A monolithic optoelectronic device structure with the potential to enable VCSEL-based photonic integrated circuits on GaAs is presented. Using integrated diffraction gratings, the device structure enables the optical output of VCSELs to be coupled to an internal horizontal waveguide, while the optical signals in the waveguide are tapped off to resonant cavity detectors. Since horizontal waveguides are used to route the optical signals between devices, the output mirror transmission of the VCSELs can be eliminated, although we have chosen to retain a small amount of transmission in the top DBR to enable on-wafer testing. The design and fabrication of the monolithically integrated structure, including epitaxial regrowth, is discussed and initial device characteristics are presented.
Epitaxial regrowth techniques, using molecular beam epitaxy, were optimized for the inclusion of submicron diffraction gratings within a vertically resonant structure. Various growth conditions including chemical surface preparation, growth rate, and regrown interfacial structure were studied to determine the quality of the regrown materials and structures. Characteristics such as dislocation density and growth planarity (flatness of the regrown layers) were of particular importance due to the vertical geometry and resonance requirements of the structure. Threading dislocation densities of ≈3×106cm−2 were measured, by means of transmission electron microscopy, in the regrown structures using optimized regrowth processes. Layer thickness variations, due to growth on nonplanar surfaces (diffraction gratings), were characterized using modeling and optical reflectometry. With these results, inclusion of diffraction gratings has been demonstrated with the accurate control over layer thickness needed for use in vertically oriented devices such as vertical-cavity, surface-emitting lasers, and resonant cavity photodetectors.
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