Interface morphology and composition of Ga(NAsP) quantum well structures for monolithically integrated LASERs on silicon substrates

Highly efficient light sources are the remaining item required for the realization of optoelectronically integrated circuits on exactly oriented Si(0 0 1). Here, we present—using transmission electron microscopy—an investigation on the structure and stability of Ga(NAsP), which is a direct bandgap semiconductor. It is shown that Ga(NAsP) can be grown on Si(0 0 1) substrates at a wide range of growth temperatures. No sign of defect formation and phase separation is observed even for the highest growth temperatures used. The interfaces of the quaternary alloys with the GaP barriers roughen significantly with increasing growth temperature. On the contrary, the material deposited at high temperatures is more homogeneous than the one deposited at low temperatures. This is highly surprising as dilute nitride III/V alloys are commonly thought to be metastable. This is resolved by density functional theory calculations, which show that Ga(NAsP) becomes significantly more stable when grown on substrates which have a smaller lattice constant than the equilibrium lattice constant of the alloy. This stability together with the strong room-temperature photoluminescence shown by all samples, make the Ga(NAsP) material system highly promising for laser applications on Si substrates.