We have developed the detached Bridgman process for growth of CdZnTe crystals. Detachment of the solidification
interface from the growth ampoule results in a low density of dislocations in the grown material and large single crystal
grains. The detached Bridgman process also provides for direct control of the melt composition close to the growth front,
allowing for accurate control of both the density of the Te/Cd precipitates as well as the majority carrier concentration in
the grown material. The influence of melt-composition control and compensation by shallow and deep donors on
detector performance is presented.
We characterized samples cut from different locations in as-grown CdZnTe (CZT) ingots, using Automated Infrared (IR)
Transmission Microscopy and White Beam X-ray Diffraction Topography (WBXDT), to locate and identify the extended
defects in them. Our goal was to define the distribution of these defects throughout the entire ingot and their effects on
detectors' performance as revealed by the pulse-height spectrum. We found the highest- and the lowest- concentration of
Te inclusions, respectively, in the head and middle part of the ingot, which could serve as guidance in selecting samples.
Crystals with high concentration of Te inclusions showed high leakage current and poor performance, because the
accumulated charge loss around trapping centers associated with Te inclusions distorts the internal electric field, affects
the carrier transport properties inside the crystal, and finally degrades the detector's performance. In addition, other
extended defects revealed by the WBXDT measurements severely reduced the detector's performance, since they trap
large numbers of electrons, leading to a low signal for the pulse-height spectrum, or none whatsoever. Finally, we fully
correlated the detector's performance with our information on the extended defects gained from both the IR- and the
WBXDT-measurements.
Infrared (IR) transmission spectra and IR microscopy images were measured to evaluate the sliced
CdZnTe crystals grown under different starting charges using modified vertical Bridgman method.
Upon comparing the corresponding electric properties and charge transport performance, IR
absorption within the wave-number range from 500 to 2500 cm-1 was potentially attributed to the
free carrier absorption caused by the ionized impurities. The size and density of Te particles were
not sensitive to IR transmission spectra over the same wave-number range. However, the electric
field was modified around isolated Te particles, in such a way that the impurities gettering in the
Te inclusions. With respect to the high resistive CdZnTe crystals, IR transmission measurements
demonstrated that the mean transmittance is higher than 60% in the wave-number region from 500
to 4000 cm-1. IR microscopy shown the typical diameters of Te particles present in the material
were in the range of 6-9 μm, and the density of the particles was 1-4×105 cm-3. The obtained
electron mobility lifetime product (μτ)e value was in the range of 1-3×10-3 cm2·V-1 by using
well-known alpha particle spectra at room temperature. The fabricated CdZnTe thin planar
detector showed the typical energy resolution was approximately 5.7% for the 59.5 keV peak at
room temperature, without any additional signal processing.
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