L. A. Kosyachenko

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.

Open-circuit voltage, fill factor, and efficiency of a CdS/CdTe solar cell

The dependences of the open-circuit voltage, fill factor, and efficiency of the thin-film CdS/CdTe solar cell on the resistivity ρ and carrier lifetime τ in the absorbing CdTe layer were studied. In the common case in which the uncompensated acceptor concentration and the electron lifetime in the CdTe layer are within 1015–1016 cm−3 and 10−10–10−9 s, the calculation results correspond to the achieved efficiency of the best thin-film CdS/CdTe solar cells. It was shown that, by decreasing ρ and increasing τ in the absorbing CdTe layer, the open-circuit voltage, fill factor, and efficiency can be substantially increased, with their values approaching the theoretical limit for such devices.

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...

The COCAE Detector: An Instrument for Localization—Identification of Radioactive Sources

The COCAE project develops an instrument for localization and identification of radioactive sources. For the localization task it will exploit the Compton scattered photons within its detecting layers. It is based on pixilated Cd(Zn)Te matrices in a stacked configuration. Progress has been achieved in all the components necessary for this technology. CdZnTe crystals have been grown of up to 75mm in diameter. A wafer level pixelization process has been tested and its results will be presented. Very good energy resolution is necessary for the performance of the instrument. For this reason we have investigated planar M-p-n diodes fabricated by using laser-induced solid phase doping and Schottky diodes created by Arion etching of the Cd(Zn)Te surfaces before deposition of Ni electrodes. The energy resolution achieved is better than 1% FWHM @662KeV. The development of a pixel CMOS readout integrated circuit has been undertaken. It outputs in digital format address, energy and time information for the pixels which have collected charge above a given threshold. Simulation studies of the minimum detection limits, of the Compton sequencing algorithms, of the angular resolution and of the reconstruction of the radioactive source position have been performed. Experiments with a precursor setup using an existing hybrid detector are underway.

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.

\gamma

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

The electrical characteristics of CdTe-based Schottky-diode detectors of X-ray and γ radiation are studied. Experimental data are obtained for Al/p-CdTe diodes with a substrate resistivity from 102 to 109 Ωcm (300 K). The obtained results are interpreted in the context of the Sah-Noyce-Shockley theory of generation-recombination, taking into account the special features of the Schottky diode. It is shown that, when semi-insulating CdTe is used, the considerable forward currents observed are caused by electron injection into the substrate.

p

Cd0.9Zn0.1Te:In crystals with semi-intrinsic conductivity have been investigated. Temperature dependence of their electrical characteristics shows a number of unconventional peculiarities: the thermal activation energy of conductivity is “anomalously” low (0.60–0.62 eV); the resistivity at elevated temperatures is greater than its intrinsic value for Cd0.9Zn0.1Te; the inversion of the conduction from n- to p-type occurs at a temperature slightly above 300 K, etc. The observed features are explained in terms of statistics of electrons and holes in a semiconductor containing a self-compensation complex, whose concentration is much higher than those of uncontrolled (background) impurities and defects. Comparison of the calculation results and experimental data leads to the conclusion that the donor level, which is far distant from the middle of the band gap, dominates in the conductivity of the material and its compensation is virtually complete (Na/Nd = 0.99996–0.99998) as predicted by theory.