Title:Growth Process of Cd0.8Zn0.2Te Crystal to Enhance its Performance as
Detector for High-energy Radiation
Volume: 5
Issue: 2
Author(s): Ching-Hua Su*
Affiliation:
- EM31, NASA/Marshall Space Flight Center, Huntsville, Alabama, USA
Keywords:
Phase diagram, Cd0.8Zn0.2Te crystals, directional solidification, controlled overpressure, Te precipitate, IR transmission micrographs.
Abstract:
Introduction: In the applications of room temperature detector for high-energy radiation,
there are two critical requirements for the semiconducting material cadmium zinc telluride (CdZnTe):
(1) high electrical resistivity to reduce the bulk leakage current and (2) low levels of structural defects
which hinder the detectivity as trapping and recombination centers for the carriers. To enhance the performance
of the detectors, an optimal single process has been developed in the melt growth of
Cd0.8Zn0.2Te by directional solidification with controlled Cd overpressure to maximize the electrical resistivity
as well as minimize the structural defects, including Te precipitates/inclusions of the grown
CdZnTe crystals.
Methods: Using the phase diagram data of pressure-temperature-composition (P-T-X), melt growth of
Cd0.8Zn0.2Te crystals by directional solidification from a starting melt at 1145oC has been performed
with various Cd overpressures controlled by the temperature of a Cd reservoir. The grown crystals were
sliced and were characterized by electrical resistivity measurements and chemical analysis of Glow
Discharge Mass Spectroscopy (GDMS). The structural defects were studied by the infrared (IR) transmission
images taken by an IR microscope.
Results: By doping of In (4-6 ppm, atomic) and growing with a Cd reservoir in the range of 785 to
825oC, the electrical conductivity was consistently higher than 109 W-cm and up to 2x1011 W-cm.
From the trend of the Te precipitates density observed by the IR micrographs, it was concluded that a
Cd reservoir temperature of 820+10oC resulted in the lowest precipitate density.
Conclusion: The employment of a Cd reservoir temperature of 820+5oC during the growth process will
provide the optimal Cd pressure over the melt at 1145oC to maximize the electrical resistivity as well as
minimize the structural defects, including Te precipitates/inclusions of the grown Cd0.8Zn0.2Te crystals.
Discussion: Since the solids of different compositions, x in the Cd1-xZnxTe system, have different
liquidus/solidus temperatures as well as different homogeneity ranges. The procedure presented here
for the Cd0.8Zn0.2Te solid may not be applicable to other compositions.
