Using nearby parts of a vertical cavity ln0.2Ga0.8As surface emitting microlaser1 wafer, four samples of etch depths, 1.3 μm (μlaser13), 1.8 μm (μlaser18), 3.1μm (μlaser31), 6.5 μm (μlaser65), were prepared. The active In0.2Ga0.8As layer is located 2.0 μm deep from the top gold film. For μlaser18, a F-ion implant at 100 keV at a dose of 5 × 1012 cm-2 was used to render ~0.25 μm of unmasked GaAs/AlAs highly resistive. For μlaser13, the same implant as above was performed followed by an additional 200-keV F-ion implant at a dose of 2 × 1012 cm-2 to make the unmasked GaAs/ AlAs resistive to a ~0.5-μm depth. Using 200-kHz 100-ns pulses, L-l curves and V-l curves are taken simultaneously. In general, μlaser31 operates at a slightly higher current than μlaser65. The difference between these two deep etched samples is attributed to the small diffraction losses present in μlaser31 but not in μlaser65. The minimum threshold current (1.3 mA) among all the μlasers tested was obtained with 5-μm square μlaser65, where complete optical guiding was achieved by the deep etch, and the active gain medium volume was smallest. It is interesting to note that the thresholds of 10-μm square μlaserl8 are generally lower than those of μlaser31 and the same as μlaser65. Because the active In0.2Ga0.8As region is not open to air, μlaser18 seems to experience less surface recombination than, μlaser31 and μlaser65 but more diffractive losses. For μlaser18, the minimum threshold current (2.0 mA) was obtained with 6.2-μm square μlasers with threshold currents being higher for the smaller and larger μlasers. At this point, the optical diffraction loss due to shallow etching becomes significant relative to the small gain from three In0.2Ga0.9As quantum wells and the mirror coupling losses. With shallower etch depths, threshold voltages are reduced significantly, from 7-8 to 4-6 V, mainly due to reduction of resistance by allowing the current spread to larger areas in the bottom mirror. For 9-μm square μlaser31, the cw optical output power was ~190 μW and differential quantum efficiency was ~5%. The threshold was 4.0 mA, and output power begins to drop at currents higher than 7.6 mA. The 10-μm square μlaser18 shows output power up to 710μW and the single facet differential quantum efficiency of 9%. In our stated output power we scaled the measured value to account for the 30% Fresnel reflection but did not compensate for the ~40% absorption in the n+ substrate. The cw threshold of 10-ftm square μlaser18 was 3.6 mA (with 3.7 V), and the output power kept increasing up to 12 mA. The reasons for the improved cw performance are threefold: the improved thermal dissipation, reduced resistance, and reduced nonradiative surface recombination.
© 1990 Optical Society of AmericaPDF Article