Epitaxial Growth and Characterization of Mo/NbN and W/NbN Superlattices

Anita Madan, Xi Chu and Scott Barnett
Department of Materials Science and Engineering
Northwestern University
Evanston., IL  60208

Mo/NbN and W/NbN superlattices with modulation wavelengths from 1.4 nm
to 25 nm were grown on MgO(001) substrates by reactive d.c. magnetron
sputtering. These superlattices represent a new class of
non-isostructural superlattice in which a body-centred cubic metal is
combined with a B1-structure nitride. Epitaxy is achieved because of the
small lattice mismatch(1.4% for Mo/NbN and 2.0% for W/NbN) between the
(001) planes after a 45 degree rotation about the normal. The epitaxial
relationship Mo(001)||NbN(001), Mo[110]||NbN[100] was verified by pole
figure scans and selected area electron diffraction patterns. Auger
scans showed little nitrogen in the metal layers, despite sputtering in
a nitrogen-containing ambient because of the 800C growth temperature.
High angle x-ray diffraction results show up to 25 satellite reflections
extending over a wide range ~ 30 degrees of 2theta values because of the
large difference in out-of plane lattice spacings. Computer
simulations(based on a kinematical model and a trapezoidal composition
modulation) of the high angle XRD data suggest that the interface 
widths were ~ 0.3 nm, presumably because of the immiscibility of the
metal and NbN. Low angle XRD patterns show up to 16 peaks (for~16 nm)
for the W/NbN superlattices and dynamical theory simulations estimate
the roughness to be ~ 0.2 nm. Cross-sectional transmission electron
microscope images of Mo/NbN superlattice show well-defined and
relatively planar layers.