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A REFLECTIVITY AND EXAFS STUDY OF LAYERED STRUCTURES
D. Jiang, N. Alberding, A. Seary, E. Crozier
To cite this version:
D. Jiang, N. Alberding, A. Seary, E. Crozier. A REFLECTIVITY AND EXAFS STUDY OF LAYERED STRUCTURES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-861-C8-864.
�10.1051/jphyscol:19868166�. �jpa-00226070�
JOURNAL D E PHYSIQUE
Colloque C 8 , suppl6ment a u n o 12, Tome 47, decembre 1986
A REFLECTIVITY AND EXAFS STUDY OF LAYERED STRUCTURES
D. JIANG, N. ALBERDING, A.J. SEARY and E.D. CROZIER Physics Department, Simon Fraser University,
Burnaby, B.C. V 5 A 1S6, Canada
Abstract.- By combining the total reflection of x-rays incident on a sample at grazing angles and fluorescent detection of the EXAFS signal, it is pos- sible, in principle, to examine the interfacial and bulk regions of layered structures. In this paper we first describe a simple sample positioner that permits a rapid, precise alignment of the sample. It is driven by stepper motors controlled by an microcomputer and permits an angular step of 0.07 mrad. A reflectivity curve can be obtained with it in less than one minute. Using this apparatus good agreement is obtained between experiment and theory for the angular dependence of the reflectivity of a multilayer film of Si/Cu/Au deposited on fused quartz. We also report structural re- sults as a function of depth obtained via fluorescent reflection EXAFS for Ni epitaxially grown on the (100) face of a Fe single crystal.
Introduction
The angular dependence of the reflected x-ray intensity in the region of total reflection provides information regarding the surface transition region in solids and liquids [1,2]. At angles near the critical angle the x-ray wave is confined to the interfacial region. Consequently the reflected and fluorescent x-rays provide EXAFS spectra with a high signal-to-noise [3,4]. Such spectra permit the rapid, in-situ measurement of surfaces and interfacial regions of solids or liquids. In this paper we report a portable, but accurate, sample positioner which can be readily used at synchrotron radiation sources for polarization- dependent measurements of the reflectivity, reflection EXAFS and fluorescent reflection EXAFS. Two studies of layered films demonstrate the potential of structural analysis as a function of depth.
Experimental
The sample positioner is indicated schematically in Fig.1. Single crystal samples are first mounted on a small rotation stage which is spun at 40 hz to reduce the magnitude of Bragg diffraction peaks which otherwise contaminate the EXAFS spectra. Other samples such as amorphous films or liquids are mounted directly on the platform PI an aluminum plate 30 cm long x 12 cm wide. The platform P is supported at each end by micrometers driven by stepper motors 1 and 2. A NEC microcomputer, model PC-8201A, enables the platform to be raised and lowered in steps of 1.7~ or tilted at a grazing angle 0 relative to the incident beam in angular steps of 0.07 mrad. The platform is prevented from tipping in a direction perpendicular to the plane of the figure by four rectangular bars, not shown, located at the sides of the platform and along which it glides.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868166
JOURNAL DE PHYSIQUE
Figure 1.
The sample positioner illustrating simultaneous measurement of reflectivity and fluorescent reflection EXAFS. The sample, which can be rotated at 40 hz, is mounted on the platform P. Motors 1, 2 and
S1
3 are stepper motors. Ion chambers 10, If and I, record the intensities of the incident, fluorescent and reflected x-rays. S1 is a Huber slit and S2 is a movable
straight-edged beam stop. 6 is the angle of grazing incidence.
Alignment of the sample is achieved quickly and accurately. With the sample lowered out of the x-ray beam, the.straight-edge S2 is raised in increments of lu with stepper motor 3 until the uppermost ray of the beam emerging from the slit S1 is blocked and does not enter the ion chamber I, . Using an iterative procedure the sample is then raised, lowered and tilted relative to the beam until its reflecting surface is parallel to the uppermost ray of the beam. The attainment of this condition is readily noted: if it exists , then when the sample is raised into the beam, without tilting, no signal is detected in 1,. If we neglect the divergence of the incident beam and assume that the surface of the sample is flat and coplanar with S1 and S2,then the zero angle of the surface relative to the beam direction can be located to within + 0.07 mrad.
The design of the positioner permits it to be aligned with the electric vector of the incident synchrotron radiation beam parallel or perpendicular to the surface of the sample. Also by inverting the platform P, the apparatus can position a x-ray mirror, reflecting the beam downwards instead of upwards.
This is useful when studying the free surface of liquids.
Reflectivity ,reflection and fluorescent reflection EXAFS measurements were made on two samples at room tmeperature on beamline IV-1 at the Stanford Synchrotron Radiation Laboratory. A multilayer film, 150 B of Si, 230 B of Cu and 300 A of Au deposited on an optical silica window, was prepared using an electron gun in an ambient pressure of 10-6 torr. The film thicknesses were determined during deposition with a piezoelectric crystal monitor and are known to t 5 %. A film of Ni, 50 B to 100 B thick was grown epitaxially on the (100) face of a single crystal of Fe ( B.Heinrich, unpublished) in an ambient pressure of 10-10 torr. This sample was transferred to a cylindrical plastic cell and maintained under nitrogen for the duration of the x-ray measurements.
Results
The angular dependence of the reflectivity of the Si/Cu/Au/silica multilayer is shown in Fig. 2 for the x-ray energy 11918 ev. The agreement between theory and experiment is remarkably good for angles greater than 2 mrad. At lower angles the reflectivity of the sample is reduced by surface roughness, finite sample and finite beam height. The reflectivity was calculated using the formulae derived by Parratt [l] for multilayer films on an infinitely thick substrate with each layer assumed to be homogeneous with the thickness given by experiment and specified by a complex dielectric constant. The latter was calculated from the bulk density and the anomalous scattering factors f' and f" calculated from first principles by Cromer [ 5 ] . In overlaying the data on the theoretical curve, the data were scaled by a factor of 0.95 and the zero angle position was shifted by 0.26 mrad. The decrease in the reflectivity at 3 mrad indicates the onset of penetration into the Cu layer. Penetration into the Au layer begins at 6.0 mrad. Interference in the Cu film produces the dip at 5.1 mrad. This interference effect is sensitive to the combined thickness of the Si and Cu layers. A change of 20 B in the Si thickness enhances the peaks at 4.3 and 5.3 mrad.
C8-863
Reflectivity
Figure 2.
Comparison of the angular dependence of the experimental reflectivity (x) of a Si/Cu/Au/silica glass
multilayer with theory (-).
Our second example is Ni on the (100) face of single crystal Fe. The
fluorescent reflection EXAFS interference function, acquired in 30 minutes at SSRL, is shown in Fig.3. The angle of incidence was less than the
experimentally determined critical angle so that the detected fluorescent rays emanated from the top 15 A of the sample. Examination of the magnitude of the fourier transform reveals that the film is a mixture of fee Ni and NiO.
Studies at larger angles of incidence indicated that with increasing depth the percentage of Ni increased, but its structure remained fee. When the sample was examined in the molecular beam epitaxy facility in which it was prepared, RHEED indicated the Ni to have the symmetry of bee. RHEED, which is restricted to the surface, could not determine if there was any tetragonal distortion.
Evidently oxidation of the Ni film occured before the EXAFS measurements were completed. In the study of the Si/Cu/Au/silca sample, high signal/noise EXAFS spectra were obtained for the Cu K-edge and a noisier signal was obtained at the Au Ll:lI-edge. In our next experiments, the Ni will be covered with an epitaxially deposited passive film of low atomic number to prevent oxidation of the Ni, and yet permit determination of its structure.
Figure 3. The EXAFS interference function and its Fourier transform for Ni epitaxially grown on the (100) surface of Fe. The Fourier transform of kX(k> was taken from 2.5 to 15 &-1 using a 10% Gaussian window. The solid vertical lines indicate the contributions from fee Ni, and the dashed lines the contributions from NiO.
R ( Angstroms )
C8-864 JOURNAL DE PHYSIQUE
Acknowledgements
The authors wish to acknowledge grants received from the National Sciences and Engineering Research Council, Canada. The experimental work was done at the Stanford Synchrotron Radiation Laboratory which is supported by the Department of Energy, Office of Basic Energy Sciences; and National Institute of Health, Biotechnology Research Program, Division of Research Resources.
References
1. Parratt,L.G., Phys. Rev. 95 (1954) 359.
2. Bosio,L., Cortes,R., Folcher,G., and Oumezine,M., Revue Phys. Appl. 20 (1985) 437.
3. Martens,G. and Rabe,P., J. Phys. C: Solid State Phys.,l4 (1981) 1523.
4. Bunker,B.A., Heald,S.M. and Tranquada,J., in Hcdgson,K.O., H&an,B., and Penner-Hahn,J.E., Ed., EXAFS and Near Edge Structure 111, Springer Proc. in Physics, 2 (1984) 482.
5. Cromer,D.T., J. Appl. Cryst. 16 (1983) 437.
