SCD proprietary etchant reveals “killer” defects in our T2SL test devices. They are related to both growth conditions and substrate imperfections. Adjusting these parameters allowed us to reduce “killer” defects density by several orders of magnitude. Because of their shape in cross-section HRTEM we named them “nails”. At high growth temperatures they originate close to the T2SL–GaSb interface, and develop in size during only few SL loops to a straight and narrow “column” through the whole structure.