Simulation, modeling, and crystal growth of Cd0.9Zn0.1Te for nuclear spectrometers

Abstract

High-quality, large (10 cm long and 2.5 cm diameter), nuclear spectrometer grade Cd 0.9 Zn 0.1 Te (CZT) single crystals have been grown by a controlled vertical Bridgman technique using in-house zone refined precursor materials (Cd, Zn, and Te). A state-of-the-art computer model, multizone adaptive scheme for transport and phase-change processes (MASTRAP), is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown CZT crystal and optimize the thermal profile. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The grown semi-insulating (SI) CZT crystals have demonstrated promising results for high-resolution roomtemperature radiation detectors due to their high dark resistivity (r % 2.8 3 10 11 V cm), good charge-transport properties [electron and hole mobility-lifetime product, mt e % (2-5) 3 10 ÿ 3 and mt h % (3-5) 3 10 ÿ 5 respectively, and low cost of production. Spectroscopic ellipsometry and optical transmission measurements were carried out on the grown CZT crystals using two-modulator generalized ellipsometry (2-MGE). The refractive index n and extinction coefficient k were determined by mathematically eliminating the ;3-nm surface roughness layer. Nuclear detection measurements on the single-element CZT detectors with 241 Am and 137 Cs clearly detected 59.6 and 662 keV energies with energy resolution (FWHM) of 2.4 keV (4.0%) and 9.2 keV (1.4%), respectively.

Component Overpressure Growth and Characterization of High-Resistivity CdTe Crystals for Radiation Detectors

“…[11][12][13] The ZR Cd and Te precursors were weighed in stoichiometric amounts and were vacuum sealed at 10 -6 torr in a cleaned carbon-coated quartz ampoule (!3 mm wall thickness). Cd was etched in 15% HNO 3 in methanol and Te was etched in a 20% aqueous solution of HCl, followed by repeated rinsing with DI water, and dried in a flow of purified nitrogen.…”

“…The details of EICÕs Bridgman furnaces have been described previously. [11][12][13] To simulate the Bridgman crystal growth we have assumed that the melt flow is compressible, laminar, axis-symmetric, and a Newtonian fluid; the thermo-physical properties are constant for the given phases; and the Boussinesq approximation is applicable (density is considered as constant except in the buoyancy force). The governing equations describing mass transport processes can then be written in the following dimensional forms:…”

“…[8][9][10] As a result, both CdTe and Cd 0.9 Zn 0.1 Te (CZT) have emerged as promising room-temperature semiconductor radiation detector materials. [11][12][13] Comparing CdTe and CZT, CdTe has advantages of higher average atomic number (Z % 50), higher density (5.85 g cm -3 ), lower electron-hole pair creation energy, and better hole transport properties, whereas CZT has larger bandgap (~1.6 eV vs. 1.5 eV at 300 K for CdTe) and higher resistivity (~10 11 X cm vs.~10 10 X cm for CdTe). Adding Zn to CdTe enhances bandgap and resistivity but results in non-homogeneous Zn distribution along the centerline of a grown ingot.…”

“…Adding Zn to CdTe enhances bandgap and resistivity but results in non-homogeneous Zn distribution along the centerline of a grown ingot. [10][11][12][13][14][15] A modified vertical Bridgman growth process for CdTe crystals is reported in this paper. The objective was to achieve a much lower density of Te precipitates than commercially available spectrometer-grade crystals.…”

CdTe and Cd 0.9 Zn 0.1 Te crystal growth and characterization for nuclear spectrometers

“…11 In order to reduce defect formation during the crystal growth, we have used a numerical modeling and simulation that combines formulation of global heat transfer and thermal elastic stresses. Using this model, the temperature distribution in the furnace and the resulting flow patterns in the melt and the interface shapes were predicted.…”