Conformal Vapor Phase Epitaxy (CVPE) is a new technique for two-dimensional constrained growth of thin semiconductor films using a halide vapor transport process. This technique will be applied to the growth of GaAs and InP films on silicon and sapphire substrates. Epitaxial-lateral overgrowth with high aspect ratios is achieved by confining growth to a thin void space bounded by an oxide-mask coating the substrate and an adjacent overhanging mask. This technique has excellent potential for reducing the detrimental effects of lattice-mismatch and thermal stress to yield low-defect heteroepitaxial films. The feasibility of CVPE for GaAs-on-silicon, GaAs-on-sapphire, InP-on-silicon, and InP-on- sapphire will be studied with experimental emphasis on GaAs-on-silicon. Heteroepitaxial materials will be characterized and minority carrier devices, such as photodetectors and LEDs, fabricated with the heteroepitaxial films, will be evaluated.