We investigate the optimal encapsulation conditions for obtaining fully activated, double delta-layers of phosphorus in silicon. Using scanning tunneling microscopy and low-temperature magnetotransport we have explored whether it is important to optimize the maximal dopant incorporation of each individual layer or the smoothness of the surface on which we form the second dopant layer to achieve maximum total bilayer carrier density. We find maintaining crystallinity of the interstitial spacer layer is of paramount importance. In doing so, we are able to achieve double delta-layers with high dopant activation resulting in a maximum carrier density of 4.35 x 10(14) cm(-2). (C) 2010 Elsevier B.V. All rights reserved.