Design, Fabrication and Optimization of Silicon Electro-optic Modulators for Digital and Analog Applications

Silicon optical interconnects have received tremendous attentions in recent years due to their properties of high bandwidth and low power consumption. In many applications, the optical interconnect is considered as a practical solution to replace the conventional copper line based electrical interconnects for improved performance in handling jitter and bandwidth advantage. The electro-optic (EO) modulator on silicon substrate is one of the most important building blocks in constructing complementary metal-oxide-semiconductor (CMOS) compatible optical interconnects. Crystalline silicon has very weak first-order linear EO effect, and therefore free carrier plasma effect is often used to manipulate the optical refractive index of silicon. This work focuses on design, fabrication, and optimization of silicon EO modulators for both digital and analog applications. SiGe heterojunction bipolar transistors (HBTs) are high-speed electronic devices and compatible with CMOS technology. In my thesis work, the SiGe HBT structure is modified to work as an EO modulator to achieve better optical modulation efficiency. Different design trade-offs are discussed in details and three different HBT models are developed. A modulation efficiency of L pi∣Vpi = 0.00198V∣cm is achieved by adding germanium into the collector region. In addition, the linearity of SiGe HBT EO modulator is evaluated for potential analog applications. A 96 dB∣Hz2/3 dynamic range is achieved for a HBT modulator. The fabrication of SiGe HBTs in cleanroom is explored using electron beam lithography and key fabrication steps are discussed. Two new device structures are explored theoretically. The tunneling modulator structure uses tunneling layers to cascade PiNs in a silicon rib waveguide. Compared with conventional PiN modulators, the proposed tunneling modulator has reduced diffusion capacitance and does not suffer from sub-wavelength confinement problem. The other new structure explored in this work is a super junction EO modulator, which utilizes super junction structure to increase the doping level without compromising the breakdown voltage. For the designed super junction modulator, the heavily-doped device can be fully depleted with a small voltage swing, which greatly improves optical modulation efficiency. Device modeling analysis predicts that the modulation efficiency of the super junction EO modulator is ~ 20 times higher compared with the state-of-the-art depletion-type Si EO modulators.