Silicon electro-optic modulator

Abstract

In recent years, as on-chip data transmission has increased dramatically, much time and effort has been devoted to the development of compact electro-optic modulators with large bandwidth and low power consumption, which is the key component for on-chip data links. Silicon which has been widely used in electronic industry has been considered as a promising material for electro-optic modulators, due to its mature manufacturing technology and low cost. In this work, a silicon electro-optic modulator based on a new type of phase shifter is proposed. The phase shifter is in one arm of an asymmetric Mach-Zehnder interferometer (MZI) structure and operates very similar to a MOS capacitor. Indium tin oxide (ITO) is used as an electrode and flowable oxide (FOx, spin-on-glass) is used as a gate oxide. ITO is being widely used in many electro-optic applications as a transparent conductor, as it offers both electrical conductivity and optical transparency, although it has seen less use in optical waveguide devices. Electrical simulations have been completed in order to study the electrical performance of the phase shifter and also to improve the design. SiO₂ has been used as a gate oxide in the simulation. The depletion region width and capacitance-voltage (C-V) characterization have been modelled. Based on the theoretical analysis and simulation results, n-type silicon with the doping concentration of 1×10¹⁷ cm⁻³ has been chosen as the active material. ITO fabrication and its electrical and optical properties have been studied. The effect of ITO annealing on sheet resistance and transmittance has been investigated. ITO (140 nm thick) annealed at 400˚C for 30 minutes in air shows the best result. The sheet resistance is around 750 Ω/□ and the optical transmittance is about 87%. Aluminium (Al) electrical contacts to silicon have been fabricated and tested. The HF cleaning and post thermal annealing effect on the contact resistance has been investigated. The transmission line method (TLM) has been used to determine the contact resistance. The Al contact resistance is about 335 Ω on n-type silicon substrate with a doping concentration of 1×10¹⁷ cm⁻³. ITO-FOx phase shifters and modulators have been fabricated and tested. The MZI structure of the modulator shows the typical interference pattern. This is important for realising phase-intensity modulation. Al-FOx phase shifters have also been fabricated and tested. The transmission depends on the Voltage, both in terms of phase and the optical loss, clearly indicating a change in carrier density. Although ITO-FOx phase shifters and modulators did not work successfully due to high optical losses caused by waveguides sidewall roughness and fabrication process, ITO showed promise to use in electro-optic modulators as a transparent conducting layer. The ITO layer separates silicon waveguides from Al contacts and lowers optical losses due to metal absorption. Overall, successful operation of a MOS-capacitor type optical phase shifter has been achieved, but the modulation efficiency is low (1 dB at 50 V) and the insertion loss is high (over 10 dB).