Lower doping would mean less lattice strain on the epitaxial structure. This low lattice strain would allow the epi to be stable for a longer period of time, hence delay the faceting and thereby give us a taller epi. On the other hand, lowering the crystallinity creates more dislocations for the lattice to relax and creates more interstitial positions where the Phosphorus can enter. Both these mechanisms result in lower lattice strain, delay the faceting, and therefore increase the epi height.