Sensor R&D for the CMS Tracker Upgrade for the HL-LHC

At an instantaneous luminosity of $5\times 10^{34} cm^{-2} s^{-1}$, the high-luminosity phase of the Large Hadron Collider (HL-LHC) is expected to deliver a total of 3000 $fb^{-1}$ of collisions, hereby increasing the discovery potential of the LHC experiments significantly. However, the radiation environment of the tracking system will be severe, requiring new radiation hard sensors for the CMS tracker. The CMS tracker collaboration has almost completed a large material investigation and irradiation campaign to identify the silicon material and design that fulfills all requirements of a new tracking detector at HL-LHC. Focusing on the upgrade of the outer tracker region, pad diodes 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 various positions in the future tracker. The measurements performed on the structures include electrical sensor characterization, measurements of the collected charge and bulk defect characterization. In this paper, the performance and limitations of the different materials are presented.