Single crystal (SC) diamond has since years demonstrated its interest for the fabrication of radi... more Single crystal (SC) diamond has since years demonstrated its interest for the fabrication of radiation detectors, especially where the material properties are providing superior interests with respect to the detection application. Among the industrial suppliers able to provide on a commercial basis high-grade single crystal diamond, IIa-Tech has recently appeared in the market as a new player. The aim of this paper is to assess the quality of one SC sample when characterized under a-particles for the measurement of its carrier transport properties. We observed that full charge collection could be observed at biases as low as 0.11 V/mm with no space charge build-up (conventionally typical bias values used are closer to 1 V/mm). Velocity reached values of 38 mm/ns and 53 mm/ns for electrons and holes, respectively (values probed at 0.33 V/mm). Similarly, the a detection spectrum displays a sharp line demonstrating the good uniformity of the material over its surface. By combining the measurements with more conventional optical observations such as birefringence and cathodoluminescence spectroscopy, it comes that the material demonstrates its ability to be used as a detector, with properties that can compare with the highest grade materials today available on the market.
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XIV, 2012
The Cd 1-x Zn x Te (x = 0.1) crystals from two different manufacturers were studied by photocondu... more The Cd 1-x Zn x Te (x = 0.1) crystals from two different manufacturers were studied by photoconductivity (PC) measurements. The samples 1 and 2 were subjected to chemical etching and irradiation with nanosecond laser pulses. The IR transmittance spectra of the crystals before and after laser irradiation were also monitored. The PC spectrum of the sample 1 had a typical one-band shape while the spectrum of the sample 2 exhibited two bands roughly corresponding to the bandgaps of CdTe and Cd 1-x Zn x Te that could be attributed to inhomogeneities in the surface region of the crystal. The positions of the maximum and red edge of the PC spectra did not correspond to the component compositions x in the bulk of Cd 0.9 Zn 0.1 Te crystals, however chemical polishing etching of the samples in a brominemethanol solution or/and laser irradiation led to this correspondence. Moreover, depending of laser pulse energy density, irradiation of Cd 1-x Zn x Te crystals resulted in a short-wavelength shift of the PC spectra, transformation of two bands to one in the case of the sample 2, and an increase in the photosensitivity of the semiconductor. The laser processing provided equalization of parameters in the surface and bulk regions. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 8507 85071S-4 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms
ABSTRACT The electrical and structural properties of thin metallic films (Au, Pt, or Ru) on CdZnT... more ABSTRACT 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.
ABSTRACT The electrical and structural properties of thin metallic layers (Au, Pt, or Ru) on CdZn... more ABSTRACT The electrical and structural properties of thin metallic layers (Au, Pt, or Ru) on CdZnTe (CZT) deposited by electroless deposition method have been investigated by means of atomic force microscopy (AFM), scanning electron microscopy (SEM), and Rutherford backscattering spectroscopy (RBS) measurements. SEM and AFM techniques put in evidence the modification of the morphology and the contact's roughness dependence with the deposition time. The surface of the Pt or Ru layer on a CZT material presents micro-cracks at a critical thickness, whereas it does not occur with the Au layer. The thickness of the Au layer with different deposition times obtained by RBS indicates first a fast increase in thickness for short deposition times and then saturation to an asymptotic value of 120 nm after 1 h.
In this paper we report on the strong impact of the concentration of uncompensated impurities on ... more 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.
The influence of deposition methods and type of metal contacts on the defects generated at the me... more The influence of deposition methods and type of metal contacts on the defects generated at the metal/semiconductor interface has been investigated. Photoluminesccence spectra at a low temperature demonstrated that the Au electroless deposition samples exhibit the lowest I (D 0 ,X) /I A-center ratio and have the best gamma response. Rutherford backscattering spectrometry results show that the sputtering method creates the highest concentration of TeO 2 layer compared with the evaporation and electroless deposition methods. The electroless Au has more inter-diffusion between the contact and the CZT material than either sputtering or evaporation methods. The electroless Pt and Ru depositions, however, exhibit lower inter-diffusion. The contacts deposited by electroless and evaporation present more quasi-ohmic behaviour and better gamma response than those fabricated by sputtering.
ABSTRACT In the Bridgman and Vertical Gradient Freeze (VGF) techniques, it is common to use a hig... more ABSTRACT In the Bridgman and Vertical Gradient Freeze (VGF) techniques, it is common to use a high purity crucible for crystal growth. One disadvantage of pBN crucibles is associated with the crucible material interacting with the melt during the growth cycle. Using pBN crucibles for the growth of CZT, we have observed that the crucible does in fact interact with the CZT melt. To prevent this interaction, a vacuum carbon coating system for applying a carbon coating to pBN crucibles has been developed. The effects of crucible use with time are studied, as are the compositional and device properties of the bulk material. GDMS measurements are used to investigate on broron and nitrogen content, among other trace impurities found in the crystal. Devices harvested from ingots grown using cc-pBN exhibit high resistivity and good functionality as room temperature gamma ray detectors.
ABSTRACT The goal of this paper is to study the effect of superheating and fast cooling condition... more ABSTRACT The goal of this paper is to study the effect of superheating and fast cooling conditions on the defects of CdZnTe crystals especially Te inclusions.The paper reports characterization of the crystal defects using IR microscopy and Fourier Transform Infrared spectroscopy. Three ingots of CdZnTe doped with Indium were grown by Vertical Gradient Freezing. Ingot A and B were grown with superheating temperatures of 26 °C and 56 °C respectively. In the last case the growth process was followed by a fast cooling.The results show that the size of Te inclusions was greatly reduced and the crystal quality was improved after higher superheating and fast cooling.
ABSTRACT There are several challenges in producing detector grade material based on Cadmium Zinc ... more ABSTRACT There are several challenges in producing detector grade material based on Cadmium Zinc Telluride (CZT), which include material synthesis and growth of electrically compensated crystals as well as the extraction of high resistivity material suitable for device applications. One of the challenges towards producing large volumes of compensated material using the Vertical Gradient Freeze (VGF) method is the axial and radial variations in material homogeneity. Growth induced strain is one of the most important factors in the crystal growth of CZT because of the low critically resolved shear stress (CRSS) value for this material. In this work, methods to reduce the thermo-mechanical stress imparted into the crystal have been implemented. Specifically, crystals have been grown under dynamic temperature gradients to minimize the temperature gradient across the ingot, while maintaining relatively higher temperature gradients at the Solid Liquid Interface (SLI). How this adjustment affects bulk resistivity and photoconductivity has been investigated.
Cadmium zinc telluride (Cd 1 À x Zn x Te) crystals have many applications in optoelectronics and ... more Cadmium zinc telluride (Cd 1 À x Zn x Te) crystals have many applications in optoelectronics and as roomtemperature detectors. We grew bismuth-doped CZT crystals by the standard Bridgman Oscillation Method, and compared them with such crystals grown in the thermal environment of a furnace modified with a Pt coldfinger (metal rod). The coldfinger serves as a tool for stabilizing the solid-liquid interface by extracting heat from the as-grown crystal, and thereby improving the ingot's crystalline quality. We detailed the crystal's quality via high-resolution X-ray diffraction (HRXRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Synchrotron-based X-ray microfluorescence (mSXRF) images, as well as by etch-pit density (EPD) measurements. Our results demonstrated that the Pt coldfinger is an effective tool for improving the quality of CZT bulk material.
The effect of surface preparation on CdZnTe properties was investigated. Surface etching using br... more The effect of surface preparation on CdZnTe properties was investigated. Surface etching using bromine solutions enhances Te elemental composition, resulting in a Te rich surface layer that is prone to oxidize. This oxidation degrades the performance of the fabricated CZT gamma detector. Roughness results were identical for samples polished with 1 and 3 mm and subsequently etched in 2% Br-MeOH.
The influences of two different ways of crystal growth preparations by using different starting m... more The influences of two different ways of crystal growth preparations by using different starting materials on the detector properties have been investigated. The characterization has been carried out in terms of structural, optical, and electrical properties of the materials grown with the vertical gradient freeze method. The distributions of Zn and In at different axial positions with different crystals have been compared by using the Induced Coupled Plasma-Mass Spectroscopy technique. The size and density of Te inclusions within the materials have been identified by Infrared Microscopy. The luminescence behaviors have been investigated by studying the defects levels. Particularly, the current-voltage characteristics and Gamma-ray Spectroscopy have been investigated by comparing the performance of detectors achieved from different axial directions of the ingots.
Nanometric double layer properties of different metal-semiconductor-metal (MSM) depositions onto ... more Nanometric double layer properties of different metal-semiconductor-metal (MSM) depositions onto CdZnTe produced by electroless method are investigated. The mechanisms of the deposition are discussed and the theoretical chemical equations implied in the process are presented. The solutions for different time of deposition and the deposited layers are analysed by TXRF to test the proposed reactions. RBS was used to determine the thickness, the depth profiles and the composition of the layers deposited at the surface. This work showed the feasibility of depositing nanometric double layers using electroless technique.
ABSTRACT Improving the structural, optical, and electronic properties of bulk CZT remains a topic... more ABSTRACT Improving the structural, optical, and electronic properties of bulk CZT remains a topic of great interest for producing high quality nuclear imaging material. The work presented here is the result of several experiments whose geometries and materials have been chosen due to their thermal properties. Results are presented for 50 mm CZT ingots grown using furnace elements which have been shown to be conducive to a convex solid–liquid interface shape. A novel crystal growth pedestal was used in this work for in-situ measurements of temperature gradients. The electrical and optical properties of the as grown material are presented. As a result, large crystal grains and volumes have been obtained for 50 mm ingots. The benefit of using this geometry has been demonstrated by comparing CZT ingots grown with and without the SiC pedestal. Gamma spectra are presented for planar devices which have been harvested from the First to Freeze regions which are most influenced by the effects of the SiC pedestal.
ABSTRACT Improving the structural, optical, and electronic properties of bulk CZT remains a topic... more ABSTRACT Improving the structural, optical, and electronic properties of bulk CZT remains a topic of great interest for producing high quality nuclear imaging material. Consideration must be given to the thermal environment under which crystal growth is carried out. It is important that the furnace and insulation elements are chosen and arranged to promote a convex solid liquid interface. Results are presented for 50 mm CZT ingots grown using furnace elements, which have been shown to be conducive to a convex SLI.
ABSTRACT Despite these recent advancements in preparing the surface of Cd(Zn)Te devices for detec... more ABSTRACT Despite these recent advancements in preparing the surface of Cd(Zn)Te devices for detector applications, large asymmetries in the electronic properties of planar Cd(Zn)Te detectors are common. Furthermore, for the development of patterned electrode geometries, selection of each electrode surface is crucial for minimizing dark current in the device. This investigation presented here has been carried out with three objectives. Each objective is oriented towards establishing reliable methods for the selection of the anode and cathode surfaces independent of the crystallographic orientation. The objectives of this study are (i) investigate how the asymmetry in I-V characteristics of Cd(Zn)Te devices may be associated with the TeO2 interfacial layer using Rutherford backscattering to study the structure at the Au-Cd(Zn)Te interface, (ii) develop an understanding of how the concentration of the active traps in Cd(Zn)Te varies with the external bias, and (iii) propose non-destructive methods for selection of the anode and cathode which are independent of crystallographic orientation. The spectroscopic methods employed in this investigation include Rutherford backscattering spectroscopy, photo-induced current transient spectroscopy, and surface photo-voltage spectroscopy, as well as gamma ray spectroscopy to demonstrate the influence on detector properties.
The objective of this work is to analyze the effects of argon ion irradiation process on the stru... more The objective of this work is to analyze the effects of argon ion irradiation process on the structure and distribution of Te inclusions in Cd 1-x Zn x Te crystals. The samples were treated with different ion fluences ranging from 2 to 8 Â 10 17 cm À2 . The state of the samples before and after irradiation were studied by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Cathodoluminescence, Photoluminescence, and micro-Raman spectroscopy. The effect of the irradiation on the surface of the samples was clearly observed by SEM or AFM. Even for small fluences a removal of polishing scratches on the sample surfaces was observed. Likely correlated to this effect, an important enhancement in the luminescence intensity of the irradiated samples was observed. An aggregation effect of the Te inclusions seems to occur due to the Ar bombardment, which are also eliminated from the surfaces for the highest ion fluences used.
The Vertical Gradient Freeze (VGF) method has been used to grow high resistivity Cadmium Zinc Tel... more 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.
The chemical etching and the passivation processes of CdZnTe wafers were studied. The treatment e... more The chemical etching and the passivation processes of CdZnTe wafers were studied. The treatment effects were tested through an X-Ray Photoelectron Spectroscopy (XPS) analysis and I-V measurement. The chemical etching in 2%Br-MeOH solution may effectively remove the damaged layer and improve the ohmic contact between CdZnTe wafer and Au electrodes making rich the surface with Te. After different etching times, the CdZnTe wafers were passivated with NH 4 F/H 2 O 2 .CdZnTe wafer passivated immediately after etching showed the best passivation efficiency because the enriched Te on the surface was fully oxidized to TeO 2 , which results in the thickest oxide layer, and the most stoichiometric surface. Also the surface leakage current was reduced in comparison with the sample passivated 24 h after etching.
A comparative study of four different metals gold (Au), platinum (Pt), ruthenium (Ru) and rhodium... more A comparative study of four different metals gold (Au), platinum (Pt), ruthenium (Ru) and rhodium (Rh) deposited on CdZnTe(CZT) by the electroless deposition method has been carried out. Two of these materials, Ru and Rh, have been deposited for the first time by this method. In contrast to the Pt deposition, the deposition of Ru and Rh were not carried out under the optimal conditions. The metals deposited on the samples were identified by Total reflection X-ray Fluorescence (TXRF). Rutherford Backscattering Spectrometry (RBS) analyses show that Au forms the thickest layer (~160 nm) for the experimental conditions of this work. Current-voltage measurements show that Pt forms a more linear ohmic contact with the lowest leakage current. A 57 Co gamma ray spectrum gave a better detector performance with a FWHM 11 keV at 122 keV.
Single crystal (SC) diamond has since years demonstrated its interest for the fabrication of radi... more Single crystal (SC) diamond has since years demonstrated its interest for the fabrication of radiation detectors, especially where the material properties are providing superior interests with respect to the detection application. Among the industrial suppliers able to provide on a commercial basis high-grade single crystal diamond, IIa-Tech has recently appeared in the market as a new player. The aim of this paper is to assess the quality of one SC sample when characterized under a-particles for the measurement of its carrier transport properties. We observed that full charge collection could be observed at biases as low as 0.11 V/mm with no space charge build-up (conventionally typical bias values used are closer to 1 V/mm). Velocity reached values of 38 mm/ns and 53 mm/ns for electrons and holes, respectively (values probed at 0.33 V/mm). Similarly, the a detection spectrum displays a sharp line demonstrating the good uniformity of the material over its surface. By combining the measurements with more conventional optical observations such as birefringence and cathodoluminescence spectroscopy, it comes that the material demonstrates its ability to be used as a detector, with properties that can compare with the highest grade materials today available on the market.
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XIV, 2012
The Cd 1-x Zn x Te (x = 0.1) crystals from two different manufacturers were studied by photocondu... more The Cd 1-x Zn x Te (x = 0.1) crystals from two different manufacturers were studied by photoconductivity (PC) measurements. The samples 1 and 2 were subjected to chemical etching and irradiation with nanosecond laser pulses. The IR transmittance spectra of the crystals before and after laser irradiation were also monitored. The PC spectrum of the sample 1 had a typical one-band shape while the spectrum of the sample 2 exhibited two bands roughly corresponding to the bandgaps of CdTe and Cd 1-x Zn x Te that could be attributed to inhomogeneities in the surface region of the crystal. The positions of the maximum and red edge of the PC spectra did not correspond to the component compositions x in the bulk of Cd 0.9 Zn 0.1 Te crystals, however chemical polishing etching of the samples in a brominemethanol solution or/and laser irradiation led to this correspondence. Moreover, depending of laser pulse energy density, irradiation of Cd 1-x Zn x Te crystals resulted in a short-wavelength shift of the PC spectra, transformation of two bands to one in the case of the sample 2, and an increase in the photosensitivity of the semiconductor. The laser processing provided equalization of parameters in the surface and bulk regions. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 8507 85071S-4 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms
ABSTRACT The electrical and structural properties of thin metallic films (Au, Pt, or Ru) on CdZnT... more ABSTRACT 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.
ABSTRACT The electrical and structural properties of thin metallic layers (Au, Pt, or Ru) on CdZn... more ABSTRACT The electrical and structural properties of thin metallic layers (Au, Pt, or Ru) on CdZnTe (CZT) deposited by electroless deposition method have been investigated by means of atomic force microscopy (AFM), scanning electron microscopy (SEM), and Rutherford backscattering spectroscopy (RBS) measurements. SEM and AFM techniques put in evidence the modification of the morphology and the contact's roughness dependence with the deposition time. The surface of the Pt or Ru layer on a CZT material presents micro-cracks at a critical thickness, whereas it does not occur with the Au layer. The thickness of the Au layer with different deposition times obtained by RBS indicates first a fast increase in thickness for short deposition times and then saturation to an asymptotic value of 120 nm after 1 h.
In this paper we report on the strong impact of the concentration of uncompensated impurities on ... more 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.
The influence of deposition methods and type of metal contacts on the defects generated at the me... more The influence of deposition methods and type of metal contacts on the defects generated at the metal/semiconductor interface has been investigated. Photoluminesccence spectra at a low temperature demonstrated that the Au electroless deposition samples exhibit the lowest I (D 0 ,X) /I A-center ratio and have the best gamma response. Rutherford backscattering spectrometry results show that the sputtering method creates the highest concentration of TeO 2 layer compared with the evaporation and electroless deposition methods. The electroless Au has more inter-diffusion between the contact and the CZT material than either sputtering or evaporation methods. The electroless Pt and Ru depositions, however, exhibit lower inter-diffusion. The contacts deposited by electroless and evaporation present more quasi-ohmic behaviour and better gamma response than those fabricated by sputtering.
ABSTRACT In the Bridgman and Vertical Gradient Freeze (VGF) techniques, it is common to use a hig... more ABSTRACT In the Bridgman and Vertical Gradient Freeze (VGF) techniques, it is common to use a high purity crucible for crystal growth. One disadvantage of pBN crucibles is associated with the crucible material interacting with the melt during the growth cycle. Using pBN crucibles for the growth of CZT, we have observed that the crucible does in fact interact with the CZT melt. To prevent this interaction, a vacuum carbon coating system for applying a carbon coating to pBN crucibles has been developed. The effects of crucible use with time are studied, as are the compositional and device properties of the bulk material. GDMS measurements are used to investigate on broron and nitrogen content, among other trace impurities found in the crystal. Devices harvested from ingots grown using cc-pBN exhibit high resistivity and good functionality as room temperature gamma ray detectors.
ABSTRACT The goal of this paper is to study the effect of superheating and fast cooling condition... more ABSTRACT The goal of this paper is to study the effect of superheating and fast cooling conditions on the defects of CdZnTe crystals especially Te inclusions.The paper reports characterization of the crystal defects using IR microscopy and Fourier Transform Infrared spectroscopy. Three ingots of CdZnTe doped with Indium were grown by Vertical Gradient Freezing. Ingot A and B were grown with superheating temperatures of 26 °C and 56 °C respectively. In the last case the growth process was followed by a fast cooling.The results show that the size of Te inclusions was greatly reduced and the crystal quality was improved after higher superheating and fast cooling.
ABSTRACT There are several challenges in producing detector grade material based on Cadmium Zinc ... more ABSTRACT There are several challenges in producing detector grade material based on Cadmium Zinc Telluride (CZT), which include material synthesis and growth of electrically compensated crystals as well as the extraction of high resistivity material suitable for device applications. One of the challenges towards producing large volumes of compensated material using the Vertical Gradient Freeze (VGF) method is the axial and radial variations in material homogeneity. Growth induced strain is one of the most important factors in the crystal growth of CZT because of the low critically resolved shear stress (CRSS) value for this material. In this work, methods to reduce the thermo-mechanical stress imparted into the crystal have been implemented. Specifically, crystals have been grown under dynamic temperature gradients to minimize the temperature gradient across the ingot, while maintaining relatively higher temperature gradients at the Solid Liquid Interface (SLI). How this adjustment affects bulk resistivity and photoconductivity has been investigated.
Cadmium zinc telluride (Cd 1 À x Zn x Te) crystals have many applications in optoelectronics and ... more Cadmium zinc telluride (Cd 1 À x Zn x Te) crystals have many applications in optoelectronics and as roomtemperature detectors. We grew bismuth-doped CZT crystals by the standard Bridgman Oscillation Method, and compared them with such crystals grown in the thermal environment of a furnace modified with a Pt coldfinger (metal rod). The coldfinger serves as a tool for stabilizing the solid-liquid interface by extracting heat from the as-grown crystal, and thereby improving the ingot's crystalline quality. We detailed the crystal's quality via high-resolution X-ray diffraction (HRXRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Synchrotron-based X-ray microfluorescence (mSXRF) images, as well as by etch-pit density (EPD) measurements. Our results demonstrated that the Pt coldfinger is an effective tool for improving the quality of CZT bulk material.
The effect of surface preparation on CdZnTe properties was investigated. Surface etching using br... more The effect of surface preparation on CdZnTe properties was investigated. Surface etching using bromine solutions enhances Te elemental composition, resulting in a Te rich surface layer that is prone to oxidize. This oxidation degrades the performance of the fabricated CZT gamma detector. Roughness results were identical for samples polished with 1 and 3 mm and subsequently etched in 2% Br-MeOH.
The influences of two different ways of crystal growth preparations by using different starting m... more The influences of two different ways of crystal growth preparations by using different starting materials on the detector properties have been investigated. The characterization has been carried out in terms of structural, optical, and electrical properties of the materials grown with the vertical gradient freeze method. The distributions of Zn and In at different axial positions with different crystals have been compared by using the Induced Coupled Plasma-Mass Spectroscopy technique. The size and density of Te inclusions within the materials have been identified by Infrared Microscopy. The luminescence behaviors have been investigated by studying the defects levels. Particularly, the current-voltage characteristics and Gamma-ray Spectroscopy have been investigated by comparing the performance of detectors achieved from different axial directions of the ingots.
Nanometric double layer properties of different metal-semiconductor-metal (MSM) depositions onto ... more Nanometric double layer properties of different metal-semiconductor-metal (MSM) depositions onto CdZnTe produced by electroless method are investigated. The mechanisms of the deposition are discussed and the theoretical chemical equations implied in the process are presented. The solutions for different time of deposition and the deposited layers are analysed by TXRF to test the proposed reactions. RBS was used to determine the thickness, the depth profiles and the composition of the layers deposited at the surface. This work showed the feasibility of depositing nanometric double layers using electroless technique.
ABSTRACT Improving the structural, optical, and electronic properties of bulk CZT remains a topic... more ABSTRACT Improving the structural, optical, and electronic properties of bulk CZT remains a topic of great interest for producing high quality nuclear imaging material. The work presented here is the result of several experiments whose geometries and materials have been chosen due to their thermal properties. Results are presented for 50 mm CZT ingots grown using furnace elements which have been shown to be conducive to a convex solid–liquid interface shape. A novel crystal growth pedestal was used in this work for in-situ measurements of temperature gradients. The electrical and optical properties of the as grown material are presented. As a result, large crystal grains and volumes have been obtained for 50 mm ingots. The benefit of using this geometry has been demonstrated by comparing CZT ingots grown with and without the SiC pedestal. Gamma spectra are presented for planar devices which have been harvested from the First to Freeze regions which are most influenced by the effects of the SiC pedestal.
ABSTRACT Improving the structural, optical, and electronic properties of bulk CZT remains a topic... more ABSTRACT Improving the structural, optical, and electronic properties of bulk CZT remains a topic of great interest for producing high quality nuclear imaging material. Consideration must be given to the thermal environment under which crystal growth is carried out. It is important that the furnace and insulation elements are chosen and arranged to promote a convex solid liquid interface. Results are presented for 50 mm CZT ingots grown using furnace elements, which have been shown to be conducive to a convex SLI.
ABSTRACT Despite these recent advancements in preparing the surface of Cd(Zn)Te devices for detec... more ABSTRACT Despite these recent advancements in preparing the surface of Cd(Zn)Te devices for detector applications, large asymmetries in the electronic properties of planar Cd(Zn)Te detectors are common. Furthermore, for the development of patterned electrode geometries, selection of each electrode surface is crucial for minimizing dark current in the device. This investigation presented here has been carried out with three objectives. Each objective is oriented towards establishing reliable methods for the selection of the anode and cathode surfaces independent of the crystallographic orientation. The objectives of this study are (i) investigate how the asymmetry in I-V characteristics of Cd(Zn)Te devices may be associated with the TeO2 interfacial layer using Rutherford backscattering to study the structure at the Au-Cd(Zn)Te interface, (ii) develop an understanding of how the concentration of the active traps in Cd(Zn)Te varies with the external bias, and (iii) propose non-destructive methods for selection of the anode and cathode which are independent of crystallographic orientation. The spectroscopic methods employed in this investigation include Rutherford backscattering spectroscopy, photo-induced current transient spectroscopy, and surface photo-voltage spectroscopy, as well as gamma ray spectroscopy to demonstrate the influence on detector properties.
The objective of this work is to analyze the effects of argon ion irradiation process on the stru... more The objective of this work is to analyze the effects of argon ion irradiation process on the structure and distribution of Te inclusions in Cd 1-x Zn x Te crystals. The samples were treated with different ion fluences ranging from 2 to 8 Â 10 17 cm À2 . The state of the samples before and after irradiation were studied by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Cathodoluminescence, Photoluminescence, and micro-Raman spectroscopy. The effect of the irradiation on the surface of the samples was clearly observed by SEM or AFM. Even for small fluences a removal of polishing scratches on the sample surfaces was observed. Likely correlated to this effect, an important enhancement in the luminescence intensity of the irradiated samples was observed. An aggregation effect of the Te inclusions seems to occur due to the Ar bombardment, which are also eliminated from the surfaces for the highest ion fluences used.
The Vertical Gradient Freeze (VGF) method has been used to grow high resistivity Cadmium Zinc Tel... more 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.
The chemical etching and the passivation processes of CdZnTe wafers were studied. The treatment e... more The chemical etching and the passivation processes of CdZnTe wafers were studied. The treatment effects were tested through an X-Ray Photoelectron Spectroscopy (XPS) analysis and I-V measurement. The chemical etching in 2%Br-MeOH solution may effectively remove the damaged layer and improve the ohmic contact between CdZnTe wafer and Au electrodes making rich the surface with Te. After different etching times, the CdZnTe wafers were passivated with NH 4 F/H 2 O 2 .CdZnTe wafer passivated immediately after etching showed the best passivation efficiency because the enriched Te on the surface was fully oxidized to TeO 2 , which results in the thickest oxide layer, and the most stoichiometric surface. Also the surface leakage current was reduced in comparison with the sample passivated 24 h after etching.
A comparative study of four different metals gold (Au), platinum (Pt), ruthenium (Ru) and rhodium... more A comparative study of four different metals gold (Au), platinum (Pt), ruthenium (Ru) and rhodium (Rh) deposited on CdZnTe(CZT) by the electroless deposition method has been carried out. Two of these materials, Ru and Rh, have been deposited for the first time by this method. In contrast to the Pt deposition, the deposition of Ru and Rh were not carried out under the optimal conditions. The metals deposited on the samples were identified by Total reflection X-ray Fluorescence (TXRF). Rutherford Backscattering Spectrometry (RBS) analyses show that Au forms the thickest layer (~160 nm) for the experimental conditions of this work. Current-voltage measurements show that Pt forms a more linear ohmic contact with the lowest leakage current. A 57 Co gamma ray spectrum gave a better detector performance with a FWHM 11 keV at 122 keV.
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Papers by H. Bensalah