TEM 3-beam study of annealing effects in InGaNAs using ab-initio structure factors for strain-relaxed supercells

We report on a Transmission Electron Microscopy 3-beam technique based on the interference of 000, 200 and 220. Nonlinear imaging artefacts are eliminated by Fourier filtering, yielding 200 and 220 lattice fringe images, from which chemically sensitive contrast and strain are measured, respectively. In this way, In and N composition can be mapped at atomic scale in quaternary InGaNAs by comparison with simulated reference data. Our Bloch wave simulations are based on structure factors derived from supercells with 10⁶ atoms, which have been strain-relaxed by valence force field methods. Additionally, the influence of electron redistributions due to chemical bonding is accounted for by modified atomic scattering amplitudes derived from density functional theory. By comparing local compositions in an annealed In_0.28Ga_0.72N_0.025As_0.975 sample with its as-grown counterpart, we find homogenisation of InGaNAs layer thickness and –stoichiometry upon annealing.