Growth and physical properties of epitaxial metastable Hf1-xAlxN alloys deposited on MgO(001) by ultrahigh vacuum reactive magnetron sputtering

Epitaxial metastable Hf1-xAlxN alloys with 0 <= x <= 0.50 were grown on MgO(001) substrates at 600 degrees C by ultrahigh vacuum reactive magnetron sputtering from Hf and Al targets in 90% Ar+ 10%N-2 discharges at 7 mTorr. X-Ray diffraction and cross-sectional transmission electron microscopy show that Hf1-xAlxN alloys are single crystals with the Bl-NaCl structure. Rutherford backscattering spectroscopy investigations reveal that all films are slightly overstochiometric with N/(Hf+Al)= 1.05 +/- 0.05. The relaxed lattice parameter decreased linearly from 0.4519 nm with x=0 to 0.4438 nm with x=0.50, compared to 0.4320 nm expected from the linear Vegard's rule. We find a metastable single phase field that is remarkably broad given the large lattice mismatch (similar or equal to 9%) between the two alloy components. Alloying HfN with AIN leads to an increase in hardness (similar or equal to 30% to 32.4 +/- 0.7 GPa), as well as nanostructured compositional modulations due to the onset of spinodal decomposition. (C) 2007 Elsevier B.V. All rights reserved.