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)     

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.     

Growth Mechanism in Atomic Layer Epitaxy (II). A Model of the Growth Process of CdTe on CdTe (111) Substrates

A realistic model of CdTe growth by atomic layer epitaxy (ALE) has been proposed. This model is based on experimental studies concerning the isothermal re-evaporation rates of elemental Cd and Te deposits on the (lll)A and (lll)B surfaces of CdTe substrates, on a study of surface morphology and crystal structure of CdTe single crystal overlayers grown by ALE on CdTe(lll)B substrates under various crystallization conditions as well as on the existing theories related to the interaction of thermally activated atoms or molecules with hot solid surfaces. This model includes: (i) an existence of transition layers of both Cd and Te₂ species, intermediate between a chemisorbed and a bulk-like film, which create reaction zones 3–4 monolayers thick near the substrate surface, and (ii) partial re-evaporation of the first, chemisorbed monolayer of the deposited constituent elements.     

Synthesis and spectrum stability of high quality CdTe quantum dots capped with stearate groups in N-oleoylmorpholine solvent

The stearate-capped CdTe quantum dots (QDs) have been first prepared via direct reaction of cadmium stearate with Te powder in N-oleoylmorpholine solvent, which was a kind of clean, air-stable and conveniently synthesized acylamide, and can readily dissolve precursors cadmium stearate and Te powder at a relative low temperature. The as-prepared CdTe QDs exhibited size-dependent optical properties, steep absorbance edge and narrow photoluminescence full width at half maximum. The high-resolution transmission electron microscopy images and X-ray diffraction revealed that the highly monodisperse CdTe QDs were of regular spherical morphology with zinc blende crystal structure displaying mean sizes of about 4 nm. The energy dispersed spectrometry measurement indicated the presence of Cd and Te, with the Cd:Te ratio being close to 1:1. Fourier transform infrared transmission spectra confirmed the existence of stearate on the CdTe QDs surfaces. The experimental results also demonstrated that the stearate-capped CdTe QDs had an unexpected good stability.     

Calculation of Diffusion-limited Growth of Hg1−x Cdx Te Epilayers on CdTe from Te-rich Melts by Step-Cooling LPE

Calculations of layer thicknesses and composition profiles in Hg_1−xCd_xTe layers on CdTe substrates for the growth from Te-rich melts have been carried out for liquidus temperatures of 460 °C, 480 °C, and 500 °C. This has been made on the basis of the multicomponent diffusion model of SMALL and GHEZ and the solid-liquid phase relation of BRICE . It could be shown that growth velocity increases only slightly with rising liquidus temperature. On the other hand, the interdiffusion velocity of Hg and Cd in the solid increases remarkably at a higher temperature. Therefore, to get layers with a constant x-value a higher supersaturation of the melt is necessary. The x-value decreases with rising supercooling by about 0.003 K⁻¹. To demonstrate the thermodynamically and kinetically advantageous properties of CdTe as substrate material, comparative calculations for a “hypothetical” HgTe substrate have been involved.     

Growth Mechanism in Atomic Layer Epitaxy (I) Re-evaporation of Cd and Te from CdTe(111) Surfaces Monitored by Auger Electron Spectroscopy

The mechanism of atomic layer epitaxy (ALE) of cadmium telluride has been studied. Auger electron spectroscopy is used to measure the isothermal re-evaporation rates of elemental Cd and Te deposits on the (111)A and (111)B surfaces of CdTe substrates. The results include an observation that the sticking coefficients of Cd and Te are smaller than unity at the growth temperatures typical of CdTe ALE. After desorption the substrates are left partially covered: 35% by a Cd overlayer on the (111)B surface and 72% by Te on the (111)A surface. The re-evaporation rates of Cd and Te experience a drastic change near the substrate-deposit interface. These rates appear two orders of magnitude smaller than those of bulk-like amorphous Cd and Te solids. The activation energies for reevaporation of the near-interface layer region are estimated to be: 1.5 eV for Te on the (111)A face, 1.0 eV for Te on (111)B and 0.5 eV for Cd on (111)B. It has also been shown that AES can be used to identify the polarity of the CdTe(111) surfaces. The relative difference in peak-to-peak intensity ratios of Cd MNN to Te MNN for (111)A and (111)B is (11 ± 2)%.     

Ultrahigh Vacuum Atomic Layer Epitaxy of Ternary II-VI Semiconductor Compounds

A concise discussion concerning the UHV ALE growth of ternary II-VI compounds is presented in this paper. Simultaneous reflection mass spectrometry (REMS) and reflection high energy electron diffraction (RHEED) measurements of the surface kinetic and structural parameters, respectively, governing the UHV ALE growth of Cd_1-xZn_xTe and Cd_1-xMn_xTe heteroepitaxial films are reported. In addition, a Monte-Carlo-based method for simulation of the UHV ALE process of CdTe (the model-compound for this growth technique) has been used for investigation of the Cd cation's fluxes reflected from the growing epilayer surface in different phases of the ALE process. The Cd⁺ ion-related REMS signals measured during CdTe growth have been compared with the simulation results.     

Growth mechanism in atomic layer epitaxy (III) reevaporation of Cd and Te from CdTe (111) surfaces and thick elemental deposits monitored by quadrupole-mass spectrometry

Quadrupole-mass spectroscopical (QMS) studies on isothermal reevaporation of Cd and Te species from the CdTe (111) surface have been performed in two extreme cases. The first concerns the reevaporation of thick, bulk-like non-crystalline Cd and Te films deposited in high vacuum at room temperature on the (111) surface, whereas in the second case evaporation of the constituent species from the bare single crystalline (111) surface has been investigated. The fluxes of the species desorbing in high vacuum (10⁻⁶ Pa) have been monitored with QMS and the desorption temperatures have been measured with a thermocouple mounted as near as possible to the sample surface. The following values E_a(Cd)_bulk = 1.13 ± 0.12 eV, E_a(Te)_bulk = 1.64 ± 0.18 eV and E_a(Cd)₍₁₁₁₎ = 1.13 ± 0.06 eV, E_a(Te)₍₁₁₁₎ = 1.92 ± 0.13 eV of the activation energies for these two cases have been determined from the slopes of the Arrhenius plots. Using these experimental values, the numbers of atomic bonds N_Cd and N_Te occurring in the atomic aggregates of quasi-gas molecules forming the near surface quasi-gas transition layers have been estimated. For Cd quasi-gas molecules 2 ≤ N_Cd ≤ 5, whereas for Te molecules 3 ≤ N_Te ≤ 10. However, no prediction concerning the number of atoms creating the quasi-gas molecules could be made on the basis of the QMS investigations. It has also been shown that Cd atoms evaporate from the bare single crystalline CdTe (111) surface with an activation energy that is equal (in the limits of the experimental error) to the activation energy for sublimation of Cd atoms from pure, non-crystalline, bulk Cd pieces. The analogous activation energy measured for Te atoms is about 20% larger than that of the relevant sublimation process. This result confirms the fact that Te atoms are bound much stronger in the CdTe crystal lattice than Cd atoms.     

Self-selecting vapour growth of bulk crystals – Principles and applicability

A method of self-selecting vapour growth (SSVG) for bulk binary and multernary crystals of semiconducting materials is reviewed comprehensively for the first time. Although it has been developed over three decades, the method is less well known – even though it is physically distinct from the more widely used ‘Piper–Polich’ and ‘Markov–Davydov’ vapour transport bulk growth methods. The means by which growth takes place on a polycrystalline source to form a crystal free from the walls is described. Modelling and empirical observations have been used to establish the characteristics of the almost isothermal temperature fields that drive the transport in SSVG. It is demonstrated that precise control of thermal radiation is a fundamental requirement for tailoring the temperature distribution—a fact that has been used well in the design of horizontal tube furnace growth rigs. Achievements in the growth of useful PbS, PbSe, PbTe, CdTe and ZnTe compound crystals are described. The SSVG method has proved to be particularly well suited to the growth of solid solutions, and the results of growth experiments, and of compositional and structural analysis, are presented for Pb(Se,S), (Pb,Sn)Se, (Pb,Sn)Te, (Pb,Ge)Te, Cd(Te,Se), Cd(Te,S) and (Cd,Zn)Te. The excellent compositional uniformity delivered is attributed to entropy driven mixing in the low thermal gradients present in SSVG.

To date, most SSVG has been done at the <50 g level for research or small scale production use. Prospects for scaling up the growth are considered, there being no barriers identified in principle. However, there is a limitation in that the shape of the grown crystals is not accurately controlled at present. To overcome this, and to offer an alternative method of scaling up, the use of vertical tube systems is explored. A significant additional advantage of the vertical configuration is that it allows for continuous recycling of the source/crystal mass so as to continuously self-refine the increasingly uniform – and crystalline – product. Achievements to date in growing II–VI and IV–VI crystals are described for prototype vertical SSVG systems. Finally, future prospects for the SSVG method in terms of further developments to the method, and the specific materials that will benefit from it are highlighted.     

Synthesis and epitaxial growth of CdTe films by neutral and ionized beams

The growth of CdTe thin films has been studied by epitaxial processes on the cleavage surface of rock salt in vacuum, using electron microscopic and electron diffraction techniques. The crystallinity and structure of the films depend largely upon the intensities and species of the incident beams. The use of two beams effused separately from the Knudsen cells resulted in the growth of films of good crystallinity when the intensity ratio N_Cd : N_Te was 10 : 1. The epitaxial relationships were studied over the range of substrate temperatures between room temperature and 350°C. The co-existence of α-hexagonal and β-cubic modifications of CdTe and their proportions in the film were revealed as a function of the growth processes and substrate temperature. If two beams were ionized by electron bombardment inside the cells and were incident upon the substrate by applying a dc voltage between source and substrate, the epitaxial temperature can be lowered to near room temperature, giving good epitaxy. The epitaxial relationships in the CdTe/NaCl system have been studied.     

Optimal crystal growth conditions of thin films of Bi2Te3 semiconductors

Crystal growth conditions of Bi₂Te₃ narrow bandgap semiconductors have been studied using molecular beam epitaxy method. It was applied to the growth of Bi₂Te₃ on Bridgman single-crystal substrate Sb₂Te₃. Substrate ingots were taken from the natural cleavage along the (0001) plane. The deposited conditions have been studied as a function of substrate temperature (T_s) and flux ratio (F_R=F(Te)/F(Bi)). The quality of deposited layers was controlled by X-ray diffraction, scanning electron microscope (SEM), secondary ion mass spectroscopy (SIMS) depth profiling and energy-dispersive X-ray (EDX) microanalyser. The sticking coefficients K_s(Te) and K_s(Bi) of the elements that compose Bi₂Te₃ were determined. It was found that the stoichiometry of deposited layers depended on substrate temperature and flux ratio. It was observed that all deposited layers were single-crystal in the orientation of their substrates with a small shift due to the stress in layer.     

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.     

Charge transfer in CdTe at 200 and 300 K

Using a perfect single crystal sample of CdTe grown using PVT method, the electronic charge transfer in the II–VI compound semiconductor CdTe at 200 and 300 K has been evaluated using two different approaches: (1) by solving a quadratic equation involving the observed structure factors of h+k+l=4n+2 type reflections; and (2) by a graphical approach in which the observed and calculated atomic form factors are extrapolated to sinθ/λ=0, to determine the transferred charge. Precise X-ray structure factors collected using MoK radiation have been used for the analysis. The results obtained are reasonable and clearly indicate the ionicity by which charge is transferred from Cd to Te in CdTe.     

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.     

A study of the Growth-Temperature-dependent Surface Morphology in InSb Liquid Phase Epitaxy

InSb LPE layers were grown on (111) InSb substrates, their phase diagram and growth rates studied. The InSb activity coefficient α(T), determined by fitting the experimental data, had a value of 2556–11.11 · T cal · mole⁻¹. The terraces on the surface were reduced by increasing the temperature to 350 °C. Hall measurements of InSb/CdTe heterostructure failed due to the high Te segregation. The carrier concentration of these InSb epilayers, however, was determined by C-V measurements with values between 7.5 · 10¹⁴ cm⁻³ and 5 · 10¹⁵ cm⁻³.     

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)     

Synthesis and structural analysis of angled Te nanocrystals

In this paper, a new direct method to prepare angled Te nanocrystals (NCs) in the isotropic thiol‐ligand system from the transformation of MA‐stabilized CdTe nanoparticles (NPs) induced by the post‐addition of L‐cysteine was presented, without removing aforehand the protective shell of organic stabilizer. Besides nanorods, angled Te nanocrystals, including nanocheckmarks, X‐shaped, nanomoths, y‐shaped, and so on, also were obtained. By means of high‐resolution transmission electron microscopy (HRTEM), selected‐area electron diffraction (SAED) and powder X‐ray diffractometry (XRD), we further investigated intensively the internal crystal structure of angled Te NCs and the growth direction of the arms in this study. The experimental results obtained show that the preferential growth direction of either arms in nanocheckmarks is along the [001] direction of trigonal Te, and the corresponding contact plane should be (112) of hexagonal lattice. In the meantime, these results also confirm that L‐cysteine not only can be used as the stabilizer for the synthesis of aqueous CdTe NPs as reported previously, but also can act as a strong complexing agent like EDTA, inducing the decomposition of CdTe NPs, i.e. the transformation from CdTe NPs to Te nanocrystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comprehensive characterization of CdTe and (Cd,Zn)Te single crystals by a chemical etchant

Using a known chemical etchant low- and high angle boundaries and lamellar twins can be seen on CdTe and (Cd, Zn)Te crystal ingots as a whole as well as on slices with naked eyes. Also the polarity of {111} samples can be determined in this way. Etch pits are produced on cut and polished surfaces independent of their crystallographic orientation. A new modified etchant was used to study the low angle subgrain structure on (111)Te surfaces.     

Correlation between growth orientation and growth temperature for bismuth tri‐iodide films

This paper reports the growth of bismuth tri‐iodide thick films intended for direct and digital X‐ray imaging. Films were grown by the vertical physical vapor deposition method, onto glass substrates 2″x 2″ in size, with gold previously deposited as rear electrode. The film thickness was up to 33 μm (±5 %). Optical microscopy and SEM were performed on the films and grain size resulted to be up to 40 μm. A strong correlation was found between the microcrystals growth orientation and the growth temperature. At low temperatures, microcrystals grow with their c axis parallel to the substrate, whereas at higher temperatures, they grow with their c axis perpendicular to the substrate. The higher the growth temperature, the lower the dark current of the film, and the higher the resistivity, which was from 10¹³ to 10¹⁵ Ωcm. A sensitivity to X‐rays of 6.9 nC/R.cm² was measured irradiating the films with X‐rays from a mamographer. Film properties were correlated with the growth temperature, with previous results for bismuth tri‐iodide films and monocrystals and with data for films of alternative materials such as lead and mercuric iodide. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Processes of vapor phase growth of AIIBVI single crystals and structures based there upon

Results of investigations of semiconductor A^IIB^VI compounds (for example of CdTe and ZnTe) grown by the chemical vapor transport (CVT) method in a closed volume using three transfer agents containing a halogen, compound NH₄X (X = Cl, Br, I) are presented. The processes of vapor phase growth (composition of the vapor phase and mass transfer) in Me(Cd, Zn)Te–NH₄X (X = Cl, Br, I) systems have been calculated theoretically and the results are have been verified in growth experiments. Optoelectronic properties of the grown materials and barrier structures based there upon are discussed.