Ion beam mixing in uranium nitride thin films studied by Rutherford Backscattering Spectroscopy

Nuclear Instruments and Methods in Physics Research B 268 (2010) 1875–1879

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Ion beam mixing in uranium nitride thin films studied by Rutherford Backscattering Spectroscopy N.-T.H. Kim-Ngan a,*, A.G. Balogh b, L. Havela c, T. Gouder d a

Institute of Physics, Pedagogical University, Podchorazych 2, 30-084 Kraków, Poland Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany c Faculty of Mathematics and Physics, Charles University, 12116 Prague 2, Czech Republic d European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, 76125 Karlsruhe, Germany b

a r t i c l e

i n f o

Article history: Available online 25 February 2010 Keywords: Uranium nitride films RBS Sputtering Ion beam mixing

a b s t r a c t Thickness, composition, concentration depth profile and ion irradiation effects on uranium nitride thin films deposited on fused silica have been investigated by Rutherford Backscattering Spectroscopy (RBS) using 2 MeV He+ ions. The films were prepared by reactive DC sputtering at the temperatures of 200 °C, +25 °C and +300 °C. A perfect 1U:1N stoichiometry with a layer thickness of 660 nm was found for the film deposited at 200 °C. An increase of the deposition temperature led to an enhancement of surface oxidation and an increase of the thickness of the mixed U–N–Si–O layers at the interface. The sample irradiation by 1 MeV Ar+ ion beam with ion fluence of about 1.2–1.7  1016 ions/cm2 caused a large change in the layer composition and a large increase of the total film thickness for the films deposited at 200 °C and at +25 °C, but almost no change in the film thickness was detected for the film deposited at +300 °C. An enhanced mixing effect for this film was obtained after further irradiation with ion fluence of 2.3  1016 ions/cm2. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Increasing interest has been recently paid to actinide nitrides due to their high application potential as advanced fuels and targets for fast reactors [1,2]. Uranium nitride (UN) having a high melting point (2850 °C), high density (14.32 g/cm3) and high thermal conductivity (13 W/mK) is considered as nuclear fuel. The stability and interactions with environment of this type of material on atomic scale are still to be studied. It was reported that the uranium mononitride is more resistant to oxidation and hydrolysis. Moreover, studies of actinide nitrides are motivated by fundamental research issues. Photoelectron spectroscopy results [3] indicate an itinerant character of the 5f states in UN. Also, only a small ordered moment of 0.75lB/U was found in the antiferromagnetic state below TN = 53 K [4] proving 5f itinerancy. Dynamical properties of nanoscaled thin film systems are often significantly different from those of corresponding bulk materials and there is a strong correlation between the film crystallinity and physical properties and important processes at the interfaces. Especially the thin film deposition techniques enable samples to be prepared with very different amount of structure defects depending on the deposition conditions. Preparation of UN thin films by

* Corresponding author. Tel.: +48 12 662 6314; fax: +48 12 635 8858. E-mail address: [email protected] (N.-T.H. Kim-Ngan). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.02.015

reactive sputter deposition and investigations of the relationship between the real structure of UN layers deposited at various conditions and their magnetic properties have been reported previously [3,5]. A large influence of the microstructure on the formation of magnetic moments and magnetic ordering was revealed, e.g. the long-range antiferromagnetism was suppressed by increasing disorder with decreasing the deposition temperature (Ts). In this work we investigate the interface properties of the UN films by ion beam techniques (IBA). Thickness and composition of the layers as well as the concentration depth profiles and the diffusion and/or interfacial reaction of the UN films will be determined using Rutherford Backscattering Spectroscopy (RBS). Since the magnetic properties of the films are sensitive to the atomic disorder in the films, we are interested in modifying the interface properties of the films by using ion beams (i.e. Ion Beam Modification of Materials (IBMM)). Ion irradiation effects on those films will be investigated using 1 MeV Ar+ ion beam.

2. Experiments The uranium nitride films were deposited by a reactive DC sputtering on quartz glass (fused silica) substrates at various temperatures (between 200 °C and +400 °C) [5]. The film characterization by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) has revealed that in all films under study, the stoichiometric

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UN phase (with a cubic-faced-centred structure (space group  Fm 3m) and a lattice parameter a = 4.8897 Å) was dominant. Measurements of magnetization and magnetic susceptibility have revealed that the antiferromagnetism known from bulk UN is replaced by a glassy magnetic behaviour in the UN film deposited at high temperatures (up to Ts = 350 °C). The increasing disorder for lower deposition temperatures leads gradually to the loss of magnetic moments, and Pauli paramagnetic behaviour was found in the film deposited at lowest temperatures (Ts = 200 °C). The RBS experiments were performed at the Institute of Nuclear Physics of the University Frankfurt/Main using 2 MeV He+ ion beam with a 171° backscattering angle and 3.4 msr detector solid angle [6,7]. The energy resolution of the detector was in the range of 17 keV (full width at half-maximum). The detector signal was amplified and registered by a multichannel analyser with 1024 channels. The incident ion beam was directed along the normal to the sample surface. The beam spot size on the target had a square shape of 1.0  1.0 mm2 and the beam current was typically in the range of 10–20 nA. For the data evaluation the computer code SIMNRA [8] was used taking into account the electronic stopping power data by Ziegler and Biersack [9], Chu + Yang’s theory [10,11] for electronic energy-loss straggling and Andersen’s screening function [12] to Rutherford cross-section. Due to the fact that it is more convenient to have the estimated layer thickness in nm, the simulated RBS areal density values (at./cm2) were converted into the layer thickness value (nm) by using the bulk density of the stoichiometric UN compound (14.23 g/cm3). We note here that the real layer thickness of different layers would be somewhat different, since they do not have the same mass density as that for UN. Besides, the sputter-deposited films have some porosity and thus the mass density of the film is certainly smaller than that the bulk UN. The sample irradiation was performed by 1 MeV Ar+ ion beam with different ion fluences in the range of 1016 ions/cm2. (The exact value of ion fluence for each irradiated sample is indicated in the figures.) The sample temperature was kept at room temperature. The beam power was estimated to be in the range 0.3–0.5 W and thus the beam heating of the samples was negligible. RBS experiments have been performed for each sample in the as-deposited state and after each ion irradiation step. We have performed ion beam mixing experiments on three films deposited at 200 °C, at room temperature (+25 °C) and at +300 °C. For a convenience, the following film-notation was used throughout the work: UN/SiO2 (200 °C), UN/SiO2 (+25 °C) and UN/SiO2 (+300 °C) where the values in the brackets indicated the deposited temperatures.

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Channel 3. Results and discussion The measured and simulated RBS spectra of the three investigated UN films are shown in Fig. 1. It is characterized by a very broad signal in the energy range from 700 to 1870 keV as a result of uranium signal from the films (U-peak), a small up-turn around 620 keV, a visible rising-edge around 400 keV and a non-zero background between the rising edge and the U-peak. The SIMNRA fits are in a good agreement with the measured RBS spectra. The sharp edge at around 1870 keV is related to the backscattered signal from uranium atoms in the front surface of the film. The energy position of the back-edge of the U-peaks related to the backscattered signal from the rear-surface of the film varies in the energy range of 700– 1200 keV depending on the film thickness. The non-zero background is considered as a consequence of uranium diffusion into the SiO2 substrate. The small up-turn visible on the non-zero background around 620 keV indicated the backscattered signal from nitrogen atoms located on the front surface of the film. The N-sig-

Fig. 1. Comparison of the random RBS (markers) and SIMNRA simulated (lines) spectra for UN film deposited on SiO2 substrate at 200 °C, at +25 °C and +300 °C in the as-deposited state and after irradiations by 1 MeV Ar+ ion beam. The ion fluence (1016 ions/cm2) is given in the figure. Some curves are shifted up for a better guide to eyes. The inset in the upper figure showed the enlarged Si-edge, N-signal from the film, O-signal from the surface layers of the film (see text).

nal from the entire film was thus induced only an additional increase of the background below 620 keV and no clear backscattered signal from the nitrogen atoms from the rear-surface of the film was shown. The Si signal from the substrate was revealed as the visible rising-edge below 400 keV (indicated in the insert of Fig. 1), whereas no distinguished feature related to the oxygen signal from the substrate was observed, since it was mixed with that from nitrogen in the film and thus ‘hidden’ in the risingedge below 400 keV. A very small peak was observed at around 720 keV. Since the energy position of such a peak was well fitted to the helium–oxygen binary collision (which was well separated

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the film thickness was obtained (Fig. 3, bottom panel). The Ucomposition in different layers varies around 50%. The change in composition and thickness of individual layer of the target can be seen clearly by a transformation of the RBS spectra to the depth profile of the elements. For all three films, the concentration profile consists of a few different zones with distinguished different compositions: the surface zone with mixed UNy–UOx layers, the UNy layer and the UNy–SiO2 interface zone. Fig. 2 shows the concentration profile of UN/SiO2 (200 °C) film. The estimated layer thickness (d) and composition of the films was given in Table 1. Detailed analysis of the film composition in the as-deposited state indicated that: 1. The surface zone (d = 56 nm) consists of the stoichiometric UN layer (y = 1) mixed with UOx as a consequence of surface oxidation. (The existence of oxygen in the surface zone was revealed by a small peak around 720 keV in RBS spectra.) 2. The main film-layer is the pure stoichiometric UN layer (660 nm; marked by a thick solid line in Fig. 2a). 3. The interface zone is the mixed UNy–SiO2 layers (78 nm, y < 1) and the U–SiO2 layers (60 nm, the U-content >2% as a consequence of U-diffusion into the substrate). The results show that the as-deposited UN film at Ts = 200 °C has a perfect stoichiometric composition. Even extended air exposure for some long time implies only oxidization of the topmost

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from the helium–nitrogen one at 620 keV), it was contributed to the oxygen signal from the surface layers. As an example, the enlarged Si-edge (Si sub.), N-signal from the film (N(film)) and O-signal from the surface layer (O(surf.)) for the UN/SiO2 (200 °C) film were shown in the insert of Fig. 1. For the UN/ SiO2 (200 °C) film (Fig. 1, upper panel), the broad signal with linearly-increasing plateau in the energy range of 1000– 1900 keV revealed a good stoichiometric UN film (i.e. the U:N ratio is exactly 1:1 or the U-content and N-content both equal to 50%. The element concentration was estimated by using SIMNRA, see next section.) For the UN/SiO2 (+25 °C) film, the similar plateau is shorter indicating a thinner stoichiometric UN layer (Fig. 1, middle panel). The strong decrease in the peak-intensity observed in the energy range of 1600–1900 keV indicated the existence of the layers with lower uranium content (or in other words the nitrogen-excess layers), which were as a consequence of a short circuit of the U-target at the end of the film deposition. For this film, a distinguished additional peak-like feature was observed at energy of 1200–1300 keV related to the U-excess layers (i.e. the U-content is higher than 50%) on the SiO2 substrate (and beneath the stoichiometric UN layer). For the UN/SiO2 (+300 °C) film (Fig. 1, bottom panel), the U signal was much wider (800–1870 keV). The broader slope of the U-edge at energies 1000 keV and 1800 keV has shown that the film has a thicker oxidized surface layer as well as a broader interface zone due to a stronger U-diffusion. Moreover, on the SiO2 substrate is thicker U-excess layers revealed by additional signal in the energy range of 800–1000 keV. Such a signal for the film deposited at +300 °C is visibly broader than that for the film deposited at +25 °C. The (measured and simulated) RBS spectra of the irradiated films are also included in Fig. 1. The irradiation of the UN/SiO2 (200 °C) film (Fig. 1, upper panel) with an ion fluence of / ¼ 1:2  1016 ions=cm2 induced apparently a large increase of the film thickness. Moreover, a large change in the layer composition under ion irradiation was observed for this film revealed by a non-linearly-increased plateau. A larger interface zone was also found shown by an increase of the slope of the U-edge below 800 keV. A large increase of the film thickness induced by ion irradiation (with / ¼ 1:7  1016 ions=cm2 Þ was also observed for the UN/SiO2 (+25 °C) film (Fig. 1, middle panel). The surprising fact in this case was an enlargement of the linearly-increased plateau indicating that ion irradiation leads to an establishment of the uranium content of 50% up to the topmost surface layer, i.e. the thickness of the stoichiometric UN layer increased largely upon ion irradiation. Moreover, ion irradiation induced a large change in the layer composition and enhanced diffusion in the interface zone implying a disappearance of the U-excess layer on the SiO2 substrate. The main features of the RBS spectra of the UN/SiO2 (+300 °C) film did not change upon irradiation with a similar ion fluence. Although the U-content in different layers has changed, the total film thickness was almost unchanged and the U-excess layer does still exist upon ion irradiation. No change in the slope of the U-edge at around 800 keV was observed indicating no visible influence from ion irradiation to the interface mixed zone of UN/SiO2 (+300 °C) film. In other words, deposition at high temperatures implied a large mixing and diffusion in the interface so that no further visible mixing effect was observed upon irradiation with an ion fluence of / ¼ 1:2  1016 ions=cm2 . However, such the ion fluence has created a large decrease of the U composition of the surface and sub-surface layers leading to a step decrease in the RBS spectrum in the energy range of 1600–1900 keV. In other words, the Ncontent in those layers are much enhanced indicating the nitrogen out-diffusion. Upon a further irradiation of this film by the next ion fluence of / ¼ 2:3  1016 ions=cm2 , a large increase of

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Table 1 Estimated thickness of different layers (in nm) for three UN films deposited at 200 °C, +25 °C and +300 °C in the as-deposited state and after ion irradiations. The surface zone was defined as the mixed U–N–O layer, the main film is UNy layer (with y < 1) including the stoichiometric UN layer (given in bracket), the U-excess layer is the UNy layer (with y < 1) revealed by the additional peak-like feature in the RBS spectra, the interface zone includes the mixed UNy–SiO2 and U–SiO2 layers. U–N–O layer

UNy (UN) layer

U-excess layer

UNy–SiO2 layer

U–SiO2 layer

56 56 170 110 253 290 1050 (mixed U–N–O

660 830 (37) 402 (240) 500 (500) 515 (515) 330 (294) layer)

– – 50 – 128 296

78 103 66 176 22 22 96

60 74 80 88 130 130 103

layer. The reason can be seen in the fact that the O2 molecule does not dissociate on the surface covered by UO2 [13] and cracking and decrepitation is the way how oxidation normally proceed into a bulk material. Large compressive residual strains observed for UN films and in particular for the low-T deposited ones (nearly 5 GPa [2]) can be responsible for the inhibition of cracking. Since the oxygen was observed only in the surface zone and beneath it was a very thick pure UN layer, the migration of oxygen from the SiO2 substrate can be ruled out. According to the present investigation, the uranium forms the precisely stoichiometric mononitride even in the surface and sub-surface layers, but the existence of a mixed uranium oxinitride cannot be excluded. In fact, our results revealed that in the surface mixed UNy–UOx zone, the oxide phase UOx covered most of the surface, but its concentration is quickly reduced in the sub-surface layers. Assuming the uranium mononitride UN have been well preserved up to the topmost surface layer, the RBS analysis indicated that UO2+x (with x = 0.67–1.0) is the oxidization product, i.e. the mixed UO2 and UO3 phase. UO2+x component quickly decreased in the sub-surface layer. The thickness of this mixed layer is about 26 nm and beneath it is the 30 nm-thick UO2 layer. Moreover, from the extrapolation of the UO2+x content in the concentration depth profile, one may expect that the topmost surface layer can be a pure UO2+x layer with a thickness of a few nm (i.e. the thickness range which cannot be well resolved by a standard RBS). For this film deposited at lowest temperature, no separated U-excess layer was found. However, due to U-diffusion into the SiO2 substrate, the U-excess layer (i.e. UNy layer with y < 1) did exist which is mixed with SiO2 layer at the interface. Upon irradiation, a smooth increase of the U-content and a smooth decrease of the N-content through the entire film were observed. A comparison of the U- and N-content in the films after irradiation with those in the as-deposited state is shown in Fig. 2b. We note here that irradiation implied almost no thickness change of the surface zone and only a small thickness change of the interface zone. However, the ion beam mixing in this case provided a large change of the U- and N-content and thus violated the U–N stoichiometry and caused a large increase of the film thickness. Namely, the main layer is the 830 nm-thick UNy layer with U-content increased smoothly from 30% to 55% including only 37 nmthick stoichiometric UN layer. (The U-excess layers do exist in this case (U-content >50%.) However, no additional peak was appeared in the RBS spectra, only a smooth increase of the slope of the plateau was observed. The total film thickness of the as-deposited film (including the interface U–N–SiO2 layer) is about 854 nm. It increases to 1054 nm upon irradiation. In other words, on one hand, the deposition at low temperatures provided a good stoichiometry of UN. On other hand, the film was not in a stable thermodynamical state. Thus ion irradiations promoted a large rearrangement of the U and N atoms leading to a large atomic disorder in the film as well as an ‘expanding’ of the film thickness by about 25%. A comparison of the U-content in the other two films between as-deposited state and irradiated state is shown in Fig. 3. For the

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as-deposited UN/SiO2 (+25 °C) film, it was visible that the surface oxidized layer are thicker (170 nm). The main layer is consisted of 162 nm-thick N-excess layer (y > 1), due to the lack of U-target during the deposition, and 240 nm-thick stoichiometric UN layer. Unlike the film deposited at low temperature, an U-excess layer was present beneath the UN layer revealed by a distinguished peak-like feature in the RBS spectra (Fig. 1b). However, U-diffusion into the SiO2 substrate in this case is almost the same as that in the film deposited at 200 °C: the thickness of the interface zone was almost the same (140–150 nm). Except of some small difference in the U-content, the slope of the U-edge in the depth profile of the two films in the as-deposited state was similar. The most interest-

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ing feature of irradiation effect on UN/SiO2 (+25 °C) film is the formation of a very thick layer with the exact value of 50% for U-content. Namely, as a consequence of atomic rearrangements in both surface region and interface, a 500 nm-thick stoichiometric UN layer was obtained. Moreover, upon irradiation, the U-excess layer has disappeared. The spread of uranium atoms implies that the mixed UNy–SiO2 layer was enlarged (to 176 nm in the as-deposited and irradiated film, respectively). Investigations of as-deposited UN/SiO2 (+300 °C) film have revealed that a thick stoichiometric UN layer (515 nm) was produced by deposition at high temperature. However, the high deposition temperature clearly promoted the oxidization of the film surface and a larger uranium diffusion shown respectively by a large increase of the thickness in the surface (253 nm) and interface zone (150 nm). Moreover, the nitrogen was probably less sticking during the first phase of deposition at high temperature. As a consequence, a much larger U-excess layer (128 nm) and a much thinner UNy–SiO2 layer (22 nm) was formed (i.e. less nitrogen content in those layers). For this film, the total film thickness (1060 nm) is almost unchanged upon irradiation by ion fluence of 1.2  1016 ions/cm2. After irradiation, however, the composition of different layers was changed leading to a large decrease in the thickness of the stoichiometric UN layer (from 515 nm to 294 nm) and a large increase of the U-excess layer (from 128 nm to 296 nm). However, no change in the concentration as well as in the thickness was observed for the interface zone (of the mixed U–N–SiO2 layer). This film was further irradiated with the second ion fluence of 2.3  1016 ions/cm2. Such an ion fluence leads to a large increase of the film thickness by about 18%. The U-content in different layers varies between 45% and 55%. But in this case, the U-excess layer is thin so that it did not provide a visible peak-like feature in RBS spectrum. We notice here that, due to the strong mixing and the small mass difference between nitrogen and oxygen and the irregular change of U-content, it was difficult to make separation of the surface zone (with the presence of oxygen) and the UNy film (without oxygen). It was possible in this case to distinguish only two layers: the film with mixed U–N–O composition and the interface with mixed UNy–SiO2 with a thickness of 1050 nm and 96 nm, respectively. For all films, the non-zero background between N-edge and Upeak was as result of a small amount of U-diffusion (1%) deep into the SiO2 substrate. 4. Conclusions Deposition at 200 °C provided a thick stoichiometric UN film. This film was found stable in exposing to air. The surface oxidation is much enhanced (either during film deposition or due to exposing to air) and the oxidized surface layer becomes gradually thicker in films deposited at higher temperature (+25 °C and +300 °C). The stoichiometric UN part is therefore gradually thinner. The

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enhanced uranium diffusion leads to a higher U-content (apparently in the form of UO2) at the interface zone. In particular, the U-excess layer is already present in the film deposited at room temperature (+25 °C) and becomes thicker in film deposited at +300 °C. A large influence of the ion irradiation on the film structure and layer composition was observed. The more dramatic effect of ion irradiation for the low-temperature deposited film is a good proof that the structure of such films is more remote from the thermodynamic equilibrium, as expected. Although ion beams can induce extensive damage, at the same time the lattice dynamics induced by the interaction with ion beams can help to release the energy stored during the low-temperature sputter deposition. Deposition at high temperatures has led to already a non-stoichiometric film with a large interface zone as a result of a strong diffusion. In this case ion irradiation would create a much less displacement of lattice atoms away from their initial sites. The most essential point is that our investigations indicate that it is possible to establish the required uranium content of 50% (i.e. the stoichiometric UN film) and/or to obtain the required film thickness by ion irradiation. It opens a possibility for modification and/or tailoring of the as-deposited films in order to obtain the films with required properties. Acknowledgements The financial support from German Academic Exchange Service (DAAD) – D/08/07729 project (between Germany and Poland) is highly acknowledged. A.G.B. acknowledges the financial support by German Research Foundation (DFG) – SFB-595 project. L.H. acknowledges the financial support by the Czech Research Plan MSM 0021620834 and the Grant No. IAA100100912. References [1] H. Matzke, Science of Advanced LMFBR Fuels, North-Holland, Amsterdam, 1986, p. 106. [2] H. Blank, J. Nucl. Mater. 153 (1988) 171. [3] L. Black, F. Miserque, T. Gouder, L. Havela, J. Rebizant, F. Wastin, J. Alloys Compd. 315 (2001) 36. [4] N. Curry, Proc. Phys. Soc. 86 (1965) 1193. [5] D. Rafaja, L. Havela, R. Kuzˇel, F. Wastin, E. Colineau, T. Gouder, J. Alloys Compd. 386 (2005) 87. [6] N.-T.H. Kim-Ngan, A.G. Balogh, J.D. Meyer, J. Brötz, S. Hummelt, M. Zajac, T. Slezak, J. Korecki, Surf. Sci. 602 (2008) 2358. [7] N.-T.H. Kim-Ngan, A.G. Balogh, J.D. Meyer, J. Brötz, M. Zajac, T. Slezak, J. Korecki, Surf. Sci. 603 (2009) 1175. [8] SIMNRA: Simulation Program developed by M. Mayer, . [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Matter, vol. 1, Pergamon, New York, 1985, . [10] W.K. Chu, Phys. Rev. A 13 (1976) 2057. [11] Q. Yang, R.J. McDonald, Nucl. Instrum. Meth. B 83 (1993) 303. [12] H.H. Andersen, F. Besenbacher, P. Loftager, W. Möller, Phys. Rev. A 21 (1980) 1891. [13] T. Gouder, A. Seibert, L. Havela, J. Rebizant, Surf. Sci. 601 (2007) L77.