The antimonide based vertical external cavity surface emitting lasers (VECSELs) operating in the 1.8 to 2.8 μm wavelength range are typically based on InGaAsSb/AlGaAsSb quantum wells on AlAsSb/GaSb distributed Bragg reflectors (DBRs) grown lattice-matched on GaSb substrates. The ability to grow such antimonide VECSEL structures on GaAs substrates can take advantage of the superior AlAs based etch-stop layers and mature DBR technology based on GaAs substrates. The growth of such III-Sb VECSELs on GaAs substrates is non-trivial due to the 7.78% lattice mismatch between the antimonide based active region and the GaAs/AlGaAs DBR. The challenge is therefore to reduce the threading dislocation density in the active region without a very thick metamorphic buffer and this is achieved by inducing 90° interfacial mist dislocation arrays between the GaSb and GaAs layers. In this presentation we make use of cross section transmission electron microscopy to analyze a variety of approaches to designing and growing III-Sb VECSELs on GaAs substrates to achieve a low threading dislocation density. We shall demonstrate the failure mechanisms in such growths and we analyze the extent to which the threading dislocations are able to permeate a thick active region. Finally, we present growth strategies and supporting results showing low-defect density III-Sb VECSEL active regions on GaAs.