Doping of silicon via phosphine exposures alternating with molecular beam epitaxy overgrowth is a path to Si:P substrates for conventional microelectronics and quantum information technologies. The technique also provides a well-controlledmaterial for systematic studies of two-dimensional lattices with a half-filled band. We show here that for a dense (n(s) = 2.8 x 10(14) cm(-2)) disordered two-dimensional array of P atoms, the full field magnitude and angle-dependent magnetotransport is remarkably well described by classic weak localization theory with no corrections due to interaction. The two-to three-dimensional crossover seen upon warming can also be interpreted using scaling concepts developed for anistropic three-dimensionalmaterials, whichwork remarkably exceptwhen the applied fields are nearly parallel to the conducting planes.