Vladislav V Krisyuk

Alain N Gleizes

Lyacine Aloui

Asiya E Turgambaeva

Bartosz Sarapata

Nathalie Prud'homme

François Senocq

Diane Samélor

Anna Zielinska-Lipiec

Dominique De Caro

Constantin Vahlas

Chemical vapor deposition of iron, iron carbides, and iron nitride films from amidinate precursors

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Iron-based metallic and ceramic films have a wide range of applications and there is an unfailing interest in developing and improving techniques for the processing of such films with innovative phase compositions. To mention but a few examples, the use of Fe films is foreseen in metallurgical applications; 1 films containing iron nitrides present not only mainly magnetic but also tribological and corrosion-resistant properties ͑Ref. 2 and references therein͒. Iron carbides have attracted attention due to their high hardness and melting point. 3 Although physical vapor deposition is widely used for these purposes, metallorganic chemical vapor deposition ͑MOCVD͒ is reputed to be a flexible and economically competitive method because it combines conformal coverage with high deposition rates and mild deposition conditions. [START_REF] Hitchman | Chemical Vapor Deposition: Principles and Applications[END_REF] However, MOCVD of nitride and of metallic films has not been widely applied yet. Typical drawbacks are the contamination with carbon and the difficulty to control the composition and the microstructure of the films.

In the frame of an ongoing project, the authors are presently investigating the preparation of multimetallic films by co-depositing Fe and Al with the MOCVD technique. Such intermetallic systems can be used for the processing of coatings containing iron aluminides and more generally complex metallic alloys presenting unique combinations of properties. [START_REF] Dubois | Useful Quasicrystals[END_REF] To meet this objective, the selection of appropriate precursors is not simply limited to identifying metal complexes with similar volatilities and similar decomposition temperatures. Ideally, precursors for the chemical vapor deposition ͑CVD͒ of metals should be selected and treated so as to be cleanly decomposed ͑clean cleavage, stable ligands, ligand fragments, etc.͒ ͑see, for instance, Ref. 6͒. Co-depositing several metals requires considering the different chemistries of the metals. This is the case for a system containing aluminum and d-transition metals such as the Fe-Al couple. The highly electropositive character of aluminum excludes the presence of the most electronegative elements such as oxygen or fluorine in the precursors of both aluminum and the companion metal͑s͒. Similarly, the affinity of iron to carbon leads to set Fe-C bonds containing precursors aside. Finally, ligand exchange reactions between the precursors must be avoided because they may induce dramatic volatility decrease. It is therefore necessary to test the selected precursors separately and then in pairs ͑see, for instance, Ref. 7͒. Consequently, one motivation of this work was to investigate an original process for the MOCVD of the Fe films.

Compared with other metals, few open scientific publications refer to the thermal MOCVD of pure iron films. Fe͑Cp͒ 2 , 1,8,9 Fe͑CO͒ 5 , [10][11][12][13][14] Fe 2 Cp 2 ͑CO͒ 4 , 15 Fe͑COT͒͑CO͒ 3 , 16 and ͓͑arene͒ ͑diene͒Fe 0 ͔ 17 are known as iron precursors, but they do not meet the prerequisites of no oxygen and no Fe-C bond. Fe͓N͑SiMe 3 ͒ 2 ͔ 3 is worth testing, but the reported deposition temperature of 325°C 18 may be too high for the purpose of the co-deposition with Al. Fe͑II͒ dihydride H 2 Fe͓P͑CH 3 ͒ 3 ͔ 4 yielded pure ␣-Fe films between 230 and 280°C, but limited information was provided on the process itself and the obtained material. 19 Finally, Lim et al. showed that some transition-metal amidinates are volatile 20 and can be used as precursors for the atomic layer deposition ͑ALD͒ of transition-metal thin films. 21 Based on the latter papers, copper amidinates have been recently successfully tested for the deposition of copper. 22 A parallel motivation of this work concerns the investigation of innovative MOCVD routes for the processing of iron nitrides. The motivation originates from the intrinsic magnetic properties of such materials, especially ␥Ј-Fe 4 N, combined with their high electrical conductivity and their chemically inert and mechanically hard surfaces ͑Ref. 23 and 24 and references therein͒. Line-of-sight techniques such as thermal evaporation, reactive sputtering, and laser ablation have been employed for the processing of iron nitride films ͑Ref. 25 and references therein͒. Alternatively, vapor-phase epitaxy involving iron trichloride ͑FeCl 3 ͒ and ammonia ͑NH 3 ͒ allowed the production of Fe 4 N epitaxial layers, with the noticeable drawbacks of high processing temperature ͑600°C͒ and aggressive halide-based chemistry. 26 Few papers deal with the CVD of iron nitrides. Funakubo et al. reported on the formation of ␥Ј-Fe 4 N from biscyclopentadienyl iron and NH 3 /H 2 /CO 2 above 700°C. 27 Roberson et al. reported on the growth of Fe 3 N from iron acetylacetonate and anhydrous NH 3 above 600°C. 2 The present paper deals with the investigation of two iron amidinates as precursors for the MOCVD of Fe and Fe 4 N: ͑1͒ dinuclear iron bis͑N,NЈ-diisopropylacetamidinate͒ ͓Fe 2 ͑-i Pr-MeAMD͒ 2 ͑ 2 -i Pr-MeAMD͒ 2 ͔ and ͑2͒ mononuclear iron bis͑N,NЈ-di-tert-butylacetamidinate͒ ͓Fe͑ t Bu-MeAMD͒ 2 ͔. 20 These are nonoxygenated metallorganic complexes with ligandmetal bonding through Fe-N bonds ͑Fig. 1͒. Lim et al. showed that compound 2 could be used for the ALD of pure iron films. 21 In what follows, information on the thermal behavior of the two precursors, investigated by in situ mass spectrometry, is first reported. The characteristics of the MOCVD processed films are then detailed, focusing on their elemental and phase composition, and morphology. Finally, the room-temperature magnetic properties of the films are reported and discussed.

Experimental

Precursor investigation.-Compounds 1 and 2 were synthesized by NanoMePS e by appropriately adapting the protocol of Lim et al. 20 They were used without further purification. Both compounds are extremely sensitive to air and light. Upon storage in a metallic glove box under a continuously purified argon flow, they slowly decomposed: The green powder of precursor 1 tarnished and the white powder of precursor 2 turned gray. The compounds must be kept in sealed ampoules at 4°C.

The gas-phase thermal decomposition of the precursors was studied by a technique which combines Knudsen effusion and mass spectrometric measurements of the gas-phase composition. The experimental details have been described elsewhere. 28 This investigation used a time of flight mass spectrometer directly fed by an input system that imitates a CVD reactor. 5 mg of the compound was sealed in a glass ampoule under inert gas and heated in the evaporator at 90 and 70°C for compounds 1 and 2, respectively. The evaporator temperature was kept constant, and the vapors were transported to the miniature CVD reactor through a heated pipeline. During the experiment, the reactor temperature was progressively raised from the temperature of evaporation to 400°C with a heating rate of 5°/min. The reaction mixture was sampled through the effusion hole ͑0.2 mm͒ from the reactor into the mass analyzer. Mass spectra were recorded at ionizing electron energies ca. 70 eV. The temperature dependence of the gas-phase composition was derived from the mass spectra.

Deposition.-The CVD experiments were conducted in two, stagnant-flow, vertical, tubular, cold wall reactors of similar configurations. Reactor A was composed of a glass tube containing an inductively heated, 20 mm diameter stainless steel susceptor. It was used for the initial screening of the two precursors. Reactor B was used for the processing of multimetallic coatings. It was composed of a stainless steel body and contained a resistively heated, 60 mm diameter stainless steel susceptor. A detailed description of the setup is provided in Ref. 29. A showerhead above the susceptor ensured homogeneous flow of the reactants on the surface of the substrates. The larger susceptor in reactor B allowed expanding the number and types of substrates processed in one run. The deposition from compound 1 was investigated in reactor A; the deposition from compound 2, which proved to be the more promising of the two precursors for the co-deposition of multimetallic films, was investigated in both reactors. The base pressure for both reactors was 1.0 ϫ 10 -6 Torr. Flow thermal and hydrodynamic characteristics were calculated so as to get laminar flows and similar operation conditions in either reactor ͑Ref. 29 and unpublished results͒.

Polycrystalline copper disks ͑10 mm in diameter, 1 mm thick͒ were used as substrates. The disappearance of copper color was optically tracked to access film growth onset. Before deposition, the substrates were polished with a diamond paste down to 5 m. They were cleaned ultrasonically in acetone and anhydrous ethanol. Pure nitrogen ͑99.998%͒ and pure hydrogen ͑99.999%, Air Products͒ were fed through computer-driven mass flow controllers. The former was used both as carrier gas through the precursor container and as dilution gas. The deposition pressure was 10 Torr and the temperature of the substrates was in the range 150-450°C. The precursor container was maintained at 95 and at 85°C for precursors 1 and 2, respectively. Deposition was run for 1-4 h, depending on the operating conditions. Table I summarizes the flow rate conditions. The main difference in the processing conditions between reactors A and B is the ratio of the hydrogen to precursor concentration ͑hereafter referred to as H/P͒. Because the saturated vapor pressure vs temperature values are not actually available for these new compounds, it is not possible to quantify P and, subsequently, the H/P ratio. For this reason, it can only be mentioned that the H/P ratio is 3 to 4 times higher in reactor B than in A.

The deposited phases were identified by grazing incidence X-ray diffraction ͑GIXRD͒ with a Siefert 3000 diffractometer ͑Cu K␣ radiation; grazing angle = 2-3°; 2 step = 0.04°͒. Surface and cross section morphologies were investigated by a scanning electron mie www.nanomeps.fr croscope ͑SEM͒ with a LEO 435 VP instrument and a field-emission gun ͑FEG͒ equipped JEOL JSN6700F instrument. Elemental analyses of the films were performed by electron probe microanalysis ͑EPMA͒ using a CAMECA SX50 instrument. The films were submitted to X-ray photoelectron spectrometry ͑XPS͒ on an ESCALAB-MkII ͑VG Scientific͒ instrument. The photoelectron spectra were excited using non-monochromatized Mg K␣ radiation ͑h = 1253.6 eV͒ with a total instrumental resolution of Ϯ0.5 eV.

N N N N Fe Fe N N N N C H 3 CH 3 C 3 H 7 C 3 H 7 C 3 H 7 C 3 H 7 CH 3 CH 3 C 3 H 7 C 3 H 7 C 3 H 7 C 3 H 7 (a) Fe N N N N C 4 H 9 C 4 H 9 C 4 H 9 C 4 H 9 CH 3 C H 3 (b)
Analyses were performed after etching of the film superficial layers with Ar ions. The room-temperature magnetic properties of the films were characterized using a Quantum Design MPMS2 superconducting quantum interference device magnetometer. The magnetic field was applied parallel to the film plane. The diamagnetic contribution from the substrate was negligibly small compared to the magnetization of the films.

Results

Characterization and thermal behavior of the precursors.-The mass spectra of the two compounds are characterized by the occurrence of relatively intense molecular peaks corresponding to the monomer ͓FeL 2 ͔ + and to the free ligand ͓HL͔ + . There are no peaks at mass-to-charge ͑m/z͒ ratios higher than the value expected for FeL 2 , i.e., 338 amu for compound 1 and 394 amu for compound 2. This was expected for compound 2, which is monomeric in the solid state. 20 Compound 1 is dimeric in the solid state, 20 so that additional studies such as vapor pressure measurement are needed to confirm the monomeric structure in the gas phase.

Figures 2 and3 show the mass spectra ion peak intensity vs temperature for compounds 1 and 2, respectively. Before commenting, it is worth noting that the observed gaseous products did not depend on whether hydrogen had been added or not.

For compound 1 ͓Fig. 2, L = ͑C 3 H 7 ͒NC͑CH 3 ͒N͑C 3 H 7 ͔͒, no major changes occur in the temperature range 110-160°C ͑not shown͒. Further temperature increase results in the decomposition of the compound and in the ensuing intensity decrease in the ͓FeL 2 ͔ + peak. At 250°C, the intensity is close to the background, indicating that the decomposition is maximal. The main gaseous compounds are the free ligand HL ͑m/z = 142͒, acetonitrile ͑m/z = 41͒, and ͑CH 3 ͒ 2 CHNH ͑m/z = 58͒.

Figure 3 shows the mass spectra ion peak intensity vs temperature for compound 2 ͓L = ͑C 4 H 9 ͒NC͑CH 3 ͒N͑C 4 H 9 ͔͒ in the presence of hydrogen. Decomposition of compound 2 starts at 150°C as proved by a considerable decrease in the intensity of the initial compound ion peaks. Decomposition is maximal at 250°C. The main gaseous compounds are the free ligand HL ͑m/z = 170͒, CH 3 CN ͑m/z = 41͒, C 4 H 8 ͑m/z = 56͒, C 4 H 9 ͑m/z = 57͒, and ͑CH 3 ͒ 2 CHNH ͑m/z = 58͒. The evolution of the intensity of the molecular peak is not sharp. This may be attributed to the relatively low vaporization stability of the compound. Similarly, the curves corresponding to ͓HL͔ + , ͓CH 3 CN͔ + , and ͓͑CH 3 ͒ 2 CHNH͔ + have complex profiles because the ions originate both from the thermal decomposition of FeL 2 when the appropriate temperature is reached and from the fragmentation of FeL 2 and of some products ͑e.g., HL͒ under electron impact.

Elemental analysis of the films.-Table II shows the carbon and nitrogen contents of the films as determined by EPMA and the main phases identified by GIXRD ͑see next paragraph͒. In this and the following tables, sample code shows the precursor ͑1 or 2͒, the reactor ͑A or B͒, and the temperature of deposition in degrees Celsius. The average thickness of films was approximately 250-300 nm. The oxygen content was below the EPMA detection limit. XPS was performed on as-processed films. From the decomposition of XPS Fe 2p and C 1s peaks and from literature data for ironcontaining films, [START_REF] Wagner | NIST X-ray Photoelectron Spectroscopy Database[END_REF] the films contained iron oxide ͑Fe 2p 3/2 at ϳ712 eV͒ and graphitic and organic carbon as minority components. Iron oxides are considered as surface contaminants because they were most probably formed when the samples were exposed to ambient atmosphere. Depth profile analyses revealed uniform composition in the films ͑to the extent of the zones sputtered with low energy Ar ions͒ except surface contaminants. From GIXRD and XPS ͑vide infra͒, the main iron-based deposited species are iron carbides ͑Fe 3 C, Fe 4 C͒, iron nitride Fe 4 N, and metallic iron. Table II shows that nitrogen was detected in four films grown from precursor 2 at low H/P ratio at 320 and 350°C, and at high H/P ratio at 280 and 300°C.

XRD.-GIXRD patterns for films prepared from compounds 1 and 2 are presented in Fig. 4 and5, respectively. Table III is a synoptic of the 2 angles calculated for copper ͑a = 3.6078 Å, F-type lattice͒ and measured on the X-ray patterns up to 80°. There is a strong fit between the calculated and observed 2 values for the diffraction lines ͑111͒, ͑200͒, and ͑220͒ from the copper substrate, and this allows using it as an internal 2 standard. Neither iron nor copper oxides were detected. The patterns contain lines attributable to ␣-Fe ͑JCPDS file no. 6-696͒ and Fe 3 C ͑JCPDS file no. 34-1͒. They also contain lines attributable to Fe 4 C or Fe 4 N depending on the sample. Fe 4 C has two allotropic forms. 3,31,[START_REF] Villars | Pearson's Handbook of Crystallographic Data for Intermetallic Phases[END_REF] The observed form is isostructural with ␥Ј-Fe 4 N ͑JCPDS file no. 6-0627͒: primitive There is an ambiguity in the determination of metallic iron because the ␣-Fe strongest reflection ͑110͒ at 44.67°and Fe 3 C strong reflection ͑220͒ at 44.59°almost overlap. Therefore, the reflection observed at ca. 44.6-44.7°in every pattern can be attributed to either Fe and Fe 3 C. Moreover, the Fe 3 C strongest reflections at 43.77 and 42.90°may be hidden by the Cu͑111͒ reflection at 43.40°.

From the GIXRD patterns shown in Fig. 4 and the data extracted from them and analyzed in Table III, the films processed from compound 1 systematically contain Fe 3 C. For the film deposited at 400°C ͑1A400͒, two weak reflections at 2 = 65.06 and 82.40°can be attributed to ␣-Fe͑200͒ and ␣-Fe͑211͒, respectively. For the film deposited at 350°C ͑1A350͒, reflection ͑200͒ does not show, and reflection ͑211͒ may be included in the wide reflections observed at 82.95°. For the film deposited at 450°C ͑1A450͒, there is no iron reflection except for the one that is also attributable to Fe 3 C. Finally, for the films deposited at 400 and 450°C, there is a reflection at 41.48°that matches the Fe 4 C strongest reflection ͑111͒.

Despite the narrow temperature range investigated, the films deposited from compound 2 present noticeable differences ͑Fig. 5, Table III͒. The possible presence of metallic iron can only be deduced from the line ca. 44.7°, which may also belong to Fe 3 C. Lines at 39.7 and 45.9°indicate the presence of Fe 3 C in the films prepared at 320 and 350°C at low H/P, and at 300°C at high H/P ratio. The patterns of the films prepared at 280 and 300°C at low H/P ratio show lines from Fe 4 C, while the other four patterns for the films prepared both at high and low H/P ratios show lines from Fe 4 N. From the intensity of reflection ͑111͒, Fe 4 N appears as the main component and is markedly ͑111͒-oriented. The films showing the X-ray pattern of Fe 4 N are those in which nitrogen was evidenced by EPMA ͑Table II͒.

The weak intensity of most reflections and the overlap among some of them preclude any attempt to use the XRD patterns to quantitatively or even semiquantitatively analyze the three-phase mixture in the films. IV shows the decomposition of Fe 2p 3/2 peaks for five films prepared from compound 2 at low H/P values in the temperature range 260-350°C. The Fe peak was decomposed into four peaks. The first one ͑Fe1͒ at ca. 707-708 eV is attributed to metallic iron and Fe 4 N because it has been reported that the Fe 2p 3/2 binding energy ͑BE͒ in iron nitrides is too close to that of the metal to be used to distinguish a nitride from ␣-Fe. 34 This point has been confirmed from the XPS analyses of amorphous ␥Ј-Fe 4 N thin films by Fehlner et al. 35 and from the investigation of the electronic structure of ultrathin ␥Ј-Fe 4 N͑100͒ films by Navio et al. 36,37 The second peak ͑Fe2͒ at ca. 708-709 eV was attributed to the carbides Fe n C ͑n = 3, 4͒. The other two peaks at ca. 710-712 and 713-714 eV were attributed to iron oxides ͑Fe3͒ and to the shake-up satellite, respectively. The C 1s peak was decomposed in three peaks ͑Table IV͒, namely carbides ͑C1͒, graphitic carbon ͑C2͒, and other, e.g., organic carbon ͑C3͒.

XPS analysis.-Table

The atomic ratios Fe1/Fe total , Fe2/Fe total , Fe3/Fe total , C1/C total , C2/C total , and C3/C total , where Fe total and C total stand for total iron and carbon contents, respectively, were deduced from their respective peak surface areas. They are reported in Table IV. The surfacegenerated iron oxide covers the other iron species. Therefore, the percentage of iron oxides ͑Fe3/Fe total ͒ is most likely overestimated with regard to other iron species ͑Fe1/Fe total and Fe2/Fe total ͒ due to the relatively weak information depth of XPS.

From the XRD, the films deposited at 260, 280, and 300°C do not contain Fe 4 N. The peak observed at ca. 707.0 eV ascertains the presence of metallic iron. The ratio Fe2/Fe total increases with deposition temperature. The films deposited at 320 and 350°C do contain Fe 4 N from the XRD. The presence of nitrogen in as-deposited films was proved by EPMA ͑Table II͒. However, no N 1s XPS peak was observed, probably because Fe 4 N was decomposed upon etching the surface of the films. The XPS peak at ca.707 eV may thus be attributed to metallic iron present before the etching and to metallic iron formed upon etching. Table IV shows that the formation of Fe 4 N at 320°C is marked by the decrease in the atomic ratios Fe2/Fe total and C1/C total from 0.45 to 0.32 and from 0.71 to 0.52, respectively.

Film morphology.-The films deposited on well-polished copper present a gray, mirrorlike metallic surface. This morphology is more pronounced for the films processed from precursor 2. Figure 6 shows surface SEM micrographs of the films deposited from precursor 1, at 350°C ͑Fig. 6a͒ and at 400°C ͑Fig. 6b͒. Figure 7 shows the micrographs for the films deposited from precursor 2, at 280°C ͑Fig. 7a͒ and at 350°C ͑Fig. 7b͒. When deposited at a temperature below 300°C, the films are made of densely packed nanocrystallites ͑size Ͻ 100 nm͒, independently of the precursor. This nanostructure results in films with a remarkably smooth surface. A comparison between the films deposited at 350°C from precursors 1 and 2 ͑Fig. 6a and 7b, respectively͒ reveals different microstructures. The film deposited from precursor 1 shows poorer crystallinity than the one deposited from precursor 2. The former mainly contains Fe 3 C, while the film prepared from precursor 2 at 350°C ͑Fig. 7b͒ is well crystallized and shows the ͗111͘ texture of Fe 4 N evidenced by the XRD pattern ͑Fig. 5͒. The use of different deposition temperatures and of two precursors results in striking differences among the microstructures of the processed films. This observation is correlated with the change in the composition of the films as discussed below.

Magnetic properties.-Figure 8 shows 4M-H loops for the 1A400 and 2A320 films. For the 1A400 film which contains Fe 3 C and ␣-Fe as a presumably additional phase, saturation magnetization ͑4M s = 4.1 kG͒ and coercive field ͑H c = 180 Oe͒ values are similar to those recently reported for Fe 3 C/␣-Fe powders obtained by reaction milling. 38 The magnetization curve recorded for the ͗111͘-textured ␥Ј-Fe 4 N film ͑2A320͒ exhibits a saturation magnetization of 4M s = 13.6 kG and a coercive field of H c = 130 Oe. The saturation magnetization value is lower than that of the bulk value for pure ␥Ј-Fe 4 N ͑18-21 kG͒. 39 This difference may be attributed to size effects, because at room temperature, the 4M s drops with particle size. 40 It may also be due to the presence of free carbon in the film. The decrease in the saturation magnetization due to the presence of free carbon has already been demonstrated for the MOCVD processed Ni films. 41 The presently reported H c value ͑130 Oe͒ is much higher than that for the ␥Ј-Fe 4 N͑001͒-oriented films grown on MgO͑001͒ or MgO͑111͒ substrates ͑in the 35-45 Oe range͒. 42 This difference can be assigned to the morphology of the films. Oriented films on MgO͑001͒ or MgO͑111͒ exhibit large grains in which spins are easily arranged. The MOCVD processed films containing ␥Ј-Fe 4 N exhibit small crystallites for which domain walls are probably harder to move.

µm

Discussion

The films deposited from compound 2 contain less carboncontaining species than those processed from compound 1. Moreover, compound 2 gives a lower onset deposition temperature and higher growth rate ͑detailed results on growth rates from the two precursors were published in Ref. 43͒. For these reasons, Fe͑ t Bu-MeAMD͒ 2 appears as a more interesting precursor for Fecontaining films than Fe 2 ͑ i Pr-MeAMD͒ 4 . The affinity of Fe to C is responsible for the formation of Fe 3 C in most investigated conditions. There exist, however, processing conditions for which either metallic Fe or Fe 4 N and Fe 4 C are also formed. Fe 4 N and Fe 4 C are isostructural compounds. Their crystal structure is a primitive cubic lattice with four Fe atoms and one N ͑C͒ atom per cell, in a semiantiperovskite-type structure ͑FeXFe 3 with X = N or C͒. The unit cell constants differ by ca. 3% only. Therefore, their identification by means of XRD is not straightforward. In this study, the use of X-ray reflections from the copper substrate as Bragg angles internal standard removed this difficulty. This is a report on CVD of Fe 4 C while, as mentioned in the Introduction, there are but a few papers on CVD of Fe 4 N. Both Funakubo et al. 27 and Takahashi et al. 26 operated at high temperature and atmospheric pressure conditions with complex gas phases involving separate precursors for Fe and N. The process reported in the present work operates in significantly simpler conditions.

The formation of Fe 4 N in the film is compatible with the analysis of the mass spectrometric data on gas-phase composition upon thermal decomposition of compound 2. Based on these data, Fig. 9 resumes the following mechanism which is proposed for the decomposition of compound 2 ͑all conversions take place on the surface͒: ͑i͒ migration of H from one ligand to another, dissociation of the Fe-N bond, and release of free ligand HL into the gas phase; ͑ii͒ decomposition of remaining ͓Fe͑L-H͔͒ with ͑a͒ release of the radical HNCH͑CH 3 ͒ 2 ͑m/z = 58͒ in the gas phase, formation of carbon and of the surface intermediate ͓Fe-C͑CH 3 ͒ = NR͔, and ͑b͒ release of methylpropene ͑CH 3 ͒ 2 CvCH 3 ͑m/z = 56͒ in the gas-phase and formation of the surface intermediate ͓Fe-NvC͑CH 3 ͒NR͔; and ͑iii͒ decomposition of the surface intermediates with the release of acetonitrile CH 3 CN ͑m/z = 41͒ and tert-butyl radical R ͑m/z = 57͒ and formation of ͑a͒ Fe-N and ͑b͒ Fe and C on the surface. Migration of H from one ligand to another after chelate-ring opening is the typical process for metal-chelate complexes with organic ligands. 28 The formation of organic products with an m/z lower than for the ligand indicates that a pure reduction of the compound by hydrogen does not take place. This is also supported by recent literature data on the decomposition of copper amidinates on Ni surface under high vacuum conditions. 44

Conclusion

This reported study of iron amidinates tentatively used as precursors for the MOCVD of iron and iron nitrides. Iron amidinates were chosen because they contain no oxygen atoms and no Fe-C bonds. Between the two alkylacetamidinates retained, the ͑1͒ isopropyl derivative is dinuclear in the solid state and the ͑2͒ tert-butyl derivative is mononuclear. Mass spectrometry showed them to be mononuclear in the gas phase. Compound 1 proved to provide more carbon-based species than compound 2.

Fe and Fe 3 C were present in the films. The films also contained either Fe 4 C or Fe 4 N depending on the temperature and the hydrogen to precursor concentrations in the input gas.

This work may be considered as a screening approach for the use of iron amidinates as precursors for preparing iron-based materials by MOCVD. The determination of the appropriate processing window for the deposition of either metallic Fe or Fe 4 N, or even iron carbides, requires further investigation of the influence of reactive atmospheres, of surface chemistry, and possibly of appropriate postdeposition treatments. More precisely, a prerequisite for the stabilization of pure Fe should be good venting conditions at the growing surface. Such conditions are ultimately illustrated by the ALD process and should involve a reducing gas such as hydrogen, preferably in the atomic state.

Figure 1 .

 1 Figure 1. Developed formulas of ͑a͒ bis͑N,NЈ-diisopropylacetamidin-ate͒iron͑II͒ ͑1͒, and of ͑b͒ bis͑N,NЈ-di-tert-butylacetamidinate͒iron͑II͒ ͑2͒.

Figure 2 .Figure 3 .

 23 Figure 2. Temperature dependence of gas-phase composition upon decomposition of compound 1 ͓L = ͑C 3 H 7 ͒NC͑CH 3 ͒N͑C 3 H 7 ͔͒. The lines are guides for the eyes.

Figure 4 .

 4 Figure 4. ͑Color online͒ XRD analysis of the films deposited from compound 1 on copper ͑ ↓ ͒ substrates.

Figure 5 .

 5 Figure 5. XRD analysis of the films deposited from compound 2 on copper ͑ ↓ ͒ substrates.

Figure 6 .

 6 Figure 6. SEM-FEG surface micrographs of films deposited from compound 1 at ͑a͒ 350 and ͑b͒ 400°C.

Figure 7 .

 7 Figure 7. SEM-FEG surface micrographs of films deposited from compound 2 at ͑a͒ 280 and ͑b͒ 350°C.

Figure 8 .

 8 Figure 8. Magnetization ͑4M͒ and magnetic field ͑H͒ loops of films 1A400 ͑squares͒ and 2A320 ͑circles͒.

Figure 9 .

 9 Figure 9. Proposed decomposition scheme on the growing surface for precursor 2.

Table I . Flow rate conditions for reactors A and B.

 I

	Flow rate
	͑sccm͒	Q͑H 2 ͒			Q͑N 2 , carrier͒		Q͑N 2 , dilution͒	Q͑total͒
	Precursor	1	2	1	2	1	2	1	2
	Reactor A	50	80	20	20	30	0	100	100
	Reactor B	-	285	-	20-40	-	20-0	-

Table II . Carbon and nitrogen content and identified phases of the films, determined by EPMA and XRD, respectively (see text for sample code).

 II

		T deposition	C	N
	Code	͑°C͒	͑atom %͒	͑atom %͒	Identified phases	Possible additional phases
	1A350	350	24	0	Fe 3 C	F e
	1A400	400	28	0	Fe, Fe 3 C, Fe 4 C	-
	1A450	450	33	0	Fe 3 C, Fe 4 C	F e
	2A280	280	18	0	Fe 4 C	F e , F e 3 C
	2A300	300	18	0	Fe 4 C	F e , F e 3 C
	2A320	320	15	7	Fe 3 C, Fe 4 N	F e
	2A350	350	17	7	Fe 3 C, Fe 4 N	F e
	2B280	280	12	9	Fe 4 N	F e , F e 3 C
	2B300	300	19	7	Fe 3 C, Fe 4 N	F e

Table III . Peak positions in GIXRD patterns of deposited films.

 III

	Code	1A350	1A400	1A450	2A280	2A300	2A320	2A350	2B280	2B300
	Presence of N ͑EPMA͒	No		No	No	No	Yes	Yes	Yes	Yes
						2
	Components					͑°͒
	Fe 3 C͑121͒	37.80	37.76	37.80
	Fe 3 C͑002͒	39.74	39.85	39.78				39.77		39.73
	Fe 3 C͑201͒	40.74	40.75	40.71
	Fe 4 C͑111͒		41.48	41.48	41.49	41.44
	Fe 4 N͑111͒						41.27	41.29	41.24	41.31
	Cu͑111͒: 43.40	43.43	43.41	43.42	43.45	43.40	43.41	43.42	43.41	43.45
	␣-Fe͑110͒; Fe 3 C͑220͒	44.69	44.73	44.62	44.76	44.70	44.62	44.69	44.67	44.69
	Fe 3 C͑112͒	45.86	45.93				45.95	45.92		45.89
	Fe 4 C͑200͒				48.25	48.11
	Fe 4 N͑200͒						47.96	47.94	48.01	48.02
	Fe 3 C͑221͒	49.20	49.23	49.11
	Cu͑200͒: 50.55	50.55	50.55	50.56	50.57	50.51	50.54	50.54	50.52	50.56
	Fe 3 C͑301͒	58.14	58.10	58.08
	␣-Fe͑200͒		65.06
	Fe 4 N͑220͒						70.20	70.12	70.10	70.21
	Cu͑220͒: 74.29	74.22	74.21	74.29	74.26	74.23	74.22	74.22	74.21	74.22
	Fe 3 C͑401͒	78.23
	␣-Fe͑211͒	82.95͑wide͒	82.40

Table IV . Decomposition of the Fe 2p 3Õ2 and C 1s XPS peaks of films prepared from compound 2: BE and energy drifts are in eV. Energy drifts are measured from peak Fe1 for Fe peaks and peak C2 for C peaks.

 IV = 3.790 Å for Fe 4 N[START_REF] Jacobs | [END_REF] and a = 3.678 Å for Fe 4 C.3,31 There is no recorded X-ray diffraction ͑XRD͒ pattern for Fe 4 C in the JCPDS database. The calculated Fe 4 C diffraction pattern can be deduced from Fe 4 N by considering the difference in the cell constant. Using the copper substrate as an internal 2 standard, the lines at 41.25 Ϯ 0.02 and 47.99 Ϯ 0.02°were indexed as Fe 4 N͑111͒ and Fe 4 N͑200͒ reflections and those observed at 41.45 Ϯ 0.02 and 48.19 Ϯ 0.04°as Fe 4 C͑111͒ and Fe 4 C͑200͒ reflections ͑Table III͒. The systematic discrepancy of 0.2°between corresponding diffraction angles is significantly larger than the differences between the calculated and observed copper angles ͑0.05°a t most͒. Therefore, the distinction between Fe 4 C and Fe 4 N becomes unambiguous.

	cubic lattice, with a
		Fe1vFe + Fe nitrides	Fe2vFe carbides	Fe3vFe oxides	C1vC carbides	C2 = graphitic C	C3 = organic C
	Peak	2p 3/2 BE	2p 3/2 BE ͑E drift͒	2p 3/2 BE ͑E drift͒	1s BE ͑E drift͒	1s BE	1s BE ͑E drift͒
	code	atom % Fe1/Fe total	atom % Fe2/Fe total	atom % Fe3/Fe total	atom % C1/C total	atom % C2/C total	atom % C3/C total
	Sample
	2A260	706.6	708.3 ͑1.7͒	710.6 ͑4.0͒	-	-	-
		46	32	22
	2A280	706.5	708.3 ͑1.8͒	710.7 ͑4.2͒	282.5 ͑Ϫ1.5͒	284.0	285.9 ͑1.9͒
		43	38	19	50	36	14
	2A300	706.6	708.3 ͑1.7͒	710.8 ͑4.2͒	283.1 ͑Ϫ1.9͒	285.0	287.6 ͑2.6͒
		42	45	13	71	17	12
	2A320	707.6	709.3 ͑1.7͒	711.2 ͑3.6͒	283.9 ͑Ϫ1.9͒	285.8	289.1 ͑3.3͒
		43	32	25	52	32	16
	2A350	707.7	709.6 ͑1.9͒	711.7 ͑4.0͒	283.3 ͑Ϫ1.4͒	284.7	No peak
		36	39	25	37	63

z E-mail: constantin.vahlas@ensiacet.fr

Acknowledgments

We are indebted to Yannick Thébault and Djar Oquab, Institut National Polytechnique, Toulouse for the SEM studies, and to Sophie Gouy and Philippe de Parseval, Observatoire Midi-Pyrénées, Toulouse for the EPMA analyses. This work was supported by the EC under contract no. NMP3-CT-2005-500140 and no. MC-IIF-39728, by the French Agence Nationale de la Recherche ͑ANR͒ under contract no. NT05-3_41834, and by CNRS through a grant awarded to V.K.

Nikolaev Institute of Inorganic Chemistry SB RAS assisted in meeting the publication costs of this article.