V. A. Gnatyuk

Solid-liquid phase transitions in CdTe crystals under pulsed laser irradiation

Phase transitions in CdTe crystals irradiated with nanosecond KrF (248 nm) excimer laser pulses were investigated by a numerical simulation and time-resolved reflectivity method. By solving the time-dependent heat flow equation and taking into account the temperature dependences of semiconductor parameters, the time dependence of the surface temperature in a CdTe crystal was calculated as a function of laser pulse intensity. The dynamics of laser-induced melting was directly monitored by measurements of the reflection coefficient of a probe laser beam (532 nm). The melting and ablation thresholds have been determined as 2.5 and 7.3 MW/cm2, respectively.

Laser-induced shock wave stimulated doping of CdTe crystals

Action of a laser-induced plane shock wave has been considered as the mechanism of doping of CdTe surface region with In. CdTe crystals coated with a relatively thick In film were subjected to irradiation with KrF excimer laser pulses. The In film was not completely evaporated under irradiation and it served further as an electrode in the fabrication of nuclear radiation detectors. Dopant atoms, implicated by laser-induced stress and shock waves, penetrated into CdTe. An In-enriched region was formed and a built-in p-n junction arose at the depth where a stress wave was converted to a shock wave.

Electrical characteristics of Schottky diodes based on semi-insulating CdTe single crystals

Electrical properties of Schottky diodes fabricated by vacuum evaporation of Al on the surface of semi-insulating p-like CdTe single crystals have been investigated. The current-voltage characteristics (CVCs) of the diodes have an unconventional pattern which is manifested in the absence of rectification at both low forward bias voltages ( 5V) is associated with the injection of minority carriers into the bulk of the crystals.

Higher Voltage Ni/CdTe Schottky Diodes With Low Leakage Current

A significant improvement in electrical characteristics of Schottky diodes designed for X- and gamma-ray detectors has been achieved using semi-insulating CdTe single crystals and unified technology, where both Schottky and near-ohmic contacts were formed by the deposition of the same metal (Ni) on the opposite surfaces of the crystal pre-treated by chemical etching and Ar ion bombardment with different parameters. Reduction of injection of minority carriers from the near-ohmic contact in the neutral part of the diode and high Schottky barrier for holes provides low leakage current even at high bias voltage (<50 nA/cm2 at 2000 V and at room temperature). The current-voltage characteristics of the detectors with Ni/CdTe/Ni electrode configuration in the low-voltage range are described by the generation-recombination Sah-Noyce-Shockley theory. The results of the reproducibility and time stability of the fabricated diodes are reported.

Super high voltage Schottky diode with low leakage current for x- and γ-ray detector application

A significant improvement in x-/γ-ray detector performance has been achieved by forming both rectifying and near-Ohmic contacts by the deposition of Ni on opposite surfaces of semi-insulating CdTe crystals pretreated by special chemical etching and Ar-ion bombardment with different parameters. The reduced injection of the minority carriers from the near-Ohmic contact in the neutral part of the diode provides low leakage current even at high bias (<50 nA/cm2 at 2000 V and 293 K). The electrical properties of the detectors are well described quantitatively by the generation-recombination Sah–Noyce–Shockley theory excepting the high reverse voltage range where some injection of minority carriers takes place.

Optimal width of barrier region in X/γ-ray Schottky diode detectors based on CdTe and CdZnTe

The spectral distribution of quantum detection efficiency of X- and γ-ray Schottky diodes based on semi-insulating CdTe or Cd0.9Zn0.1Te crystals is substantiated and obtained in analytical form. It is shown that the width of the space charge region (SCR) of 6–40 μm at zero bias in CdTe (Cd0.9Zn0.1Te) Schottky diode is optimal for detecting radiation in the photon energy range above 5–10 keV. Based on the Poisson equation, the relationship between the SCR width and the composition of impurities and the degree of their compensation are investigated. It is shown that the presence of deep levels in the bandgap leads to a considerable increase in space charge density and electric field strength near the crystal surface. However, this effect contributes a small error in the determination of the SCR width using the standard formula for the Schottky diode. It is also shown that the concentration of uncompensated impurities in CdTe and Cd0.9Zn0.1Te crystals within the 4 × 1011–1013 cm–3 range is optimal for the de...

Transport Properties of CdTe X/

Charge transport mechanism in X- and γ-ray detectors based on CdTe diodes with a p-n junction is studied. Shallow p-n junctions were formed in semi-insulating p -like CdTe crystals by laser-induced doping of a thin semiconductor layer with In atoms and, finally, In/CdTe/Au diode structures were fabricated. The energy diagram was developed to explain the reverse I-V characteristics of the diodes particularly increased leakage current. It was shown that the I-V characteristics at low bias voltages were described by the Sah-Noyce-Shockley theory. At higher voltages, an additional increase in leakage current was observed and it was attributed to injection of minority carriers (electrons) from the forward-biased Au/CdTe Schottky contact to the reverse-biased p -n junction (near the In/CdTe contact) through the CdTe crystal. Spectral properties of In/CdTe/Au diode detectors have also been analyzed.

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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.

-Ray Detectors With

Native and impurity point defects, complexes and extended defects which are formed during CdTe crystal growth and fabrication of diode structure are crucial for CdTe-based X-ray and gamma-ray detectors, cause deterioration of parameters and limit widespread practical application. Therefore, control of defect formation in CdTe crystals and device structures is important to achieve excellent charge collection efficiency and high energy resolution. Photoelectric, electrical and spectral properties of M-p-n structured CdTe diodes fabricated by the optimized excimer laser doping technique have been studied at different conditions. To make the diodes, a relatively thick In film was deposited on the surface of CdTe crystal and then it was irradiated with a laser pulse. The film served as an n-type dopant source as well as an electrode after laser irradiation. A Schottky contact was deposited on the opposite side of CdTe crystals. The In/CdTe/Au detectors have showed promise for nuclear radiation devices. However, the variations of I-V and C-V characteristics, fluctuations of time dependences of leakage current and degradation of spectral characteristics were evidences that electrically active defects cause non-uniform carrier trapping and induce excessive noise, deteriorating the detector performance.

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By investigating the photoluminescence (PL), we have studied laser-induced doping and defect formation in high-resistivity Cl-compensated p-like CdTe crystals coated with and without a thin In dopant film. A detailed analysis of the PL spectra has been performed to discuss the laser-stimulated modification of the defect structure in CdTe crystals subjected to irradiation with nanosecond KrF excimer laser pulses with the energy density above the melting threshold. In particular, laser irradiation increased VCd as a result of the dissociation of (Cd-X) complexes and laser desorption of Cd atoms. Laser-stimulated diffusion of In atoms at Cd vacancies provided doping of CdTe crystals. Fast freezing of a large number of the point defects InCd impeded the formation of compensating acceptors (VCd–InCd). This allows us to suppress the self-compensation mechanism, and to introduce and activate the In impurity into a thin surface layer of the crystals without damaging of the structure and properties of the underlying region.