Yi-Gao Sha

Mass flux of ZnSe by physical vapor transport

Abstract Mass fluxes of ZnSe by physical vapor transport (PVT) were measured in the temperature range of 1050 to 1160°C using an in-situ dynamic technique. The starting materials were either baked out or distilled under vacuum to obtain near-congruently subliming compositions. Using an optical absorption technique Zn and Se 2 were found to be the dominant vapor species. Partial pressures of Zn and Se 2 over the starting materials at temperatures between 960 and 1140°C were obtained by measuring the optical densities of the vapor phase at the wavelengths of 2138, 3405, 3508, 3613, and 3792 A. The amount and composition of the residual gas inside the experimental ampoules were measured after the run using a total pressure gauge. For the first time, the experimentally determined partial pressures of Zn and Se 2 and the amount and composition of the residual gas were used in a one-dimensional diffusion limited analysis of the mass transport rates for a PVT system. Reasonable agreement between the experimental and theoretical results was observed.

Growth of HgZnTe by directional solidification in a magnetic field

Abstract Hg0.84Zn0.16Te crystals were grown vertically by directional solidification in a 5 T axial magnetic field. Growth in an identical thermal environment but without the presence of magnetic field was also performed. The ingots were partially grown and quenched to preserve the growth interface shapes. The effects of magnetic damping on the buoyancy-driven convective flows in the melt were investigated by examining the quenched-in interface shapes and by measuring the composition variations along the axial and radial directions of the grown ingots and comparing them with those of ingots grown without a field.

Selenium precipitation in ZnSe crystals grown by physical vapor transport

Abstract The morphology of freshly cleaved ZnSe surfaces was investigated by atomic force microscopy and the results were correlated with differential scanning calorimetry (DSC) data. Selenium precipitates in undoped ZnSe crystals grown by the physical vapor transport method were determined. The Se inclusions have a size of about 20 nm. A transition temperature at 221°C in the DSC measurement is interpreted as the eutectic temperature of Se-saturated ZnSe. The total amount of the ZnSe Se-rich second phase was 0.8 wt%, and some segregation effect along low angle grain boundaries was evident.

Gibbs Energy of Formation and Partial Pressure of Hg over Te‐Saturated HgTe ( c ) Between 385 and 724 K

The partial pressure of Hg over Te-saturated HgTe(c) has been determined between 385 K, where it is 1.0×10 −8 atm, and 724 K, where it is 3×10 −2 atm, by measuring the optical absorbance at 253.7 nm of the vapor phase. The data below the 686 K eutectic with Te(c) are used to calculate the standard Gibbs energy of formation

Mass flux and partial pressures of ZnSe by physical vapor transport

Abstract Mass fluxes of ZnSe by physical vapor transport (PVT) were measured in the temperature range of 1050–1140°C using an in situ dynamic technique. The starting materials were heat treated by a hydrogen reduction process followed by the dynamic bake-out method. The amount and composition of the residual gas inside the experimental ampoules were measured after the transport experiments using a total-pressure gauge. The total residual gas pressures measured were about one order of magnitude lower than the values reported on similar processed ZnSe ampoules which were not treated with a hydrogen reduction process. As a result of the reduction in the residual gas pressure, the measured fluxes were 3–7 times higher than previously reported and correspond to a growth rate higher than 10 mm/day at 1120°C. The simultaneous measurements of partial pressures of transport species and the mass fluxes were also performed on a ZnSe optical ampoule. The partial pressures of Zn and Se 2 were obtained by measuring the optical densities of the vapor at the wavelengths of 2138, 3405, 3508, 3613, and 3792 A. For the first time, the experimentally obtained mass fluxes were compared with those calculated, without any adjustable parameters, from a one-dimensional diffusion model which uses all the measured parameters (thermal field, partial pressures of transport species, and residual gas) as inputs, and good agreement was observed.

Vapor-phase stoichiometry and heat treatment of CdTe starting material for physical vapor transport

Abstract Six batches of CdTe, having total amounts of material from 99 to 203 g and gross mole fraction of Te, XTe, 0.499954–0.500138, were synthesized from pure Cd and Te elements. The vapor-phase stoichiometry of the assynthesized CdTe batches was determined from the partial pressure of Te2, PTe2, using an optical absorption technique. The measured vapor compositions at 870°C were Te-rich for all of the batches with partial pressure ratios of Cd to Te2, P Cd P Te 2 , ranging from 0.00742 to 1.92. After the heat treatment of baking under dynamic vacuum at 870°C for 8 min, the vapor-phase compositions moved toward that of the congruent sublimation, i.e., P Cd P Te 2 = 2.0 , with the measured P Cd P Te 2 varying from 1.84 to 3.47. The partial pressure measurements on one of the heat-treated samples also showed that the sample remained close to the congruent sublimation condition over the temperature range 800–880°C.

Phase diagram of HgTe–ZnTe pseudobinary and density, heat capacity, and enthalpy of mixing of Hg1−xZnxTe pseudobinary melts

In this article, the solidus temperatures of the Hg1−xZnxTe pseudobinary phase diagram for several compositions in the low x region were measured by differential thermal analysis and the HgTe–ZnTe pseudobinary phase diagram was constructed. The densities of two HgZnTe melts, x=0.10 and 0.16, were determined by an in situ pycnometric technique in a transparent furnace over, respectively, 110 and 50 °C ranges of temperature. The thermodynamic properties of the melts, such as the heat capacity and enthalpy of mixing, were calculated for temperatures between the liquidus and 1500 °C by assuming an associated solution model for the liquid phase.

Viscosity of Hg0.84Zn0.16Te pseudobinary melt

An oscillating‐cup viscometer was developed to measure viscosity of molten HgZnTe ternary semiconductor alloys. Data were collected for the pseudobinary Hg0.84Zn0.16Te melt between 770 and 850 °C. The kinematic viscosity was found to vary from approximately 1.1 to 1.4×10−3 cm2 s−1. A slow relaxation phenomena was also observed for temperatures from the melting point of 770 to ∼800 °C. Possible mechanisms for this effect are discussed.

Segregation Coefficients of Impurities in Selenium by Zone Refining

Abstract The purification of Se by zone refining process was studied. The impurity solute levels along the length of a zone-refined Se sample were measured by spark source mass spectrographic analysis. By comparing the experimental concentration levels with theoretical curves the segregation coefficient, defined as the ratio of equilibrium concentration of a given solute in the solid to that in the liquid, k = x s / x l , for most of the impurities in Se are found to be close to unity, i.e., between 0.85 and 1.15, with the k value for Si, Zn, Fe, Na and Al greater than 1 and that for S, Cl, Ca, P, As, Mn and Cr less than 1. This implies that a large number of passes is needed for the successful implementation of zone refining in the purification of Se.

Vapor Growth and Characterization of ZnSeTe Solid Solutions

Abstract Six ZnSe 1−x Te x crystals were grown by the physical vapor transport technique. For each of the source material compositions, x =0.10, 0.20 and 0.30, two crystals were grown — one under horizontal and the other under vertical stabilized configurations. The axial and radial compositional uniformity were determined by precision density measurements, wavelength dispersive X-ray spectroscopy (WDS) and optical transmission mappings. The measured radial ZnTe content was quite uniform for all grown crystals except the horizontally grown crystal for x =0.30. WDS results on this crystal indicated a core with uniform ZnTe content, about 0.38, surrounded by a thin region of high ZnTe content with x =0.8. This feature was confirmed by the SEM back scattering electron images. For the three source compositions the axial compositional variations for the vertically grown crystals were more uniform than those for the horizontally grown crystals. The measured compositions in the crystals grown from source composition of x =0.10 suggest that the transport mechanism in the system cannot be interpreted by a simple one-dimensional diffusion limited model.