Irradiation study of different silicon materials for the CMS tracker upgrade

Around 2022, an upgrade of the LHC collider complex is planned to significantly increase the luminosity (the High Luminosity LHC, HL-LHC). This means that the experiments have to cope with a higher number of collisions per bunch crossing and survive in a radiation environment much harsher than that at the present LHC. Especially the tracking detectors have to be improved for the HL-LHC. The increased number of tracks requires an increase of the number of readout channels while the higher radiation makes new sensor materials necessary. Within CMS, a measurement campaign was initiated to study the performance of different silicon materials in a corresponding radiation environment. To simulate the expected radiation the samples were irradiated with neutrons and with protons with two different energies. Radiation damage can be divided in two categories. First, ionizing energy loss in the surface isolation layers of the sensor leads to a change of the concentration of charged states in the sensor surface and therefore alters the distribution of the electrical fields in the sensor. Second, non-ionizing energy loss in the bulk of the sensor material leads to a variety of de- fects in the silicon lattice. Electrically active defects can influence the material properties. The three properties under investigation are the reverse current, the full depletion voltage and the charge collection. While the reverse current and full depletion voltage influence the power dissipation and the noise of the detector, the charge collection directly influences the measurement. The material properties were studied using pad and strip sensor. The structures were elec- trically characterized before and after irradiation with different fluences of neutrons and protons, corresponding to the expected fluences at different radii of the outer tracker after 3000 fb 1 . The charge collection measurements were mainly performed using the ALiBaVa readout system and the charge was induced with b -sources and lasers. In addition to the bulk properties, surface effects were studied. It was found that surface conditions have a great influence on the charge collection as well as on the noise and there- fore have to be considered in the design of silicon sensors.