Fully strained epitaxial Ti1-xMgxN(001) layers

Ti1-xMgxN(001) layers with 0.00 <= x <= 0.49 are deposited on MgO(001) by reactive magnetron co-sputtering from titanium and magnesium targets in 5 mTorr pure N-2 at 600 degrees C. X-ray diffraction omega-2 theta scans, omega-rocking curves, phi-scans, and high resolution reciprocal space maps show that the Ti1-xMgxN layers are rock-salt structure single crystals with a cube-on-cube epitaxial relationship with the substrates: (001)(TiMgN) parallel to(001)(MgO) and [100](TiMgN) parallel to[100](MgO). Layers with thickness d= 35-58 nm are fully strained, with an in-plane lattice parameter a(parallel to)= 4.212 +/- 0.001 angstrom matching that of the MgO substrate, while the out-of-plane lattice parameter a(perpendicular to) increases with x from 4.251 angstrom for TiN(001) to 4.289 angstrom for Ti0.51Mg0.49N(001). This yields a relaxed lattice parameter for Ti1-xMgxN(001) of a(o)=(1-x)(aTiN) + xa(MgN) - bx(1-x), where a(TiN)= 4.239 angstrom, a(MgN)= 4.345 angstrom, and the bowing parameter b= 0.113 angstrom. In contrast, thicker Ti1-xMgxN(001) layers with d= 110-275 nm are partially (pure TiN) or fully (x= 0.37 and 0.49) relaxed, indicating a critical thickness for relaxation of 50-100 nm. The inplane x-ray coherence length is large (100-400 nm) for fully strained layers with 0.00 <= x <= 0.45 but drops by an order of magnitude for x= 0.49 as the composition approaches the phase stability limit. It is also an order of magnitude smaller for thicker (d >= 110 nm) layers, which is attributed to strain relaxation through the nucleation and growth of misfit dislocations facilitated by glide of threading dislocations.