Trapping in proton irradiated p⁺-n-n⁺silicon sensors at fluences anticipated at the HL-LHC outer tracker

Abstract

The degradation of signal in silicon sensors is studied under conditions expected at the CERN High-Luminosity LHC. 200 μm thick n-type silicon sensors are irradiated with protons of different energies to fluences of up to 3 · 1015 neq/cm2. Pulsed red laser light with a wavelength of 672 nm is used to generate electron-hole pairs in the sensors. The induced signals are used to determine the charge collection efficiencies separately for electrons and holes drifting through the sensor. The effective trapping rates are extracted by comparing the results to simulation. The electric field is simulated using Synopsys device simulation assuming two effective defects. The generation and drift of charge carriers are simulated in an independent simulation based on PixelAV. The effective trapping rates are determined from the measured charge collection efficiencies and the simulated and measured time-resolved current pulses are compared. The effective trapping rates determined for both electrons and holes are about 50% smaller than those obtained using standard extrapolations of studies at low fluences and suggest an improved tracker performance over initial expectations.

Performance of the ATLAS Pixel Detector at the Start of LHC Run 3

“…The trapping rate (1/𝜏 𝑒 ) for electrons, indicated by simulation as the responsible for the majority of the collected charge, has been estimated using the data collected on IBL sensors during the bias voltage scan perfomed at the beginning of Run 3 in 2022. Fits to the cluster charge and to the slope of the increase of this charge with the applied bias voltage (HV) above the point of depletion give results in good agreement with the values used in the radiation damage digitiser, based on previous measurements [14][15][16]. The evolution of the computed electric field profile and (Right) Electron trapping rate (1/𝜏 𝑒 ) estimated from fits to the cluster charge (filled squares) and to the slope of the cluster charge increase with the applied bias voltage above depletion (filled circles) as a function of the fluence on IBL sensors.…”

Displacement Damage in Silicon Detectors for High Energy Physics

“…In a more recent work focusing on high-fluence irradiations, deviations from the linear behavior shown in Fig. 11 for particle fluences mentioned above about 3 × 10 14 n eq cm −2 were reported [60]. The inverse trapping time (trapping rate) increased slower than expected from the linear extrapolation from low-fluence data and gave, e.g., a 2-3 times lower value at 3 × 10 15 n eq cm −2 .…”

“…The relation between the basic defect parameters (electron and hole cross sections, position in the bandgap, charge states, and concentration) and the macroscopic detector properties (space charge, generation current, and trapping) was already discussed in Section I-D. We furthermore distinguish between point defects and extended defects (or cluster defects). Irradiation with 60 Co-gammas or low-energy electrons up to some MeV can only lead to single atom displacements, and thus to the creation of point defects only. Irradiation with neutrons leads predominantly to defect clusters and a few point defects.…”

“…27. DLTS spectra for neutron and 60 Co-gamma irradiated identical high resistivity FZ devices. The spectra are scaled to the signal of the vacancyoxygen level VO i which is overlapping with the C i C s level [35], [104].…”

P-Type Silicon Strip Sensors for the new CMS Tracker at HL-LHC

“…[40]. With increasing fluence, trapping effects [41,42] become more and more dominant and hole collection (at p-type strips) is a clear disadvantage due to the much lower mobility and therefore longer drift time of the holes. In figure 8 a lower seed signal is observed for n-in-p type sensors at the first irradiation step of 3 × 10 14 n eq /cm 2 , which is due to wider clusters in those sensors resulting in a smaller seed signal.…”