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

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

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 nucleation and growth mechanisms for optimized and large-scale synthesis of aligned ZnO nanorods for photovoltaic applications

The growth of ordered ZnO nanostructures is very important for many application fields, as optoelectronics, photovoltaics, piezo-electric power generation, etc. In particular, vertically aligned ZnO nanorods can be employed as 3D electron-harvesting TCO, capable of inducing multiple absorptions by light scattering phenomena in hybrid third generation solar cells. However, for photovoltaic application it is necessary to produce large area substrates uniformly covered by nanorods with controlled dimensions. Aligned ZnO nanorods can be obtained in highly ordered arrays over different substrates both by wet chemical methods and by vapour-phase processes. Not-catalyzed vapour growth processes are those that can supply nanostructures with the lowest concentration of undesired impurities. This aspect is fundamental for photovoltaic application in order to prevent negative effects on electron transport properties. In this work, we report the preparation of a large area (few square centimetres) nanostructured TCO on commercial glass substrate, constituted by a highly conducting ZnO:Al layer (by Pulsed Electron Deposition) and vertically aligned ZnO nanorods homogeneous in length and diameter over the whole substrate. The latter have been obtained by a vapour-phase process, starting from metallic Zn where only argon and oxygen flows are used, with a maximum temperature lower than 500?C (which is compatible with low-cost and application-oriented glass substrates). This goal has been achieved after an in-depth study of the nucleation and growth mechanisms of ZnO nanorods. The influence of substrate material and its crystallographic properties, as well as the results obtained with different growth parameters (temperature, flows, time) are discussed. Above all, it has been observed that the condensation of small Zn clusters on the polar surface of a ZnO film can reproducibly nucleate the growth of ZnO well-aligned rods, perpendicularly to the substrate face. These clusters can be formed by a proper control of temperature gradients in order to obtain locally a zinc vapour pressure larger than the equilibrium one of liquid phase. Once nucleation has occurred, the growth of the nanorods follows until zinc and oxygen are supplied. Diameter of nanorods can thus be intentionally modified in the 20-200 nm range, and length up to 3-4 microns. Moreover, it has been evidenced that during the growth process a ZnO wetting layer of controllable thickness can be deposited between the ZnO:Al layer and nanorods

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

Enhanced aldehydes detection by ZnO nano-tetrapod based gas sensors

Metal oxides are very important materials in gas-sensing and the possibility to obtain them as crystalline nanostructures represents an essential chance to improve sensors sensitivity an lifetime. Zinc oxide (ZnO) is a versatile material that is today widely studied because of the large number of possible application fields. The availability of this material in a large number of nanostructures makes it very interesting for the realization of gas sensors. In this field ZnO nano-tetrapods can find a suitable and reliable application, since they can be obtained by vapour phase growth, starting from metallic Zn, with very large yield and low production costs. In the present work authors report the excellent results obtained in: (i) developing an optimized growth process for the production of ZnO tetrapods, (ii) realizing a gas sensing device based on these nanostructures and (iii) the very promising results obtained in the detection of some volatile organic compounds (VOC). In particular a very high response and a remarkable sub-ppm detection limit is demonstrated for aldehydes. Furthermore, the reaction mechanisms, which take place on the surface of ZnO tetrapods, are discussed as a function of temperature and it is shown that the response curves measured at different temperatures can provide a powerful tool for adding selectivity to aldehydes detection towards particular interfering compounds (e.g. alcohols)

Method for large area deposition of ZnO tetrapod nanostructures

Among several morphologies ZnO presents one quite characteristic nano-crystalline structure, usually reported as "tetrapod" (TP), which consists of four needle-shaped "legs" connected at one common end and respectively arranged as axes of a tetrahedron (Fig. 1). Using an appropriate vapor-solid process, ZnO TPs are produced in large quantity (tens of mg per run) and, when removed from the growth reactor, they appear well assembled like a light white-grey "sponge". These aggregate structures mainly consist of TP nanocrystals, though usually they might also include unreacted Zn metal particles, nanosized ZnO powders and/or partially oxidized ZnO1-x nanostructures/powders. In order to "purify" as-grown TPs from the other undesired structures, the authors propose a multi-step process summarized as following: (1) post growth thermal annealing in vacuum (evaporation of metal Zn) and, subsequently, in oxygen atmosphere (oxidation of not completely reacted particles of ZnO1-x); (2) suspension of all the reaction products in appropriate liquids, in which ZnO is insoluble, in order to decant and to separate TPs from spurious structures; (3) room-temperature deposition of purified ZnO TPs on proper substrates (glass, silicon, alumina, etc., depending on the final application), whose sizes can vary from a few square mm up to many square cm; (4) heating at moderate temperature under low vacuum to remove traces of organic solvent and to favour the sticking of ZnO TPs on the substrates. The described procedure is highly valuable as it allows the achievement of homogeneous distribution of purified ZnO nanostructures on large substrates and a room-temperature deposition process which avoids detrimental interaction of ZnO TPs with substrate material, or with metal contacts previously deposited on the substrates. In practice, the proposed method is a new way to prepare large area films of metal oxides nanostructures, ready for device production. Application of the process to gas sensor fabrication and hybrid compounds (ZnO-MeS) preparation is also reported

Characterization of CZT crystals grown by the Boron Oxide Encapsulated Vertical Bridgman technique for the preparation of X-ray imaging detectors"

CdZnTe crystals are employed for the preparation of X- and Gamma- ray detectors. However, the large-scale production of these detectors is limited by the low yield of material with the required material properties, i. e. high resistivity, high carrier mobility lifetime product, and low inclusion density. In this frame, a new technique for the growth of CZT crystals has been developed based on a modification of the vertical Bridgman technique consisting in the encapsulation of the molten charge by a layer of boron oxide [1]. The authors have found that due to a chemical interaction with the quartz ampoule [2], a boron oxide layer fully encapsulates the crystal during growth preventing the direct contact of the crystal and the crucible walls. As a consequence, 2-inches CZT crystals characterized by large single grains and extremely low dislocations density have been grown. More recently, x-ray detectors with spectroscopic characteristics and good mobility lifetime product have been obtained using these crystals [3]. In this work we present an extensive characterization of these crystals with different techniques, ranging from energy dispersion x-ray analysis, photoluminescence mapping, contactless resistivity mapping, and infrared transmission. As a result, it is determined the interface shape and the zinc concentration distribution, that is related to the fluido-dynamic properties of the melt during growth. Moreover, the good homogeneity of resistivity is demonstrated. Finally, the typical inclusion density of these crystals is studied, hence the possibility to use them for the preparation of imaging and high flux x-ray detectors

Crystal Defects and Charge Collection in CZT X-Ray and Gamma Detectors

CdZnTe (CZT) is very promising material for room-temperature x-ray detectors proposed for medical, environmental and astrophysics applications. CZT can potentially provide high resistivity, low leakage current and high charge-collection efficiency. However, commercial CZT material is affected by crystal defects limiting the use of CZT in the large-scale production of x-ray and gamma detectors. In this work we have tested several CZT samples grown at IMEM-CNR Institute, in order to understand the roles of defects on the charge transport. Different optical and electrical techniques were used and the results will be reported. Te inclusions were studied with an IR microscope and accurate measurement of the electron mu-tau product u were conducted using an alpha particle source. We also studied the effect caused by grain boundaries and dislocations, which were identified using White beam X-ray Diffraction Topography. The homogeneity of the device response and the uniformity of the electric field were examined in the National Synchrotron Light Source using a 25-KeV highly collimated X-ray beam and raster scanning the device. A strong correlation between the extended defects and the detector response was found

Interface shape control and tellurium inclusion concentration distribution in CdZnTe crystals grown by vertical Bridgman for X-ray detector applications

In spite of the efforts devoted to the task, many problems connected with the growth of CdZnTe (Zn>0) crystals are still unresolved, in particular tellurium inclusion density control, large single crystalline yield, seeding, and interface shape control. Moreover, also the electrical properties of the crystals (high resistivity and mobility-lifetime product) must be taken into account if detector performances have to be improved. In this work, the authors report on the growth and characterization of several CdZnTe crystals (Zn=10%) by vertical Bridgman, with and without the use of boron oxide as encapsulant. Different techniques were used to characterize the crystals: i) PL mapping for determining interface shape and to study the nucleation ii) a novel IR mapping apparatus to obtain fully 3D reconstruction of the inclusion distribution iii) X-ray detector characterization by means of nuclear sources to study the transport properties of the material (with mobility-lifetime product for electrons up to 6x10-3 cm2/V)