Optimized GaAsSb/GaAsN type-II superlattices studied by cross-sectional scanning tunneling microscopy

Short period GaAsSb/GaAsN type-II superlattices (SLs) with a bandgap between 1.00 and 1.15 eV can be grown lattice-matched to GaAs, making them suitable sub-cells to be implemented in the design of monolithic multi-junction solar cells based on III–V semiconductors. In the past, the challenge in obtaining molecular-beam-epitaxy-(MBE)-grown GaAsSb/GaAsN SLs with sharp composition profiles was limited by Sb’s strong tendency to segregate, resulting in a non-zero Sb content throughout the entire SL structure. However, different growth approaches aimed to prevent Sb segregation have been recently proposed. These methods typically involve a growth interruption and Sb soaking prior to the growth of the GaAsSb layer and annealing processes (Sb desorption or flash-off) or As soaking after it. In this work, the studied GaAsSb/GaAsN SL sample was grown by MBE employing Sb soaking, to promote incorporation, and Sb flash-off, to promote desorption of Sb adsorbed on the growth surface. In order to verify whether these growth approaches result in square Sb composition profiles, the SL structure was analyzed by cross-sectional scanning tunneling microscopy (X-STM). From the X-STM study, it was observed that the employed growth strategies result in the intended square Sb profiles. The N and Sb experimental content was obtained from the X-STM images and found to be comparable to the nominal value. Moreover, short-range (dis)order in terms of N–N and Sb–Sb nearest neighbor (NN) pair formation was investigated. It was found that N has a strongly diminished tendency to form first and, in lesser extent, fourth NN pairs.