Silicon-on-germanium waveguide photodetectors: a multiphysics computer-aided design approach

The past few years have witnessed a staggering increase of information traffic, growing incessantly with the complexity and quality of the services provided to users around the world. A remarkable example can be found in the social media history: starting from Facebook, people swept to Instagram, and at present one of the most fashionables media is TikTok. The paradigm enabling this evolution has been “centralized computation”, the data processing being processed in data centres, where traffic is localized, and then delivered to the end users. In this context, the role of short-range interconnects is becoming comparable or even greater than that of telecommunications, leading to the requirement of fast, low-power optoelectronic devices. One of the main limitations of this architecture is the interface between the optical domain and the electrical domain. Until an all-optical computer will be technologically feasible, one of the most promising solutions to attack this bottleneck is silicon photonics (SiPh), which promises a synergistical, low-cost and fully CMOS-compatible integration of optical and electronics systems. Exploring new device concepts or even just optimizing consolidated geometries requires extensive and expensive trial-and-error prototyping campaigns, each prototype requiring manufacturing a wafer from masks to epitaxy. Aiming to overcome the state of the art SiPh receiver subsystems, the scope of this thesis is developing a computer-aided design framework for waveguide photodetectors. Also from a simulation perspective, dealing with the optical and electrical domains is delicate, as it requires a joint treatment of Maxwell’s equations and of a carrier transport model, resulting in a self-consistent multiphysics picture: in the first instance, the spatially-resolved photogenerated carrier distribution is evaluated from absorbed photon density through a full-wave electromagnetic simulation (optical model); then, this is used as a source term in an electrical simulation obtained from the solution of the electron and hole continuity equations with drift-diffusion constitutive relations, coupled to the quasistatic Poisson’s equation (electrical model). The thesis is organized as follows: - The first chapter reviews the state of the art of waveguide photodetectors, introducing the figures of merit necessary to assess the electro-optical performance (average photogeneration, responsivity, modulation response). - The second chapter describes the details of the simulators used in this work. The electromagnetic section is based on Synopsys RSoft FullWave: a 3D finite-difference time-domain (FDTD) solver of Maxwell’s equations. The electrical simulator is Synopsys Sentaurus Device, which solves the Poisson-drift-diffusion system with a finite-box method based on the Scharfetter-Gummel discretization of the drift-diffusion relations. The coupling strategy behind the multiphysics approach is described in detail, with emphasis on possible model simplifications allowing to reduce the computational burden (low carrier generation rates). - The third chapter is focused on two silicon-germanium waveguide photodetectors operating in both O (1.31 µm) and C (1.55 µm) bands: one based on mode-evolution, one on butt coupling. After describing their operation by means of approximate semi-analytical models, the two devices are optimized and compared on the basis of multiphysics simulations.