Simulation study of signal formation in position sensitive planar p-on-n silicon detectors after short range charge injection

Abstract

Segmented silicon detectors (micropixel and microstrip) are the main type of detectors used in the inner trackers of Large Hadron Collider (LHC) experiments at CERN. Due to the high luminosity and eventual high fluence of energetic particles, detectors with fast response to fit the short shaping time of 20-25 ns and sufficient radiation hardness are required.Charge collection measurements carried out at the Ioffe Institute have shown a reversal of the pulse polarity in the detector response to short-range charge injection. Since the measured negative signal is about 30-60% of the peak positive signal, the effect strongly reduces the CCE even in non-irradiated detectors. For further investigation of the phenomenon the measurements have been reproduced by TCAD simulations.As for the measurements, the simulation study was applied for the p-on-n strip detectors similar in geometry to those developed for the ATLAS experiment and for the Ioffe Institute designed p-on-n strip detectors with each strip having a window in the metallization covering the p + implant, allowing the generation of electron-hole pairs under the strip implant. Red laser scans across the strips and the interstrip gap with varying laser diameters and Si-SiO 2 interface charge densities (Q f ) were carried out. The results verify the experimentally observed negative response along the scan in the interstrip gap. When the laser spot is positioned on the strip p + implant the negative response vanishes and the collected charge at the active strip increases respectively.The simulation results offer a further insight and understanding of the influence of the oxide charge density in the signal formation. The main result of the study is that a threshold value of Q f , that enables negligible * Work performed in the framework of the CERN-RD50 collaboration. losses of collected charges, is defined. The observed effects and details of the detector response for different charge injection positions are discussed in the context of Ramo's theorem. IntroductionSegmented silicon detectors, like micropixel and microstrip, are the primary detector types used in inner trackers of the Large Hadron Collider (LHC) experiments [1,2,3]. Due to the high luminosity of the proton beam and the high counting rate the detectors with fast response to fit the short shaping time of 20-25 ns are required. The detectors must operate in a high-radiation environment for at least 10 years, maintaining a sufficient performance regardless of the accumulating degradation of the silicon properties.The development and testing of advanced detectors include detailed studies of the detector current and charge responses using e.g. sub-nanosecond lasers, which enable to simulate the interaction of short or long range (MIP) particles with the detector. This gives information on the carrier transport in the detector bulk and the electric field distribution which are the input data for the prediction of the long-term scenario of the detector position sensitivity and charge collection efficiency (CC...