3-D mapping of tellurium inclusions in CdZnTe crystals by means of improved optical microscopy

CdZnTe crystals are employed for the preparation of room temperature operating X-ray detectors. High resistivity is usually reached by contemporarily doping with group III or group VII elements and using tellurium deviated charge. This latter condition is responsible for the presence in crystals of a large number of tellurium inclusions. These can be incorporated at the growing interface or can form during cooling as a result of the retrograde behavior of the liquidus curve. Unfortunately, inclusions severely limit the performances of CdZnTe-based detectors, in particular in the case of imaging devices. This is why, monitoring tellurium inclusion density has become very important: i) for assessing the material quality ii) for studying the formation mechanisms of inclusions during growth iii) for checking the effectiveness of post-growth thermal treatments to reduce inclusion concentration. Tellurium inclusions are typically revealed by transmission optical microscopy in the near infrared. However, determination of the concentration of inclusions is complicated by the fact that at high magnification, the depth of field is much less then the sample thickness, so that in a single photograph only few inclusions appear really sharp. In order to overcome this problem, it is possible to take a set of photographs at different focal planes and, by means of specific software, reconstruct all inclusions on a single focal plane. This technique, also provided with some commercial microscopes, suffers two main problems: i) if one inclusion is present beneath a second one, only one is detected ii) any information about the depth in the sample of each inclusion is lost. For this reason, we have developed an instrument for the 3D mapping of the inclusions. The system is mounted on a standard optical microscope with automatic vertical movement. Pictures are taken at different focal planes. Images are then elaborated by dedicated software that ascribes each inclusion to the proper focal plane. As a result, all the inclusions are counted and precisely localized in 3D. By using the different objective lenses of the microscope is possible to tune the desired compromise between resolution and width of the monitored area. However, at high magnification it is possible to map inclusions down to 1 micron diameter. The system can be practically installed on any optical microscope that can operate in transmission mode

A New Model to Derive the Transport Parameters in CZT Detectors with a Liner Decreasing Internal Electric Field

Charge collection efficiency and material transport properties in CdTe and CZT planar devices for X and gamma ray detection are commonly determined by means of the Hecht equation, under the limiting approximation of a uniform internal electric field. As an alternative, the Manys theory is helpful when surface recombination velocity of carriers has to be taken into account. Experiments carried out on planar detectors have shown a non constant, linearly decreasing profiles of the internal electric field and, in several cases, the electric field does not decrease down to zero but to a fixed value different and afterwards it result to be constant. A similar non-uniform electric field could be also expected when a high radiation flux hits the surface near the detector contacts. This fact could explains as photoconductivity measurements on these kinds of devices are often not so easy described by means of the Hecht theory. Last of all, the use of the Hecht or Manys equations to fit photoconductivity curves could lead to wrong conclusionsin the determination of μτ product. Starting from the Ramo-Shockley theorem, the authors calculate a new relation between charge collection efficiency and applied bias in the case of an electric field decreasing linearly along the detector thickness, being satisfied all the other Hecht hypotheses. The new functional dependence of charge collection efficiency on applied bias contains the slope of the electric field as a parameter and provides for the Hecht model as the special solution when the electric field is uniform. Moreover this model allows to fit with a very good accuracy experimental results on several our CZT detectors. The authors believe that this shape of field should be widespread in presence of diffused spatial charge and think that this model could be an important instrument for interpreting the data arising from pulsed photocurrent measures not only for CdTe and CZT detectors but also for other materials

Charge Correction in X and Gamma Ray Detectors Based on CZT

CZT, show some prerogatives as materials for X and gamma ray detection because of its properties, as high stopping power, good transport properties, and low leakage current even at room temperature. Nevertheless, despite the improvement in the grown techniques, its transport properties are limited, if compared with those of traditional materials for radiation detection. This fact leads to a reduction of spectroscopic properties especially when increases the penetration length of gamma-rays at high energies. Several methods have been proposed to link the interaction depth and the charge losses achieving good improvements in the spectroscopy but, in most cases, they produce only heuristic corrections without any clearly visible relation with some physical models. Bargholtz et al. have improved the spectroscopy digitizing the signals and fitting them with a model, but this method is very expensive in term of computing power because of the high number of fitting paramiters. In this work, we propose an hybrid approach to the problem. Once known the read-out electronic transfer function, the detector signal could be corrected using the photon impact depth (extracted from the discontinuity in the current signal) and the material transport parameters, obtained from two simple fitting procedure with only four free parameters for each fit. This method, based on the Zanio model, is not only useful to correct the charge losses but also to achieve the characteristic properties of CZT, because takes into account also the detrapping contribution. In the last year we have presented the results obtained with a simplified model, now we show those obtained with the complete model, with a more accurate fit that lead to a more precise measure of detrapping time. The transfer function used in the calculations is the one measured in our system; nevertheless the procedure employed and the calculation of the integral terms are immediately extendable to many other transfer function

Comparative Analysis of Proteins Regulated during Cadmium Sulfide Quantum Dots Response in Arabidopsis thaliana Wild Type and Tolerant Mutants

In previous work, two independent Arabidopsis thaliana Ac/Ds transposon insertional mutant lines, atnp01 and atnp02, were identified that showed a higher level of tolerance than the wild type (wt) line to cadmium sulfide quantum dots (CdS QDs). The tolerance response was characterized at physiological, genetic and transcriptomic levels. In this work, a comparative analysis was performed on protein extracts from plantlets of the two mutants and of wt, each treated with 80 mg L-1 CdS QDs. A comparative protein analysis was performed by 2D-PAGE, and proteins were characterized by MALDI-TOF/TOF mass spectrometry. Of 250 proteins identified from all three lines, 98 showed significant changes in relative abundance between control and CdS QD-treated plantlets. The wt, atnp01, and atnp02 control-treated pairs respectively showed 61, 31, and 31 proteins with differential expression. The two mutants had a different response to treatment in terms of type and quantity of up- and downregulated proteins. This difference became more striking when compared to wt. A network analysis of the proteins differentially expressed in atnp01 and atnp02 included several of those encoded by putative genes accommodating the transposons, which were responsible for regulation of some proteins identified in this study. These included nifu-like protein 3 (Nfu3), involved in chloroplast assembly, elongator complex 3 (Elo3), involved in transcriptional elongation, magnesium-chelate subunit-2 (Chli2), involved in chlorophyll biosynthesis, and protein phosphatase 2C (PP2C) which mediates abiotic stress response

Characterization of tellurum inclusions in CdZnTe ingots grown by vertical bridgam technique

CdZnTe (CZT) crystals are employed for the preparation of room temperature operating X-ray detectors [1]. The functioning of the devices without refrigeration is made possible by growing high resistivity (>1010 Ohm.cm) ingots. This is usually reached by contemporarily doping with group III or group VII elements and using tellurium deviated charge. This second condition is responsible for the presence in crystals of a large number of tellurium inclusions. These can be incorporated at the growing interface or can form during cooling as a result of the retrograde behavior of the liquidus curve [2]. Unfortunately, inclusions severely limit the performances of CZT-based detectors, in particular in the case of imaging devices. In fact because of the role of diffusion of the electrons drifting from the cathode to the anode, tellurium inclusions act as traps for the charge carriers. Consequently the detector response close to the inclusions is deteriorated [3]. Hence, monitoring tellurium inclusion density is very important for assessing the material quality, selecting the best region in CZT wafer and for studying the formation mechanisms of inclusions during growth. Tellurium inclusions presence can be revealed by means of optical transmission microscopy in the near-infrared, in fact tellurium inclusions are opaque to the IR, while the CZT matrix is transparent. We developed an instrument for 3D mapping of inclusions mounted on a standard optical microscope with automatic vertical movement. Pictures are taken at different focal planes. Images are then elaborated by a dedicated software that ascribes each inclusion to the proper focal plane. As a result, all inclusions are counted and precisely localized in 3D. Using different objective lenses of the microscope it is possible to choose the optimal compromise between resolution and extent of the monitored area. However, at high magnification it is possible to map inclusions down to 1 micron diameter. The spatial position information of tellurium inclusion obtained by 3D IR mapping was used to select a single inclusion in the sample and then acquire photoluminescence (PL) map in the selected region. The inclusion was placed very close to the surface (few microns) by etching the sample. A correlation was set between the PL spectra emission and the presence of tellurium inclusion. [1] Knoll GF (2000) Radiation Detection and Measurements. Wiley [2] Rudolph P (1995), J. Cryst. Growth 147:297-304. [3] Carini GA (2006), Appl. Phys. Lett. 88:143515-143526. [4] Zambelli N (2011), J. Cryst. Growth 318:1167-1170

The study of growth mechanism as a key for large-scale vapor-phase synthesis of ZnO nanostructures

The growth of zinc oxide (ZnO) nanostructures is one of the main topics in today\u27s Material Science. These nanostructures have several proved or potential application in many fields, such as optoelectronics, photovoltaics, transparent electronics, gas-sensing, "piezo-tronics", etc. For real-world technological application of these nanostructures, large-scale and low-cost synthesis processes are strongly required. Many different growth processes have been presented in literature for ZnO nanostructures, "wet" chemical syntheses probably represent the easiest way to obtain them with high yields and low cost. However, ZnO nanostructures with a large variety of morphologies can be obtained also by vapor-phase growth processes, with high crystal perfection grade. If no catalyst is used in the growth process, a very low dopant/impurity content is also obtained (a fundamental request for some applications), but reproducibility and yield are generally much lower than those of chemical syntheses. In this presentation a deep-study of the vapor-phase nucleation and growth mechanisms of selected ZnO nanostructures (nanorods, nanotetrapods, nanowires and nanobelts) is presented. By mean of the obtained results, it has been possible to enhance the growth procedures and strongly improve the synthesis yields. Since the higher purity is generally one of the main advantages of vapor-phase growth process, only zinc and oxygen have been used as reagents, avoiding any catalyst or precursor. Metallic Zn has been preferred to ZnO as starting material to keep growth temperature as low as possible (generally <500?C). Large amounts of ZnO tetrapods have been obtained by a continuous streaming reaction, directly in the vapor phase. On the other side, thin aligned ZnO nanorods rods have been grown over some square centimeters areas, by mean of a ZnO layer. Then, also randomly oriented long nanowires have been obtained on several substrates, with homogeneous distribution. The role of Zn/O ratio, supersaturation and seeding-substrates is discussed in the different cases and results are compared, together with structural and optical characterizations. Moreover, some examples of functionalization of the obtained nanostructures and feasible applications are described

Growth mechanism of aligned ZnO nanorods by vapour phase process

ZnO is an important and versatile functional semiconducting material. In recent years one-dimensional nanostructures (wires, tubes, tetrapods, etc.) have received increased attention not only for their specific properties but also for the fabrication of nanoscale devices. Among these structures the parallelly aligned, column-shaped nanorods, orthogonal to the growth substrate plane, are particularly interesting for many applications such as dye sensitized solar cells, transistors, nanogenerators, short-wavelength nanolasers, etc. Many different techniques have been reported for the growth of ZnO nanostructures but thermal evaporation turns out to be one of the most convenient when considering the high quality and purity of the grown crystals, the simplicity of the growth apparatus and the easiness in the scaling-up of the process. In this communication the authors report on a selective growth process as regards well-aligned ZnO nanorods arrays extended up to a few cm2. The method, which is based on thermal evaporation and controlled oxidation, includes nucleation and growth kinetics control, adjustment of local growth temperature, selection of appropriate source materials and chemical composition of the substrate. The optimized growth parameters allowed to obtain arrays of (0001)-oriented, vertically aligned single crystals with length and diameters within 1-3 microns and 20-10 nm respectively. It is in particular pointed out: (a) the formation of sub-micrometric metal Zn clusters during the metal source evaporation; (b) their adhesion to a ZnO buffer layer (about 300 nm thick) previously deposited on the substrate (glass, Si, ...), which enables to keep an appropriate cluster distribution while avoiding larger clusters/drops formation thanks to suitable "surface wettability" conditions; (c) the subsequent growth of ZnO nanorods owing to the contemporary presence of Zn vapours, oxygen and Zn clusters on the substrate, these last ones acting as selective nucleation points. On the ground of what above and consequent process re-adjustments, the here proposed method, with its elevated yields and reproducibility as well as low production costs, turns out to be especially well-suited for large-scale application requirements

Study of the interface shape of CdZnTe crystals grown by Vertical Bridgman for X-ray detector applications

CdZnTe crystals are currently used for the preparation of X-ray detectors. However, the large-scale exploitation of CdZnTe-based detectors is limited by the low single crystalline yield of the available growth techniques. In particular, two problems connected with the growth process seem to be critical. The first one concerns with the first part of the growth: due to the presence of tellurium structures in the melt above the melting point, superheating is necessary, causing difficulties to standard seeding procedures. For this reason, unseeded growth is usually preferred, with the consequence that at least the first part of the growth is characterized by polycrystalline material. The second problem is connected with the difficulty to obtain a convex growth interface, basically because of the low thermal conductivity of CdZnTe crystals. This fact favors the development of spurious nuclei at the crucible walls. Some of the authors have recently proposed a modification of the vertical Bridgman technique that makes use of a boron oxide layer covering the melt during the growth. In this work, the growth interface of several CdZnTe crystals grown by the vertical Bridgman technique, with and without the use of boron oxide to cover the charge, has been studied mainly by means of pholuminescence mapping, optical microscopy, and EDS microanalysis. The results show that, even in the presence of a vertical thermal gradient of about 10?C/cm, considered ideal for achieving a good crystallization of CdZnTe crystals, nucleation often starts not from the lower tip of the crucible, but rather from lateral crucible walls. This seems to be due to a local modification of the thermal gradient due to the presence of the molten charge, the crucible, and the crucible support. Moreover, while the first part of the main body of the crystal is characterized by a convex interface, the second half is characterized by a concave interface in the case of crystals grown without encapsulant. On the contrary, if the melt is covered by boron oxide, the interface is convex up to the end of the growth. The explanation of this experimental evidence can be found in the different thermal conductivity of boron oxide and vapor and in the fact that boron oxide separates the melt from the convective flows in the vapor

Luminescence Properties of CZT Crystals in the Presence of Tellurium Inclusions

CZT is a widespread material for the realization of room temperature radiation detectors. The presence of defects and in particular secondary phases, like Te inclusions, represents nowadays a limit in the realization of high resolution devices. For the development of CZT detectors, in particular for high-flux applications, is very important to understand the role of deep levels, the influence of Te inclusions on the device performance and their correlations between Te inclusions and deep levels. Using a IR microscope recently developed at IMEM, it is possible to identify the 3D position of each inclusion in the bulk and reconstruct a 3D plot describing the spacial position of every inclusion. This permits to select a sigle inclusion in the sample, to place the inclusion very close to the surface (few microns) by etching the sample and hence to study the selected inclusion. In this way it is possible to perform photoluminescence and cathodoluminescence mapping in the inclusion region and investigate the behavior of the crystal-inclusion interface. The correlation between the deep level emission acquired at the micro-scale and the presence of tellurium inclusion is discussed