- Email: [email protected]
Si bilayers with N2+ ions
Thin Solid Films 459 (2004) 23–27
Formation of iron-nitrides by irradiation of ions
57
FeySi bilayers with N2q
ˇ ´ a, D. Perusko ˇ a, N. Bibic´ a,b,*, V. Milinovic´ a,b, S. Dharb, K.P. Liebb, M. Milosavljevic´ a,b, M. Siljegovic P. Schaafb ˇ Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia and Mantenegro VINCA ¨ Gottingen, ¨ ¨ II. Physikalisches Institut, Universitat Bunsenstr. 7-9, D-37073 Gottingen, Germany
a b
Abstract This article reports on the formation of iron nitrides during nitrogen ion irradiation of FeySi bilayers at room temperature. The 30 nm 57FeySi bilayers were prepared by pulsed laser ablation and irradiated with 22 keV N2q ions at fluences of (0.6– 2)=1017 ionsycm2. The structural changes and phases formed by irradiation were analyzed by Rutherford backscattering ¨ spectroscopy (RBS), X-ray diffraction analysis (XRD) and conversion electron Mossbauer spectroscopy (CEMS). The results clearly show the formation of several iron nitrides. At the lower fluences, the samples contain a mixture of ´-Fe2qx N and a-Fe with nitrogen interstitials. For the highest fluence, the paramagnetic ´-Fe2 N (61%) phase predominated. It can be concluded that the elemental iron almost fully transforms to an iron–nitride layer, but that no iron silicides are formed. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 75.50.Bb; 76.80.qy; 77.84.Bw; 82.80.Yc Keywords: Ion irradiation; Thin film; Nitride; RBS; XRD; CEMS
1. Introduction Iron nitride compounds have been studied extensively due to their remarkable magnetic properties, as well as their mechanical properties, such as high hardness, high corrosion and wear resistance, which are better than those of pure iron w1–3x. The iron–nitrogen phase diagram exhibits several interstitial solutions (a,g,´), chemical compounds (g9-Fe4N and z-Fe2N) and metastable phases (a9-martensite, a0-Fe16 N2 ). A specific a0Fe16N2 phase exhibiting giant magnetic moment has attracted considerable attention by virtue of possible applications in magnetic recording. However, the hexagonal ´-nitride is of importance due to its high hardness and improved corrosion resistivity. Therefore, this phase is suitable for applications in steel production and surface hardening. There are various experimental approaches to the synthesis of iron nitrides: thermal evaporation in nitrogen gas w4x, reactive sputtering w5,6x, laser nitriding w7x, *Corresponding author. Tel.: q381-11-455-451; fax: q381-11344-01-00. ´ E-mail address: [email protected] (N. Bibic).
molecular-beam epitaxy w8x, ion beam assisted deposition w9x and ion implantation w10–12x. Among these different nitriding processes, ion implantation offers several advantages, such as precise control of the impurity concentration and depth distribution. In addition, ion irradiation induces the formation of new phases that cannot be produced by other conventional techniques. Nitrogen implantation in thin Fe films particularly enhances the formation of the nitride phases as compared to thicker iron layers or bulk a-Fe w13x. In the case of thin films, the formation of nitrides is more complete and a specific nitride phase can be achieved at lower nitrogen fluences than in the bulk samples. It is generally observed that the formation of different phases induced by ion irradiation in iron-based bilayer or multilayer structures strongly depends on the thickness of thin films w14x. This study is part of the systematic investigation of chemical effects during ion beam mixing of bilayers, including nitrideymetal and nitrideySi systems w15,16x. The aim of the experiments described in this article is to discuss in more detail the formation of specific phases formed during the irradiation of FeySi bilayer structures
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.12.077
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N. Bibic´ et al. / Thin Solid Films 459 (2004) 23–27
with 22 keV N2q ions. Also, we will outline the possible effects of the thin layer structure and thickness on the processes of the phase transformation induced by nitrogen irradiation. 2. Experimental methods The FeySi bilayers were prepared by pulsed laser deposition (PLD) of 95% enriched 57Fe on Si substrates. The films were deposited at room temperature to a thickness of 30 nm at an average deposition rate of 0.5 nmys and a pressure of 1=10y4 Pa. Before deposition, the Si (100) substrates were cleaned with an HF solution and in de-ionized water. After deposition, the samples were irradiated with 22 keV N2q ions at room temperature (RT). N2q ions were supplied by an electron cyclotron resonance (ECR) ion ˇ The implanted source of the TESLA facility in Vinca. fluences were in the range from 0.6=1017 to 2=1017 ionsycm2, which is typical for compound formation by implantation. The irradiation energy was chosen in such a way that the nitrogen ions penetrate the 57FeySi interface, according to the calculation done with the SRIM code w17x. The RBS analyses were performed with a 900 keV He2q ion beam provided by the IONAS facility in ¨ Gottingen, with two Si-surface detectors positioned at a 1658 backscattering angle. Backscattering spectra were taken at normal incidence and the changes in the concentration profiles of Fe, Si and N were analyzed with the WiNDF w18x and RUMP codes w19x. Further structural investigations were performed with the timeof-flight elastic recoil detection analysis (TOF-ERDA). We used a 53 MeV 127I10q beam provided by the EOG10 tandem accelerator and the ERDA time-of-flight spectrometer of the University of Helsinki. CEMS and XRD were used for the phase identification in the irradiated 57FeySi bilayers. The CEMS spectra were taken at room temperature with a 57CoyRh source. The conversion electrons emitted from a depth of approximately 150 nm were detected with a Hey CH4 gas-flow proportional counter. The spectra were stored in a multichannel scaler and fitted according to a least squares fit routine by superimposing Lorentzian line shapes. The velocity calibration was performed with a a-Fe foil, and isomer shifts are given relative to aFe. All the samples were analyzed by X-ray diffraction (XRD), at 58 incidence, using Cu-Ka emission. 3. Results and discussion The RBS spectra presented in Fig. 1a were taken from an as-deposited FeySi sample and from irradiated samples for ion fluences of 0.6=1017, 1=1017 and 2=1017 ionsycm2. The presented RBS spectra clearly illustrate that the nitrogen ion irradiation induces com-
Fig. 1. RBS spectra (a): Fe and Si concentration profiles (b) of FeySi bilayers irradiated with 0.6=1017 –2=1017 N2q ionsycm2 at 22 keV. The profiles of nitrogen and oxygen for the high fluence irradiated sample are also presented.
positional changes at the FeySi bilayer interface. This can be concluded from a decrease in height of the signals from Fe and spread at the FeySi interface, with increasing ion fluence. After irradiation to the highest fluence, a well-defined step indicates the formation of a compound layer, thus hinting at the formation of iron nitride phases. The changes are also clearly visible from the deduced Fe and Si profiles shown in Fig. 1b. The deduced depth profiles of nitrogen and oxygen for the high fluence irradiated sample are also presented in Fig. 1b. It can be seen that the nitrogen ions penetrate the FeySi interface, as determined by the calculations done by the SRIM code. As shown in this figure the implanted nitrogen ions are distributed mostly within the Fe layer. The nitrogen concentration in the surface layer is up to 20 at.%, and reaches a maximum of 50 at.% near the FeySi interface. Approximately 20 at.% of oxygen was found at the surface of the sample, but less than 2 at.% within the Fe layer. The depth profiles of implanted nitrogen and of possible contaminants during the thin film deposition process and irradiation were confirmed by ERDA depth profiling (not shown). The results showed that the nitrogen concentration increases up to 57 at.% within the Fe layer. The hydrogen and carbon
N. Bibic´ et al. / Thin Solid Films 459 (2004) 23–27
Fig. 2. XRD analysis of the
25
57
Fe(30 nm)ySi samples before and after irradiation with 22 keV N2q ions to 0.6, 1 and 2=1017 ionsycm2.
concentrations throughout the Fe layers were less than 1 at.%. Information on the phases appearing in the samples was obtained from the X-ray diffraction measurements. Fig. 2 shows the grazing angle incidence X-ray diffraction spectra of the 57FeySi samples before and after irradiation to 0.6=1017, 1=1017 and 2=1017 ionsycm2. The spectrum of the as-deposited sample shows the (110) broad peak corresponding to elemental iron. For increasing ion fluences a small shift and broadening of the characteristic line at 2uf448 could be assigned to the formation of iron nitride phases. It can be associated with the (101) reflection of the ´-Fe2qxN phase. The characteristic reflection is rather broad, suggesting that the formed iron nitride phases have very small crystalline grains. The presence of a broad peak of elemental iron indicates that the formation of nitride phases was not completed. No other peaks were observed, which indicates that the silicon substrate did not take part in the reaction with Fe layer. When the irradiated fluence was increased to 2=1017 ionsycm2, the formation of the well-defined iron nitride phase ´-Fe2 N was achieved according to the XRD analysis. At the same time the small and broad reflection corresponding to elemental iron was observed. The analysis confirms that Fe is consumed mostly by the nitride formation during irradiation at highest fluence. With the increase in ion fluence, the small shift of the characteristic iron nitride peak towards the higher values of the interplanar distances d could be attributed to the expansion of the iron sublattice, due to the high concentration of nitrogen interstitial atoms.
A more sensitive phase analysis of the modified layers was performed by means of CEMS, especially supplying information on the nature and relative fractions of the formed nitrides. This enabled us to clarify the results obtained from the above RBS and XRD analyses. Fig. 3 shows the CEMS scans (large velocity range) for 57 FeySi samples irradiated with increasing nitrogen fluence. The spectrum taken from the sample irradiated at 0.6=1017 ionsycm2 revealed one broad doublet and up to four sextets. One sextet, with the hyperfine field Hs 33.0 T and the isomer shift ds0 mmys, is attributed to the presence of a-Fe with a relative contribution of 32.6%. Another sextet with a fraction of approximately 51% was identified as the nitride phase ´-Fe2qxN. Finally, one doublet may represent unresolved nitride phases. These phases cannot be identified precisely by the CEMS analysis, but the fraction of material in the sample responsible for the doublet splitting in the CEMS scan is approximately 15.5%. After irradiation to a fluence of 1=1017 ionsycm2, CEMS showed one singlet, one doublet and up to five sextets. In this case, the relative fractions of the ´-Fe2qxN phase and a-Fe phase slowly decreased to 49% and 26.5%, respectively. At the same time, the a9-(FeN) phase starts to appear, with a relative contribution of 10%. It is a broad sextet with a hyperfine field of 29.4 T and an isomer shift dsy 0.039 mmys. Nevertheless, a contribution of the g9Fe4N phase cannot be excluded. Therefore, we attribute this part of the spectrum to both the a9-(FeN) and the g9-Fe4N phases. The possible presence of the unresolved nitride phase could be seen from one doublet in CEMS
N. Bibic´ et al. / Thin Solid Films 459 (2004) 23–27
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spectrum, with the relative fraction of this component decreasing to 14%. For the highest nitrogen fluence of 2=1017 ionsy cm2, the CEMS spectrum was fitted with two doublets and four sextets. The doublets can be assigned to the formation of the ´-Fe2N phase with a relative abundance of 61%. It should be noted that the mixture of the ´Fe2qxN phase with a low nitrogen concentration and a9(FeN) could also be assigned to this sample. The spectral contribution of this mixture is 19%. Again, the presence of the a-Fe phase with a low fraction of approximately 7.6% was identified. Additionally, one sextet in the spectrum may originate from the presence of a small amount of oxide, in agreement with the results of RBS and ERDA analyses. It is then evident that even at highest nitrogen fluence of 2=1017 ionsy cm2, the ´-Fe2qxN and a9-(FeN) phases are formed. This can be due to the difference in the implanted nitrogen distribution along the thickness of the layers. From these CEMS analyses, the evolution of the nitride phase fractions has been derived and is shown in Fig. 4 as a function of the ion fluence. It is clearly visible that the fraction of a-Fe decreased with increasing fluence. The relative fractions of ´-Fe2qxN were almost constant after irradiation with the lower ion fluences. Then, the relative fraction of ´-Fe2qxN decreased, but nitrogenricher ´-Fe2N became the dominant nitride phase for the highest fluence. It must be emphasized that this is an abrupt and almost complete transformation into the nitrogen rich ´-Fe2N phase. The presented results clearly indicate that we have achieved almost complete transformation of a-Fe into nitrides for the highest irradiation fluence. It should be noted that in irradiated bulk iron samples the abundance of the a-Fe phase is always approximately 20%, irre-
Fig. 3. CEMS measurements of a
57
Fig. 4. Fraction of the nitride phase as observed by CEMS as a function of the ion fluence.
spective of the applied ion fluence w11x. This is a significant difference in the efficiency of the nitriding process in the FeySi bilayers, as compared to the bulk iron. It is very likely that the high concentration of defects incorporated in the thin films during the deposition process, contributes to the enhanced reaction of iron and nitrogen atoms. Moreover, the pronounced ioninduced damage accumulated in the thin film may cause a decrease in the short-range migration of nitrogen atoms, the mechanism that is responsible for the persistence of the a-Fe phase in bulk samples w13x. In this case, the conditions for the formation of nitrogen precipitates within the iron layer could not be fulfilled, and as a consequence, higher relative fractions of nitrides occur. Another important aspect of the possible influence of the thin film structure is the level of the stress, which
Fe(30 nm)ySi bilayers irradiated at RT to 0.6=1017, 1=1017 and 2=1017 ionsycm2.
N. Bibic´ et al. / Thin Solid Films 459 (2004) 23–27
may cause the acceleration of the new phase formation w14x. Besides, ion irradiation induces an additional energy transfer, which provides a further increase of the stress level in the iron layers. Therefore, the structural transformation into nitrides, especially the rather abrupt change towards ´-Fe2N, can be interpreted in the frame of the stress-related effects. Presumably, the stress relaxation in the thin layers may enhance the structural rearrangements, leading to the formation of the new phases. 4. Conclusions In conclusion, we have shown that the nitriding effect occurred during the irradiation of the FeySi bilayer with 22 keV N2q ions. The consumption of pure iron has been observed during the formation of the nitride, accompanied by an abrupt transformation into ´-Fe2N phase after irradiation to the highest fluence of 2=1017 ionsycm2. The enhanced efficiency of the nitriding process in the FeySi bilayers can be attributed to the high concentration of defects and the stress in the thin film structure. Surprisingly, no iron silicides have been observed. Acknowledgments The authors gratefully acknowledge the help of Detlef Purschke during the ion irradiation and ion beam anal¨ yses at Gottingen. The help of T. Sajavaara and J. Keinonen during the ERDA measurements is highly appreciated. N.B. and M.M. acknowledge support by the Deutsche Forschungsgemeinschaft during their stay ¨ in Gottingen. The financial support for this work and for project no. 1960 from the Ministry of Science,
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Technologies and Development of Republic of Serbia is gratefully acknowledged. References w1x C.R. Brooks, Principles of the Surface Treatments of Steels, Technomic Publishing Company, Lancaster, Basel, 1992. w2x W.Y. Lai, Q.C. Zheng, W.Y. Hu, J. Phys.-Condens. Mat. 6L (1994) 259. w3x H. Sawada, A. Nogami, T. Matsumiya, Phys. Rev. B 50 (1994) 10 004. w4x M. Takahashi, H. Shoji, M. Tsunod, J. Magn. Mater. 134 (1994) 403. w5x J.-F. Bobo, H. Chatbi, M. Vergnat, L. Hennet, O. Leonble, Ph. Bauer, M. Piecuch, J. Appl. Phys. 77 (1995) 5309. w6x P. Schaaf, C. Illgner, M. Niederdrenk, K.P. Lieb, Hyperfine Interact. 95 (1995) 199. w7x P. Schaaf, Prog. Mater. Sci. 47 (1) (2002) 1. w8x A. Morisako, M. Matsumoto, M. Naoe, J. Appl. Phys. 69 (1991) 5619. w9x H. Jiang, Q.L. Wu, K.T. Tao, H.D. Li, J. Appl. Phys. 78 (1995) 3299. w10x K. Nakajima, S. Okamoto, T. Okada, J. Appl. Phys. 85 (1989) 4367. w11x M. Kopcewicz, J. Jagielski, A. Turos, D.L. Williamson, J. Appl. Phys. 71 (1992) 4217. w12x B. Rauschenbach, A. Kolitsch, K. Hohmuth, Phys. Status Solidi A 80 (1983) 471. w13x J. Jagielski, M. Kopcewicz, G. Gawlik, W. Matz, L. Thome, ´ J. Appl. Phys. 91 (2002) 6465. w14x M. Kopcewicz, Jagielski, W. Matz, Hyperfine Interact. 139– 140 (2002) 369. w15x L. Rissanen, S. Dhar, K.P. Lieb, Phys. Rev. B 63 (2001) 155411. w16x S. Dhar, P. Schaaf, N. Bibic, E. Hooker, M. Milosavljevic, K.P. Lieb, Appl. Phys. A 76 (2003) 773. w17x J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. w18x N.P. Baradas, C. Jeynes, R. Webb, Appl. Phys. Lett. 71 (1997) 291. w19x L.R. Doolittle, Nucl. Instrum. Meth. B 9 (1985) 344.
Formation of iron-nitrides by irradiation of ions
57
FeySi bilayers with N2q
ˇ ´ a, D. Perusko ˇ a, N. Bibic´ a,b,*, V. Milinovic´ a,b, S. Dharb, K.P. Liebb, M. Milosavljevic´ a,b, M. Siljegovic P. Schaafb ˇ Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia and Mantenegro VINCA ¨ Gottingen, ¨ ¨ II. Physikalisches Institut, Universitat Bunsenstr. 7-9, D-37073 Gottingen, Germany
a b
Abstract This article reports on the formation of iron nitrides during nitrogen ion irradiation of FeySi bilayers at room temperature. The 30 nm 57FeySi bilayers were prepared by pulsed laser ablation and irradiated with 22 keV N2q ions at fluences of (0.6– 2)=1017 ionsycm2. The structural changes and phases formed by irradiation were analyzed by Rutherford backscattering ¨ spectroscopy (RBS), X-ray diffraction analysis (XRD) and conversion electron Mossbauer spectroscopy (CEMS). The results clearly show the formation of several iron nitrides. At the lower fluences, the samples contain a mixture of ´-Fe2qx N and a-Fe with nitrogen interstitials. For the highest fluence, the paramagnetic ´-Fe2 N (61%) phase predominated. It can be concluded that the elemental iron almost fully transforms to an iron–nitride layer, but that no iron silicides are formed. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 75.50.Bb; 76.80.qy; 77.84.Bw; 82.80.Yc Keywords: Ion irradiation; Thin film; Nitride; RBS; XRD; CEMS
1. Introduction Iron nitride compounds have been studied extensively due to their remarkable magnetic properties, as well as their mechanical properties, such as high hardness, high corrosion and wear resistance, which are better than those of pure iron w1–3x. The iron–nitrogen phase diagram exhibits several interstitial solutions (a,g,´), chemical compounds (g9-Fe4N and z-Fe2N) and metastable phases (a9-martensite, a0-Fe16 N2 ). A specific a0Fe16N2 phase exhibiting giant magnetic moment has attracted considerable attention by virtue of possible applications in magnetic recording. However, the hexagonal ´-nitride is of importance due to its high hardness and improved corrosion resistivity. Therefore, this phase is suitable for applications in steel production and surface hardening. There are various experimental approaches to the synthesis of iron nitrides: thermal evaporation in nitrogen gas w4x, reactive sputtering w5,6x, laser nitriding w7x, *Corresponding author. Tel.: q381-11-455-451; fax: q381-11344-01-00. ´ E-mail address: [email protected] (N. Bibic).
molecular-beam epitaxy w8x, ion beam assisted deposition w9x and ion implantation w10–12x. Among these different nitriding processes, ion implantation offers several advantages, such as precise control of the impurity concentration and depth distribution. In addition, ion irradiation induces the formation of new phases that cannot be produced by other conventional techniques. Nitrogen implantation in thin Fe films particularly enhances the formation of the nitride phases as compared to thicker iron layers or bulk a-Fe w13x. In the case of thin films, the formation of nitrides is more complete and a specific nitride phase can be achieved at lower nitrogen fluences than in the bulk samples. It is generally observed that the formation of different phases induced by ion irradiation in iron-based bilayer or multilayer structures strongly depends on the thickness of thin films w14x. This study is part of the systematic investigation of chemical effects during ion beam mixing of bilayers, including nitrideymetal and nitrideySi systems w15,16x. The aim of the experiments described in this article is to discuss in more detail the formation of specific phases formed during the irradiation of FeySi bilayer structures
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.12.077
24
N. Bibic´ et al. / Thin Solid Films 459 (2004) 23–27
with 22 keV N2q ions. Also, we will outline the possible effects of the thin layer structure and thickness on the processes of the phase transformation induced by nitrogen irradiation. 2. Experimental methods The FeySi bilayers were prepared by pulsed laser deposition (PLD) of 95% enriched 57Fe on Si substrates. The films were deposited at room temperature to a thickness of 30 nm at an average deposition rate of 0.5 nmys and a pressure of 1=10y4 Pa. Before deposition, the Si (100) substrates were cleaned with an HF solution and in de-ionized water. After deposition, the samples were irradiated with 22 keV N2q ions at room temperature (RT). N2q ions were supplied by an electron cyclotron resonance (ECR) ion ˇ The implanted source of the TESLA facility in Vinca. fluences were in the range from 0.6=1017 to 2=1017 ionsycm2, which is typical for compound formation by implantation. The irradiation energy was chosen in such a way that the nitrogen ions penetrate the 57FeySi interface, according to the calculation done with the SRIM code w17x. The RBS analyses were performed with a 900 keV He2q ion beam provided by the IONAS facility in ¨ Gottingen, with two Si-surface detectors positioned at a 1658 backscattering angle. Backscattering spectra were taken at normal incidence and the changes in the concentration profiles of Fe, Si and N were analyzed with the WiNDF w18x and RUMP codes w19x. Further structural investigations were performed with the timeof-flight elastic recoil detection analysis (TOF-ERDA). We used a 53 MeV 127I10q beam provided by the EOG10 tandem accelerator and the ERDA time-of-flight spectrometer of the University of Helsinki. CEMS and XRD were used for the phase identification in the irradiated 57FeySi bilayers. The CEMS spectra were taken at room temperature with a 57CoyRh source. The conversion electrons emitted from a depth of approximately 150 nm were detected with a Hey CH4 gas-flow proportional counter. The spectra were stored in a multichannel scaler and fitted according to a least squares fit routine by superimposing Lorentzian line shapes. The velocity calibration was performed with a a-Fe foil, and isomer shifts are given relative to aFe. All the samples were analyzed by X-ray diffraction (XRD), at 58 incidence, using Cu-Ka emission. 3. Results and discussion The RBS spectra presented in Fig. 1a were taken from an as-deposited FeySi sample and from irradiated samples for ion fluences of 0.6=1017, 1=1017 and 2=1017 ionsycm2. The presented RBS spectra clearly illustrate that the nitrogen ion irradiation induces com-
Fig. 1. RBS spectra (a): Fe and Si concentration profiles (b) of FeySi bilayers irradiated with 0.6=1017 –2=1017 N2q ionsycm2 at 22 keV. The profiles of nitrogen and oxygen for the high fluence irradiated sample are also presented.
positional changes at the FeySi bilayer interface. This can be concluded from a decrease in height of the signals from Fe and spread at the FeySi interface, with increasing ion fluence. After irradiation to the highest fluence, a well-defined step indicates the formation of a compound layer, thus hinting at the formation of iron nitride phases. The changes are also clearly visible from the deduced Fe and Si profiles shown in Fig. 1b. The deduced depth profiles of nitrogen and oxygen for the high fluence irradiated sample are also presented in Fig. 1b. It can be seen that the nitrogen ions penetrate the FeySi interface, as determined by the calculations done by the SRIM code. As shown in this figure the implanted nitrogen ions are distributed mostly within the Fe layer. The nitrogen concentration in the surface layer is up to 20 at.%, and reaches a maximum of 50 at.% near the FeySi interface. Approximately 20 at.% of oxygen was found at the surface of the sample, but less than 2 at.% within the Fe layer. The depth profiles of implanted nitrogen and of possible contaminants during the thin film deposition process and irradiation were confirmed by ERDA depth profiling (not shown). The results showed that the nitrogen concentration increases up to 57 at.% within the Fe layer. The hydrogen and carbon
N. Bibic´ et al. / Thin Solid Films 459 (2004) 23–27
Fig. 2. XRD analysis of the
25
57
Fe(30 nm)ySi samples before and after irradiation with 22 keV N2q ions to 0.6, 1 and 2=1017 ionsycm2.
concentrations throughout the Fe layers were less than 1 at.%. Information on the phases appearing in the samples was obtained from the X-ray diffraction measurements. Fig. 2 shows the grazing angle incidence X-ray diffraction spectra of the 57FeySi samples before and after irradiation to 0.6=1017, 1=1017 and 2=1017 ionsycm2. The spectrum of the as-deposited sample shows the (110) broad peak corresponding to elemental iron. For increasing ion fluences a small shift and broadening of the characteristic line at 2uf448 could be assigned to the formation of iron nitride phases. It can be associated with the (101) reflection of the ´-Fe2qxN phase. The characteristic reflection is rather broad, suggesting that the formed iron nitride phases have very small crystalline grains. The presence of a broad peak of elemental iron indicates that the formation of nitride phases was not completed. No other peaks were observed, which indicates that the silicon substrate did not take part in the reaction with Fe layer. When the irradiated fluence was increased to 2=1017 ionsycm2, the formation of the well-defined iron nitride phase ´-Fe2 N was achieved according to the XRD analysis. At the same time the small and broad reflection corresponding to elemental iron was observed. The analysis confirms that Fe is consumed mostly by the nitride formation during irradiation at highest fluence. With the increase in ion fluence, the small shift of the characteristic iron nitride peak towards the higher values of the interplanar distances d could be attributed to the expansion of the iron sublattice, due to the high concentration of nitrogen interstitial atoms.
A more sensitive phase analysis of the modified layers was performed by means of CEMS, especially supplying information on the nature and relative fractions of the formed nitrides. This enabled us to clarify the results obtained from the above RBS and XRD analyses. Fig. 3 shows the CEMS scans (large velocity range) for 57 FeySi samples irradiated with increasing nitrogen fluence. The spectrum taken from the sample irradiated at 0.6=1017 ionsycm2 revealed one broad doublet and up to four sextets. One sextet, with the hyperfine field Hs 33.0 T and the isomer shift ds0 mmys, is attributed to the presence of a-Fe with a relative contribution of 32.6%. Another sextet with a fraction of approximately 51% was identified as the nitride phase ´-Fe2qxN. Finally, one doublet may represent unresolved nitride phases. These phases cannot be identified precisely by the CEMS analysis, but the fraction of material in the sample responsible for the doublet splitting in the CEMS scan is approximately 15.5%. After irradiation to a fluence of 1=1017 ionsycm2, CEMS showed one singlet, one doublet and up to five sextets. In this case, the relative fractions of the ´-Fe2qxN phase and a-Fe phase slowly decreased to 49% and 26.5%, respectively. At the same time, the a9-(FeN) phase starts to appear, with a relative contribution of 10%. It is a broad sextet with a hyperfine field of 29.4 T and an isomer shift dsy 0.039 mmys. Nevertheless, a contribution of the g9Fe4N phase cannot be excluded. Therefore, we attribute this part of the spectrum to both the a9-(FeN) and the g9-Fe4N phases. The possible presence of the unresolved nitride phase could be seen from one doublet in CEMS
N. Bibic´ et al. / Thin Solid Films 459 (2004) 23–27
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spectrum, with the relative fraction of this component decreasing to 14%. For the highest nitrogen fluence of 2=1017 ionsy cm2, the CEMS spectrum was fitted with two doublets and four sextets. The doublets can be assigned to the formation of the ´-Fe2N phase with a relative abundance of 61%. It should be noted that the mixture of the ´Fe2qxN phase with a low nitrogen concentration and a9(FeN) could also be assigned to this sample. The spectral contribution of this mixture is 19%. Again, the presence of the a-Fe phase with a low fraction of approximately 7.6% was identified. Additionally, one sextet in the spectrum may originate from the presence of a small amount of oxide, in agreement with the results of RBS and ERDA analyses. It is then evident that even at highest nitrogen fluence of 2=1017 ionsy cm2, the ´-Fe2qxN and a9-(FeN) phases are formed. This can be due to the difference in the implanted nitrogen distribution along the thickness of the layers. From these CEMS analyses, the evolution of the nitride phase fractions has been derived and is shown in Fig. 4 as a function of the ion fluence. It is clearly visible that the fraction of a-Fe decreased with increasing fluence. The relative fractions of ´-Fe2qxN were almost constant after irradiation with the lower ion fluences. Then, the relative fraction of ´-Fe2qxN decreased, but nitrogenricher ´-Fe2N became the dominant nitride phase for the highest fluence. It must be emphasized that this is an abrupt and almost complete transformation into the nitrogen rich ´-Fe2N phase. The presented results clearly indicate that we have achieved almost complete transformation of a-Fe into nitrides for the highest irradiation fluence. It should be noted that in irradiated bulk iron samples the abundance of the a-Fe phase is always approximately 20%, irre-
Fig. 3. CEMS measurements of a
57
Fig. 4. Fraction of the nitride phase as observed by CEMS as a function of the ion fluence.
spective of the applied ion fluence w11x. This is a significant difference in the efficiency of the nitriding process in the FeySi bilayers, as compared to the bulk iron. It is very likely that the high concentration of defects incorporated in the thin films during the deposition process, contributes to the enhanced reaction of iron and nitrogen atoms. Moreover, the pronounced ioninduced damage accumulated in the thin film may cause a decrease in the short-range migration of nitrogen atoms, the mechanism that is responsible for the persistence of the a-Fe phase in bulk samples w13x. In this case, the conditions for the formation of nitrogen precipitates within the iron layer could not be fulfilled, and as a consequence, higher relative fractions of nitrides occur. Another important aspect of the possible influence of the thin film structure is the level of the stress, which
Fe(30 nm)ySi bilayers irradiated at RT to 0.6=1017, 1=1017 and 2=1017 ionsycm2.
N. Bibic´ et al. / Thin Solid Films 459 (2004) 23–27
may cause the acceleration of the new phase formation w14x. Besides, ion irradiation induces an additional energy transfer, which provides a further increase of the stress level in the iron layers. Therefore, the structural transformation into nitrides, especially the rather abrupt change towards ´-Fe2N, can be interpreted in the frame of the stress-related effects. Presumably, the stress relaxation in the thin layers may enhance the structural rearrangements, leading to the formation of the new phases. 4. Conclusions In conclusion, we have shown that the nitriding effect occurred during the irradiation of the FeySi bilayer with 22 keV N2q ions. The consumption of pure iron has been observed during the formation of the nitride, accompanied by an abrupt transformation into ´-Fe2N phase after irradiation to the highest fluence of 2=1017 ionsycm2. The enhanced efficiency of the nitriding process in the FeySi bilayers can be attributed to the high concentration of defects and the stress in the thin film structure. Surprisingly, no iron silicides have been observed. Acknowledgments The authors gratefully acknowledge the help of Detlef Purschke during the ion irradiation and ion beam anal¨ yses at Gottingen. The help of T. Sajavaara and J. Keinonen during the ERDA measurements is highly appreciated. N.B. and M.M. acknowledge support by the Deutsche Forschungsgemeinschaft during their stay ¨ in Gottingen. The financial support for this work and for project no. 1960 from the Ministry of Science,
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