Inclusions and micro-defects revealed by etch pits are the main defects of CdZnTe crystal, which can not be eliminated
thoroughly at present. These defects will affect the photoelectric characteristic of CdZnTe and the quality of the MCT
epilayer used for the infrared detectors.
Many technologies are employed to suppress or eliminate the defects of CdZnTe materials, and one of them is ampoule
coating technology. In the paper, ordinary carbon coating, improved carbon coating and BN (Boron Nitride) coating
technology were used to study their effects on defects in CdZnTe crystal.
After carbon coating technology was improved, the size of inclusion in the crystal had an obvious decrease from more
than 30 μm to less than 15 μm, the density of inclusion reduced from 3~6×104cm-3 to 2×104cm-3, and etch pits
density(EPD) reduced from 1×105cm-2 to less than 5×104cm-2. This meant carbon film breaking off from the ampoule
was major factors to form inclusion with high density and large size, and high EPD might be relative with impurities in
the material.
The size of inclusions and the densities of inclusion and etch pits could be further decreased to less than 5 μm, 4×103cm-3
and 1×104cm-2 respectively after taking BN coating technology, while cell structure of etch pits found in the previous
CdZnTe wafers disappeared. This result further showed that high density of inclusion and EPD originated from carbon
impurities in the materials.
Cadmium zinc telluride(CdZnTe) crystals are widely used in the fabrication of γ-ray detectors and IR focal plane
arrays(IRFPA). But the existence of the inclusion defects in the materials have limited performances and yield of the
detectors for quite long time. So it is very important and emergent to study the characteristic of the inclusion in CdZnTe
crystal and how to suppress them.
In this paper, the distribution characteristic of inclusion densities in the CdZnTe Crystal were described by the IR-transmission
microscope technique. And the sizes and densities of the inclusions were found to be relative with the melt
stoichiometry and cooling process of the growth. It was also found that the post-annealing of CdZnTe samples can
eliminate the inclusions by the element complementarity under the action of thermal drive. In order to investigate the
origin of the inclusions, a quick cooling growth experiment was completed by quenching the ampoule into the water. The
result showed that there isn't any inclusion in the region of the material where the material grows in very fast growth
rate. But the transmissivity of the material has an obvious decrease in long wave band. This means that a great deal of
point defects generate instead of the form of the inclusions. The mechanism to form the inclusions has been analyzed
based on the above mentioned phenomena and the phase diagram of CdZnTe. Then it can be indicated that the controls
of the melt stoichiometry and cooling process of the growth are the two main factors to eliminate the inclusions of
CdZnTe materials.
The Cd-annealing effects on Cd1-xZnxTe wafers were studied by means of IR transmission and micro-Raman spectrum. The experiments and theoretical analysis demonstrated that the free carrier absorption related to the Cd vacancies resulted in the IR extinction as observed in the transmission spectra. The Raman spectra showed that Raman scattering is a more sensitive method to detect the fine Te precipitates in the Cd1-xZnxTe substrates. The Raman scattering peaks related to the Te precipitates could be found in both the as grown and the annealed samples. The relative intensity of the Te scattering peaks became weaker after Cd-annealing. This result also indicated that it was quite difficult to eliminate the fine Te precipitates entirely through annealing process.
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