The upgrade of the LHC to the High-Luminosity LHC (HL-LHC) is expected to increase the LHC design luminosity by an order of magnitude. This will require silicon tracking detectors with a significantly higher radiation hardness. The CMS Tracker Collaboration has conducted an irradiation and measurement campaign to identify suitable silicon sensor materials and strip designs for the future outer tracker at the CMS experiment. Based on these results, the collaboration has chosen to use n-in-p type silicon sensors and focus further investigations on the optimization of that sensor type. This paper describes the main measurement results and conclusions that motivated this decision.

Silicon sensors in next generation hadron colliders will face a tremendously harsh radiation environment. Requirement to study rarest reaction channels with statistical constraints has resulted in a huge increment in radiation flux, resulting in both surface damage and bulk damage. For sensors which are used in a charged hadron environment, both of these degrading processes take place simultaneously. Recently it has been observed in proton irradiated n+-p Si strip sensors that n+ strips had a good inter-strip insulation with low values of p-spray and p-stop doping densities which is contrary to the expected behaviour from the current understanding of radiation damage. In this work a simulation model has been devised incorporating radiation damage to understand and provide a possible explanation to the observed behaviour of irradiated sensors.

The expected increments in the radiation fluences to which the Si sensors will be exposed after future upgrades of LHC demands the systematic investigation of radiation damage of silicon sensors. The campaigns to produce radiation hard Si sensors have already been initiated by CMS and ATLAS. The experimental investigation of radiation damage should be complemented by simulations of silicon sensors with proper radiation damage modeling. The radiation damage modeling not only provides insight in the understanding of the radiation damage but, it is also helpful in the sensor design optimization. The radiation damage simulation of silicon sensors are needed to be carried out by simultaneous incorporation of appropriate bulk and surface damages since both the strip and pixel sensors undergo these degrading effects. The use of either bulk damage or surface damage alone can lead to wrong conclusions. In this work, simulation of irradiated silicon sensors incorporating the bulk and surface damages, using TCAD tool (Silvaco), are discussed. The bulk damage is parametrized by two trap model while the surface damage is incorporated in the simulations using oxide charge density (Q F) and interface trap density (N it).

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During the scheduled high luminosity upgrade of the LHC, the world's largest particle physics accelerator at CERN, the position sensitive silicon detectors installed in the vertex and tracking part of the CMS experiment will face a more intense radiation environment than the present system was designed for. To upgrade the tracker to the required performance level, extensive measurements and simulation studies have already been carried out. A defect model of Synopsys Sentaurus TCAD simulation package for the bulk properties of proton irradiated devices has been producing simulations closely matching to measurements of silicon strip detectors. However, the model does not provide the expected behavior due to the fluence increased surface damage. The solution requires an approach that does not affect the accurate bulk properties produced by the proton model, but only adds to it the required radiation induced properties close to the surface. These include the observed position dependency of the strip detector's charge collection efficiency (CCE).