Paper TF-WeM2
Morphology Evolution during Growth of Epitaxial Ti_1-xAl_xN and Cr_1-xAl_xN Films onto MgO(100) and MgO(111)

Wednesday, October 17, 2007, 8:20 am, Room 613/614

Metastable Ti_1-xAl_xN has found widespread industrial use as hard coating for cutting and forming applications, while the knowledge base for Cr_1-xAl_xN on structure-property relations still evolves. Here, we report on growth studies of epitaxial Ti_1-xAl_xN and Cr_1-xAl_xN films deposited onto MgO(100) and MgO(111) substrates by reactive magnetron sputter epitaxy. Both orientations promote cube-on-cube epitaxial growth. However, transmission electron microscopy reveals a smooth single-crystal morphology for the MgO(100) and a facetted columnar morphology for the MgO(111) substrate. This can be ascribed to the highly anisotropic step energies for the corresponding nitride growth surfaces, resulting in a switch between layer-by-layer and island growth mode as characterized by in-situ x-ray diffraction experiments. High-resolution x-ray reciprocal space maps display that films deposited onto MgO(111) grow fully relaxed with lattice parameters that correspond well to literature values at the given compositions. The growth mode on MgO(100) substrates depends on the Al fraction. Ti_1-xAl_xN films at low Al fractions that are well lattice-matched to MgO show pseudomorphic strained growth with a small percentage of in-plane strain relaxation due to interfacial misfit dislocations, and exhibit almost defect-free single-crystal morphology. On the contrary, less lattice-matched (Ti,Cr)_1-xAl_xN films with x close to the AlN precipitation threshold show an initial pseudomorphic strained layer that relaxes for increasing film thickness. The relaxation starts with interfacial misfit dislocations that gradually evolve into a dislocation network along the {111}<110> slip system. For Cr_1-xAl_xN films with an Al fraction of 0.6, this dislocation network is superimposed by crystals of first-order twins about [111] with the orientation relationship Cr_1-xAl_xN (122) // MgO (100). The twins overgrow the primary (100) orientated film, likely due to the angular vicinity of fast growing (111) planes. These diverse relaxation mechanisms might be attributed to changed stacking fault energies for different Al fractions. Since polycrystalline Ti_1-xAl_xN and Cr_1-xAl_xN show AlN precipitation at grain boundaries during annealing, the observed morphology changes for different substrate orientation and stoichiometries have impact for the understanding of age hardening in these systems.