Radiation effects on surface and bulk properties of ATLAS18 silicon sensors under low- and high-dose gamma irradiation and annealing

Silicon strip detectors for the ATLAS Inner Tracker (ITk) at the HL-LHC must withstand harsh radiation conditions, including fluences of up to 1.6E15 1 MeV n$_{eq}$/cm$^{2}$ and total ionizing doses (TID) of up to 66 Mrad.

These requirements are met using radiation-hard n+-in-p technology implemented in the ATLAS18 silicon strip sensors currently under production.

This work presents a combined study of gamma-irradiation effects in ATLAS18 silicon sensors, including both segmented miniature strip sensors (minis) and unsegmented MD8 diodes fabricated on ATLAS18 production wafers.

The samples were irradiated with a $^{60}$Co gamma source to multiple low TIDs between 0.5 and 100 krad, corresponding to the dose range relevant for the early operational phase of the ITk tracker.

Additional measurements extending up to a few Mrad were performed to investigate the saturation of surface related damage effects.

Post-irradiation characterization included measurements of total, bulk, and surface leakage currents, as well as capacitance-voltage measurements used to extract the full depletion voltage.

The thermal stability of radiation-induced defects was studied using isochronal annealing between 80°C and 300°C and isothermal annealing at 60°C and 160°C.

In addition, complementary studies of MD8 diodes irradiated to ultra-high doses of several hundred Mrad, well beyond the ATLAS ITk requirements, are included to investigate possible bulk-related effects induced by pure gamma irradiation and their annealing behavior.

The combined analysis of low- and ultra-high-dose irradiation provides a comprehensive picture of surface- and bulk-related gamma-induced effects in ATLAS18 silicon sensors and their thermal evolution.