Cadmium Vapor Research Articles

Thermodynamic investigations in the lanthanum–cadmium system

An isopiestic vapor pressure method was developed which allows vapor pressure measurements in systems containing elements that are air and/or moisture sensitive. With this method, cadmium vapor pressures were determined for the intermetallic compound La 13Cd 58 and the neighboring two-phase field with LaCd 2 between about 750 and 1020 K. From these, activities and partial molar enthalpies were derived, and integral Gibbs energies of formation were obtained by a Gibbs–Duhem integration. It was found that the compound La 13Cd 58 shows some non-stoichiometry extending between about 79 and 81.5 at.% Cd. The enthalpy of formation of La 13Cd 58 was determined by a calorimetric method; it was found to be Δ H 298=−32.0±2.0 kJ mol −1.

Hydride generation atomic absorption spectrometry with different flow systems and in-atomizer trapping for determination of cadmium in water and urine—overview of existing data on cadmium vapour generation and evaluation of critical parameters

An overview of literature data on vapour generation techniques for cadmium and comparison with own experiments by means of several different types of hydride generation–electrothermal atomic absorption spectrometric systems (HG–ETAAS) (batch, semi-batch (SB), continuous-flow (CF) and flow-injection (FI) as well as different gas–liquid separators (GLS) exhibits apparent variations and inconsistency. However, if data for optimal chemical conditions are re-plotted in another coordinates: C HCl (mol l −1) vs. the ratio of reductant-to-acid molar input rates (i.e. millimoles per minute), [BH 4 −]:[H +], much better consistency of data is revealed: more than half of data are clustered around 0.2–0.3 mol l −1 HCl which appears an optimal acidity at moderate BH 4 − concentrations; the tetrahydroborate molar input rates should always be in excess versus the H + molar input rates (1.1 to tenfold); relatively high flow rates of argon purge gas are required (≥120 ml min −1); special attention to the blank control at ng l −1 levels as well as to the construction of gas–liquid separator and vapour transfer lines should be paid. ‘Milder’ conditions for HG could be provided with some of the examined systems and GLSs, thus minimizing reagent consumption, blanks, vigorous reactions, foaming, aerosol production and drift in measurements: e.g. 0.4 mol l −1 HCl—3% m/v NaBH 4 with the semi-batch system and 0.25 mol l −1 HCl—2% m/v NaBH 4 in continuous flow mode. Experimental system is based on the Transversely Heated Graphite Atomizer coupled with flow injection system FIAS 400. Integrated platforms are treated for permanent modification with Zr (110 μg) or W (240 μg) and then with Ir (8 μg). Temperatures of trapping, pyrolysis and atomization are 350, 500 and 1300 °C, respectively. The best overall efficiency of HG, transportation and trapping is 41%. The characteristic mass for peak area measurements is m o=2.8 pg and the limit of detection is 0.002 μg l −1. The long-term stability of characteristic mass (within-day, 8 h) is m o=2.8±0.1 pg (R.S.D. 4.0%, n=8), whereas the corresponding between-day figures (1 mo) are m o=2.8±0.2 pg (R.S.D. 6.6%, n=6). The linear range is 0.002–0.12 μg l −1 with a sample loop of 1.8 ml, being strongly impaired with smaller sample volumes in FI mode. The sample throughput rate is 10 h −1 with the semi-batch system. Applications to real human and bovine urine samples and CRMs of sea water (CASS-3), river water (SLRS-1 and SLRS-3) and urine (SRM 2670) are presented.

Luminescent properties of CdS crystals doped with gallium and tellurium in cadmium vapor

A technique is devised for vapor-phase doping CdS in the quaternary system Cd–Ga–Te–S. CdS crystals are doped with Ga and Te via four-zone annealing in a vapor phase containing Ga, Te, and Cd (the more volatile component of CdS). The luminescence spectra of the CdS〈Ga,Te〉[Cd] crystals are found to contain orange and red emission bands, in contrast to those of CdS[Cd] crystals, which are dominated by green emission.

Phase Diagram and Luminescent Properties of CdTe–MnTe Solid Solutions

The T–x phase diagram of the CdTe–MnTe system is mapped out. The CdTe–MnTe join is shown to be nonpseudobinary. The system contains an extended series of CdTe-based solid solutions (0–71.4 mol % MnTe at 1070 K). At 1070 K, α-MnTe dissolves up to 0.3 mol % CdTe. Data are presented on the composition dependences of lattice parameters for the solid solutions studied and the luminescent properties of Cd1 – x Mn x Te single crystals annealed in cadmium or tellurium vapor.

The role of extended defects in the formation of the spectrum of electronic states in high-purity p-CdTe polycrystals with stoichiometric composition

The spectra of the low-temperature photoluminescence in the cases of band-to-band and subgap laser excitation and the luminescence-excitation spectra of high-purity [111]-textured p-CdTe polycrystals with stoichiometric composition were measured. The spectrum of electronic states in the 1.3–1.6 eV energy range and the changes in this spectrum as a result of annealing of the samples in cadmium vapor were studied. It is shown that the electron levels located in the CdTe band gap that are responsible for the Z and Y luminescence bands (ℏω=1.36 and 1.47 eV) are formed as a result of interaction of extended defects with background impurities.

Semiconductor crystal optimization of gamma detection

Semiconductor gamma detector systems, that are not sensitive to hole trapping, are improved by trading off the bulk electrical resistivity and the gamma electrons’ lifetime. CdTe and CdZnTe detectors are prepared from donor doped crystals, grown by the low pressure Bridgman method with cadmium vapor pressure control. Donor doping, and annealing in tellurium vapors, adjusts the crystal's electrical resistance to high n-type value. The level of doping optimizes detector operation. Uniform detector arrays with good energy resolution are achieved.

Basic principles of postgrowth annealing of CdTe:Cl ingot to obtain semi-insulating crystals

The process of annealing of a CdTe:Cl ingot during its cooling after growth was studied. The annealing was performed in two stages: a high-temperature stage, with an approximate equality of chlorine and cadmium vacancy concentrations established at the thermodynamic equilibrium between the crystal and vapors of volatile components, and a low-temperature stage, with charged defects interacting to form neutral associations. The chlorine concentrations necessary to obtain semi-insulating crystals were determined for various ingot cooling rates in the high temperature stage. The dependence of the chlorine concentration [Cl+Te] in the ingot on the temperature of annealing in the high-temperature stage was found. The carrier lifetimes and drift mobilities were obtained in relation to the temperature and cadmium vapor pressure in the postgrowth annealing of the ingot.

High-temperature point defect equilibrium in CdTe modelling

High-temperature (673–1173 K) electrical property ( σ, R H) measurements in undoped and In-doped (up to 1×10 20 at/cm 3) CdTe were performed under different stoichiometric conditions. Cadmium (tellurium) vapor pressure or temperature electron (hole) density dependences were obtained. The results are described by a computed defect structure model, which optimizes the defect reaction constants. A new In incorporation defect reaction is proposed, explaining the dopant self-compensation mechanism.

Mechanisms for formation of a population inversion in the levels of metal atoms and ions in a plasma jet

The mechanisms for formation of population inversions in plasma jets of lithium, sodium, cadmium, and strontium vapor are studied. The primary contribution to forming a population inversion over the transitions of the alkali atoms is found to be three-body electron-ion recombination, and for transitions between the ion levels of strontium and cadmium, inelastic collisions with the buffer gas play an important role. Using helium instead of argon as a buffer leads to a substantial increase in the magnitude of the inversion.

Synchronously pumped laser without inversion in cadmium

We report a lasing without inversion (LWI) experiment on the basis of coherent population trapping, realized in low-pressure cadmium vapor. A single-pass inversionless amplification by more than a factor of 10 has been measured, a significant improvement over previous experiments. In addition, we report synchronously pumped laser oscillation without inversion. Using a simple model to fit the gain/absorption curves as a function of magnetic field, we have obtained values for the populations in the system, showing that there is no inversion in the atomic basis. Combined with a simple optical method to measure absolute densities of a gas, this model results in a completely optical method to determine the absolute populations and density in the system.

The dominant pumping mechanism of the 441.6 nm Cd II line in a three-colour hollow-cathode discharge laser

The three most probable processes for pumping of the upper lasing level of the 441.6 nm Cd II line under hollow-cathode discharge conditions, namely Penning ionization, `direct' and `stepwise' electron excitation, have been analysed. Atomic ionization and excitation cross sections and the electron distribution function in the energy interval up to 300 eV have been calculated. Radial dependencies of pumping rates of laser transition for a cylindrical cathode cavity and an integral pumping rate have been determined versus the cadmium vapour and helium pressures. It has been shown that the contributions of Penning ionization and direct and stepwise excitations to the pumping rate are strongly dependent on the pressures of discharge components. Contributions of these processes are found to reach up to about 70, 30 and less than 1%, respectively, under optimal conditions for the laser discharge. The comparison of calculated radial pumping rate distributions with experimental data confirmed that the Penning ionization plays the dominant role.

Partial pressure monitoring in cadmium telluride vapour growth

A simple system, based on optical absorption, which allows the partial pressures of cadmium and tellurium vapour over CdTe to be monitored under typical vapour-growth conditions is described. Use of the 326, 479 and 508 nm lines of a cadmium spectral lamp allows the narrow 326nm elemental absorption of cadmium together with the broad (350–600 nm) absorption of the tellurium dimer to be easily monitored with silicon photodiodes and optical bandpass filters. As the two absorptions do not overlap significantly their optical densities may be determined and related to the concentration and hence partial pressures of cadmium and tellurium. The present apparatus has been calibrated for cadmium pressures over the range 1.4–95.2 mbar and for tellurium pressures from 0.2 to 80.3 mbar.

ESR studies of doping C60 fullerene by zinc, cadmium, copper, gallium, and indium vapors

In the ESR spectra recorded at the beginning of the doping reaction of C60 fullerene by zinc, gallium, and indium vapors, signals corresponding to C60− were observed. In the course of reduction, these signals disappeared and reversibly transformed into the signals of higher charged C60n− anions. In the case of cadmium and copper, only the spectra of C60n− were recorded.

Control of Defects and Impurities in Production of CdZnTe Crystals by the Bridgman Method

AbstractCadmium zinc telluride crystals were grown by vertical Bridgman processes using in situ compdunding from high purity elements into pyrolytic boron nitride crucibles within sealed fused quartz ampoules containing cadmium vapor at a pressure of roughly one atmosphere. These conditions produce material having the low etch pit density, low precipitate density, high infrared transmission and high purity required for use as substrates for infrared focal plane detector arrays fabricated in epitaxial mercury cadmium telluride. Similar processes should be satisfactory for producing cadmium zinc telluride for gamma ray detectors.

Control of Defects and Impurities in Production of CdZnTe Crystals by the Bridgman Method

AbstractCadmium zinc telluride crystals were grown by vertical Bridgman processes using in situ compounding from high purity elements into pyrolytic boron nitride crucibles within sealed fused quartz ampoules containing cadmium vapor at a pressure of roughly one atmosphere. These conditions produce material having the low etch pit density, low precipitate density, high infrared transmission and high purity required for use as substrates for infrared focal plane detector arrays fabricated in epitaxial mercury cadmium telluride. Similar processes should be satisfactory for producing cadmium zinc telluride for gamma ray detectors.

Electrical activity of iodine-diffused CdTe

The electrical activity of iodine in CdTe is discussed when iodine is introduced into the CdTe by diffusion from the vapour phase. It is compared both with the total concentration of iodine in the diffused CdTe slices and with the electrical activity in CdTe slices which have been annealed under Cd- and Te-saturated vapour pressures. Iodine-diffused slices of CdTe were profiled by secondary ion mass spectrometry (SIMS) to obtain the total iodine concentration and by capacitance-voltage (C-V) techniques to obtain the net concentration of electrically active iodine. After annealing with iodine in the form of Cdl2, the slices were p-type, similar to those obtained when CdTe was annealed in either excess Te or Cd vapour, and they showed no significant increase in electrical activity. If Cd was added to the Cdl2 diffusion source or the CdTe was given a subsequent anneal in cadmium vapour, the slices became n-type. The results indicated that in all cases a neutral layer composed of Cd nIm (m and n are integers) formed on the surface layers, whereas if Cd was involved in the diffusion, some of the iodine existed in an electrically active form deeper into the slice with a maximum concentration of active carriers given by ND - NA ≈ 1017 cm−3. © 1997 John Wiley & Sons Ltd.

Cadmium purification by vacuum distillation in the presence of its oxide: thermodynamic modelling and experiment

We describe a process of cadmium vacuum distillation in the presence of a small quantity of its oxide. A thermodynamic model of the process which describes the phase composition in the evaporator and condensor during evaporation of cadmium was established. The model is used to separate impurities into classes according to their behaviour in the purification process. Both the experimental equipment and results of the purification process of cadmium grade 99.9 by one-stage and two-stage distillation are discussed.

Separation of Cd3(PO4)2, Cd(NO3)2 and CdCl2 by Sequential Metal Vapor Elution Analysis

Abstract The separation of cadmium compounds, (Cd3(PO4)2, Cd(NO3)2 and CdCl2), by sequential metal vapor elution analysis (column temperature; >1050 K) with helium carrier gas is described. The column consists of a molybdenum capillary tube inserted tungsten coil. Each cadmium vapor was separated at a column temperature of 1050 K and an vaporization temperature of 1050 K. Under the experimental conditions, the retention times of Cd3(PO4)2, Cd(NO3)2 and CdCl2 were 20.4, 32.0 and 35.0 s, respectively. The relative standard deviation for the retention times was better than 2.5%.

A study on the mechanism of the hole density depletion in p-type cadmium telluride during heat treatment

The phonomenon of hole density decrease in p-type cadmium telluride (CdTe) after annealing was investigated in order to identify the responsible mechanism. A model is suggested for the mechanism. By successive etching and capacitance-voltage (C-V) profiling, we proved that the effect was a reality and not an artifact of C-V measurements. It was shown that the hole density decrease was related with evaporation of CdTe at the surface but not with the out-diffusion of the dopant impurity. Annealing under various cadmium vapor pressures showed results which indicate that the dopant impurity taking cadmium sites are responsible for the carrier loss. The suggested mechanism was confirmed by annealing cesium-doped CdTe which showed increase in hole density. The depletion of surface hole density was reversed when further evaporation of CdTe at the surface was suppressed.

Dispersion of the refractive index of cadmium vapor and the dipole polarizability of the atomic cadmium 1S0 state.

We report measurements of the frequency dependence of the refractive index and polarizability of cadmium vapor in the frequency range between \ensuremath{\omega}=0.045 and 0.091 a.u. The quasicontinuous refractive index spectra have been measured at T\ensuremath{\approxeq}1050 K with a high-temperature Michelson interferometer using the technique of dispersive Fourier-transform spectroscopy. The dispersion of the dipole polarizability \ensuremath{\alpha}(\ensuremath{\omega}) of Cd obtained in our experiments can be represented as \ensuremath{\alpha}(\ensuremath{\omega})=\ensuremath{\alpha}(0)${\mathrm{\ensuremath{\omega}}}_{0}^{2}$/(${\mathrm{\ensuremath{\omega}}}_{0}^{2}$-${\mathrm{\ensuremath{\omega}}}^{2}$), with a static polarizability of \ensuremath{\alpha}(0)=(49.65\ifmmode\pm\else\textpm\fi{}1.46\ifmmode\pm\else\textpm\fi{}0.16) a.u. and an effective transition frequency of ${\mathrm{\ensuremath{\omega}}}_{0}$=(0.250 61\ifmmode\pm\else\textpm\fi{}0.000 23) a.u. In addition, these measurements yield approximations for the dispersion interaction energy constant ${\mathit{C}}_{6}$=466 a.u. and the second hyperpolarizability \ensuremath{\gamma}(0)=1.1\ifmmode\times\else\texttimes\fi{}${10}^{5}$ a.u.