A Hall mobility as high as 153,800 cm2 V-1 s-1 at 77 K with N(d) - N(a) = 1.5 x 10(14) cm-3 has been obtained by adjusting the growth temperature and the phosphine cracking temperature. The 4 K photoluminescence spectra show finely resolved excitonic transitions for layers grown above 535-degrees-C with linewidths as narrow as 0.07 meV. The use of substrates misoriented 2-degrees and 3.5-degrees towards (111)A significantly improves the morphology at the cost of a slight increase in the impurity incorporation.