Cd Vapor Pressure Research Articles

Contributions to the development of crystal growth technologies

An overview of selected contributions to the development of crystal growth technology of the Laudise Prize awardee 2023 is presented. First some results on shaped crystal growth are given, such as layers with eutectic periodic structures, casting of high-resistivity CdTe sheets, growth of in situ core doped laser rods by double die EFG, and Czochralski growth of Si crystals with rectangular cross section. Then the correlation between melt structure and quality of II-VI crystals showing high bond ionicity is discussed. The importance of marked melt overheating for growth of high-quality CdTe and (Cd,Zn)Te crystals is derived. Also the in situ stoichiometry control for reducing intrinsic point defects, precipitates and inclusions is demonstrated by the examples of Bridgman and VGF CdTe growth with Cd extra source and vapor pressure controlled Czochralski method of 6 inch GaAs crystals. Next, the reason and general appearance of dislocation cell patterns in all growing thermomechanical stressed crystals and their possible prevention are explained. The contribution of point defect diffusion for dislocation climb at high temperatures to form globular cells is underlined. Finally, melt growth experiments under travelling magnetic field (TMF) generated in coil-shaped heater-magnet modules around the crucibles are discussed. The importance of global numeric modeling for optimization the TMF convection mode in the melt is accentuated. It is shown that the growing interface can be levelled, the solution flux at vertical liquid phase epitaxy can be homogenized, and the IR transmission and minority carrier live time in 640 kg mc-Si ingots for photovoltaics can be markedly enhanced and homogenize by such external field application. The close cooperation between academics, engineers and industry is particularly emphasized.

CdTe synthesis and crystal growth using the high-pressure Bridgman technique

Abstract Efficient, safe and cost-effective synthesis of CdTe from elements is rather challenging in silica sealed ampoules due to the high vapor pressure of Cd. In this article, we report on the integrated synthesis and crystal growth of high-purity CdTe using the high pressure Bridgman (HPB) technique that is scalable to large volumes. The process lends itself for cost competitive industrial production of polycrystalline feedstock material for photovoltaics, sensors and electro-optic applications. Cadmium telluride (CdTe) crystals exceeding 1 kg in size were synthesized from elemental Cd and Te sources with purity comparable to state-of-the-art gamma ray detector crystals. In addition, synthesis of highly-doped CdTe feedstock for thin film photovoltaics applications demonstrating effective incorporation of group V (As, Sb) dopants was achieved at growth speeds of ~500 mm/hr. The technique may be applicable to produce other II-VI compounds with volatile components.

A thermodynamic study of the cadmium-neodymium system.

Cd vapor pressures were determined over Cd–Nd samples by an isopiestic method. The measurements were carried out in the temperature range from about 690 to 1200 K and over a composition range between 48 and 92 at % Cd. From the vapor pressures, thermodynamic activities of Cd were derived for all samples at their respective sample temperatures, and partial molar enthalpies of Cd were obtained from the temperature dependence of the activities. With these partial molar enthalpies, the Cd activities were converted to a common temperature of 873 K. By means of a Gibbs–Duhem integration Nd activities and integral Gibbs energies were calculated, using a literature value of ΔfG for the phase Cd6Nd as integration constant. A minimum of ΔfG ≈ −38 kJ g-atom−1 at 873 K was obtained for the phase CdNd, a value that compares well with other CdRE compounds.Graphical abstract

Thin film growth of Cd1‐xZnxTe and its application to X‐ray sensor

AbstractCd1‐xZnxTe (x∼0.5) thin films with less than 300μm are grown on ITO/glass substrate by Hot‐wall and PVT methods, one of the sublimation methods. In order to obtain the films with high resistivity and good quality for X‐ray sensor, three attempts have been tried in the experiments. The one is stoichiometry control of CZT (x = 0.04) films, which were grown under controlled Cd vapor pressure. The second is excess chlorine doping to the films (x = 0.04) by controlling CdCl2 vapor pressure during growth process. The third is to change composition x = 0.04‐0.50, which increases band gap. Through these methods, best films with good X‐ray response was obtained for the third case of the films with the composition x = 0.20. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ChemInform Abstract: Thermochemical Investigations in the System Cd—Gd.

AbstractThe vapor pressure of Cd in the Cd—Gd system is determined between 693 and 1045 K, and the thermodynamic activities of Cd are derived as a function of temperature for the composition range 52—86 wt.% Cd.

Surface stoichiometry of pulsed ultraviolet laser treated polycrystalline CdTe

The effects of nanosecond pulsed ultraviolet laser annealing on the surface stoichiometry of close-space sublimated polycrystalline thin films are investigated using angle-resolved x-ray photoemission spectroscopy (XPS). The raw data suggest the formation of a Cd-rich surface layer, but this is counter to the expectation based on Cd and Te vapor pressures above CdTe that predicts a Te-rich layer and to direct observation of elemental Te at the surface. In order to explain this apparent discrepancy, we analyze our XPS data in the context of prior reports of lateral segregation of Cd and Te at the surface after pulsed laser treatments with a simple model of angular dependent XPS in the presence of surface roughness. This analysis reveals that a uniform Te layer cannot explain our results. Instead, our analysis suggests that Te enrichment occurs near grain boundaries and that a sub-monolayer Cd layer exists elsewhere. These complex yet repeatable results underscore the challenges in measuring surface stoichiometry to high precision on films relevant for polycrystalline CdTe devices. It also suggests that the Cd and Te vapor pressures above grain boundaries may differ from those above grain interiors and that ohmic contact may be made preferentially at the grain boundaries after pulsed laser annealing.

Elimination of Te Inclusions in ${\rm Cd}_{1-x}{\rm Zn}_{x}{\rm Te}$ Crystals by Short-term Thermal Annealing

The presence of Te inclusions degrades the quality of today's CdZnTe (CZT) crystals used for Xand gamma-ray detectors; both their sizes and concentrations densities must be reduced. Over the past years, many researchers proposed using long-term annealing (>;24 h) under Cd vapor pressure to reduce or even eliminate the inclusions visible under IR microscopes. We annealed detector-grade CZT samples for periods of 15 to 60 min under Cd-, Zn-, or Te-overpressure or in vacuum at 1000-1200 K. We determined the optimal temperature, duration, and the vapor atmosphere for such high-temperature annealing, typically at ~1100 K for 0.5-1.0 h. The results were very promising in eliminating Te-rich inclusions, even on twins where the inclusions are more stable than in the unperturbed lattice; indeed, we saw almost no inclusions whatsoever by IR transmission microscopy after such annealing. We note that eliminating inclusions at lower temperatures takes much longer. However, annealing under a Cd vapor pressure at temperatures above ~1170 K generates a large quantity of irregular Cd inclusions. The samples' resistance after annealing was estimated by I-V curves.

Effect of ${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te\!:\!In}$ Crystals Annealing on Their High-Temperature Electrical Properties

We studied the electrical properties of Cd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Te:In (CZT) single crystals with [In]=3*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> at/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> at its high-temperature point-defect (PD) equilibrium state under a Cd overpressure (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cd</sub> ). We detailed the influence of thermal treatment and the deviation of stoichiometry on the electron concentration, observing unexpectedly high conductivity and an increase in free-electron density ( ~ 1.5-2 orders of magnitude) when annealing the sample at 770 K under a Cd vapor pressure (0.01 atm.). Prolonged exposure of the samples under these conditions lowered the electron density by two approximately orders-of-magnitude until it approached the intrinsic value. The electron mobility after such treatment increased to CZT's maximal values at ~ 460 K (650-700 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /(V*s)). Therefore, such annealing can be effective in assuring high-resistive CZT detectors after crystal growth, or by special treatment, thereby eliminating the inclusions. We analyzed these data in the framework of Kröger's theory of quasi-chemical reactions, and compared the findings to those obtained for undoped CdTe.

The growth and the interfacial layer of CdZnTe nano-crystalline films by vacuum evaporation

Nano-crystalline CdZnTe films were fabricated by thermal vacuum evaporation. The structure and surface morphology of the CdZnTe films were determined by XRD and SEM. The CdZnTe films were poly-crystalline with preferential (111) orientation. The CdZnTe films exhibited a typical particle size of 15 nm and a blue shift in the absorption with an effective band gap of 2.26 eV. An amorphous Te interfacial layer with thickness of 3 nm was observed with high resolution transmission electron microscopy, which should be formed at the initial stage of the growth. This is because the equilibrium vapor pressure of Cd is largely higher than that of Te 2 at the growth temperature, and the desorption rate of Cd atoms is much higher than that of and Te 2 molecules. The amorphous interfacial layer should be favorable for the formation of nano-crystalline CdZnTe film.

Dopant Content and Thermal Treatment of ${\rm Cd} _{1-{\rm x}} {\rm Zn} _{\rm x} {\rm Te} \langle {\rm In}\rangle $: Effects on Point-Defect Structures

We measured, in the 873-1173 K temperature range, the temperature- and Cd vapor-pressure-dependences of the free electron density in single CdTe(In) crystals with different In contents. Increasing the cooling rate of the crystals and/or decreasing the well-defined Cd vapor pressure reduced the free-electron density. We interpreted and modelled these phenomena and the crystal's high-temperature electrical properties within the framework of Kroger's point-defect theory. Our experiments demonstrated the possibility of controlling the free-electron density in CdTe(In) crystals by changing the cooling rate. We supplemented a point-defect structural study of CZT(In) crystals by low-temperature (80-420 K) electrical measurements. These findings allowed us to identify the nature of the point defects responsible for free-carrier scattering, which is an important parameter influencing the mutau-product value in detector-grade material.

On the Formation of Unusual Diffusion Profiles in Cd&lt;sub&gt;x&lt;/sub&gt;Zn&lt;sub&gt;1-x&lt;/sub&gt;Te Crystals after Implantation of Different Elements

It is known that the diffusion of Ag and Cu in Cd1 xZnxTe crystals exhibits unusual concentration profiles depending strongly on the external vapor pressure of Cd during diffusion. Recent experiments show that the dopant Na forms qualitatively the same diffusion profiles including the phenomenon of uphill diffusion. Also the transition elements Ni and Co show a strong dependence of the diffusion behavior on the external Cd pressure, but the shapes of the concentration profiles differ significantly from those known for Ag and Cu. The different behavior of Ag, Cu, and Na, on the one hand, and Ni and Co, on the other hand, are proposed to be connected to the respective charge states of the dopants at interstitial positions in Cd1 xZnxTe. For the dopants K and Au, unusual diffusion properties have not been observed. The respective diffusion coefficients are DK = 1.2(2)•10 10 cm2/s (750 K) and DAu = 8(2)•10 8 cm2/s (800 K).

Rational synthesis and characterization of heterostructures of ZnO nanocombs with (Zn,Cd)O nanocaps

ZnO/(Zn,Cd)O nanoscale heterostructures were synthesized by a two-stage thermal evaporation method. The heterostructures consisted of ZnO nanocombs and (Zn,Cd)O nanocaps. Two types of nanocaps, wurtzite and cubic (Zn,Cd)O with different Cd contents, could be produced by tuning the growth conditions. The Cd vapour pressure played an important role in determining the morphology, structure and size of the (Zn,Cd)O nanocaps on the branches. Temperature-dependent PL measurements indicated that band-gap engineering of ZnO could be realized in the heterostructures of ZnO nanocombs with cubic (Zn,Cd)O nanocaps. This method can be used to synthesize ZnO nano-heterostructures with other materials.

High Temperature Properties of CdTe Crystals, Doped by Sb

Electrical properties of CdTe single crystals doped by Sb were studied in situ at high temperature point defect equilibrium under well defined Cd and Te vapour pressure. Up to samples revealed p-type conductivity both under Cd and Te saturation. The position of the deep acceptor level was determined using hole density temperature dependency at 350-650 K. The point defects responsible for hole density are supposed to be acceptors. The n-type conductivity above is determined by intrinsic point defects: electrons under Te saturation due to native disorder and Cd interstitials under Cd saturation.

The nature of point defects in CdTe

High temperature in-situ Hall effect measurements in CdTe single crystals grown by different techniques at 500–1,200 K under well-defined Cd and Te vapor pressure were made. It was established that native point defects (PD) Cd i 2+ and V Te 2+ are dominant at T>770 K in Cd-rich CdTe. Their formation enthalpies were determined (ΔH VTe 2+ =1.3 eV; ΔH Cdi 2+ =2.5 eV). It was shown that V Te 2+ dominate at low temperatures and Cd i 2+ begin to prevail at T>930 K. In Te-saturated CdTe at heating up to ∼800 K, the hole density was Te vapor pressure independent. At higher temperatures (HT), the conductivity changed to intrinsic type, turning then into n-type. The results were explained in the framework of Kroger’s quasichemical formalism assuming the presence of an electrically active foreign point defect, the oxygen acceptor. The PD structure modeling demonstrated satisfactory agreement with experimental results both for temperature and component vapor pressure dependencies.

Electrical characteristics of CdTe:Pb single crystals at high temperatures

The electrical properties of CdTe:Pb single crystals at high temperatures (400–900°C) and under controlled Cd vapor pressure (0.001–3 atm) were investigated for the first time. The temperature and baric dependences of the conductivity and Hall coefficient were measured. Low (in comparison with undoped CdTe) electron concentration indicates an increase in the number of impurity point defects related to the Pb impurity. The results obtained are explained within the Kroger theory of quasi-chemical reactions of defect formation on the assumption that lead may exist in the isolated state (PbCd+) and as a component of (PbCd+VCd2−)− associates.

CdTe native point defects under Cd saturation

The high temperature point defect (PD) structure of undoped CdTe under well-defined Cd vapour pressure was studied. The single crystals were grown by different techniques (vapor phase, THM, classical and high pressure Bridgman methods). By using in situ Hall effect measurements it was established that the native double charged PDs Cd 2+ i and V 2+ Te are the dominant donors at t > 500 °C. The formation enthalpies of these PDs were determined from [e∼] temperature dependencies (ΔHv 2+ Te =1.3 eV; ΔH Cd 2+ i =2.5 eV). The PD structure modelling demonstrated satisfactory agreement with experimental results both for temperature and Cd vapor pressure dependencies. It showed that V 2+ Te dominates at low temperatures and Cd 2 i begins to prevail at t >930 K.

Synthesis and Ring Size Effect of Macrocyclic Ethynylhelicene Oligomers

[reaction: see text] The cyclization of acyclic ethynylhelicene oligomers with decyl 3,5-diiodobenzoate under optimized conditions gave the corresponding optically active [n+n]cycloalkynes (n = 4-8) in high yields. Their structures were compared in terms of ring size by using (1)H NMR, UV-vis, and CD spectroscopies and vapor pressure osmometry (VPO). The UV-vis spectra exhibited an increase in absorbance in proportion to n. In contrast, the CD spectra of the macrocycles exhibited a large ring size effect, comparable Deltaepsilon values despite the increase in n and temperature-dependent properties of the [8+8]cycloalkyne. It was concluded that [4+4]cycloalkyne, [5+5]cycloalkyne, [6+6]cycloalkyne, and [7+7]cycloalkyne have rigid structures, while [8+8]cycloalkyne has a flexible structure.

The Role of Intrinsic Defects for the Diffusion of Ag and Cu in CdTe

Ag and Cu diffused into CdTe exhibit unusual shapes of their concentration profiles, especially if the diffusion was performed under Cd vapor pressure. The shapes of the concentration profiles at low concentrations of Ag or Cu are well described by a model based on the interaction with intrinsic defects. In that model the characteristic features of the profiles can be independently reproduced by appropriate parameters. It turns out that the resulting profiles are determined by the diffusion of interstitial Cd atoms and reflect the actual distribution of the intrinsic defects in the crystal, i.e. the deviation from stoichiometry.

Point defect structure of CdTe〈Cl〉 crystals at high temperatures

High temperature Hall effect measurements in CdTe〈Cl〉 single crystals grown by THM and Bridgman techniques at 200–900 °C under well defined Cd and Te vapor pressure were made. Associates decay at heating from 500 to 700 °C in THM grown samples was observed. Cd vapour pressure influence on electron concentration [e–] in all samples was established. The main point defects present in crystals were: at T 700 °C - chlorine donors, Cd vacancies and Cd interstitials. Bridgman grown CdTe〈Cl〉 crystals had [e–] similar to undoped CdTe and differed in Cd vapor pressure dependencies by line slopes. Under Te vapor pressure the samples demonstrated p-type conductivity up to 550 °C. In this case the hole density changed from [h+] = 1016 cm–3 to 3 × 1017 cm–3 and did not depend on Te vapor pressure. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exact Self-Compensation of Conduction in Cd[sub 0.95]Zn[sub 0.05]Te:Cl Crystals in a Wide Range of Cd Vapor Pressures

The process of self-compensation in Cd0.95Zn0.05Te:Cl solid-solution crystals has been studied by annealing single crystals under a controlled Cd vapor pressure, with subsequent measurements of the Hall effect, photoluminescence, carrier lifetime and mobility, and photocurrent memory in the annealed crystals. By means of this annealing, conditions of thermal treatment that make it possible to fabricate low-conductivity samples with a low carrier density, 107–1011 cm−3, are defined. In these samples, a p → n conduction inversion is observed at a higher free-carrier density (n, p ≈ 109 cm−3) and the dependence of the electron density on the Cd vapor pressure exhibits a more gentle slope than in the case of CdTe:Cl crystals. The obtained data are discussed in terms of a self-compensation model in which intrinsic point defects act as acceptors with deep levels. This level is attributed to a Zn vacancy, which remains active at high Cd pressure.