Effects of low energy ion bombardment on the formation of cubic iron mononitride thin films

Thin Solid Films 539 (2013) 35–40

Contents lists available at SciVerse ScienceDirect

Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Effects of low energy ion bombardment on the formation of cubic iron mononitride thin films Pilar Prieto a,⁎, Juan de la Figuera b, José M. Sanz a, José F. Marco b a b

Departamento de Física Aplicada M-12, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain Instituto de Química-Física “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 13 July 2012 Received in revised form 25 April 2013 Accepted 26 April 2013 Available online 7 May 2013 Keywords: Iron nitride Dual ion beam sputtering Rutherford backscattering spectroscopy X-ray diffraction Mössbauer spectroscopy

a b s t r a c t The formation of cubic nitrides with stoichiometry close to FeN obtained by ion assisted sputter deposition has been studied as a function of deposition parameters. In particular, we have explored the influence of the energy deposited by the assistant beam per deposited Fe atom to understand changes in composition, phase formation and nanocrystallinity of the films. An optimum N2+ ion energy and a JN/JFe ratio (JN and JFe represent the current density of N2+ ions and Fe atoms respectively) have been determined in order to obtain only iron mononitride phases. X-ray diffraction and Mössbauer spectroscopy revealed a phase evolution from ε-Fex(x ≈ 2)N to γ″ and γ‴-FeN as the N2+ ion energy and the JN/JFe flux ratio increase. Pure nanocrystalline iron mononitride, with nitrogen content close to 50%, is obtained when JN/JFe ratio reaches 5.9 and the N2+ ion energy is 63.4 eV. Further increments of N2+ energies and JN/JFe values reverse this behavior and a phase evolution from γ″ and γ‴-FeN to ε-Fex(x ≈ 2)N is found. This behavior is attributed to energy damage and resputtering phenomena. It has also been found that γ‴-FeN phase coexists with γ″-FeN phase when the deposition is performed at room temperature. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The Fe–N system shows a very complex phase diagram [1] so that iron nitrides can exist in many different phases with different structures and properties. Nevertheless, the growth of any of these iron-nitrides as pure phases is not easy. The nitrogen-poor phases with N atomic concentration below 33%, i.e. α′-Fex(x ≥ 8)N, γ′-Fe4N and ε-Fex(3 ≤ x b 2)N, are ferromagnetic compounds and have been proposed as alternative materials to pure iron in magnetic applications due to their good tribological behavior and chemical inertness [2,3]. The ε-phase occurs over a wide nonstoichiometric range and its hexagonal iron sublattice expands with increasing nitrogen content. When the nitrogen content reaches 33 at.%, paramagnetic ζ-Fe2N, with an orthorhombic structure is formed [4]. For higher nitrogen contents close to 50 at.%, iron mononitrides (FeN), which have received increasing attention because of their potential spintronics applications [5], are formed. They can exist in two phases: γ″-FeN with a ZnS-type structure and a lattice constant in the range 0.429– 0.434 nm [6,7] and γ‴-FeN with the NaCl structure [8] and a lattice constant of 0.450 nm. Experimental [6–14] and theoretical [15,16] studies have been performed to grasp a better understanding of the grown conditions and properties of iron mononitrides. γ″-FeN and γ‴-FeN are difficult to produce as pure phase so that most of the published studies claim

⁎ Corresponding author. Tel.: +34 914975265; fax: +34 914973969. 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.04.141

for a coexistence of γ″- and γ‴-FeN [9–11]. However a discussion about their crystal structure and the coexistence of these two phases, is lacking [13,14]. Iron mononitrides have been often grown in the form of thin films by physical vapor deposition methods [5–14]. Fe evaporation in a radio frequency nitrogen plasma discharge [5–12] produces iron nitrides with a γ″-FeN ZnS structure. Pulsed laser deposition (PLD) is also used to grow iron mononitride thin films, but in this case the γ″ and γ‴-FeN phases coexist [11]. Magnetron sputtering [7–10,13,14] has been used more extensively to grow these materials. Most of the works found either the coexistence of γ″ and γ‴-FeN phases [9,10] or the presence of γ″-FeN with a ZnS-type structure [7,9]. More recently, M. Gupta et al. [13] and I. Jouanny et al. [14], using also magnetron sputtering based-techniques, have found only one phase in iron mononitride thin films that they have assigned to a γ‴-FeN phase with ZnS-type structure, on the basis of their lattice parameter and (111) to (200) diffraction peak intensity ratio. Theoretical [15,16] studies have also been performed trying to understand the electronic structure and magnetism of the iron mononitride phases. They predict that the iron nitride with NaCl structure should be ferromagnetic or antiferromagnetic, while that with ZnS structure should be paramagnetic. Ion assisted deposition (IAD) methods have been used in industrial applications because it allows in obtaining denser films and microstructures far from equilibrium. In a previous work [17], we have demonstrated the capability of dual ion beam sputtering to obtain iron nitride thin films in a wide range of compositions and different structures by controlling the deposition conditions (i.e. Fe deposition rate,

36

P. Prieto et al. / Thin Solid Films 539 (2013) 35–40

flux and energy of the nitrogen ions and substrate temperature). One of the advantages of this system with respect to other sputtering techniques is the possibility of controlling independently the flux of Fe atoms, coming from the sputtering of Fe target, and the flux of nitrogen atoms, coming from the beam of N2+ bombarding the sample. The purpose of this work is to investigate the formation, by ion assisted sputter deposition, of FeNx thin films in the composition range (0.6 ≤ x ≤ 1) and to show the influence of the assistance beam of N2+ in the formation of the iron mononitride phases (i.e. γ″ and γ‴). We have studied the nanocrystallinity, the phase and chemical compositions of the films in terms of the N2+ energy deposited per arriving Fe atom, i.e. EFe, defined as the product of the N2+ ion energy, E, and the JN/JFe flux ratio (JN and JFe represent the current density of N2+ ions and Fe atoms respectively). 2. Experimental details Iron nitride thin films were deposited on Si (100) substrates using a dual ion beam sputtering system. The films were obtained by Ar + sputtering with a 3 cm Kaufmann-type ion source of a pure iron (99.99%) target and the simultaneous bombardment of the growing film with low energy nitrogen ions from an end-Hall ion source. Although this kind of source allows using high density fluxes and low energy ions, an independent control of the energy and the nitrogen flux is not possible. Therefore, to achieve different values of N2+ flux, small changes in the energy of these ions are required. The films were deposited in a vacuum chamber with a residual pressure of 2 × 10 −5 Pa. During the deposition the pressure was maintained at 3 × 10 −2 Pa and the substrates were rotated at 2 rpm to increase the homogeneity of the deposit. The temperature of the substrates was kept constant at 20 or 150 °C. The first three columns of Table 1 summarize the deposition conditions for each film obtained at room temperature. JN and JFe represent the current density of N2+ ions and Fe atoms respectively. JFe was estimated from the thickness of Fe films deposited without assistance. The energy, E, is the energy of the assisting beam which in the case of N2+ ions is shared by the two atoms. Table 1 also includes the nitrogen-ion to Fe-atom arrival ratio, defined as JN/JFe and the N2+ energy deposited per arriving Fe atom, i.e. EFe. The average composition of the films, with an accuracy better than 5%, was determined by resonant Rutherford backscattering spectrometry (RBS), [12,18] using a 5 MV tandem accelerator. The RBS analysis of the films was performed with an incident beam of 3.7 MeV 4He+ ions whereas the backscattering ions were detected at scattering angles of 165°. The energy of the 4He+ ions is selected to enhance resonantly the scattering cross section of the nitrogen atoms and, therefore, to enable a better quantification of the average nitrogen composition in a non-destructive way. The distribution and quantification of the elements were determined by simulation (RUMP program) using the 14N(α,α)14N elastic-scattering cross sections, as reported by Feng et al. [18], and taking into account the background of protons from the reaction 14N(α,p) 17O contributing to the N signal, as determined by Andrzejewska et al. [12] and Feng et al. [18]. RBS also allowed us to

determine the thickness of the films and the real deposition rate, clearly different for the Fe atoms' deposition rate. The nitrogen content of the films (i.e. x in FeNx) and the deposition rate, both determined by RBS, have been also included in Table 1. The crystalline structure of the different films was analyzed by X-ray diffraction (XRD) at a grazing incidence of 1° with respect to the surface plane, using CuKα radiation (λ = 0.15408 nm) in a Siemens 5000 spectrometer. Integral Conversion Electron Mössbauer spectroscopy (ICEMS) measurements were performed using a 57Co(Rh) source and a parallel plate avalanche counter (PPAC) [19]. The spectra were recorded at room temperature in the constant acceleration mode. All the spectra were computer fitted to determine the hyperfine parameters and the relative areas of the various components. All the isomer shifts are referred to the centroid of the spectrum of α-Fe at room temperature. The spectra corresponding to the γ″ and γ‴ iron nitrides have been deconvoluted into two singlets and a quadrupole-split doublet. The two singlets were associated to Fe atoms pertaining to the ZnS-structure (i.e. γ″-FeN), and to Fe atoms belonging to the NaCl structure (i.e. γ‴-FeN), all having the nearest nitrogen neighbor sites occupied. The χ2 values of the different fits were always reasonably good and ranged between 1.3.and 1.6. 3. Results and discussion Fig. 1 shows typical RBS spectra for samples S4 and S6 measured with 4He + particles at 3.7 MeV where differences at the N and Fe channels, indicated by arrows, are clearly observed. The channel corresponding to the Si substrate is also included in that figure. The analysis of the RBS spectra for different iron nitride thin films is listed in Table 1 in terms of nitrogen concentration. In addition, from the areal density and the deposition time we can also estimate the deposition rate, as we will discuss later. Fig. 2 shows the XRD patterns of the different iron nitride thin films obtained under the deposition conditions shown in Table 1. The broad peak at around 56° corresponds to the (100) reflection of the silicon wafer substrate. Except for samples S1 and S6, the XRD patterns are rather complex, with the presence of broad lines that are compatible with the presence of different iron nitride phases. Sample S1 shows the main diffraction peak at 2θ = 42.5° that is assigned to the (111) diffraction plane of ε-Fe2N phase. The (110), (112) and (300) diffraction planes of this phase are also observed with peaks at 37.2, 56.3 and 67.3° respectively. The ε-phase occurs over a wide nonstoichiometric range, and its hexagonal iron nitride sublattice expands with increasing nitrogen content. When the stoichiometry reaches FeN0.5, an orthorhombic distorted ζ-Fe2N phase accompanied by a rearrangement of the nitrogen atoms and a slight expansion of the iron sublattice along the b-axis is formed. Therefore the ε- and ζ-iron nitrides resemble each other in their diffraction patterns [4] so that the diffraction peaks observed in the diffractogram recorded from sample S1 can also be assigned to the (021), (121), (221) and (040) reflections from the ζ-Fe2N phase. The diffraction peaks are quite broad and, assuming that the broadening is only related to grain size effects, a crystal size

Table 1 Main deposition parameters of iron nitride thin films deposited at room temperature. The table includes the N2+ ion energy, E, N2+ ion current density (JN), Fe atoms current density (JFe), JN/JFe ratio and the N2+ energy deposited per Fe atom, EFe, defined as E · JN/JFe. The table also included the nitrogen concentration in the films as well as the deposition rate, both estimated from the RBS data. Sample

N2+ energy E (eV)

JN (N2+ ions/cm2s)

S1 S2 S3 S4 S5 S6

57.5 63.4 65.5 63.4 73.5 81.4

1.4 1.9 2.6 2.2 3.3 4.6

× × × × × ×

1014 1014 1014 1014 1014 1014

JFe (Fe atoms/cm2s) 5.5 5.5 5.5 3.8 5.5 5.5

× × × × × ×

1013 1013 1013 1013 1013 1013

JN/JFe

EFe (eV/atom)

x (FeNx)

Deposition rate (Å/s)

2.6 3.4 4.7 5.9 5.9 8.2

150 216 312 374 433 673

0.64 0.85 0.84 0.98 0.81 0.76

0.10 0.08 0.07 0.06 0.05 0.03

P. Prieto et al. / Thin Solid Films 539 (2013) 35–40

2000 S4 S6

Intensity (arb. units)

N

Fe

1500 Si 1000

500

0 400

600

800

1000

Channel number Fig. 1. RBS spectra of samples S4 and S6 obtained with 3.7 MeV 4He+ ions. The arrows indicate the surface channels of Fe, N and Si.

of ~12.0 nm is obtained by applying the Scherrer formula [20] (d = 0.9λ/FMHW · cosθ where λ is the wavelength of the radiation used) to the most intense diffraction peak, i.e. (111). The phase composition of the films proved to be very sensitive to the deposition conditions. Increasing the energy deposited by Fe atom, EFe, up to 216 eV/atom, a film is obtained (S2) whose diffraction pattern differs completely from that recorded from S1. That diffraction pattern is very complex and shows the co-existence of ε, γ″ and γ‴ phases. The peak at 39.1°, clearly observed in the diffractogram, is assigned to the (200) diffraction plane of the NaCl crystal structure of γ‴-FeN. The peak at 56.9° can be also assigned to the (220) diffraction plane of γ‴-FeN. Nevertheless, the presence of a small contribution of the (112) diffraction plane of the ε phase, around 56.3°, cannot be disregarded. The lines due to the (111) diffraction planes from γ″ and γ‴-FeN appear strongly overlapped in the diffractogram because of their large line broadening. The line due to the (111) diffraction plane of γ″-FeN appears at 35.4°, while a shoulder at low angles, roughly ≈ 33.8°, indicates that the (111) peak of the γ‴-FeN can

Fig. 2. XRD diffraction patterns measured at grazing incidence for the different iron nitride thin films.

37

also be present. The broad peak at ≈42.2° corresponds mainly to the (111) reflection plane of the ε phase. However, we cannot exclude the presence of a small contribution from the (200) diffraction plane of γ″-FeN. We would like to remark that the two more intense peaks at 39.1° and 35.4° cannot be assigned to a single mononitride phase. The intensity ratio I(111)/I(200), of the (111) and (200) respective reflections, has been used to identify the structure type in iron mononitride thin films [13,14]. In general, I(111) > I(200) corresponds to a ZnS-type structure while I(200) > I(111) usually corresponds to a NaCl-type structure. In the present case, the large I(200)/I(111) intensity ratio supports the presence of γ‴-FeN phase with a NaCl-type structure, while for γ″-FeN with a ZnS-type structure only the (111) diffraction peak can be clearly distinguished in the diffractogram. We have estimated a lattice parameter of 0.459 ± 0.005 nm for γ‴-FeN using the (200) and (220) peaks, and a lattice parameter of 0.439 ± 0.005 nm for γ″-FeN using the (111) diffraction plane. In both cases the estimated lattice parameter is slightly higher that that obtained by other sputtering techniques [7–9]. The average crystal size obtained by applying the Scherrer's formula to γ‴-FeN is around 13 nm while the resulting size for γ″-FeN is slightly lower around 8 nm. If EFe, increases, the nanocrystalline character of the films (S3 and S4) is clearly enhanced and this is evidenced by the presence of quite broad peaks in the corresponding diffraction patterns (Fig. 2). Thus, sample S3 shows three broad peaks at 35.5°, 39.1° and 42.3° which can be assigned to γ″(111), γ‴(200) and ε(111), the most intense one corresponding to γ″(111). The estimated crystallite size for γ″ and γ‴ is around 4 nm while for the ε phase it is slightly higher, 7 nm approximately. This effect is even stronger for higher energy and nitrogen flux. So sample S4, which was obtained with EFe = 374 eV/atom, shows the highest nitrogen concentration as well as the highest nanocrystalline character. The corresponding diffractogram does not show any evidence of the presence of ε-phase, and iron mononitride (i.e. γ″ + γ‴) becomes the main phase as suggested by the presence of very broad peaks, roughly around 35 and 39°, mainly from the (111) reflection of γ″ and (200) reflection of γ‴ respectively. The estimated crystallite size is, in this case, lower than 2 nm. In addition, an inspection of the relative intensities of the (111) and (200) reflections for samples S2, S3 and S4, indicates an increase of (111) peak, related mainly with γ″, versus (200) peak due to γ‴-FeN, as EFe increases. The evolution observed in nitrogen composition and crystallite size is reversed when EFe keeps increasing. The diffractogram of sample S5, corresponding to EFe = 433 eV/atom, shows mainly the ε-phase with small contributions around 35° and 39° characteristics of iron mononitrides, while the diffractogram of sample S6 (EFe = 673 eV/atom) only shows the presence of the ε-phase. The estimated average crystallite size increases up to 7.5 and 9.5 nm for samples S5 and S6 respectively. Summarizing, with the increase of the energy deposited per Fe atom, the phase evolution ε → γ‴ + γ″ → ε is observed in our deposition process. We cannot confirm the presence of only a single mononitride phase, i.e. γ‴ or γ″ as it has been reported by other deposition methods [5–7,9,13,14]. However, as EFe increases, γ″-FeN seems to become the major phase in the mononitride (γ‴ + γ″). These results are in agreement with Easton et al. [9], who also observe an increase of concentration of γ″ with respect to that of γ‴ phase with increasing nitrogen flux in FeN films deposited by magnetron sputtering. The energy of the assistant beam and the ratio JN/JFe, have an important influence in the composition, phase structure and nanocrystalline character of the films. The evolution of average grain size related to the main phases present in the different films (i.e. ε-phase for samples S1, S5 and S6 and γ″ + γ‴ phases for samples S2, S3 and S4) and the nitrogen content of the films, as a function of EFe, is shown in Fig. 3. As EFe increases, the nitrogen concentration in the films also increases until a stoichiometry close to FeN. Once this

38

P. Prieto et al. / Thin Solid Films 539 (2013) 35–40

1.0 x(FeN)

110 S6

0.9

10

100 8

0.8

6 0.7 4

110

x in FeNx

S5 100 110

0.6

2 0 200

300

400

500

600

Effect (%)

grain size (nm)

12

0.5 700

EFe (eV/atom)

S4 100 110

S3

100

Fig. 3. Mean grain size determined by the broadening of diffraction patters and nitrogen composition of the films determined by resonant backscattering spectroscopy as a function of the energy deposited per Fe atom, EFe (eV/atom).

110 S2 100 120

stoichiometry has been reached, higher EFe values cause a decrease of nitrogen content in the films. This fact suggests that the incorporation of nitrogen in the films is limited by resputtering effects of nitrogen atoms. In addition, since the thickness of the films also decreases with the energy and JN/JFe ratio, the resputtering of Fe atoms is also taking place as we will discuss later. It is generally found that, in sputtering processes, the grain size increases with increasing substrate temperature or sputtering power as a consequence of the faster atom mobility and diffusion at the surface. In an IAD process, an increase of ion energies and ion fluxes generally brings about an increase of the nucleation rate of the growing film induced by the ion generated defects. It is evident that the dependence of grain size with the reduced energy (eV/atom) shown in Fig. 3 has both effects mixed. The decrease of the grain size with the deposited energy per Fe (eV/atom) up to ≈ 380 eV/atom is the expected behavior for an ion assisted deposition process due to an increase of the number of nuclei on the increasing number of surface defects that act as nucleation centers. Larger nucleation rates are generally associated with smaller grain sizes. For EFe values higher that 380 eV/atom, the increase of the grain size observed might be related to a higher surface mobility and diffusion of the adatoms. Due to the nanocrystalline character of the films, it is very difficult to carry out an accurate determination from the XRD data, characterized by very broad and overlapping peaks, of the relative concentration of the ε-phase and iron mononitride phases in the films. In order to get a more precise quantification of the different phases formed, Integral Conversion Electron Mössbauer spectra were recorded at room temperature from the deposited films (Fig. 4). The ICEM spectrum recorded from sample S6 is constituted by an asymmetric doublet which was best-fitted considering two quadrupole contributions. The corresponding Mössbauer parameters (Table 2) match those reported for ε-Fe2.12N [21]. The spectrum of sample S4, with a stoichiometry very close to FeN and without any contribution due to a ε-phase as determined by XRD, has been deconvoluted into two singlets and a quadrupole-split doublet. We must point out that, although being apparently simple, there is no unique, unambiguous way of fitting this spectrum and several fitting models can be applicable. Similar spectra have been fitted in the literature considering two singlets assignable to ZnS and NaCl-type structures [8,22], two singlets assigned to ZnS-type structure [13], one singlet assigned to ZnS-type structure plus one doublet assigned to the presence of vacancies in such structure [12] or two singlets assigned to the ZnS and NaCl-type structures plus a doublet assigned again to the presence of vacancies or crystalline defects in the lattice [10,11,17]. In any case the singlet with the lower isomer shift is related to the ZnS-type structure.

110

S1

100 -2

-1

0

1

2

Velocity (mms-1) Fig. 4. Room temperature ICEMS spectra recorded from the different iron nitride films as labeled (cf. Table 1). The Mössbauer parameters used for the fit of the different phases are included in Table 2.

In view of the XRD data recorded from sample S4, that show evidence of the presence of both the γ″ ZnS and γ‴ NaCl-type structures, but also due to possible defects induced by the ion assistance, the fitting of Mössbauer spectrum was carried out assuming a mixture of the γ″ and γ‴ phases (i.e. two singlets) plus a doublet to account for the existence of Fe sites with neighboring vacancies. The ICEM spectra recorded from samples S1, S2, S3 and S5 have been deconvoluted in terms of one singlet due to γ‴, one singlet plus one doublet due to

Table 2 Mössbauer parameters obtained from the fit of the ICEMS spectra. Sample

δ (mm s−1)

Δ (mm s−1)

Γ (mm s−1)

Area (%)

Assignment

S1

0.43 0.30 0.65 0.15 0.22 0.43 0.30 0.65 0.15 0.22 0.43 0.30 0.65 0.15 0.22 0.65 0.15 0.22 0.43 0.30 0.65 0.15 0.22 0.43 0.30 0.65 0.15 0.22

0.29 0.47 − − 0.50 0.29 0.47 − − 0.50 0.29 0.47 − − 0.50 − − 0.50 0.29 0.47 − − 0.50 0.29 0.47 − − 0.50

0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30

75 8 6 4 7 18 4 26 31 21 16 4 28 32 20 25 35 40 39 4 17 22 18 68 18 8 5 1

ε-Fe2.12

S2

S3

S4

S5

S6

γ‴-FeN γ″-FeN γ″_vacancies ε-Fe2.12 γ‴-FeN γ″-FeN γ″_vacancies ε-Fe2.12 γ‴-FeN γ″-FeN γ″_vacancies γ‴-FeN γ″-FeN γ″_vacancies ε-Fe2.12 γ‴-FeN γ″-FeN γ″_vacancies ε-Fe2.12 γ‴-FeN γ″-FeN γ″_vacancies

P. Prieto et al. / Thin Solid Films 539 (2013) 35–40

% iron nitride phase

100 80 60 40 20 0 0.6

0.7

0.8

0.9

1.0

x in FeNx Fig. 6. ε and γ″ + γ‴ iron nitride (at.%) determined by XRD and ICEMS as a function of nitrogen content in the different iron nitride thin films.

they impact with the surface of the film at the employed energies, then: N ¼ Ni −NO ¼ 2JN −rN ·JN Fe ¼ Fei −FeO ¼ JFe −rFe ·JN: From these equations, the resputtering yield can be calculated (Fig. 7b). It can be observed that rN increases with the energy deposited per Fe atom (eV/atom) reaching very large values at high EFe values, indicating that under these deposition conditions it is difficult to incorporate nitrogen into the films. The quantitative values are close to those reported by Quirós et al. for CN compounds [22]. On the contrary, the resputtering yield of iron is low and remains approximately constant down to 0.1. In this model we have neglected the relationship between the nitrogen flow and the iron sputtering yield from

0.10

Deposition rate (Å/s)

γ″ and two doublets corresponding to the presence of the ε-phase. For clarity, the area under the doublet of ε-phase is filled in the figure. In Table 2 we have included the Mössbauer parameters used for the fit of the ICEMS spectra as well as the relative areas obtained for each phase. As pointed out above, the nanocrystalline character of the iron nitride thin films makes it difficult to accurately determine the amount of the ε, γ″ and γ‴ phases from the XRD results. However, we have performed a rough fit of the XRD data in the 2θ range between 30 and 45°, by subtracting the XRD pattern corresponding to the pure ε-phase recorded from sample S6 from each of the rest of diffractograms and by subsequently fitting the resulting diffraction patterns using elementary Lorentzian lines, which include one peak at ≈35° related mainly to γ″ (111) and one peak at ≈39° related to γ‴ (200). Fig. 5 shows the relative concentrations of iron mononitride (γ″ + γ″) and ε-iron nitride determined both by XRD and ICEMS as a function of EFe. This figure shows a good agreement between the XRD and Mössbauer data, which show the same evolution with EFe. It follows from these results that iron mononitride thin films can only be obtained at intermediate EFe values. It is also interesting to investigate the evolution of the different phases formed (iron mononitride (γ″ + γ‴) and ε-nitride) as a function of the nitrogen concentration (i.e. x in FeNx) in the films. This evolution is presented in Fig. 6 and shows that the iron mononitride phases can be grown and the phase transformation ε → γ″ + γ‴ takes place when the nitrogen content is equal to or above 43 at.%. The deposition rate was obtained from the areal densities determined by RBS and by using the mass density of 6 g/cm 3 determined experimentally by M. Gupta et al. for FeN0.7 [23]. The deposition rate values for the different iron nitride thin films have been included in Table 1 and have been represented as a function of EFe (eV/atom) in Fig. 7a. An increase of N2+ ion energy and JN/JFe flux ratio leads to a progressive decrease of the deposition rate. The explanation of this effect is probably related to the existence of resputtering processes that remove material from the growing film and slow down the growth rate. In order to quantify the resputtering processes, a simple empirical growing model was applied [24] by using the N/Fe composition and thickness as determined by RBS and the nitrogen ion and Fe atom current densities determined from the experiment. We define: Ni (Fei), NO (FeO) and N (Fe) as the nitrogen (iron) atoms arriving, ejected from and incorporated in the film respectively. In addition we define the resputtering yield for iron and nitrogen, i.e. rFe and rN respectively. rFe is the number of Fe atoms ejected per N2+ arriving ion and rN is the number of nitrogen atoms ejected per N2+ arriving ion. Assuming that most of the N2+ ions dissociate into two nitrogen atoms when

39

0.08 0.06 0.04 0.02 0.00 0

200

400

600

800

EFe (eV/atom) 100

resputtering yield

% iron nitride phase

2.0 80 60 40 20 0

rN r Fe

1.8

0.2 200

300

400

500

600

700

EFe (eV/atom) Fig. 5. Iron mononitride and ε-nitride phase concentrations (in at.%), determined by XRD and Mössbauer spectroscopy, as a function of the energy deposited per Fe atom, EFe (eV/atom).

0.0 0

200

400

600

800

EFe (eV/atom) Fig. 7. Deposition rate and resputtering yields of iron and nitrogen atoms as a function of the energy deposited per Fe atom, EFe (eV/atom).

40

P. Prieto et al. / Thin Solid Films 539 (2013) 35–40

the target, modeled recently by Puech et al. for iron oxide deposition [25]. If this effect were taking place in our experiments, the observed reduction of the deposition rate as the nitrogen flux increases could be also related to a lower iron sputtering yield from the target and therefore a lower iron resputtering yield could be expected.

3.1. Effect of substrate temperature

Intensity (arb. units)

In order to clarify the coexistence of γ″ and γ‴ and to compare our results with those reported in other works [9–14], we have investigated the effect of substrate temperature to reach growth conditions closer to equilibrium. The effect of substrate temperature during growth, maintaining constant the other deposition parameters, is shown in Fig. 8. This figure shows the diffraction patterns of samples grown with EFe = 216 eV/atom while keeping the substrate at room temperature (sample S2) and at 150 °C (sample S7). The XRD pattern of the sample grown at room temperature shows mainly the coexistence of γ″ and γ‴ phases as it has been discussed before. However, the XRD pattern of the iron nitride thin film grown at a substrate temperature of 150 °C can be indexed as belonging to the ε and γ″ phases. The presence of γ″-FeN is clearly identified by the diffraction peaks observed at 36.3, 61.0 and 73.1° that correspond to the (111), (220) and (311) planes, respectively. An additional small bump at ~ 42°, that can be associated with γ″(200), is also visible in the diffractogram. The diffraction intensities follow the trend I(111) > I(220) > I(200), as expected for a ZnS-type structure [14]. The lattice parameter calculated from the (111), (220) and (311) diffraction peaks is 0.429 nm, being in good agreement with the value reported in the literature [6,7,9]. The XRD pattern of this sample, i.e. S7, also shows peaks at 37.5, 42.9, 56.5 and 67.6°, which correspond to the (110), (111), (112) and (300) diffraction planes of the ε-iron nitride phase. For clarity, we have included the diffraction peak position corresponding to a γ″ phase with lattice parameter of 0.429 nm as well as the one corresponding to a pure ε-phase obtained at 150 °C in our deposition system. From the peak areas γ″(111) + γ″(200) and ε(110) + ε(111), we estimated that this sample contains about 42% γ″-FeN. The data do not show evidence of the γ‴-FeN phase suggesting that γ‴-FeN with a NaCl-type structure is probably a metastable phase and it transforms in γ″-FeN + ε-Fe2N phases when the sample is grown at 150 °C. As discussed before, a significant amount of γ‴-FeN with NaCl-type structure can only be reached at room temperature and at relatively low EFe values. Previous works based on samples grown by magnetron sputtering found that γ‴-FeN with a NaCl structure coexists with γ″-FeN with a ZnS structure [9,10] and when a single phase of iron mononitride is obtained (γ‴ or γ″), this phase has a ZnS-type structure [7,9,13,14].

o

S7(150 C)

S2 (RT)

30

40

50

60

70

Fig. 8. X-ray diffraction patterns at grazing incidence of iron nitride thin films obtained with a JN/JFe flux ratio of 3.4 and N2+ ion assistant energy of 63.4 at room temperature and at 150 °C as labeled.

The comparison with our results shows the difficulty to produce pure γ‴-FeN with a NaCl structure. 4. Conclusions Iron nitride thin films with nitrogen contents in the range 39– 50 at.%, have been deposited by dual ion beam sputtering. The influence of deposition parameters was investigated in order to find the conditions which lead to the formation of nanocrystalline γ″ and γ‴ nitride phases. We have observed that the grain size decreases and the content of N-rich phases increases as the ion energy and flux ratio JN/JFe increase. The use of N2+ energies around 63 eV and JN/JFe values around 5.9 gives the highest nitrogen composition and the lowest crystallite size of the deposited iron nitride thin films with only iron mononitride phases (i.e., γ‴ and γ″). However, a further increase of the ion energy and JN/JFe values induces a decrease of nitrogen concentration and an increase of grain size. The phase structure depends of the N2+ energy deposited per arriving Fe atom, EFe, and a phase evolution ε → γ‴ + γ″ → ε has been observed as EFe increases. It was not possible to reach only pure γ″ or γ‴-FeN phases, however as the energy and flux ratio JN/JFe increase, γ″-FeN seems to become the major phase in the mononitride. For EFe = 216 eV/atom we have observed that the γ‴-FeN coexists with γ″-FeN at room temperature deposition conditions, however only γ″-FeN phase is present in the mononitride when the substrate temperature is kept at 150 °C during deposition. The change in nitrogen composition and the observed decrease of the deposition rate as EFe increases can be explained in terms of resputtering phenomena. Acknowledgments We acknowledge the financial support from the Spanish Government under project number MAT2009-14578-CO3-01 and MAT2012-38045. References [1] E.H. du Marchie van Voorthuysen, N.C. Chechenin, D.O. Boerma, Metall. Mater. Trans. A 33A (2002) 2594. [2] P. Schaaf, Prog. Mater. Sci. 47 (2002) 1. [3] G.F. Gomes, M. Ueda, H. Reuther, J. Appl. Phys. 94 (2003) 5379. [4] M. Kano, T. Nakagawa, T.A. Yamamoto, M. Katsura, J. Alloys Compd. 327 (2001) 43. [5] C. Navío, J. Alvarez, M.J. Capitan, F. Indurain, R. Miranda, Phys. Rev. B78 (2008) 155417. [6] W. Lin, J. Pak, D.J. Ingram, A.R. Smith, J. Alloys Compd. 463 (2008) 257. [7] V. Demange, T.H. Loi, P. Weisbecker, E. Bauer-Grosse, Thin Solid Films 494 (2006) 184. [8] A. Oueldennaoua, E. Bauer-Grosse, M. Foos, C. Frantz, Scr. Metall. 19 (1985) 1503. [9] E.B. Easton, Th. Buhrmester, J.R. Dahn, Thin Solid Films 493 (2005) 60. [10] L. Risanen, M. Neubauer, L. Lieb, P. Schaaf, J. Alloys Compd. 274 (1998) 74. [11] R. Usui, Y. Yamada, Y. Kobayashi, Hyperfine Interact. 205 (2012) 13. [12] E. Andrzejewska, R. González-Arrabal, D. Borsa, D.M. Boerma, Nucl. Instrum. Meth. Phys. Res. B 249 (2006) 838. [13] M. Gupta, A. Tayal, A. Gupta, V.R. Reddy, M. Horisberger, J. Stahn, J. Alloys Compd. 509 (2011) 8283. [14] I. Jouanny, P. Weisbecker, V. Demange, M. Grafouté, O. Peña, E. Bauer-Grosse, Thin Solid Films 518 (2010) 1883. [15] A. Houari, S.F. Matar, M.A. Belkhir, J. Magn. Magn. Mater. 312 (2007) 298. [16] Y. Kong, J. Phys. Condens. Matter 12 (18) (2000) 4161. [17] P. Prieto, J.F. Marco, J.M. Sanz, Surf. Interface Anal. 40 (2008) 781. [18] Y. Feng, Z. Zhuying, Z. Guoqing, Y. Fujia, Nucl. Instrum. Meth. Phys. Res. B 94 (1994) 11. [19] J.R. Gancedo, J.Z. D´avalos, M. Gracia, J.F. Marco, Hyperfine Interact. 110 (1997) 41. [20] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, MA, 1978. [21] P. Schaaf, Hyperfine Interact. 111 (1998) 113. [22] D.M. Borsa, D.O. Boerma, Hyperfine Interact. 151 (152) (2003) 31. [23] M. Gupta, A. Gupta, S.M. Chaudhari, D.M. Phase, V. Ganessan, M.V.R. Rao, T. Shripathi, B.A. Dasannacharya, Vacuum 60 (2001) 305. [24] C. Quirós, P. Prieto, E. Elizalde, R. Pérez-Casero, V. Gómez, P. Herrero, J.M. Sanz, Surf. Coat. Technol. 125 (2000) 366. [25] L. Puech, C. Dubarry, G. Ravel, E. De Vito, J. Appl. Phys. 107 (2010) 054908.