P. Veeramani

Radiation induced control of electric field in Au/CdTe/In structures

In this contribution we demonstrate both experimentally and theoretically the possibility to control the profile of internal electric field by compensation of the space charge formed by carriers trapped at deep levels QT with space charge present due to band bending at the metal/CdTe interface QM. The demonstrated mechanism represents a promising way to decrease the problems associated with charge collection efficiency in CdTe x-ray detectors operated at high fluxes of x-ray photons.

An ASIC for the Study of Charge Sharing Effects in Small Pixel CdZnTe X-Ray Detectors

An Application Specific Integrated Circuit (ASIC) has been developed at the Rutherford Appleton Laboratory (RAL) to study the small pixel effect in spectroscopic CdTe and CdZnTe detectors. The PIXIE ASIC consists of four arrays of 3 × 3 channels flip chip bonded directly to the detector pixels. The active circuitry of each channel is a charge sensitive preamplifier and an output buffer which is multiplexed directly off chip. Each of the four detector arrays has a different anode geometry. The HEXITEC series of small pixel detectors developed at RAL have demonstrated energy resolutions of ~1 keV per pixel for both CdTe and CdZnTe, however, charge sharing events account for between 30-40% of the total count rate and can lead to degradation of the spectroscopy if not corrected for. The PIXIE ASIC will be used to study the effect of anode geometry on charge sharing and other aspects of the small pixel effect.

Study on temperature dependent resistivity of indium-doped cadmium zinc telluride

Indium-doped CdZnTe crystals, grown by the modified vertical Bridgman method, were characterized by temperature dependent resistivity measurements in the range from 220 to 320 K. The Fermi level, pinned near the midgap, was confirmed by fitting ln(ρ) versus 1/k0T plots, giving energies of 0.63 eV and 0.72 eV above the valence band for the high resistivity samples, with doping levels of 5.0 × 1016 at cm−3 and 4.8 × 1017 at cm−3, respectively. Different dominant deep level defects or complexes for pinning the Fermi level, and hence producing high resistivity, were expected when comparing the charge transport behaviours of the materials and the dopant concentration. However, two energies of 0.24 eV and 0.33 eV, below the conduction band for the Fermi level, were calculated at positive and negative bias voltages, respectively, for a low resistivity sample doped by 9.7 × 1017 at cm−3 In.

Characterization of CdZnTe Crystals Grown Using a Seeded Modified Vertical Bridgman Method

In this paper, several 60 mm diameter CdZnTe crystal ingots, containing large-size single crystals, were grown using (111) or (211) orientation seeds by the modified vertical Bridgman method. The Zn concentration distribution along the axial direction of the ingots was measured by near-infrared transmission spectroscopy at room temperature. The partition coefficient of Zn during the growth was calculated to be about 1.2. Zinc distribution uniformity measurements were carried out using photoluminescence mapping measurement at 80 K, which showed that the band gap variation in the CdZnTe wafers is less than 0.004 eV. IR microscopy showed that the diameters of the Te inclusions present in the material are in the range 6-9 mum, and the density of the inclusions is 1~3times105 cm-3. IR transmission measurements in the wave number region from 500 to 4000 cm-1 demonstrate that the IR transmittance of CdZnTe wafers is higher than 60%. Current-voltage measurements were performed on test structures fabricated using thermally evaporated Au contacts deposited on as-grown crystals, which revealed bulk resistivity values of 2~5times1010 Omegamiddotcm. Typical leakage currents for the planar devices were ~ 4 nA at a field strength of 1500 Vcm-1. The electron and hole mobility-lifetime products were evaluated using alpha particle irradiation. The obtained typical (mutau)e and (mutau)h values for the as-grown CdZnTe were 2.3times10-3 cm2V-1 and 1.5times10-4 cm2V-1, respectively.

Investigation of the internal electric field distribution under in situ x-ray irradiation and under low temperature conditions by the means of the Pockels effect

The internal electric field distribution in cadmium zinc telluride (CdZnTe) x-ray and γ-ray detectors strongly affects their performance in terms of charge transport and charge collection properties. In CdZnTe detectors the electric field distribution is sensitively dependent on not only the nature of the metal contacts but also on the working conditions of the devices such as the temperature and the rate of external irradiation. Here we present direct measurements of the electric field profiles in CdZnTe detectors obtained using the Pockels electo-optic effect whilst under in situ x-ray irradiation. These data are also compared with alpha particle induced current pulses obtained by the transient current technique, and we discuss the influence of both low temperature and x-ray irradiation on the electric field evolution. Results from these studies reveal strong distortion of the electric field consistent with the build-up of space charge at temperatures below 250 K, even in the absence of external irradiation. Also, in the presence of x-ray irradiation levels a significant distortion in the electric field is observed even at room temperature which matches well the predicted theoretical model.

Epitaxial Growth of High-Resistivity CdTe Thick Films Grown Using a Modified Close Space Sublimation Method

This paper reports the growth of high-resistivity CdTe thick epitaxial films of single crystal nature using a modified close space sublimation method (MCSS) in a Te-rich environment. We propose that the high Te2 partial pressure results in an increased concentration of TeCd antisites acting as deep donors to produce the high-resistivity CdTe, as well as improved quality of thick films. This is in agreement with the deep-donor model introduced by Fiderele et al. [Cryst. Res. Technol. 38 (2003) 588]. The thick films have a µeτe value in the order 10-4 cm2 V-1 and as expected, the TeCd antisites appeared not to act as electron traps.

Performance characteristics of CdTe drift ring detector

CdTe and CdZnTe material is an excellent candidate for the fabrication of high energy X-ray spectroscopic detectors due to their good quantum efficiency and room temperature operation. The main material limitation is associated with the poor charge transport properties of holes. The motivation of this work is to investigate the performance characteristics of a detector fabricated with a drift ring geometry that is insensitive to the transport of holes. The performance of a prototype Ohmic CdTe drift ring detector fabricated by Acrorad with 3 drift rings is reported; measurements include room temperature current voltage characteristics (IV) and spectroscopic performance. The data shows that the energy resolution of the detector is limited by leakage current which is a combination of bulk and surface leakage currents. The energy resolution was studied as a function of incident X-ray position with an X-ray microbeam at the Diamond Light Source. Different ring biasing schemes were investigated and the results show that by increasing the lateral field (i.e. the bias gradient across the rings) the active area, evaluated by the detected count rate, increased significantly.

Development of a CZT drift ring detector for X and γ ray spectroscopy

CdTe and CZT detectors are considered better choices for high energy γ and X-ray spectroscopy in comparison to Si and HPGe detectors due to their good quantum efficiency and room temperature operation. The performance limitations in CdTe and CZT detectors are mainly associated with poor hole transport and trapping phenomena. Among many techniques that can be used to eliminate the effect of the poor charge transport properties of holes in CdTe and CZT material, the drift ring technique shows promising results. In this work, the performance of a 2.3 mm thick CZT drift ring detector is investigated. Spatially resolved measurements were carried out with an X-ray microbeam (25 and 75 keV) at the Diamond Light Source synchrotron to study the response uniformity and extent of the active area. Higher energy photon irradiation was also carried out at up to 662 keV using different radioisotopes to complement the microbeam data. Different biasing schemes were investigated in terms of biasing the cathode rear electrode (bulk field) and the ring electrodes (lateral fields). The results show that increasing the bulk field with fixed-ratio ring biases and lateral fields with fixed bulk fields increase the active area of the device significantly, which contrasts with previous studies in CdTe, where only an increasing lateral field resulted in an improvement of device performance. This difference is attributed to the larger thickness of the CZT device reported here.

Comparison of the x-ray spectroscopy response and charge transport properties of semi-insulating In/Al doped CdZnTe crystals

The x-ray spectroscopy performance of In/Al doped CdZnTe planar detectors based on as-grown crystals were investigated at room temperature, using a Tb x-ray source with a principle energy of 44.2 keV. The observed broadening in the photopeak resolution was attributed to incomplete charge carrier collection due to carrier trapping and scattering by the defects in the crystal. Alpha particle spectroscopy and pulse shape rise time analysis were used to measure the electron mobility lifetime product (μτ), as well as the mobility (μ) of the CdZnTe material grown with different dopant concentrations. To further clarify the role of the dopant and associated trapping states, temperature dependent alpha particle spectroscopy and pulse shapes were investigated at various applied bias fields over a temperature range from 200 to 300 K. CdZnTe doped with 1.5 ppm In exhibits excellent x-ray spectral resolution and charge transport properties, which implies a lower density of trapping centers in the crystal. The deep le...

Investigating the small pixel effect in CdZnTe Hard X-ray detectors — The PIXIE ASIC

An Application Specific Integrated Circuit (ASIC) has been developed at the Rutherford Appleton Laboratory (RAL) to study the small pixel effect in spectroscopic CdTe and CdZnTe detectors. The PIXIE ASIC consists of four arrays of 3×3 channels flip chip bonded directly to the detector pixels. The active circuitry of each channel is a charge sensitive preamplifier and an output buffer which is multiplexed directly off chip. Each of the four arrays has different anode geometry. The HEXITEC series of small pixel detectors developed at RAL have demonstrated energy resolutions of ∼1keV per pixel for both CdTe and CdZnTe, however, charge sharing events account for between 30–40% of the total count rate and can lead to degradation of the spectroscopy if not corrected for. The PIXIE ASIC will be used to study the effect of anode geometry on charge sharing and other aspects of the small pixel effect.