Next-generation Ge-on-Si waveguide photodetectors: modelling, design and optimization strategies

Ge-on-Si waveguide photodetectors achieving remarkable bandwidths exceeding 250 GHz and compatible with mature CMOS technology have been demonstrated recently (S. Lischke et al., Nat. Photon. 2021). The present thesis leverages commercial 3D multiphysics simulation tools, specifically Synopsys TCAD Sentaurus and RSoft Photonic Device Tools, to develop a model for the computer-aided design of this novel concept of device. In the perspective of providing guidance to manufacturers, this model has been tailored to predict key performance metrics of the detector, namely modulation bandwidth and responsivity. After validating and calibrating the model versus experimental results in the literature, the performance of the detector has been related to its technological parameters, such as geometry and doping, and to various operating conditions, including power and wavelength of the light source and bias. Such physics-based model has also been used to synthesize a small-signal high-frequency analytical model to identify the mechanisms limiting the broadband operation of the device. The rest of the thesis explores strategies for optimizing the photodetector: an inductive peaking-based approach to extend the bandwidth further has been suggested and the design of a tapered waveguide to improve light coupling with the device has been proposed. The thesis is structured as follows: Chapter 1 provides a general introduction to photodetectors, Chapter 2 details the adopted simulation methodologies, Chapter 3 presents the outcome of the conducted analyses.