We investigate the propagation of dislocations in InAs-channel inserted high electron mobility transistor-structures with step graded metamorphic buffers grown by molecular beam epitaxy on GaAs-substrates. Applying a new focused ion beam technique for the preparation of very large cross-sectional and deep-selective plan-view transmission electron microscopy samples we are able to study the density of threading dislocation arms and the lattice relaxation due to misfit dislocation segments in the upper interfaces of the buffer. We find a very strong influence of an additional AlAs/GaAs short-period superlattice (SL) grown directly on top of the GaAs substrate. A conventional device without SL exhibits a non-complete lattice relaxation in the metamorphic buffer and threading dislocation arms that penetrate the active regions. This results in a significantly reduced electron mobility in comparison to a device with a SL, where lattice relaxation is nearly complete and no threading dislocations are observed. As an explanation, we assume that the SL buries residual impurities of the as-delivered wafer surface that otherwise inhibit the gliding of threading dislocation arms on their way to the edge of the sample. Using the optimised design with SL, electron mobilities up to 400.000 cm2/Vs are achieved.