Monolithic heteroepitaxial integration of III-V semiconductor lasers on Si substrates

Abstract

Monolithic optoelectronic integration on silicon-based integrated circuits has to date been limited to date by the large material differences between silicon (Si) and the direct-bandgap GaAs compounds from which optoelectronic components are fabricated. Graded Ge/GeSi buffer layers grown on standard Si substrates have been shown to produce near-lattice matched virtual substrates for GaAs integration on Si. This study investigated the crystal growth conditions and device fabrication techniques necessary for successful GaAs-based laser integration on Ge/GeSi buffer layers on Si substrates. The nucleation conditions for GaAs on Ge/GeSi/Si substrates have been comprehensively examined. High-temperature ( 2 700⁰ C) initiation with properly chosen V/III gas flow ratio yields high-quality, stacking fault-free GaAs films on Ge/GeSi/Si substrates, but also encourages the vapor-phase transport of Ge from the substrate into the active regions of integrated GaAs devices. A new two-step GaAs nucleation process was developed that enabled the first demonstration of high-quality Ge-free GaAs light-emitting diodes on Ge/GeSi/Si substrates. The large thermal expansion mismatch between Si, Ge, and GaAs introduces additional strain to integrated device layers on Ge/GeSi/Si substrates grown at high temperatures. This study conclusively demonstrated the link between thermal mismatch strain and increased misfit dislocation formation in InxGa(lx)As/GaAs quantum well structures integrated on Ge/GeSi/Si substrates.

(cont.) The thermal mismatch strain was successfully countered by the introduction of compressive InGaAs graded buffer layers above the Ge/GeSi/Si substrate surface, and strain-free GaAs layers at growth temperatures suitable for laser integration have been demonstrated. The integration of edge-emitting heterostructure lasers on Ge/GeSi/Si substrates introduces additional waveguide design issues addressed by this study. Low-index Alo.6Gao.4As cladding layers, along with a graded-index separate confinement heterostructure, were introduced to reduce photon losses. Interfacial roughness transmitted from the Ge/GeSi/Si substrate was reduced with a pre-growth chemical-mechanical polishing step, and smooth mirror facets on integrated devices were fabricated by cleaving thinned lasers parallel to the substrate offcut direction. Continuously operating edge-emitting GaAs/AlGaAs quantum well lasers on Ge/GeSi/Si substrates were demonstrated at room temperature with an operating wavelength of 858 nm. Series resistance heating in early devices was reduced by the introduction of a top-contact geometry and optimized cladding layer structure, and improved laser diodes had a differential quantum efficiency of 40%, a threshold current density of 269 A/cm2, and a characteristic temperature of 129 K. Identical devices fabricated on GaAs substrates had similar performance characteristics. Lasers on Ge/GeSi/Si substrates fell below threshold after 4 hours of continuous operation-a dramatic improvement over early measured lifetimes of less than 20 minutes ...