The In/CdTe/Au p-n junction-diode X/γ-ray detectors, formed by frontside laser irradiation doping, were studied using IV characteristics, measured at different temperatures, and spectra of 241Am, 57Cs, and 137Cs isotopes, obtained in a wide bias range V = 60-380 V. A key feature of the technology was low-temperature ( ~90 ºC) vacuum annealing of polished in a Br-methanol solution detector-grade (111) oriented p-like CdTe crystals prior to the deposition of an In dopant film and formation of electrodes. After laser-induced doping of a layer near the In/CdTe interface and deposition of an Au electrode (ohmic contact), the In/CdTe/Au structures showed high rectification. The I-V measurements and calculations revealed that the dominant charge transport mechanism at low reverse bias was generation-recombination in the space charge region. It was noteworthy that the reverse current linearly increased at higher V ≥ 50 V when the depletion region extended over the entire crystal thickness. A sharp increase in I at higher V, that was inherent in diode structures (I ~ V n , n < 1), was not observed that evidenced a perfect ohmic contact, i.e. no injection of minority carriers from the Au/CdTe contact occurred. The detectors formed on the preliminary annealed CdTe crystals showed high energy resolution ((FWHM = 0.99 %@662keV at V = 300 V). Furthermore, high spectroscopic characteristics (detection efficiency, energy resolution, true energy position of the 662 keV peak) were observed (with a deviation < 20 %) at V =150-400V.
The defect structure of p-CdTe:Cl single crystals and MoOx/p-CdTe/MoOx heterostructures based on them were investigated by high-resolution X-ray diffractometry methods. Different models of dislocation systems were applied, according to which the densities of dislocations were estimated from the Wilson-Hall plot. It is shown that the application of the MoOx layer significantly affects the density of dislocations and their influence on the electrical and spectroscopic properties of heterostructures is estimated
The electrical and spectroscopic properties of the CdTe-based p-n junction-diode X/γ-ray detectors have been studied by the measurements of I-V characteristics and emission spectra of 137Cs, 57Co, and 241Am radioisotopes. The In/CdTe/Au diodes were fabricated by the frontside laser irradiation doping technique. Detector-grade p-like CdTe(111) crystals, pre-coated with an In dopant (electrode) film, were irradiated with nanosecond KrF laser pulses in distilled water. Laser stimulated solid-phase doping was attributed to generation and propagation of stress and shock waves, barodiffusion of In dopant atoms into the thin CdTe surface region and thus, creation of high concentration donors. The second contact (quasi-ohmic) was formed on the opposite (Cd-terminated) crystal side by vacuum evaporation of Au. In addition, some diodes were created on CdTe crystals, which were preliminary annealed in vacuum prior to deposition of metal films. The I-V characteristics of the In/CdTe/Au p-n junction diodes evidenced that the reverse dark current in the initial part was described by the Sah-Noyce-Shockley theory. Creation of the favorable conditions for efficient collection of photogenerated charge carriers, decrease in the reverse dark current in the In/CdTe/Au detectors ensured obtaining the high-energy resolution spectra. Based on the electric field dependence of the 137Cs isotope spectrum, it was established that the applied bias V = 800 V was the optimal for the diodes formed on preliminary annealed CdTe crystals which ensured the energy resolution FWHM = 0.62 % at the 662 keV line
The defects structure, charge collection, and detection efficiency of the Ni(NiO)/p-CdTe/Au/Cu Schottky-diode detector have been investigated. The spectroscopic properties of the obtained heterostructures have been studied experimentally and analyzed theoretically. The optimal reverse bias voltage for higher performance of the detectors under study was determined. The reasons of poor charge collection in the detectors and low detection efficiency of photons emitted by an 241Am (59.5 keV) radioisotope have been established and discussed. The techniques of increasing the functional parameters of Ni(NiO)/p-CdTe/Au/Cu Schottky diodes have the investigated and the optimal ways for improvement of the detector performance have been formulated.
Epitaxial layers and complex heterostructures on the base of the CdTe and Cd1-xMnxTe crystals were obtained by pulsed laser irradiation both in their transparency and absorption regions. Structural perfection of the laser-epitaxial layers, transition regions and distribution of the Te and Cd inclusions in the area of the laser action were studied. Analysis of the U-V and C-V characteristics of the obtained structures indicates a decrease in influence of surface states at the hetero-boundaries as a result of laser treatment in optimal mode, and also a reduction of reverse dark current and loss current. The spectral characteristics of the laser-epitaxial structures on the base of CdTe and Cd1-xMnxTe show, that they are promising materials for X- and gamma-radiation detectors.
Peculiarities of morphology and electric properties of X-ray and γ-ray detectors developed by graphene deposition onto commercially available (111) oriented CdTe:Cl wafers has been investigated. Laser treatment of graphene contact using millisecond YAG-laser with an energy density of 0.1–4.5 J/cm2 has been carried out in order to modify and improve their structure and phase state.
The degree of structural perfection of CdTe:Cl single crystals was estimated by methods of high-resolution Xray diffractometry. Two possible systems of dislocations that consists of two sets of complete 60-degree dislocations and Frank partial dislocations were investigated with the use of Krivoglaz kinematic theory and Monte Carlo method. The density of dislocations that provides correspondence between experimental and simulated reciprocal space maps is determined.
The electrical and photoelectric properties of thin-film CdS/Cd1-xMgxTe (x = 0-0.07) solar cells are investigated. The measured I-V characteristics of the heterostructures are described in terms of well-known theoretical models. The quantum efficiency spectra are analyzed taking into account the drift and diffusion components, recombination on the front and rear surfaces of the Cd1-xMgxTe absorber layer. Comparison of the calculation results with the experimental data allows to determine the main parameters of the Cd1-xMgxTe layer and diode structure.
The present paper analyzes the сharge transport mechanisms and spectrometric properties of In/CdTe/MoOx heterojunctions prepared by magnetron sputtering of indium and molybdenum oxide thin films onto semi-insulating p-type single-crystal CdTe semiconductor, produced by Acrorad Co. Ltd. Current-voltage characteristics of the detectors at different temperatures were investigated. The charge transport mechanisms in the heterostructures under investigation were determined: the generation-recombination in the space charge region (SCR) at relatively low voltages and the space charge limited currents at high voltages. The spectra of 137Cs and 241Am isotopes taken at different applied bias voltages are presented. It is shown that the In/CdTe/MoOx structures can be used as X/γ-ray detectors in the spectrometric mode.
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)
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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