Radiation hard silicon sensors for the CMS tracker upgrade

At an instantaneous luminosity of 5 × 10³⁴ cm^-2 s^-1, the high-luminosity phase of the Large Hadron Collider (HL-LHC) is expected to deliver a total of 3 000 fb^-1 of collisions, hereby increasing the discovery potential of the LHC experiments significantly. However, the radiation dose of the tracking systems will be severe, requiring new radiation hard sensors for the CMS tracker. The CMS tracker collaboration has initiated a large material investigation and irradiation campaign to identify the silicon material and design that fulfils all requirements for detectors for the HL-LHC. Focussing on the upgrade of the outer tracker region, pad sensors as well as fully functional strip sensors have been implemented on silicon wafers with different material properties and thicknesses. The samples were irradiated with a mixture of neutrons and protons corresponding to fluences as expected for the positions of detector layers in the future tracker. Different proton energies were used for irradiations to investigate the energy dependence of the defect generation in oxygen rich material. The measurements performed on the structures include electrical sensor characterization, measurement of the collected charge injected with a beta source or laser light and bulk defect characterization. Measurements are performed at different annealing times. In this paper, results from the ongoing campaign are presented which led to the decision to take p-type silicon sensors for the CMS strip tracker.