Synthesis and optical properties of (GaAs)_yGe_5-2y alloys assembled from molecular building blocks

Monocrystalline alloys of GaAs and Ge with compositions (GaAs)_yGe_5-2y have been synthesized following a chemical vapor deposition approach that promotes the incorporation of Ga and As atoms as isolated donor-acceptor pairs. The structural and optical properties show distinct behavior relative to (GaAs)_1-xGe_2x counterparts produced by conventional routes. Strong band gap photoluminescence is observed in the 0.5-0.6 eV range for samples whose compositions approach the GaAsGe₃ limit for isolated Ga-As pairs. In such samples, the Ge-like Raman modes appear at higher frequencies and are considerably narrower than those observed in samples with higher Ge concentrations. These results suggest that the growth mechanism may favor the formation of ordered phases comprising Ga-As-Ge₃ tetrahedra. In contrast with the diamond-to-zincblende ordering transition previously reported for III-V-IV alloys, ordered structures built from Ga-As-Ge₃ tetrahedra feature III-III and V-V pairs as third-nearest neighbors, and therefore both the III- and V-components are equally present in each of two fcc sublattices of the average diamond-like structure. These bonding arrangements likely lead to the observed optical response, indicating potential applications of these materials in mid-IR technologies integrated on Si.