Speaker
Description
Inverse Low Gain Avalanche Diodes (iLGAD) can improve the signal-to-
noise ratio for X-ray detection in photon science applications. PSI collaborates
with FBK for the development of iLGAD sensors with a thin entrance window
(TEW) targeting soft X-rays. For this development a first batch of wafers,
consisting of diodes and pixelated sensors, was fabricated and is currently under
test.
In this talk, we will present measurements performed in the photon energy
range from 200 eV to 1000 eV at the Surface/Interface Microscopy (SIM) beam-
line at the Swiss Light Source (SLS).
Quantum efficiency measurements were exploited to determine the thickness
of the passivation layers and to estimate the charge collection efficiency. From
the measurements of the average gain of the iLGAD diodes, the multiplication
factor was also modeled as a function of the photon absorption depth.
In addition, soft X-ray detection with iLGAD sensors bump-bonded to the
Mönch readout chip has been investigated. The spectral response shows double
peaks due to the dependence of the multiplication factor on the position where
the photon is absorbed. Namely, if the absorption occurs before the gain layer,
the avalanche is initiated by holes that drift to the readout side, resulting in a
lower multiplication factor. If the photon is absorbed after the gain layer, the
avalanche is induced by electrons drifting to the backplane, leading to a higher
multiplication factor.
The origin of the observed spectrum has been confirmed with Monte Carlo
simulations. The simulations consider photon absorption, drift and diffusion of
carriers, as well as charge multiplication. In particular, the model used for the
multiplication factor as a function of the absorption depth has been extracted
from the iLGAD diodes. The results have been compared to the measurements,
providing not only a qualitative, but also a quantitative interpretation of the
spectrum.
