Fully strained epitaxial Ti1−xMgxN(001) layers

Abstract

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 N2 at 600 °C. X-ray diffraction ω-2θ scans, ω-rocking curves, φ-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║(001)MgO and [100]TiMgN║[100]MgO. Layers with thickness d = 35–58 nm are fully strained, with an in-plane lattice parameter a|| = 4.212 ±0.001 Å matching that of the MgO substrate, while the out-of-plane lattice parameter a⊥ increases with x from 4.251 Å for TiN(001) to 4.289 Å for Ti0.51Mg0.49N(001). This yields a relaxed lattice parameter for Ti1−xMgxN(001) of ao = (1-x)aTiN + xaMgN – bx(1-x), where aTiN = 4.239 Å, aMgN = 4.345 Å, and the bowing parameter b = 0.113 Å. 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 in-plane 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.