J. Crocco

Study of the Effects of Edge Morphology on Detector Performance by Leakage Current and Cathodoluminescence

The Vertical Gradient Freeze (VGF) method has been used to grow high resistivity Cadmium Zinc Telluride (CZT) for high energy radiation applications. In this work, the effect of lapping and polishing the lateral edges of planar detectors is studied. Expectations that improved surface morphology of the edges should correlate with reduced surface leakage current are shown to be erroneous. The effect of various types of lateral edge treatments on detector performance was observed before and after each surface modification. Complementary results were obtained using I-V, Cathodoluminescence (CL), and gamma ray response measurements using 133Ba. As a result, a quick and easy method is reported which minimizes leakage current and actually enhances detector performance through the introduction of surface defects. It is demonstrated that the introduction of radiative recombination centers helps reduce surface leakage current in the detector by a factor of up to 200%, depending on the surface treatment. The purpose of this work is to identify material processing steps for fabricating planar devices based on CZT for gamma ray spectroscopy.

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

In this paper we report on the strong impact of the concentration of uncompensated impurities on the detection efficiency of CdTe and Cd0.9Zn0.1Te Schottky diodes. The results of our study explain the observed poor detection properties of some Cd0.9Zn0.1Te 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 Cd0.9Zn0.1Te 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 Cd0.9Zn0.1Te 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 × 1010 to 3 × 1012 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 Cd1−xZnxTe crystals.

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.

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.

Electroless plating of Au, Pt, or Ru thin film layer on CdZnTe

The electrical and structural properties of thin metallic films (Au, Pt, or Ru) on CdZnTe (CZT) deposited by electroless deposition method have been investigated by means of Atomic Force Microscopy (AFM), Scanning Electron Microscope (SEM), Energy Dispersion X-ray (EDX), and Rutherford Backscattering Spectroscopy (RBS) measurements. SEM and AFM techniques put in evidence the modification of morphology and the change of roughness versus deposition time on the surface of contacts. The surface of Pt or Ru layer on CZT material present micro-cracks at a critical thickness, whereas it does not occur with Au layer. The thickness of Au layer with different deposition time was obtained by RBS depth profiles, which indicates a fast increase of thickness at a short deposition time and a saturation to an asymptotic value of 120 nm after 1 hour deposition.