Concentration of uncompensated impurities as a key parameter of CdTe and CdZnTe crystals for Schottky diode x\ssty{/}γ-ray detectors

Abstract

In this paper we report on the strong impact of the concentration of uncompensated impurities on the detection efficiency of CdTe and Cd 0.9 Zn 0.1 Te Schottky diodes. The results of our study explain the observed poor detection properties of some Cd 0.9 Zn 0.1 Te detectors with resistivity and lifetime of carriers comparable to those of good CdTe detectors. We show that the concentration of uncompensated impurities in a highly efficient CdTe Schottky diode detector is several orders of magnitude higher than that of a CdZnTe, which does not register the gamma spectra of commonly used isotopes (59-662 keV) by using photoelectric measurements. The significant difference of the concentration of uncompensated impurities between CdTe and Cd 0.9 Zn 0.1 Te crystals is confirmed by our study of the temperature change of the resistivity and of the Fermi level energy. The degree of compensation of the donor complex, responsible for the electrical conductivity of the material, is much lower in the CdTe crystal compared to that in the Cd 0.9 Zn 0.1 Te crystal. The calculations of the detection efficiency of x/γ -radiation by a Schottky diode result in a dependence on the concentration of uncompensated impurities described by a curve with a pronounced maximum. The position of this maximum occurs at a concentration of uncompensated impurities which ranges from 3 × 10 10 to 3 × 10 12 cm −3 depending on the registered photon energy of x/γ -rays and on the lifetime of the charge carriers. Our measurements and calculations lead to the conclusion that the concentration of uncompensated impurities in this range is a necessary condition for the effective operation of x-and γ -ray Schottky diode detectors based on CdTe and Cd 1−x Zn x Te crystals.

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

“…The findings from the fabricated CdZnTe detectors with a Schottky diode, demonstrated in our room-temperature measurements, show that the spectroscopic performances in the low energy range definitely are better than the previously published data for CdZnTe detectors with barrier contacts [21]. Fig.…”

Diodes based on semi‐insulating CdTe crystals with Mo/MoOx contacts for X‐ and γ‐ray detectors

“…The best coincidence of the calculated (solid lines) and experimentally measured (circles) I–V curves at high reverse bias is observed at N t = 1.5 × 10 12 cm −3 , E t = 0.612 eV. The value of N t correlates with the concentration of uncompensated impurities in the CdTe crystals used in the study . Therefore, the total reverse current through the Mo‐MoO x /р‐CdTe/MoO x ‐Mo detector consists of the generation current I gen and the space charge limited current I SCLC : …”

Mechanisms of Charge Transport and Photoelectric Conversion in CdTe-Based X- and Gamma-Ray Detectors

“…At the same decreasing N d -N a , the detection efficiency of 57 Co (hν = 122 kеV) isotope varies within one order of magnitude and the detection efficiency of 133 Ba (356 kеV) and 137 Cs (662 kеV) isotopes vary relatively weak. An important feature of the results is that the dependences η (N d -N a ) for all the isotopes are described by a curve with maximum (Figure 6c) [6,10,11,25]. As seen, in all cases, the detection efficiency rather rapidly increases as the SCR widens starting at high uncompensated impurity concentrations (10 15 cm −3 ).…”

“…In addition, recombination losses in the SCR also increase and ultimately become so significant that the detection efficiency decreases with a further increase in N d -N a . The obtained results for the measurements and calculations show that, together with high resistivity, lifetime and mobility of charge carriers, the concentration of uncompensated impurities in the range 10 11 -10 13 cm −3 can be considered also necessary condition for the efficient operation of X/γ-rays detectors based on CdTe and Cd 0,9 Zn 0,1 Te [10,11]. It is the concentration of uncompensated impurities of 10 12 cm −3 in CdTe crystals made it possible to obtain 137 Cs radioisotope energy spectrum by an Ni/CdTe/Ni diode detector at applied reverse bias voltage of 1200 V with the record values of energy resolution at room temperature (2.8 keV of FWHM at 662 keV) (Figure 6c, inset).…”

“…The results of the research reveal important features of the electrical characteristics of the crystals. Figure 1a presents the typical temperature dependences of the resistivity ρ(T) of the CdTe [5][6][7][8] and CdZnTe [9,10] crystals under study.…”