A new synthetic strategy for (III-V)y(IV)5-2y materials is introduced to avoid III-V/IV phase separation. As a proof of principle, crystalline AlPSi3 with thickness <900 nm was grown directly, and virtually lattice matched, on Si(100) substrates by combining Al atoms and the well-known P(SiH3)3 molecule as co-reactants using gas-source MBE techniques at temperatures < 600 °C. Intermediate "Al:P(SiH3)3" complexes containing (Al-P)-Si3 cores are presumed to form, and incorporate intact into highly stable diamond-like solid devoid of phase segregation and defects, and a composition with maximal III-V content in which "donor-acceptor" pairs are isolated within an Si matrix. This strategy is generalized to: (i) expand the composition space for this class of materials (ii) grow bulk-like films for detailed comparative characterization and (iii) alloy on the III-V and/or IV sublattice as a means of tuning lattice constants and band gaps. Assembly mechanisms, thermochemistry and key materials properties are elucidated via molecular/solidstate simulation.