Stress mapping of a strain superlattice using scanning moiré fringe imaging

Accurate adjustment of the stress/strain field can significantly affect the physical properties of a strain compensation superlattice structure in quantum cascade lasers. Therefore, precise evaluation of the stress/strain is of significant importance at the nanoscale. In this investigation, nanometer-scale scanning moiré fringes (SMFs) with two directions in an In_0.6Ga_0.4As/In_0.56Al_0.44As superlattice structure are obtained simultaneously by precisely adjusting the formation parameters of SMF using scanning transmission electron microscopy (STEM). Starting from the principle of STEM imaging, the fundamental formation principle and forming condition of STEM moiré are systematically studied. The 2D strain/stress distributions parallel and vertical to the growth direction are measured simultaneously, indicating that the maximum absolute value of strain/stress is close to the interface, with the peak stress at the gigapascal level, whereas the minimum absolute value of strain/stress is near the middle of each layer. The calculated resultant force indicates that each In_0.56Al_0.44As layer provides effective strain compensation for the adjacent In_0.6Ga_0.4As layers. The active region is properly strain-balanced to provide a nearly net-zero strain within a single period, reducing the possibility of forming dislocations.