Characterisation and optimisation of radiation-tolerant silicon sensors with intrinsic gain

Abstract

The research work of this PhD thesis is performed in the framework of the RD50 Collaboration, working on the development and characterisation of radiation tolerant silicon detectors for high luminosity colliders. The main focus is on silicon sensors for tracking applications within the HL-LHC experiments. Sensors with intrinsic charge multiplication are presently under investigation within RD50. In this thesis, Low Gain Avalanche Detectors (LGADs) and Deep Diffused Avalanche Particle Detectors (DD-APDs) are studied, as they allow to multiply the charge generated by traversing particles. The subsequent increase in the signal produced by the detectors has the potential to improve the signal-to-noise ratio, and with it the efficiency and the time resolution of the detectors. Unfortunately, these devices suffer from serious radiation damage when exposed to particle fluences higher than about $10^{14}\text{ particles/cm}^2$. The aim of this thesis is to characterise LGADs and DD-APDs before and after exposure to radiation, and study the performance degradation (loss of signal, loss of gain, inhomogeneous spatial response, etc.) as a function of radiation fluence. The origin of the differences in performance is evaluated, and new optimised designs are proposed based on some of the results obtained. The performed work helps understand the fundamental physical processes that are leading to the deterioration of the gain mechanism in silicon devices with intrinsic gain. Whilst aiming for performance optimisation, clear application limits for LGAD and DD-APD devices in terms of radiation hardness are obtained. From a solid-state physics point of view, this thesis sheds light on the many phenomena occurring inside silicon devices after heavy irradiation, both with protons and neutrons. From the perspective of detector physics and detector technology, these findings will serve the HL-LHC and other experiments as guidelines for considering the use of devices with intrinsic gain for tracking, calorimetry, or timing applications.