Diffuse scattering in multilyers: a tool for the study of interfaces 
defects and disordered alloys

Helene Fischer*, Henry Fischer#, Michel Bessiere#,, Michel Piecuch*

*LMPSM, Univ. Nancy I, B.P. 239, 54506 Vandoeuvre-les-Nancy Cedex, France
#LURE (CNRS/CEA/MESR), Bat. 209d, Univ. Paris.Sud, 91405 Orsay Cedex, France

Diffuse scattering of X rays is a particularly useful tool for studying
interface and surface defects in single layer films.  We have extended this
technique to the study of metallic multilayers.  X-ray diffraction
measurements were carried out using a 4-circle diffractometer high resolution
goniometer at the D23A anomalous dispersion beamline at the DCI synchrotron
(LURE, Orsay).  The use of both specular and off-specular measurements has
provided probes of the samples structure in the vertical (growth) direction,
and in the horizontal (parallel to interfaces) direction.

Small angle specular simulations:

To perform the small angle specular simulations, we employed the matrix method
technique of Vidal and Vincent [1] for calculating the specular electric
fields at each interface of the sample.  The diffuse scattering contribution
at the specular condition was first removed from the data by subtracting the
diffuse longitudinal scan having omega equal to 2theta/2 + d(theta), where
d(theta) is small compared to theta.  The advantages of high intensity and
anomalous dispersion available with synchrotron radiation have greatly
contributed to the quality of our data, for which the use of this simulation
program has allowed a robust and precise extraction of several structural
parameters, such as layers thicknesses and interfacial thicknesses "sigma".

Small angle off specular simulations:

To simulate the small-angles off-specular scans, we use the specular electric
fields calculated at each interface of the samples, and then we apply the
theoretical results of Daillant and Belorgey [2] for calculating the diffused
scatterred intensity from these electric fields.  The two additional
structural parameters that can be extracted from these off-specular scans are
the horizontal and the vertical correlation length of the interfacial
roughnesses, ksi(x) and ksi(z) respectively.  For za(x) and zb(x) being the
vertical displacements (about average altitudes of z(0,a) and z(0,b) of two
interfaces in the sample along the x direction, we supposed a gaussian profile
of interfaces and defined our interface correlation function where
(za(x)zb(0)) is equal to 
sigma(z,a)sigma(z,b).exp-(x/ksi(x))(x/ksi(x)).exp-((z(0,a)-z(0,b))/ksi(z))((
z(0,a)-z(0,b))/ksi(z)) [3] [4].  The parameter ksi(x) serves to distinguish
interfacial roughness (ksi(x) bigger than arround 1.5nm) from interfacial
diffusion (ksi(x) smaller than arround 1.5nm) [5].

Results:

The samples are Mn/Ir(111) superlattices where Mn is pseudomorphic to Ir.
Using a M.B.E.  chamber in the 10-10 vaccum range, we have grown three typical
samples very similar with respect to period, relative thicknesses of Mn and
Ir, and number of bilayers.  The only parameter very different from one sample
to the another one is the growth temperature :  either room temperature, 373K
or 573K.  The small angle measurements and simulations show clearly that the
interfaces of the superlattice grown at RT are poorly correlated (ksi(z) is
close to 15nm) and rough (sigma varies from 0.7 to 1.1nm from the bottom to
the top of the sample, ksi(x) is close to 20nm), and that the superlattice
grown at 573K has rather well correlated (ksi(z) is close to 20nm) and
interdiffused (sigma varies from 1.7nm to 1.3nm from the bottom to the top of
the sample, ksi(x) is close to 2nm) interfaces.  The superlattice grown at
373K presents intermediate structural properties, with less disorder (sigma
varies from 0.3 to 0.8nm from the bottom to the top of the sample, ksi(x) is
close to 5nm, ksi(z) is close to 15nm).  All these structural results have
been confirmed by transmission electron microscopy.  Moreover, we show that
large length-scale interfacial roughness is mainly due to the formation of
terraces during growth at low deposition temperature, whereas small
lengh-scale interfacial roughness occurs preferably at high deposition
temperature and is mainly due to an atomic interdiffusion (i.e.  formation of
an interface alloy) which manages to maintain a high degree of
crystallographic order [6].

        At the present time, we want to extend our technique to the study
of disordered alloys. Then, we have produced pseudomorphic
=46e(x)Mn(1-x)/Ir(100) superlattices having different stoichiometries (x
varies from 0.8 to 0.3) [7]. The alloy crystalline structure is body
centered tetragonal with a c/a ratio between 1.18 and 1.26. Our goal is to
describe how a disordered alloy in the layers modify the specular and the
off-specular intensity. We will also use the anomalous signal available on
synchrotron radiation to distinguish Mn and Fe disorder.

References : [1] B. Vidal and P. Vincent, Appl. Opt. 23 (1988) 2297.
[2] J. Daillant and O. Belorgey, J. Chem. Phys. 97 (1994) 10668.
[3] S.K. Sinha, E.B. Sirota, S. Garoff and H.B. Stanley, Phys. Rev. B 38
(1988) 2297.
[4] S.K. Sinha, J. Phys. III France 4 (1994) 1543.
[5] Henry. Fischer, Helene Fischer, O. Durand, O. Pellegrino, M. Piecuch,
S. Lefebvre, M. Bessiere, Nuclear Instruments and Methods B97, 402, (1995).
[6] Thesis of Helene Fischer, Universite Henri Poincare de Nancy, France,
(1995).
[7] Helene Fischer, S. Andrieu, P. Bauer and M. Piecuch, M.R.S. Proc. 384,
(1995).