Boron Removal Effect in p-type Silicon Sensors

Abstract

Silicon detectors operated at collider experiments like for instance the Large Hadron Collider at CERN, experience radiation damage due to the intense flux of charged and neutral particles through them. In the innermost part of the detector, the particle fluence can reach up to Φeq = 3.5 × 10^16 cm^{−2}. To increase radiation tolerance, the silicon sensors currently developed for the LHC detectors upgrade are boron-doped p-type silicon, in contrast to the n-type sensors currently in operation. In addition to planar pixel sensors, also devices with an additional gain layer, under the name of Low Gain Avalanche Detectors (LGADs), will be employed in the upgraded LHC detectors. These devices feature a p-type layer with higher boron concentration than in the bulk of p-type sensors. This work focuses on one particular radiation damage effect in p-type silicon sensors known as the boron removal effect. This effect is characterized by the removal of boron dopants from their lattice sites, causing them to lose their acceptor properties. The boron removal effect is particularly significant for p-type silicon sensors. For example, the reduction in gain value observed in LGADs after radiation exposure is commonly attributed to the deactivation of boron dopants during the initial irradiation. This effect is closely associated with radiation-induced defects. In general, the types of bulk defects induced by radiation, whether point-like or cluster-related, can vary depending on the type of radiation. For example, both hadrons and leptons can induce either defect type, but leptons are more likely to introduce point defects, while high-energy photons only generate point-like defects. The properties of these defects are determined by their components, i.e. the elements that make up the defect, and these elements strongly depend on the concentration of dopants and impurities like Oxygen or Carbon. In this work, diodes with different dopants and impurities concentrations (boron, oxygen and carbon) were studied after being exposed to three different types of radiation (23 GeV protons, 5.5 MeV electrons and 60 Co γ-ray). The microscopic techniques of Thermally Stimulated Current (TSC) and Thermally Stimulated Capacitance (TS-Cap) were employed to characterize radiation-induced defects, especially focusing on BiOi (boron interstitial and oxygen interstitial) and CiOi (Carbon interstitial and Oxygen interstitial) defects. The presented results include the analysis of defect concentrations, energy levels within the band gap, and their charged states at room temperature. The changes in macroscopic properties, such as leakage current and space charge density, after irradiation, are evaluated through current-voltage (I–V) and capacitance-voltage (C–V) measurements, respectively. To assess the impact of microscopic defects on macroscopic properties, isothermal annealing experiments at a temperature of 80 ◦C with varying annealing times, and isochronal annealing experiments for a duration of 15 minutes with varying annealing temperatures up to 300 ◦C have been performed. In the end, the development of the boron removal rate with initial doping concentration, irradiation fluence (for different radiation sources) and impurities are presented and discussed together with the radiation-induced defects in chapter 6-8. Besides that, the annealing behaviour of Bi Oi including the candidate state of BiOi after it anneals out is given.