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Accepted Manuscript Interdiffusion processes at irradiated Cr/Si interfaces L. Luneville, L. Largeau, C. Deranlot, J. Ribis, F. Ott, N. Moncoffre, G. Baldinozzi, D. Simeone PII: DOI: Reference:
S0925-8388(14)02769-8 http://dx.doi.org/10.1016/j.jallcom.2014.11.121 JALCOM 32675
To appear in:
Journal of Alloys and Compounds
Please cite this article as: L. Luneville, L. Largeau, C. Deranlot, J. Ribis, F. Ott, N. Moncoffre, G. Baldinozzi, D. Simeone, Interdiffusion processes at irradiated Cr/Si interfaces, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.11.121
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Interdiffusion processes at irradiated Cr/Si interfaces L. Lunevillea,∗, L. Largeaub , C. Deranlotc , J. Ribisg , F. Ottd , N. Moncoffree , G. Baldinozzif , D. Simeoneg a DEN/DANS/DM2S/SERMA/LLPR/LRC-CARMEN,
CEA Saclay, 91191 Gif-sur-Yvette, France Route de Nozay, 91460 Marcoussis, France c Unite Mixte de Physique CNRS/Thales, 1 Avenue Augustin Fresnel, 91767 Palaiseau, France d DSM/IRAMIS/LLB/CEA/CNRS, CEA Saclay, 91191 Gif-sur-Yvette, France e IPNL/CNRS, Domaine scientifique de la Doua, 69622 Villeurbanne, France f CNRS-SPMS/UMR 8580/ LRC CARMEN Ecole Centrale Paris, 92295 Chatenay-Malabry, France g DEN/DANS/DMN/SRMA/LA2M/LRC-CARMEN, CEA Saclay, 91191 Gif-sur-Yvette, France b LPN-UPR20/CNRS,
Abstract Chromium silicon CrSi alloys are foreseen as possible materials for spintronic devices. Ion beam mixing could be an efficient technique to produce thin films of such alloys at room temperature while avoiding thermal diffusion. In order to assess this point, we have irradiated 20 nm Cr layer on a (100) Si wafer with 70 keV Kr ions. The X ray reflectometry technique combined with Transmission Electron Microscopy and Energy Dispersive X-ray analysis was applied to analyze, at the nanometric scale, the formation of Cr/Si blurred interfaces induced by ion beam mixing. From the analysis of reflectivity curves, it appears that nanometric Cr5 Si3 and CrSi2 phases are produced at the early stage of the process. The existence of these two paramagnetic phases gives some clues to explain the reason why the experimentally observed ferrimagnetism was weaker than predicted. Keywords: X-ray reflectometry, Cr/Si interfaces, interdiffusion, ion beam mixing
1. Introduction As promising materials for spintronic devices, Cr/Si systems have attracted much attention in recent years [1] as possible diluted magnetic semiconductor systems. Ab initio calculations on the Cr doped Si system predict a ferrimagnetic behavior for Cr doped Si solid [2]. However, measurements on Si wafers implanted with Cr ions clearly display that this ferrimagnetism is weaker than predicted [3]. On the other hand, previous investigations on the annealing of Cr/Si bilayers have clearly shown the formation of different phases at the interface between Cr and Si atoms [1, 4, 5]. A possible explanation for this weakness of the magnetization may be due to the appearence of these phases at the interface on a nanometric scale (few nanometers). In order to synthesize high purity buried interfaces between two solids, ion beam implantation was extensively used in the well known top down approach [6, 7, 2, 4, 8]. The slowing down of impinging atoms with high kinetic energy (few hundreds of KeV) leads to the mixing of atoms at the atomic scale in alloys, semi conductors or insulators [9, 10, 11, 12]. By changing the energy of the incident beam, it is possible to create buried interfaces at well defined depths. Moreover, the thickness of the produced interface is directly related to the fluence of impinging ∗ Corresponding
author Email address: [email protected] (L. Luneville)
Preprint submitted to Journal of Alloys and Compounds
atoms [13]. Some authors [14, 15, 16] claim the formation of CrSi2 under implantation at room temperature. According to these authors, the composition of the phase produced under implantation and extracted from RBS measurements is similar to that of CrSi2 phase produced during thermal annealing. Different measurements [17] point out that the formation of CrSi2 occurs only above 523K under implantation. These authors were unable to identify the phase produced at room temperature. More recently, other authors [4] claim that chromium atoms diffuse in silicon at room temperature during implantation without evoking a mechanism associated either with high energy collisions induced by impinging ions or an enhanced diffusion induced by vacancies produced under irradiation. From these contradictory reports, an accurate concentration profile derived from a depth profiling technique at the nanometric scale could be useful to identify which phases are created. Many experimental techniques offer the opportunity to probe free surfaces and interfaces such as Transmission Electron Microscopy (TEM), X ray Photoelectron Spectroscopy (XPS), Rutherford Back Scattering (RBS) and Low and Medium Energy Ion Scattering spectroscopy. However, the preparation of thin foils for TEM studies implies the relaxation of strain fields produced by the layers surrounding the interface. Moreover, the depth resolution of the RBS technique highly decreases with the penetration depth of the incident ion beam[18]. The X-ray reflectometry technique (XRR) has been extensively used over twenty years to characterize November 21, 2014
stratified media like semi conductors, super lattices [19], Langmuir-Blodgett films [20, 21, 22] and thin film heterostructures [23]. The main interest of this technique is to probe nondestructively the buried interfaces with a few angstrom resolution [22, 24, 25, 26]. Neither relaxation of stress field nor pollution by hydrogenation can induce spurious effects which can occur during the preparation of thin foils for TEM studies. Moreover, the resolution of the XRR does not depend on the probed depth. In this work, X ray reflectometry technique as well as TEM studies were thus combined to track new phases in the buried Cr/Si interface.
a thermocouple deposited on the chromium surface and maintained out of the irradiation flux. This temperature did not exceed 350K, well below the temperature associated with point defect migration, that is significant above 450 K in this system [28]. Within these conditions, no competition between collisions and thermal diffusion can occur because of the large gap in temperature [29]. From SRIM, it was alse possible to estimate the penetration depth of Cr into the Si layer. This penetration depth is about 5 nanometers. On the other hand, the cross section associated with the vacancies formation in the Si layer extracted from SRIM calculations is equal to 7 × 10−14 cm2 . Assuming a recombination volume equal to the silicon unit cell (0.543 nm), we can compute the atomic concentration of interstitial and vacancies induced by 70 KeV Kr irradiation in the silicon wafer, from the knowledge of the entropy and free enthalpy energies associated with the migration of interstitial and vacancies in silicon [30]. To compute this atomic fraction, we only assume a mutual recombination of interstitials and vacancies produced by the irradiation. This atomic fraction of interstitials and vacancies is equal to 1.5 × 10−4 and remains larger than equilibrium values [30]. From this analysis, the mechanism of mixing seems to result from an enhancement of the diffusion due to elevated number of ion-induced vacancies.
2. Experimental measurements 2.1. Samples elaboration The studied Cr/Si samples were grown by Physical Vapor Deposition (PVD) technique. From this process, we deposited only one Cr thick layer (20 nanometers) on a (100)Si wafer. Prior to the Cr deposition, the wafer surface was cleaned with a hydrofluoric acid solution to remove the native SiO2 oxide. To avoid corrosion of the chromium surface, a thin layer of platinium (2nm) was deposited on the Cr layer. The thickness of the Pt layer was chosen to be sputtered by the Kr ions at the beginning of the mixing, i.e. for the lowest fluence. The Cr and Pt thin films have been deposited by magnetron sputtering in a pure Ar discharge. The distances between targets and substrate was around 12 cm and the purities of Cr and Pt targets were equal to 99.95% and 99.99% respectively. The rates of deposition were equal to 0.15 nm/s for Cr and 0.4 nm/s for Pt. The base pressure, working pressure and the substrate temperature were equal to 5 × 10−6 Pa, 2.5 × 10−1 Pa and 300 K. The thichness and the density of the Cr and Pt layers were determined from the analysis of the reflectivity curve. From the measurement of the critical angle, the density of the deposited Cr layer could be computed and was equal to 97% of the theoretical density of chromium. Moreover, the thickness of the chromium layer, derived from a Fourier transform of the reflectivity curve, was equal to 20 ± 1 nanometers. Such a layer explains the kink observed on the XRR curve of the pristine sample near 2.5◦ , as plotted on figure 1(a) (black lines). In order to maximize the mixing of atoms at the Cr/Si interface, all samples were irradiated with 70 keV Kr ions at room temperature under a flux of 7.7 × 1012 cm−2 s−1 . The fluences vary from 5 × 1014 to 2 × 1016 cm−2 . Based on SRIM-2003[27] calculations, the density of deposited energy by the ion beam, Fd , is maximum at the Cr/Si interface. The maximum value of Fd is equal to 0.8 keV/nm and similar Fd values were reported in previous works [14, 15, 4]. Moreover, the implantation peak of Kr ions lies far inside the Si wafer and these atoms do not contaminate the Cr/Si interface, even at high fluences. For all implantations, the temperature of the Cr layer was monitored by
2.2. X Ray Reflectometry and Energy Dispersive X ray analysis studies Cross sectional TEM lamella were extracted using the Focussed Ion Beam technique. TEM diffraction patterns performed on the Cr layer and the Si substrate (not shown) clearly exhibit well defined spots assessing that the Cr layer deposited was polycristalline. Well defined spots observed on TEM patterns performed at each fluence clearly show that this chromium layer remains crystalline even at high fluence. On one hand, XRR was performed to obtain accurate measurements of the atomic concentration profiles of chromium and silicon induced by ion implantation of thin Cr films on Si wafers. As chromium and silicon possess very different electron density, they provide a good contrast in X-ray diffraction. On the other hand, the morphology and chemical characterization of the different interfaces produced under implantation were studied by Scanning Transmission Electron Microscopy (STEM) in cross sectional view using a JEOL 2200FS equipped with a probe aberration corrector. Different high angle annular dark field pictures (STEM-HAADF) were collected as a function of the fluence. The local Cr/Si ratio was also measured along the incident ion beam direction by Energy Dispersive X-Ray analysis (EDX). This technique provides a direct measurement of the atomic fractions of these two elements and was used to validate the refinement of the reflectivity curves versus the fluence. XRR specular diagrams were collected with a Bruker D8 Advance diffractometer in symmetric reflexion condi2
tions [31]. In this geometry, the scattering vector remains always normal to the surface of the sample. The experimental setup is equipped with a parabolic Gobel mirror and a line focus CuKα radiation tube (40 kV, 4O A). A 12 micron thick Ni filter was used to attenuate the CuKβ radiation. The footprint of the beam, limited by a primary 0.1 millimeter slit at small incident angles, allows calibrating the scattering angle θ. It thus remains smaller than the sample length (3 cm) for θ > 0.19◦ . The photons intensity of the X ray beam was equal to 2108 counts per second. A secondary 0.4 millimeter slit was used in front of a 0D detector. 2θ scans were collected in specular conditions over 10 degrees with a scan step of 0.01 degree. The instrumental resolution of the experimental setup is 7 ◦ about 1000 [31]. Each scan was collected 10 times with a time per step of 10 seconds. Moreover, 2θ scans with an offset of 0.3◦ were performed to correct the raw data from diffuse scattering. A simple subtraction of raw data provides the XRR specular signal. 3. Results 3.1. Refinements of XRR diagrams XRR diagrams are shown in Fig. 1(a) for the evolution of the reflectivity curves as a function of the scattering angle θ for all fluences. The Kiessig fringes are conspicuous on the pristine sample over eight orders of magnitude assessing the high quality of the Cr layer. Even on the most irradiated sample, Kiessig fringes due to interferences between distinct well-defined layers remain clearly visible. This point implies that well defined interfaces are created during implantation. As expected, the slope of the reflectivity curve is a decreasing function of the fluence. This effect is mainly due to an increase of the roughness of the surface of the sample due to sputtering. Increasing the fluence, reflectivity curves exhibit different kinks due to the appearance of extra layers produced by ion implantation. Refinements were performed with the Parratt’s recursive formalism [32]. The least square method weighting was applied to Ln(R(2θ)) to give more weight on the large 2θ points, i.e. to track thin layers. The classical least square technique was used to fit the XRR curves. A minimum value of 10−7 for R(θ) allows fixing the 2θ range for the refinement. For each fitting curve, the thickness, the density and the roughness of the different layers were fitted. A 20 nanometer thick Cr layer was used as the initial guess to fit the pristine sample. To assess the stability of the refinements, the simulated annealing technique was used to avoid local minima during the minimization process. The best refinement of the reflectivity curve for a given fluence becomes the initial guess for the next fluence. This recursive approach avoids any spurious effects due to ad hoc models [23, 22] and constrains the number of free parameters in the Parratt’s formalism. Fig. 1(b) displays the comparison between the measured and the refined reflectivity curves for the pristine
Figure 1: (Color online)(a) Plot of different reflectivity curves versus the scattering angle for different fluences (black: pristine, red: 5 × 1014 cm−2 , green: 2 × 1015 cm−2 , blue: 5 × 1015 cm−2 , cyan: 8 × 1015 cm−2 , magenta: 1 × 1016 cm−2 , olive: 2 × 1016 cm−2 ). The inset displays the variation of the critical angle θc (dashed lines), derived from the relation R(2θc ) = 1/2, between the pristine sample (black) and the most irradiated sample (olive). (b) Refinements of the XRR experimental curves (black circles) are performed with the Simulreflec code [22] (red lines) on the pristine sample (bottom) and the most irradiated sample (top). In order to asses the quality of the fit, the error function (W = [log(Rsimul ) − log(Rexp )]Rsimul /Rexp ) was plotted (green line) for each refinement.
sample and the most irradiated sample. The good agreement of the measured and computed reflectivities validate the characterization approach. 3.2. Comparison with EDX measurements From XRR refinements, it is possible to extract the Scattering Length Density (SLD). The translation from the SLD to the atomic fraction profiles implies the knowledge of the density. To extract the atomic fraction profiles of Si and Cr, we assume that the density of layers formed by Si and Cr atoms follows a Vegard law. The silicon atomic fractions extracted from XRR refinements are displayed versus the depth for each fluence in Fig. 2. They are in excellent agreement with those obtained from EDX measurements (squares on Fig. 2), thus justifying our procedure. 3
investigations [15, 17, 4]. Figure 3 displays high resolution HAADF STEM cross sectional view on the most irradiated sample. The Z contrast on this picture clearly shows two caracteristic layers of one nanometer with well defined Si atomic fractions. The EDX analysis (red squares Fig. 2) allows to determine the atomic compositon of these two limit phases associated with Cr5 Si3 and CrSi2 phases. These results assess the existence of two plateau extracted from the analysis of reflectometry data (green line on Fig. 3). All these techniques are in good agreement and prove that two limit Cr5 Si3 and CrSi2 phases are created under irradiation. It must be noticed that these compositions are not associated with the eutectic point in the equilibrium phase diagram. This last point disagrees with previous theoretical assumptions [14, 33] on the phase stability of silicides under ion implantation. Between these two phases with fixed compositions, the Si atomic fraction evolves linearly in the mixing layer L as pointed out by the green line on Fig. 3. This linear evolution can be understood as the creation by ion beam mixing of a crystalline solid solution between these two limit phases.
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Figure 2: (Color online) Comparison between the Si atomic fractions extracted from reflectivity curves (full lines) and EDX analyses (squares) as a function of the depth for all fluences (black: pristine, red: 5 × 1014 cm−2 , green: 2 × 1015 cm−2 , blue: 5 × 1015 cm−2 , cyan: 8 × 1015 cm−2 , magenta: 1016 cm−2 , olive: 2 × 1016 cm−2 ). The error bars are associated with EDX data points. The Si atomic fractions are in fair agreement. For the highest fluence, all Cr atoms have diffused. This point explains the value of the Si atomic fraction at the surface. In the inset, the evolution of the Si atomic fraction is plotted for low fluences. Two distinct plateau clearly appear on Si profiles at low fluences. These plateau exist for all fluences (dashed horizontal lines) and are associated with Si atomic fractions equal to 0.375 and 0.67 respectively.
Fig. 3 displays the HAADF STEM variation contrast in cross sectional view for the sample irradiated with a fluence of 1016 cm−2 . The silicon concentration profile extracted from the reflectivity curve (green solid line) and the EDX measurements (red squares) are drawn on the same scale. They are in fair agreement. The maximum of the Z contrast pointed out on HAADF STEM picture lies at the same depth as the inflexion points of the reflectivity curve assessing the quality of the concentration profile derived from the reflectivity curve. The first inflexion point of the reflectivity curve is associated with the Cr rich interface. The last inflexion point is associated with the Si rich interface. The STEM cross sectional view clearly shows that the interface perpendicular to the ion beam does not exhibit a large waviness for each sample (below one nanometer). This point assesses that the global roughness of XRR curves is mainly due to the sputtering of the surface by Kr ions during the ion beam mixing.
Figure 3: (Color online) HAADF STEM cross sectional view of the 1016 cm−2 irradiated sample. Even at this high fluence, this picture clearly displays that blurred interfaces remain abrupt. On this graph, the silicon atomic fraction profiles extracted from XRR analysis (green line) and resulting from EDX measurements (red squares) agree quite well with the Z contrast HAADF STEM picture, assessing our analysis. In order to study the cristalline nature of the mixing layer, diffraction patterns were collected in this layer. The intensity is uniformaly distributed along a ring on the diffraction pattern. The position of this ring from the origine in the reciprocical space defines a caracteristic distance of 0.25nm in the real space.
4. Discussion Two distinct plateau are clearly visible on the Si atomic profiles (inset in Fig. 2), i. e. during the early stages of the mixing (F= 5 × 1014 cm−2 ) up to the highest fluence (F= 2 × 1016 cm−2 ). Even if the two plateau are difficult to detect for high fluences, they are still present. It thus appears that the Cr/Si mixing layer cannot be described by a single gaussian interdiffusion process, as reported on previous
To study the crystalline structure of the solid solution produced under irradiation, diffraction patterns were collected on the most irradiated sample as pointed out in the inset on Fig. 3. The distance in real space extracted from the position of the ring from the center of the reciprocical space is equal to 0.25nm and corresponds to the nearest 4
Cr-Si bond length in Cr/Si compounds. The uniform distribution of intensities along the ring in the diffraction pattern may be the signature of an amorphous solid solution. However, this ring may also result from the diffraction of a large number of nanodomains. To overcome this difficulty in the analysis of the diffraction pattern, different patterns were calculated within a reduced window (4x4nm2 ) at different points inside the mixing layer of the HAADF STEM cross sectional view plotted on Fig. 3. Figure 4 is an example of these calculated diffraction patterns. Although a Wiener filter was used to reduce the noise, the calculated diffraction patterns display speckled intensities due to the drastic reduction of the probed area. Despite a faint ring can be observed on Figure 4, the intensity is not uniform along this ring but exhibits more intense spots. These non random spots may be the signature of nanodomains in the mixing layer. From this analysis, the solid solution in the mixing layer could be a mixture of two crystalline Cr5 Si3 and CrSi2 limit phases explaining thus the linear variation of the Si atomic fraction in this layer. However, the small resolution of the HAADF STEM picture (about 2 nanometers) does not allow calculating accurately the angular correlation function [34] and then assessing the existence of crystalline nanodomains. On the other hand, nanodomains are smaller than few nanometers and the distinction between amorphous and crystalline nanodomains is not so clear at this lengthscale. Moreover, previous investigations [35, 36] have clearly shown that amorphous Cr5 Si3 and CrSi2 phases are produced in thin Cr/Si layers. Whatever the crystalline nature of phases produced under irradiation is, two plateau are the signature of a diffusion reactive process [37]. Such a process was not expected to occur under irradiation [14, 33].
Figure 4: Numerical Fourier transform performed on a selected part (4 nm x 4nm) of the mixing area. The intensity is not uniform along the ring but exhibits more intense spots circled in red on the picture. These large symetric spots may be the signature of Cr5 Si3 and CrSi2 nano crystalline domains in the mixing layer.
calculations thus pointed out that Cr5 Si3 and CrSi2 crystalline phase are not ferrimagnetic [3]. It must be noticed that neutron reflectometry can also provide a magnetization profile in the mixing layer [38]. However, the neutron resolution is always smaller than the X-ray resolution and density profiles are more difficult to extract from reflectometry measurements as clearly pointed by previous authors[38]. The atomic fraction profiles extracted from XRR experimental curves displayed on Figure 2 nicely demonstrate that conventional laboratory sources can now be easily used for assessing the structural properties of buried thin films exhibiting thicknesses of few nanometers. This nondestructive technique thus allows extracting the atomic concentration profiles associated with chemical reactions between interfaces or stress induced segregation effects.
5. Conclusion From the analysis of the reflectivity curves, it clearly appears that ion beam mixing leads to the formation of a Cr rich phase, Cr5 Si3 , near the Cr layer, and a Si rich phase, CrSi2 , near the Si layer, even at the lowest fluence. Moreover, it clearly appears from different analyses that the CrSi phase, the most stable phase at room temperature out of irradiation, is not produced under irradiation even at high fluence where the thickness of the mixing layer is about 20 nanometers. The production of such Crrich and Si-rich phases under irradiation is not surprising and have already been observed [2, 4, 19]. However their exact chemical compositions remain up to now subject to debate. In this work, we have shown that the non destructive XRR technique accurately allowed quantifying the chemical compositions of these very thin phases. The existence of these phases, extracted from XRR measurments may explain the weakness of measured magnetization of the Cr/Si system synthesized by ion beam mixing. Ab initio
Acknowledgements We acknowledge A. Traverse for helpful and fruitful advices during the writting of the manuscript. We also acknowledge A. Perrat-Mabilon from IPNL (France) for different implantations performed and D. Troadec from IEMN (France) for performing FIB samples. This work has been supported by Triangle de la Physique contract INSTRUMAT Nr 2011-074T. References References [1] P. Wetzel, C. Pirri, J. C. Peruchetti, D. Bolmont, G. Gewinner, Formation of CrSi and CrSi2 upon annealing of Cr overlayers on Si(111), Phys. Rev. B 35(11) (1987) 5880–5883.
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* Interdiffusion at Cr/Si interfaces induced by ion beam mixing at room temperature * Creation of Cr/Si alloy metastable phases * Reconstruction of Cr/Si interdiffusion profile by X-ray reflectometry * Quantitative correlation between Cr and Si profiles extracted from XRR and measured by EDX-TEM.
S0925-8388(14)02769-8 http://dx.doi.org/10.1016/j.jallcom.2014.11.121 JALCOM 32675
To appear in:
Journal of Alloys and Compounds
Please cite this article as: L. Luneville, L. Largeau, C. Deranlot, J. Ribis, F. Ott, N. Moncoffre, G. Baldinozzi, D. Simeone, Interdiffusion processes at irradiated Cr/Si interfaces, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.11.121
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Interdiffusion processes at irradiated Cr/Si interfaces L. Lunevillea,∗, L. Largeaub , C. Deranlotc , J. Ribisg , F. Ottd , N. Moncoffree , G. Baldinozzif , D. Simeoneg a DEN/DANS/DM2S/SERMA/LLPR/LRC-CARMEN,
CEA Saclay, 91191 Gif-sur-Yvette, France Route de Nozay, 91460 Marcoussis, France c Unite Mixte de Physique CNRS/Thales, 1 Avenue Augustin Fresnel, 91767 Palaiseau, France d DSM/IRAMIS/LLB/CEA/CNRS, CEA Saclay, 91191 Gif-sur-Yvette, France e IPNL/CNRS, Domaine scientifique de la Doua, 69622 Villeurbanne, France f CNRS-SPMS/UMR 8580/ LRC CARMEN Ecole Centrale Paris, 92295 Chatenay-Malabry, France g DEN/DANS/DMN/SRMA/LA2M/LRC-CARMEN, CEA Saclay, 91191 Gif-sur-Yvette, France b LPN-UPR20/CNRS,
Abstract Chromium silicon CrSi alloys are foreseen as possible materials for spintronic devices. Ion beam mixing could be an efficient technique to produce thin films of such alloys at room temperature while avoiding thermal diffusion. In order to assess this point, we have irradiated 20 nm Cr layer on a (100) Si wafer with 70 keV Kr ions. The X ray reflectometry technique combined with Transmission Electron Microscopy and Energy Dispersive X-ray analysis was applied to analyze, at the nanometric scale, the formation of Cr/Si blurred interfaces induced by ion beam mixing. From the analysis of reflectivity curves, it appears that nanometric Cr5 Si3 and CrSi2 phases are produced at the early stage of the process. The existence of these two paramagnetic phases gives some clues to explain the reason why the experimentally observed ferrimagnetism was weaker than predicted. Keywords: X-ray reflectometry, Cr/Si interfaces, interdiffusion, ion beam mixing
1. Introduction As promising materials for spintronic devices, Cr/Si systems have attracted much attention in recent years [1] as possible diluted magnetic semiconductor systems. Ab initio calculations on the Cr doped Si system predict a ferrimagnetic behavior for Cr doped Si solid [2]. However, measurements on Si wafers implanted with Cr ions clearly display that this ferrimagnetism is weaker than predicted [3]. On the other hand, previous investigations on the annealing of Cr/Si bilayers have clearly shown the formation of different phases at the interface between Cr and Si atoms [1, 4, 5]. A possible explanation for this weakness of the magnetization may be due to the appearence of these phases at the interface on a nanometric scale (few nanometers). In order to synthesize high purity buried interfaces between two solids, ion beam implantation was extensively used in the well known top down approach [6, 7, 2, 4, 8]. The slowing down of impinging atoms with high kinetic energy (few hundreds of KeV) leads to the mixing of atoms at the atomic scale in alloys, semi conductors or insulators [9, 10, 11, 12]. By changing the energy of the incident beam, it is possible to create buried interfaces at well defined depths. Moreover, the thickness of the produced interface is directly related to the fluence of impinging ∗ Corresponding
author Email address: [email protected] (L. Luneville)
Preprint submitted to Journal of Alloys and Compounds
atoms [13]. Some authors [14, 15, 16] claim the formation of CrSi2 under implantation at room temperature. According to these authors, the composition of the phase produced under implantation and extracted from RBS measurements is similar to that of CrSi2 phase produced during thermal annealing. Different measurements [17] point out that the formation of CrSi2 occurs only above 523K under implantation. These authors were unable to identify the phase produced at room temperature. More recently, other authors [4] claim that chromium atoms diffuse in silicon at room temperature during implantation without evoking a mechanism associated either with high energy collisions induced by impinging ions or an enhanced diffusion induced by vacancies produced under irradiation. From these contradictory reports, an accurate concentration profile derived from a depth profiling technique at the nanometric scale could be useful to identify which phases are created. Many experimental techniques offer the opportunity to probe free surfaces and interfaces such as Transmission Electron Microscopy (TEM), X ray Photoelectron Spectroscopy (XPS), Rutherford Back Scattering (RBS) and Low and Medium Energy Ion Scattering spectroscopy. However, the preparation of thin foils for TEM studies implies the relaxation of strain fields produced by the layers surrounding the interface. Moreover, the depth resolution of the RBS technique highly decreases with the penetration depth of the incident ion beam[18]. The X-ray reflectometry technique (XRR) has been extensively used over twenty years to characterize November 21, 2014
stratified media like semi conductors, super lattices [19], Langmuir-Blodgett films [20, 21, 22] and thin film heterostructures [23]. The main interest of this technique is to probe nondestructively the buried interfaces with a few angstrom resolution [22, 24, 25, 26]. Neither relaxation of stress field nor pollution by hydrogenation can induce spurious effects which can occur during the preparation of thin foils for TEM studies. Moreover, the resolution of the XRR does not depend on the probed depth. In this work, X ray reflectometry technique as well as TEM studies were thus combined to track new phases in the buried Cr/Si interface.
a thermocouple deposited on the chromium surface and maintained out of the irradiation flux. This temperature did not exceed 350K, well below the temperature associated with point defect migration, that is significant above 450 K in this system [28]. Within these conditions, no competition between collisions and thermal diffusion can occur because of the large gap in temperature [29]. From SRIM, it was alse possible to estimate the penetration depth of Cr into the Si layer. This penetration depth is about 5 nanometers. On the other hand, the cross section associated with the vacancies formation in the Si layer extracted from SRIM calculations is equal to 7 × 10−14 cm2 . Assuming a recombination volume equal to the silicon unit cell (0.543 nm), we can compute the atomic concentration of interstitial and vacancies induced by 70 KeV Kr irradiation in the silicon wafer, from the knowledge of the entropy and free enthalpy energies associated with the migration of interstitial and vacancies in silicon [30]. To compute this atomic fraction, we only assume a mutual recombination of interstitials and vacancies produced by the irradiation. This atomic fraction of interstitials and vacancies is equal to 1.5 × 10−4 and remains larger than equilibrium values [30]. From this analysis, the mechanism of mixing seems to result from an enhancement of the diffusion due to elevated number of ion-induced vacancies.
2. Experimental measurements 2.1. Samples elaboration The studied Cr/Si samples were grown by Physical Vapor Deposition (PVD) technique. From this process, we deposited only one Cr thick layer (20 nanometers) on a (100)Si wafer. Prior to the Cr deposition, the wafer surface was cleaned with a hydrofluoric acid solution to remove the native SiO2 oxide. To avoid corrosion of the chromium surface, a thin layer of platinium (2nm) was deposited on the Cr layer. The thickness of the Pt layer was chosen to be sputtered by the Kr ions at the beginning of the mixing, i.e. for the lowest fluence. The Cr and Pt thin films have been deposited by magnetron sputtering in a pure Ar discharge. The distances between targets and substrate was around 12 cm and the purities of Cr and Pt targets were equal to 99.95% and 99.99% respectively. The rates of deposition were equal to 0.15 nm/s for Cr and 0.4 nm/s for Pt. The base pressure, working pressure and the substrate temperature were equal to 5 × 10−6 Pa, 2.5 × 10−1 Pa and 300 K. The thichness and the density of the Cr and Pt layers were determined from the analysis of the reflectivity curve. From the measurement of the critical angle, the density of the deposited Cr layer could be computed and was equal to 97% of the theoretical density of chromium. Moreover, the thickness of the chromium layer, derived from a Fourier transform of the reflectivity curve, was equal to 20 ± 1 nanometers. Such a layer explains the kink observed on the XRR curve of the pristine sample near 2.5◦ , as plotted on figure 1(a) (black lines). In order to maximize the mixing of atoms at the Cr/Si interface, all samples were irradiated with 70 keV Kr ions at room temperature under a flux of 7.7 × 1012 cm−2 s−1 . The fluences vary from 5 × 1014 to 2 × 1016 cm−2 . Based on SRIM-2003[27] calculations, the density of deposited energy by the ion beam, Fd , is maximum at the Cr/Si interface. The maximum value of Fd is equal to 0.8 keV/nm and similar Fd values were reported in previous works [14, 15, 4]. Moreover, the implantation peak of Kr ions lies far inside the Si wafer and these atoms do not contaminate the Cr/Si interface, even at high fluences. For all implantations, the temperature of the Cr layer was monitored by
2.2. X Ray Reflectometry and Energy Dispersive X ray analysis studies Cross sectional TEM lamella were extracted using the Focussed Ion Beam technique. TEM diffraction patterns performed on the Cr layer and the Si substrate (not shown) clearly exhibit well defined spots assessing that the Cr layer deposited was polycristalline. Well defined spots observed on TEM patterns performed at each fluence clearly show that this chromium layer remains crystalline even at high fluence. On one hand, XRR was performed to obtain accurate measurements of the atomic concentration profiles of chromium and silicon induced by ion implantation of thin Cr films on Si wafers. As chromium and silicon possess very different electron density, they provide a good contrast in X-ray diffraction. On the other hand, the morphology and chemical characterization of the different interfaces produced under implantation were studied by Scanning Transmission Electron Microscopy (STEM) in cross sectional view using a JEOL 2200FS equipped with a probe aberration corrector. Different high angle annular dark field pictures (STEM-HAADF) were collected as a function of the fluence. The local Cr/Si ratio was also measured along the incident ion beam direction by Energy Dispersive X-Ray analysis (EDX). This technique provides a direct measurement of the atomic fractions of these two elements and was used to validate the refinement of the reflectivity curves versus the fluence. XRR specular diagrams were collected with a Bruker D8 Advance diffractometer in symmetric reflexion condi2
tions [31]. In this geometry, the scattering vector remains always normal to the surface of the sample. The experimental setup is equipped with a parabolic Gobel mirror and a line focus CuKα radiation tube (40 kV, 4O A). A 12 micron thick Ni filter was used to attenuate the CuKβ radiation. The footprint of the beam, limited by a primary 0.1 millimeter slit at small incident angles, allows calibrating the scattering angle θ. It thus remains smaller than the sample length (3 cm) for θ > 0.19◦ . The photons intensity of the X ray beam was equal to 2108 counts per second. A secondary 0.4 millimeter slit was used in front of a 0D detector. 2θ scans were collected in specular conditions over 10 degrees with a scan step of 0.01 degree. The instrumental resolution of the experimental setup is 7 ◦ about 1000 [31]. Each scan was collected 10 times with a time per step of 10 seconds. Moreover, 2θ scans with an offset of 0.3◦ were performed to correct the raw data from diffuse scattering. A simple subtraction of raw data provides the XRR specular signal. 3. Results 3.1. Refinements of XRR diagrams XRR diagrams are shown in Fig. 1(a) for the evolution of the reflectivity curves as a function of the scattering angle θ for all fluences. The Kiessig fringes are conspicuous on the pristine sample over eight orders of magnitude assessing the high quality of the Cr layer. Even on the most irradiated sample, Kiessig fringes due to interferences between distinct well-defined layers remain clearly visible. This point implies that well defined interfaces are created during implantation. As expected, the slope of the reflectivity curve is a decreasing function of the fluence. This effect is mainly due to an increase of the roughness of the surface of the sample due to sputtering. Increasing the fluence, reflectivity curves exhibit different kinks due to the appearance of extra layers produced by ion implantation. Refinements were performed with the Parratt’s recursive formalism [32]. The least square method weighting was applied to Ln(R(2θ)) to give more weight on the large 2θ points, i.e. to track thin layers. The classical least square technique was used to fit the XRR curves. A minimum value of 10−7 for R(θ) allows fixing the 2θ range for the refinement. For each fitting curve, the thickness, the density and the roughness of the different layers were fitted. A 20 nanometer thick Cr layer was used as the initial guess to fit the pristine sample. To assess the stability of the refinements, the simulated annealing technique was used to avoid local minima during the minimization process. The best refinement of the reflectivity curve for a given fluence becomes the initial guess for the next fluence. This recursive approach avoids any spurious effects due to ad hoc models [23, 22] and constrains the number of free parameters in the Parratt’s formalism. Fig. 1(b) displays the comparison between the measured and the refined reflectivity curves for the pristine
Figure 1: (Color online)(a) Plot of different reflectivity curves versus the scattering angle for different fluences (black: pristine, red: 5 × 1014 cm−2 , green: 2 × 1015 cm−2 , blue: 5 × 1015 cm−2 , cyan: 8 × 1015 cm−2 , magenta: 1 × 1016 cm−2 , olive: 2 × 1016 cm−2 ). The inset displays the variation of the critical angle θc (dashed lines), derived from the relation R(2θc ) = 1/2, between the pristine sample (black) and the most irradiated sample (olive). (b) Refinements of the XRR experimental curves (black circles) are performed with the Simulreflec code [22] (red lines) on the pristine sample (bottom) and the most irradiated sample (top). In order to asses the quality of the fit, the error function (W = [log(Rsimul ) − log(Rexp )]Rsimul /Rexp ) was plotted (green line) for each refinement.
sample and the most irradiated sample. The good agreement of the measured and computed reflectivities validate the characterization approach. 3.2. Comparison with EDX measurements From XRR refinements, it is possible to extract the Scattering Length Density (SLD). The translation from the SLD to the atomic fraction profiles implies the knowledge of the density. To extract the atomic fraction profiles of Si and Cr, we assume that the density of layers formed by Si and Cr atoms follows a Vegard law. The silicon atomic fractions extracted from XRR refinements are displayed versus the depth for each fluence in Fig. 2. They are in excellent agreement with those obtained from EDX measurements (squares on Fig. 2), thus justifying our procedure. 3
investigations [15, 17, 4]. Figure 3 displays high resolution HAADF STEM cross sectional view on the most irradiated sample. The Z contrast on this picture clearly shows two caracteristic layers of one nanometer with well defined Si atomic fractions. The EDX analysis (red squares Fig. 2) allows to determine the atomic compositon of these two limit phases associated with Cr5 Si3 and CrSi2 phases. These results assess the existence of two plateau extracted from the analysis of reflectometry data (green line on Fig. 3). All these techniques are in good agreement and prove that two limit Cr5 Si3 and CrSi2 phases are created under irradiation. It must be noticed that these compositions are not associated with the eutectic point in the equilibrium phase diagram. This last point disagrees with previous theoretical assumptions [14, 33] on the phase stability of silicides under ion implantation. Between these two phases with fixed compositions, the Si atomic fraction evolves linearly in the mixing layer L as pointed out by the green line on Fig. 3. This linear evolution can be understood as the creation by ion beam mixing of a crystalline solid solution between these two limit phases.
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Figure 2: (Color online) Comparison between the Si atomic fractions extracted from reflectivity curves (full lines) and EDX analyses (squares) as a function of the depth for all fluences (black: pristine, red: 5 × 1014 cm−2 , green: 2 × 1015 cm−2 , blue: 5 × 1015 cm−2 , cyan: 8 × 1015 cm−2 , magenta: 1016 cm−2 , olive: 2 × 1016 cm−2 ). The error bars are associated with EDX data points. The Si atomic fractions are in fair agreement. For the highest fluence, all Cr atoms have diffused. This point explains the value of the Si atomic fraction at the surface. In the inset, the evolution of the Si atomic fraction is plotted for low fluences. Two distinct plateau clearly appear on Si profiles at low fluences. These plateau exist for all fluences (dashed horizontal lines) and are associated with Si atomic fractions equal to 0.375 and 0.67 respectively.
Fig. 3 displays the HAADF STEM variation contrast in cross sectional view for the sample irradiated with a fluence of 1016 cm−2 . The silicon concentration profile extracted from the reflectivity curve (green solid line) and the EDX measurements (red squares) are drawn on the same scale. They are in fair agreement. The maximum of the Z contrast pointed out on HAADF STEM picture lies at the same depth as the inflexion points of the reflectivity curve assessing the quality of the concentration profile derived from the reflectivity curve. The first inflexion point of the reflectivity curve is associated with the Cr rich interface. The last inflexion point is associated with the Si rich interface. The STEM cross sectional view clearly shows that the interface perpendicular to the ion beam does not exhibit a large waviness for each sample (below one nanometer). This point assesses that the global roughness of XRR curves is mainly due to the sputtering of the surface by Kr ions during the ion beam mixing.
Figure 3: (Color online) HAADF STEM cross sectional view of the 1016 cm−2 irradiated sample. Even at this high fluence, this picture clearly displays that blurred interfaces remain abrupt. On this graph, the silicon atomic fraction profiles extracted from XRR analysis (green line) and resulting from EDX measurements (red squares) agree quite well with the Z contrast HAADF STEM picture, assessing our analysis. In order to study the cristalline nature of the mixing layer, diffraction patterns were collected in this layer. The intensity is uniformaly distributed along a ring on the diffraction pattern. The position of this ring from the origine in the reciprocical space defines a caracteristic distance of 0.25nm in the real space.
4. Discussion Two distinct plateau are clearly visible on the Si atomic profiles (inset in Fig. 2), i. e. during the early stages of the mixing (F= 5 × 1014 cm−2 ) up to the highest fluence (F= 2 × 1016 cm−2 ). Even if the two plateau are difficult to detect for high fluences, they are still present. It thus appears that the Cr/Si mixing layer cannot be described by a single gaussian interdiffusion process, as reported on previous
To study the crystalline structure of the solid solution produced under irradiation, diffraction patterns were collected on the most irradiated sample as pointed out in the inset on Fig. 3. The distance in real space extracted from the position of the ring from the center of the reciprocical space is equal to 0.25nm and corresponds to the nearest 4
Cr-Si bond length in Cr/Si compounds. The uniform distribution of intensities along the ring in the diffraction pattern may be the signature of an amorphous solid solution. However, this ring may also result from the diffraction of a large number of nanodomains. To overcome this difficulty in the analysis of the diffraction pattern, different patterns were calculated within a reduced window (4x4nm2 ) at different points inside the mixing layer of the HAADF STEM cross sectional view plotted on Fig. 3. Figure 4 is an example of these calculated diffraction patterns. Although a Wiener filter was used to reduce the noise, the calculated diffraction patterns display speckled intensities due to the drastic reduction of the probed area. Despite a faint ring can be observed on Figure 4, the intensity is not uniform along this ring but exhibits more intense spots. These non random spots may be the signature of nanodomains in the mixing layer. From this analysis, the solid solution in the mixing layer could be a mixture of two crystalline Cr5 Si3 and CrSi2 limit phases explaining thus the linear variation of the Si atomic fraction in this layer. However, the small resolution of the HAADF STEM picture (about 2 nanometers) does not allow calculating accurately the angular correlation function [34] and then assessing the existence of crystalline nanodomains. On the other hand, nanodomains are smaller than few nanometers and the distinction between amorphous and crystalline nanodomains is not so clear at this lengthscale. Moreover, previous investigations [35, 36] have clearly shown that amorphous Cr5 Si3 and CrSi2 phases are produced in thin Cr/Si layers. Whatever the crystalline nature of phases produced under irradiation is, two plateau are the signature of a diffusion reactive process [37]. Such a process was not expected to occur under irradiation [14, 33].
Figure 4: Numerical Fourier transform performed on a selected part (4 nm x 4nm) of the mixing area. The intensity is not uniform along the ring but exhibits more intense spots circled in red on the picture. These large symetric spots may be the signature of Cr5 Si3 and CrSi2 nano crystalline domains in the mixing layer.
calculations thus pointed out that Cr5 Si3 and CrSi2 crystalline phase are not ferrimagnetic [3]. It must be noticed that neutron reflectometry can also provide a magnetization profile in the mixing layer [38]. However, the neutron resolution is always smaller than the X-ray resolution and density profiles are more difficult to extract from reflectometry measurements as clearly pointed by previous authors[38]. The atomic fraction profiles extracted from XRR experimental curves displayed on Figure 2 nicely demonstrate that conventional laboratory sources can now be easily used for assessing the structural properties of buried thin films exhibiting thicknesses of few nanometers. This nondestructive technique thus allows extracting the atomic concentration profiles associated with chemical reactions between interfaces or stress induced segregation effects.
5. Conclusion From the analysis of the reflectivity curves, it clearly appears that ion beam mixing leads to the formation of a Cr rich phase, Cr5 Si3 , near the Cr layer, and a Si rich phase, CrSi2 , near the Si layer, even at the lowest fluence. Moreover, it clearly appears from different analyses that the CrSi phase, the most stable phase at room temperature out of irradiation, is not produced under irradiation even at high fluence where the thickness of the mixing layer is about 20 nanometers. The production of such Crrich and Si-rich phases under irradiation is not surprising and have already been observed [2, 4, 19]. However their exact chemical compositions remain up to now subject to debate. In this work, we have shown that the non destructive XRR technique accurately allowed quantifying the chemical compositions of these very thin phases. The existence of these phases, extracted from XRR measurments may explain the weakness of measured magnetization of the Cr/Si system synthesized by ion beam mixing. Ab initio
Acknowledgements We acknowledge A. Traverse for helpful and fruitful advices during the writting of the manuscript. We also acknowledge A. Perrat-Mabilon from IPNL (France) for different implantations performed and D. Troadec from IEMN (France) for performing FIB samples. This work has been supported by Triangle de la Physique contract INSTRUMAT Nr 2011-074T. References References [1] P. Wetzel, C. Pirri, J. C. Peruchetti, D. Bolmont, G. Gewinner, Formation of CrSi and CrSi2 upon annealing of Cr overlayers on Si(111), Phys. Rev. B 35(11) (1987) 5880–5883.
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* Interdiffusion at Cr/Si interfaces induced by ion beam mixing at room temperature * Creation of Cr/Si alloy metastable phases * Reconstruction of Cr/Si interdiffusion profile by X-ray reflectometry * Quantitative correlation between Cr and Si profiles extracted from XRR and measured by EDX-TEM.