Homogenitätsuntersuchungen und Bestimmung der molaren Konzentration x an CdxHg1–xTe mit Hilfe der Elektronenstrahlmikrosonde

The semiconductor Cd_xHg_1–xTe serves as a starting material for the production of infrared detectors with a high detectivity and a small time constant. Cd_xHg_1–xTe single crystals are necessary to manufacture detectors efficiently. The single crystals should not contain segregations and have to include a homogeneous molar concentration x. An electron microprobe X-ray analyzer meets all requirements expected from the measuring method for checking the homogeneity of Cd_xHg_1–xTe crystals. — Results of qualitative and quantitative studies in the microscopical and macroscopical range are presented. Typical defects of the crystals were detected by using the qualitative electron microprobe X-ray analysis. The quantitative analysis was carried out with standard samples. The method for determination of the molar concentration x of the standard samples is described.     

Herstellung homogener CdxHg1−xTe-Barren als Ausgangsmaterial für infrarotempfindliche Fotowiderstände

The basis material for high sensitive infrared-photo-conductive cells today is often cadmium-mercury telluride (Cd_xHg_1−xTe). In order to get Cd_xHg_1−xTe-infrared detectors with nearly constant elektrical properties a good homogeneity of the material has to be demanded. This can't be achieved by conventional methods of crystal growing (f. i. Bridgman technique). But the technique of high-speed quenching of the melt offers good results.     

Vickers indentation hardness of Hg1−xCdxTe

The Vickers indentation hardness of Hg_1−xCd_xTe has been measured as a function of composition x using monocrystalline samples of Bridgman and THM crystals and polycrystalline starting material at room temperature. The microhardness varied between 220 MPa (x=0) and 440 MPa (x=1), showing a maximum of 850 MPa at x ∼ 0.75, and was different between monocrystalline and polycrystalline samples. The “hardening rate” dH/dx is strongly dependent on the composition range and is discussed in context with solid solution hardening due to elastic interactions of solute atoms with gliding dislocations and ordering effects.     

Bridgman growth of CdxHg1−xTe— A review

Recent developments in the bulk Bridgman growth method for Cd_xHg_1−xTe are reviewed. Both melt mixing and heat flow control techniques have been applied in attempts to produce more uniform material in terms of composition. In the U.K. work has concentrated on application of the Accelerated Crucible Rotation Technique (ACRT) to achieve the required uniformity improvements. Elsewhere, various means to control isotherm shape have been used with the same aim. The ultimate use of the material is in infra-red detectors and Bridgman grown Cd_xHg_1−xTe has produced these successfully for both photoconductive and photovoltaic applications.     

Liquid-Phase Epitaxy of (Hg,Cd)Te on CdTe Substrates by a Vertical Dipping Technique

Hg_1−xCd_xTe layers on CdTe substrates were grown from Te-rich melt solutions by a vertical dipping technique using a special quasi-closed system with ground-glass sealing. Results are good reproducibilities of the electrical properties after annealing in Hg-rich atmosphere (p77 ≈ 2 · 10¹⁶ cm⁻³ μ77 ≈ 500 cm² V⁻¹ s⁻¹) and of the x-value, respectively. A horizontal position of the substrate downwards to the melt solution yields, in difference to a vertical one, to homogeneous layer thicknesses. Short meltback steps before growth leads to sharper profiles of composition.     

High temperature arsenic doping of CdHgTe epitaxial layers

Experimental results on solid‐state arsenic doping of the n‐type bulk and ISOVPE epitaxial Cd_XHg_1‐XTe (X = 0.19 ÷ 0.3) alloys are presented. The arsenic doped thin epitaxial Cd_xHg_1‐xTe films (n_As ≈ 5 · 10¹⁶ ÷ 1 · 10²⁰ cm^‐3; d = 2 ÷ 5 μm) obtained by RF sputtering in a mercury glow discharge were used as As diffusion sources. The arsenic diffusion and activation were carried out at temperatures T = 500 ÷ 600 °C under Hg vapour pressure. Immediately after the high temperature treatment all samples were annealed to annihilate point defects. The SIMS analysis was used for determination of the quantitative admixture distribution of As in the diffusion area. The arsenic electrical activity has been evaluated by means of differential Hall, resistivity and thermoemf measurements. The analysis of experimental data obtained as well as their comparison with previously obtained results has been performed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X-Ray Diffraction Studies on Point Defects in II–VI Compounds

The point defect concentration in Te-rich CdTe and Hg_1-xCd_xTe annealed at various temperatures has been estimated from precision lattice parameters using a simple continuum inclusion model and compared with densities of electrically charged defects determined by high-temperature Hall and conductivity measurements. The nonstoichiometry is realized by cation vacancies. Dependent on the CdTe content, the ratio of total to charged defect concentrations varies between about unity for HgTe-rich composition and 75 for CdTe. Therefore, it is necessary to distinct between “electrical” and “chemical” stability regions.     

Calculation of Diffusion-limited Growth of Hg1−x Cdx Te Epilayers on CdTe from Te-rich Melts by Step-Cooling LPE

Calculations of layer thicknesses and composition profiles in Hg_1−xCd_xTe layers on CdTe substrates for the growth from Te-rich melts have been carried out for liquidus temperatures of 460 °C, 480 °C, and 500 °C. This has been made on the basis of the multicomponent diffusion model of SMALL and GHEZ and the solid-liquid phase relation of BRICE . It could be shown that growth velocity increases only slightly with rising liquidus temperature. On the other hand, the interdiffusion velocity of Hg and Cd in the solid increases remarkably at a higher temperature. Therefore, to get layers with a constant x-value a higher supersaturation of the melt is necessary. The x-value decreases with rising supercooling by about 0.003 K⁻¹. To demonstrate the thermodynamically and kinetically advantageous properties of CdTe as substrate material, comparative calculations for a “hypothetical” HgTe substrate have been involved.     

Investigations for Sn diffusion in Pb1−xSnxTe and PbTe

By annealing Pb_1−xSn_xTe and PbTe isothermally in a quartz ampoule Sn diffused from Pb_1−xSn_xTe into PbTe. The profiles obtained have been investigated by means of an electron beam microanalyser, and the coefficients of diffusion have been determined at various temperatures. The diffusion of Sn can be explained by the expressions: D_PbSnTe = 1.5 · 10⁻¹ exp (−1.8 eV/kT) cm² s⁻¹ (0,14 < x < 0,18) D_PbTe = 5,5 · 10⁻⁴ exp (−1.5 eV/kT) cm² s⁻¹. N-type layers are observed at the surface of Pb_1−xSn_xTe specimens.     

X-ray investigation of the mechanisms of phase transformations in single crystals of ZnS,ZnxCd1−xS,and ZnxMn1−xS (II). Comparison of observed and calculated diffraction effects

The experimentally observed intensity profiles recorded from ZnS, Zn_xCd_1−xS and Zn_xMn_1−xS crystals at different stages of the 2H-3C and 2H-6H transformations are compared with those computed theoretically from the three parameter model in the previous paper. It is found that the 2H-3C and 2H-6H transformations occur by the nonrandom nucleation of deformation faults. The probability of the growth of a nucleus is much greater than that of the creation of a fresh nucleus. At least two different fault probabilities have therefore to be employed in computing the diffraction effects in each case. The transformation behaviour of Zn_xCd_1−xS and Zn_xMn_1−xS crystals is strongly influenced by variations in stoichiometry (x). Large values of x (x > 0.95) favour the 2H-3C transformation, whereas smaller values (x ≦ 0.94) favour the formation of the 6H phase. A comparison of the calculated and observed intensity distributions indicates that in some crystals the 2H-3C and 2H-6H transformations occur simultaneously in different regions of the same single crystal.     

Thermoelectrical properties of In1‐xGaxAs and InAs crystals irradiated with fast electrons

The thermoelectric power in In_1‐xGa_xAs (x = 0,01;0,04) solid solutions and InAs crystals irradiated with fast electrons by the energy of 6 MeV and dose of 10¹⁶‐ 2 x 10¹⁷ el/cm^‐2 on the interval 80‐400 K have been investigated. It is revealed that in the all crystals the value of the thermoelectric power is decreased under irradiation that resulted from the growth of the free electron concentration to form radiation induced defects of the donor type. It has been determined that in the initial InAs after irradiation, the charge carriers scatter on optical phonons and in In_1‐xGa_xAs solid solutions they do on optical phonons and ionized impurities. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tempern von Cd0,2Hg0,8Te im Quecksilberdampf

N-type Cd_0,2Hg_0,8Te with a minimum concentration and a maximum mobility of charge carriers serves as starting material for the production of photoconductive infrared detectors with high sensitivity. The number of the defects, originating in Cd_0,2Hg_0,8Te during crystal growth, can be considerably diminished by a heat treatment in saturated mercury vapour. Details of the preparation and the accomplishment of the annealing essays as well as the results of the experiments are described.     

Composition,morphology, and properties of sodium-bismuth tungstate crystals

A correlation has been revealed between the formation conditions, composition, morphology, and physicochemical properties of NaBi_{1 − x} (W_{1 + y} O₄)₂ crystals (0 ≤ x ≤ 0.16, 0 ≤ y ≤ 0.11). The effect of melt deviation from stoichiometry on the decomposition rate of grown crystals under electron beam irradiation is shown by transmission electron microscopy. A nonuniform dopant distribution over the crystal boule cross-section is found by microprobe analysis. The Raman spectra of the samples depend on their crystallographic orientation.

     

Growth of bulk zinc chalcogenide single crystals from the vapour phase and their use for laser screens of projection colour television

Results on ZnSe, ZnSe_xS_1−x and ZnS crystal growing from the vapour phase up to 7.5 cm³ in volume are described. Crystals were grown on sapphire, ZnS, ZnSe_xS_1−x and quartz glass substrates without a contact of the growing crystal with a growth ampoule and using the molten tin as a heating medium. Large high-purity crystals with a density of etch pits of 10³ cm⁻² were obtained They exhibited an effective exciton luminescence and rather high radiation efficiency (30 ± 10% for ZnSe at T = 77 K). This made it possible to use these crystals for fabricating laser screens for a cathode ray tube. The main laser parameters obtained on a ZnSe screen at T = 80 and 300 K using a 75 keV electron beam excitation are presented. The light power output reached 0.8 W at T = 80 K; this allowed to obtain a 10 cd · m⁻² TV image of 1.5 × 2 m² in area.     

Properties of Hg1‐xCdxTe epitaxial films grown on (211)CdTe and (211)CdZnTe

Hg_1‐xCd_xTe (MCT) epitaxial films have been grown employing single crystalline substrates of CdTe and Cd_0.96Zn_0.04Te with (211)Cd and (211)Te crystalline orientations. The Isothermal Vapor Phase Epitaxy (ISOVPE) technique without Hg overpressure has been used for the epitaxial growth. Substrates and films were characterized by optical microscopy, chemical etching and x ray diffraction (Laue technique). The electrical properties were determined by Hall effect measurements. The characterization results allowed to evaluate the crystalline quality of MCT films. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature-Induced structural phase transformations in Cu<Subscript>1.50</Subscript>Zn<Subscript>0.30</Subscript>Te and Cu<Subscript>1.75</Subscript>Cd<Subscript>0.05</Subscript>Te single crystals

The results of studying the temperature-induced polymorphic phase transformations in Cu_1.50Zn_0.30Te and Cu_1.75Cd_0.05Te single crystals are presented. The single crystals have been investigated in the range of 290–1100 K using X-ray diffraction analysis. The temperature dependences of the unit-cell parameters, X-ray density, and coefficients of thermal expansion for each polymorphic modification of Cu_1.50Zn_0.30Te and Cu_1.75Cd_0.05Te single crystals are presented. The influence of cationic substitution (replacement of Cu²⁺ with Zn²⁺ and Cd²⁺ cations) on the phase-formation processes, number of polymorphic modifications, and temperature of structural phase transition in single crystals of these solid solutions is demonstrated.     

Growth and defect structure of CdF2 and nonstoichiometric Cd1?xRxF2 + x phases (R = rare earth elements and In). I. Growth of Cd1?xRxF2 + x single crystals (R = La-Lu, Y)

This article begins a series of publications on growth of single crystals of nonstoichiometric Cd_{1− x} R _x F_{2 + x} phases (R = La-Lu, Y, In) with the defect CaF₂-type structure, their crystal structures, and some properties. The present article is dedicated to the phase diagrams of the CdF₂-RF₃ systems in the region of Cd_1−x R _x F_{2 + x} formation. Their analysis shows that it is possible to synthesize homogeneous Cd_0.9 R _0.1F_2.1 crystals. The dependence of the defect structure of the crystals on the type and concentration of rare earth elements is studied on specially grown Cd_0.9 R _0.1F_2.1 (R = La-Lu) and Cd_1−x Y_xF_{2 + x} (x = 0.1, 0.15, 0.20) crystals. It is shown that, despite the fact that all the Cd_0.9 R _0.1F_2.1 crystals melted congruently, irrespectively of the rare earth elements used, they were rather homogeneous. The chemical compositions of the Cd_1−x R _x F_{2 + x} phases (R = Sm, Gd, Tb, Ho, and Lu) determined by the method of inductively coupled plasma atomic emission spectroscopy (ICP-AES) turned out to be close to Cd_0.9 R _0.1F_2.1. __________ Translated from Kristallografiya, Vol. 49, No. 3, 2004, pp. 566–574. Original Russian Text Copyright ? 2004 by Buchinskaya, Ryzhova, Marychev, Sobolev.     

Ultrahigh Vacuum Atomic Layer Epitaxy of Ternary II-VI Semiconductor Compounds

A concise discussion concerning the UHV ALE growth of ternary II-VI compounds is presented in this paper. Simultaneous reflection mass spectrometry (REMS) and reflection high energy electron diffraction (RHEED) measurements of the surface kinetic and structural parameters, respectively, governing the UHV ALE growth of Cd_1-xZn_xTe and Cd_1-xMn_xTe heteroepitaxial films are reported. In addition, a Monte-Carlo-based method for simulation of the UHV ALE process of CdTe (the model-compound for this growth technique) has been used for investigation of the Cd cation's fluxes reflected from the growing epilayer surface in different phases of the ALE process. The Cd⁺ ion-related REMS signals measured during CdTe growth have been compared with the simulation results.     

Extrinsic doping of CdxHg1−xTe— A review

Extrinsic doping by elements which are stable to subsequent processing will become increasingly important in future infra-red device structures based on Cd_xHg_1−xTe. This paper reviews the incorporation and activation of dopants in the most widely used bulk and epitaxial growth methods. Stoichiometry at the growth temperature is shown to be the critical factor affecting dopant activation. Various factors, including stoichiometry, can affect the as-grown electrical properties and the importance of determining the type of conduction in the as-grown state, if successful extrinsic doping is to be accomplished, is stressed. Data on dopant segregation behaviour, in growth from liquids, acceptor ionization energies and carrier lifetimes are also presented and their importance is discussed.     

Validity of the Lattice-Parameter Vegard-Rule in Cd1-xZnxTe Solid Solutions

Cd_1-xZn_xTc crystals of different composition (0 ⩽ x ⩽ 1) were grown by the vertical Bridgman method and by synthesis in Te excess. After careful surface preparation of crystal slices, their Zn:Cd ratios were ascertained by wavelength-dispersive X-ray spectroscopy (WDXS), and the lattice parameters of the same slice regions were measured by X-ray diffractometry. The Vegard rule concerning the linear lattice-parameter dependence on composition is – in contrast with earlier literature data – exactly valid within the limits of error (Δx ⩽ ±0.01 and Δa/a ⩽ 3.7. 10⁻⁴) and follows the equation a(x) = (0.64822 - 0.03792x) nm. After annealing some slices at different temperatures and controlled partial pressure conditions in ordcr to find possible phase separations or cation ordering effects, neither the lattice parameters were changed nor additonal interferences were found. A published occurrence of a rhomhohedral phase could not be confirmed by means of powder diffraction analysis. The results are discussed in relation to own electron diffraction investigations and to EXAFS literature data.