Phase-field simulations of Te-precipitate morphology and evolution kinetics in Te-rich CdTe crystals

Te precipitates are one of principal defects that form during cooling of melt-grown CdTe or CZT crystals when grown Te-rich. Many factors such as the kinetic properties of intrinsic point defects (vacancy, interstitial, and antisite defects); stresses associated with the lattice mismatch between precipitate and matrix; temperature gradients and extended defects (dislocations, twin and grain boundaries); non-stoichiometric composition; thermal treatment history all affect the formation and growth/dissolution of Te precipitates in CdTe. A good understanding of these effects on Te precipitate evolution kinetics is technically important in order to optimize material processing and obtain high-quality crystals. This research develops a phase-field model capable of investigating the evolution of coherent Te precipitates in a Te-rich CdTe crystal undergoing cooling from the melt. Cd vacancies and Te interstitials are assumed to be the dominant diffusing species in the system, which is in two-phase equilibrium (matrix CdTe and liquid Te inclusion) at high temperatures and three-phase equilibrium (matrix CdTe, Te precipitate, and void) at low temperatures. Using available thermodynamic and kinetic data from experimental phase diagrams and thermodynamic calculations, the effects of Te interstitial and Cd vacancy mobility, cooling rates and stresses on Te precipitate, and void evolution kinetics are investigated.     

Melt growth and post‐grown annealing of semiinsulating (CdZn)Te by vertical gradient freeze method

We present results of development of CdZnTe semi‐insulating crystals prepared by Vertical Gradient Freeze method in a 4‐zone furnace. We applied the way of growth of the crystal from the top when the first crystallization seed is created on the surface of the melt. The typical height of the crystals is 5 cm. Resistivity and photoconductivity profiles measured along the growth axis by contactless method are compared and their mutual correlation is explained based on a model of relative shift of the Fermi level and the midgap level present in the material. The influence of the Fermi level on electron trapping and recombination is summarized. We present here results of a two‐step annealing method aimed at reduction of Te inclusions while keeping the resistivity high. We employed CdTe:Cl VGF grown samples to eliminate Te inclusions observed in as grown crystals by two‐step post grown annealing in Cd and Te atmosphere and present a model of the processes leading to high resistivity material after annealing.     

Optical property analysis of CZT:In single crystals after annealing by a two-step method

Indium-doped Cd_1−xZn_xTe (CZT:In) single crystals were annealed by a two-step method, including a high-temperature step and a low-temperature step in sequence. IR transmittance spectrum, I–V curve and PL spectrum were used to characterize the CZT single crystals. After annealing, the opto-electrical properties of the CZT:In crystals were improved obviously. The average IR transmittance was remarkably increased by about 23%, and the resistivity was enhanced by as high as four orders of magnitude. In the PL spectra, the intensity of the (D⁰, X) peak prominently increased, and the full-width-at-half-maximum was reduced. Meanwhile, the intensity of the DAP peak decreased greatly, and the structure became practically indistinguishable from the background. Moreover, the intensity of the D_complex peak also decreased. The investigation shows that these improvements in the physical properties after annealing are due to variations in the micro-structures. The two-step annealing method can eliminate precipitates/inclusions, remove impurities, compensate Cd vacancies, decrease dislocations and reduce internal stress.     

The influence of annealing atmosphere on the optical properties of flower‐like ZnO

The influence of annealing atmosphere on the optical properties of flower‐like ZnO is investigated. The flower‐like ZnO is composed of nanosheets. Annealing at 500°C results in the increase of the thickness of nanosheets and the enhancement of UV emission. PL spectra results show that the annealing atmosphere affects the visible emission band, which is sensitive to intrinsic and surface defects in the flower‐like ZnO. The vibrational properties of the flower‐like ZnO annealing in different atmospheres are characterized by Raman spectra. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

TEM analysis of the container effect of Au‐based catalyst droplets during vapour‐liquid‐solid growth of axial ZnTe/CdTe nanowires

Axial heterostructure nanowires (NWs) of ZnTe/CdTe were grown by vapour‐liquid‐solid growth realized in a molecular beam epitaxial chamber. By alternative supply of Zn or Cd and constant Te the heterostructure was generated. The liquid phase is provided by a Au‐based eutectic droplet which stays at the tip of the NW during the entire growth. For structural and chemical characterization by TEM the NWs were harvested from the substrate and transferred to a holey carbon film. The NWs exhibit an expansion of the diameter correlated with the interface region between ZnTe and CdTe. Idiomorphic growth of the CdTe is evident from electron diffraction experiments. The growth rate of CdTe appears to be smaller compared to that of ZnTe at the same temperature. Both, quantitative high‐resolution TEM and energy dispersive X‐ray spectroscopy line scans reveal a smeared ZnTe/CdTe interface along about 200 nm. The smearing is due to both, the liquid catalyst which buffers the supply of Cd instead of Zn at the liquid/solid interface and to the strain which is induced by the lattice mismatch. It forces the system to consume the remnant Zn for the NW growth in favour of Cd. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth‐Sector Boundaries and Growth‐Rate Dispersion in Potassium Alum Crystals

Crystals of potassium alum, pure and slightly doped with Cr³⁺, were grown from aqueous solution by slow temperature lowering. In addition, short re‐dissolution periods were introduced in order to provoke growth defects and changes of growth rates. Crystal slices of about 1 mm thickness were studied by conventional LANG X‐ray diffraction topography using MoKα radiation. For Cr‐doped crystals, boundaries between {100}, {100} and {111} growth sectors appear by pronounced dynamical X‐ray topographic contrast similar to that of stacking faults. Re‐dissolution experiments provoke the formation of inclusions on {100} faces, followed by an increase of the {100} growth rate by the factor of about six, relative to the neighboured {111} faces. X‐ray topographs show that this increase is correlated with the formation of dislocations, which interestingly have pure‐edge character. During further growth these dislocations penetrate the {100}‐{111} growth sector boundary and vanish from the {100} face, which slows down and finally adopts its former growth rate before re‐dissolution.     

Surfactant‐assisted synthesis of novel star‐like PbWO4 hierarchical architectures

Large‐scale star‐like PbWO₄ hierarchical architectures were controllably synthesized by a facile surfactant‐assisted technology under mild conditions in the presence of a mixed solvent of ethylene glycol and water. The morphology, structure, and phase composition of PbWO₄ architectures were characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FE‐SEM), field emission transmission electron microscopy (FE‐TEM), and nitrogen adsorption‐desorption isotherms. The possible formation mechanism of the star‐like PbWO₄ architectures (initial nucleating stage and a subsequent self‐assembly stage) was proposed based on the observations from a time‐dependent morphology evolution process, which may pave the way to shape‐controlled synthesis of inorganic nanocrystals with the complex structures. This route provides a facile strategy to fabricate complex hierarchical PbWO₄ structures. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Te溶剂-Bridgman法Cd0.9Mn0.1Te晶体生长习性研究

采用Te溶剂-Bridgman法生长了尺寸为φ30 mm× 60 mm的Cd0.9Mn0.1Te:In晶锭,通过淬火得到了生长界面形貌.测试了晶片在近红外波段的透过率和电阻;采用化学腐蚀的方法观察了晶片中位错,Te夹杂和孪晶界;采用光学显微镜和红外成像显微镜观察了生长界面处附近的形貌.测试结果表明,晶锭中部结晶质量较好的晶片红外透过率达到60％,电阻率达到2.828×1011Ω · cm.位错密度在106 cm-2数量级,Te夹杂密度为1.9×104 cm-2,同时孪晶密度明显低于Bridgman法生长的晶锭.生长界面宏观形貌平整,呈现微凹界面.但由于淬火过程的快速生长,界面微观形貌发生变化,呈现不规则界面,并在界面附近形成富Te相的包裹.     

CdZnTe晶片表面化学抛光研究

采用不同浓度的Br2-MeOH作为抛光液对CdZnTe进行化学抛光,发现用2％Br2-MeOH腐蚀时速率平稳且易于控制,能有效去除表面划痕,获得光亮表面。AFM分析发现,抛光后表面粗糙度降低30％,平整度增加。XPS分析发现CdZnTe的（111）Cd极性面变成了富Te非极性表面。PL分析发现表面陷阱态密度降低,表面晶格的完整性增强。     

Atomic force microscopy studies on liquid inclusions and induced defects of cadmium mercury thiocyanate crystal

Liquid inclusions and various defects accordingly induced on a nonlinear optical material of CMTC crystal were investigated by atomic force microscopy. Liquid inclusions are chiefly caused by formation of macrosteps, which result from impurity‐induced inhibiting of step growth and meeting of step trains advancing along different directions. Liquid inclusions induce generation of dislocations and even cracks within the crystal by three‐dimensional nucleation growth. Liquid inclusions also provide screw dislocation growth sources, leading to formation of spiral hillock trains with ridged tails. Etching experiments reveal circular hollow cores, indicative of screw dislocation growth, and negative crystals resulting from further crystallization in the liquid inclusions. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Centrifugal separation of primary silicon crystal in solvent refining of silicon using Al‐30% Si alloy

This study describes the formation of a primary silicon network during separation of an Al‐30% Si melt and the process conditions to make bigger primary silicon crystals. As the crucible rotates in the centrifugal separation, the unfrozen aluminum‐rich phase and small silicon particles are pushed outside through the openings in the silicon network. As a result, primary silicon crystals are separated in the form of a foam after centrifugation. The recovery of the silicon ranged from 13 to 18% depending on the location in the crucible. The size of the primary silicon achieved by changing the cooling rate and quenching temperature during solidification is also measured using a quenching furnace. Primary silicon particles exhibit a coarse, plate‐like morphology, although small star‐like silicon particles are also found in the aluminum‐rich matrix. The fraction of plate silicon decreases, while the fraction of small globular silicon increases with an increasing cooling rate. The thickness of the primary silicon plate also decreases with an increasing cooling rate in the samples quenched at various temperatures during solidification.     

CdTe(110)表面原子与电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算了CdTe(110)表面的原子和电子性质.结果表明,CdTe(110)理想表面在禁带中出现两个明显的表面态,弛豫后表层Cd原子和Te原子p态电子发生转移,Cd原子趋向于sp2平面杂化构型,Te原子趋向p3杂化的锥形构型.经过表面弛豫大大降低了表面能,增大了表面功函数,表面占据态和表面空态分别被推进价带顶之下和导带底之上,导致弛豫表面没有明显的表面态.     

Effect of rapid thermal annealing on Zn1‐xCdxO layers grown by radio‐frequency magnetron co‐sputtering

Zn_1‐xCd_xO layers were deposited on the sapphire substrate using the radio‐frequency magnetron co‐sputtering system. The grown Zn_1‐xCd_xO layers were carried out in the post‐annealing treatment for 1 min at the 800 °C oxygen‐ambient by the rapid thermal annealing (RTA) method. X‐ray diffraction (XRD) experiment shows that the Zn_1‐xCd_xO layers are changed from the single phase of the hexagonal structure at 0≤x ≤0.08 to the double phase of hexagonal‐and‐cubic structure at x =0.13. Thus, the maximum Cd‐composition ratio with the hexagonal structure was found out to be x =0.08. Also, the crystallinity of Zn_1‐xCd_xO layers at x =0.13 was remarkably improved by the RTA annealing treatment. This crystal quality improvement was thought to be associated with the relaxation of the compressive strain remaining in the Zn_1‐xCd_xO layers. Therefore, the results of XRD and transmittance lead that the crystal quality of the Zn_1‐xCd_xO layers forming the hexagonal ZnO phase is better than that forming the cubic CdO phase. Consequently, the reliable formation and the crystallinity of the Zn_1‐xCd_xO layers were achieved by using the RTA method of short‐time thermal‐annealing at the high temperature. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

稀释磁性半导体Cd0.9Mn0.1Te晶体的退火改性

运用缺陷化学原理近似计算了Cd0.9Mn0.1Te晶体的点缺陷浓度,得到了晶体成分与理想化学计量比偏离最小时的退火条件.利用该退火条件,指导了Cd0.9Mn0.1Te晶体的两温区退火实验,并分析了退火对晶片性能的影响.结果表明:在973 K,Cd气氛下对Cd0.9Mn0.1Te晶片退火140 h后,晶片(111)面的X射线回摆曲线的FWHM值由退火前的168.8' 降至108',红外透过率由退火前48;提升到64;,接近晶体的理论透过率,电阻率也由退火前的2.643×105 Ω·cm提高到4.49×106 Ω·cm.由此可见,对生长态的Cd0.9Mn0.1Te晶体进行退火实验能提高晶体的结晶质量,补偿晶体的Cd空位点缺陷,使晶体成分接近理想的化学计量比.     

Raman scattering study on Ga1–x Mnx As prepared by Mn ions implantation,deposition and post‐annealing

Raman scattering measurements have been performed in Ga_1–x Mn_x As crystals prepared by Mn ions implantation, deposition, and post‐annealing. The Raman spectrum measured from the implanted surface of the sample shows some weak phonon modes in addition to GaAs‐like phonon modes, where the GaAs‐like LO and TO phonons are found to be shifted by approximately 4 and 2 cm^‐1, respectively, in the lower frequency direction compared to those observed from the unimplanted surface of the sample. The weak vibrational modes observed are assigned to hausmannite Mn₃O₄ like. The coupled LO‐phonon plasmon mode (CLOPM), and defects and As related vibrational modes caused by Mn ions implantation, deposition, and post‐annealing are also observed. The compositional dependence of GaAs‐like LO phonon frequency is developed for strained and unstrained conditions and then using the observed LO_GaAs peak, the Mn composition is evaluated to be 0.034. Furthermore, by analyzing the intensity of CLOPM and unscreened LO_GaAs phonon mode, the hole density is evaluated to be 1.84×10¹⁸ cm^‐3. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Strain history effects at low temperature deformation

The work hardening coefficient of hep metals like Zn or Cd depends on the temperature even at very low temperatures. Additional information of the processes involved can be derived from specially designed annealing experiments. If deformed single crystals of Zn, Zn + 0.2 at.% Ga and Zn + 0.2 at.% Ag are annealed the work hardening increases. The amount of increase depends on deformation temperature and annealing time. The effects observed suggest that the work hardening is determined at least partly by the mobility of points defects.     

Effect of annealing on an amorphous GexTe1−x matrix with Te cristallites

Annealing shows very different behaviour for Te crystallites in an amorphous Ge_xTe_1?x matrix as a function of x (x = 0.1 … 0.5). For x ? 0.2, annealing at increasing temperatures increases the number (size) of the Te crystallites with subsequent GeTe + Te crystallization. However for x ? 0.3 there is first a disappearance of Te crystallites, then an appearance of GeTe crystallites, and subsequently GeTe + Te crystallization. Crystallites of either Te or GeTe act as extrinsic defects which add to the intrinsic ones. Activatioon energy decreases (increases) and conductance increases (decreases) as the number of defects increases (decreases). In all cases the final metallic state is obtained only when both GeTe and Te crystallites are present.     

Morphology‐photocatalytic properties‐growth mechanism for ZnO nanostructures via microwave‐assisted hydrothermal synthesis

ZnO nanostructures with various morphologies including rod‐like, sheet‐like, needle‐like and flower‐like structures were successfully synthesized via a fast and facile microwave‐assisted hydrothermal process. Reaction temperature, reaction time and the addition of NaOH were adjusted to obtain ZnO with different morphologies. Scanning electron microscopy(SEM), transmission electron microscope(TEM), X‐ray diffraction (XRD) and ultraviolet spectrophotometer (UV) were used to observe the morphology, crystal structure, ultraviolet absorption and photocatalytic activity of the obtained ZnO. The results indicated that growth rate of ZnO nanostructure along [001] direction was more sensitive to temperature compared with those along [101] and [100] directions. The competition between anionic surfactant and OH⁻ played an important role in the formation of ZnO with various morphologies. Flower‐like ZnO had better ultraviolet absorption property and excellent photocatalytic activity than ZnO in the other morphologies. On the basis of the above results, a possible growth mechanism for the formation of ZnO nanostructures with different morphologies was described.     

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.     

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)