Massimiliano Zanichelli

Aldehyde detection by ZnO tetrapod-based gas sensors

Zinc oxide (ZnO) tetrapods have been synthesized and prototypal gas sensors for volatile organic compounds (VOCs) have been realized with this sensing nanomaterial. Sensor characterization has been focused on aldehydes, such as acetaldehyde (or ethanal) and propionaldehyde (or propanal), whose detection at ppb levels is extremely important for monitoring environmental, domestic and food pollution, as well as for some sickness diagnosis and other medical applications. The response of gas sensor prototypes has been measured as a function of concentration, temperature, relative humidity and time. Good response values at ppb levels and some peculiar behaviours have been pointed out.

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.

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.

CZT X-ray detectors obtained by the boron encapsulated vertical Bridgman method

Recently, some of the authors showed that it is possible to grow CZT crystals by the boron oxide encapsulated vertical Bridgman method. The most important feature of the technique is that the crystal, during the growth, is fully encapsulated by a thin layer of liquid boron oxide, so that the crystal-crucible contact is prevented. The stress of the crucible to the crystal is strongly reduced also during the cooling, because the boron oxide layer is molten down to about 500°C. A number of detectors have been prepared out of these crystals. The transport properties (μτ product) have been studied by photoconductivity measurements as well as by determining the response to hard X-ray irradiation. The transport properties have been studied as a function of the indium content and of the position of the wafer which the detector was cut out.

Spectroscopic response of CZT detectors obtained by the boron encapsulated vertical Bridgman method

A great effort is being presently devoted in growing and studying CdTe and CdZnTe detectors which can be used in a large variety of applications, such as the basic, medical, industrial, and space research.

Development status of a CZT spectrometer prototype with 3D spatial resolution for hard x-ray astronomy

The development of new focusing optics based on wide band Laue lenses operating from ~60 keV up to several hundred keV is particularly challenging. This type of hard X-ray or gamma ray optics requires a high performance focal plane detector in order to exploit to the best their intrinsic capabilities. We describe a three dimensional (3D) position sensitive detector prototype suitable as the basic module for a high efficiency Laue lens focal plane detector. This detector configuration is currently under study for use in a balloon payload dedicated to performing a high significance measurement of the polarization status of the Crab between 100 and 500 keV. The prototype is made by packing 8 linear modules, each composed of one basic sensitive unit bonded onto a thin supporting ceramic layer. Each unit is a drift strip detector based on a CZT crystal, irradiated transversally to the electric field direction. The anode is segmented into 8 detection cells, each comprising one collecting strip and 8 surrounding drift strips. The drift strips are biased by a voltage divider. The cathode is divided into 4 horizontal strips for the reconstruction of the Z interaction position. The detector readout electronics is based on RENA-3 ASIC and the data handling system uses a custom electronics based on FPGA to provide the ASIC setting, the event handling logic, and the data acquisition. This paper mainly describes the components and the status of the undergoing activities for the construction of the proposed 3D CZT prototype and shows the results of the electronics tests.

Development of a 3D CZT detector prototype for Laue Lens telescope

We report on the development of a 3D position sensitive prototype suitable as focal plane detector for Laue lens telescope. The basic sensitive unit is a drift strip detector based on a CZT crystal, (~19×8 mm2 area, 2.4 mm thick), irradiated transversally to the electric field direction. The anode side is segmented in 64 strips, that divide the crystal in 8 independent sensor (pixel), each composed by one collecting strip and 7 (one in common) adjacent drift strips. The drift strips are biased by a voltage divider, whereas the anode strips are held at ground. Furthermore, the cathode is divided in 4 horizontal strips for the reconstruction of the third interaction position coordinate. The 3D prototype will be made by packing 8 linear modules, each composed by one basic sensitive unit, bonded on a ceramic layer. The linear modules readout is provided by a custom front end electronics implementing a set of three RENA-3 for a total of 128 channels. The front-end electronics and the operating logics (in particular coincidence logics for polarisation measurements) are handled by a versatile and modular multi-parametric back end electronics developed using FPGA technology.

Charge transport properties in CZT detectors grown by the vertical bridgman technique

Great efforts are being presently devoted to the development of CdTe and CdZnTe detectors for a large variety of applications, such as medical, industrial, and space research. We present the spectroscopic properties of some CZT crystals grown by the standard vertical Bridgman method and by the boron oxide encapsulated vertical Bridgman method, which has been recently implemented at IMEM-CNR (Parma, Italy). By this technique the crystal is grown in an open quartz crucible fully encapsulated by a thin layer of liquid boron oxide. This method prevents contact between the crystal and the crucible thereby allowing larger single grains with a lower dislocation density to be obtained. Several mono-electrode detectors were realized each with two planar gold contacts. The samples are characterized by an active area of about 7 mm × 7 mm and thicknesses ranging from 1 to 2 mm. The charge transport properties of the detectors have been studied by mobility-lifetime (μt) product measurements, carried out at the European Synchrotron Radiation Facility (Grenoble) in the PTF configuration, where the impinging beam direction is orthogonal to the collecting electric field. We have performed several fine scans between the electrodes with a beam spot of 10 μm × 10 μm at various energies from 60 keV to 400 keV. In this work we present the test results in terms of μt product of both charge carriers.