Fully strained epitaxial Ti_{1-x}Mg_xN(001) layers

TL;DR

This study investigates the epitaxial growth, structural properties, and strain relaxation mechanisms of Ti_{1-x}Mg_xN(001) layers on MgO substrates, revealing critical thicknesses and lattice parameter variations with composition.

Contribution

It provides detailed characterization of epitaxial Ti_{1-x}Mg_xN layers, including strain states, lattice parameters, and relaxation behavior as a function of composition and thickness.

Findings

Fully strained layers with x up to 0.45

Critical relaxation thickness of 50-100 nm

Lattice parameter bowing parameter b = 0.113 Å

Abstract

Ti1-xMgxN(001) layers with 0.00 {\leq} x {\leq} 0.49 are deposited on MgO(001) by reactive magnetron co-sputtering from titanium and magnesium targets in 5 mTorr pure N2 at 600 {\deg}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} (001)MgO and [100]TiMgN{\parallel} [100]MgO. Layers with thickness d = 35-58 nm are fully strained, with an in-plane lattice parameter of 4.212{\pm}0.001 {\AA} matching that of the MgO substrate, while the out-of-plane lattice parameter increases with x from 4.251 {\AA} for TiN(001) to 4.289 {\AA} 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 {\AA}, aMgN = 4.345 {\AA}, and the bowing parameter b = 0.113 {\AA}. In contrast, thicker Ti1-xMgxN(001) layers with d = 110-275 nm are partially (pure TiN) or fully (x = 0.37 and 0.39) 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 {\leq} x {\leq} 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 {\leq} 110 nm) layers, which is attributed to strain relaxation through the nucleation and growth of misfit dislocations facilitated by glide of threading dislocations.