ELSEVIER Thin Solid Films 281-282 (1996) 289-293

Carbon nitride thin films .deposited by the reactive ion beam sputteringtechnique

Satoshi Kobayashi a.*, Shinji Nozaki n, Hiroshi Morisaki a, Shigeo Fukui b, Susumu Masaki b

• Department 0/Communications andSystems, The University ofElectro-Communications. /-5-1 Chofugaoka, Chofu, Tokyo 182. Japanh Department ofElectronic Engineering, Tokyo National College ofTechnology, 1220-2 Kunugida-machi, Hachiojl, Tokyo 193, Japan

Abstrllct

Carbon nitride (C1-xNx) thin films were deposited at room temperature onSiand Gesubstrates by the reactive ion beam sputter deposition(IBSD) technique. A pure graphite target has been sputtered with a nitrogen ion beam extracted from a Kaufman-type ion source. The filmswere characterized byRaman spectroscopy, IR absorption spectroscopy and X-ray photoelectron spectroscopy (XPS). There was no clearindication of the presence of C-N single bonds in the films. Raman and IR absorption spectra show two characteristic bands; a broad bandcomposed of graphite G-band and disordered D-band of carbon, and the other associated with C=N triple bonds. TheDvband suggests Ihepresence of an amorphous carbon network. The XPS spectra show the presence of C=N triple and C=N conjugated double bonds. As areliable structural model of the C,_xNxfilms, an amorphous network with C=N triple bond terminations, as well assubstitutions ofnitrogeninto the network, have been proposed. Reactive IBSD with high acceleration energy andlor the irradiation of the primary ions onto thesubstrate have been found tobe important to form a sp)·rich amorphous C-Nnetwork.

Keywords: Carbon; Deposition process; Nitrides; Sputtering

1. Introduction

Preparation (;i carbon nitride (CI-xNx) and itshydrogen­ated (C,-xNx:H) films has been attempted by various dep­osition processes, such as glow discharge decomposition ofhydrocarbon [1], de and rf sputtering [2,3], etc. It has beenfound that the produced materials contain various forms oforganically J:'rynded structures being analogous topolymericcompounds, Recently, Liuand Cohen [4,5] have predictedthat carbon nitride canform stoichiometric C3N4with acrys­talline structure similar to that of I3-Si3N4. They have alsostudied the electronic structure of I3-C3N4' predicting thebandgap energy to be 3.2eV. This semiconducting propertyof f3-C3N4 hasboosted the study on this material. Inorder toinvestigate the semiconducting property of carbon nitride, itis essential to develop a suitable deposition technique todeposit this material. In the present study, we applied thereactive ion beam sputter deposition (IBSD) technique forthe preparation ofCt-xN'x thin films, using nitrogen gas andcarbon target. Thereactive IBSD allows anindependent con­trol of the sputtering ion energy and the conditions of thesputtering atmosphere.

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In this report, we will show some irradiation effects ofnitrogen ions on the chemical bonds of carbon and nitrogenwithin the deposited films.

2. Experimental

C'-xNx thin films were deposited at room temperature oneither Si or Ge substrates by the reactive IBSD technique.The scheme of the film deposition arrangement used in thiswork is shown inFig. I. Ion source, target and substrate arearranged with a configuration as illustrated in this figure inavacuum chamber with a background pressure of I X 10- 6

Torr. The deposition chamber is the same as the one forgrowing diamond films [6], where argon and hydrogen gasinstead ofnitrogen ill introduced. The flow rate ofthenitrogengas into the ion source was varied between 4-5 seem toobtaina designated pressure, using a mass flow controlling system(Tylan general FC-2900v: A pure graphite target with 10em diameter was sputtered with the nitrogen ion beamextracted from a multipole-anode Kaufman-type ion source.The ion source has a 3 em-diameter accelerator grid, beingable to accelerate the nitrogen ions up to I keY. Both theincident angle of the ion beam towards the target and theangle between the target and thesubstrate were set to "" 45°.

290 S. Kobayashi etal./TilitlSolid Fi/ms 281-282 (1996) 289-293

3. Results and discussion

10 11

Non-irradiated

0.8 1.0 0.6 0.8 1.0

11 10 10

Ion-lrradimed

Table IlBS deposition conditions ofCI-xNxfilms

can be ignored Ui <10- 20 A cm- 2). The deposited filmswere characterized by infrared (IR) absorption spectroscopy,Raman spectroscopy and X-ray photoelectron spectroscopy(XPS).

3.1. Composition

N2 PNX 10-4 (Torr) 3.0gas ED (keV) 0.6

Van (V) 120jl (rnA cm-2) 0.5ion source outletjdA cm") _10- 12

substrateNitrogen 11concentration(at.%)

p=3.0e-4 Torr

~.-si,L~ substrate

/.s, \~~~

Fig. 1.Scheme of the reactive ionbeam sputtering deposition apparatus.

O.2'l"M"'l""l'"I"'l""l'"I"'l""l'"I"M'T'l"'I"T"lTT"n-T"IlTMMT'1m

ee=800eV

6" 0.16Eu

I 0.12

~c.g 0.08

~::s 0.04c.,) d,-10-IINcmt d.<10-tOA'cm'

~ ~'12-1 0 1 2 3 4 5 6 7 8 9 101112

position d. (em)Fig.2.Spatlsl profile ofion beam current density. Arrows indicate substratepositions.

At first, a series of samples was prepared varying the ionbeam energy (EB) from 0.6-1.0 keV, while the ion currentdensity Vj) at the immediate outlet of the ion source, theanode bias (Van) and thegas pressure (PN) during the dep­osition experiments were fixed at 0.5 rnA em-2, 120 V, and3.0X10- 4 Torr, respectively. Thedistance between the sub­strate and the center of the axis of the ionbeam d, was fixedto7 cm.A second series ofsamples was preprared by chang­ing d, to 12em, in order to avoid the effect of primary ionirradiation onto thesubstrate. According to the beam profilemeasurement of iii it showed the Gaussian shape with aFWHM of 2.7 em with the maximum of the beam profile atthe center of the target plane as illustrated in Fig. 2. Whends=7 em, the primary ion current density at the surface ofthe substrate isestimated tobeinthe order of 10-12 Aem-2.

On the other hand (ds =12 ern), the effect of primary ions

The carbon nitride films deposited by reactive IBS werefound tobe oxygen-free; only nitrogen and carbon peaks wereobserved in a wide scan XPS-spectrum. Thechemical com­position of each sample was estimated from the XPS peakareas. It should benoticed that the total nitrogen concentrationof the films were always about 10at.% independent of thevarious sputtering conditions, summarized inTable 1.

3.2. Raman spectroscopy

The Raman spectrum of a typical sample, which isobserved using the 488 om line of an argon ion laser with apower of2 mW, isshown inFig. 3. There aretwo character­istic bands in the spectrum: one is a broad band between1100-1700 cm- I, and the other is a peak in the range of2100-2300 em- I. The former can be divided in two peakswhich are known tobe associated with graphitic sp2-bondedcarbon (G-band): 1575 cm - I and disordered carbon (D­band): 1360cm-1[7,8], [seeFig. 3(a)]. Thepeak at 2200cm-) is assigned to CaN triple bonds [see Fig. 3(b)]. Theformer band suggests the presence ofanamorphous network,and the latter for cyano radicals [8]. Concerning the originof the D-band, there is anexplanation that is associated withSp2 graphitic microdomains [9].

3.3. IR absorption spectroscopy

Fig. 4 shows the IR absorption spectrum of the sampledeposited ona crystalline Ge (c-Ge) substrate. It is remark­able that the IR absorption spectrum peak in the range of1100-1700 cm- I

, which is corresponding to the 'G-band'and 'D-band' in the Raman spectrum (Fig. 3), is also

S. KobayaslJi etal.ITIJill Solid Films 281-282 (1996) 289-293 291

Df

+

ES",1.0kaV

.....'.'......".::

~:;~......

'.: '.2000 1800 1600 1400 1200 1000 600

wavenumber (em-I)

Fig. 6. IR absorption spectra of nitrogen ion-irradiated CI_~N.. subtractedbackground.

The IR absorption spectra of ion irradiated and non-irra­diated samples, which have been plotted after background­subtraction) am summarized in Fig. 5 and Fig. 6. A series ofnon-irradiated samples show a drastic change of the absorp­tion between 900-1800 em - J when the ion energy Eo

increases from 0.6 to0.8 keV, asseen inFig. 5.When ED =0.6keY, a broad D-band a~ about 1350 em -l, which isobservedwhen a beam energy is higher than 0.8 keY, is not observedat an. In comparison to the series of ion-irradiated samples,even inthe case ofED=0.6 keV,astrong D-band isexhibited,as illustrated in Fig. 6. Furthermore, it was found that theintegrated intensity increases as ED increases. The absorptionpeak found at 1100 em- I is due to silicon oxide on theopposite surface of the c-Si substrate.

3.4. X-ray photoelectron spectroscopy

The NIS XPS spectra ofthe respective samples, which wereobserved after argon ion etching. are shown in Fig, 7 and

:e-t:

:0:- :3

'2 .e::J.Q

lG ~-.. Q)C\1 0- faQ)0 -et:ttl 0.a w.. .c

Es=0.6kaV0 lGf/)

~~

2200 2000Raman shift (em-I)

Fig. 3.Typical Raman spectrum ofCI-xN..: incident light is488 nm line ofan argon ion laser with powerof2 mW. Dashed lines are Gaussian fits.

4000 3000 2000 1500 1000 500

Wavenumber (cm'")Fig. 4. TypicallR absorption spectra ofC1-.lN...

observed. Note thattheG- and Dsbands are IR-forbidden inthe carbon network [10]; these bands become IR activebecause of symmetry-breaking of the network byincorpora­tion ofnitrogen atoms. Anotherabsorption peakatabout 2200em - I, which isassociated with C=N stretching mode, isalsoobserved in the spectrum [11].

If the IRabsorption band between 1100 and 1700 em- I isdivided in the G- and D-bands, as in thecase of the Ramanspectrum, we note that the area of the Gaussian-curve of theG-band is smaller than the D-band, although they are almostthe same amplitude in thecase of theRaman peak. Kaufmanet aI. have alsostudied IR absorption and Raman spectra ofcarbon nitride films; the nitrogen-doped samples with a Nconcentration upto 20at.% have contained mostly graphiticnetworks showing strong G-bands. They have found that theincorporation of nitrogen in the networks forms an aromaticring-like (pyridine-like) structure, which makes the G-bandIR-active [8] . If this is thecase, the strong IRD-band wouldbe associated with the termination of amorphous networksby C=N triple bends,

292 S. Kobayashi etal./Thifl Solid Films 281-282 (1996) 289-293

fig. 8. Here theconditions of ion etching areas follows: ionenergy; 2 keY, ioncurrent density; 20 rnA and the incidentangle to the sample surface; 70°. Each spectrum is dividedinto four elements peaked at 398.4-398.9, 399.9-400.3,400.1-400.7 and 402.9-403.9 eV. The first and third onesare attributed to N=C triple bond and N=C conjugated dou­ble bonds [12], respectively, the second and fourth beingunknown. Thefact that thepeak at399.0 eV which isassignedtoa N simple substance [13) is not observed gives theclearevidence that the nitrogen atoms in the samples are notoccluded but bonded with carbon atoms. Though the inte­grated intensities of theN=C conjugated double bond peaksof the ion-irradiated samples are slightly larger than thatofnon-irradiated ones, themolar fractions ofC=N triple bondsand C=N conjugated double bonds in most of the samplesare about 45% and 40% of the N1s spectrum area,respectively.

eB=1.0keV

Ea=O.6keV

Recently, Marton et at. have made an XPS study on theircarbon nitride films prepared by various methods andassigned thepeakaround 398.5 eVas thenitrogen componentwhich istetrahedrally bonded with carbon [ '14). Theirassign­ment isbased onthefact that the similarXPS peak isobservedin hexamethylenetetramine which has tetrahedrally bondednitrogen. However, it should be noticed that this moleculehas a trigonal strained Sp2 orbital covalent with tetrahedralSp3 ofcarbon [15J.This means thathexamethylenetetraminemay notbea suitable material for the identification ofa C-Nsingle bond within the hypothetical ~~C3N4 crystal. Thedetails will be reported elsewhere.

The IR D-band appears only when ED is higher than 0.6keVorprimary ions irradiate the surface ofthesubstrate, Thisfact indicates thatthe carbon sputtering by nitrogen ions withhigh enough primary energy and/or the irradiation of theprimary nitrogen ions onto thesubstrate areimportant toforma nitrogen-incorporated amorphous carbon network. AtEn =0.6 keV without primary ion irradiation, the film ismainly composed of low molecular compounds with C=Ntermination. The effect of primary-ion irradiation is underinvestigation. Some of theeffects of primary-ion irradiationwould becleaning ofthesurface ofthesubstrate andenhancedmigration of incident substances at thesurface.

4. Conclusions

CI-xNx thin films were deposited at room temperature bythe reactive IBSD technique. A pure graphite target was sput­tered with a nitrogen ion beam. Raman and IR absorptionspectra indicated two distinct bands, onecharacteristic-bandof the amorphous carbon network and the other associated toC=N triple bonds. The XPS spectra showed the presence ofC=N triple and C=N conjugated double bonds. No clearindication of the presence of C-N single bonds has beenobtained inthe C1-xNx films prepared by the IBSD technique.The actual structure of the CI-xNx is predominantly com­posed of an amorphous network with C:::::N triple-bond ter­mination. Reactive IBSD, with an ion energy between 0.6and 1.0 keV andlor the irradiation of the primary ions ontothe substrate, were found to be important to form the amor­phous C-N films.

Acknowledgements

Wewould like to thank Ms. Y. Kubo, Jaseo Corp. for hercooperation on Raman measurements.

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