A Hall mobility as high as 176,200 cm2V-1s-1 at 77 K with N(d)-N(a) = 1.3 x 10(14) cm-3 has been obtained by adjusting the substrate and the phosphine cracker temperatures. The 2 K photoluminescence spectra show finely resolved excitonic transitions for layers grown above 535-degrees-C with linewidths of 0.07 meV. Excitonic transitions have been recorded between 2 and 250 K. The free exciton energy position leads to a highly accurate expression of the band gap energy, which extrapolates to E(g) = 1.347 eV at 300 K.