Abstract
Selectively oxidized vertical-cavity surface-emitting laser (VCSEL) now is regarding as a very important light source for many optoelectronic applications, such as high-speed LANs, computer links, and optical interconnects, etc. However, due to the device with intentional mesa geometry, which causes an obstacle for its versatile applications. In this dissertation, a state of the art processing technique is presented to fabricate a planar-type oxide-confined 850-nm VCSEL. The planarized process of VCSELs was to use the silicon oxide (SiOx) as the buried layer. As a result, these devices exhibit excellent static characteristics, including a threshold voltage (Vth) of 2.05 V corresponding to a threshold current of 0.88 mA, a minimum threshold current of 0.7 mA near 60℃, a maximum output power of 4.28 mW at 11 mA, a maximum external differential quantum efficiency (ηex) of 43 % just above threshold, and an operation temperature beyond 130℃. In addition, the transverse modes of the device initially are low-order, while high-order modes appear at elevated current levels. The fundamental transverse mode at the longest wavelength increases with injected current with a red shift of 0.49 nm/mA due to joule effect. Since the thermal resistance of the VCSEL with a SiOx buried layer is less than that of device without it, the VCSEL with a SiOx buried layer displays less red shift and better performance. Finally, the VCSEL with a SiOx buried layer shows a clear eye-opening feature as operating at 2.488 Gbit/s with a bias current of 2 mA. Further increasing the current level, the device can work at the maximum bit rate of 8 Gbit/s and a bias current of 3.7 mA. Because of the strong optical confinement and the larger transverse dimension, VCSELs made with oxide apertures tend to lase in high-order Laguerre-Gaussian modes at elevated current levels. This in turn leads to a problem in fiber coupling because the beam divergence from these high-order modes is much higher than the fundamental mode. In addition to poor overall fiber coupling efficiency, the quality of the optical signal is also influenced by the amount of light coupled, and how each of the individual modes is sampled. The mode dynamics in the device can cause another problem, i.e., mode partition noise, which will further deteriorate the optical signal under system data transmission. With reduction the size of the oxide aperture to the point where only the fundamental mode is supported, we have succeeded in fabricating a high-efficiency SiOx-planarized single-transverse emission 850-nm VCSEL. These devices with an oxidized aperture of 3 μm in diameter exhibit a single-transverse mode behavior throughout the operation current range. In addition, the static characteristics of VCSELs at 300K include a threshold current of 0.52 mA corresponding to a threshold voltage of 2.2 V, a maximum single transverse-mode light output power of 1.13 mW at 4.5 mA, and an external differential quantum efficiency of 35%. On the other hand, this TO-packaged planar-type 850-nm VCSEL for back-to-back test shows a wide open along with symmetric eye diagram and could also pass the 10 Gb/s mask as operating at 10.3 Gb/s and 4 mA. Furthermore, the VCSEL can still keep the eye diagram open and symmetric after the 66-m multi-mode fiber (MMF) transmission and has a power penalty of 6.6 dB because of fiber dispersion for 10.3 Gb/s data rate at a bit error rate of 10-11. These results confirm the excellent high-speed performance of SiOx-planarized VCSELs as compared to the polyimide-planarized VCSELs. On the other hand, for improved transmission distance over MMF and for extended reach over single-mode fiber, it is desirable to operation around 1.3-μm. Based upon the SiOx-planarized technique, we report on a novel high-efficiency planar-type oxide-confined 1.3-μm GaInNAs VCSEL. The devices exhibit excellent static characteristics at room temperature, including a threshold voltage of 2 V corresponding to a threshold current of 3.5 mA, a maximum light output power of 1.86 mW measured at 15 mA. To our knowledge this output power is the best when compared to those obtained with conventional VCSEL processes for the similar epitaxial structure design. The VCSELs show a threshold current density of 3100 A/cm2, a differential resistance at half of maximum power of 110 Ω, a slope efficiency of 0.22 W/A above the threshold, and a continuous wave operation temperature up to 80℃. In addition, when operating at 4 mA these devices exhibit a single-mode emission with the transverse-mode suppression of more than 20 dB and an output power of 0.12 mW. The wavelength of the strongest emission peak, which corresponds to the fundamental transverse mode, increases with injection current at a red shift of 0.45 nm/mA from 1280.6 nm at 4 mA to 1284.4 nm at 12 mA due to a joule effect. Finally, this planar-type 1.3-μm VCSEL shows a clear and symmetric eye diagram operating at 2.488 Gb/s at 12 mA. These results confirm the SiOx-planarized GaInNAs VCSELs have the potential capacity for fiber optic applications.