InAsSb submonolayer (SML) islands separated by GaAs spacer layers show three-dimensional charge carrier localization centers of very high density. We advance the understanding of the submonolayer growth technique by combination of structural and optical data. Our analysis of the Sb incorporation by Langmuir-type adsorption model reveals involvement of a slow reaction component. In search for the governing mechanism for electronic coupling we monitored structural parameters such as interface roughness by X-ray reflection measurements. Interestingly, no significant structural changes with respect to the spacer thickness are found. Still, a critical upper thickness limit to observe electronic changes of 1.8-1.9 MLs GaAs is found. Below this critical thickness, three-dimensional charge carrier localization is present in the SML stack and the average localization depth can be controlled through spacer thickness. (C) 2018 Elsevier B.V. All rights reserved.