We describe an optimized growth sequence for the overgrowth of quantum cascade laser ridge sidewalls with semi-insulating InP:Fe. A thin In0.52Al0.48As spacer layer grown on the laser ridge sidewalls before InP:Fe prevents the formation of void defects at this interface, which appear otherwise. Elimination of these voids at the sidewalls has led to more than a two-fold improvement of the thermal conductivity in a 7 mu m wide buried-heterostructure quantum cascade laser overgrown with InP:Fe by gas source molecular beam epitaxy, and has allowed the continuous wave operation of the laser up to 210 K. The measured thermal conductance is G(th) = 500 W/K cm(2) at 210 K and G(th) = 1020 W/K cm(2) at 127 K, comparable to the state of the art literature values obtained with regrowth by metal organic vapor phase epitaxy. (C) 2014 Elsevier B.V. All rights reserved,