Large-size CZT single crystals of up to 300 cm3 have been grown at Yinnel Tech. These crystals were produced into radiation detectors with excellent performances, leading to enhancements in both energy resolution and detector efficiency.
Various passivating agents that reduce the surface leakage current of CZT crystals have been previously reported. In none of the studies, NH4F/H2O2 was identified as a promising passivation agent for CZT. We now present a study that includes the effect of NH4F/H2O2 treatment on the surface properties and detector performance. An elemental depth profile was obtained via Auger Electron Spectroscopy. Furthermore, X-ray Photoelectron Spectroscopy acquired at different processing times to identify the chemical states of the elemental species that composed the dielectric layer. It was found that the NH4F/H2O2 surface passivation significantly improved the sensitivity and energy resolution of CZT detectors. Furthermore, the NH4F/H2O2 treatment did not attack the Au electrodes, which eliminated the need to protect the contacts in the detector fabrication process.
KEYWORDS: Sensors, Temperature metrology, Luminescence, Thermoelectric materials, Signal detection, Chemical species, Crystals, Zinc, Lithium, Spectroscopy
Semi-insulating CdZnTe radiation detectors from five leading crystal growers and universities were characterized by thermally stimulated conductivity (TSC), thermoelectric voltage spectroscopy (TEVS), dark conductivity, current- voltage, and variable temperature time-resolved and spatially-resolved photoluminescence (PL). By TEVS, which is an extension of the hot-probe method, all of the samples were found to have n-type electrical conductivities at room temperature and this implied that the dominant deep level is a donor level. The TSC and TEVS spectra showed that all of the samples had a dominant deep electron trap, a series of shallow electron traps, and a deep hole trap. Some of the samples showed large concentrations of shallow hole traps. A two level model of compensation is proposed which is consistent with the observed resistivities, electrical conductivities at room temperature, observed trap level energies, and observed trapping behavior. It consists of a dominant deep donor level compensating a smaller concentration of a hole acceptor level that may be shallow or deep. The model showed that the electrical conductivity type of the stably compensated materials at RT is determined by the dominant level of the compensation, which is a deep donor level for CdZnTe. Preliminary results from the variable temperature time-resolved and spatially-resolved PL showed that the emission from the traps dominate the photoluminescence spectra from these materials and that there is much spatial variation in the trap concentrations.
The literature of radiation damage measurements on cadmium zinc telluride (CZT), cadmium telluride (CT), and mercuric iodide is reviewed and supplemented in the case of CZT by new alpha particle data. CZT strip detectors exposed to intermediate energy proton fluences exhibit increased interstrip leakage after 1010 p/cm2 and significant bulk leakage after 1012 p/cm2. CZT exposed to 200 MeV protons shows a two-fold loss in energy resolution after a fluence of 5 X 109 p/cm2 in thick planar devices but little effect in 2 mm devices. No energy resolution effects were noted from moderated fission spectrum neutrons after fluences up to 1010 n/cm2, although activation was evident. Exposures of CZT to 5 MeV alpha particle at fluences up to 1.5 X 1010 (alpha) /cm2 produced a near linear decrease in peak position with fluence and increases in FWHM beginning at about 7.5 X 109 (alpha) /cm2.
We have studied the ability of different oxidizing agents, other than H2O2 to reduce the surface leakage current of CdZnTe devices. All chemical treatments were performed in aqueous solutions, at room temperature, with weight percent concentrations of 2.5g/25ml. Before and after I-V curves were obtained. It was found that by increasing the basicity of the chemical treatment, greater reduction in surface leakage current occurred. The result show that these alternative chemical treatments reduced the surface leakage current as well as or better than H2O2 chemical treatment.
Polarized transmission optical profiles were employed to characterize the CdZnTe (CZT) room-temperature radiation detectors. 2D images reflecting the internal electric field intensity changes were obtained utilizing the Pockels electro-optic effect. Varieties of different types of CZT detectors, i.e., planar and P-I-N detectors, were investigated under different operating bias voltages, respectively. Single crystal and polycrystalline CZT detectors were also studied and compared. Nonuniform internal electric field distributions throughout the detector volumes were observed and analyzed. The grain- boundary effects to the internal electric fields will be presented and discussed, along with a theoretical simulation. A semiconductor energy band model associated with depletion layer width will be emphasized and discussed.
The electric properties of CdZnTe radiation detectors are largely determined by the electron and hole traps in this material. The traps, in addition to degrading the detector performance, can function as dopants and determine the resistivity of the material. Thermoelectric emission spectroscopy and thermally stimulated conductivity are used to detect these traps in a commercially available spectrometer-grade CdZnTe detector, and the electrical resistivity is measured as a function of temperature. A deep electron trap having an energy of 695 meV and cross section of 8 X 10-16 cm(superscript 2$ is detected and three hole traps having energies of 70 +/- 20 meV, 105 +/- 30 meV and 694 +/- 162 meV are detected. A simple model based on these traps explains quantitatively all the data, including the electrical properties at room temperature and also their temperature dependence.
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