X-ray diffraction characterization of mo[m]/v[n] superlattices with
ultrathin layers (m+n<~ 4 monolayers) prepared by sequential sputter
deposition

Jens Birch# and Jan-Eric Sundgren

Thin Film Physics Division, Department of Physics, 
Linkoping University, S-581 83 Linkoping, Sweden

# Present adress: ESRF, B.P. 220, F-38 043 Grenoble, France. e-mail:
birch@esrf.fr


Artificial superlattices with sub-unit cell layer thicknesses have
attracted an increasingly amount ofinterest during the last years. For
example, in the III-V semiconductor systems such layers can beproduced
routinely with the goal of spatially confining electrons in 1, 2, or 3
dimensional quantumwells. Also in metallic systems there is an
interest in realizing such structures. For example, 3d bcc transition
metals have shown to have strongly enhanced ferromagnetic moments when
the dimensionality  is reduced from 3 to 2.  In particular, a
monolayer of V(001) is predicted to be magnetic if it is prepared on
Ag(001) or Au(001) while the surface layer of a bulk V crystal is
non-magnetic.  In this work, we have used Mo/V(001) to show that it is
indeed possible to prepare metallic superlattices with layer
thicknesses down to sub-monolayer dimensions even though the phase
diagram predicts a solid solution over the entire composition range
for this material system.  Single crystal Mo/V(001) superlattices were
grown on MgO(001) substrates held at 700C by sequential dc magnetron
sputtering using Ar as the sputtering gas.  Four Mo[m]/V[n] samples
wereprepared: Mo[0.6]/V[1.5], Mo[1]/V[1], Mo[1.1]/V[3.0], and
Mo[2]/V[2] where m and n are the nominal number ofmonolayers in each
layer of Mo and V respectively.  All samples were investigated by
x-ray diffraction (XRD) using low resolution reciprocal space mapping
(RSM) as well as conventional Bragg-Brentano geometry producing
specular theta-2theta scans with medium resolution.  Superlattice
reflections in the theta-2theta scans confirmed the chemical
modulations in the growth direction in the Mo[0.6]/V[1.5],
Mo[1.1]/V[3.0], and Mo[2]/V[2] superlattices while for the Mo[1]/V[1]
sample no signs of compositional modulation were observed.  RSM did
not give any indications of any long range ordering of the Mo and V
atoms in the plane of the layers.  The Mo[0.6]/V[1.5] superlattice
reflections,associated with two neighboring 00l  Bragg-peaks did not
coincide, instead two narrow peaks appeared close to each other.  This
is due to the incommensurability between the superlattice period and
the MoV bcc sublattice.  Moreover, this splitting shows that the
growth mode is 2 dimensional layer-by-layer and that the interfacial
roughness is controlled (by the ability of accurately syncronizing the
shutters with the growth of complete monolayers) rather than random.
The Mo[0.6]/V[1.5] superlattice is thus equivalent to an artificially
layered alloy in the [001] direction where each second monolayer
consists of V and each second monolayer is a 2 dimensional mixture of
Mo and V.  The lattice parameters determined by RSM were consistent
with complete accomodation of the misfit strain in the the layers. 
This was confirmed by cross-sectional HREM images from the Mo[1]/V[1]
and Mo[2]/V[2] superlattices which showed that the films were of a
very good crystalline quality.  A few threading dislocations were
present while no misfit dislocations could bee seen. The Mo[2]/V[2]
superlattice exhibited clear Z-contrast from the layers of Mo and Vand
selected area electron diffraction gave strong superlattice
reflections, which is in agreement with the XRD results.