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Low dose gamma irradiation study of ATLAS ITk MD8 diodes

M. Mikeštíková, V. Fadeyev, P. Federičová, P. Gallus, J. Kozáková, J. Kroll, M. Kůtová, J. Kvasnička, P. Tůma, M. Ullán, Y. Unno

TL;DR

This study investigates how low-dose $^{60}$Co gamma irradiation affects leakage currents in ITk diodes used for ATLAS, focusing on separating surface and bulk contributions and understanding their evolution during annealing and temperature changes. Using MD8 and MD8p diodes (the latter including a p-stop implant) irradiated from 0.5 to 100 krad, the authors perform precise I–V measurements to track bulk, surface, and total currents, coupled with systematic annealing and temperature scans. They find that surface leakage rises significantly after irradiation and dominates the total current, while bulk leakage stays largely constant up to 100 krad; annealing at moderate temperatures slightly increases currents, whereas high-temperature annealing effectively restores pre-irradiation levels. Temperature dependence fits yield activation energies $E_A$ around $1.19$–$1.20$ eV for all current components, suggesting similar underlying defect dynamics, with the p-stop implant aiding in isolating the pertinent currents. These results have direct implications for the initial operation and design of the ATLAS ITk tracker in the HL-LHC environment, particularly regarding surface damage mitigation and thermal treatments.

Abstract

Silicon strip detectors developed for the Inner Tracker (ITk) of the ATLAS experiment will operate in a harsh radiation environment of the HL-LHC accelerator. The ITk is thus designed to endure a total fluence of 1.6E15 1MeV n_eq/cm2 and a total ionizing dose (TID) of 66 Mrad in the strip detector region. A radiation-hard n^+-in-p technology is implemented in the ITk strip sensors. To achieve the required radiation hardness, extensive irradiation studies were conducted during sensor development, primarily performed up to the maximal expected total fluence and TID to ensure a full functionality of the detector at its end-of-life. These studies included irradiations of sensors with various particle types and energies, including the Co60 gamma-rays. Our previous results obtained for gamma-irradiated diodes and strip sensors indicate a linear increase of bulk current with TID, while the surface current saturates at the lowest TID levels checked (66 Mrad), preventing a determination of the exact TID for which the observed saturation occurs. This work presents the results coming from irradiations by Co60 gamma-rays to multiple low TIDs, ranging from 0.5 to 100 krad. The detailed study of total, bulk, and surface currents of diodes explores an unknown dependence of surface current on the TID, annealing, and temperature. Additionally, the effect of the p-stop implant between the bias and the guard ring of measured samples is shown. The observations are relevant for the initial operations of the new ATLAS tracker.

Low dose gamma irradiation study of ATLAS ITk MD8 diodes

TL;DR

This study investigates how low-dose Co gamma irradiation affects leakage currents in ITk diodes used for ATLAS, focusing on separating surface and bulk contributions and understanding their evolution during annealing and temperature changes. Using MD8 and MD8p diodes (the latter including a p-stop implant) irradiated from 0.5 to 100 krad, the authors perform precise I–V measurements to track bulk, surface, and total currents, coupled with systematic annealing and temperature scans. They find that surface leakage rises significantly after irradiation and dominates the total current, while bulk leakage stays largely constant up to 100 krad; annealing at moderate temperatures slightly increases currents, whereas high-temperature annealing effectively restores pre-irradiation levels. Temperature dependence fits yield activation energies around – eV for all current components, suggesting similar underlying defect dynamics, with the p-stop implant aiding in isolating the pertinent currents. These results have direct implications for the initial operation and design of the ATLAS ITk tracker in the HL-LHC environment, particularly regarding surface damage mitigation and thermal treatments.

Abstract

Silicon strip detectors developed for the Inner Tracker (ITk) of the ATLAS experiment will operate in a harsh radiation environment of the HL-LHC accelerator. The ITk is thus designed to endure a total fluence of 1.6E15 1MeV n_eq/cm2 and a total ionizing dose (TID) of 66 Mrad in the strip detector region. A radiation-hard n^+-in-p technology is implemented in the ITk strip sensors. To achieve the required radiation hardness, extensive irradiation studies were conducted during sensor development, primarily performed up to the maximal expected total fluence and TID to ensure a full functionality of the detector at its end-of-life. These studies included irradiations of sensors with various particle types and energies, including the Co60 gamma-rays. Our previous results obtained for gamma-irradiated diodes and strip sensors indicate a linear increase of bulk current with TID, while the surface current saturates at the lowest TID levels checked (66 Mrad), preventing a determination of the exact TID for which the observed saturation occurs. This work presents the results coming from irradiations by Co60 gamma-rays to multiple low TIDs, ranging from 0.5 to 100 krad. The detailed study of total, bulk, and surface currents of diodes explores an unknown dependence of surface current on the TID, annealing, and temperature. Additionally, the effect of the p-stop implant between the bias and the guard ring of measured samples is shown. The observations are relevant for the initial operations of the new ATLAS tracker.
Paper Structure (7 sections, 1 equation, 6 figures, 1 table)

This paper contains 7 sections, 1 equation, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Samples and irradiation
  3. Experimental methods and results
  4. Bulk, surface, and total currents measured before and after irradiation
  5. Annealing studies of bulk, surface, and total currents
  6. Temperature dependence of total, bulk, and surface currents
  7. Conclusions

Figures (6)

  • Figure 1: (a) The test structure "Mini&MD8" includes a miniature strip sensor, an MD8 diode (0.8 cm $\times$ 0.8 cm), and several other smaller diodes. (b) The layout of MD8 and MD8p diodes, which is the same except for the extra p-stop implant between BR and GR ring in MD8p. bib:unno
  • Figure 2: (a) Dependence of total, bulk, and surface currents of MD8 and MD8p diodes on bias voltage measured before irradiation. (b) Total, bulk, and surface currents of irradiated MD8 and MD8p diodes measured at $\mathrm{V_{bias}}$ = 300 V after their annealing for 80 minutes at 60°C. All measurements were done at 20 $^\circ$C with a relative humidity <1%.
  • Figure 3: Dependence of the bulk (a) and surface currents (b) on the bias voltage of the MD8p diode irradiated to 25 krad measured after the specified times of annealing at 60 $^\circ$C. Measurements were performed at 20 $^\circ$C with a relative humidity of <1%.
  • Figure 4: (a) The evolution of the total, bulk, and surface currents with the time of annealing at 60 $^\circ$C measured for the MP8p diode irradiated to 25 krad and biased to 300 V. (b) Comparison of the total, bulk, and surface currents of not irradiated MD8p diode and MD8p diode irradiated to 15 krad and annealed up to 300 $^\circ$C. All measurements were performed at 20 $^\circ$C with a relative humidity of <1%.
  • Figure 5: (a) The surface currents of the diode MD8p diode irradiated to 15 krad measured after its isochronal annealing with 20-minute steps at temperatures ranging from 80 $^\circ$C to 300 $^\circ$C. Measurements were performed at 20 $^\circ$C with a relative humidity of <1 %. (b) The evolution of total, bulk, and surface currents of MD8p diode irradiated to 15 krad and biased to 300 V with annealing temperature.
  • ...and 1 more figures