Wanqi Jie

Solidification structure formation in undercooled Fe-Ni alloy

Abstract The Fe alloy melts containing 7.5, 15, 22.5 and 30 at% Ni were bulk undercooled to investigate the structure evolution. When the undercooling of the four melts is lower than the critical value 110, 125, 175 and 325 K, respectively, only the stable face-centered cubic phase crystallizes. In this case a grain refinement caused by solid superheating is observed in all the alloys, but another grain refinement induced by recrystallization can merely occur in the Fe–30 at%Ni alloy undercooled by 190–325 K. Alternate crystallization of the metastable body-centered cubic phase occurs above the critical undercooling. It is indicated that the subsequent heterogeneous nucleation of the stable phase in the metastable solid and remaining liquid coexisting system is influenced not only by the morphology and surface area of the metastable solid, but also by the effective undercooling of the remaining liquid. On the basis of the experimental results and the theoretical analyses, a structure evolution map for bulk Fe–Ni system is constructed.

Chemical mechanical polishing and nanomechanics of semiconductor CdZnTe single crystals

(1 1 1), (1 1 0) Cd0.96Zn0.04Te and (1 1 1) Cd0.9Zn0.1Te semiconductor wafers grown by the modified vertical Bridgman method with dimensions of 10 mm × 10 mm × 2.5 mm were lapped with a 2–5 µm polygonal Al2O3 powder solution, and then chemically mechanically polished by an acid solution having nanoparticles with a diameter of around 5 nm, corresponding to the surface roughnesses Ra of 2.135 nm, 1.968 nm and 1.856 nm. The hardness and elastic modulus of (1 1 1), (1 1 0) Cd0.96Zn0.04Te and (1 1 1) Cd0.9Zn0.1Te single crystals are 1.21 GPa, 42.5 GPa; 1.02 GPa, 44.0 GPa; and 1.19 GPa, 43.4 GPa, respectively. After nanocutting is performed by the Berkovich nanoindenter, the surface roughness Ra of the (1 1 1) Cd0.9Zn0.1Te single crystal attains a 0.215 nm ultra-smooth surface. The hardness and elastic modulus of three kinds of CdZnTe single crystals decrease with the increase of indentation load. When the nanoindenter departs the surface of the crystals, the adherence effects are obvious for the three kinds of single crystals. This is attributed to the plastic sticking behavior of CdZnTe material at a nanoscale level. When the indentation load of the three kinds of CdZnTe single crystals is in the range of 4000–12 000 µN, the adhered CdZnTe material on the nanoindenter falls onto the surface and accumulates around the nanoindentation.

Effect of weak convection on lamellar spacing of eutectics

Abstract An asymptotic expansion method is proposed to solve the convection–diffusion equation which governs the directional solidification of eutectic alloys under the condition of weak convection at low Peclet number. The so-called weak convection is defined here as the condition for which the velocity of convective flow and its gradient ahead of the S/L interface are both relatively small. Our analysis shows that such weak convective flow will give rise to an increase of eutectic lamellar spacing since the convection produces a slight shift of solute concentration profile at the interface and a decrease of eutectic growth undercooling. Our analytical results are compared with other models and have been applied to some practical cases such as weak forced convection induced by a rotating disk and natural convection induced by solutal and thermal differences in solidification processing. Calculations for two typical eutectic alloys, Pb–Sn and Al–Cu, are performed in order to reveal schematically the effect of weak convective flow on eutectic growth.

Cd1−xZnxTe: Growth and characterization of crystals for X-ray and gamma-ray detectors

Abstract The choice a suitable crystal growth method and a reasonable x value is of profound importance in the preparation of high quality Cd1−xZnxTe crystals for x-ray and gamma-ray detectors. The present paper reviews the evolution and development of Cd1−xZnxTe crystal growth for x-ray and gamma-ray detectors. At the same time, emphasis is put upon finding the relationship between the x value and the quality of the Cd1−xZnxTe. Three sets of Cd1−xZnxTe ingots with different x values, specifically 0.10, 0.15, and 0.20 were grown by the vertical Bridgman method (VBM) and characterized. Their x specification was then correlated with their dislocation densities, Te precipitates, inclusions, IR transmission, resistivities, and impurity concentrations, respectively. It was found that VBM Cd0.85Zn0.15Te as grown in this paper possessed the best choice of qualities with respect to defects and impurities.

Enforced c-axis growth of ZnO epitaxial chemical vapor deposition films on a-plane sapphire

To enforce perfect c-axis orientation of ZnO epitaxial films grown on a-plane sapphire, we first grew perfectly perpendicularly aligned (i.e., c axis oriented) ZnO nanopillars on such sapphire substrates, and then over-grew these by a closed epitaxial film using a modified chemical vapor deposition process at atmospheric pressure. X-ray diffraction and low temperature photoluminescence measurements confirm the desired epitaxial relationship and very high crystalline quality. This growth scheme is an efficient method to suppress dislocations and polycrystalline growth of ZnO and could work equally well for other heteroepitaxial epilayer/substrate systems.

Study on temperature dependent resistivity of indium-doped cadmium zinc telluride

Indium-doped CdZnTe crystals, grown by the modified vertical Bridgman method, were characterized by temperature dependent resistivity measurements in the range from 220 to 320 K. The Fermi level, pinned near the midgap, was confirmed by fitting ln(ρ) versus 1/k0T plots, giving energies of 0.63 eV and 0.72 eV above the valence band for the high resistivity samples, with doping levels of 5.0 × 1016 at cm−3 and 4.8 × 1017 at cm−3, respectively. Different dominant deep level defects or complexes for pinning the Fermi level, and hence producing high resistivity, were expected when comparing the charge transport behaviours of the materials and the dopant concentration. However, two energies of 0.24 eV and 0.33 eV, below the conduction band for the Fermi level, were calculated at positive and negative bias voltages, respectively, for a low resistivity sample doped by 9.7 × 1017 at cm−3 In.

Characterization of CdZnTe Crystals Grown Using a Seeded Modified Vertical Bridgman Method

In this paper, several 60 mm diameter CdZnTe crystal ingots, containing large-size single crystals, were grown using (111) or (211) orientation seeds by the modified vertical Bridgman method. The Zn concentration distribution along the axial direction of the ingots was measured by near-infrared transmission spectroscopy at room temperature. The partition coefficient of Zn during the growth was calculated to be about 1.2. Zinc distribution uniformity measurements were carried out using photoluminescence mapping measurement at 80 K, which showed that the band gap variation in the CdZnTe wafers is less than 0.004 eV. IR microscopy showed that the diameters of the Te inclusions present in the material are in the range 6-9 mum, and the density of the inclusions is 1~3times105 cm-3. IR transmission measurements in the wave number region from 500 to 4000 cm-1 demonstrate that the IR transmittance of CdZnTe wafers is higher than 60%. Current-voltage measurements were performed on test structures fabricated using thermally evaporated Au contacts deposited on as-grown crystals, which revealed bulk resistivity values of 2~5times1010 Omegamiddotcm. Typical leakage currents for the planar devices were ~ 4 nA at a field strength of 1500 Vcm-1. The electron and hole mobility-lifetime products were evaluated using alpha particle irradiation. The obtained typical (mutau)e and (mutau)h values for the as-grown CdZnTe were 2.3times10-3 cm2V-1 and 1.5times10-4 cm2V-1, respectively.

Photoluminescence analysis on the indium doped Cd0.9Zn0.1Te crystal

Photoluminescence spectra are used to characterize high resistivity of In-doped CdZnTe crystal. Shallow level donor-acceptor pair (DAPs) peak at 1.6014eV is found due to shallow donor and acceptor compensation related defects, which forms the [VCd–InCd]− singly negative complex and the neutral complex [VCd–2InCd]. Cd vacancy acceptors are compensated dominantly due to indium doped [InCd]+ donor and Te antisite [TeCd]4+ donor, which improves the resistivity of CdZnTe:In crystal. Energy level model diagram of CdZnTe:In crystal is discussed. Current-voltage measurement results confirm that Cd vacancy compensation by doped indium improves the resisitivity of CdZnTe crystal.

Investigation of Te inclusion induced glides and the corresponding dislocations in CdZnTe crystal

Te inclusion induced dislocations were identified using FE-SEM imaging on CdZnTe (111) B face, after etching by Everson solution. Hexagram etching pits rosette was observed, which suggests a noticeable enrichment of dislocations surrounding a Te inclusion. The spatial distribution of the etch pits possibly correlates with the screw and edge dislocations, respectively. The corresponding dislocations attributed to the tetrahedral and tangential glide, respectively, occurred by local deformation of CdZnTe crystal. This deformation was contributed by the contraction of Te inclusions during cooling down to room temperature, since the thermal expansion coefficient of Te is significantly higher than that of CdZnTe crystal. Both tetrahedral and tangential glides were dominated by the plastic deformation mechanism for zinc-blende structure material. In addition, the volume of the dislocation enriched region was found to be significantly bigger than the volume of Te inclusion itself, and was proportional to the volume of the originating Te inclusion.

Dependence of film texture on substrate and growth conditions for CdTe films deposited by close-spaced sublimation

The texture of CdTe films deposited via close-spaced sublimation (CSS) was studied. Different substrates were used, including Si(100), fluorine-doped SnO2 (FTO), and CdS, and different growth conditions were applied. The texture behaviors of each sample were evaluated based on its XRD spectrum and are found to be dependent on both the substrate and the growth conditions. The texture strength is found to be, in order, Si(100) > FTO > CdS at a substrate temperature of 763 K under 100 Pa Ar, which is the opposite of the order of the surface roughness of the substrates. The textures of the films on FTO and chemical bath deposition (CBD)-CdS substrates, especially on CBD-CdS, are very sensitive to the growth conditions, whereas those on Si(100) are not. It is found that the texture is strengthened at elevated temperatures and suppressed under decreased ambient pressure. It is also found that the textured films are composed of both (111) and (511) texture components, which is believed to be the result of the tw...