Laura Marchini

Study of Surface Treatment Effects on the Metal-CdZnTe Interface

The quality of a CdZnTe-based X-ray detector is highly related to the interface between semiconductor and metal contact. One of the factors that increase leakage currents in CdZnTe based X-ray detectors is the presence of a conductive surface layer. In this paper the result of the passivation of the CdZnTe surface by means of an aqueous solution of NH4F/H2O2 is studied by optical ellipsometry and by the current-voltage characteristics of gold contacts deposited on the oxidized surface. Collected data show that leakage currents can be reduced and contact stability improved by the combined use of the passivation layer and a guard ring.

Growth and Characterization of CZT Crystals by the Vertical Bridgman Method for X-Ray Detector Applications

CdZnTe crystals were grown by the vertical Bridgman method in closed quartz ampoules. The crystalline quality and the impurity content of these crystals were studied. Several X-ray detectors were cut out of these crystals. The resistivity, emission spectra, μτ product, and spectroscopic characteristics of these detectors were extensively measured and compared with the characteristics of detectors obtained from CdZnTe crystals grown by the boron oxide encapsulated vertical Bridgman technique. The detectors prepared from crystals grown without boron oxide show good μτ value, spectroscopic resolution, and higher reproducibility. The influence of growth method on impurity content and on detector response was discussed.

Characterization of Bulk and Surface Transport Mechanisms by Means of the Photocurrent Technique

The transport properties of ternary alloys, as CdZnTe, affect heavily the features required in X and Gamma spectroscopic detectors. The density and the nature of bulk defects and the lacking quality of contacts, characterized by high recombination center densities near the surface, damage strongly the quality of these materials. The photocurrent technique is a powerful tool to study the bulk properties and to investigate contact and surface quality. From steady-state photocurrent spectra information about the bulk trap defects and about the kinds of surface states far and near the contacts, has been obtained. By varying the bias at fixed wavelength, instead, it can be calculated the transport parameters like the product mobility-lifetime ¿¿ and the ratio s/¿, this last one related to the surface underneath the metal contacts. This paper gives some contribution in the interpretation of high energy region of the spectra. The authors correlate the response of samples illuminated with energies above the band gap to the surface states and show the different effects of this states on the electron and the hole transport case. Moreover in this work the authors compare the photocurrent measurement with the X ray spectroscopy and also discuss several phenomena like the shape difference between the spectra taken at different photon flux and bias polarities. Also the band edge shift and the modification of S/¿ parameter observed on varying the bias polarity have been discussed giving new a possible interpretations.

Preparation of bent crystals as high-efficiency optical elements for hard x-ray astronomy

Curved crystals, instead of flat mosaic crystals, can be used as optical elements of a Laue lens for hard x- and gamma-ray astronomy to increase the diffraction efficiency. We propose to achieve the bending of the crystals by a controlled surface damaging, which introduces defects in a layer of few tens nanometers in thickness undergoing a highly compressive strain. Several oriented silicon and gallium arsenide wafer crystals have been treated. The local and mean curvature radii of each sample have been determined by means of high resolution x-ray diffraction measurements in Bragg condition at low energy (8 keV). Silicon samples showed spherical curvatures, whereas GaAs-treated samples evidenced elliptical curvatures with major axes corresponding to the 〈110〉 crystallographic directions. Curvature radii between 3 and 70 m were easily obtained in wafers of different thicknesses. The characterization of GaAs samples performed in Laue geometry at gamma-ray energy of 120 keV confirmed the increase of the diffraction efficiency in the bent crystals.

Study of surface treatment effects on the metal-CdZnTe interface

The quality of a CdZnTe-based X-ray detector is highly related to the interface between semiconductor and metal contact. One of the factors that increase leakage currents in CdZnTe based X-Ray detectors is the presence of a conductive surface layer. In this paper the result of the passivation of the CdZnTe surface by means of an aqueous etching solution of NH4F/H2O2 is studied by optical ellipsometry and by the current-voltage characteristics of gold contacts deposited on the etched surface. Collected data show that leakage currents can be reduced and contact stability improved by the combined use of the passivation layer and a guard ring.

High-resolution x-ray characterization of mosaic crystals for hard x-ray astronomy

GaAs, Cu, CdTe, and CdZnTe crystals have been studied as optical elements for lenses for hard x-ray astronomy. High-resolution x-ray diffraction at 8 keV in Bragg geometry and at synchrotron at energies up to 500 keV in Laue geometry has been used. A good agreement was found between the mosaicity evaluated in Bragg geometry at 8 keV with x-ray penetration of the order of few tens of micrometers and that derived at synchrotron in transmission Laue geometry at higher x-ray energies. Mosaicity values in a range between a few to 150 arcsec were found in all the samples but, due to the presence of crystal grains in the cm range, CdTe and CdZnTe crystals were found not suitable. Cu crystals exhibit a mosaicity of the order of several arcmin; they indeed were found to be severely affected by cutting damage which could only be removed with a very deep etching. The full width at half maximum of the diffraction peaks decreased at higher x-ray energies showing that the peak broadening is affected by crystallite size. GaAs crystals grown by Czochralski method showed a mosaic spread up to 30 arcsec and good diffraction efficiency up to energies of 500 keV. The use of thermal treatments as a possible method to increase the mosaic spread was also evaluated.

Spectroscopic Response of CZT Detectors Obtained by the Boron Oxide Encapsulated Vertical Bridgman Method

Great efforts are being presently devoted to the development of CdTe and CdZnTe detectors for a large variety of applications, such as the basic, medical, industrial, and space research. The purpose of this work is to present the spectroscopic proper ties of some CZT crystals grown by the boron oxide encapsulated vertical Bridgman method, which has been recently implemented at IMEM-CNR. By this technique the crystal, during the growth, is fully encapsulated by a thin layer of liquid boron oxide, so that the crystal-crucible contact is prevented, thus allowing larger single grains with lower dislocation density to be obtained. Several detectors were realized about 4 mm x 4 mm x 1 mm in size, with two planar gold contacts on both the surfaces realized by an electroless technique. The behavior of these detectors was studied as a function of the bias voltage, irradiation geometry and energy of the interacting photons. Good electron charge collection properties have been demonstrated and electric field distribution has been investigated using the Pockels effect.

Crystal Defects in CdZnTe Crystals Grown by the Modified Low-Pressure Bridgman Method

Cadmium Zinc Telluride (CZT) is among the most promising materials for room-temperature X- and gamma-ray detectors. However, crystal defects such as Te inclusions and subgrain boundaries significantly hamper their performances. In this work, we evaluated CZT crystals grown by the modified low-pressure Bridgman technique at the IMEM Institute, Parma. We characterized the crystals by IR microscopy to identify the sizes and concentrations of the Te inclusions, along with high spatial resolution X-ray response mapping to measure the uniformity of their charge-transport properties. In addition, we employed white X-ray beam diffraction topography to analyze their crystalline structure.

Characterization of bulk and surface transport mechanisms by means of the photocurrent technique

Crucial points for the ternary materials, as CdZnTe, for X and Gamma detectors are the transport properties related with bulk defect density and nature, and, on the other hand, the quality of contacts, surfaces with high defect density, hence strong recombination rate. The photocurrent technique allows both to study bulk material properties and to obtain contact quality information. From steady-state photocurrent spectra information were obtained about the bulk trap levels, about the kinds of contact layer defects, and on the distributions of electric field in the sample volume. By varying the bias, with constant wavelength, instead, we can obtain the value of transport parameters like the product mobility-lifetime μt and the ratio S/μ, this last one related to the contact surface properties of the sample. The authors exploit the peculiarity of this technique by studying planar samples, comparing the achieved results with the results from X and Gamma ray spectroscopy.

Boron oxide encapsulated vertical Bridgman grown CdZnTe crystals as X-ray detector material

CdZnTe crystals doped with indium were grown by the boron oxide vertical Bridgman technique, with a diameter of 2-inches. The crystals showed large single crystalline yield and low etch pit density. The background impurity content was dominated by boron in concentration lower than 1 ppm. High resistivity was obtained and a procedure for contact preparation was developed. The mobility-lifetime product of the material was determined by both X-ray irradiation and photocurrent spectroscopy. The X-ray detector prepared with this material showed good spectroscopic characteristics.