We demonstrate a way to reduce the room temperature wafer bow of AlAs/GaAs super-lattices (SL) by the addition of phosphorus forming an AlAsyP1-y/GaAs SL. In-situ monitoring of the curvature not only allows for a determination of the AlAsyP1-y composition, but also to assess the onset of relaxation by means of the strain-thickness product in conjunction with curvature transient simulation. Based on this, we were able to realize an approach where the effective strain within one SL period is lowered by increasing the GaAs thickness while maintaining a high reflectivity at design wavelength. This lowers the cool down curvature swing and thus yields a RT curvature reduction from - 142 km(-1) to - 76 km(-1). (C) 2014 Elsevier B.V. All rights reserved