Investigation of radiation defects in electron irradiated Hg1−xCdxTe crystals using positron annihilation

The electron characteristics of defects in the initial and electron irradiated Hg_1−xCd_xTe (2–3 MeV, 10¹⁸ cm⁻², 300 K) crystals using the positron annihilation method have been investigated. The data of electric measurements are confirmed on connection of p-type conductivity with vacancy defects of metal sublattice initial crystals Hg_1−xCd_xTe. An analysis of correlation curves of irradiated crystals has shown a possibility of formation of associations of initial defects and radiation damages of vacancy type during radiation process. The presence of narrow component on correlation curves in the region of small angles is associated with formation of positronium states localized in the region of radiation defect complex of vacancy type. Identification of positron-sensitive defects with electrically active radiation induced ones has been carried out according to the results of isochronal annealing of irradiated crystals.     

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.     

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.     

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.     

Zn1−xCdx polytype single crystals with various degree of order

The results of X-ray investigations of the polytype structures formed in solid solutions of Zn_1–xCd_xSe are presented. For compositions of 0.22 < x < 0.35 polytypes 8H, 6H, 4H, and DS (disordered structure) are present. The Zn_1–xCd_xS single crystals of 0 ≦ x ≦ 0 were obtained by Bridgman's method under high protective argon pressure from the melt. The structures are characterised by FARKASZ -JAHNKE statistical parameters π(m,p), and by parameters P_LH of the polytype unit cell formation probability.     

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)     

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)     

Structural and electrical properties of vapour phase grown Cd1 − xFexTe single crystals

Cd_1 − xFe_xTe single crystals were prepared by vapour phase growth method in the composition range of 0 ≤ x ≤ 0.03. Chemical analysis, surface morphology, structural investigations and electrical properties were carried out by EDAX, SEM, XRD, TEM and transport technique, respectively. Microscopic variations between the target and actual compositions were noticed. Morphology studies revealed that dislocation aided growth is active in the present crystals. TEM and XRD studies confirmed that the samples of all compositions crystallized in zinc blende structure, and the lattice parameters varied almost linearly decreases with Fe content. At room temperature, the resistivity of the Cd_1 − xFe_xTe crystals of all compositions (x = 0.01, 0.015, 0.02, 0.025 and 0.03) lies in the range of 3.5-6.5 M Ω, the activation energies lie in the range of 63-133 meV, and the samples were show the ‘p’ type conductivity.     

Electron-beam microprobe analysis of epitaxial GaxIn1−xP solid solutions

Ga_xIn_1–xP epitaxial layers were investigated by means of a scanning electron microscope X-ray microanalyser. The conditions for quantitative X-ray microprobe analysis are discussed. The growth of Ga_xIn_1–xP layers is possible on GaP substrates with x > 0,8 and on GaAs with 0,3 < x < 0,6. For the deposition of layers with 0,5 < x < 0,8 two substrate materials are possible: GaAs_1–xP_x or Ga_xIn_1–xP with suitable compositions. These materials must be epitaxially deposited by step by step layer growth or by vapour phase epitaxy, respectively.     

Lattice Parameter Study in the Bi1–xSbx Solid-solution System

The lattice parameters of Bi_1–xSb_x single crystals were measured for 0 ≦ x ≦ 1 by a special X-ray diffractometer technique using reflections chosen so that minimum errors were achieved. Corrections for thermal expansion and refraction were applied. The antimony concentration was determined by means of electron microprobe analysis. The dependences of the lattice parameters on x (in nm) can be described with good approximation by a = 0.45469 – 0.02398x and c = 1.186294 – 0.058632 [1 + 1.26 (x⁻¹ – 1)]⁻¹, resp. The behaviour of the parameter a is in agreement with the earlier study by CUCKA and BARRETT , whereas their linear expression for c(x) (0 ≦ x ≦ 0.3) was not confirmed.     

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.     

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.     

Atomic structure of Cd1 − x Zn x Te solid solution single crystals and structural prerequisites of their ferroelectricity

Precision X-ray structural investigation of Cd_{1 ? x} Zn _x Te single crystals (x = 0.04, 0.10, 0.21, 0.30, 0.40) is performed at temperatures below and above the ferroelectric phase transition. The regularities associated with the increase in the concentration of Zn atoms in solid solutions are revealed. It is established that the unit cell parameter of the crystals varies according to the Vegard law. The thermal parameters of Te and Zn atoms are strongly overestimated within the model of a sphalerite-type average structure. It is found that the positions of the Zn cations are split and the anion sublattice is disordered in such a way that the Te anions are predominantly displaced along the threefold axes. The Zn atoms can serve as off-center impurities. This phenomenon offers strong possibilities for manifesting the ferroelectric properties in the solid solutions under investigation.     

Electrical and optical properties of Cd1−xZnxS (0x0.18) grown by chemical bath deposition

Cd_1−xZn_xS films with 0x0.18 were grown by chemical bath deposition technique on glass substrates from an aqueous solution containing cadmium and zinc sulfate, ammonia and thiourea. Microstructural features, obtained from X-ray diffraction and scanning electron microscopy (SEM) measurements, reveal a predominance of Wurtzite structure and an homogenous microstructure formed by densely microcrystallines for all the samples studied. Cd_1−xZn_xS semiconductor was found to be resistive and of n-type. Also, the electron density decreases with increased x and the mobility reaches a maximum around x=0.12. Which means that the Cd_1−xZn_xS films at this composition are of high crystalline quality, i.e. having reduced intrinsic defect concentrations.     

Spektrochemische Bestimmung von Silizium in AlxGa1–xAs und physikalische Eigenschaften der Kristalle

A direct spectrochemical method for the quantitative determination of silicon in Al_xGa_1–xAs (x ≈ 0,8) crystals was elaborated. The results of the analysis of highly doped samples (10¹⁹ … 10²⁰ cm⁻³) were discussed in connection with photoluminescence spectra and electrical measurements.     

Bestimmung der Energielücke von (Pb1–x*Snx)1–yTe y-Einkristallen. Vergleich indirekter mit direkten Methoden

For applications of Pb_1–xSn_xTe in infrared light emitting and light detecting devices it is necessary to control the energy gap. For determining the gap in as-grown Bridgman crystals indirect methods have to be used (measurement of lattice constant and density). The results of these indirect methods are compared with results obtained by more direct measurements (spectral photoresponse, intrinsic absorption edge).     

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.     

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.

     

Influence of silver concentration in Agx(Sb0.33S0.67)100−x thin amorphous films on photoinduced crystallization

One of the recent applications of thin chalcogenide films is in rewritable optical data recording. This technology is based on reversible phase transition between crystalline and amorphous state. Currently, the primary materials for rewritable optical are Ge–Sb–Te and Ag–In–Sb–Te alloys, but materials research still continues due to the need for increased storage capacity and data recording rates. (Ag)–Sb–S thin films were prepared by thermal evaporation of Sb₃₃S₆₇ bulk and optically induced diffusion and dissolution of thermally evaporated Ag films. Prepared samples were characterized by electron microprobe (SEM-EDX), differential scanning calorimetry (DSC) and by UV–Vis–NIR and Raman spectroscopy. The phase-change recording processes in (Ag)–Sb–S films were carried out by photocrystallization experiments done by Ar⁺ ion laser. The laser exposed dots were studied by scanning electron microscopy (SEM) and transmission optical microscopy. Micro X-ray diffraction (μ-XRD) was used for the exposed dots crystallinity study. Photocrystallization kinetic curves (showing the dependence of optical transmission on laser exposure time) were also established. Crystallization mechanism of Ag_x(Sb_0.33S_0.67)_100−x samples was discussed.