Heribert Wiedemeier

Vapor transport and crystal growth in the mixed system MnS-MnSe

Abstract Mass transport rate studies in the multi-component multi-reaction system MnS-MnSe-iodine were performed on atomic mixtures of high purity elements in the temperature gradient 945 → 850 ° C and with iodine pressures ranging from 0.7 to 7.8 atm (based on diatomic iodine). Experimental data evaluated in terms of material flux in moles/cm 2 · sec as a function of iodine pressure show that the mass transfer process is predominantly by convection within the applied pressure range. Growth studies of single crystals of MnS, MnSe, and solid solution by vapor transport of the elements with iodine yielded crystals of 2–5 mm edge length (octahedra and platelets) using iodine concentrations of 1–3 mg/cm 3 tube volume and small temperature gradients ( ΔT C ). The as-grown surfaces of the platelets are (111) planes. Thermoelectric measurements indicate p-type conductivity of the single crystals. The effects of different iodine concentrations, temperature gradients, and etching of the transport ampules on crystal growth are discussed.

The high temperature structure of ß-SnS and ß-SnSe and the B16-to-B33 type λ-transition path

The high temperature modifications ß-SnS and ß-SnSe crystallize in the orthorhombic Til-type structure [Cmcm, No. 63; SnS at 905 Κ : a — 4.148(2), b = 11.480(5), c = 4.177(2)Â; SnSe at 825 K: a = 4.410(6), b = 11.705(7), c = 4.318(4) A]. For the A-type phase transition α-> β from the GeS-type (B16) to the Til-type (B33) the atomic shifts were determined from about 200° below the critical temperature to Tc (878 Κ and 807 K). They show a continuous transition from threefold to fivefold bonded atoms. This change in topology and bond character corresponds to the model of a SN2 chemical reaction. There is an indication of a possible first-order transition near Tc.

Diffusive and convective vapor transport in the GeSe—GeI4 system☆

Abstract Mass transport rate studies in the GeSe—Ge 4 chemical vapor transport system in the absence and presence of inert atmospheres under predominantly diffusive and diffusive-connective conditions provided experimental evidence for the existence of a diffusion barrier in closed tube systems. For low additions of inert gas to given pressures of initial transport agent, the mass flux can be described as being limited by diffusion through a “boundary” layer of constant thickness. For larger additions of inert gas the thickness of the diffusion barrier decreases. These observations are generally consistent with estimated relevant fluid dynamic parameters (Rayleigh numbers) of the respective gas mixtures. The obsrvation of a diffusion barrier is in agreement with a fluid dynamic model by Klosse and Ullersma and with general descriptions convective flow in closed, horizontal containers. However, different from Klosse anf Ullersmas model, which predicts a minimum duffusion barrier thickness, no such minima were observed in the present study. The calculated barrier thicknesses werer considerably smaller than the predicted minima for the highest transport agent pressures employed. A simple, conceptual explanation is presented for the growth of dendritic type crystals in the convection dominated mass transport regime.

The thermal expansion and high temperature transformation of SnS and SnSe

The thermal expansion of SnS and SnSe has been studied above room temperature up to the melting point of 1163 ± 5 Κ and 1135 ± 5 K, respectively, by X-ray diffraction techniques using a 190 mm Unicam high temperature camera. The changes of the lattice parameters indicate that the atomic positions in the (010) plane approach a square planar arrangement with increasing temperature. The transformation of SnS and SnSe from orthorhombic to a pseudotetragonal orthorhombic modification with a & c < b is observed at 878 ± 5 Κ and 807 ± 5 K, respectively. The lattice parameters of the high temperature modifications are for SnS a^c = 4.162 ± 0.006 Â, 6 = 11.517 ± 0.015Â at 932K and for SnSe Ö7c = 4.313 ± 0.006 A, b = 11.703 ± 0.015 Â at 820 K. Both compounds maintain this structure type up to the melting points.

Crystal growth and transport rates of GeSe and GeTe in micro-gravity environment☆

Abstract Six vapor transport experiments on the systems GeSe-GeI 4 and GeTe-GeI 4 were performed on Skylab to determine the effects of micro-gravity on crystal growth and transport rates. Based on a direct comparison of crystals and transport data obtained on earth and in space, employing X-ray diffraction, microscopic and etching techniques, the results demonstrate a considerable improvement of the space grown crystals in terms of growth morphology and bulk perfection. The observation of greater mass transport rates than expected in micro-gravity for diffusion-controlled transport could indicate the existence of other transport modes in a reactive solid-gas phase system. The combined results show that the interference of gravity-driven convection with the transport process causes negative effects on crystal growth as observed on earth for otherwise identical conditions. This points to the unique environment of weightlessness for the observation of basic transport phenomena.

Single-crystal studies of β-Ag2HgI4

The structure and transformation behavior of β-Ag2HgI4 have been investigated by X-ray diffraction methods using single crystals. It has been verified that a single crystal of the β-form, stable at room temperature, transforms to a single crystal of the α-phase, stable above 50°C. The reverse transformation, α → β, on the other hand, is found to result in a multidomain arrangement of the β-phase and not to yield a new cubic modification. The β-structure is tetragonal: a = 6.322 (2) A, c = 12.605 (15) A, with space group I4 and Z = 2. The chalcopyrite type of cation ordering, proposed by Hahn et al. (7), is confirmed. The iodine arrangement is considerably distorted from ideal cubic close packing with preservation of normal cation-iodine distances. No significant amount of disorder or occupancy of vacant sites has been found.

Physical vapor transport of cadmium telluride in closed ampoules

Abstract The dependence of the mass flux on the source material pretreatment, on the source temperature, and on the temperature difference between the source and transport product have been systematically investigated. Procedures have been developed yielding source materials of composition near that of the “congruently vaporizing” phase of CdTe. With this material, mass fluxes at moderate temperatures are observed which are up to about an order of magnitude larger than the highest reported in the literature for comparable conditions. Minute deviations from the “stoichiometric” composition reduce the mass flux by orders of magnitude. The experimental results are compared with theoretically predicted mass fluxes based on a diffusion limited transport process, and with other possible limitations reported in the literature. The high mass fluxes observed in this work at relatively low temperatures enable the vapor growth of large CdTe single crystals under conditions of practical importance.

Chemical vapor transport and thermal behavior of the GeSe-GeI4 system for different inclinations with respect to the gravity vector; Comparison with theoretical and microgravity data

The diffusive and convective mass flux in closed tube chemical vapor transport of the GeSe-GeI4 system has been investigated as a function of inclination of the temperature gradient with respect to the gravity vector. A strong effect of inclination on the degree of convective contribution to mass transport was observed. Computed streaming-diffusive transport rates of this system are compared with experimental data observed for the vertical, stabilizing configuration and for microgravity conditions. At low to medium pressures, good agreement between experimental and predicted results is obtained, at higher pressures experimental transport rates are about 40% greater than predicted. Microgravity flux data of this system are about 300% greater at the highest pressure compared to experimental (vertical, stabilizing) rates on earth and about 400% greater relative to those predicted by diffusion. Transport rates for other ampoule orientations reveal progressively increasing convective contributions with increasing inclination relative to the vertical, stabilizing orientation, reaching in general the highest rates for the vertical, destabilizing orientation. Studies of the thermal behavior inside closed transport ampoules indicated changes of the temperature profile owing to natural convection. In addition, small amplitude (~ ±1°C) temperature oscillations were observed inside transport ampoules under both horizontal and vertical, stabilizing conditions. These could possibly be related to gas phase reactions. Very large oscillations (±25°C) observed under vertical, destabilizing conditions and at 1.5 atm pressure are caused by natural convection.

Growth by CVT and characterization of Hg 1- x Cd x Te epitaxial layers

Single crystal epitaxial layers of Hg1-xCdxTe were grown on CdTe substrates employing the chemical vapor transport technique. Different growth temperatures, substrate orientations, and various pressures of Hgl2 as a transport agent were used while the source materials had a fixed composition ofx = 0.2. The epilayers are of nearly uniform composition to a depth of about one-half of the layer thickness. Chemical etching of the as-grown epilayers revealed low etch pit densities in the range of 103–104 cm−2. Rectangle-shaped etch pits are observed for the first time on the (100) oriented epilayers of this material. The growth temperature and Hgl2 pressure used for the growth experiments have significant effects on the layer morphology and composition.

Effects of growth conditions on the composition of CVT and PVT grown hg 1- x Cd x Te epitaxial layers

Single crystal epitaxial layers of Hg1-xCdxTe were grown on CdTe substrates employing the chemical and physical vapor transport techniques. Different growth temperatures and various pressures of HgI2 as a transport agent were used while the source materials had compositions of eitherx = 0.4 orx = 0.6. The epilayers are of nearly uniform composition to a depth of about one-half of the layer thickness. The Hgl2 pressure and the growth temperature used for the growth experiments have significant effects on the layer composition. The desired epilayer composition ofx = 0.2 can be achieved with either source compositions by properly adjusting the HgI2 pressure and the growth temperature.