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Research ArticleReceived: 13 August 2007 Revised: 25 October 2007 Accepted: 25 October 2007 Published online in Wiley Interscience: 15 January 2008

(www.interscience.com) DOI 10.1002/sia.2658

Synthesis and characterization of iron nitrides.An XRD, Mossbauer, RBS and XPScharacterizationP. Prieto,a∗ J. F. Marcob and J. M. Sanza

In this work, we demonstrate the capability of a dual ion beam sputtering method to obtain iron nitride thin films, FeNx , in awide composition range (0.1 < x < 0.5) and structures (α′, γ ′, ε, ζ , γ ′′ and γ ′′′) by controlling the deposition conditions (i.e.deposition rate, flux and energy of the assisting nitrogen ions and substrate temperature). The average composition of thefilms has been determined by resonant RBS to enhance the nitrogen signal, whereas the structural characterization and thephase identification have been carried out by X-ray diffraction (XRD) and Mossbauer spectroscopy. We present a thorough andcomplementary analysis of the XRD and Mossbauer data as a function of the composition of the film for all the iron nitridephases. Furthermore, it is shown that the iron Auger parameter (AP) increases up to 0.8 eV with respect to that for pure α-Fewhen the nitrogen concentration increases up to 46 at.% (i.e. γ ′′ and γ ′′′ phases) so that it might serve to analyze samples ofunknown composition. Copyright c© 2008 John Wiley & Sons, Ltd.

Keywords: iron nitride; Mossbauer spectroscopy; resonant RBS; Auger parameter

Introduction

The Fe–N system shows a very complex phase diagram[1] andthat the iron nitrides can exist in many different phases withdifferent structures and properties. The nitrogen-poor phases,i.e. α′-Fex(x≥8)N, γ ′-Fe4N and ε-Fex(3≤x<2)N are ferromagneticcompounds.[2] In particular,α′-Fex(x≥8)N is of great interest becauseit exhibits a higher magnetization than that of pure α-Fe.[2,3] Theγ ′-Fe4N and ε-Fex(3≤x<2)N phases show a lower magnetizationthan that of pure iron but they are still magnetic and show agood tribological behavior.[2,4] A higher nitrogen content inducesthe formation of paramagnetic nitrides at room temperature,i.e.ζ -Fe2N, and the recently discovered cubic phases, γ ′′-ZnS andγ ′′′-NaCl type, with a stoichiometry close to FeN.[5 – 7]

We report here on the synthesis and characterization ofFeNx films obtained by dual ion beam sputtering (DIBS) at lowtemperature (<400 ◦C) covering the entire compositional range(0 ≤ x < 0.5) of the phase diagram.

XPS and AES have been extensively used to characterize thesurface of iron nitrides.[8] However, the common surface oxidationof the samples when exposed to the atmosphere, and modificationof the surface composition when they are sputter cleaned, is a clearhandicap of these techniques when they are used to determinethe composition of the films. In a previous study,[9] iron nitridethin films obtained by the same method were characterized byRBS, AES-depth profiling, X-ray diffraction (XRD) and vibratingsample magnetometry (VMS). In that case, the low sensitivityof common RBS for nitrogen and preferential sputtering effectsduring the AES-Depth profiling analysis hindered an appropriatedetermination of the composition. In the present paper, we havefabricated new samples and extended the range of nitrogencontent to reach the γ ′′ and γ ′′′ phases. In this case, we haveperformed resonant RBS[7,10] to enhance the nitrogen signal andto obtain a reliable quantitative analysis of the nitrogen content inthe film. In addition, we have used the advantages of Mossbauer

spectroscopy in combination with XRD to improve the distinctionbetween different phases present in the films.[5,6] The Augerparameter (AP) allows characterizing chemical effects in terms ofthe extra atomic relaxation energy Rea. In fact, the variation in theAP (i.e �AP) is usually approximated by �AP ≈ 2 �R.[11] Finally,it is shown that the iron AP increases up to 0.8 eV with respectto that for pure α-Fe when the nitrogen content increases up to50 at.% and therefore could serve to identify samples of unknowncomposition even though the method is very surface sensitive andtherefore dependent on the state of the surface.

Experimental Details

Iron nitride thin films were deposited on Si (100) substrates usinga dual ion beam sputtering system. The films were obtained byAr+ sputtering of a pure iron (99.99%) sputtering target and thesimultaneous bombardment of the growing film with a controlledmixture of argon and nitrogen ions, which assist the deposition.Table 1 summarizes the deposition conditions for each filmcharacterized in this work. During the deposition, the temperatureof the substrate was kept constant but varied for each sample asgiven in Table 1. The values of the deposition rate, included inTable 1, correspond to the deposition of pure Fe and allow theestimation of the number of Fe atoms arriving at the substrate.

∗ Correspondence to: P. Prieto, Departamento de Física Aplicada C-XII,Universidad Autonoma de Madrid, Cantoblanco, E-28049 Madrid, Spain.E-mail: pilar.prieto@uam.es

a Departamento de Física Aplicada C-XII, Universidad Autonoma de Madrid,Cantoblanco, E-28049 Madrid, Spain

b Instituto de Química Física Rocasolano-CSIC, c/Serrano 119, E-28006 Madrid,Spain

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Table 1. Main deposition parameters of iron nitride films: substrate temperature during deposition, %N2+, current density (mA/cm2) and energy of

the assisting ion beam, deposition rate [nm/s] and arrival rate ratio of N2+ ions to Fe atoms (ARRN). The areal density (atoms/cm2) and thickness of

the films, as determined by RBS, are also given

AssistanceSputtering RBS areal density

Sample[composition]

Substratetemperature (◦C)

%N2+

ionsCurrent density

(mA/cm2)Energy

(eV)Depositionrate (nm/s)

(×1015 atoms/cm2)[thickness (nm)]a ARRN

S1[FeN0.12] 150 33 0.018 71 0.0180 820[97] 0.6

S2[FeN0.24] 280 33 0.018 71 0.0180 1188[140] 0.6

S3[FeN0.38] 360 60 0.033 71 0.0180 300[35] 1.2

S4[FeN0.59] 150 100 0.029 60 0.0058 1020[120] 3.5

S5[FeN0.75] 20 100 0.030 63 0.0056 1050[124] 3.5

S6[FeN0.87] 150 100 0.040 60 0.0038 790[93] 6.4

a Estimated thickness assuming the density of pure Fe (7.87 g/cm3).

Table 1 includes nitrogen-ion to Fe-atom arrival ratio (ARRN) andthe areal density (atoms/cm2) of the film determined by RBS.

RBS was carried out using a 5-MV tandem machine. A completedescription of the experimental setup is described in Ref. [12]. Wehave used resonant RBS[7,10] to determine the average compositionof the films, with an accuracy better than 5%, by using an incidentbeam of 3.7 MeV 4He+ ions and detecting the backscattered ionsat scattering angles of 165◦. The energy of the 4He+ ions isselected to enhance resonantly the scattering cross section of thenitrogen atoms and, therefore, to enable a better quantificationof the average nitrogen composition in a nondestructive way. Thedistribution and quantification of the elements was determinedby simulation (i.e. SIMNRA program) using the 14N(α, α)14Nelastic-scattering cross sections, as reported by Feng et al.,[10]

and taking into account the background of protons from thereaction 14N(α, p)17O contributing to the N signal, as determinedby Andrzejewska et al.[7] and Feng et al.[10]

The structure of the different films was analyzed by XRD at agrazing incidence of 1◦ with respect to the surface plane, usingCuKα emission (λ = 0.15408 nm) in a Siemens 5000 spectrometer.Integral Conversion Electron Mossbauer spectroscopy (ICEMS)measurements were performed using a 57Co(Rh) source and aparallel plate avalanche counter (PPAC).[13] The spectra wererecorded at room temperature in the constant acceleration mode.All the spectra were computer fitted to determine the isomershift [δ], the magnetic hyperfine field [H], the quadrupole shift orsplitting [2ε or �] and the relative areas of the components. Allthe isomer shifts were referred to the centroid of the spectrum ofα-Fe at room temperature.

The modified AP for Fe in the different iron nitrides wascalculated using the formula AP = BE (Fe 2p) + KE (Fe L3M4,5M4,5),where both, the binding energy (BE) of Fe 2p and the kineticenergy (KE) of the Auger transition Fe L3M4,5M4,5, were measuredfor each sample in a PHI 3027 spectrometer equipped with adouble-pass cylindrical mirror analyzer, a nonmonochromaticMgKα radiation source (hν = 1253.6 eV). The pass energy ofthe spectrometer was 50 eV and the respective energies weremeasured with an accuracy of 0.1 eV.

Results and Discussion

Resonant Rutherford backscattering spectrometry

Figure 1 shows typical RBS spectra for samples S4 and S5 measuredwith 4He+ particles of 3.7 MeV. The arrows indicate the respective

200 400 600 800 1000 12000

500

1000

1500

2000

2500

Inte

nsity

(a.

u.)

Channel

S4

S5

W

Fe

Ar

Si

N

Figure 1. RBS spectra of samples S4 and S5 (cf Table 1) obtained with3.7 MeV 4He+ ions. The arrows indicate the surface channels of Fe, N, Ar,W and Si.

channels for N, Si, Ar, Fe and W, whereby Si is the substrateand W a contaminant (<0.1 at.%) mainly due to some sputteringof the neutralizer and gas distributor of the assisting ion gunduring the deposition process. Ar (<1 at.%) is also incorporatedduring the sputtering process. The enhancement of the nitrogensignal between S4 and S5 samples is clear. A quantitativeanalysis of the different spectra using the scattering cross sectionsreported by Feng et al.[10] for the 14N(α, α)14N reaction consideringthe appropriate corrections for the protons from the reaction14N(α, p)17O, which contribute to the N signal,[7,10] leads to thefollowing average composition for the respective samples, i.e.FeN0.12, FeN0.24, FeN0.38, FeN0.59, FeN0.75 and FeN0.87 (cf Table 1).The areal density of the films was also determined by RBS and itwas used to estimate their thickness. As the atomic density of thedifferent iron nitrides is not well known, we have used that of pureα-Fe to obtain the thickness included in Table 1.

X-ray diffraction and integral conversion electron Mossbauerspectrometry

Figure 2 shows the XRD patterns of the different iron nitride thinfilms obtained under the deposition conditions shown in Table 1.As the ARRN ratio is increased (cf Table 1), a rapid evolutionof phases appears in sequence in good agreement with the

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30 40 50 60 70 80

ζ(040)ζ(221)ζ(121)

ζ(021)

γ ''–γ '''-(111)

γ ''-γ '''(200)γ ''–γ '''(220)

ε(300)ε(112)ε(110)ε(111)

ε(110) ε(111)

γ '(111)γ '(220)

γ '(200)

Inte

nsity

(a.

u.)

2θ(degree)

S1

S2

S3

S4

S5

S6

α'(100)

Figure 2. XRD diffraction patterns measured at grazing incidence for thedifferent iron nitride films, as labeled (cf Table 1).

phase diagram at low temperatures,[1] i.e. α′-Fex(x≥8)N, γ ′-Fe4N,ε-Fex(3≤x<2)N, ζ -Fe2N and the cubic γ ′′-FeN and γ ′′′-FeN.

Figure 3 shows the ICEM spectra of the ferromagnetic (Fig. 3(a))and the paramagnetic (Fig. 3(b)) iron nitrides studied in this work(cf Table 1). As expected, nitrogen contents below 33 at.% (i.e. 11,20 and 27 at.% for samples S1, S2 and S3) lead to ferromagneticnitrides (Fig. 3(a)), whereas nitrogen contents above 33 at.% (i.e.

37, 43 and 46 at.% for samples S4, S5 and S6) cause paramagneticnitrides (Fig. 3(b)).

Sample S1 shows a main diffraction peak at 2θ = 44.6◦ and asmall peak at 2θ = 64.9◦ that are assigned to the (110) and (200)planes of the α′-Fex(x≥8)N phase, indicating that this is the mainphase of the film deposited under these conditions. However, theasymmetry of the main peak and the presence of a small featureat ∼69.4◦ assigned to the ε(300) diffraction plane clearly indicatethe presence of small amounts of ε phase. This coexistence isconfirmed by the ICEM spectrum recorded from this sample (cfFig. 3(a)). The spectrum shows a main sextet assigned to α′-FexN(the parameters are given in Table 2), which accounts for the 62%of the signal and a second sextet (33% of the total signal) whoseparameters (cf Table 2) agree rather well with those expected fora ε-Fe3−xN phase.[5] Finally, a third minor contribution (5%) iscompatible with the presence of some α′′-Fe16N2 phase.

The diffraction pattern of sample S2 clearly indicates thepresence of the γ ′-Fe4N phase with diffraction peaks at 41.2◦,47.9◦ and 70.0◦ associated with the (111), (200) and (220) planesrespectively. The presence of a small amount ofε-Fe3N is suggestedby the diffraction peak at 43.8◦ [i.e. (111) plane]. The correspondingICEMS spectrum agrees very well with that reported for γ ′ Fe4N,[6]

except for the presence of a small paramagnetic doublet (∼3%)in the center of the spectrum. The fcc structure of γ ′-Fe4N ischaracterized by three different nonequivalent iron sites,[6] whichlead to three hyperfine magnetic sextets. The parameters of thethree sextets are collected in Table 2.

In the diffraction pattern of sample S3, we have identified the(111) and (110) planes of ε-Fe3N with a small contribution of γ ′-FeN. However, the analysis of the ICEM spectrum indicates a rathercomplex mixture of phases. The corresponding ICEM spectrum isexplained as being due to a sextet (30%) assigned to ε-Fe3N,[5]

two additional sextets associated with the presence of ε-Fe3−xN[5]

(intensity 43%) and the three characteristic sextets of

a'-Fe16N2

a'-FeNε-Fe3-xN

e-Fe3-xNe-Fe3N

100

102

104

106

Velocity (mms-1)

S1

Eff

ect

(%)

100

102

104

106

S2

-9 -6 -3 0 3 6 9100

101

102

103

S3

g'-Fe4N

g'-Fe4N

Fe3+

Fe3+

e-Fe2.10N

100

105

110

115

120

Velocity (mms-1)

S4

Eff

ect

(%)

100

105

110

115

S5

-4 -2 0 2 4100

105

110

S6

g'''-FeNg''-FeN with vacancies

z-FeN

g''-FeN

g'''-FeNg''-FeN

g''-FeN with vacancies

(a) (b)

Figure 3. Room temperature ICEMS spectra recorded from the different iron nitride films, as labeled (a) and (b) (cf Table 2).

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Table 2. Mossbauer parameters obtained from the fit of the ICEMSspectra

Sampleδ

(mm/s)2ε (mm/s)

or � (mm/s)a H(T) Area (%) Assignment

S1 0.00 −0.02 31.9 62 α′-Fex N

0.31 0.10 21.0 33 ε-Fe3−x N

0.09 −0.12 27.8 5 α′′-Fe16N2

S2 0.23 0.02 34.4 25 γ ′-Fe4N

0.29 −0.40 21.9 22

0.31 0.18 21.9 50

0.22 1.01 – 3 ?

S3 0.23 0.02 34.4 4 γ ′-Fe4N

0.29 −0.40 21.9 4

0.31 0.18 21.9 9

0.34 −0.02 24.7 30 ε-Fe3N

0.33 0.01 22.6 34 ε-Fe3−x N

0.41 −0.04 10.6 9

0.40 1.02 – 10 Fe–O–N

S4 0.43 0.29 – 84 ε-Fe2.12N

0.30 0.45 – 16

S5 0.11 – – 26 γ ′′-FeN

0.20 0.47 – 16 γ ′′-FeN withvacancies

0.40 0.32 – 33 ζ -FeN

0.67 – – 25 γ ′′′-FeN

S6 0.14 – – 39 γ ′′-FeN

0.25 0.55 – 34 γ ′′-FeN withvacancies

0.66 – – 27 γ ′′′-FeN

a 2ε is the quadrupole shift and stands for sextets; � is the quadrupolesplitting and stands for doublets.

γ ′-Fe4N (17%). In addition and in order to obtain a satisfactoryfit, the spectrum requires a paramagnetic doublet (10%) whose

parameters (cf Table 2) could correspond to a paramagnetic Feoxynitride.[6]

The Fe2N phase is well represented by the diffraction patternof sample S4, where the diffraction planes of ε-phase [i.e. (100),(111), (112) and (300)] or ζ -phase [i.e. (021), (121), (221) and(040)] are clearly identified. The ICEM spectrum consists ofan asymmetric doublet, which was best fitted considering twoquadrupole contributions (cf Table 2) that match those reportedfor ε-Fe2.12N.[5]

Increasing the ARRN value from 0.6 to 6.4 (cf Table 1) allows toreach nitrogen contents close to 50 at.%, in the region of the phasediagram where γ ′′ and γ ′′′ are expected. The diffraction patternsfor S5 and S6 indicate rather clearly the presence of both the γ ′′and γ ′′′ phases as well as small contributions from the ζ phase.The ICEM spectrum of sample S5 consists of a broad asymmetricdoublet. In cases as the present one, the analysis of the spectrum isnot unique and different reasonable combinations of singlets anddoublets can lead to similar χ2 values. This fact has raised somecontroversy about the real existence of the γ ′′′ phase.[6,14] Table 2gives the fitting parameters in terms of a singlet (δ = 0.11 mm/s)and a doublet (δ = 0.20 mm/s;� = 0.47 mm/s), which account forthe presence of the γ ′′ phase. In this case, the singlet correspondsto Fe atoms surrounded by four nitrogen atoms (no vacancies)and the doublet would be associated with the presence of Fe siteswith vacancies in the first coordination sphere.[7] A second doublet(δ = 0.20 mm/s; � = 0.47 mm/s) indicates the presence of ζ -FeN,whereas a second singlet (δ = 0.66 mm/s) would account for thepresence of the γ ′′′ phase. Finally, the spectrum of sample S6 (i.e.nitrogen concentration of 46 at.%) was well fitted in terms of justthe γ ′′ and γ ′′′ phases.

Auger parameter

In this work, we have also studied the modified AP = BE(Fe 2p) +KE(L3M4,5M4,5) of iron in the different nitrides. For a given chemicalstate, the AP is defined as the sum of the BE of a core electron andthe KE of an Auger electron, mostly derived for a core–core–coretransition. In the case of the iron-nitrides studied here, we have

Inte

nsity

(a.

u.)

730 725 720 715 710 705

Binding energy (eV)

S6Fe

Fe 2p

565 560 555 550 545

S6Fe

Binding energy (eV)

Inte

nsity

(a.

u.)

Fe L3M4,5M4,5

(a)

(b)

0 10 20 30 40 50

Fe A

uger

Par

amet

er (

eV)

N at.%

α-Fe α'-FexN γ'-Fe4N ε-Fe3N ζ,ε-Fe2N γ''-γ'''-FeN

1410.6

1410.4

1410.2

1410.0

1409.8

(c)

Figure 4. (a) and (b) Fe 2p and Fe LMM XPS spectra respectively for pure Fe and for sample S6, as labeled. (c) Auger parameter of Fe as a function of thenitrogen concentration in different iron nitride films, as determined by RBS. The main phase present in the films is also included at the x-axis.

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analyzed the AP associated with the Fe 2p core level and theFe L3M45M45 Auger transition. XPS has been used to measure theFe 2p BE and the KE of the L3M4,5M4,5 Auger transition. Figure 4(a)shows the Fe 2p spectra for pure iron and sample 6, whereas thecorresponding L3M4,5M4,5 Auger spectra are depicted in Fig. 4(b).The spectra have been measured after sputter cleaning with 1 KeVAr+ ions and therefore correspond to surfaces whose compositionhas been modified by preferential sputtering effects.[9] In spite ofthis, the systematic comparison of the AP in all the samples givesinteresting results.

The measured APs have been depicted in Fig. 4(c) as a functionof the nitrogen content in the film, as determined by RBS. In thefigure, the main phase present in the film as determined by XRDand Mossbauer spectroscopy is also included. We have includedthe AP of pure iron for completeness. Figure 4(c) shows a clearvariation of the AP of up to 0.8 eV as the nitrogen concentrationincreases up to 46 at.%. In the case of iron oxide, the AP increases upto 2.8 eV with respect to that of pure iron value.[15] It is interestingto note here that we are using an Auger transition involving twicethe valence band. In order to assure the invariance of the lineshapeof the peaks as the chemical environment is modified, it is commonto measure the AP in terms of Auger transitions that involve onlycore levels. In the case of Fe, the Auger transition is quasiatomic innature[16 – 18] and therefore is invariant in form upon nitriding. Inthe case of Auger transitions involving the valence band, it can beshown[11] that

AP = BE(2p) + KE(L2,3VV) ≈ 2[BE(2p) − BE(V)] − U(3d 3d)

where U(3d 3d) is the hole–hole interaction energy in the finalstate. If U W , where W is the width of the local density ofstates, the Auger spectrum is atomic like, then the two holesform a localized state and U become important and even thedetermining factor in the variation of the AP between the differentnitrides. The observed variation of the AP would be associatedwith changes in the localization of the 3d states as the nitrogenconcentration increases.

Conclusions

We report here the synthesis and characterization of FeNx filmsobtained by DIBS at low temperature (<400 ◦C) covering theentire compositional range (0.1 < x < 0.5) of the phase diagram.Increasing the ARRN ratio, we observe a rapid evolution of phases,

i.e. α′-Fex(x≥8)N, γ ′-Fe4N, ε-Fex(3≤x<2)N, ζ -Fe2N and the cubicγ ′′-FeN and γ ′′′-FeN, which appear in sequence and in goodagreement with the Fe–N phase diagram at low temperatures.The use of 4He+ particles of 3.7 MeV to perform RBS allowed areliable quantitative determination of the nitrogen concentrationin the different films. The complementary use of XRD andMossbauer spectroscopy also enabled a better characterizationand identification of the different nitride phases. A careful studyof the AP using the Fe(2p) BE and the KE of the Fe(LVV) Augertransition in the different nitrides has shown a shift of up to0.8 eV between Fe and FeN, which depends on the nitrogenconcentration in the film and could serve to analyze samples ofunknown composition.

Acknowledgements

This work has been funded by the MEC (MAT2006-08158 andMAT2004-01451).

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