  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.
