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

TL;DR

This study investigates the charge collection efficiency and trapping rates in irradiated p-on-n silicon sensors at fluences relevant to the HL-LHC, revealing better-than-expected performance due to reduced trapping.

Contribution

It provides new measurements of trapping rates in irradiated silicon sensors and compares them with simulations, indicating improved sensor performance at high fluences.

Findings

Effective trapping rates are about 50% smaller than standard extrapolations.

Charge collection efficiency remains relatively high at high fluences.

Sensor performance exceeds initial expectations based on previous models.

Abstract

The degradation of signal in silicon sensors is studied under conditions expected at the CERN High-Luminosity LHC. 200 $\mu$m thick n-type silicon sensors are irradiated with protons of different energies to fluences of up to $3 \cdot 10^{15}$ neq/cm$^2$. 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 suggests an improved tracker performance over initial expectations.