Quantitative assessments of the possibility to improve the energy resolution of detectors based on CdTe crystals with two
Ohmic contacts by varying the crystal thickness, applied voltage, temperature and lifetimes of charge carriers are
presented. It is shown that a super-linear increase in the conductivity of Pt/CdTe/Pt detectors at larger voltages is caused
by the space charge limited current, and the excess component over the linearly approximated current is virtually
temperature independent. Due to such behavior of the excess current, cooling of the detector does not give the desired
result. The calculations show that the effect of charge carrier recombination at the crystal surfaces can be ignored when
V < 9-10 V, while trapping carriers in the crystal bulk for thick CdTe detectors can significantly degrade the charge
collection efficiency. Thinning the crystal and lowering the applied voltage to maintain the same current value leads to
considerable improving of the charge collection efficiency. In the case of the detector thickness of 0.25 mm, the
collection efficiency is higher than 97-98% in the photon energy range of 10-1000 keV. The collection efficiency could
exceed 99% if to perfect the crystal growth technology and thereby increase the charge carrier lifetime by one order of
magnitude as compared to that in the currently used CdTe crystals (2-3 μs)
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.
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.
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 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.
The electrical properties of Al-Hg1-xMnxTe (x = 0.08 - 0.1) Schottky barriers are investigated. The main parameters of the diode structure and charge transport mechanisms responsible for their characteristics, tunneling and over-barrier passage of carriers, are determined. The features of the diode electrical characteristics caused by the narrow bandgap and big difference between effective masses of carriers are revealed. The results obtained experimentally and theoretically testify of the high detectivity of the diodes studied.
Photodiodes with p-n junctions formed by ion etching of p-Hg1-xMnxTe (x approximately equals 0.1) are reported. The absorption curves of the crystals found from optical measurements are treated within the Kane theory for semiconductors with highly nonparabolic energy bands. The diode responsivity spectra are interpreted in the framework of model taking into account the generation of photocarriers in -regions as well as in the depletion layer. The diode photoresponsivity in the region of the photon energies below semiconductor bandgap is shown to be caused by the gentle slope of the observed absorption edge.
A possibility of CdTe substrate purification from impurities by structure-breakdown layer gettering, formed by laser irradiation, is considered. For profile calculation of diffusive distribution of point defects during heat treatment, and also substrate purification degree after heat treatment, a model, based on diffusion equation with consideration of impurity absorption by dislocations, is proposed. Impurity redistribution task in structure CdTe-CdHgTe during annealing is solved also. Investigations, carried out on specially prepared samples, confirmed CdTe purification effectiveness by gettering: impurity concentration decreased in 5 - 10 times.
A new optoelectronic sensor whose spectral responsivity to UV radiation is almost identical with that of the human skin or of the eyes is presented. The sensor comprises two closely-spaced UV-sensitive Au-SiC diode, one of which is fitted with a glass filter. The photodiodes are connected to electronics that amplifies, combines and subtracts electrical signals generated by radiation in the photodiodes. The responsivity of the Au-SiC diode structure with a semitransparent gold electrode covers the whole UV spectrum, with the long-wavelength end bounded by the semiconductor bandgap. The photodiode with a filter absorbing wavelengths shorter than 315-320 nm is responsive in the UV-A region, while the difference between the electrical signals generated in the filter-containing and filter-free diodes is determined by the UV-B + IV-C radiation. The measuring of biologically effective radiation over the entire UV spectral range is achieved through combining the signal generated by UV-A radiation and the previously amplified difference signal generated by UV-B + UV-C radiation. The sensor spectral responsivity thus obtained is very close to the tabular curve of the relative spectral effectiveness of UV radiation on the normal human skin or eyes.
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