Room temperature In/CdTe/Au diode detectors have been developed with record energy resolution (0.7% FWHM at the
662 keV peak of 137Cs) and electrical and detection properties of the detectors have been investigated. The detectors were
fabricated using laser-induced doping of a thin surface layer of semi-insulating CdTe crystals with In. High resistivity p-like
CdTe(111) crystals pre-coated with an In dopant film were irradiated with nanosecond laser pulses in a water
environment that made it possible to introduce and activate In dopant atoms with high concentration and form a steep p-n
junction in the surface region of the crystals. Multiple laser irradiation of the samples from the In-coated side increased
forward current of the diodes and decreased reverse one. The special surface processing was used, including chemical
and thermal procedures, to modify the surface state of the CdTe crystals before deposition of an In dopant film and
electrodes. Finally, the room temperature CdTe-based X- and γ-ray radiation detectors with a p-n junction have been
obtained with extremely high energy resolution.
Charge transport mechanism responsible for leakage current in X/γ-ray detectors with a p-n junction formed in
semi-insulating p-like CdTe single crystals by laser-induced doping is studied. The In/CdTe/Au diodes showed
high rectification and good spectral response to high-energy radiation, however samples were suffering from an
increase in leakage current and deterioration of the characteristics with time. The proposed energy diagram allows
to explain the reverse I-V characteristic of the diodes. At low voltages, the Sah-Noyce-Shockley theory describes
well both the shape of the I-V characteristic and its temperature changes. At higher voltages, measured currents deviate
from the theoretically calculated values toward increasing. An additional current increase is attributed to injection of
electrons from the "near-ohmic" Au/CdTe contact and their diffusion to the p-n junction. When the current
increases, the drift component is also included in injection of electrons. This leads to a rapid rise in the current
contribution with increasing bias voltage and limits possibility to extend the detector active region by increasing
the applied voltage.
The temperature dependences of the resistivity of detector-grade semi-insulating CdTe and Cd0.9Zn0.1Te single crystals were
investigated. The investigations have revealed that the thermal activation energy can be higher than Eg/2 at T → 0 K or
considerably less than this value, although the Fermi level is located near the middle of the band gap. It is shown that such
an "anomalous" behavior of the electrical characteristics is explained in detail by the features of the compensation of deep
acceptor levels in the semiconductor band gap. A method based on the electroneutrality equation is proposed for the
determination of the ionization energy and compensation degree of the impurity (defect), which is responsible for the
conductivity of the material. The results extracted with the use of this method lead to the prediction that the inversion of the
conductivity type of the semiconductor under certain conditions can occur as the temperature varies during operation of a
Cd(Zn)Te-based device.
The possibilities for the application of CdTe layer with a Schottky barrier as an X-ray photoconductor in direct-conversion
flat-panel X-ray image detectors for digital fluoroscopy and radiography are analyzed. Conditions for
obtaining a low dark current, high charge collection and detection efficiency of X-ray radiation are investigated in the
photon energy range of 10-100 keV.
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