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Current Applied Physics 12 (2012) 712e717

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Current Applied Physics

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Effect of nitrogen addition on hydrogen incorporation in diamond nanorodthin films

A.R. Sobia a,*, S. Adnan b, A. Mukhtiar b, A.A. Khurram a, A.A. Turab a, A. Awais a, A. Naveed a, Q.J. Faisal a,H. Javaid a, G.J. Yu c

a Experimental Physics Labs, NCP, Quaid-i-Azam University, Shahdrah Valley Road, Islamabad 45320, PakistanbDepartment of Physics, Hazara University, Mansehra 21300, Pakistanc Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, PR China

a r t i c l e i n f o

Article history:Received 3 July 2011Received in revised form7 October 2011Accepted 13 October 2011Available online 24 October 2011

Keywords:NanocrystallineDiamond filmHydrogen incorporationIon beam analysis techniques

* Corresponding author. Tel.: þ92 51 2896085; fax:E-mail address: sobiaqau@gmail.com (A.R. Sobia).

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2011.10.008

a b s t r a c t

The effect of nitrogen addition in the feed gas on the finally incorporated amount of hydrogen in thediamond nanorods (DNRs) thin films has been investigated. The Raman spectroscopy measurementshelped to understand the structural and quality changes with increasing nitrogen gas flow rate duringCVD deposition. The hydrogen concentration was measured with 3.0 MeV He2þ beam using elastic recoildetection analysis technique and it was found that with the addition of nitrogen, the hydrogenconcentration was increased. The results of non-Rutherford backscattering spectroscopy (NRBS) used tomeasure the amount of nitrogen in the DNRs thin films have shown that the incorporated nitrogen isbelow the detection limit of NRBS technique. Our results suggested that the addition of nitrogen hasaffected the overall quality of diamond films in two ways; increasing the thickness of diamond films byincreasing the non-diamond carbon content and increasing the hydrogen impurity incorporation. Therole of nitrogen additive on diamond growth and hydrogen incorporation is discussed.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Nanocrystalline diamond (NCD) has been the subject of highscientific interest in recent years due to their unique physicalproperties [1,2]. In this sense, diamond nanorods (DNRs) are verypromising nanostructured carbon materials. DNRs have alreadybeen used for bio-chemical sensing [3] and enhancements inthermal management [4]. Other promising applications of DNRs arein microelectronics and electron field emission devices [5,6] wherethe negative electron affinity is utilized [7]. It is well known that thecrystalline quality of CVD diamond films and the content of defectsand impurities in these films strongly depend on the growthconditions. In order to achieve a tailored growth of NCD/DNRs filmswith well controlled properties, it is important to study the influ-ence of growth parameters on the formation of impurities and oncrystalline quality of DNRs films. The nitrogen is considered one ofimportant feed gas among argon, methane and hydrogen for thegrowth of DNRs. The effect of nitrogen addition on the morphologyof diamond nanocrystallites has been investigated, extensively by

þ92 51 2896084.

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many researchers [8e11] and it has been recognized that nitrogengas addition in the plasma has a strong effect on the growthmorphology of diamond nanocrystalline grains. In our previouswork [12] no noticeable changes in the morphology of the DNRswere observed with the addition of N2 rather the addition of highmethane content observed to be influential in terms of morphologychanges.

Hydrogen impurity content is a key to obtain a wide optical gapand high electrical resistivity [13e15] in diamond films. Hydrogenimpurity arises from the CH4 and H2 gases added in the CVDchamber for the deposition of diamond nanocrystallites. Thepresence of hydrogen, may affect the electrical and electronicproperties of DNRs thin films. Many sophisticated techniques[16e20] including elastic recoil detection analysis (EDRA) [21,22],have been employed to measure the total amount of hydrogenimpurity in diamond films. It is also important to investigate therole of nitrogen addition on hydrogen impurity incorporation inDNRs to understand how the growth parameters influence thehydrogen incorporation. However, only few reports has addressedthe effect of nitrogen addition on hydrogen impurity incorporationin CVD diamond films [23e26], especially in DNR thin films. Tanget al. studied the role of oxygen [23] and nitrogen additives [24,25]on hydrogen impurity incorporation in NCD films using infrared

A.R. Sobia et al. / Current Applied Physics 12 (2012) 712e717 713

spectroscopy. Michaelson et al. [26] measured much higherhydrogen content, (w15e20 at. %) in NCD by second ion massspectroscopy (SIMS) which is further supported by the diamondnucleation and growth mechanism associated with hydrogenparticipation proposed by Lifshitz et al. [27].

The objectives of this work are to understand the nitrogen effectson growth mechanism of DNRs. Here, we have determined thehydrogen content to observe the role of nitrogen addition on thegrowth of nanorods and the crystalline quality of the films. We havealso shed light onto the incorporation mechanism of hydrogenimpurity inDNRfilms. These studieswere carried out using ionbeamanalysis techniques (IBA). IBA techniques are generally used todetermine the total elemental concentrationof thinfilms. Rutherfordbackscattering spectrometry (RBS)usingMeVHe ionbeamshasbeenused extensively for the last thirty years [28]. It is a well establishedanalytical tool with specific capabilities still unsurpassed by newemerging analytical techniques. Non-Rutherford backscatteringspectroscopy (NRBS) is one of the useful IBA techniques in the case ofquantification of light elements (i.e. lighter than the substrate). Anaccessible and generally used method to determine the totalhydrogen content in thin films is Elastic Recoil Detection Analysis

Fig. 1. Plane view SEM images of DNRs films with (a) 0%, (b) 15% and (c) 2

(ERDA) [29]. ERDA is one of the few analytical techniques that areused for quantitativemeasurement of hydrogen concentration in thematerial using simulation software [30]. Kayani et al. [31] utilizedNon-RBS and ERDA techniques to observe the effects of depositionparameter on incorporated elements in amorphous carbon thinfilms. Kulisch et al. [32] studied hydrogen incorporation in ultra-nanocrystalline diamond (UNCD) films using nuclear reaction anal-ysis (NRA) and Fourier transform infrared (FTIR) spectroscopy. Theyemphasized on the role of hydrogen species incorporation on theformation of UNCD films. Buljan et al. [33] investigated the influenceof implanted dose and annealing on carbon and hydrogen concen-trations, clustering, and bonding using RBS and ERDA.

2. Experimental details

Thin films of diamond nanorods were deposited on siliconwafers with (111) orientation using Ar/H2/CH4/N2 mixtures, ina 2.45 GHz microwave plasma system. Nominal gas composition;10% CH4 and 15% H2 was used with variable Ar and N2 flow rates sothat the total gas flow was always 100 SCCM. The nitrogen contentwas varied in the range between 0% and 20%. The substrate

0% addition of nitrogen. The inset shows high magnification images.

A.R. Sobia et al. / Current Applied Physics 12 (2012) 712e717714

temperature was maintained at 800 �C during the depositionprocess, while total ambient pressure was kept at 150 Torr.

The as-grown films synthesized from different concentration ofN2 were characterized by scanning electron microscopy (SEM, LEO1530), and by micro-Raman spectroscopy (Ranishaw Raman micro-scope with the 514.5 nm line from an argon ion laser). Elementalanalyses of the sampleswere carried out using IBA techniques; NRBSand ERDA available at 5UDH-2 Pelletron Tandem accelerator facilityat NCP, I. R. Pakistan. The NRBS spectra were recorded using1.756 MeV proton beam focused to a diameter of 2 mm. The beamwas incident on the sample at the normal to the sample surface andthe backscattered particleswere collected on a silicon surface barrierdetector at a scattering angle of 152� with respect to the incidentdirection of the beam. The spectra were recorded for beam dose of20 mC. The setup chamber was maintained at a background pressureof approximately 10�6 Torr. The purpose of using proton beam at thisenergy is to enhance thedetectionsensitivity fornitrogenandcarbondue to an increase in the scattering cross section of these elements at1.756 MeV.

Recoil spectra of the samples were obtained using 3 MeV He2þ

(alpha) beam, incident at 75� from the normal to the sample surfaceand the forward scattered ions were collected using a silicon surfacebarrier detector at a scattering angle of 30�; the beam spot size was2 mm. An Aluminum stopper foil of 10 mm thickness was placed infront of the detector to stop alpha particles from entering thedetector. The samples’ surfaces were cleaned with ethanol beforeloading in test chamber. The hydrogen recoil spectra shown herewere recorded for He2þ beam dose of 20 mC. SIMNRA program wasused to simulate the experimental data for quantitative analysis.

Fig. 2. Raman spectra of DNRs films with (a) 0%

3. Results and discussion

3.1. Characterization of as deposited samples

Fig. 1aec shows the SEM images of three deposited samples.These micrographs demonstrate continuous film of DNRs withlength up to 1e2 mm. Fig. 1a shows the low (left side) and highmagnification SEM images of sample S1 deposited with 10%methane and 15% hydrogen in Ar rich plasma without the additionof nitrogen in feed gases. Fig. 1b and c shows the SEM images ofDNRs deposited with 15e20% nitrogen addition and are designatedas samples S2 and S3, respectively. It can be seen from Fig.1aec thatthe morphology of the DNRs is unaffected by the addition ofnitrogen gas. Raman spectra of samples S1 (0%), S2 (15%) and S3(20%) are shown in Fig. 2aec. The spectra show the presence ofdiamond phase (peak around 1332 cm�1), and sp2 bonded carbonphase (“G” band around 1586 cm�1). A shoulder around 1120 cm�1

is due to the trans-polyacetylene (TPA) or poly-CHx segments at thegrain boundaries of DNRs surface. The band associated with the sp2

bonded carbon phase has been broadened and shifted to higherfrequency after the N2 addition. Moreover, the relative intensity ofdiamond to graphite peak has also been reduced. The FWHM of thediamond peak was increased in the nitrogen doped samples rela-tive to the samples with undoped nitrogen, which is an indicationof the deterioration of diamond phase purity. These changes areconsistent with an increase in sp2 bonding character of the grainboundary carbon due to the incorporation of nitrogen. Similarchanges with the addition of nitrogen have been observed by otherresearchers [34]. The overall Raman signal intensity decreases with

, (b) 15% and (c) 20% addition of nitrogen.

A.R. Sobia et al. / Current Applied Physics 12 (2012) 712e717 715

nitrogen addition. This is due to the increased optical absorption inthe films with nitrogen content in plasma [35].

3.2. Ion beam analysis

Fig. 3 shows the NRBS spectra of the deposited samples. Atypical experimental and simulated NRBS spectrum of sample S3(with 20% N2 addition) is shown in Fig. 3a. In this figure, the peakresulting fromnear surface carbon is due to 12C(p,p)12C reaction. Nosignal of nitrogen (i.e peak due to reaction 14N(p,p)14N) wasobserved from the spectra of samples S1 and S3 as shown in theFig. 3b. The near surface carbon signal has been depicted from thepeak around 1350 keV. Thewidth of the carbon signal for sample S3is larger than that of the S1 indicating that the thickness of sampleS3 deposited with high nitrogen flow rate is higher. The resultspresented here suggested that incorporated nitrogen concentrationin the grain boundaries of DNRs or some other defective site issmall enough not to be detected by the NRBS. The present resultsupports our previously suggested formation mechanism of DNRsthat formation is not due to the presence of nitrogen at grainboundaries instead it is due to higher content of methane presentduring the deposition of DNRs in MPCVD system [12]. The higher

Fig. 3. a: Experimental and simulated NRBS spectrum of a sample S3 obtained using1.756 MeV Hþ beam. b. NRBS spectra of samples S1 and S3 obtained using 1.756 MeVHþ beam.

content of methane favors the formation of transpolyacetylenchains (TPA) i.e. formation of CeH chains and these chains governthe formation mechanism of DNRs instead of CeN radicals/chains.The high flow rate of nitrogen is utilized to increase the growth rateof diamond film and the contents of sp2ebonded graphite/non-diamond carbon. The increase in growth rate of DNR films asa function of the increased nitrogen flow rate in the feed gases isattributed to several factors. Nitrogen based radicals can act as C-containing precursors during diamond synthesis [36] as N-basedradicals can act as precursors for diamond nucleation sites. Fromthe results of Raman spectra it can be said that the addition ofnitrogen is more influential in term of structural changes ratherthan in morphology changes.

The ERD result for the sample S3 is shown in Fig. 4a and forcombined three samples is shown in Fig. 4b. Area under the peak isthe number of He2þ encounters with hydrogen atom that isproportional to the concentration of hydrogen in the film. Thehydrogen contents determined from the simulation of themeasuredspectra using the SIMNRA simulation code are listed in Table 1. The

Fig. 4. a: Experimental and simulated forward scattering spectrum of sample S3 ob-tained using 3 MeV He2þ beam. b: ERD spectra of samples S1, S2 and S3 obtained using3.0 MeV He2þ beam.

Table 1Summary of nitrogen addition in feed gases in MPCVD system, film thickness andhydrogen incorporation in DNRs films determined using NRBS and ERDA.

Samples N addition Film thickness H at. %

S1 0% w4.2 mm � 0.02 mm 1.8 � 1021 (1.6%)S2 15% w7.3 mm � 0.02 mm 4.3 � 1021 (3.9%)S3 20% w8.8 mm � 0.02 mm 5.7 � 1021 (5.2%)

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hydrogen concentration is estimated to be approximately 1.6 atomic% (1.8 � 1021 atoms/cm3) for S1 (shown by the line connected withsquares), approximately 3.9 atomic % (4.3 � 1021 atoms/cm3) for S2(shown by the line connected with circles) and approximately 5.2atomic % (5.7�1021 atoms/cm3) for S3 (shownby the line connectedwith triangles). The concentrations in atoms/cm3 have been deter-mined by taking the density of carbon as 1.1 � 1023 atoms/cm3.Hydrogen incorporation in these films has been increased with theincrease in flow of nitrogen during the growth process (Table 1).This observation suggests that the addition of nitrogen in the plasmasignificantly enhances the incorporation of hydrogen in DNRs. Themaximum incorporation of hydrogen about 5.2% was found insample S3. Sample S3 was deposited from 20% addition of nitrogenin the Ar/CH4/H2 gas mixture and had high sp2 content, as wasobserved from the Raman spectrum (Fig. 2c). The addition ofnitrogen in gas phase for the deposition of samples resulted inincreasing the non-diamond content as was also observed by otherresearchers [10]. The non-diamond content includes mixed sp2 andsp3 bonding states. Thus, there is a possibility for incorporatedhydrogen to make bonds with non-diamond carbon atoms. Theargument for incorporated hydrogen being bonded to carbon is alsosupported by the high intensity TPA peaks around 1105 cm�1

present in Fig. 2c. By comparing Raman spectra of samples S1 andS3, one can conclude that 20% N2 addition degrade the crystallinequality of DNRs films and heavily provoke large amount of hydrogenimpurity incorporation. In other words, the crystalline quality andhydrogen incorporation in diamond films is sensitive to the amountof N2 addition.

It is important to investigate H incorporation in DNRs due to thereason that the growth mechanism depends on ratio of H and C.Experimental evidence for its importance arises from investiga-tions in H2/CH4/Ar gas plasmas [37,38]. If too little hydrogen ispresent, or the H:C ratio is lowered to w2, only non-diamondcarbon films are formed [39]. The different conditions, i.e., gasmixture, temperature, and pressure reported in the literature forthe diamond growth simply serve to establish the ratio ofhydrogen to methane to optimize the filmmorphology and growthrate.

The main steps in DNRs growth includes hydrogen absorptionfor the formation of TPA (like CeH) and then removal/abstraction ofhydrogen for the rearrangement of CeC chain like structures [12].Hydrogen is involved in major steps of DNRs growth, if any of thesesteps are not run thoroughly, then hydrogenwill be buried into thediamond lattice as an impurity. In other words, the probability for Has an impurity in a diamond film during the growth processstrongly depends on the rate of removal of H from the films. Withthe addition of nitrogen gas in MPCVD chamber, non-diamondcarbon content and TPA chains increased as can be observed bythe high intensity of graphite and TPA peaks in Fig. 2c as comparedto Fig. 2a. The presence of nitrogen also strongly favor theadsorption of CH2 species [25], the chance for H being buried asimpurity in the diamond lattice is higher for the samples depositedwith nitrogen addition as compared to that without nitrogenaddition. This is the reason for high incorporation of hydrogen insamples S2 and S3 deposited for 15% and 20% nitrogen. Theseresults, combined with our previous investigations on similar

systems, show that with nitrogen addition, the amount of hydrogenimpurity is significantly increased due to low crystalline quality ofDNRs films.

4. Conclusions

The DNR thin films with variable nitrogen concentration wereanalyzed using IBA techniques and using Raman spectroscopy.NRBS results showed that negligible amount of incorporatednitrogen is present in the finally deposited samples. The additionof nitrogen in gas phase has been utilized to increase the non-diamond content in the films. The incorporation of hydrogen inthe samples was found to increase with increasing the addition ofnitrogen in the feed gases in deposition chamber. The increase inincorporated amount of hydrogen is related to the low crystallinequality of the film due to increase in non-diamond contentpresent in the samples as is supported by Raman spectroscopymeasurements.

Acknowledgments

The support of the Key Laboratory of Nuclear Analysis andTechniques Chinese Academy of Sciences Shanghai China fordeposition of films is gratefully acknowledged. The correspondingauthor acknowledges the support of Higher Education Commissionof Pakistan.

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