Effects of Bulk and Surface Conductivity on the Performance of CdZnTe Pixel Detectors

We studied the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe (CZT) pixel detectors with Pt contacts. We emphasize the difference in mechanisms of the bulk and surface conductivity as indicated by their different temperature behaviors. In addition, the existence of a thin (10-100 A) oxide layer on the surface of CZT, formed during the fabrication process, affects both bulk and surface leakage currents. We demonstrate that the measured I-V dependencies of bulk current can be explained by considering the CZT detector as a metal-semiconductor-metal system with two back-to-back Schottky-barrier contacts. The high surface leakage current is apparently due to the presence of a low-resistivity surface layer that has characteristics which differ considerably from those of the bulk material. This surface layer has a profound effect on the charge collection efficiency in detectors with multi-contact geometry; some fraction of the electric field lines originated on the cathode intersects the surface areas between the pixel contacts where the charge produced by an ionizing particle gets trapped. To overcome this effect we place a grid of thin electrodes between the pixel contacts; when the grid is negatively biased, the strong electric field in the gaps between the pixels forces the electrons landing on the surface to move toward the contacts, preventing the charge loss. We have investigated these effects by using CZT pixel detectors indium bump bonded to a custom-built VLSI readout chip

Properties of Pt Schottky Type Contacts On High-Resistivity CdZnTe Detectors

In this paper we present studies of the I-V characteristics of CdZnTe detectors with Pt contacts fabricated from high-resistivity single crystals grown by the high-pressure Brigman process. We have analyzed the experimental I-V curves using a model that approximates the CZT detector as a system consisting of a reversed Schottky contact in series with the bulk resistance. Least square fits to the experimental data yield 0.78-0.79 eV for the Pt-CZT Schottky barrier height, and <20 V for the voltage required to deplete a 2 mm thick CZT detector. We demonstrate that at high bias the thermionic current over the Schottky barrier, the height of which is reduced due to an interfacial layer between the contact and CZT material, controls the leakage current of the detectors. In many cases the dark current is not determined by the resistivity of the bulk material, but rather the properties of the contacts; namely by the interfacial layer between the contact and CZT material.Comment: 12 pages, 11 figure

Use of a pulsed laser to study properties of CdZnTe pixel detectors

We have investigated the utility of employing a short (<4 ns) pulsed laser with wavelength tunable between 600 - 950 nm as a tool for studying and characterizing CdZnTe detectors. By using a single mode optical fiber and simple optics, we can focus the beam to a spot size of less than 10 micrometers and generate the number of the excess carriers equivalent to a several MeV gamma-ray either at the surface or deep inside the sample. The advantages of this technique over use of a collimated X-ray or alpha particle source are strong induced signal, precise pointing, and triggering capability. As examples of using this technique, we present the results of measurements of the drift velocity, electron lifetime, and electric field line distribution inside CZT pixel detectors

Development of CdZnTe pixel detectors for astrophysical applications

Over the last four years we have been developing imaging Cadmium Zinc Telluride pixel detectors optimized for astrophysical focusing hard X-ray telescopes. This application requires sensors with modest area (~2cm X 2 cm), relatively small (≾ 500µm) pixels and sub-keV energy resolution. For experiments operating in satellite orbits, low energy thresholds of ~1 - 2 keV are also desirable. In this paper we describe the desired detector performance characteristics, and report on the status of our development effort. In particular, we present results from a prototype sensor with a custom low- noise VLSI readout designed to achieve excellent spectral resolution and good imaging performance in the 2 - 100 keV band

Growth of detector-grade CZT by Traveling Heater Method (THM): An advancement

In this present work we report the growth of Cd{sub 0.9}Zn{sub 0.1}Te doped with In by a modified THM technique. It has been demonstrated that by controlling the microscopically flat growth interface, the size distribution and concentration can be drastically reduced in the as-grown ingots. This results in as-grown detector-grade CZT by the THM technique. The three-dimensional size distribution and concentrations of Te inclusions/precipitations were studied. The size distributions of the Te precipitations/inclusions were observed to be below the 10-{micro}m range with the total concentration less than 10{sup 5} cm{sup -3}. The relatively low value of Te inclusions/precipitations results in excellent charge transport properties of our as-grown samples. The ({mu}{tau}){sub e} values for different as-grown samples varied between 6-20 x 10{sup -3} cm{sup 2}/V. The as-grown samples also showed fairly good detector response with resolution of {approx}1.5%, 2.7% and about 3.8% at 662 keV for quasi-hemispherical geometry for detector volumes of 0.18 cm{sup 3}, 1 cm{sup 3} and 4.2 cm{sup 3}, respectively

First test results from a high-resolution CdZnTe pixel detector with VLSI readout

We are developing a CdZnTe pixel detector with a custom low- noise analog VLSI readout for use in the High-Energy Focusing Telescope balloon experiment, as well as for future space astronomy applications. The goal of the program is to achieve good energy resolution (< 1 keV FWHM at 60 keV) and low threshold in a sensor with approximately 500 micrometers pixels. We have fabricated several prototype detector assemblies with 2 mm thick, 680 by 650 micrometers pitch CdZnTe pixel sensors indium bump bonded a VLSI readout chip developed at Caltech. Each readout circuit in the 8 X 8 prototype is matched to the detector pixel size, and contains a preamplifier, shaping amplifiers, and a peak stretcher/discriminator. In the first 8 X 8 prototype, we have demonstrated the low-noise preamplifier by routing the output signals off-chip for shaping and pulse-height analysis. Pulse height spectra obtained using a ^(241)Am source, collimated to illuminate a single pixel, show excellent energy resolution of 1.1 keV FWHM for the 60 keV line at room temperature. Line profiles are approximately Gaussian and dominated by electronic noise, however a small low energy tail is evident for the 60 keV line. We obtained slightly improved resolution of 0.9 keV FWHM at 60 keV by cooling the detector to 5 degree(s)C, near the expected balloon- flight operating temperature. Pulse height spectra obtained with the collimated source positioned between pixels show the effect of signal sharing for events occurring near the boundary. We are able to model the observed spectra using a Monte-Carlo simulation that includes the effects of photon interaction, charge transport and diffusion, pixel and collimator geometry, and electronic noise. By using the model to simulate the detector response to uncollimated radiation (including the effect of finite trigger threshold for reconstruction of the total energy of multi-pixel events), we find the energy resolution to be degraded by only 10% for full-face illumination, compared to the collimated case. The small value of the degradation is due directly to the low readout noise and amplifier threshold

Structural and sub-structural features of chemically deposited Zinc-oxide thin films

We investigated the structural and sub-structural characteristics of ZnO films obtained by chemical bath deposition from solutions of zinc sulfate, thiourea, and ammonia. The duration of deposition ranged from 20- to 120-minutes. The concentration of thiourea was varied from 0.1- to 3-mol. We detailed the structural and sub-structural characteristics of these films using high-resolution scanning electron microscopy and x-ray diffraction. This research enables us to study features of the films’ structural formation, and to determine their basic characteristics, viz., phase analysis, texture quality, lattice constants, grain size, and size of the coherent scattering domain. Regimes were identified for depositing films with optimal structural characteristics for eventual use in solar-energy applications. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3340

Use of a pulsed laser to study properties of CdZnTe pixel detectors

We have investigated the utility of employing a short (<4 ns) pulsed laser with wavelength tunable between 600 - 950 nm as a tool for studying and characterizing CdZnTe detectors. By using a single mode optical fiber and simple optics, we can focus the beam to a spot size of less than 10 micrometers and generate the number of the excess carriers equivalent to a several MeV gamma-ray either at the surface or deep inside the sample. The advantages of this technique over use of a collimated X-ray or alpha particle source are strong induced signal, precise pointing, and triggering capability. As examples of using this technique, we present the results of measurements of the drift velocity, electron lifetime, and electric field line distribution inside CZT pixel detectors

Development of CdZnTe pixel detectors for astrophysical applications

Over the last four years we have been developing imaging Cadmium Zinc Telluride pixel detectors optimized for astrophysical focusing hard X-ray telescopes. This application requires sensors with modest area (~2cm X 2 cm), relatively small (≾ 500µm) pixels and sub-keV energy resolution. For experiments operating in satellite orbits, low energy thresholds of ~1 - 2 keV are also desirable. In this paper we describe the desired detector performance characteristics, and report on the status of our development effort. In particular, we present results from a prototype sensor with a custom low- noise VLSI readout designed to achieve excellent spectral resolution and good imaging performance in the 2 - 100 keV band

Investigation optimal contact geometry for CdZnTe pixel detectors

We are developing CdZnTe pixel detectors for use as focal plane sensors in astronomical hard X-ray telescopes. To optimize the spectral response and imaging performance, we are investigating the effect of contact geometry on charge collection. Specifically, we have studied contact designs with orthogonal thin strips placed between pixel contacts. We apply a negative bias on the grid with respect to the pixel potential to force charge to drift toward the contacts. The grid bias is selected to be just sufficient to avoid charge collection on the grid, while increasing the transverse electric field on the surface between contacts. In contrast to focusing electrodes designed to force field lines to terminate on the pixel contact, our approach allows us to overcome the effects of charge loss between the pixels without significant increase of the leakage current, improving the overall energy resolution of the detector. In this paper we describe the performance of a CdZnTe pixel detector containing a grid electrode, bonded to a custom low-noise VLSI readout. We discuss the advantages of this type of detector for high spectral resolution applications