V. M. Sklyarchuk

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...

Special features of charge transport in Schottky diodes based on semi-insulating CdTe

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.

Self-compensation limited conductivity in semi-insulating indium-doped Cd0.9Zn0.1Te crystals

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.

Improving and Characterizing (Cd,Zn)Te Crystals for Detecting Gamma-Ray Radiation

Cd0.9Zn0.1Te ingots were synthesized from pure components (6N purity Cd, Zn, and Te with In as the dopant) and subsequently grown from the melt under an argon overpressure. Graphite crucibles (with and without an inner coating of pyrolytic BN) were used. The temperature gradient in the solidification zone was 7-30 K/cm, and the growth rate was 0.6-1.0 mm/hour. We investigated the chemical composition, structure, and electrical properties of the as-grown crystals, and established relationships between the crystal properties and the growth conditions. The bottom, middle, and top of the ingots had n-type conductivity, but slightly different properties. Resistivity reached a maximum in the middle of the ingots ((2.5 - 5.0) ×1010 Ohm-cm), and was less at the edges ~ 0.8 ×1010 Ohm-cm. The value of the bandgap was minimal in the middle of the ingots (~ 1.5 eV), and 1.53-1.55 eV at the edges. The compensation degree ( Nd/Na) of the energy level responsible for the low dark conductivity showed a maximum value at the bottom of the ingots (~ 60 - 90%), and a minimum in the middle part (1-2%). The crystals were then used to fabricate Cd(Zn)Te detectors for gamma-ray radiation.

The energy dependence of the sensitivity for planar CdZnTe gamma-ray detectors

Considerable variations in the charge-carrier transport parameters necessitate individual calibration of CdZnTe gammaray detectors for many applications. We carried out a set of experiments wherein we determined that the main region of interest for the energy dependence of CdZnTe-detectors’ sensitivity lay in the gamma-quantum energy range of 0.03- to 3-MeV. This finding was satisfactorily verified and reconstructed from our measurements of pulse-height distributions using 241Am-, 137Cs-, and 60Co-sources. We discuss our comparison of the quality-of-fit of the approximation formulae with our detailed calculations of the sensitivity of CdZnTe detectors via a Monte-Carlo method.

Spectroscopic response of Cd(Zn)Te radiation detectors with a Schottky diode

We investigated the spectroscopic properties of several Cd(Zn)Te detectors with a Schottky contact and simulated them via a computer code. The responses were determined of 0.5-mm-thick surface-barrier Ni/Cd(Zn)Te/Ni detectors to gamma-rays from reference sources of 241Am, 133Ba, 152Eu, 137Cs and 60Co. The best measured energy-resolution at 661.67 keV (137Cs) of these detectors under 800 V of displacement voltage was better than 1.5%. The detectors’ response functions, simulated with Geant4 toolkit, agreed satisfactorily with our experimental data.

Effect of side-surface passivation on the electrical properties of metal-Cd(Zn)Te-metal structures

We explored the influence of Cd(Zn)Te detectors on the detector’s dark current for different methods of contact formation and passivation of the side surfaces. Our findings suggest that the dark current of a homogeneous detector with ohmic contacts is limited by the detector’s resistivity and the operating voltage. Detectors with a rectifying barrier have a markedly lower dark current at the same voltage and contact geometry than those without such a barrier, and their sides have a larger space charge than those of untreated ones. The major factor lowering the detector’s dark current is the formation of a rectifying barrier that occurs while creating contacts to the detector; the role of passivation of the lateral surface in this case is minimal. However, passivation plays the main role in the formation of leakage current in homogeneous detectors with ohmic contacts, where the uniformity of the electric field is important inside the detector, or in other studies used for determining the bulk resistivity of the detector material. We formed a surface-barrier structure on a semi-insulating Cr-Cd(Zn)Te-Cr crystal (n-type) with a resistivity of 1010 Ohm-cm at room temperature. The measured leakage current of this detector was less than 3 nA at 1500 V. We discuss our findings on this detector’s structural properties.

Development of CdTe-based nuclear radiation sensors and related devices

The portable nuclear radiation sensor devices have been developed based on the compact spectrometers equipped with the CdTe-based X/γ-ray detectors in a stacked mode. The stacks were assembled using four Ni/CdTe/Au Schottky diodes mounted on separate thin printed circuit boards (PCB) electrically connected in parallel to provide higher detection efficiency and then were connected to the common electrodes and fixed on the small PCB. The Ni/CdTe/Au diode detectors with a Schottky barrier and near Ohmic contact at the Ni-CdTe and Au-CdTe interfaces, respectively, were created using the Ar-ion bombarding technique. The fabricated stacks of four Ni/CdTe/Au diode detectors were tested by measuring of 137Cs isotope spectra. Energy resolution of Schottky diode detectors, mounted in a stacked mode, was slightly lower (∼ 3.0%@662 keV) compared with the separate detectors however, the number of counts in the spectra significantly increased evidencing an increase in detection efficiency (sensitivity) of the stacks. The fabricated portable sensors can be employed for identification and localization of radioactive sources in environment monitoring and high-energy radiation spectroscopic measurements.