Synthesized amorphous selenium (a-Se) alloy materials have been characterized for room temperature high-energy
nuclear radiation detector and x-ray detection applications. The alloy composition has been optimized to ensure good
charge transport properties and detector performance. The synthesis of a-Se (As, Cl) alloys has been carried out by
thoroughly mixing zone-refined (ZR) Se (~7N) with previously synthesized a-Se(As) and a-Se(Cl) master alloys (MS).
The synthesized alloys have been characterized by x-ray diffraction (XRD), glow discharge mass spectroscopy (GDMS),
differential scanning calorimetry (DSC), x-ray photoelectron spectroscopy (XPS), and current-voltage (I-V)
characteristics measurements. Raman spectroscopy demonstrated that the a-Se(As) master alloy samples were in
metastable monoclinic Se8 states, in which seven vibrational modes are located at 40(41), 59(60), 77, 110, 133, 227(228)
and 251(252) cm-1. However, a-Se(Cl) master alloy samples are in stable form of trigonal structure of Se8 ring, in which two modes at 142 and 234 cm-1 were found. Both Raman and energy dispersive spectroscopy (EDS) exhibited that a
small amount of tellurium (Te) existed in a-Se (As, Cl) master alloy samples. DSC measurements showed that a-Se (Cl)
MS and a-Se (As) MS samples have one melting point, located at ~219.6°C, whereas a-Se-As (0.52%)-Cl and Se-
As(10.2%)-Cl(60 ppm) both possess two melting points, located at 221 and 220.3°C respectively. The a-Se alloy plate
detectors have been fabricated and tested and the results showed high dark resistivity (1012 - 1013 Ω-cm) with good
charge transport properties and cost-effective large-area scalability.
Different rotation profiles were applied to the ampoule during Vertical Bridgman (VB) growth of Cd0.9Zn0.1Te (CZT) for
enhancing compositional homogeneity, minimizing radial and axial segregation and stabilizing growth interface. The
radial Zinc (Zn) segregation was significantly minimized. Improvements in the axial Zn segregation were also
noticeable. The modifications in interface shapes during the growth were prominent from the photoluminescence (PL)
studies. Improved grain structure was observed with growths containing lower (0.5 wt. %) initial excess Te; however
with higher amount of initial excess Te (7.5 wt. %) high resistivity ingots with improved carrier properties were grown
consistently but with lower single crystal yield. With more intense rotations, resistivity values within the ingot varied by
an order of magnitude showing non uniformity in distribution of dopants.
CdZnTe, commonly known as CZT is the material of paramount importance for
hard X-ray and gamma ray spectroscopy and imaging applications at room temperature.
Over the years, the quality of CZT crystals and its charge transport properties has
improved significantly making it an attractive detector material especially for homeland
security applications. The applications for homeland security demand large and thick
detectors to provide a sufficient stopping power for fast detection of high energy
gamma photons. In this present report we have grown two inch diameter CZT by
Traveling heater method (THM) technique. The as-grown crystals were characterized
through photoluminescence (PL) mapping for composition uniformity, growth interface
study, Te precipitations/inclusions studies. In order to evaluate our as-grown samples,
charge transport characteristics have been studied for thick samples up to ~16mm thick.
It has been demonstrated that by controlling the growth interface, visualization of large
and thick (~16mm thick) detectors with fairly good response is possible from as-grown
CZT detectors grown by THM technique.
The layered anisotropic chalcogenide semiconductors GaSe and GaTe single crystals have been grown by a modified
vertical Bridgman technique using high purity Ga (7N) and in-house zone refined (ZR) precursor materials (Se and Te).
The crystals harvested from ingots of up to 10 cm length and up to 2" diameter, have been characterized by measuring
resistivity through current-voltage (I-V) characteristics and bulk carrier concentration and mobility through Hall effect
measurements. Micro-hardness, infrared microscopy, etching characteristics, low-temperature photoluminescence (PL)
and contact resistivity studies have also been performed to further characterize the grown crystals.
We find that the high-Z crystal Barium Iodide is readily growable by the Bridgman growth technique and is less
prone to crack compared to Lanthanum Halides. We have grown Barium Iodide crystals: undoped, doped with
Ce3+, and doped with Eu2+. Radioluminescence spectra and time-resolved decay were measured. BaI2(Eu)
exhibits luminescence from both Eu2+ at 420 nm (~450 ns decay), and a broad band at 550 nm (~3 μs decay)
that we assign to a trapped exciton. The 550 nm luminescence decreases relative to the Eu2+ luminescence
when the Barium Iodide is zone refined prior to crystal growth. We also describe the performance of BaI2(Eu)
crystals in experimental scintillator detectors.
High quality detector grade GaSe and GaTe single crystals have been grown by a modified vertical Bridgman
technique using high purity Ga (7N) and in-house zone refined (ZR) precursor materials (Se and Te). A state-of-the-art
computer model, MASTRAPP, is used to model heat and mass transfer in the Bridgman growth system and to predict the
stress distribution in the as-grown crystals. The model accounts for heat transfer in the multiphase system, convection in
the melt, and interface dynamics. The crystals harvested from ingots of 8-10 cm length and 2.5 cm diameter, have been
characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, low temperature
photoluminescence (PL), atomic force microscopy (AFM), and optical absorption/transmission measurements. Single
element devices up to 1 cm2 in area have been fabricated from the crystals and tested as radiation detectors by measuring
current-voltage (I-V) characteristics and pulse height spectra using 241Am source. The crystals have shown high promise
as nuclear detectors with their high dark resistivity (≥109 Ω.cm), good charge transport properties (μτe ~ 1.4x10-5 cm2/V and μτh ~ 1.5x10-5 cm2/V), and relatively good energy resolution (~4% energy resolution at 60 keV). Details of numerical modeling and simulation, detector fabrication, and testing using a 241Am energy source (60 keV) is presented
in this paper.
We explored the dependence of electron mobility-lifetime product (μτ) and surface recombination velocity ( S/μ) on
surface processing and temperature for Cd1-xZnxTe (CZT) radiation detectors. The surfaces of the CZT crystals were
mechanically polished and then chemically etched with 2% bromine-in-methanol. Different regions at the Au contacts
were illuminated with light at a wavelength of 650-, 800-, 900-, and 940-nm. The position-dependent and temperature-
dependent values of the electron mobility-lifetime product and surface recombination velocity were measured using a
direct current (DC) photoconductivity technique. The study revealed that this technique is a valuable and practical
method to measure the spatial- and temperature-variations of the surface recombination velocity and bulk lifetime for
CZT detectors. We discuss the correlation between the DC photoconductivity and measurements of the detector
performance.
In this paper, we report on the growth of a relatively new scintillator from the crystal K2LaX5 family (X=Cl, Br, I), K2LaBr5:Ce3+, and its scintillation properties for γ-ray spectroscopy. The material was synthesized by reacting stoichiometric amounts of LaBr3 and KBr. Prior to synthesis, the material was heated under dynamic vacuum to remove water molecule from the starting material. The synthesized material was then mixed with 10 mol % CeBr3, subsequently sealed in an evacuated quartz ampule. The crystals were grown by vertical Bridgman growth technique. An energy resolution of ~ 5% (FWHM) for the 662 KeV gamma ray was obtained with an encapsulated K2LaBr5:Ce3+ scintillator. The light yield non-proportionality achieved versus the energy range of 31 KeV to 1333 KeV was about 3%.
Silver gallium diselenide (AgGaSe2) is a semiconductor compound having an energy bangap of 1.7 eV, a value that is favorable for the room temperature radiation detection application. The starting material was synthesized from high purity elemental starting materials: 5N purity Se, 6N purity Ag, and 7N purity Ga. The crystals were grown at 880 °C in a three-zone semi-transparent gold-coated horizontal furnace. High resistivity (1.4 x 1011 ohm-cm) material was obtained and radiation detectors were fabricated. The response to gamma and alpha particles will be reported along with an analysis of the mobility - trapping time product for this novel material.
High quality CdTe crystals with resistivities higher than 108 Ω cm were grown by the 'contactless' PVT technique. Group III elements In and Al, and the transition metal Sc were introduced at the nominal level of about 6 ppm to the source material. Low-temperature photoluminescence (PL) has been employed to identify the origins of PL emissions of the crystals. It was found that the emission peaks at 1.584 eV and 1.581 eV exist only in the In-doped crystal. The result suggests that the luminescence line at 1.584 eV is associated with Cd-vacancy/indium complex. The intensity of the broadband centered at 1.43 eV decreases dramatically with introduction of Sc.
Detector grade, charge compensated Cd1-xZnxTe (CZT) crystals were treated by mechanical polishing, chemical solution etching, and radio-frequency gold sputtering. The crystals and detectors were next studied using direct current (DC) photoconductivity and low-temperature photoluminescence (PL). The DC photocurrent was analyzed with a steady state continuity equation and the results were correlated with PL and gamma-ray detector performance tests. The dependence of the electron lifetime on the light intensity of the CZT detector was consistent with a Shockley-Read one-center model. It was found that the surface recombination velocities, the effective mobility-lifetime products, and gamma-ray performances of CZT crystals are modified dramatically by the choice of chemical etchant.
The focus of this paper is the characterization of electro- optic properties of single crystal thin films of organic material NPP grown by the plate-guided method. Characterization was performed using the longitudinal a.c. modulation technique. Half-wave voltage V(pi ), figure-of- merit F, and electro-optic coefficient r63 were estimated to be 3.24 kV, 0.98 X 10-10 m/V and 25.8 MUL 10-12 m/V respectively. We found that crystalline z-axis is off the normal to the plane of the film at an angle of 70. We also proposed a transverse version of a thin film electro-optic modulator with low driving voltage, which is based on a single-arm thin film waveguide interferometer.
Organic materials with large electro-optic coefficients are promising for fabrication of fast electro-optic modulators with low driving voltage. In this paper we describe the growth of micron-thick single crystal films of various electro-optic organic materials from melt using the plate- guided method. The compounds such as mNA, COANP, NPP, PNP, MBANP and DAST will be considered. The films are grown using a homemade facility, which allows to control melting and recrystallization rate and to observe the process with an optical polarizing microscope. A single crystal films will make up a planar optical waveguide integrated with a coupling prism. The light will be injected into the film using the prism. Then, after traveling a certain distance within the film, the light will be decoupled. Combination of high electro-optic coefficients of the film (10 to 100 pm/V) with relatively long travel distance of the light beam (5 mm and more) potentially gives a driving voltage of the phase modulation of the beam of 5 V and less. This is very important for the incorporation of the modulator into a standard low voltage electronic circuitry. The phase modulation can be converted into amplitude modulation using various interferometer schemes such as Mach-Zehnder (M-Z) scheme with external arm, M-Z with internal arm, polarimetric waveguide interferometer, and dual-mode single- arm interferometer. We will provide some experimental data showing the advantage of the dual-mode single-arm scheme. Potential modulation rate of the system can be 10 dB and higher.
Pockels and Kerr cells were fabricated by growing N-(4- nitrophenyl)-L-prolinol crystal films between two transparent fused quartz plates on which indium-tin-oxide layers were deposited. Both linear and quadratic electro- optic effects of the crystals with thickness of 5 and 10 micrometers were examined by an ac modulation method. Large modulation signals were observed with a low driving voltage, the figure of merit of the electro-optic phase retardation were estimated to be 8.0 X 10-11 m/V. It is suggested that the electro-optic cells are promising for application because of their durability, relative easy manipulation during crystal growth and high figure of merit.
Meta-nitroaniline (mNA) and 2-cyclo-octylamino-5- nitropyridine (COANP) single crystal films with thickness of 5 to 20 micrometers were grown between two transparent fused quartz plates on which indium-tin-oxide layers were deposited. Both linear and quadratic electro-optic effects of the crystals were examined by an ac modulation method. The figures of merit for mNA and COANP are estimated to be 3.2 X 10-2 m/V and 4.2 X 10-12 m/V, respectively. The crystals were of high quality due to the excellent protection provided by the cells and the figure of merits of the crystals did not decrease with time. It is believed that the method adopted in the paper is applicable to electro-optic cell fabrication with other compounds if those crystals are grown from melt are feasible. It is suggested that the electro-optic cells have excellent potential use for device fabrication because of their durability and relative easy manipulation during crystal growth.
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