Effect of the concentration of uncompensated impurities on the properties of CdTe-based X- and γ-ray detectors

Abstract

Measurements of the 55 Fe isotope emission spectra and the photosensitivity of CdTe detectors with a Schottky diode, and also the temperature dependence of the resistivity of a CdTe crystal ((2-3) × 10 9 Ω cm at 300 K) have been used to determine the concentration of uncompensated donors (1-3) × 10 12 cm -3 . Similar measurements performed for Cd 0.9 Zn 0.1 Te crystals with the resistivity (3-5) × 10 10 Ω cm at 300 K have shown that the concentration of uncompensated donors in this case is lower by approximately four orders of magnitude. The results of calculations show that, due to such a significant decrease in the con centration of uncompensated donors, the efficiency of X and γ ray radiation detection in the photon energy range 59 to 662 keV can decrease by one-three orders of magnitude (depending on the photon energy and the lifetime of charge carriers in the space charge region). The results obtained account for the apparent poor detecting properties of the Cd 0.9 Zn 0.1 Te detectors.

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 ).…”

“…If a CdTe detector with a Schottky contact is subjected to the radiation of the 137 Cs isotope, the peak height at the photon energy 662 keV (α γ ≈ 0.1 cm −1 ) is practically the same when different sides of the sample are irradiated, the side of the Schottky contact or the side of the Ohmic contact. If the isotope 55 Fe is used (α γ ≈ 4000 cm −1 ), the peak height in the case of irradiation of the Schottky contact side is by two orders of magnitude, than that in the case of irradiation of the Ohmic contact side, as is shown in Figure 6a (inset) [11]. This is accounted for by the fact that, at α γ ≈ 4000 cm −1 , the effective depth of radiation penetration into the material is smaller than 1 μm, therefore, in the case of irradiation of the Ohmic contact side, a significant portion of electrons, which appeared as a result of γ-photon absorption, do not reach the SCR by diffusion [17].…”

“…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).…”

“…Thus, there is a significant difference between the concentrations of uncompensated impurities in CdTe and Cd 0,9 Zn 0,1 Te crystals, which are used in the fabrication of X/γ-rays detectors. The concentration of uncompensated impurities is ~(1-3) × 10 12 cm −3 for CdTe crystals and (1-5) × 10 8 cm −3 (i.e., four orders of magnitude lower) for Cd 0,9 Zn 0,1 Te crystals [11]. The low concentration of uncompensated impurities (10 8 -10 9 сm −3 ) is the reason for the unsatisfactory detectivity of Cd 0,9 Zn 0,1 Te detectors regardless of a fully acceptable resistivity of crystals (> 10 9 Ω•сm) and the lifetime of charge carriers (> 10 −6 s) [10].…”

“…An analysis of the statistics of electrons and holes in a semiconductor, in which donor defects form self-compensated complexes, provides additional information on the compensation mechanism in the CdTe and Cd 0,9 Zn 0,1 Te crystals. Consideration of the features of self-compensated semiconductors minimizes the number of independent parameters in the calculations [11]. It is important that the concentration of the donor impurity (Cl or In) is significantly higher than the concentration of background impurities and defects.…”