Utpal N. Roy

Use of high-granularity position sensing to correct response non-uniformities of CdZnTe detectors

CdZnTe (CZT) is a promising medium for room-temperature gamma-ray detectors. However, the low production yield of acceptable quality crystals hampers the use of CZT detectors for gamma-ray spectroscopy. Significant efforts have been directed towards improving quality of CZT crystals to make them generally available for radiation detectors. Another way to address this problem is to implement detector designs that would allow for more accurate and predictable correction of the charge loss associated with crystal defects. In this work, we demonstrate that high-granularity position-sensitive detectors can significantly improve the performance of CZT detectors fabricated from CZT crystals with wider acceptance boundaries, leading to an increase of their availability and expected decrease in cost.

An array of virtual Frisch-grid CdZnTe detectors and a front-end application-specific integrated circuit for large-area position-sensitive gamma-ray cameras

We developed a robust and low-cost array of virtual Frisch-grid CdZnTe detectors coupled to a front-end readout application-specific integrated circuit (ASIC) for spectroscopy and imaging of gamma rays. The array operates as a self-reliant detector module. It is comprised of 36 close-packed 6 × 6 × 15 mm(3) detectors grouped into 3 × 3 sub-arrays of 2 × 2 detectors with the common cathodes. The front-end analog ASIC accommodates up to 36 anode and 9 cathode inputs. Several detector modules can be integrated into a single- or multi-layer unit operating as a Compton or a coded-aperture camera. We present the results from testing two fully assembled modules and readout electronics. The further enhancement of the arrays performance and reduction of their cost are possible by using position-sensitive virtual Frisch-grid detectors, which allow for accurate corrections of the response of material non-uniformities caused by crystal defects.

Topographic Evaluation of the Effect of Passivation in Improving the Performance of CdZnTe Detectors

Surface passivation reportedly is an effective technique for controlling surface leakage current and its related electronic noise. Several chemical agents have been effectively used for passivating cadmium zinc telluride (CdZnTe or CZT) surfaces; however, the long-term stability and the adverse effect on the metal contacts remain questionable. In this study, we reviewed two popular passivating agents, viz. hydrogen peroxide (H2O2) and a mixture of ammonium fluoride and hydrogen peroxide (NH4Fxa0+xa0H2O2). Our aim was to identify an ideal one that can effectively and permanently lower surface leakage currents without adversely affecting the metal contacts. We characterized their topographic features and their long-term effectiveness in terms of detector performance, and compared the results to understand their nature. We determined which chemical species were formed, and recorded the peaks of elemental Cd and Te via x-ray photoelectron spectroscopy (XPS) and micron-scale spatial resolution x-ray fluorescence (μ-XRF). We describe in detail the formation of new chemical species and the material nonuniformity of differently treated surfaces. Their effectiveness was assessed from experimental findings of their electrical properties and the spectral response. Our results imply that both passivating agents lowered the surface leakage current, and improved the detection efficiency of the CZT detectors, but their effectiveness was unstable over time.

Point defects: Their influence on electron trapping, resistivity, and electron mobility-lifetime product in CdTexSe1−x detectors

In this research, we assessed the abundance of point defects and their influence on the resistivity, the electron mobility-lifetime (μτe) product, and the electron trapping time in CdTeSe crystals grown under different conditions using the traveling heater method. We used current-deep level transient spectroscopy to determine the traps energy, their capture cross-section, and their concentration. Further, we used these data to determine the trapping and de-trapping times for the charge carriers. The data show that detectors with a lower concentration of In-dopant have a higher density of A-centers and Cd double vacancies (VCd-u2009-). The high concentrations of VCd-u2009- and A-centers, along with the deep trap at 0.86u2009eV and low density of 1.1u2009eV energy traps, are the major cause of the detectors low resistivity, and most probably, a major contributor to the low μτe product. Our results indicate that the energy levels of point defects in the bandgap, their concentrations, capture cross-sections, and their trapping and de-trapping times play an important role in the detectors performance, especially for devices that rely solely on electron transport.

Carbon Coating and Defects in CdZnTe and CdMnTe Nuclear Detectors

CADMIUM zinc telluride (CdZnTe) and cadmium manganese telluride (CdMnTe) are prime materials for detecting X-rays and gamma-rays at room temperature due to their high average atomic numbers that are essential to having high stopping-power for incident high-energy electromagnetic radiations. A major obstacle in developing CdZnTe and CdMnTe detectors lies in growing crystals free from defects, such as Te inclusions, dislocations, sub-grain boundary networks, and precipitates. We present the results of our study of the relationship between carbon coating of the growth ampoule and dislocations in CdZnTe and sub-grain boundary networks in CdMnTe, grown by Bridgman method. For the CdZnTe crystals, a carbon-coating of 2 μm on the ampoule generated fewer dislocations than did a thinner 0.2 - μm carbon-coated one. Furthermore, the ampoules design (normal- or tapered-shape) did not affect the densities of etch pits as much as did the thickness of the carbon-coating. For a CdMnTe ingot with a carbon coating of about 2 μm, created by cracking spectroscopic-grade acetone at ~ 900° C, we observed very few grain boundaries and grain-boundary networks.

An analysis of point defects induced by In, Al, Ni, and Sn dopants in Bridgman-grown CdZnTe detectors and their influence on trapping of charge carriers

In this research, we studied point defects induced in Bridgman-grown CdZnTe detectors doped with Indium (In), Aluminium (Al), Nickel (Ni), and Tin (Sn). Point defects associated with different dopants were observed, and these defects were analyzed in detail for their contributions to electron/hole (e/h) trapping. We also explored the correlations between the nature and abundance of the point defects with their influence on the resistivity, electron mobility-lifetime (μτe) product, and electron trapping time. We used current-deep level transient spectroscopy to determine the energy, capture cross-section, and concentration of each trap. Furthermore, we used the data to determine the trapping and de-trapping times for the charge carriers. In In-doped CdZnTe detectors, uncompensated Cd vacancies (VCd−) were identified as a dominant trap. The VCd− were almost compensated in detectors doped with Al, Ni, and Sn, in addition to co-doping with In. Dominant traps related to the dopant were found at Evu2009+u20090.36u2009eV an...

A comparison of point defects in Cd1−xZnxTe1−ySey crystals grown by Bridgman and traveling heater methods

In this paper, the properties of point defects in Cd1−xZnxTe1−ySey (CZTS) radiation detectors are characterized using deep-level transient spectroscopy and compared between materials grown using two different methods, the Bridgman method and the traveling heater method. The nature of the traps was analyzed in terms of their capture cross-sections and trap concentrations, as well as their effects on the measured charge-carrier trapping and de-trapping times, and then compared for the two growth techniques. The results revealed that Se addition to CdZnTe can reduce the VCd− concentration. In Travelling Heater Method (THM) and Bridgman Method (BM) grown CZTS detectors, besides a few similarities in the shallow and medium energy traps, there were major differences in the deep traps. It was observed that the excess-Te and lower growth-temperature conditions in THM-grown CZTS led to a complete compensation of VCd− and two additional traps (attributed to Tei− and TeCd++ appearing at around Ev + 0.26u2009eV and Ec − ...

Growth and characterization of CdTeSe for room-temperature radiation detector applications

The near-unity segregation coefficient of Se in a CdTe matrix ensures the compositional homogeneity, both axial- and radial, of the CdTeSe ternary compound, so making it a material of choice for room- temperature radiation detectors. In this study, we grew CdTeSe crystals by the Traveling Heater Method (THM), using Te as the solvent, and characterized the crystals by IR transmission microscopy, white-beam X-ray diffraction topography, and low-temperature photoluminescence. The total average concentration of the secondary phases obtained for the CdTeSe sample was about 7x104 cm-3 for crystals grown at two different laboratories. The best resistivity registered was 5x109 ohm-cm, and the estimated μτ product for the electrons was 3-4x10-3 cm2/V.

Al-doped ZnO contact to CdZnTe for x- and gamma-ray detector applications

The poor adhesion of common metals to CdZnTe (CZT)/CdTe surfaces has been a long-standing challenge for radiation detector applications. In this present work, we explored the use of an alternative electrode, viz., Al-doped ZnO (AZO) as a replacement to common metallic contacts. ZnO offers several advantages over the latter, such as having a higher hardness, a close match of the coefficients of thermal expansion for CZT and ZnO, and better adhesion to the surface of CZT due to the contact layer being an oxide. The AZO/CZT contact was investigated via high spatial-resolution X-ray response mapping for a planar detector at the micron level. The durability of the device was investigated by acquiring I–V measurements over an 18-month period, and good long-term stability was observed. We have demonstrated that the AZO/CZT/AZO virtual-Frisch-grid device performs fairly well, with comparable or better characteristics than that for the same detector fabricated with gold contacts.

Structural, electrical, and optical properties of CdMnTe crystals grown by modified floating-zone technique

Cadmium manganese telluride (CdMnTe or CMT), a compound semiconductor, is considered a promising material for the fabrication of high-performance room-temperature x-ray and gamma-ray detectors. The presence of material defects, e.g., high density of Te inclusions, has been a long-standing issue in CMT crystals grown by various Bridgman methods, since these defects degrade the device performance via charge-trapping. To address this issue, we employed the modified floating-zone method (MFZ) to grow CMT crystals and obtained as-grown crystals free of Te inclusions. This represents a new and distinct feature, absence of Te inclusions, compared to CMT crystals grown by Bridgman methods. White-beam x-ray diffraction topography (WBXDT) measurements demonstrated the existence of a high stress field within the MFZ-grown CMT crystals, which originates from the steep temperature gradient near the growth interface. Furthermore, we achieved a resistivity of 109 Ωcm for the MFZ-grown CMT crystals. The low-temperature photoluminescence (PL) measurements show that the intensity of the dislocation-related Y band is much higher than that of the principal exciton peaks, (D0,X) and (A0,X), confirming that the crystalline quality is affected by the high stress field. A long-term in-situ or post-growth thermal annealing will help to release such stress to improve the crystalline quality.