Stabilization of cubic AlN in epitaxial TiN/AlN superlattices

Ilwon Kim, Anita Madan, Scott Barnett, Shang-Cong Cheung, and Vinayak Dravid 

Department of Materials Science and Engineering 
Northwestern University. 

There have been predictions of a high-pressure cubic (NaCl/B1 structure)
phase (a_AlN ~ 0.405 nm)of AIN, which at ambient pressures crystallizes
in a hexagonal (wurtzite) phase with a=0.3108 nm and c/a=1.601.
Epitaxial TiN/B1 AlN superlattices with the modulation wavelengths 1.8
nm to 8 nm were grown on MgO(001)by D.C. reactive magnetron sputtering.
AlN grew in the B1 structure at layer thicknesses <= 2.0 nm. For larger
layer thicknesses, the AlN grows in the wurtzite structure and epitaxy
is lost. Satellite reflection (up to 6) were observed in high angle XRD
patterns. Dynamical theory (Paratt's formalism) was used to simulate the
low angle XRD patterns and to estimate the layer thickness ratio and the
modulation wavelength. These parameters were then input in a simulation
program (based on a kinematical model and a trapezoidal composition
modulation) used to fit the high angle XRD pattern. The lattice spacing
of B1 AIN was determined to be 0.40 +/- 0.002 nm. Random lattice spacing
fluctuations(Gaussian width ~ 0.002 nm) and layer thickness variations
(~ 0.1 nm) were incorporated to match the  observed broadening of the
superlattice peaks. The interface widths were estimated to be ~ 0.1 nm.
XRD simulations also ruled  out the possibility that other Ti-Al-N
phases e.g. Ti_(1-x)Al_xN, TiAl_3N or zinc-blende AlN formed during
growth. Cross-sectional transmission electron microscopy showed that the
layers were well-defined and planar. Selected area electron diffraction
patterns and HREM confirmed that the AlN was in a cubic phase.