Magnetic properties of carbonitride films elaborated by plasma enhanced chemical vapor deposition1

Magnetic properties of carbonitride films elaborated by plasma enhanced chemical vapor deposition1

Carbon 37 (1999) 457–462 Magnetic properties of carbonitride films elaborated by plasma enhanced chemical vapor deposition 1 M. Trinquecoste a , *, E...

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Carbon 37 (1999) 457–462

Magnetic properties of carbonitride films elaborated by plasma enhanced chemical vapor deposition 1 M. Trinquecoste a , *, E. Daguerre a , L. Couzin a , J. Amiell a , A. Derre a , P. Delhaes a , L. Ion b , B. Held b a

Centre de Recherche Paul Pascal — C.N.R.S. and Universite´ Bordeaux I, Avenue du Docteur Albert Schweitzer, 33600 Pessac, France b Laboratoire d’ Electronique des Gaz et des Plasmas, Universite´ de Pau et des Pays de l’ Adour, 64000 Pau, France Received 30 October 1997; accepted 11 April 1998

Abstract Several amorphous carbonitride films have been elaborated by plasma enhanced chemical vapor deposition. A radio frequency discharge has been used under standard conditions with different graphitic nitrogenated compounds. It is shown that the influence of molecular precursor is important concerning in particular the magnetic properties. The films issued from pyrrole and cyanuric chloride exhibit a large number of spins. However, in contrast to previous works, no cooperative magnetic effects have been found at very low temperature. Finally, it is argued that a further heat-treatment at 10008C of a film, prepared from cyanuric chloride, seems to lead to a crystalline phase of C 3 N 4 .  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Doped carbons; Pyrolytic carbon; B. Chemical vapor deposition; Plasma deposition; D. Magnetic properties

1. Introduction A rich polymorphism, associated with different types of bondings, is observed in solid carbons. Other phases than the thermodynamically stable one, i.e. the hexagonal graphite, are known belonging to diamond structures, but also to fullerenes, nanotubes and even carbynes [1]. These recent observations are essentially due to new experimental approaches where several metastable compounds are synthesized thanks to non-conventional methods affording an excess of energy to the system. In particular, the substitution of carbon atoms by neighbouring atoms (inside the periodic classification) as boron or nitrogen, separately or together in ‘B-N type’ isoelectronic to carbon compounds, has expanded the search for new classes of materials. The research and development of novel materials, with either extreme thermal and mechanical properties as in cubic diamond, or

*Corresponding author. Fax: 133-(0)556-845-600. This paper was presented at the European Materials Research Society 1997 Meeting, Symposium A: Fullerenes and Carbon based Materials, Strasbourg, France, June 1997. 1

high electrical characteristics as in hexagonal graphite, are very active areas. Among all the current trends, materials containing carbon and nitrogen are the focus of active research. Interest in these covalent carbon nitrides has been initially stimulated by a theoretical prediction due to Cohen and Liu who indicated that a b-C 3 N 4 phase (analogous to b-Si 3 N 4 hexagonal phase), could exist as a new hard material [2]. Following this work, experimental efforts to produce carbon nitrides have risen, giving birth to the deposition of more or less crystalline C–N films with different composition [3,4]. More recent theoretical approaches have demonstrated that at least three different phases are predicted with the composition C 3 N 4 and moreover, other nitrogen–carbon compositions are possible [5]. It turns out that experimental approaches with a large excess of energy such as ion beam deposition and reactive sputtering techniques allow formation of carbon–nitrogen simple bonding as in crystalline b-C 3 N 4 phase, but this is not a general situation. It turns out that often multiple bonded states (i.e. including p-orbitals) are more stable than the simple one with s-orbitals only. This has been evidenced in particular by IR spectroscopic analysis of RF plasma enhanced chemical

0008-6223 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 98 )00212-7

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vapor deposition (PECVD) materials [6]. Indeed, graphitic type phases have also been postulated by a few theoretical calculations [7]. The interest of such phases is not in their hardness as in other cases [8], but rather in their electronic and magnetic properties. In particular, it has been predicted by Ovchinnikov, a long time ago, that, due to a lone electron pair on nitrogen, a ferromagnetic ordering of spins could exist in such p delocalized systems [9]. Recently, experimental results have been published, based on thermal CVD of organic nitrogenated precursors as amines or other compounds [10–12]. Starting from the PECVD techniques, some authors have also presented amorphous carbons containing both nitrogen and hydrogen atoms and called a-C:N:H [6,13]. One noteworthy point is the detected presence of a permanent magnetization in these amorphous-like nitrogenated carbons [12,14]. It turns out from this short analysis that the nature of deposited carbonitrides, their elemental composition as well as their structural organization, is basically a function of two experimental parameters. They are respectively the way to afford the excess of energy for getting a metastable state and the nature of the precursors [1]. Starting from a usual radio-frequency reactive plasma experimental set-up, we have developed a chemical approach, looking for the role played by gaseous (or evaporated solids / liquids) molecular precursors. In a previous investigation [6], the feed gases were a mixture of N 2 , CH 4 and Ar; they gave rise to a limited content of nitrogen. In order to improve this result and to get a better insight about the plasma chemistry, we have selected several nitrogenated heterocycles as starting materials. Following this idea, we would like to deposit graphitictype carbonitrides, in order to examine their physical properties. We have therefore divided our presentation in two parts, firstly relative to the experimental technique with the presentation of the ‘as deposited’ films, then the further influence of a heat-treatment temperature. Secondly we report and discuss the magnetic properties (ESR spectroscopy and bulk magnetization measurements) of the obtained samples.

2. Experimental technique and characterization The PECVD technique which has been already described [6], is mainly based on an asymmetrical RF discharge with two external electrodes over a Pyrex tubular reactor (Fig. 1). The usual processing parameters are located around the following functional point: RF power Total inlet flow rate Pressure Substrates

100 W (0 to 300 W at 13.56 MHz) 5 l/h 1000 Pa silica and Zr plates

The distance from the end of the excitation coil to the substrates is around 20 cm

With the reference gas mixture (CH 4 , N 2 , Ar), we have already examined the main dissociation and recombination products by optical emission spectroscopy (OES), chemical titration, actinometry and on-line mass spectroscopy (MS) [15]. Systematic analyses were made around the functional point (varying electric power, total and partial pressures of precursor gases) and correlations between the plasma chemistry processes and film characteristics have been suggested. The deposition mechanism is based on polyatomic C x H y N z deposition in the electrode region (d#20 cm) and methane partial pressure governs its efficiency. These results explain why we have been therefore interested in the change of precursors, mainly by those based on nitrogen heterocycles to improve the nitrogen content in these graphitic films. The selected compounds, presented in Table 1, are liquids but also solids (as cyanuric chloride) at room temperature; a controlled preheating step is necessary to evaporate / sublimate them with a regular flow-rate before going inside the plasma reactor which is at a bulk temperature between 200 and 2508C [15]. The stable and excited discharge products of methylamine and pyridine gaseous phases (with or without argon) were detected and analyzed by OES and MS. Even if some dissociation and recombination species are difficult to identify because the emission and mass spectra are very complicated and unknown in the literature, we observe the creation of several carbonitride species as CN, HCN, C 2 N 2 . We notice that the same discharge products were also detected in (CH 4 , N 2 , Ar) mixture [15] and our attention was focused on a comparative study between the emission or mass intensities associated with commonly detected species and nitrogen content in the film using the proposed precursors. For instance, ex-CH 3 NH 2 (i.e. films prepared from CH 3 NH 2 as precursor) and ex-(CH 4 , N 2 , Ar) films, presenting, respectively, 12 and 14% of nitrogen in the film at P5200 W and p51 mbar, have given important ratios INH /ICH by OES and IH 2 /IHCN by MS. These results suggest that the deposition mechanism efficiency may be followed by OES and MS (via the specified intensity ratios). The main chemical reactions are through radical mechanisms but, up to now, it was not possible to establish a relationship between the nature of the precursor and the composition of the deposited film. It is noteworthy to quote that the cyanuric chloride presents a rather different behavior as explained in the following part. The characterization of these ‘as deposited’ films has been carried out firstly by elemental analysis and density determination (Table 1), then secondly using IR spectroscopy to find the main chemical bonds. As presented in Table 1, we observe a rather low nitrogen content associated with the large presence of hydrogen for the three first compounds. The density (Micromeritics ‘Accupyc 1330’ helium pycnometer) is increasing with the nitrogen content, indicating that different types of bonding between carbon

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Fig. 1. Diagram of the experimental set-up for RF plasma enhanced CVD.

and nitrogen are present. They are evidenced by IR spectroscopy. Absorption measurements have been obtained on pellets of compressed powder diluted in a KBr matrix by using an FT-IR spectrometer (Nicolet MX1). For all the hydrogenated samples, the absorption spectra are similar, characterized by several intense vibrational bands presenting features in agreement with the work of Kaufmann et al. [16]. We observe in particular the following modes:

N–H stretching modes at n53300 cm 21 , C–H stretching bands at n52900 cm 21 ,

C≡N (nitrile group) at n5 2200 cm 21 , C=N (imino group) at n51600 cm 21 with also C=C (sp2) and C–N vibrations at n51400 cm 21 .

For the ex-cyanuric chloride film, we observe only the vibrational bands associated with the different carbon / nitrogen bonding types. The main result is, in agreement with previous observations [6], all kinds of bondings are present, particularly those associated with the C–N and the C=N types which are included in a broad band between 1200 and 1600 cm 21 , due to the presence of aromatic clusters, basic units of ‘graphitic crystallites’.

Table 1 Composition and density of the ‘as deposited’ films Precursor

C (%at.)

N (%at.)

H (%at.)

O (%at.)

C/N

C/H

d

Methylamine (CH 3 NH 2 ) Pyridine (C 5 H 5 N) Pyrrole (C 4 H 5 N) Cyanuric chloride (C 3 N 3 Cl 3 )

36.2 53.3 50.6 51.6

12.1 7.5 7.9 32.8

45.9 37.9 42.7 15.6 (Cl)

2.8 1.3 1.0

2.4 7.1 6.8 1.6

0.8 1.4 1.2

1.7 1.3 1.3 1.7

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3. Magnetic properties of as deposited films In this class of rather nitrogenated amorphous carbons, the electronic and magnetic properties are associated with both more or less delocalized p electrons and unpaired electrons due to dangling bonds. The investigations about transport properties have shown that they are associated to the presence of a Mott–Anderson pseudo-transition with a sharp change from an insulating to a conducting state [6] associated to the hetero-atoms content. Currently, we are mainly focused on the magnetic properties of these aC:N:H conducting compounds because there are theoretical predictions [8], as well as experimental approaches [12,14] related to the presence of a possible cooperative magnetic state. We have therefore examined their ESR spectra thanks to an X-band spectrometer (Brucker ESP 300E) equipped with a He 4 low temperature accessory (Oxford Instruments) and their bulk magnetization between 2 and 300 K, using a SQUID magnetometer (Quantum Design, MPMS5). At room temperature, we have observed a unique and not air-sensitive ESR line, of Lorentzian shape, the characteristics of which are gathered in Table 2. An isotropic g-factor is determined with a somewhat larger value than for the free electron. It is associated with a line-width broader than for pure hydrogenated carbons. These observations have to be correlated to the hyperfine interactions between the electronic and the nuclear spins of proton and nitrogen atoms [17]. The most drastic difference between these deposited films is the spin susceptibility value. It turns out that the pyrrole type a-C:N:H exhibits a room temperature spin susceptibility more than 10 times larger than for the other hydrogenated films. The origin of this larger number of supposed unpaired spins is due to the high chemical reactivity of pyrrole which leads to high deposition rates. This observation is in agreement with the experiments carried out to prepare doped fullerenes in the presence of pyrrole [18]. Moreover, with cyanuric chloride as precursor, we observe also a broad ESR line with a different g-factor. This new resonance, attributed to the presence of chlorine, presents a very large spin susceptibility comparable to those quoted in previous studies [12]. We have therefore investigated the temperature dependence of the bulk magnetic susceptibility thanks to the

Fig. 2. Temperature dependences of the bulk total magnetic susceptibility of the different a-C:N:H deposits.

SQUID magnetometer. On every compound, we have carried out the temperature dependence between 4 and 300 K (Figs. 2 and 3). On each compound, we observe a constant magnetism with, for only the ex-pyrrole and ex-cyanuric chloride films, a strong Curie tail at low temperature (also evidenced by low temperature ESR spectra).

Fig. 3. Temperature dependence of the paramagnetic susceptibility of the ex-cyanuric chloride deposit. The plot x 21 vs. T exhibits a p classical Curie–Weiss law.

Table 2 ESR room temperature characteristics of the ‘as deposited’ a-C:N:H(/ Cl) films Precursor

Line width DH (gauss)

g-factor

Spin susceptibility x s 310 8 emu CGS g 21

Standard gas mixture (CH 4 , N 2 , Ar) Methylamine Pyridine Pyrrole Cyanuric chloride

10.4 12.0 7.0 8.0 220.0 (5.0)

2.0040 2.0038 2.0030 2.0028 2.1700 (2.0028)

0.16 0.11 0.09 2.25 70.00

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This bulk value is the algebraic sum of a core diamagnetic constant value, equal to 25310 27 emu CGS g 21 for atomic carbon in the absence of any delocalized p electrons, and a paramagnetic term. We observe immediately that this paramagnetic component is very weak and almost negligible for all the samples except for the two last films. These ‘as deposited’ films exhibit a Curie law at low temperature (obtained by plotting x 21 vs. temperature, see p Fig. 3) with a Curie constant C¯1.2310 25 emu CGS g 21 K, and C¯22.6310 25 emu CGS g 21 K, respectively, for pyrrole and cyanuric chloride precursors. Furthermore, we did not observe any remnant magnetization at 4 K by changing the sign of the applied magnetic field. We have compared this result to those obtained by Murata et al. on ex-1,2-diamino propane films [12] who observed a permanent magnetization corresponding to 2310 22 spin per carbon atom instead of about 10 23 spin per carbon atom in our last carbonitride film. Besides, one further comment can be made about the presence of a magnetic ordered state in these compounds. It is necessary to have a ferromagnetic exchange interaction J which will give rise to a Curie temperature at T c ~J. In a general case, for the molecular magnetic materials, the antiferromagnetic coupling is more efficient than the ferromagnetic one. Besides, it is estimated that the exchange integral in p systems is decreasing exponentially with the mean distance between two spins [19]. In the present situation, with 10 22 to 10 23 spins per carbon atom for a graphitic system, uJu¯1 K for a mean distance ˚ It turns out that a magnetic between spins of about 10 A. transition temperature would occur at a temperature lower than 1 K, this being in agreement with the Weiss temperature found for the last compound (see Fig. 3 where u5 23 K). It turns out that the presence of a bulk magnetic ground state is quite unlikely in these compounds. However, one possibility to observe such an intrinsic behavior would be to consider a non-homogeneous distribution of unpaired spins (as in spin glasses) with in particular a high surface concentration [20]. A definite proof associated with the study of other surface properties is still necessary to confirm these rather curious results.

4. Influence of a further heat treatment Concerning the a-C:N:H films obtained from heterocyclic precursors, an annealing process around 10008C is associated to the weight-loss due to hydrogen, but also to some nitrogen removal as previously observed [6]. In order to prevent any hydrogen content, we have also prepared thin films from the decomposition of cyanuric chloride. Indeed, we observe a bulk nitrogen content higher than for previous heterocyclic precursors but still with the presence of chloride which can also be removed by means of an annealing treatment. Thermogravimetric

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analysis has shown that at 10008C under inert atmosphere, the weight-loss is greater than 50%; after solid state reactions, the final compound is a carbonitride with a nitrogen atomic content of around 10%. We have examined the X-ray diffraction pattern of such heat-treated films which appears completely different to those observed for a-C:N:H films. The diffraction pattern ˚ due to shown in Fig. 4 exhibits a first broad peak (4.44 A) ˚ typical of the sample holder and a second one (at 3.54 A) an amorphous carbon phase. Of special interest are how˚ to 0.94 ever the sharp and strong lines ranging from 2.66 A ˚ It has been recently shown in the literature that different A. polymorphs of C 3 N 4 can be theoretically predicted [21]. The best agreement of our X-ray diffraction pattern with these calculations would be a cubic phase of C 3 N 4 crystals. Indeed, our experimental result is possibly interpreted as the presence of a crystalline C 3 N 4 phase embedded inside an amorphous carbon matrix. Finally, a strong spin susceptibility is still detected after the heat-treatment at 10008C which obeys a Curie law 4 down to the liquid He temperature. We attribute this unique ESR line to the majority carbon phase, rather than the supposed crystalline C 3 N 4 phase.

5. Conclusion On the way to prepare and to characterize new materials, it turns out that lamellar amorphous as well as crystalline carbonitrides are challenging. In this study, we have shown that plasma enhanced CVD is a versatile technique to deposit a-C:N:H films. The nitrogen and hydrogen contents are drastically dependent on the precursor origin, in particular, starting from cyanuric chloride which is very reactive because of the presence of atomic chlorine [6] we have deposited a thin film which is a good precursor for a crystalline type carbonitride after annealing treatment. Indeed we have shown by X-ray diffraction the presence of a crystalline phase, which is not of graphitic type but tentatively identified by comparison with a theoretical prediction as crystalline C 3 N 4 [21]. This result has to be confirmed but it surely opens the way to prepare larger quantities of new carbonitrides. Concerning the physical properties, here we have been interested in the magnetic ones which where not explored previously. We have shown that the number of unpaired spins is a function of the chemical reactivity under plasma constraints. However in contrast to previous reports on other nitrogenated carbons obtained from a thermal decomposition, no long range magnetic order is detected. From these data analysis, it turns out that the theoretical prediction [9] of a bulk ferromagnetic state is not confirmed. It rather appears that we might be in the presence of surface effects which have to be controlled because these magnetic centers, associated with dangling bonds,

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Fig. 4. X-ray diffractogram of the ex-cyanuric chloride deposit, heat-treated at 10008C (indexed according to Teter and Hemley [21] for C 3 N 4 ).

should play a fundamental role in surface adsorption and reactivity processes.

Acknowledgements This work has been supported by a D.G.A.-D.R.E.T. (France) contract.

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