UNIVERSIT . . AT BONN Physikalisches Institut - Prof. Dr. Norbert ...

2.5. Direct converting sensor materials 21
signals i(t), where Q0 is the total generated charge, which is known from the energy of
the impinging α-particles.
� t ′
Q(t ′ ) =
0
i(t)dt = Q0
D · (x(t′ ) − x(0)) (2.19)
⇒ x(t ′ ) = Q(t′ ) · D
− x(0) (2.20)
Q0
In combination with (2.11), these equations yield a position-dependent description of the
electric field in a single channel detector.
E(x(t)) =
i(x(t)) · D
Q0 · µ
(2.21)
The electric field in the Si p-n diode shows the expected linear decrease, which furthermore
agrees nicely with a theoretical calculation of the field profile at room temperature and
96 V depletion voltage [39].
Based on this validation of the experimental method the electric field profiles in CdTe and
CdZnTe were analyzed. As shown in Fig. 2.14, CdZnTe has a field profile, which can be,
over a large section of the detector, approximated by a linear decrease in field strength
from cathode to anode. This infers that these detectors contain a homogeneous, positive
space charge, which is most likely caused by trapped holes.
A similar situation is found for the 500 µm thick CdTe sensors. However, here the limitations
of the TCT method become obvious as the the electric field profile close to the
cathode shows an artificial breakdown. This artifact is a result of a delay in the charge
carrier movement right after their generation. The dense electron-hole cloud partially
shields the sensor’s electric field [2] thus influencing the charge carrier velocities and as
a consequence also the reconstruction of the field profile. This measurement induced origin
of the breakdown is supported by a recent study [42], in which electric field profiles
obtained with the TCT method were compared to measurements based on the Pockel’s
effect. While the TCT measurements yield field breakdowns similar to the ones shown
in Fig. 2.14, the transmission of polarized laser light through the crystals reveals no such
effects.
In summary, the electric field in CdTe as well as CdZnTe deviates from the idealized model
of a solid state ionization chamber with a constant electric field. To be precise, both materials
show a significant internal space charge and an accompanying non-constant electric
field.
Polarization
In general polarization describes the decrease of a sensor’s detection efficiency due to the
accumulation of space charges inside the detector volume. A detailed model of this effect
can be found in [43] from which Fig. 2.15 was taken. The figure shows that the trapped
hole charge as a function of the position inside the detector follows the intensity profile
of the impinging X-ray beam. In that case the large space charge close to the cathode
causes a sharp drop in the electric field and thereby introduces a zero field pinch point.
This weak field region greatly hinders the charge carrier movement and leads to electronhole
recombination, i.e. an additional charge loss or a reduced charge collection efficiency.
Previous measurements [2,44] have shown that these polarization effects are prominent in
CdTe sensors with Schottky contacts.
In addition to this, recent measurements have also shown lateral polarization in CdZnTe