CdZnTe Material Uniformity and Coplanar-Grid Gamma-Ray Detector Performance Page: 2 of 8
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Presented at the 1999 IEEE Nuclear Science Symposium
characterization procedure which consists first of lapping the
crystal surfaces with a fine-grit SiC powder in water slurry on
a glass plate. The scattering of light from the lapped crystal
surfaces is dependent on the crystallographic orientation
thereby allowing the identification of grain boundaries through
simple visual inspection of the surfaces. We crudely
categorize a boundary as either a twin if it is straight or
random if it is not. Following this, the crystal is mechanically
polished with a water-based slurry of sub-micron alumina
powder on a fabric pad in order to produce smooth surfaces.
The crystal is then characterized using infrared transmission
microscopy through which pipes, precipitates, and inclusions
are identified. The fabrication of the crystal into a simple
planar detector is continued by chemically etching the crystal
in an approximately 2 % bromine-methanol solution in order
to remove the surface damage introduced by the mechanical
processing. Immediately following the etch, full-area Au
electrodes approximately 80 - 90 nm thick are deposited onto
two opposing detector surfaces through thermal evaporation.
The completed planar detector is then mounted into a vacuum
chamber where one of the detector electrodes is illuminated
with alpha particles from an mAm source. A thin windowless
alpha-particle source is used, and the measurements are made
under vacuum to ensure that the alpha particles entering the
detector have a narrow energy distribution. A bias is applied
across the detector to cause the collection of the electrons and
holes generated by the alpha-particle interaction events within
the detector. Finally, each of the charge pulses induced on the
detector electrodes by the collection of the alpha-particle
generated charge is measured with a charge-sensitive
preamplifier and standard pulse-processing electronics chain.
The use of an alpha-particle source provides the advantages of
large signals, a well-defined energy deposition and interaction
region (within 20 m of the entrance contact), and ease of
collimation. These properties provide the means for highly
detailed and accurate characterization of nonuniformities in
charge generation and transport.
The measurement configuration just described is used to
extract both the electron and hole mobilities and lifetimes of
the CdZnTe crystal. For the electron properties, the cathode
of the detector is illuminated with alpha particles, and pulses
measured with a charge-sensitive preamplifier connected to the
anode are acquired as a function of the detector bias. From
each pulse a rise time tr and a pulse height V, are measured.
This is done over a bias range of about 500 V to 2000 V.
Assuming a constant field in the detector, the electron mobility
e is determined from the rise time of each pulse using
e =d/trVb (1)
where d is the detector thickness and Vb is the magnitude of
the bias applied across the detector. Typically the mobility is
nearly constant over the bias range chosen. The electron
mobility of the crystal is then taken as the average over the
values extracted at the different biases. The electron lifetime is
extracted assuming a single lifetime re with no detrapping
effects. The Hecht equation describes such a situation:
V = b t e2t ) (2)
f Jrwhere Qo is the amount of charge generated by an alpha-
particle interaction event at the cathode and Cf is the feedback
capacitance of the charge-sensitive preamplifier. Equation (2)
is fitted to the measured V and t data with Qo/Cf and r. as the
fitting parameters thereby determining the electron lifetime of
the crystal.
The hole mobility Ph and lifetime 'h of a CdZnTe crystal
are determined using a similar measurement setup as that for
the measurement of the electron properties. For the
measurement of the hole properties, however, the anode of the
detector is illuminated with alpha particles and the pulses
induced on the cathode are measured. In this geometry the
pulses are produced from the collection of the alpha-particle
generated holes from the anode to the cathode. Since hole
transport is poor in CdZnTe, the resultant small signal levels
and long collection times complicate the extraction of the
mobility and lifetime from this pulse data. For this reason, the
mobility is determined from the initial slope of the pulse. The
initial pulse slope isdV Q
dt Cf tr(3)
Substituting t = d2/p4Vb into (3) and rearranging, we get the
following for the hole mobility:2
_d C1 dV
hVb Q0 dit(4)
Equation (4) allows us to directly determine the mobility from
the initial slope of the induced charge signal pulse since the
remaining parameters in the equation are known from the
electron lifetime measurement.
The extraction of the hole lifetime is accomplished using a
version of the Hecht equation which includes the effect of the
exponential decay of our resistive-feedback charge-sensitive
preamplifier. Using this equation, tr = d2/p V, the measured
pulse height, and the previously extracted value of Ph, we
obtain an equation which we solve numerically to determine
'h. We note that this method neglects detrapping effects which
can be significant over the typical 20 - 50 s rise times of the
pulses. Consequently, the hole lifetime is overestimated in
some cases. The extraction of Ph and 'h is made at a number
of different biases, and the averages of these values are taken
as the hole mobility and lifetime for the crystal.
Following these measurements of the average charge
transport properties of the crystal, the overall electron
generation and transport uniformity of the material is
characterized. This is accomplished by uniformly illuminating
the full cathode area of the crystal with an z41Am alpha-particle
source and measuring the resultant pulse-height spectrum at a
typical detector bias of 1000 V. The spectral line shape and
any background that may be present provide a measure of
uniformity.
One concern with this characterization method is that
energy straggling from a dead layer at the entrance contact or a
nonuniform dead layer could lead to a substantial broadening
of the full-energy peak. If such broadening is large, then the
full-energy peak width will be a measurement dominated by
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Amman, M.; Luke, P. N. & Lee, J. S. CdZnTe Material Uniformity and Coplanar-Grid Gamma-Ray Detector Performance, article, October 1, 1999; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc720525/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.