus1 and 1775 cmminus1 [18]There are two other possibilities of assignment The bands1707 cmminus1 1694 cmminus1 and 1710 cmminus1 can be correlated withbenzene-related vibrationsThe corresponding band assigned

to benzene-related vibrations in carbonfilms is placed around1740 cmminus1 [19] The other possible assignment is concernedwith bands maxima at 1795 cmminus1 and 1792 cmminus1These valuesare not far from the maximum of vibrations assigned toiTALO mode The reported maximum of iTALO band isplaced at about 1860 cmminus1 [21] The correlation with bandreported for cumulene chains placed around 1980 cmminus1 ismuch worse [14] To sum up the bands 1707 cmminus1 1694 cmminus1and 1710 cmminus1 can be assigned to 2oTO modes or benzene-related vibrations and bands 1795 cmminus1 and 1792 cmminus1 shouldbe correlated with 2oTO or iTALOmodes

Table 3 summarizes the features of Raman spectra mea-sured for nsc1 3 and nsc1 1 with top excitation in the rangecorresponding to 2119863 band Each component is characterizedby maximum position FWHM and normalized intensityThe intensity of the most intense Gaussian profile in thespectrumof each sample in particular 27044 cmminus1 for nsc1 3and 27004 cmminus1 for nsc1 1 was normalized to unity Thelast column of the table contains proposed assignment 2119863band appears in double resonance mechanism [36] and issusceptible to the carbon structure For example Ramanspectrum of so-called two-dimensional graphite (withoutstacking order) has 2119863 band which can be reproduced withsingle Lorentzian function centred at 2707 cmminus1 [37] Thistype of structure is called turbostatic graphite To describe2119863 band of three-dimensional graphite with mathematicalmodel two Lorentzian functions are necessary [37] Theyhave maxima at 2687 cmminus1 and 2727 cmminus1 The shape of 2119863band is very sensitive to the number of layers in graphitesample especially for so-called ldquofew-graphene layersrdquo [22] In


Table 3 Summary of results obtained for 2119863 band Spectra were measured for top excitation

nsc1 3 nsc1 1AssignmentMax


NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

26596 670 031 26418 529 016ldquoNarrowrdquo component overtone

accompanied by interaction with ldquobroadrdquocomponent

27044 562 100 27004 616 100 ldquoNarrowrdquo component overtone27246 619 086 27199 489 089 ldquoBroadrdquo component overtone

27665 515 018 27522 606 039ldquoBroadrdquo component overtone

accompanied by interaction withldquonarrowrdquo component

the case of graphene 2119863 band can be modeled with singleLorentzian function [22] The band splits into multiprofilestructure due to splitting of electronic band structure andinteraction between ldquographenerdquo layers if the number oflayers increases [38] This splitting causes four processescontributing to 2119863 band Two of them introduce componentswith relatively strong intensity while two others give a riseto weaker components The most spectacular changes of2119863 band are observed in the case of numbers of layersbelow ten [39] ldquoGraphene bilayerrdquo requires four Lorentziancomponents for proper mathematical representation of 2119863band [40] In the case of ldquographene three-layerrdquo it is necessaryto use even six Lorentzian functions to reproduce properlythe band shape [16] The analysis of 119866 band observed forsamples nsc1 1 and nsc1 3 suggests that carbon few-layercan be divided into two sublayers one with dominant ABCstacking order and the other with dominant ABA stackingorder Graphite with ABA stacking order (Bernal type) showsmetallic-like behaviour [41] The rhombohedral graphiteshows the properties of narrow-band semiconductor [41]which has the band-gap equal to about 6meV around 119870point [41] The band-gap in 119899-ldquographenerdquo-layer depends onthe number of layers 119899 and the stacking order For examplecalculated band-gap from first principle for rhombohedralgraphite changes from 18meV to 48meV if sequence andnumber of layers change from ABC to ABCA [42] The four-layer with stacking order ABAC should have two band-gapsldquoquasidirectrdquo equal to 88meV and secondary (around 119870point) equal to 115meV [42] Reported values of band-gap48meV18meV correspond to the range 39 cmminus1145 cmminus1This range of band gap corresponds to the energy of twovibrational modes in carbon structures 119864

2g and 1198612g [43] 1198642gvibrational mode contributes to Raman spectrum of HOPGas weak line centred around 42 cmminus1 [43ndash45] In the case ofcarbon nanotubes the reported frequency of this mode isequal to 49 cmminus1 or 58 cmminus1 depending on the tube diameter[44]The 119861

2g mode was observed in Raman spectra of HOPGas weak line with maximum at 127 cmminus1 [43ndash45]

We will adopt the concept of interacting layers toexplain the origin of four Gaussian components of 2119863 bandInstead of ldquographenerdquo layers we have two graphite sub-layers with ABA- and ABC-dominant stacking orders Onehas to take into account that scattering accompanied by

interaction between carbon sub-layers with different stackingorder requires in the energy balance the quantum relatedto small band-gap in part of carbon layer with dominantABC stacking order Let us now come back to the measuredRaman spectra The components of 2119863 band recorded fornsc1 1 and centred around 2700 cmminus1 and 2720 cmminus1 canbe assigned to overtones ldquonarrowrdquo and ldquobroadrdquo parts of 119863band respectively Similar assignment can be carried outfor strong components of 2119863 band (maxima 2704 cmminus1and 2725 cmminus1) observed for nsc1 3 Weak components withfrequencies 2660 cmminus1 for nsc1 3 and 2642 cmminus1 for nsc1 1can be assigned to overtone of ldquonarrowrdquo component of 119863band shifted due to energy exchangewith ldquobroadrdquo componentof the same band Symmetrically the components centredaround 2767 cmminus1 for nsc1 3 and 2752 cmminus1 for nsc1 1 canbe treated as overtone of ldquobroadrdquo component of 119863 bandarising in scattering accompanied with energy exchange withldquonarrowrdquo component of the same band ldquoWeakrdquo componentsin Raman spectrum of 2119863 band observed for nsc1 1 areshifted towards smaller values of Raman shift in comparisonwith spectrum obtained for nsc1 3 This shift is equal toabout 15 cmminus118 cmminus1 and is due to changes in stackingcaused by higher annealing temperature An example ofelectronic excitation and vibrationalmodes joined action wasreported in the case of intramolecular excited state protontransfer reaction [46] The tautomerization mechanism of[22-bipyridine]-331015840-diol was explained in [46] in terms ofwavepacket evolution triggered by electronic excitation

5 Summary

Let us state at the beginning that application of Gaussian pro-files significantly improved the quality of the fitting procedurein comparison with former approach based on Lorentzianfunctions [10] This is probably due to complicated characterof the spectra Gaussian function is focused on the informa-tion from the vicinity of maximum of the band whereas incase of Lorentzian profile the tails far from the maximumplay much more important role than for Gaussian function

The part of carbon layer place at the SiCC interface hascomplex structureThe existence of such amorphous compo-nent probably cannot be avoided because of lattice mismatchbetween silicon carbide and graphite [47] and differences


between in-plane thermal expansion of silicon carbide andgraphite [23] The complicated character of Raman spectrasuggests that the achieving low-resistance ohmic contacts isnot only dependent on graphitization process understood asdecrease of the concentration of defects in graphite lattice Italso includes more subtle processes like changes in stackingorder or creation of structures like carbon nanotubes It isespecially important in the vicinity of SiCC interface Thenanotube-like structure should have much better electricproperties [48] than amorphous carbon placed on the SiCCinterface [49]

Acknowledgment

The research was partially supported by the European Unionwithin European Regional Development Fund throughgrant Innovative Economy (POIG010301-00-15908ldquoInTechFunrdquo)

References

[1] J A Cooper and A Agarwal ldquoSiC power-switching devices-thesecond electronics revolutionrdquo Proceedings of the IEEE vol 90no 6 pp 956ndash968 2002

[2] R C Clarke and J W Palmour ldquoSiC microwave power tech-nologiesrdquo Proceedings of the IEEE vol 90 no 6 pp 987ndash9922002

[3] H Matsunami ldquoCurrent SiC technology for power electronicdevices beyond Sirdquo Microelectronic Engineering vol 83 no 1pp 2ndash4 2006

[4] J Crofton P G McMullin J R Williams and M J BozackldquoHigh-temperature ohmic contact to n-type 6H-SiC usingnickelrdquo Journal of Applied Physics vol 77 no 3 pp 1317ndash13191995

[5] M G Rastegaeva A N Andreev A A Petrov A I BabaninMA Yagovkina and I P Nikitina ldquoThe influence of temperaturetreatment on the formation of Ni-based Schottky diodes andohmic contacts to n-6H-SiCrdquoMaterials Science and EngineeringB vol 46 no 1ndash3 pp 254ndash258 1997

[6] W Lu W C Mitchel C A Thornton G R Landis and WEugene Collins ldquoCarbon structural transitions and ohmic con-tacts on 4H-SiCrdquo Journal of Electronic Materials vol 32 no 5pp 426ndash431 2003

[7] W Lu W C Mitchel G R Landis T R Crenshaw and W ECollins ldquoElectrical contact behavior of NiC604H-SiC struc-turesrdquo Journal Vacuum Science amp Technology A vol 21 no 4pp 1510ndash1514 2003

[8] A Kuchuk V Kladko M Guziewicz et al ldquoFabrication andcharacterization of nickel silicide ohmic contacts to n-type 4Hsilicon carbiderdquo Journal of Physics vol 100 no 4 Article ID042003 2008

[9] W Norimatsu and M Kusunoki ldquoSelective formation of ABC-stacked graphene layers on SiC(0001)rdquo Physical Review B vol81 no 16 Article ID 161410 2010

[10] P Borowicz A Kuchuk Z Adamus et al ldquoVisible and deep-ultraviolet Raman spectroscopy as a tool for investigation ofstructural changes and redistribution of carbon in Ni-basedohmic contacts on silicon carbiderdquo ISRN Nanomaterials vol2012 Article ID 852405 11 pages 2012

[11] P Borowicz T Gutt T Małachowski and M Latek ldquoCarbonicinclusions on SiCSiO

2

interface investigated with Raman

ScatteringrdquoDiamond and RelatedMaterials vol 20 no 5-6 pp665ndash674 2011

[12] A C Ferrari and J Robertson ldquoInterpretation of Raman spectraof disordered and amorphous carbonrdquo Physical Review B vol61 no 20 pp 14095ndash14107 2000

[13] A C Ferrari and J Robertson ldquoRaman spectroscopy of amor-phous nanostructured diamond-like carbon and nanodia-mondrdquo Philosophical Transactions of the Royal Society A vol362 no 1824 pp 2477ndash2512 2004

[14] L Ravagnan F Siviero C Lenardi et al ldquoCluster-beam deposi-tion and in situ characterization of carbyne-rich carbon filmsrdquoPhysical Review Letters vol 89 no 28 pp 2855061ndash28550642002

[15] C H Lui Z Li Z Chen P V Klimov L E Brus and T F HeinzldquoImaging stacking order in few-layer graphenerdquo Nano Lettersvol 11 no 1 pp 164ndash169 2011

[16] C Cong T Yu K Sato et al ldquoRaman characterization of ABA-and ABC-stacked trilayer graphenerdquo ACS Nano vol 5 no 11pp 8760ndash8768 2011

[17] C Cong T Yu R Saito G F Dresselhaus and M S Dressel-haus ldquoSecond-order overtone and combination raman modesof graphene layers in the range of 1690-2150 cm-1rdquo ACS Nanovol 5 no 3 pp 1600ndash1605 2011

[18] V W Brar G G Samsonidze M S Dresselhaus et al ldquoSecond-order harmonic and combination modes in graphite single-wall carbon nanotube bundles and isolated single-wall carbonnanotubesrdquo Physical Review B vol 66 no 15 Article ID 155418pp 1554181ndash15541810 2002

[19] N Radic B Pivac F Meinardi and T Koch ldquoRaman study ofcarbon clusters in W-C thin filmsrdquoMaterials Science and Engi-neering A vol 396 no 1-2 pp 290ndash295 2005

[20] Z Y Chen J P Zhao T Yano T Ooie M Yoneda and JSakakibara ldquoObservation of sp3 bonding in tetrahedral amor-phous carbon using visible Raman spectroscopyrdquo Journal ofApplied Physics vol 88 no 5 pp 2305ndash2308 2000

[21] R Rao R Podila R Tsuchikawa et al ldquoEffects of layer stackingon the combination ramanmodes in graphenerdquo ACS Nano vol5 no 3 pp 1594ndash1599 2011

[22] A C Ferrari ldquoRaman spectroscopy of graphene and graphitedisorder electron-phonon coupling doping and nonadiabaticeffectsrdquo Solid State Communications vol 143 no 1-2 pp 47ndash572007

[23] R Colby M L Bolen M A Capano and E A Stach ldquoAmor-phous interface layer in thin graphite films grown on the carbonface of SiCrdquo Applied Physics Letters vol 99 Article ID 1019042011

[24] F Tuinstra and J L Koenig ldquoRaman spectrum of graphiterdquoJournal of Chemical Physics vol 53 no 3 pp 1126ndash1130 1970

[25] R J Nemanich and S A Solin ldquoFirst- and second-order Ramanscattering from finite-size crystals of graphiterdquo Physical ReviewB vol 20 no 2 pp 392ndash401 1979

[26] J Maultzsch S Reich and C Thomsen ldquoDouble-resonantRaman scattering in graphite interference effects selectionrules and phonon dispersionrdquo Physical Review B vol 70 no15 Article ID 155403 pp 155403ndash9 2004

[27] M S Dresselhaus G Dresselhaus A Jorio A G S Filho andR Saito ldquoRaman spectroscopy on isolated single wall carbonnanotubesrdquo Carbon vol 40 no 12 pp 2043ndash2061 2002

[28] M Nishitani-Gamo I Sakaguchi K P Loh H Kanda and TAndo ldquoConfocal Raman spectroscopic observation of hexago-nal diamond formation from dissolved carbon in nickel under


chemical vapor deposition conditionsrdquo Applied Physics Lettersvol 73 no 6 pp 765ndash767 1998

[29] A M Rao E Richter S Bandow et al ldquoDiameter-selectiveRaman scattering from vibrational modes in carbon nan-otubesrdquo Science vol 275 no 5297 pp 187ndash190 1997

[30] J R Shi X Shi Z Sun E Liu B K Tay and S P Lau ldquoUltravio-let and visible Raman studies of nitrogenated tetrahedral amor-phous carbon filmsrdquoThin Solid Films vol 366 no 1-2 pp 169ndash174 2000

[31] H Riascos G Zambrano E Camps and P Prieto ldquoInfluence ofnitrogen gas pressure on plume-plasma and chemical bondingof carbon nitride films synthesized by pulsed laser depositionrdquoRevista Mexicana de Fisica vol 53 no 7 pp 274ndash278 2007

[32] W Lu W C Mitchel G R Landis T R Crenshaw and W ECollins ldquoCatalytic graphitization andOhmic contact formationon 4H-SiCrdquo Journal of Applied Physics vol 93 no 9 pp 5397ndash5403 2003

[33] V Y Osipov A V Baranov V A Ermakov et al ldquoRaman cha-racterization and UV optical absorption studies of surfaceplasmon resonance in multishell nanographiterdquo Diamond andRelated Materials vol 20 no 2 pp 205ndash209 2011

[34] C Fantini M A Pimenta and M S Strano ldquoTwo-phononcombination Ramanmodes in covalently functionalized single-wall carbonnanotubesrdquo Journal of Physical Chemistry C vol 112no 34 pp 13150ndash13155 2008

[35] T L Weiss H J Chun S Okada et al ldquoRaman spectro-scopy analysis of botryococcene hydrocarbons from the greenmicroalga Botryococcus brauniirdquo Journal of Biological Chem-istry vol 285 no 42 pp 32458ndash32466 2010

[36] P H Tan C Y Hu J Dong andW C Shen ldquoDouble resonanceRaman scattering of second-order Raman modes from anindividual graphite whiskerrdquo Physica E vol 37 no 1-2 pp 93ndash96 2007

[37] M A Pimenta G Dresselhaus M S Dresselhaus L GCancado A Jorio and R Saito ldquoStudying disorder in graphite-based systems by Raman spectroscopyrdquo Physical ChemistryChemical Physics vol 9 no 11 pp 1276ndash1291 2007

[38] R Narula and S Reich ldquoDouble resonant Raman spectra ingraphene and graphite a two-dimensional explanation of theRaman amplituderdquo Physical Review B vol 78 no 16 Article ID165422 2008

[39] D Yoon H Moon H Cheong J S Choi J A Choi and BH Park ldquoVariations in the Raman spectrum as a function ofthe number of graphene layersrdquo Journal of the Korean PhysicalSociety vol 55 no 3 pp 1299ndash1303 2009

[40] A C Ferrari J C Meyer V Scardaci et al ldquoRaman spectrumof graphene and graphene layersrdquo Physical Review Letters vol97 no 18 Article ID 187401 2006

[41] P Xu Y Yang I D Qi et al ldquoA pathway between Bernaland rhombohedral stacked graphene layers with scanning tun-nelling microscopyrdquo Applied Physics Letters vol 100 Article ID201601 2012

[42] S Latil and L Henrard ldquoCharge carriers in few-layer graphenefilmsrdquo Physical Review Letters vol 97 no 3 Article ID 0368032006

[43] Y Wang D C Alsmeyer and R L McCreery ldquoRaman spectro-scopy of carbon materials structural basis of observed spectrardquoChemistry of Materials vol 2 no 5 pp 557ndash563 1990

[44] P C Eklund J M Holden and R A Jishi ldquoVibrational modesof carbon nanotubes Spectroscopy and theoryrdquoCarbon vol 33no 7 pp 959ndash972 1995

[45] R Kostic M Miric T Radic M Radovic R Gajic and ZV Popovic ldquoOptical characterization of graphene and highlyoriented pyrolytic graphiterdquo Acta Physica Polonica A vol 116no 4 pp 718ndash721 2009

[46] K Stock C Schriever S Lochbrunner and E Riedle ldquoReactionpath dependent coherent wavepacket dynamics in excited stateintramolecular double proton transferrdquo Chemical Physics vol349 no 1ndash3 pp 197ndash203 2008

[47] L B Biedermann M L Bolen M A Capano D Zemlyanovand R G Reifenberger ldquoInsights into few-layer epitaxial gra-phene growth on 4H-SiC (000 1) substrates from STM studiesrdquoPhysical Review B vol 79 no 12 Article ID 125411 2009

[48] P R Bandaru ldquoElectrical properties and applications of carbonnanotube structuresrdquo Journal of Nanoscience and Nanotechnol-ogy vol 7 no 4-5 pp 1239ndash1267 2007

[49] L Kumari and S V Subramanyam ldquoStructural and electricalproperties of amorphous carbon-sulfur composite filmsrdquo Bul-letin of Materials Science vol 27 no 3 pp 289ndash294 2004

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


of carbon structures with preferred ABC stocking order[9] Annealing above 800∘C results in structural changes incarbon structure andmigration of carbon atoms towards freesilicide surface

The main topic discussed in this work is the comparisonof the structure observed for carbon layer at different sidesSiCC interface and CNi

2

Si interface As an experimentaltechnique Raman spectroscopy was chosen Due to largepenetration depth of visible light into silicide layer and siliconcarbide substrate it was possible to observe Raman spectraexcited from both sides of carbon film introduced betweenNiSiNiSi sequence and SiC substrate

2 Experimental

21 Samples The preparation of the samples was alreadydescribed in the paper concerning visible and ultravioletRaman study of carbon properties in ohmic contacts [10]

The samples will be called hereafter nsc1 2 (temperatureof the second annealing step 800∘C) nsc1 3 (temperature ofthe second annealing step 950∘C) and nsc1 1 (temperature ofthe second annealing step 1000∘C)

22 Apparatus and Experimental Conditions Raman scat-tering was measured with MonoVista 2750i micro-Ramanconfocal spectrometer (Spectroscopy and Imaging GmbHGermany)Themicroscopic part was based onOlympusBX51microscopeThe images frommicroscopewere recordedwithTM 2040GE imaging camera (JAI Japan) Motorized stage(Marzhauser GmbH Germany) allowed sample positioningwith accuracy equal to 100 nm in 119909 119910 and 119911 directions Thepositioning mechanism worked in feedback loop in order tostabilize the position of the stage Spectral part of the Mono-Vista was based on Princeton Instruments spectral devicesspectrograph SpectraPro 2750i (focal length 750mm) andspectral nitrogen-cooled CCD camera LN2048 times 512BIUVAR Spec-10 System with maximum efficiency at 250 nm

As an excitation light the Ar+ laser INNOVA 90C(Coherent USA) line (120582 = 488 nm) was used The power oflaser light on the sample was below 1mW in order to avoidthermal effects caused by focused laser light The gratingwith 1800 linesmm and blazed for visible spectral range wasused to measure Raman spectra Two types of excitationswere used through the silicide layer hereafter called also topexcitation and through SiC substrate hereafter called alsobottom excitation Although confocal configuration stronglyreduced signal generated outside the focal plane bottomexcitation introduces strong component of two-phonon scat-tering from 4H-SiC Huge difference between thickness ofSiC substrate and carbon film (sim1mm versus 3 nm) causedlarge signal from 4H-SiC substrate placed outside focal planeIn order to separate Raman scattering generated by carbonlayer from two-phonon SiC background two measurementswere necessary to get ldquopurerdquo spectrum of carbon layer Thefirst measurement was taken from the area covered withcarbon layer and the second one outside of this area Theldquopurerdquo carbon spectrumwas calculated by subtraction of two-phonon SiC spectrum measured outside of the covered areafrom the spectrum collected from area covered with carbon

and silicide layers In order to obtain good signal-to-noiseratio long irradiation time about 1 hour was applied formeasurement of single spectrum

23 Spectra Analysis The spectra were mathematically pro-cessed before analysis and interpretationMathematical treat-ment included offset removal baseline correction smooth-ing and normalization Measured spectra were normalizedto unityThe procedure used to calculate pure carbon spectrafrom bottom excitation measurements was similar to theprocedure used in the study of carbon inclusions at SiCSiO

2

interface [11]

3 Results

Raman spectra measured with visible excitation (120582exc =488 nm) and irradiation through the silicide layer are pre-sented in

Figure 1(a)mdashdata obtained for nsc1 2Figure 1(b)mdashfor nsc1 3Figure 1(c)mdashnsc1 1

The main plot of each panel shows the experimental datatogether with fitted Gaussian profilesThe upper inset in eachpanel compares experimental points with fitted function (thesumofGaussian profiles used to reconstruct the experimentaldata) The lower inset presents autocorrelation functionwhich attests to quality of fitting procedure The maximaof Gaussian profiles fitted to experimental data are given inthe main plot of each panel Random distribution of pointsaround zero level in autocorrelation function bears testimonyto the quality of fitting procedure Qualitative descriptionof the spectra measured for top excitation was alreadypresented in the paper comparing visible and deep-ultravioletRaman investigation of these samples and it will be notrepeated here in details [10]Themain difference between theresults presented here and in [10] is the number of profilesnecessary for reconstruction 119863 and 119866 bands in the case ofsamples nsc1 3 and nsc1 1 In [10] one Lorentzian componentwas sufficient to reconstruct each band whereas here twoGaussian profiles are necessary The detailed description ofthe results obtained from mathematical analysis performedin this work will be reported in the part Discussion

Subtracted spectra obtained from bottom excitation arepresented in Figure 2 The data in Figure 2 are presented inthe same way as in Figure 1 It means that the main plot ofeach panel shows experimental points and Gaussian profilesused in fitted function upper inset compares the whole fittedfunction with experimental data and lower inset presents theautocorrelation function The maxima of Gaussian profilesare given in the main plot of each panel In the case ofeach sample different numbers of Gaussian profiles arenecessary to reconstruct the course of experimental pointsFour profiles are necessary to reproduce the shape of thesubtracted spectrum in the case of nsc1 2 for nsc1 3 as manyas seven profiles are necessary and for nsc1 1 sixThe detaileddiscussion of the mathematical analysis will be presented innext the part of this paper In this chapter the description of


Ram

an in

tens

ity (a

u)

0

02

04

06

08

1

1603

1533

1356

002040608

1

1200 1400 1600 1800

1300 1400 1500 1600 1700 1800

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua 0

00501

015





(a)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


1355

1363

1604

1591

1582

1622



0002004006

002040608

1

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua


(b)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


1355

1361

1587

1582

1620




0

002004

0

0204

0608

1

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

minus002minus004

(c)





1525

1602

1394

0

Ram

an in

tens

ity (a

u)

002

004

006

008

01



0002004006008

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua 0

002001

minus002minus001


1707

(a)

Ram

an in

tens

ity (a

u)

0

001

002

003

004


17951694

1610

1566

1537

1397

1319


0

003

002

001

004

Wei

ghte

dre

sidua

0

003002001

Ram

anin

tens

ity (a

u)


minus001

minus003minus002

(b)

Ram

an in

tens

ity (a

u)

0

001

002

003

004

005


1792171016

44

1580

1517

1378


0

003002001

004005

Wei

ghte

dre

sidua

0001

Ram

anin

tens

ity (a

u)



minus001

(c)






0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

01

Ram

an in

tens

ity (a

u)

0

02

04

06

08


2704

2725

2660

2767

2716



minus01

(a)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

004



2642

2715

2720

2752

2700


minus004

(b)


4 Discussion








NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

13553 129 13545 334 131 13554 293 068 119863 band632 13627 761 069 13614 684 059 119863 band


16029 729 100 15912 403 100 15868 262 100 119866 band





















NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt


13935 966 023 13970 880 020 13782 1040 019 119863-band15253 588 100 15371 664 100 15175 392 100 119886-C


15800 330 079 119866-band graphite


16103 477 117


16444 331 013 119863



17952 424 014 17922 1592 017 iTALO or 2oTOmode










NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt











5 Summary





Acknowledgment


References












2




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


Ram

an in

tens

ity (a

u)

0

02

04

06

08

1

1603

1533

1356

002040608

1

1200 1400 1600 1800

1300 1400 1500 1600 1700 1800

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua 0

00501

015





(a)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


1355

1363

1604

1591

1582

1622



0002004006

002040608

1

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua


(b)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


1355

1361

1587

1582

1620




0

002004

0

0204

0608

1

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

minus002minus004

(c)





1525

1602

1394

0

Ram

an in

tens

ity (a

u)

002

004

006

008

01



0002004006008

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua 0

002001

minus002minus001


1707

(a)

Ram

an in

tens

ity (a

u)

0

001

002

003

004


17951694

1610

1566

1537

1397

1319


0

003

002

001

004

Wei

ghte

dre

sidua

0

003002001

Ram

anin

tens

ity (a

u)


minus001

minus003minus002

(b)

Ram

an in

tens

ity (a

u)

0

001

002

003

004

005


1792171016

44

1580

1517

1378


0

003002001

004005

Wei

ghte

dre

sidua

0001

Ram

anin

tens

ity (a

u)



minus001

(c)






0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

01

Ram

an in

tens

ity (a

u)

0

02

04

06

08


2704

2725

2660

2767

2716



minus01

(a)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

004



2642

2715

2720

2752

2700


minus004

(b)


4 Discussion








NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

13553 129 13545 334 131 13554 293 068 119863 band632 13627 761 069 13614 684 059 119863 band


16029 729 100 15912 403 100 15868 262 100 119866 band





















NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt


13935 966 023 13970 880 020 13782 1040 019 119863-band15253 588 100 15371 664 100 15175 392 100 119886-C


15800 330 079 119866-band graphite


16103 477 117


16444 331 013 119863



17952 424 014 17922 1592 017 iTALO or 2oTOmode










NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt











5 Summary





Acknowledgment


References












2




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


1525

1602

1394

0

Ram

an in

tens

ity (a

u)

002

004

006

008

01



0002004006008

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua 0

002001

minus002minus001


1707

(a)

Ram

an in

tens

ity (a

u)

0

001

002

003

004


17951694

1610

1566

1537

1397

1319


0

003

002

001

004

Wei

ghte

dre

sidua

0

003002001

Ram

anin

tens

ity (a

u)


minus001

minus003minus002

(b)

Ram

an in

tens

ity (a

u)

0

001

002

003

004

005


1792171016

44

1580

1517

1378


0

003002001

004005

Wei

ghte

dre

sidua

0001

Ram

anin

tens

ity (a

u)



minus001

(c)






0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

01

Ram

an in

tens

ity (a

u)

0

02

04

06

08


2704

2725

2660

2767

2716



minus01

(a)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

004



2642

2715

2720

2752

2700


minus004

(b)


4 Discussion








NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

13553 129 13545 334 131 13554 293 068 119863 band632 13627 761 069 13614 684 059 119863 band


16029 729 100 15912 403 100 15868 262 100 119866 band





















NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt


13935 966 023 13970 880 020 13782 1040 019 119863-band15253 588 100 15371 664 100 15175 392 100 119886-C


15800 330 079 119866-band graphite


16103 477 117


16444 331 013 119863



17952 424 014 17922 1592 017 iTALO or 2oTOmode










NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt











5 Summary





Acknowledgment


References












2




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

01

Ram

an in

tens

ity (a

u)

0

02

04

06

08


2704

2725

2660

2767

2716



minus01

(a)

Ram

an in

tens

ity (a

u)

0

02

04

06

08

1


0

1

02040608

Ram

anin

tens

ity (a

u)

Wei

ghte

dre

sidua

0

004



2642

2715

2720

2752

2700


minus004

(b)


4 Discussion








NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

13553 129 13545 334 131 13554 293 068 119863 band632 13627 761 069 13614 684 059 119863 band


16029 729 100 15912 403 100 15868 262 100 119866 band





















NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt


13935 966 023 13970 880 020 13782 1040 019 119863-band15253 588 100 15371 664 100 15175 392 100 119886-C


15800 330 079 119866-band graphite


16103 477 117


16444 331 013 119863



17952 424 014 17922 1592 017 iTALO or 2oTOmode










NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt











5 Summary





Acknowledgment


References












2




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of





NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

13553 129 13545 334 131 13554 293 068 119863 band632 13627 761 069 13614 684 059 119863 band


16029 729 100 15912 403 100 15868 262 100 119866 band





















NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt


13935 966 023 13970 880 020 13782 1040 019 119863-band15253 588 100 15371 664 100 15175 392 100 119886-C


15800 330 079 119866-band graphite


16103 477 117


16444 331 013 119863



17952 424 014 17922 1592 017 iTALO or 2oTOmode










NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt











5 Summary





Acknowledgment


References












2




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of














NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt


13935 966 023 13970 880 020 13782 1040 019 119863-band15253 588 100 15371 664 100 15175 392 100 119886-C


15800 330 079 119866-band graphite


16103 477 117


16444 331 013 119863



17952 424 014 17922 1592 017 iTALO or 2oTOmode










NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt











5 Summary





Acknowledgment


References












2




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of





NormInt

Max(cmminus1)

FWHM(cmminus1)

NormInt











5 Summary





Acknowledgment


References












2




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



Acknowledgment


References












2




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

Research Article Structure of Carbonic Layer in Ohmic Contacts:...

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