Radiation-induced point- and cluster-related defects with strong impact on damage properties of silicon detectors

Abstract

This work focuses on the investigation of radiation induced defects responsible for the degradation of silicon detectors. Comparative studies of the defects induced by irradiation with 60 Co-γ rays, 6 and 15 MeV electrons, 23 GeV protons and reactor neutrons revealed the existence of point defects and cluster related centers having a strong impact on damage properties of Si diodes. The detailed relation between the "microscopic" reasons as based on defect analysis and their "macroscopic" consequences for detector performance are presented. In particular, it is shown that the changes in the Si device properties after exposure to high levels of 60 Co-γ doses can be completely understood by the formation of two point defects, both depending strongly on the oxygen concentration in the silicon bulk. Specific for hadron irradiation are the annealing effects which decrease respectively increase the originally observed damage effects as seen by the changes of the depletion voltage. A group of three cluster related defects, revealed as deep hole traps, proved to be responsible specifically for the reverse annealing. Their formation is not affected by the oxygen content or silicon growth procedure suggesting that they are complexes of multi-vacancies located inside extended disordered regions.

Investigation of point and extended defects in electron irradiated silicon—Dependence on the particle energy

“…The oxygen enrichment for the DOFZ process was achieved by a 72 h post-oxidation O diffusion at 1150 C resulting in an average oxygen concentration of 1.2 Â 10 17 cm À3 , $20 times larger than in the STFZ ones. 10 The oxygen content in EPI diodes is 2 Â 10 17 cm À3 , similar to the DOFZ samples. The Carbon content in all of the investigated materials is $2 Â 10 15 cm À3 .…”

“…Donor with energy level in the upper part of the bandgap, strongly generated by irradiation with charged particles 10,29 Linear fluence dependence (this work) Amphoteric defect generated via a second order process (quadratic fluence dependence), strongly generated in Oxygen lean material [22][23][24] (this work) Not identified extended defect. Acceptor with energy level in the lower part of the bandgap 10,29 Linear fluence dependence (this work)…”

“…Any promising attempt for radiation hardening of the silicon material as well as improvements by modifying the detector processing will rely on a thorough knowledge of the generation of electrically active defects in the bulk material. Macroscopic effects induced by irradiation in silicon detectors as seen from the device electrical characteristics measured at room temperature (RT) are: [3][4][5][6][7][8][9][10] (i) Strong increase of the detector leakage current (LC). This fact requires cooling to cope with the otherwise unavoidable heating due to the dissipation power in large area installations thus to ensure a tolerable signal/noise ratio for the detection of minimum ionizing particles.…”

“…Among the multitude of radiation induced electrically active defects only some proved to have a direct impact on the "macroscopic" behavior of the sensors operating at ambient temperatures. These point-or extended-defects are labeled in the literature as: I p -a deep acceptor strongly generated in oxygen lean, standard float-zone material (STFZ) [22][23][24] and BD-a bistable thermal donor (TDD2) 24,26,27 strongly generated in oxygen rich float-zone material (DOFZ), both associated with point defects that are stable at room temperature and determining the N eff in silicon diodes irradiated with Co 60 g-rays; E(30K)-a shallow donor contributing to the a) R. Radu beneficial annealing after hadron irradiation, strongly generated in diodes irradiated with charged particles; 10,29 H(116K), H(140K), H(152K)-cluster-related hole traps with enhanced field emission (acceptors in the lower part of the Si bandgap), contributing fully with their concentration to N eff and causing the long term annealing effects (reverse annealing) in hadron irradiated silicon diodes; 10,29 the bistable E4 and E5 energy levels-identified with the double and single acceptor charge state of the V 3 defect in a configuration part of a hexagonal ring (PHR), respectively, [30][31][32][33] a defect contributing to the magnitude of the leakage current in the Si sensors and bipolar transistors upon irradiation with high energy particles. 28,31 The most important characteristics of these defects, including some features resulted from the present work, are summarized in Table I.…”

Simulation of radiation-induced defects

“…Measurements with methods like Thermally Stimulated Current technique (TSC), Deep Level Transient Spectroscopy (DLTS), Transient Current Technique (TCT), Capacitance-Voltage (C-V) and Current-Voltage (I-V) have revealed a multitude of defects (11 different energy levels listed in [13]) after irradiations with hadrons or higher energy leptons. The microscopic defects have been observed to influence the macroscopic properties of a silicon sensor by charged defects contributing to the effective doping concentration [14,15,16,17,18,19] and deeper levels also to trapping and generation/recombination of the charge carriers (leakage current) [19,20,21,22]. Such quantity of defect levels set up a vast parameter space that is neither practical nor purposeful to model and tune.…”