Defect characterization studies on neutron irradiated boron-doped silicon pad diodes and Low Gain Avalanche Detectors

TL;DR

This study investigates radiation-induced defects in neutron-irradiated boron-doped silicon diodes and LGADs, revealing measurement challenges and defect behaviors affecting detector performance in high-energy physics applications.

Contribution

It provides detailed defect spectroscopy analysis of irradiated LGADs and silicon diodes, highlighting the impact of the gain layer on defect measurement accuracy and electric field effects.

Findings

Defect properties are straightforward in silicon diodes but complex in LGADs.

Capacitance measurements in LGADs are frequency and temperature dependent.

Radiation induces residual electric fields affecting carrier signals in LGADs.

Abstract

High-energy physics detectors, like Low Gain Avalanche Detectors (LGADs) that will be used as fast timing detectors in the High Luminosity LHC experiments, have to exhibit a significant radiation tolerance. Thereby the impact of radiation on the highly boron-doped gain layer that enables the internal charge multiplication, is of special interest, since due to the so-called Acceptor Removal Effect (ARE) a radiation-induced deactivation of active boron dopants takes place. In this paper we present defect-spectroscopy measurements (Deep-Level Transient Spectroscopy and Thermally Stimulated Current technique) on neutron irradiated p-type silicon pad diodes of different resistivity as well as LGADs irradiated at fluences up to 1 x 10^15 neq/cm2. Thereby we show that while for the silicon pad diodes irradiated with electrons, neutrons or protons the determination of defect electronic properties and defect introduction rates is straightforward, DLTS and TSC measurements on LGADs are strongly influenced by the impact of the gain layer. It is shown that the measurability of the capacitance of the gain layer shows a strong frequency and temperature dependence leading to a capacitance drop in DLTS and non-reliable measurement results. With TSC defects formed in the LGADs can be very nicely observed and compared to the defects formed in the silicon pad diodes. However the exact assignment of defects to the gain layer or bulk region remains challenging and the charge amplification effect of the LGADs impacts the exact determination of defect concentrations. Additionally, we will demonstrate that depending on the TSC measurement conditions defect induced residual internal electric fields are built up in the irradiated LGADs that are influencing the current signal of carriers emitted from the defect states.