Selective oxidation allows integration of photodetectors and vertical-cavity lasers

Selective oxidation allows integration of photodetectors and vertical-cavity lasers

Bottom-emitting vertical-cavity lasers (VCLs) and bottom-illuminated resonant-cavity photo detectors have been fabricated adjacent to each other on the same substrate without regrowth by researchers at the University of California (Santa Barbara, CA). Monolithic integration of lasers and detectors is a prerequisite for successful development of free-space optical interconnects. In this case, both devices are made with the same epitaxial structure and material--they differ in the reflectivity of the bottom mirror.

In both devices, three aluminum gallium arsenide (AlGaAs) layers form three 80-Å quantum wells in the bottom distributed Bragg reflector (DBR). As originally grown, the DBR structure provides a reflectivity value of 80% to form the bottom mirror of a resonant-cavity photodetector. By oxidizing selected areas of the AlGaAs layers, however, the researchers were able to increase the reflectivity of the DBR to the 99.3% needed for it to act as an output mirror for the VCL. Hence, regrowth was not required to monolithically fabricate the photo detectors and VCLs on the same silicon gallium arsenide substrate. The detectors exhibited responsivity of about 0.43 A/W, which corresponds to 56% optical absorption and an optical bandwidth of 5.9 nm, say the researchers. For single-mode and multimode VCLs, respectively, threshold currents as low as 180 and 450 µA were obtained, as well as slope efficiencies of 33% and 60%.