Investigation of optical and concentration profile changes of InGaNAs/GaAs heterostructures induced by thermal annealing

In this contribution we compare optical and structural properties of In_0.2Ga_0.8N_0.024As_0.976 quantum wells before and after annealing at 550 °C and 600 °C in an N₂ atmosphere. We measure strain and chemically sensitive contrast to determine local indium and nitrogen concentrations using a TEM 3-beam image formed by the 000, 220 and 200 beams. For quantification of the concentrations Bloch wave simulations are used, which include bonding and static atomic displacements. The samples were grown by metal-organic vapour phase epitaxy, prepared with focused ion beams (FIB), thinned with low energy ion milling and investigated in a Cs-corrected Titan 80/300 microscope using an L-shaped objective aperture. Imaging conditions with the Laue circle centre at (0 4.2 0) are used as they show a weak dependence on the lamella thickness.

Absorption measurements show a blueshift of the band gap of 19 ± 7meV (550 °C) and 36 ± 7 meV (600 °C) after annealing. The average nitrogen concentration was found to be 2±1% and is unaffected by the annealing temperature. In contrast, the mean indium concentration appears to decrease from 18.5 ± 2% before to 15 ± 1% after annealing. Together with the blueshift, this observation is discussed in terms of a modification of electron structure factors, caused by preferred coordination of N to In atoms.