Extrinsic Parasitics and Design Considerations on Modulation Bandwidth of 850-nm Vertical Cavity Surface Emitting Lasers

The extrinsic parasitics generally limit the modulation bandwidth of vertical cavity surface emitting lasers (VCSELs) for optical data links and interconnects. In this article, the extrinsic parasitics of 850-nm VCSELs are investigated by three different designs: oxide-only (design A), oxide-implant (design B), and oxide-implant with benzocyclobutene (BCB) (design C). The static and dynamic characteristics of all the designs with~7- $\mu \text{m}$ aperture are measured and compared. The maximum modulation bandwidth of design A is only about 12 GHz mainly limited by the large parasitic capacitances. With a proper proton implantation, a thick nonconducting layer is created inside the device with design B, which dramatically reduces the parasitic capacitances, and thus the bandwidth is increased to about 18 GHz. Based on design B, a 4- $\mu \text{m}$ -thick BCB layer with low dielectric constant is employed underneath the bond pad of design C to further reduce the pad capacitance, leading to the highest bandwidth about 24 GHz and the highest modulation current efficiency factor (MCEF) of 10.8 GHz/mA ^1/2 . The microwave reflection coefficient ( ${S}_{{11}}$ ) parameters of the three designs are measured and compared. Results indicate that design C has the lowest parasitic capacitances and the highest modulation bandwidth, which is preferred for high-speed VCSELs to obtain good static and dynamic performance.