Correlation between hardness and structure of carbon-nitride thin films obtained by reactive pulsed laser deposition

Thin Solid Films 388 Ž2001. 93᎐100

Correlation between hardness and structure of carbon-nitride thin films obtained by reactive pulsed laser deposition E. Gyorgy ¨ a , V. Neleaa , I.N. Mihailescu a,U , A. Perrone b , H. Pelletier c , A. Cornet c , S. Ganatsios d , J. Werckmanne a

Lasers Department, National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-54, RO-76900, Bucharest V, Romania b Department of Physics, Uni¨ ersity of Lecce and Istituto Nazionale Fisica della Materia, 73100 Lecce, Italy c Laboratoire d’Ingenierie des Surfaces de Strasbourg (LISS) ENSAIS-24 Bld de la Victoire 67000 Strasbourg, France ´ d Technical-Educational Institute, Kozani, Greece e I.P.C.M.S. (UMR 7504 du CNRS) Groupe ‘Surfaces-Interfaces’ 23, rue du Loess 67037 Strasbourg Cedex, France Received 5 May 2000; received in revised form 20 December 2000; accepted 9 February 2001

Abstract Carbon-nitride thin films were synthesized by reactive pulsed laser deposition from graphite targets in low-pressure nitrogen. X-Ray photoelectron spectroscopy and nanoindentation measurements were performed in order to establish a connection between the composition, structure and hardness of the obtained thin films. We studied the variation of the sp 3rsp 2 C bonded to N ratio with the increase of the N content of the thin layers. We found that the value of this ratio mainly determines the hardness of the carbon-nitride layers. The stability in time andror under thermal heating of the CN bonds formed was also tested. 䊚 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Nitrides; X-Ray photoelectron spectroscopy ŽXPS.; Hardness

1. Introduction Reactive pulsed laser deposition ŽRPLD. has been applied during the last few years to the synthesis of protective coatings, various types of carbides and nitrides w1᎐5x. The main advantage of lasers in synthesis and deposition of thin films is the better purity of deposited films due to the use of radiation for processing. The interest in trying to synthesize carbon-nitride thin films largely stems from the prediction made by Liu and Cohen in 1989 w6,7x. Their calculations suggested a high bulk modulus of the hypothetical covalent bonded carbon-nitride solid, ␤-C 3 N4 , with a strucU

Corresponding author. Tel.: q40-1780-5385; fax: q40-1423-1791. E-mail address: [email protected] ŽI.N. Mihailescu .

ture similar to ␤-Si 3 N4 . The hardness of such new material would have to exceed that of diamond. We reported in our previous works the synthesis and deposition of uniform and adherent, substoichiometric carbon-nitride, CN x , thin films. We conducted RPLD experiments using a nuclear grade graphite target submitted to multipulse XeClU Ž ␭ s 308 nm, ␶ FW HM ( 30 ns. excimer laser in low-pressure nitrogen or ammonia. The incident laser fluences were 3, 6, 12 and 16 Jrcm2 w8᎐11x. Our studies showed an increase in the nitrogen content of the deposited films with the increase in incident laser fluence and the pressure of the chemically active gas ŽN2 or NH 3 .. Our analyses also revealed the presence of single, double and triple bonds between the C and N atoms. In this paper we try to identify a direct connection between the incorporation of nitrogen into the films, the type of chemical bonding between C and N atoms

0040-6090r01r$ - see front matter 䊚 2001 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 0 8 4 0 - 9

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and the hardness of the structures obtained. In this way, we expect to obtain a confirmation of the initial prediction, that when reaching higher NrC ratios the CN x thin films become increasingly harder nearing the value characteristic of diamond. We performed new experiments at an incident laser fluence of 16 Jrcm2 at different ambient nitrogen pressures. We analyzed the resulting structures by X-ray photoelectron spectroscopy ŽXPS. and hardness indentation. To test the composition and mechanical stability of the structures obtained the XPS studies were repeated approximately 1 year later. The samples were meanwhile exposed to air and humidity in normal conditions. In order to remove potentially volatile species from the films, some of the structures obtained were subjected to prolonged heating in air. They were analyzed by XPS both prior to and after the thermal treatment of different durations. 2. Experimental The experimental apparatus is schematically shown in Fig. 1. The XeClU excimer laser beam Ž ␭ s 308 nm, ␶ FW HM ( 30 ns. was focused on the target surface at an incident angle of 45⬚. We used high purity Žnuclear grade. graphite targets. The incident laser fluence was set at a value of Fs s 16 Jrcm2 . The laser pulses succeeded each other at a frequency repetition rate of 10 Hz. A series of 10 4 laser pulses was applied to deposit a single film. In order to avoid fast drilling, the target was rotated during the multipulse laser irradiation with a frequency of 3 Hz. In this way, approximately 320 laser pulses were directed at the same

location inside the ring crater, which was dug into the target’s depth. The synthesized material was collected on Si ²111: wafers. Prior to the deposition, the collectors were cleaned with acetone, rinsed with distilled water, etched in an HF bath, then again rinsed with distilled water. During depositions the Si collectors were kept at room temperature or heated at 250⬚C. The target-collector separation distance was 4 cm. Before each deposition the irradiation chamber was evacuated down to a residual pressure of ; 10y4 Pa. The residual gas was monitored with a quadrupole mass spectrometer ŽAmetek MA 100.. Particular attention was paid to water vapor traces due to their very aggressive contaminant action. The chamber was then filled with high purity nitrogen. The gas pressure was dynamically set at values within the 1᎐100 Pa range. Most depositions were performed at 1, 5, 10, 50 or 100 Pa. We applied a laser cleaning treatment to the graphite targets surface. For this purpose, the collector surface was shielded during the action of the first 1000 laser pulses with a shutter, which was interposed between target and collector, parallel to their surfaces. To obtain information about the composition and the chemical bonding between atoms in the carbonnitride layers, we recorded XPS spectra. We used Mg K ␣ Ž1253.6 eV. radiation for excitation and a concentric VSW100 hemispherical analyzer working at a base pressure of 5 = 10y8 Pa. The integral spectra were recorded in the C1s, N1s and O1s regions. More accurate recordings were also made for the study of the C1s and N1s lines by the deconvolution peak synthesis method. In some cases the samples were sputtercleaned in order to remove the oxygen contamination

Fig. 1. Experimental setup.

E. Gyorgy ¨ et al. r Thin Solid Films 388 (2001) 93᎐100

from the near-surface region. The cleaning was performed by sputtering with an Ar ion beam for several minutes. We observed that prolonged Žseveral minutes. Ar ion beam sputtering not only removes oxygen, but also causes a nitrogen deficiency. This is due to the preferential sputtering of the nitrogen atoms over carbon during the Ar ion beam irradiation. We therefore recorded the spectra for the peak synthesis method without any surface cleaning. The samples were subjected to XPS analysis immediately after the deposition as well as about 1 year later. Also, in order to further study the stability of the chemical composition of the synthesized layers, we applied heat treatment in vacuum as follows: 1. 2. 3. 4.

from 100⬚C up to 200⬚C for 30 min; from 200⬚C up to 300⬚C for 30 min; from 300⬚C up to 350⬚C for 30 min; at 350⬚C for 1 h.

We recorded the C1s and N1s spectra of the films after each step of the heat treatment. The mechanical properties of the deposited layers Žhardness and Young modulus into the sample depth. were measured with a nanoindenter ŽXP II Nanoinstrument Inc.. equipped with a Berkovich triangular pyramidal indentor. We worked with six indentation loads: 0.5, 1.5, 2.5, 5, 7.5 and 10 mN. The characteristic hardness and Young modulus of the thin films were obtained using the data of the load᎐displacement curves. These curves were drawn by continuously increasing the applied load and measuring the penetration depth of the indentor. We made 10 indentations with the same load and give the arithmetical mean of these values. The nanoindentation analysis was carried out according to the Oliver and Pharr method w12x.

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3. Results 3.1. X-Ray photoelectron spectroscopy analysis The complete XPS spectra of the films showed a minor presence of oxygen besides the C1s and N1s peaks. After cleaning of the surface by an Ar ion dose this O1s peak became hardly distinguishable. In the same time, the N1s peak decreased due to preferential desorbtion of the nitrogen atoms from the surface regions under the action of the Ar ion beam. Fig. 2 shows the integral C1s, N1s and O1s spectrum of the film deposited at 1 Pa N2 as recorded before the Ar ion cleaning Žcurve a. and after it Žcurve b.. Our XPS analysis indicated that the N content in the films depends on the deposition conditions. From the spectra we derived the NrC atomic ratios ŽTable 1. for the films deposited at 1, 5, 10, 50 and 100 Pa of N2 . During deposition the collector was kept at room temperature Žcolumn 1. or heated at 250⬚C Žcolumn 2.. From Table 1 we observe the following trends: 1. The amount of incorporated nitrogen in the deposited layer increases with increasing nitrogen ambient pressure in the deposition chamber. Thus, the ratio NrC which is 0.2 for P s 1 Pa increased up to a maximum value of 0.7, when the collector was kept at room temperature. 2. The NrC atomic ratios are smaller when depositions are conducted on heated substrates as compared to the deposition performed at room temperature. Thus, the NrC ratio for heated collectors was 0.2 at 1 Pa and did not exceed 0.6 at 100 Pa N2 . We note a reasonable agreement of the data pre-

Fig. 2. Integral XPS spectra of the film deposited at 1 Pa N2 , recorded in the zone of the C1s, N1s and O1s lines, before cleaning Žcurve a. and after the Ar ion sputtering Žcurve b..

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Table 1 NrC atomic ratio values of the films deposited at room temperature and at 250⬚C for different ambient pressures of N2 P wPax 1 5 10 50 100

NrC ŽRT. 0.2 0.6 0.7 0.7 0.7

NrC Ž250⬚C. 0.2 0.5 0.5 0.5 0.6

sented in the Table 1 with the results of RBS measurements w10,11x performed on identical samples. The C1s spectra of the films deposited at 5, 10 and 50 Pa ambient nitrogen at room temperature are shown in Fig. 3a᎐c. We have chosen these spectra for the peak synthesis analyses because of the higher nitrogen content of these samples. The spectrum recorded in the case of the film deposited at 5 Pa N2 ŽFig. 3a. was deconvoluted into four lines, corresponding to different bonding states of the carbon atoms. The components of the C1s line having binding energies of approximately 288.0, 286.4, 285.3 and 284.6 eV were assigned to the C᎐N single, C⬅N triple, C⫽N double bonds and to a pure carbon network ŽC⫽C.. The C1s spectra corresponding to the films deposited at 10 Pa N2 ŽFig. 3b. and 50 Pa N2 ŽFig. 3c. were also deconvoluted, the lines having the maximum intensity at 289.4, 288.0, 286.4 and 285.3 eV. We assigned the peak corresponding to the highest energy, at 289.4 eV to the C⫽O bonds and the line having the maximum intensity at 288.0 eV to C᎐N single bonds. The peak at 286.4 eV could originate from sp 3 hybridized C singly bonded to C or from the presence of C⬅N bonds. Indeed, the peak corresponding to sp C bonded to N in polyacrylonitriles is positioned at 286.7 eV. Furthermore, we ascribe the 285.3 eV peak due to C⫽N double bonds. Our assignments are in good accordance with recent reports in literature w13᎐19x. The peak with the highest energy was attributed to oxide contamination, formed due to oxygen impurities in the working nitrogen gas. When the nitrogen pressure in the reaction chamber was increased, the number density of oxygen impurity atoms is higher as well w20x. The probability of an oxidation reaction was thus increased leading to the formation of oxides. This assumption is fully supported by the data in Table 2 where we give the relative change of the integrated intensities corresponding to the oxide peaks. We calculated the ratio between the oxide peak area and the total area of the C1s peaks. We notice that in the case of the film deposited at 50 Pa N2 the relative ratio of the oxide peak is doubled as compared to the one obtained at 10 Pa N2 . Fig. 4a᎐c shows the N1s spectra of the films de-

Fig. 3. Deconvoluted C1s XPS spectra of the films deposited at 5 Pa Ža., 10 Pa Žb. and 50 Pa Žc. N2 .

posited at room temperature and at the same nitrogen pressures of 5, 10 and 50 Pa, respectively, as in the sequence given in Fig. 3a᎐c. In the deconvoluted spectra we assigned the two peaks, centered at 400.6 and 399.1 eV to the C⫽N double and C᎐N single bonds, Table 2 The C⫽OrC1s ratio between the integrated intensity of the oxide peak and the total C1s peaks for 10 and 50 Pa ambient N2 pressure P wPax

C⫽OrC1s

10 50

0.08 0.16

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Table 3 The C᎐NrC⫽N ratio between the integrated intensity of the peak corresponding to the C᎐N single and C⫽N double bonds calculated from the deconvoluted C1s and N1s spectra, for different ambient pressures of N2 P wPax

C1s C᎐NrC⫽N

N1s C᎐NrC⫽N

5 10 50

0.98 0.35 0.37

1.1 0.37 0.37

284.6 eV. is prevalent in the case of the film obtained at 1 Pa N2 . The three other lines with maximum intensities at 288.0, 286.4 and 285.3 eV are assigned to the C᎐N single, C⬅N triple and C⫽N double bonds. In order to check the stability with time in open air of the obtained structures, we repeated the XPS measurements approximately 1 year after deposition. No significant changes in the NrC atomic ratio values were observed as compared with the previous results given in Table 1, obtained immediately after the depositions. We also tested the stability of the obtained structures on heating in vacuum. We recorded the XPS spectra after each step of the heat treatment. We calculated the NrC atomic ratio as well as the integrated intensity C᎐NrC⫽N ratio. These values were found to be identical with those given above. On the other hand, at elevated annealing temperatures a graphitization is taking place. According to Chowdhury et al. w22x at annealing temperatures above 550⬚C the number of C⬅N Žsp. bonds is diminished preferentially as compared to the nitrogen᎐nitrogen bonds. 3.2. Hardness measurements Fig. 6a,b shows the hardness values corresponding to different depths inside the deposited layers. Here the Fig. 4. Deconvoluted N1s XPS spectra of the films deposited at 5 Pa Ža., 10 Pa Žb. and 50 Pa Žc. N2 .

respectively. These identifications are in good agreement with recent reports in the literature w18,21x. An interesting feature is the relative changes of the intensities and peak area of the different components of the deconvoluted C1s and N1s spectra. We calculated the C᎐NrC⫽N peaks ratio for all the deconvoluted C1s and N1s spectra ŽTable 3.. From Table 3 we note that the C᎐NrC⫽N ratio decreases with the increase in nitrogen pressure. We further compare the deconvoluted C1s spectra of the films as deposited at 1 Pa ŽFig. 5. and at 5 Pa N2 ŽFig. 3a.. As can be seen from Fig. 5, the peak assigned to the pure carbon network ŽC⫽C. Žbinding energy of

Fig. 5. A typical C1s XPS deconvoluted spectrum of the film deposited at 1 Pa N2 .

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i.e. with the increase of the nitrogen content in the synthesized layers Žsee Table 1.. Similar behavior was observed by other authors, when studying the mechanical properties of carbon-nitride films deposited by various techniques w23᎐25x. Nevertheless, the hardness of the film deposited at 1 Pa Žwith the NrC ratio of 0.2. is smaller than that of the films deposited at 5 Pa ŽNrCs 0.6.. 2. The hardness values corresponding to the films deposited at 250⬚C are generally lower than that of similar films deposited at room temperature. We notice from Table 1 that these films systematically contain less nitrogen than the ones deposited at room temperature.

4. Discussion

Fig. 6. Microhardness values of the films deposited at room temperature Ža. and 250⬚C collector temperature Žb., at different depths, d, measured from the surface of the obtained structures.

distance d measured from the surface of the films represents the maximum penetration depths for different indentation loads. We determined for each load the maximum penetration depth using the load᎐displacement curve. Next, we calculated the corresponding hardness value. From Fig. 6 we notice that the hardness values stabilizes rapidly into the depth of the layers. We consider the slightly smaller values on the surface to be due to possible contamination with oxide. Table 4 shows the dependence of the stabilized hardness values on the nitrogen pressure in the deposition chamber. The most important features shown in Table 4 are: 1. The decrease of the film hardness with the increase in the nitrogen pressure in the irradiation chamber, Table 4 The microhardness values of the films deposited at room temperature and at 250⬚C for different ambient pressures N2 P wPax

H wGPax ŽRT.

H wGPax Ž250⬚C.

1 5 10 50

10 15 9 6

᎐ 14 7 4

We first note that according to the literature w26᎐28x, the hardness of carbon-based films is determined by the presence of sp 3 bonding of the carbon atoms. In carbon-nitride materials, this requirement is the same, as stated by Cohen and Liu w6,7x. Indeed, in order to have a high density, low compressibility, hard carbonnitride layer, the carbon atoms must be in an sp 3 hybridisation state. However, our studies have revealed that the increase in the nitrogen concentration in the synthesized structure leads to preferential formation of the sp 2 trigonal C bonded to N. Thus, by comparing the data in Tables 1 and 3, we observe that the C᎐NrC⫽N integrated intensity ratios calculated from the deconvoluted C1s and N1s spectra ŽFig. 3a᎐c. decrease with the increase in nitrogen pressure in the irradiation chamber and in the nitrogen content of the films. Moreover, other experimental studies w15x confirmed that the incorporation of nitrogen into the films result in a preferential formation of C⫽N double bonds. We must point out that these trends are in good agreement with the results reported in Hu et al. w29x, according to which an increase in nitrogen concentration leads to a structural transformation of carbon-nitride films to phases having predominant sp 2 C to N bonds. The ab initio Hartree᎐Fock, density functional and semi-empirical calculations reported in Hu et al. w29x suggest that thermodynamic and kinetic preferences for sp 2 vs. sp 3 bonded structures occur above 12% nitrogen. Consequently, in our opinion, attempts to increase the nitrogen concentration up to the stoichiometric value of the ␤-C 3 N4 molecule will not result in the formation of the theoretically predicted structure, in which C atoms must be in the sp 3 hybridisation state. Our results are also in good agreement with the predictions in Weich et al. w30x, namely that the addition of nitrogen lowers the sp 3 hybridized C content. Nevertheless, other recent experimental investigations w21x report the study

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of the structural properties of carbon-nitride thin films obtained by PLD. The authors found that the fraction of N᎐sp 3 C bonds increases with the increase of NrC atomic ratio. We consider that this change in structure Žthe reduction of the amount of C᎐N single bonds in favor of the formation of C⫽N double bonds. is in fact responsible for the observed decrease of hardness of the films along with the increase in nitrogen incorporation. Other authors w23x have recently attributed the changes in the mechanical properties of carbon-nitride thin films to changes in nitrogen concentration and atomic bonding structures. They also observed a decrease in film hardness with the increase in nitrogen content. On the other hand, we notice an apparent discrepancy when comparing the NrC atomic ratio data from Table 1 and the C᎐NrC⫽N integrated intensity ratio from Table 3 for films deposited at 10 and 50 Pa N2 . Indeed, despite the fact that these values are similar for the two films, the microhardness values further decreased with the increase in nitrogen pressure. We explain this apparent discrepancy with the aid of the data in Table 2. In our opinion, a hardness reduction from 9 to 6 GPa can be caused by the formation of a higher quantity of oxides at higher nitrogen pressure. We would like to remark that the general trend of hardness decrease with the increase in nitrogen content is only valid above a certain threshold of nitrogen incorporation in the structures obtained. The hardness of the film deposited at 1 Pa Žwith NrC atomic ratio of 0.2. is only 10 GPa compared with 15 GPa for the film deposited at 5 Pa ŽNrCs 0.6.. We think that this is the effect of the predominant C⫽C graphitic network formed during deposition in low-pressure nitrogen ŽFig. 5.. At 1 Pa deposition pressure the amount of incorporated nitrogen is so small that the film has a mainly graphitic structure. An interesting result is that the NrC atomic ratio values were smaller when the collector was heated during deposition, as compared with the NrC atomic ratio of the films obtained at room temperature. We showed in our previous studies w31x that the heating of the collector during deposition can favor desorbtion from the collector surface of the volatile CN radicals. These radicals are in our opinion important participants in the reaction leading to the formation of the carbon-nitride molecule. Similar results are reported in by Sjostrom et al. and Cuomo et al. w32᎐34x. The deposited structures were thermally stable. We did not observe any significant changes in the XPS spectra or mechanical properties after heat treatment or maintaining the obtained structures in open air for a long period. This supports our previous hypothesis w31x, that once formed any kind of CN bonding is stable in time andror under thermal treatment. This statement is in good agreement with the results of Niu et al. w35x.

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5. Conclusions We found a direct correlation between the composition, nitrogen content and chemical bonding on the one hand and the hardness on the other hand of carbon-nitride thin films obtained by RPLD. We consider that the increase in nitrogen concentration of the structures obtained does not lead to the formation of the theoretically predicted ␤-C 3 N4 molecule, with the carbon atoms in the sp 3 hybridisation state. The high nitrogen concentration produces a structural transformation to phases having prevalently sp 2 C to N bonds. Our results are in good agreement with recent theoretical calculations w29,30x, indicating that the addition of nitrogen reduces the sp 3 hybridized C content. According to our studies, the decrease in the hardness value of the films along with the incorporation of nitrogen is the result of this structural transformation. Acknowledgements The authors are grateful to Armando Luches from the Department of Physics, University of Lecce for encouraging discussions and for full access to the facility of the Radiation Physics Laboratory. Eniko ¨ Gyorgy ¨ acknowledges with thanks the financial support of CNR in the framework of the bilateral agreement with the Romanian Ministry of Research and Technology and to the Istituto Nazionale per la Fisica della Materia for a research contract which ensured her participation to the work. References w1x D. Chrisey, G.K. Huber ŽEds.., Pulsed Laser Deposition of Thin Films, J. Wiley, New York, 1994. w2x J. Bulir, M. Jelinek, V. Vorlicek, J. Zemek, V. Perina, Thin Solid Films 292 Ž1997. 318. w3x A. Kumar, H.L. Chan, J.S. Kapat, Appl. Surf. Sci. 127r129 Ž1998. 549. w4x I. Alexandrou, I. Zergioti, M.J.F. Healy, G.A.J. Amaratunga, C.J. Kiely, H. Davock, A. Papworth, C. Fotakis, Surf. Coat. Technol. 110 Ž1998. 147. w5x V. Craciun, D. Craciun, I.W. Boyd, Mater. Sci. Eng. B 18 Ž1993. 178. w6x A.Y. Liu, M.L. Cohen, Science 245 Ž1989. 841. w7x A.Y. Liu, M.L. Cohen, Phys. Rev. B 41 Ž1990. 10727. w8x E. D’Anna, A. Luches, A. Perrone, S. Acquaviva, R. Alexandrescu, I.N. Mihailescu, J. Zemek, G. Majni, Appl. Surf. Sci. 106 Ž1996. 126. w9x J. Zemek, A. Luches, G. Leggieri, A. Fejfar, M. Trchova, J. Electron. Spectr. Rel. Phenom. 76 Ž1995. 747. w10x A.P. Caricato, G. Leggieri, A. Luches, A. Perrone, E. Gyorgy, I.N. Mihailescu, M. Popescu, G. Barucca, P. Mengucci, J. Zemek, M. Trchova, Thin Solid Films 307 Ž1997. 54. w11x M.L. De Giorgi, G. Leggieri, A. Luches, M. Martino, A. Perrone, A. Zocco, G. Barucca, G. Majni, E. Gyorgy, I.N. Mihailescu, M. Popescu, Appl. Surf. Sci. 127r129 Ž1998. 481. w12x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 Ž1992. 1564.

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