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Description
Silicon strip detectors developed for the Inner Tracker (ITk) of the ATLAS experiment are designed to operate in the harsh radiation environment of the HL-LHC accelerator. In the strip region of the ITk, sensors must withstand a total fluence of $1.6 \times 10^{15}~\text{n}_{\text{eq}}\text{ 1 MeV}/\text{cm}^2$ and a total ionizing dose (TID) of $66~\text{Mrad}$. To meet these requirements, radiation-hard $n^+$-in-$p$ sensor technology has been implemented in the ATLAS18 silicon strip sensors currently under production. The radiation tolerance of these sensors was verified through extensive irradiation studies conducted during development, including exposures to various particle types and energies. These studies were performed up to the maximum expected fluence and TID levels to ensure end-of-life operations.
This work delivers a comprehensive investigation of radiation-induced effects, using the miniature sensors in the ATLAS18 production wafers, irradiated with a $^{60}$Co $\gamma$-source to multiple low TIDs, ranging from $0.5$ to $100~\text{krad}$—levels highly relevant for the initial operating phase of the ITk tracker. Post-irradiation characterization included detailed measurements of total, bulk, and surface currents as functions of dose, temperature, and annealing regimes. Changes in full-depletion voltage and effective dopant concentration were extracted from capacitance–voltage analyses performed over the same parameter space. To investigate the thermal stability of radiation-induced bulk and surface defects, as well as to investigate their properties, isochronal annealing was performed from $80~^\circ\mathrm{C}$ to $300~^\circ\mathrm{C}$ and at a fixed temperature of $160~^\circ\mathrm{C}$.
The surface current showed a strong nonlinear increase with dose under the applied irradiation conditions and dominated the total current contribution over most of the TID range investigated. High-temperature annealing (above $\sim 250~^\circ\mathrm{C}$) reduced the total leakage current to pre-irradiation values, while the capacitance–voltage characteristics, and thus the full depletion voltage, remained essentially unchanged after irradiation and also after annealing. These results indicate that $\gamma$-radiation–induced defects in the mini sensors are predominantly surface-related and can be completely removed by sufficiently high-temperature annealing. This behaviour contrasts with earlier observations in high-dose ($630~\text{Mrad}$) irradiated diodes, where the bulk current increased linearly with dose and high-temperature annealing had little effect on the currents, but restored the full depletion voltage close to its pre-irradiation value. These observations are under investigation.
