Nitrogen mapping from ADF imaging analysis in quaternary dilute nitride superlattices

A method for the determination of N distribution in dilute nitride GaAsSbN superlattices (SLs) by using different STEM imaging settings is proposed. The method combines the simultaneous acquisition under Low Angle (LA-) and High Angle (HA-) Annular Dark Field (ADF) conditions by exploiting two different dedicated angle intervals. On one hand, HAADF technique gives information that principally depends on the atomic number (Sb sensitive) and on the other hand, N atoms produce high static atomic displacements affecting the image intensity especially under LAADF conditions. However, the simultaneous presence of Sb and N supposes an important handicap to differentiate both elements. N distribution maps could be obtained from suitable normalization and the separation of the intensity ratios in regions with/without Sb. Semi-quantitative maps are also available by combination of the results from high-resolution X-ray diffraction and energy dispersive X-Ray spectroscopy techniques. Type-I (GaAsSbN/GaAs) and type-II (GaAsSb/GaAsN) SL structures are evaluated using the proposed methodology. Differences in the N distribution between both samples such as inhomogeneities and cluster formation are discussed. Specifically, we have found a greater number of N-rich regions in type-I structure as compared to their type-II counterparts, which could have an influence on the optical response of each design.