Improved integration of optoelectronic devices is provided by a spacer layer laterally sandwiched between distinct regions that are monolithically fabricated onto the same substrate (e.g., by selective epitaxy). An optical waveguide in one of the regions can optically couple to an optoelectronic device in another of the regions through the spacer layer, thereby providing a monolithically integrated form of butt-coupling. Preferably, the spacer layer thickness is less than about 50 nm, and is more preferably less than about 20 nm, to reduce optical loss. The spacer layer is preferably electrically insulating, to prevent shorting of devices grown by selective epitaxy.