Evaluation of antimony segregation in InAs/InAs_1-xSb_x type-II superlattices grown by molecular beam epitaxy

InAs/InAs_1-xSb_x type II superlattices designed for mid-wavelength infrared photo-detection have been studied using several electron microscopy methods, with specific attention directed towards interface chemical diffusion caused by Sb segregation. Reciprocal-space image analysis using the geometric phase method showed asymmetric interfacial strain profiles at the InAs-on-InAsSb interface. Measurement of local Sb compositional profiles across the superlattices using electron energy-loss spectroscopy and 002 dark-field imaging confirmed asymmetric Sb distribution, with the InAs-on-InAsSb interface being chemically graded. In contrast, the InAsSb-on-InAs interface showed a small intrinsic interface width. Careful evaluation of the experimental Sb composition profiles using a combined segregation and sigmoidal model reached quantitative agreement. Segregation dominated over the sigmoidal growth at the InAs-on-InAsSb interface, and the segregation probability of 0.81 ± 0.01 obtained from the two microscopy techniques agreed well within experimental error. Thus, 81% of Sb atoms from the topmost layers segregated into the next layer during growth causing the interfaces to be broadened over a length of ∼3 nm. This strong Sb segregation occurred throughout the whole superlattice stack, and would likely induce undesirable effects on band-gap engineering, such as blue-shift or broadening of the optical response, as well as weakened absorption.