J.M. Koebel

CdTe and Cd1−xZnxTe for nuclear detectors: facts and fictions

Abstract Both CdTe and Cd 1− x Zn x Te (CZT) can be considered from their physical properties as very good materials for room temperature X- and γ-rays detection. However, despite years of intense material research, no significant advance has been made to help one to choose between both semiconductors. This paper reviews a few facts about CdTe and CZT to attempt to draw a real comparison between both. THM-CdTe and HPB-CZT have been grown and characterized in Strasbourg. Crystal growth, alloying effects, transport properties and defects are reviewed on the basis of our results and the published ones. The results show that it is still very difficult to claim which one is the best.

Electric field distribution in CdTe and Cd1−xZnxTe nuclear detectors

Abstract Internal field measurement in nuclear radiation detectors has always been a very difficult task. Hopefully, II–VI semiconductors, and all zinc blende structure, have a strong linear electro-optical coefficient, which allows to make use of Pockels effect. CdTe and CdZnTe are IR transparent and polarized light transmission is strongly related to the internal electric field. Therefore, IR transmission provides us powerful tools for electric field mapping. Detectors with several geometry have been investigated for improving detection quality.

Influence of deep levels on CdZnTe nuclear detectors

We report deep level characterization on CdZnTe nuclear detectors grown by the high pressure Bridgman method. Three CdZnTe samples taken from the top, middle and tail of the same ingot were investigated using photo-induced current transient spectroscopy and thermoelectric-effect spectroscopy. Three major traps are detected and their influence on the nuclear detection properties are discussed.

Structural defects in high resistivity cadmium telluride

Abstract The defect chemistry in compound semiconductors, especially CdTe, is not completely understood presently. Native point defects, like vacancies and interstitials, as well as extended defects or defects involving impurities are numerous in this II–VI semiconductor. This paper reviews our present view on the microscopic defects in the THM grown semi-insulating cadmium telluride, generally compensated with chlorine. A large number of crystals have been investigated by an upgraded TSC technique as well as by transient current spectroscopic methods. Defect introduction by strong gamma-rays and particle bombardment was used to ascertain the models of the defect structure we consider today as most probable.

Effects of copper in high resistivity cadmium telluride

Abstract In this paper, a large number of travelling heater method (THM) grown cadmium telluride crystals of high resistivity, having various concentrations of copper incorporated, have been investigated by PICTS and TSC techniques. The main results indicate that the levels identified in the 0.15–0.18 eV range can be explained by complexes including copper, rather than substitutional doping by this element, while the 0.35 eV level may result from active copper.

Properties of Cd1-xZnxTe crystals grown by high pressure Bridgman for nuclear detection

Abstract Cd x Zn 1− x Te crystals grown by high-pressure Bridgman are promising for nuclear detection and are already widely used and studied for this application. Phase precipitation is identified for the first time in such HPB grown crystals, more or less pronounced depending on the samples studied. TEM images and electron diffraction patterns are presented. The possible relationships with nuclear detection performance and defect energy levels are discussed in the light of nuclear spectrometry, PICTS, TSC and TEES.

Thermal annealing of gold deposited contacts on high resistivity p-type CdTe nuclear detectors

Abstract Nuclear radiation detectors made from high resistivity CdTe have a solid state ionisation chamber structure, with a nearly ohmic contact. Gold contacts were formed by electroless or electrolytic deposition. In order to understand the mechanisms involved at the interface between gold and semi-insulating p-type CdTe, a systematic investigation has been undertaken with different types of thermal cycling, including annealing in various atmospheres (hydrogen, argon, vacuum) at temperatures up to 250°C. The effects of these treatments have been investigated by current-voltage ( I–V ) curves, thermally stimulated current measurements (TSC). nuclear radiation detection and Rutherford backscattering (RBS) measurements. The results of these investigations on the variations in the barrier height with different treatments will be discussed.

The interaction of copper with structural defects in THM grown cadmium telluride

Abstract The exact role played by copper on the electronic properties of cadmium telluride is still subject to discussion. Using photoinduced current transient spectroscopy (PICTS) as well as current transient spectroscopy (CTS) and other electrical measurements on a large number of ingots, intentionally doped with copper or not, in which the concentration of Cu was determined by high sensitivity atomic absorption, we have investigated the 0.10–0.20 eV depth defect level in the gap. Our results show that the 0.14–0.18 eV band present in copper doped crystals is not due to a simple substitutional copper atom, but rather to a complex involving this metal. A model will be presented, explaining the effective structures.

Real defect concentration measurements of nuclear detector materials by the combination of PICTS and SCLC methods

The measurement of the real defect concentration by Photo-induced current transient spectroscopy method (PICTS) is still unattainable owing to the presence of reflecting metal layer contacts that avoid the knowledge of the real absorbed photons number and then the real photogenerated current in the sample. The combination of the two methods PICTS and space charge limited current (SCLC) allows to the extraction of the mean apparent absorption coefficient of the excitation light in the considered sample, as a consequence that leads to solve few main problems like: the scaling of PICTS spectra in term of real defect concentration and the μτ product evolution.

Optical and thermal spectroscopy of vanadium-doped CdTe and related photorefractive effect

The energy level diagram of substitutional vanadium in CdTe (and also Cd 0.96 Zn 0.04 Te) has been investigated by using a variety of optical and thermal spectroscopy techniques: optical absorption, photoconductivity, photoluminescence, deep level optical spectroscopy, DLTS and thermally stimulated current (PICTS). This study has led to set the V 2+ /V 3+ level at 0.95 eV below the conduction band with an optical photoionization threshold at 1.03 eV. Two-wave mixing experiments were used to evaluate the photorefractive properties. Gain coefficients in excess of 10 cm -1 have been measured at λ=1.32 μm with applying a 40 Hz AC electric field