Towards microcavity vertical cavity lasers: aperture and cavity design for high efficiency and low threshold

We have fabricated /spl lambda/=980 nm vertical cavity lasers with a tapered oxide profile which is produced by placing a 10.5 nm layer of AlAs within the first p-mirror layer which is a 230 nm (3/4 /spl lambda/) thick Al/sub 0.9/Ga/sub 0.1/As layer. We have plotted the scattering loss (as determined from the differential efficiency of tapered and abrupt apertured devices) vs. the radius of the opening in the oxide. Simulations of 980 nm AlAs/GaAs VCLs with quarter-wave thick apertures tapered over more than /spl sim/3 /spl mu/m show they have no excess loss. Although for a given mode size, scattering losses from such a "long" tapered aperture are much lower than for an abrupt aperture, the apertures close off before the mode size can shrink below a l/e/sup 2/ radius of /spl sim/1.5 /spl mu/m. Tapering quarter-wave thick apertures over a shorter distance (/spl sim/1 /spl mu/m) not only lowers scattering losses at the small aperture sizes, but also confines the mode to the smallest sizes. Ultimately, the smallest size mode confined by a single aperture (obtained by using a parabolic (ideal lens) profile) is limited by the angular stop-band of the DBR mirror. For 980 nm VCLs with AlAs/GaAs mirrors the smallest mode size has a l/e/sup 2/ radius of /spl sim/0.6 /spl mu/m. To reduce this size further one would need to turn to dielectric mirrors or use multiple apertures which are thin so as not to introduce additional losses.