Radiation-tolerant Silicon Detectors for the LHC Phase-II Upgrade and Beyond: Review of RD50 Activities

At the LHC Phase-II Upgrade foreseen for 2027, the particle densities and radiation levels will increase by roughly an order of magnitude compared to the present LHC conditions, and the silicon-based inner tracking systems have to be able to withstand fluences of up to 2$\times$ 16$^{16}$ n$_{eq}$/cm$^{2}$. To mitigate the increased pileup at the HL-LHC, dedicated timing detectors are employed.Within the CERN RD50 Collaboration, a large RD program has been underway for more than a decade across experimental boundaries to develop silicon sensors with sufficient radiation tolerance for HL-LHC tracking and timing detectors. This challenge is approached simultaneously from different angles: Collaboration activities range from defect characterization and modeling to sensor development and the integration of sensors into full detector systems.One of the main objectives of the RD50 Collaboration is to improve understanding of the connection between the macroscopic sensor properties, such as radiation-induced increase of leakage current and trapping, and the microscopic properties at the defect level. An example of this isthe acceptor removal phenomenon in p-type silicon, which is affecting especially the low-gain avalanche detectors developed for fast timing applications. With increasing fluences, radiation-induced phenomena on a sensor level become increasingly complex, and call for advanced techniques and strategies to identify the mechanisms behind the observed changes in material properties. Furthermore, at very high radiation levels the differences in radiation damage caused by different types of radiation are highlighted, which complicates the definition and scaling of radiation damage.In this paper, we summarize the silicon pixel and timing detector upgrades of the LHC experiments and report on recent developments in novel silicon detector technologies, most importantly 3D detectors and low-gain avalanche detectors. Scaling of radiation damage based on leakage current and 1 MeV neutron equivalents is challenged. We also comment on considerations for silicon detectors in future collider experiments, where tracker detectors may be exposed to fluences of up to 7$\times$ 10$^{17}$ n$_{eq}$/cm$^{2}$