Quantitative evaluation of the interface lattice quality of a strain superlattice by strain analysis
The lattice quality of strain superlattice structures in Quantum Cascade Lasers (QCLs) directly influences the photoelectric properties and service life of the lasers. However, the evaluation method for lattice quality on the nanoscale is not very well developed at present, especially for interface lattice quality assessment. In this investigation, all atoms positioned in the multiple interface layers can be simultaneously and accurately determined through Subset Geometric Phase Analysis (S-GPA) combined with a Peak Finding (PF) method and an Optimal Approximation Algorithm (OAA) with a sensitivity of about 0.04 Å. Based on the determined interface location, the strain distribution in all layers of the superlattice structure was simultaneously measured using the improved S-GPA by means of the optimal selection of multiple reference areas. A quantitative evaluation of the strain/stress compensation effect was then carried out based on the theoretical model of elastic mechanics. The proposed method was successfully applied to evaluating the lattice quality of an In0.6Ga0.4As/In0.44Al0.56As superlattice structure grown by Molecular Beam Epitaxy (MBE). The obtained results show that the interface lattices are almost perfect with a uniform thickness of layers, without any defects and stress concentration. Each In0.44Al0.56As layer and adjacent In0.6Ga0.4As layers provided effective strain/stress compensation for each other, reducing the possibility of forming dislocations. In one period, the active region has been properly strain-balanced to give a nearly net zero strain. The proposed method can not only be applied in evaluating the growth quality of the superlattice structure with a large field of view, but also provide quantitative experimental data for further improving the superlattice design.