Radiation hardness of amorphous silicon particle sensors

AbstractRadiation tests of 32 lm thick hydrogenated amorphous silicon n–i–p diodes have been performed using a high-energy 24 GeV pro-ton beam up to fluences of 2 · 10 16 protons/cm 2 . The results are compared to irradiation of similar 1 lm and 32 lm thick n–i–p diodesusing a proton beam of 405 keV at a fluence of 3 · 10 13 protons/cm 2 . All samples exhibited a drop of the photoconductivity and anincrease in the dark leakage current under both high- and low-energy proton irradiation. An almost full recovery of the device perfor-mance was observed after a subsequent thermal annealing. 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Wk; 61.82.Fk; 73.61.JcKeywords: Silicon; Sensors 1. IntroductionVertical integration of hydrogenated amorphous silicon(a-Si:H) diodes on top of application-specific integratedcircuits (ASICs) is an interesting option for the next gener-ation of pixelized detectors for high-energy physics experi-ments. These innovative sensors offer advantages in termsof radiation hardness, interconnection density, integrationand cost. Recently, the authors have successfully developedsuch sensors, aiming at the detection of single charged par-ticles (b particles and protons) at the minimum ionizingenergy [1,2].Even though a-Si:H has been proven to have an excel-lent radiation hardness, most experiments have so far beenperformed in the context of testing the hardness of solarcells for space applications. In thin devices, the effects ofproton [3–5], neutron [6] and photon irradiation have allvery similar consequences on material and diode proper-ties: metastable deep defects are created than can beannealed out. The process has been very extensively studiedfor light-soaking but only few experiments have been car-ried out in the case of irradiation.In the present work, we first tried to investigate thelimits of thick a-Si:H diodes under high-energy protonbeams at the fluences that are expected for detectors infuture generations of particle colliders. Similar thickdiodes as well as thin ones were also degraded usinglow-energy proton beams and lower fluences as a meansof comparison with earlier irradiation experiments pub-lished in literature [3].2. ExperimentalThe 1.1 and 32.6 lm thick diodes were deposited by veryhigh-frequency plasma-enhanced chemical vapor deposi-tion at 70 MHz and 200 C at a deposition rate of15.6 A˚/s. Test devices (in the substrate–n–i–p configura-tion) were evaporated on Cr-coated glass and the pixel area(2 · 2or5· 5mm