- Email: [email protected]
Synchrotron radiation studies of thin films and implanted layers with the materials research endstation of ROBL
Journal of Alloys and Compounds 328 (2001) 105–111
L
www.elsevier.com / locate / jallcom
Invited original talk
Synchrotron radiation studies of thin films and implanted layers with the materials research endstation of ROBL a, a,b b b b N. Schell *, W. Matz , F. Eichhorn , F. Prokert , F. Berberich b
a Project Group ESRF-Beamline, Forschungszentrum Rossendorf e.V., P.O. Box 510119, D-01314 Dresden, Germany Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf e.V., P.O. Box 510119, D-01314 Dresden, Germany
Received 14 June 2000; received in revised form 14 June 2000; accepted 22 December 2000
Abstract The paper describes the possibilities and advantages of the in-house materials research endstation of the Rossendorf Beamline ROBL at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Some X-ray diffraction and reflectometry results mainly of thin films and interfaces show the scientific merit and versatility of the instrumentation. 2001 Elsevier Science B.V. All rights reserved. Keywords: Synchrotron radiation beamline; X-ray diffraction; Reflectometry; Thin films; Implanted layers
1. Organisation and scientific program of ROBL The ROssendorf BeamLine (ROBL) is a collaborating research group (CRG) beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France [1]. It was built and is operated by the Forschungszentrum Rossendorf (FZR). The two different experimental stations, a unique radiochemistry hutch and a multi-purpose materials research hutch, both operating alternatively, are mostly used by the FZR for research devoted to: • radioecology, i.e. determination of the chemical speciation of radionuclides interacting with geological material, natural and anthropogenic organics, and micro-organisms using X-ray absorption spectroscopy; • structural identification and characterisation of modifications of surfaces and interfaces produced by ion beam techniques for applications as hard covers, biocompatible materials or in semiconductor technology; study of interfaces in thin films and nanometer-multilayers. The main experimental techniques applied are X-ray diffraction and reflectometry. The beamline is also available for external users – either in collaboration with the FZR or by submitting a proposal to the ESRF. About 15% of total beamtime available at ROBL will be given to European research groups with *Corresponding author. E-mail address: [email protected] (N. Schell).
direct support of the EC within the framework ‘Access to Research Infrastructure’ (ARI) – all details and the calls for proposals can be found in the WEB [2].
2. Scheme of X-ray optics ROBL is located at the bending magnet BM20 of the ESRF (0.8 T, critical energy 19.6 keV) and uses horizontally a fan of 2.8 mrad of synchrotron radiation from its hard edge. The layout of the optics is sketched in Fig. 1. The main elements are a fixed-exit double crystal Si monochromator located between two mirrors with Si and Pt surface coatings, respectively. The beamline is designed for an energy range from 5 to 35 keV, where in the lower range up to 12 keV the Si coated mirrors are used. The lower energy limit is given essentially by the mandatory Be-windows. The upper energy limit was chosen to allow X-ray absorption spectroscopy experiments on all chemical elements from Ti onwards, since at least one absorption edge is in the energy range 5–35 keV. The energy resolution is around 2310 24 for Si (111) crystals as monochromator and better than 1310 24 with Si (311) crystals.
3. Experimental equipment The radiochemistry endstation is designed for studying radionuclides of environmental importance by X-ray ab-
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01330-5
N. Schell et al. / Journal of Alloys and Compounds 328 (2001) 105 – 111
106
Fig. 1. Schematic layout of the ROBL optics.
sorption spectroscopy (element specific, oxidation state, bond lengths, numbers of neighbouring atoms in the first few coordination shells) for solid and liquid samples. The ROBL CRG obtained a license from the French authorities to investigate the isotopes listed in Table 1 up to a maximum allowed activity at any given time present at ROBL of 185 MBq (5 mCi). As these elements emit mostly a- and b-particles and only weak g-radiation, the heart of the radiochemistry endstation is a glovebox without additional lead shielding. To ensure a safe handling of the radionuclides, the entire experimental station is built as a radiochemistry laboratory according to legal safety requirements (multi-barrier safety concept, separate ventilation, radiation monitoring, redundancy) [3]. In Fig. 2 the basic experimental equipment of the materials research hutch (MRH) is shown. The heart is a special six-circle diffractometer for heavy duty. It allows diffraction and reflectivity experiments with high accuracy (angular steps of 0.0018 plus additional gear boxes 1:10 at each circle). Different sample holders for sheets, wafers and powders are available. The installation of special sample environment chambers (e.g. high-temperature) is possible. Various detector systems like scintillators, energy dispersive photo-diodes, or a 2-dimensional CCD camera can be placed on the detector arm. An additional beamdeflector in front of the diffractometer makes it fit for Table 1 List of radioactive elements, which can be investigated at ROBL and the maximum amount of material to remain below the activity limit of 185 MBq (5 mCi) Isotope
Half-life (years)
Amount (g)
Isotope
Half-life (years)
Amount (g)
Np 237 Am 241 Am 243 Po 208 Po 209 Pa 231
2.1310 6 433 7370 2.9 103 3.28310 4
6.97 1.4310 23 0.025 8310 26 3.01310 24 0.106
Pu 239 Pu 242 Ra 226 Tc 99 U nat Th nat
2.4310 4 3.75310 5 1600 2.1310 5 4.47310 9 1.4310 10
0.08 1.27 0.005 29.1 1000 1000
investigations of liquids with free surfaces (for that the whole diffractometer can be lowered). Fig. 3 gives the experimental diffraction resolution Dd /d obtained on silicon powder. It can be seen that the rise of the energy of the monochromatic beam alters the resolution only moderately. The use of an analyzer crystal in front of the detector allows an increase of resolution as well as the suppression of fluorescence radiation.
4. Experimental results from materials research Profiting from the well-known properties of synchrotron radiation (esp. energy tunability and high brilliance) and the dedicated experimental equipment installed – the large majority of experiments in MRH (FZR research and external users or collaborators) has been a combination of only six main experimental possibilities: • thin film diffraction by grazing incidence / exit techniques (in- / off-plane) • high resolution diffraction (low beam divergence) • diffraction at high temperatures • reflectometry • use of anomalous scattering (energy tunability) • structural studies of liquids with free surfaces (beam deflector) As representative results so far obtained in one and a half years of operation of ROBL some scientific topics will be presented in the following, demonstrating the special advantages of this research laboratory.
4.1. Stress relaxation and precipitation of SiC in Si implanted with C SiC is regarded as a promising semiconductor for high frequency, high power and high temperature applications. One way of producing basic material for circuits is the
N. Schell et al. / Journal of Alloys and Compounds 328 (2001) 105 – 111
107
Fig. 2. Basic experimental equipment in the materials research hutch of ROBL. It consists of a six-circle diffractometer for heavy duty (a) and a deflector (b) which can be translated on a slide (c). Various detector systems and also special sample environments can be installed.
Fig. 3. Experimental diffraction resolution Dd /d for various energies obtained on silicon powder inside a glass capillary (diameter 0.4 mm; wall thickness 0.01 mm). For comparison the value from a single crystal sample is also indicated. Flat Si (111) monochromator crystals were used.
formation of SiC by C ion implantation into Si. To control the growth of SiC a detailed knowledge of the crystallisation process of SiC is necessary. In diffraction experiments for various implantation parameters we investigated the stress relaxation in the Si matrix and the formation of SiC precipitates from precursor stages onwards. The high intensity and low divergence of synchrotron radiation allow the recording of high resolution diffraction patterns from an implanted layer of about 200 nm thickness. Different samples with a large range of implantation parameters (fluences, annealing temperatures) have to be investigated in order to allow the development of a detailed growth model [4]. Implantation of C ions with an energy of 195 keV into Si wafers heated up to 8008C results in an elastic distortion of the Si host lattice and in the formation of crystalline SiC particles or their precursor stages depending on implantation fluence and temperature. Only a Si lattice deformation without growth of SiC was observed if the fluence does not exceed 5310 15 C ions / cm 2 . After implantation of C ions up to 4310 17 / cm 2 at a temperature of 5008C, agglomerations of Si–C and an altered state of Si lattice deformation are found. By implantation of 4310 17 ions / cm 2 at 8008C, particles of the 3C–SiC (b-SiC) phase grow, which are aligned to the Si matrix. Fig. 4 shows as an example
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N. Schell et al. / Journal of Alloys and Compounds 328 (2001) 105 – 111
Fig. 4. Reciprocal space maps near SiC (002) – the matrix reflex Si (004) is located at Q uu 50 nm 21 , Q ' 546.3 nm 21 . The Si wafers are implanted with 4310 17 cm 22 C 1 (195 keV) at 500 and 8008C (left and right part). The iso-intensity lines are in a logarithmic scale (bold lines mark the half-maximum-intensity).
reciprocal space maps near SiC (002) – the matrix reflex Si (004) is located at Q uu 50 nm 21 , Q ' 546.3 nm 21 – giving information about the orientation relation between Si and SiC. It was found that for implantation at 5008C the [001] axis of SiC is aligned to Si [001] with a nearly isotropic spread of 4.58 (Fig. 4, left part). For implantation at 8008C a more complex distribution is found (Fig. 4, right part): the majority of SiC crystallites is aligned in the same manner but with a reduced spread of only 2.58 (symmetric top of the SiC peak). There exists a second group of crystallites with anisotropic distribution around Si [001] with preference into the Si ,111. directions (streaks at the bottom of the SiC peak). At the lowest fluence of 5310 15 C ions / cm 2 the Si lattice expansion due to implantation is caused mainly by C interstitials and Si self-interstitials. Besides this, there are Si lattice regions with reduced spacing ((Dd /d) Si 52 0.001) which may be caused by substitutional C with an average concentration of 1.5310 20 atoms / cm 3 according to Vegard’s rule. The corresponding peak is detected up to the fluence of 5310 16 C ions / cm 2 at 5008C. Finally, crystalline 3C–SiC particles are found for a fluence of 4310 17 C ions / cm 2 at 8008C. These 3C–SiC crystallites are surrounded by an expanded Si lattice ((Dd /d) Si 5 0.0002). The latter may be the consequence of the thermal stress by cooling down the sample from the process temperature to room temperature due to the different coefficients of thermal expansion for Si (3.59310 26 / K) and SiC (4.2–4.68310 26 / K), respectively.
4.2. X-ray reflectivity and diffuse scattering on Co /Cumultilayers near the absorption edges Certain multilayers (MLs), e.g. Co / Cu, show giant magnetoresistance (GMR). This effect is sensitive to layer
thickness, interface roughness, as well as to crystallinity and texture. The study of specular and diffuse reflectivity with synchrotron radiation allows a detailed characterization of the layer and interfacial properties (roughness, correlation lengths) necessary to optimize the preparation processes and to improve the device parameters wanted. Especially, the tunability of wavelength at a synchrotron X-ray source allows to enhance the scattering contrast of Co and Cu for resonant X-ray reflectivity measurements at the K-edges of the two elements as can be seen in Fig. 5. Two types of MLs prepared by crossed beam pulsed laser deposition with different layer thicknesses of Co and Cu layers on Si substrates – Si / SiO 2 / 83[Co (4 nm) / Cu (4 nm)] and Si / SiO 2 / 43[Co (8 nm) / Cu (8 nm)] – were investigated at ROBL with energies near the Co-edge (7.71
Fig. 5. Example of contrast variation for specular reflectivity by wavelength tuning for a Co / Cu–ML of composition Si / SiO 2 (550 nm) / 83[Co (4 nm) / Cu (4 nm)] fabricated by crossed beam pulsed laser deposition (CB PLD). The chosen energies are: K-edge of Co (7.708 keV), K-edge of Cu (8.974 keV) and the Cu Ka-line (8.048 keV).
N. Schell et al. / Journal of Alloys and Compounds 328 (2001) 105 – 111
keV) and Cu-edge (8.97 keV), respectively. The samples were studied in the ‘as-deposited’ state and after annealing (5008C, 2 h). From the specular reflectivity and the distribution of the diffusely scattered intensity a characterisation of the roughnesses of the surfaces and interfaces could be given by simulations based on Sinha’s representation of the height–height correlation function. The lateral correlation length j and the Hurst parameter h have been determined and the roughness correlation between the layers could be quantified. The results are nearly the same for both types of MLs. In the ‘as-deposited’ state the roughness is dominated by a high morphological jaggedness, represented by a high fractal dimension (D532h) of about 2.75. After annealing, due to thermally induced grain growth, this ‘short-scale roughness’ is reduced (D52.5). However, the correlation length which is a lateral measure for the biggest morphologicial feature determining the roughness value is drastically shortened from |4000 to |50 nm. This is accompanied by a reduction of the roughness conformity, expressed by the ratio scorr /srms , from 40 to 10%.
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Fig. 6. In situ diffraction of Ti–6Al–4V under incidence angle 48 (E5 8.048 keV) after ion beam implantation with nitrogen up to a fluence of 6310 17 cm 22 . Clearly discernible is the continuous transformation of TiN into Ti 2 N within appr. 100 K around 7008C.
expansion were observed. The rearrangement in the base alloy material is a second mechanism for the hardness degradation at higher temperatures.
4.3. Phase transformations studied in situ during annealing of Ti– 6 Al– 4 V implanted with nitrogen
4.4. Local internal strain and stress in ultra fine grained Ni due to cyclic plastic deformation
Ti–6Al–4V alloys are commonly used industrial materials especially for their low weight. To enhance surface hardness, nitridation by ion implantation is often used. with a fluence of 6310 17 N ions / cm 2 a hardness increase by a factor of three can be achieved. The hardness gain, however, is partially lost at higher temperatures, limiting the application of Ti–6Al–4V. After annealing at 7008C for 1 h there remains a hardness increase by a factor of 2.3 compared to the non-implanted sample. To understand the underlying structural mechanisms it is therefore advantageous to follow the structural changes during annealing with in situ diffraction experiments [5]. The use of synchrotron radiation for phase determination and depth distribution is necessary since one deals with a very thin implanted layer (typically 200 nm) and the phase transformations occur in time scales of several minutes. Fig. 6 shows a temperature series of grazing incidence diffraction patterns. The process is the solution of TiN crystallites and the formation of the new phase Ti 2 N. The transformation between both phases is direct, there is no intermediate phase. From the study of the distribution of N atoms in the subsurface region of the alloy by elastic recoil detection analysis (ERDA) nitrogen concentrations below the stoichiometry of TiN and Ti 2 N, respectively, are found. In comparison with the depth distribution of both phases (recorded by grazing incidence XRD when changing the angle of incidence) it can be concluded that the nitride phases form grains in the alloy matrix. Therefore, precipitation hardening will be the dominant mechanism. Additionally, changes in the lattice constants of both alloy phases (a-Ti and b-Ti) not conforming with thermal
This example is from collaboration with the Technical University of Dresden. The deformation behavior of ultra fine grained (ufg) nickel with a typical grain size of 200 nm yielding special mechanical properties was studied depending on grain size, dislocation density and deformation parameters like pre-deformation or annealing temperature [6,7]. The technique consists of a separation of grain size and strain contributions to the broadening of Bragg reflections. For the construction of the so-called Williamson–Hall plot several Bragg reflections have to be measured with very high instrumental resolution. The low divergence of the synchrotron beam allows a resolution rendering a deconvolution of the measured line shapes unnecessary. At the same time the high flux allows a tremendous amount of samples to be investigated in a reasonable amount of time (about 35 samples within 12 to 15 shifts, each to be aligned for six or seven Bragg reflexes for a Williamson–Hall plot). A direct comparison between a laboratory X-ray source and ROBL for the same sample shows the appr. three orders of magnitude higher flux gain at ROBL for comparable slit and distance dimensions. For the highly crystal-collimated laboratory source a position sensitive detector was used while at ROBL a scintillator detector was scanned with the corresponding angular resolution [8]. Ufg Ni samples ‘as-prepared’ and after cyclic plastic deformation at room temperature and at 2008C were studied. The grain structure was observed using the orientation contrast in a scanning electron microscope. No significant changes of the grain size distribution were detected after the cyclic deformation. Measurements of
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N. Schell et al. / Journal of Alloys and Compounds 328 (2001) 105 – 111
X-ray diffraction profiles were performed at ROBL applying monochromatic synchrotron radiation with energy of 8.05 keV. The instrumental broadening was negligible in comparison to the strain induced broadening of the profiles measured at the ufg samples. To estimate the spectrum of internal strains, diffraction profiles were measured for six different hhklj-types of lattice planes parallel to the sample surface. From the changes in the shape and asymmetry of the profiles one can conclude that the width of the strain spectrum is reduced by the cyclic plastic deformation especially for higher temperatures though no recrystallisation occurs. The rms strain reduces from 1.5310 23 to 0.3310 23 . Taking into account the variation of the profile shape with the hhklj-type of reflection, long-range granular stresses should be present. With increasing deformation temperature the long-range stresses decrease as can be deduced from the increasing ratio of integral breadth to FWHM of the Bragg peaks. The mean dislocation density, calculated on the basis of the Krivoglaz–Wilkens theory, was found to be correlated with the deformation state of the samples.
4.5. Characterization of waveguides In a collaboration with the LMU Munich and the ILL Grenoble hard X-ray waveguides with single- and multimode guiding layers of Ni / C–ML-structure on floatglass substrates were characterized. These experiments did profit from the high precision of sample positioning and beam definition, though it meant a waste in intensity for a bending magnet source (beam size 0.130.1 mm 2 at 20 keV). Starting with single guiding layers to test design parameters and demonstrate the principle of resonant mode coupling by shining a parallel beam at grazing incidence onto the waveguide [9], limitations were encountered (shape of modes and angular acceptance of typically only several thousands of a degree) which limit coupling efficiencies for more divergent X-ray sources. Those restrictions could be overcome by using multiple guiding layers [10]. Essentially in high-precision mappings of incidence / exit-angles the farfield interference patterns were measured and confirmed simulations of internal and external standing electromagnetic fields as a function of the structural and geometric parameters (layer thickness, composition, density, interface roughness, angles of incidence, X-ray energy). By mode splitting into submodes (leading to a limiting band-like-behaviour for an infinite number of layers), an increase of ‘allowed’ incidence angles of appr. 0.038 (FWHM in the vertical of the interference maxima of the farfield behind the waveguide) could be reached for a sample with composition (73[Ni (2.5 nm) / C (45.7 nm)] / Ni (20.0 nm) / floatglass substrate) with an additional 1.4 nm oxide top layer (general srms |1 nm). By measuring the farfield interference patterns the
possibility was demonstrated that carefully designed waveguides can be used as tools to control the near- and farfield distributions, optimize angular acceptance and thus efficiencies for the production of nm X-ray beams.
4.6. Structure refinement of SiC polytypes Although the diffractometer was not specifically designed for single crystal studies, the high setting accuracy of the axes and the sample table as well as the versatility of the six-circle-design allow such studies for distinct problems. A first single crystal experiment was devoted to the structure refinement of the polytypes 4H and 6H of SiC crystals by a group from the University of Jena. The atomic positions show small deviations from the ideal tetrahedron. A systematic investigation of so-called ‘quasiforbidden’ reflections allows to determine these atomic relaxations since those reflections are extremely sensitive to small variations in the structure. There remains an ambiguity because the distinction between different possible structure models (from space group symmetry) calls for phase information. This additional necessary phase information was gained by measured ‘Renninger’scan (C-scan) profiles in the vicinity of three-beam cases. These profiles were compared with theoretically calculated profiles for various possible models. In the end an unambiguous determination of the structure refinement parameters was possible [11].
5. Conclusions The above overview clearly demonstrates the versatility of the materials research experimental station of the Rossendorf Beamline ROBL at the ESRF. It combines the reliable and stable general X-ray source characteristics of high flux, low divergence, energy tunability in a wide range of hard X-rays with an extremely free and precise sample positioning, enhanced by special sample environments. Methodically, a wide range of X-ray techniques is possible, e.g. HRXRD, GIXD, reflectometry. For in-house research of the Forschungszentrum Rossendorf the main fields are thin films and buried layers (including multilayers) with correspondingly suitable X-ray techniques. Via FZR-collaborations, ESRF-proposals or direct ECsupported access to research infrastructures ROBL is also accessible to outside users.
Acknowledgements We would like to thank M. Hecker from the IFW Dresden for making available unpublished data as well as A. Bauer from the University of Jena for leaving us with preprint-data concerning his single-crystal works.
N. Schell et al. / Journal of Alloys and Compounds 328 (2001) 105 – 111
References [1] W. Matz, N. Schell, G. Bernhard, F. Prokert, T. Reich, J. Claubner et al., J. Syn. Rad. 6 (1999) 1076. [2] See under http: / / www.fz-rossendorf.de / FWE; see under: EC supported access. [3] T. Reich, G. Bernhard, G. Geipel, H. Funke, C. Hennig, A. Rossberg, W. Matz, N. Schell, H. Nitsche, Acta Radiochim. 88 (2000) 633. ¨ [4] F. Eichhorn, N. Schell, W. Matz, R. Kogler, J. Appl. Phys. 86 (1999) 4184. ¨ [5] F. Berberich, W. Matz, E. Richter, N. Schell, U. Kreibig, W. Moller, Surf. Coat. Tech. 128–129 (2000) 450. [6] E. Thiele, M. Hecker, N. Schell, Mat. Sci. Forum 321–324 (2000) 598.
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[7] E. Thiele, B. Bretschneider, L. Hollang, N. Schell, C. Holste, in: T.C. Lowe, R.Z. Valiev (Eds.), Proc. Nato Adv. Res. Workshop: Invest. & Appl. of Severe Plastic Deformation, Kluwer, 2000, p. 173. ¨ Dresden, Institut [8] M. Hecker, Internal Report, Technische Universitat ¨ Physikalische Metallkunde, 1998, private communication. fur ¨ [9] F. Pfeiffer, Diploma Thesis, Ludwig-Maximilians-Universitat ¨ Munchen, 1999. [10] F. Pfeiffer, T. Salditt, P. Høghøj, I. Anderson, N. Schell, Phys. Rev. Lett. B 62 (2000) 16939. ¨ [11] A. Bauer, Ph. Reischauer, J. Krausslich, N. Schell, W. Matz, K. Goetz, Acta Cryst. A 57 (2001) 60.
L
www.elsevier.com / locate / jallcom
Invited original talk
Synchrotron radiation studies of thin films and implanted layers with the materials research endstation of ROBL a, a,b b b b N. Schell *, W. Matz , F. Eichhorn , F. Prokert , F. Berberich b
a Project Group ESRF-Beamline, Forschungszentrum Rossendorf e.V., P.O. Box 510119, D-01314 Dresden, Germany Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf e.V., P.O. Box 510119, D-01314 Dresden, Germany
Received 14 June 2000; received in revised form 14 June 2000; accepted 22 December 2000
Abstract The paper describes the possibilities and advantages of the in-house materials research endstation of the Rossendorf Beamline ROBL at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Some X-ray diffraction and reflectometry results mainly of thin films and interfaces show the scientific merit and versatility of the instrumentation. 2001 Elsevier Science B.V. All rights reserved. Keywords: Synchrotron radiation beamline; X-ray diffraction; Reflectometry; Thin films; Implanted layers
1. Organisation and scientific program of ROBL The ROssendorf BeamLine (ROBL) is a collaborating research group (CRG) beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France [1]. It was built and is operated by the Forschungszentrum Rossendorf (FZR). The two different experimental stations, a unique radiochemistry hutch and a multi-purpose materials research hutch, both operating alternatively, are mostly used by the FZR for research devoted to: • radioecology, i.e. determination of the chemical speciation of radionuclides interacting with geological material, natural and anthropogenic organics, and micro-organisms using X-ray absorption spectroscopy; • structural identification and characterisation of modifications of surfaces and interfaces produced by ion beam techniques for applications as hard covers, biocompatible materials or in semiconductor technology; study of interfaces in thin films and nanometer-multilayers. The main experimental techniques applied are X-ray diffraction and reflectometry. The beamline is also available for external users – either in collaboration with the FZR or by submitting a proposal to the ESRF. About 15% of total beamtime available at ROBL will be given to European research groups with *Corresponding author. E-mail address: [email protected] (N. Schell).
direct support of the EC within the framework ‘Access to Research Infrastructure’ (ARI) – all details and the calls for proposals can be found in the WEB [2].
2. Scheme of X-ray optics ROBL is located at the bending magnet BM20 of the ESRF (0.8 T, critical energy 19.6 keV) and uses horizontally a fan of 2.8 mrad of synchrotron radiation from its hard edge. The layout of the optics is sketched in Fig. 1. The main elements are a fixed-exit double crystal Si monochromator located between two mirrors with Si and Pt surface coatings, respectively. The beamline is designed for an energy range from 5 to 35 keV, where in the lower range up to 12 keV the Si coated mirrors are used. The lower energy limit is given essentially by the mandatory Be-windows. The upper energy limit was chosen to allow X-ray absorption spectroscopy experiments on all chemical elements from Ti onwards, since at least one absorption edge is in the energy range 5–35 keV. The energy resolution is around 2310 24 for Si (111) crystals as monochromator and better than 1310 24 with Si (311) crystals.
3. Experimental equipment The radiochemistry endstation is designed for studying radionuclides of environmental importance by X-ray ab-
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01330-5
N. Schell et al. / Journal of Alloys and Compounds 328 (2001) 105 – 111
106
Fig. 1. Schematic layout of the ROBL optics.
sorption spectroscopy (element specific, oxidation state, bond lengths, numbers of neighbouring atoms in the first few coordination shells) for solid and liquid samples. The ROBL CRG obtained a license from the French authorities to investigate the isotopes listed in Table 1 up to a maximum allowed activity at any given time present at ROBL of 185 MBq (5 mCi). As these elements emit mostly a- and b-particles and only weak g-radiation, the heart of the radiochemistry endstation is a glovebox without additional lead shielding. To ensure a safe handling of the radionuclides, the entire experimental station is built as a radiochemistry laboratory according to legal safety requirements (multi-barrier safety concept, separate ventilation, radiation monitoring, redundancy) [3]. In Fig. 2 the basic experimental equipment of the materials research hutch (MRH) is shown. The heart is a special six-circle diffractometer for heavy duty. It allows diffraction and reflectivity experiments with high accuracy (angular steps of 0.0018 plus additional gear boxes 1:10 at each circle). Different sample holders for sheets, wafers and powders are available. The installation of special sample environment chambers (e.g. high-temperature) is possible. Various detector systems like scintillators, energy dispersive photo-diodes, or a 2-dimensional CCD camera can be placed on the detector arm. An additional beamdeflector in front of the diffractometer makes it fit for Table 1 List of radioactive elements, which can be investigated at ROBL and the maximum amount of material to remain below the activity limit of 185 MBq (5 mCi) Isotope
Half-life (years)
Amount (g)
Isotope
Half-life (years)
Amount (g)
Np 237 Am 241 Am 243 Po 208 Po 209 Pa 231
2.1310 6 433 7370 2.9 103 3.28310 4
6.97 1.4310 23 0.025 8310 26 3.01310 24 0.106
Pu 239 Pu 242 Ra 226 Tc 99 U nat Th nat
2.4310 4 3.75310 5 1600 2.1310 5 4.47310 9 1.4310 10
0.08 1.27 0.005 29.1 1000 1000
investigations of liquids with free surfaces (for that the whole diffractometer can be lowered). Fig. 3 gives the experimental diffraction resolution Dd /d obtained on silicon powder. It can be seen that the rise of the energy of the monochromatic beam alters the resolution only moderately. The use of an analyzer crystal in front of the detector allows an increase of resolution as well as the suppression of fluorescence radiation.
4. Experimental results from materials research Profiting from the well-known properties of synchrotron radiation (esp. energy tunability and high brilliance) and the dedicated experimental equipment installed – the large majority of experiments in MRH (FZR research and external users or collaborators) has been a combination of only six main experimental possibilities: • thin film diffraction by grazing incidence / exit techniques (in- / off-plane) • high resolution diffraction (low beam divergence) • diffraction at high temperatures • reflectometry • use of anomalous scattering (energy tunability) • structural studies of liquids with free surfaces (beam deflector) As representative results so far obtained in one and a half years of operation of ROBL some scientific topics will be presented in the following, demonstrating the special advantages of this research laboratory.
4.1. Stress relaxation and precipitation of SiC in Si implanted with C SiC is regarded as a promising semiconductor for high frequency, high power and high temperature applications. One way of producing basic material for circuits is the
N. Schell et al. / Journal of Alloys and Compounds 328 (2001) 105 – 111
107
Fig. 2. Basic experimental equipment in the materials research hutch of ROBL. It consists of a six-circle diffractometer for heavy duty (a) and a deflector (b) which can be translated on a slide (c). Various detector systems and also special sample environments can be installed.
Fig. 3. Experimental diffraction resolution Dd /d for various energies obtained on silicon powder inside a glass capillary (diameter 0.4 mm; wall thickness 0.01 mm). For comparison the value from a single crystal sample is also indicated. Flat Si (111) monochromator crystals were used.
formation of SiC by C ion implantation into Si. To control the growth of SiC a detailed knowledge of the crystallisation process of SiC is necessary. In diffraction experiments for various implantation parameters we investigated the stress relaxation in the Si matrix and the formation of SiC precipitates from precursor stages onwards. The high intensity and low divergence of synchrotron radiation allow the recording of high resolution diffraction patterns from an implanted layer of about 200 nm thickness. Different samples with a large range of implantation parameters (fluences, annealing temperatures) have to be investigated in order to allow the development of a detailed growth model [4]. Implantation of C ions with an energy of 195 keV into Si wafers heated up to 8008C results in an elastic distortion of the Si host lattice and in the formation of crystalline SiC particles or their precursor stages depending on implantation fluence and temperature. Only a Si lattice deformation without growth of SiC was observed if the fluence does not exceed 5310 15 C ions / cm 2 . After implantation of C ions up to 4310 17 / cm 2 at a temperature of 5008C, agglomerations of Si–C and an altered state of Si lattice deformation are found. By implantation of 4310 17 ions / cm 2 at 8008C, particles of the 3C–SiC (b-SiC) phase grow, which are aligned to the Si matrix. Fig. 4 shows as an example
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Fig. 4. Reciprocal space maps near SiC (002) – the matrix reflex Si (004) is located at Q uu 50 nm 21 , Q ' 546.3 nm 21 . The Si wafers are implanted with 4310 17 cm 22 C 1 (195 keV) at 500 and 8008C (left and right part). The iso-intensity lines are in a logarithmic scale (bold lines mark the half-maximum-intensity).
reciprocal space maps near SiC (002) – the matrix reflex Si (004) is located at Q uu 50 nm 21 , Q ' 546.3 nm 21 – giving information about the orientation relation between Si and SiC. It was found that for implantation at 5008C the [001] axis of SiC is aligned to Si [001] with a nearly isotropic spread of 4.58 (Fig. 4, left part). For implantation at 8008C a more complex distribution is found (Fig. 4, right part): the majority of SiC crystallites is aligned in the same manner but with a reduced spread of only 2.58 (symmetric top of the SiC peak). There exists a second group of crystallites with anisotropic distribution around Si [001] with preference into the Si ,111. directions (streaks at the bottom of the SiC peak). At the lowest fluence of 5310 15 C ions / cm 2 the Si lattice expansion due to implantation is caused mainly by C interstitials and Si self-interstitials. Besides this, there are Si lattice regions with reduced spacing ((Dd /d) Si 52 0.001) which may be caused by substitutional C with an average concentration of 1.5310 20 atoms / cm 3 according to Vegard’s rule. The corresponding peak is detected up to the fluence of 5310 16 C ions / cm 2 at 5008C. Finally, crystalline 3C–SiC particles are found for a fluence of 4310 17 C ions / cm 2 at 8008C. These 3C–SiC crystallites are surrounded by an expanded Si lattice ((Dd /d) Si 5 0.0002). The latter may be the consequence of the thermal stress by cooling down the sample from the process temperature to room temperature due to the different coefficients of thermal expansion for Si (3.59310 26 / K) and SiC (4.2–4.68310 26 / K), respectively.
4.2. X-ray reflectivity and diffuse scattering on Co /Cumultilayers near the absorption edges Certain multilayers (MLs), e.g. Co / Cu, show giant magnetoresistance (GMR). This effect is sensitive to layer
thickness, interface roughness, as well as to crystallinity and texture. The study of specular and diffuse reflectivity with synchrotron radiation allows a detailed characterization of the layer and interfacial properties (roughness, correlation lengths) necessary to optimize the preparation processes and to improve the device parameters wanted. Especially, the tunability of wavelength at a synchrotron X-ray source allows to enhance the scattering contrast of Co and Cu for resonant X-ray reflectivity measurements at the K-edges of the two elements as can be seen in Fig. 5. Two types of MLs prepared by crossed beam pulsed laser deposition with different layer thicknesses of Co and Cu layers on Si substrates – Si / SiO 2 / 83[Co (4 nm) / Cu (4 nm)] and Si / SiO 2 / 43[Co (8 nm) / Cu (8 nm)] – were investigated at ROBL with energies near the Co-edge (7.71
Fig. 5. Example of contrast variation for specular reflectivity by wavelength tuning for a Co / Cu–ML of composition Si / SiO 2 (550 nm) / 83[Co (4 nm) / Cu (4 nm)] fabricated by crossed beam pulsed laser deposition (CB PLD). The chosen energies are: K-edge of Co (7.708 keV), K-edge of Cu (8.974 keV) and the Cu Ka-line (8.048 keV).
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keV) and Cu-edge (8.97 keV), respectively. The samples were studied in the ‘as-deposited’ state and after annealing (5008C, 2 h). From the specular reflectivity and the distribution of the diffusely scattered intensity a characterisation of the roughnesses of the surfaces and interfaces could be given by simulations based on Sinha’s representation of the height–height correlation function. The lateral correlation length j and the Hurst parameter h have been determined and the roughness correlation between the layers could be quantified. The results are nearly the same for both types of MLs. In the ‘as-deposited’ state the roughness is dominated by a high morphological jaggedness, represented by a high fractal dimension (D532h) of about 2.75. After annealing, due to thermally induced grain growth, this ‘short-scale roughness’ is reduced (D52.5). However, the correlation length which is a lateral measure for the biggest morphologicial feature determining the roughness value is drastically shortened from |4000 to |50 nm. This is accompanied by a reduction of the roughness conformity, expressed by the ratio scorr /srms , from 40 to 10%.
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Fig. 6. In situ diffraction of Ti–6Al–4V under incidence angle 48 (E5 8.048 keV) after ion beam implantation with nitrogen up to a fluence of 6310 17 cm 22 . Clearly discernible is the continuous transformation of TiN into Ti 2 N within appr. 100 K around 7008C.
expansion were observed. The rearrangement in the base alloy material is a second mechanism for the hardness degradation at higher temperatures.
4.3. Phase transformations studied in situ during annealing of Ti– 6 Al– 4 V implanted with nitrogen
4.4. Local internal strain and stress in ultra fine grained Ni due to cyclic plastic deformation
Ti–6Al–4V alloys are commonly used industrial materials especially for their low weight. To enhance surface hardness, nitridation by ion implantation is often used. with a fluence of 6310 17 N ions / cm 2 a hardness increase by a factor of three can be achieved. The hardness gain, however, is partially lost at higher temperatures, limiting the application of Ti–6Al–4V. After annealing at 7008C for 1 h there remains a hardness increase by a factor of 2.3 compared to the non-implanted sample. To understand the underlying structural mechanisms it is therefore advantageous to follow the structural changes during annealing with in situ diffraction experiments [5]. The use of synchrotron radiation for phase determination and depth distribution is necessary since one deals with a very thin implanted layer (typically 200 nm) and the phase transformations occur in time scales of several minutes. Fig. 6 shows a temperature series of grazing incidence diffraction patterns. The process is the solution of TiN crystallites and the formation of the new phase Ti 2 N. The transformation between both phases is direct, there is no intermediate phase. From the study of the distribution of N atoms in the subsurface region of the alloy by elastic recoil detection analysis (ERDA) nitrogen concentrations below the stoichiometry of TiN and Ti 2 N, respectively, are found. In comparison with the depth distribution of both phases (recorded by grazing incidence XRD when changing the angle of incidence) it can be concluded that the nitride phases form grains in the alloy matrix. Therefore, precipitation hardening will be the dominant mechanism. Additionally, changes in the lattice constants of both alloy phases (a-Ti and b-Ti) not conforming with thermal
This example is from collaboration with the Technical University of Dresden. The deformation behavior of ultra fine grained (ufg) nickel with a typical grain size of 200 nm yielding special mechanical properties was studied depending on grain size, dislocation density and deformation parameters like pre-deformation or annealing temperature [6,7]. The technique consists of a separation of grain size and strain contributions to the broadening of Bragg reflections. For the construction of the so-called Williamson–Hall plot several Bragg reflections have to be measured with very high instrumental resolution. The low divergence of the synchrotron beam allows a resolution rendering a deconvolution of the measured line shapes unnecessary. At the same time the high flux allows a tremendous amount of samples to be investigated in a reasonable amount of time (about 35 samples within 12 to 15 shifts, each to be aligned for six or seven Bragg reflexes for a Williamson–Hall plot). A direct comparison between a laboratory X-ray source and ROBL for the same sample shows the appr. three orders of magnitude higher flux gain at ROBL for comparable slit and distance dimensions. For the highly crystal-collimated laboratory source a position sensitive detector was used while at ROBL a scintillator detector was scanned with the corresponding angular resolution [8]. Ufg Ni samples ‘as-prepared’ and after cyclic plastic deformation at room temperature and at 2008C were studied. The grain structure was observed using the orientation contrast in a scanning electron microscope. No significant changes of the grain size distribution were detected after the cyclic deformation. Measurements of
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X-ray diffraction profiles were performed at ROBL applying monochromatic synchrotron radiation with energy of 8.05 keV. The instrumental broadening was negligible in comparison to the strain induced broadening of the profiles measured at the ufg samples. To estimate the spectrum of internal strains, diffraction profiles were measured for six different hhklj-types of lattice planes parallel to the sample surface. From the changes in the shape and asymmetry of the profiles one can conclude that the width of the strain spectrum is reduced by the cyclic plastic deformation especially for higher temperatures though no recrystallisation occurs. The rms strain reduces from 1.5310 23 to 0.3310 23 . Taking into account the variation of the profile shape with the hhklj-type of reflection, long-range granular stresses should be present. With increasing deformation temperature the long-range stresses decrease as can be deduced from the increasing ratio of integral breadth to FWHM of the Bragg peaks. The mean dislocation density, calculated on the basis of the Krivoglaz–Wilkens theory, was found to be correlated with the deformation state of the samples.
4.5. Characterization of waveguides In a collaboration with the LMU Munich and the ILL Grenoble hard X-ray waveguides with single- and multimode guiding layers of Ni / C–ML-structure on floatglass substrates were characterized. These experiments did profit from the high precision of sample positioning and beam definition, though it meant a waste in intensity for a bending magnet source (beam size 0.130.1 mm 2 at 20 keV). Starting with single guiding layers to test design parameters and demonstrate the principle of resonant mode coupling by shining a parallel beam at grazing incidence onto the waveguide [9], limitations were encountered (shape of modes and angular acceptance of typically only several thousands of a degree) which limit coupling efficiencies for more divergent X-ray sources. Those restrictions could be overcome by using multiple guiding layers [10]. Essentially in high-precision mappings of incidence / exit-angles the farfield interference patterns were measured and confirmed simulations of internal and external standing electromagnetic fields as a function of the structural and geometric parameters (layer thickness, composition, density, interface roughness, angles of incidence, X-ray energy). By mode splitting into submodes (leading to a limiting band-like-behaviour for an infinite number of layers), an increase of ‘allowed’ incidence angles of appr. 0.038 (FWHM in the vertical of the interference maxima of the farfield behind the waveguide) could be reached for a sample with composition (73[Ni (2.5 nm) / C (45.7 nm)] / Ni (20.0 nm) / floatglass substrate) with an additional 1.4 nm oxide top layer (general srms |1 nm). By measuring the farfield interference patterns the
possibility was demonstrated that carefully designed waveguides can be used as tools to control the near- and farfield distributions, optimize angular acceptance and thus efficiencies for the production of nm X-ray beams.
4.6. Structure refinement of SiC polytypes Although the diffractometer was not specifically designed for single crystal studies, the high setting accuracy of the axes and the sample table as well as the versatility of the six-circle-design allow such studies for distinct problems. A first single crystal experiment was devoted to the structure refinement of the polytypes 4H and 6H of SiC crystals by a group from the University of Jena. The atomic positions show small deviations from the ideal tetrahedron. A systematic investigation of so-called ‘quasiforbidden’ reflections allows to determine these atomic relaxations since those reflections are extremely sensitive to small variations in the structure. There remains an ambiguity because the distinction between different possible structure models (from space group symmetry) calls for phase information. This additional necessary phase information was gained by measured ‘Renninger’scan (C-scan) profiles in the vicinity of three-beam cases. These profiles were compared with theoretically calculated profiles for various possible models. In the end an unambiguous determination of the structure refinement parameters was possible [11].
5. Conclusions The above overview clearly demonstrates the versatility of the materials research experimental station of the Rossendorf Beamline ROBL at the ESRF. It combines the reliable and stable general X-ray source characteristics of high flux, low divergence, energy tunability in a wide range of hard X-rays with an extremely free and precise sample positioning, enhanced by special sample environments. Methodically, a wide range of X-ray techniques is possible, e.g. HRXRD, GIXD, reflectometry. For in-house research of the Forschungszentrum Rossendorf the main fields are thin films and buried layers (including multilayers) with correspondingly suitable X-ray techniques. Via FZR-collaborations, ESRF-proposals or direct ECsupported access to research infrastructures ROBL is also accessible to outside users.
Acknowledgements We would like to thank M. Hecker from the IFW Dresden for making available unpublished data as well as A. Bauer from the University of Jena for leaving us with preprint-data concerning his single-crystal works.
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References [1] W. Matz, N. Schell, G. Bernhard, F. Prokert, T. Reich, J. Claubner et al., J. Syn. Rad. 6 (1999) 1076. [2] See under http: / / www.fz-rossendorf.de / FWE; see under: EC supported access. [3] T. Reich, G. Bernhard, G. Geipel, H. Funke, C. Hennig, A. Rossberg, W. Matz, N. Schell, H. Nitsche, Acta Radiochim. 88 (2000) 633. ¨ [4] F. Eichhorn, N. Schell, W. Matz, R. Kogler, J. Appl. Phys. 86 (1999) 4184. ¨ [5] F. Berberich, W. Matz, E. Richter, N. Schell, U. Kreibig, W. Moller, Surf. Coat. Tech. 128–129 (2000) 450. [6] E. Thiele, M. Hecker, N. Schell, Mat. Sci. Forum 321–324 (2000) 598.
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[7] E. Thiele, B. Bretschneider, L. Hollang, N. Schell, C. Holste, in: T.C. Lowe, R.Z. Valiev (Eds.), Proc. Nato Adv. Res. Workshop: Invest. & Appl. of Severe Plastic Deformation, Kluwer, 2000, p. 173. ¨ Dresden, Institut [8] M. Hecker, Internal Report, Technische Universitat ¨ Physikalische Metallkunde, 1998, private communication. fur ¨ [9] F. Pfeiffer, Diploma Thesis, Ludwig-Maximilians-Universitat ¨ Munchen, 1999. [10] F. Pfeiffer, T. Salditt, P. Høghøj, I. Anderson, N. Schell, Phys. Rev. Lett. B 62 (2000) 16939. ¨ [11] A. Bauer, Ph. Reischauer, J. Krausslich, N. Schell, W. Matz, K. Goetz, Acta Cryst. A 57 (2001) 60.