Metalorganic vapour phase epitaxy of mercury cadmium telluride

The present review covers the fundamental factors involved in the the metalorganic vapour phase epitaxy of mercury cadmium telluride. In addition recent significant developments which could have an important influence on the future of the technology are discussed. The topics covering these developments include growth techniques, photoepitaxy, in-situ monitoring, “new” precursors, substrates and layer quality. The review highlights the advanced nature of the technology, which inspite of currently-addressed limitations has an important future.     

Crystallization kinetics and phase transformations of zinc- and cadmium diphosphide films

The kinetics of isothermal crystallization of amorphous ZnP₂ films in the temperature range from 730 to 810 K and amorphous CdP₂ films between 630–770 K has been investigated. The kinetic curves of isothermal transformation of amorphous films into crystalline ones have been obtained. For ZnP₂ the transformation occurs as follows: amorphous films → α-ZnP₂ (tetragonal modification) → β-ZnP₂ (monoclinic modification). For CdP₂ – amorphous films → γ-CdP₂ (cubic modification) → β-CdP₂ (tetragonal modification). From the experimentally determined kinetic curves the stability diagrams for ZnP₂ and CdP₂ films have been plotted. The most stable modification in the investigated temperature range are β-ZnP₂ and β-CdP₂ other ones are metastable. The values of effective energy of activation of transformation have been determined.     

Vapor phase growth of alumina whiskers by hydrolysis of aluminum fluoride

The vapor phase growth of alumina whiskers by the hydrolysis of aluminium flouride has been studied at an elevated temperature. The product was obtained in various forms: wool-like whiskers, needle-like whiskers, jointed needle-like crystals which consisted of linked small single crystals, tightly coagulated powder or as thin film covering the substrate. Aluminium oxi-fluoride was suggested as the intermediate gaseous species in the formation of alumina. The relation between the growth mechanism and crystal morphologies was studied.     

Vapor phase epitaxy of CdTe on sapphire substrates in dependence on the vapor-flow orientation

The growth of cadmium telluride films on a structured (0001) sapphire surface oriented at an angle of 44° to the vapor-flow direction and normal to the steps formed along the \(11\overline 2 0\) direction is studied. It is found that this geometry of the vapor source and a substrate (heated to a temperature of 300°С) provides the growth of single-crystal CdTe films if a step height on the substrate surface is more than 1 nm. The results are explained by the occurrence of a longitudinal component of the diffusion flux of СdTe molecules and atoms toward the steps from the inner side and their high density at the step edge from the outer side due to the presence of the Ehrlich–Schwoebel barrier, which ensures the efficient supply of material and minimum supersaturation necessary for the nucleation at the step edge and growth of oriented CdTe islands. The cadmium telluride films that are grown have the \(\left( {110} \right)\left[ {1\overline 1 0} \right]CdTe\left\| {\left( {0001} \right)} \right.\left[ {11\overline 2 0} \right]A{l_2}{O_3}\) orientation and a composition similar to stoichiometric CdTe.     

Surface chemistry and transport effects in GaN hydride vapor phase epitaxy

A simple quasi-thermodynamic model of surface chemistry in GaN hydride vapor phase epitaxy (HVPE) is presented. The model is coupled with the detailed 3D simulations of species transport in a horizontal-tube reactor and validated by the comparison with the data on the GaN growth rate obtained by laser reflectometry. Parametric study of the growth rate as a function of temperature and species flow rates has been performed over a wide range of growth conditions. The important role of species transport in an HVPE reactor is demonstrated. In particular, a strong effect of the natural concentration convection resulting in the formation of recirculation zones and in a non-uniform vapor composition is revealed by modeling. The impact of these effects on the GaN growth rate and V/III ratio on the growth surface is discussed in detail.     

Vibrational control of crystal growth from liquid phase

Modeling and numerical simulations of the convective flows induced by the vibration of the monocrystal during crystal growth have been performed for two configurations simulating the Cz and FZ methods. This permitted to emphasize the role of different vibrational mechanisms in the formation of the average flows. It is shown that an appropriate combination of these mechanisms can be used to counteract the usual convective flows (buoyancy- and/or thermocapillary-driven) inherent to crystal growth processes from the liquid phase. While vibrational convection is rather complex due to these identified mechanisms, the new modeling used in the present paper opens up very promising perspectives to efficiently control heat and mass transfer during real industrial applications of crystal growth from the liquid phase.     

Growth control of stoichiometry in LaMnO3 epitaxial thin films by pulsed laser deposition

We have studied structural, magnetic, and optical transport properties of LaMnO₃ (LMO) thin films grown on SrTiO₃. While the stoichiometric LMO is an insulating antiferromagnet, it tends to be a ferromagnetic insulator when grown as thin films. By exploring the majority of growth parameters, we have found that the bulk-like electronic and magnetic phases can be stabilized by growing thin films under reducing atmospheres and by using more energetic laser processes. These conditions are found to reduce the La deficiency in the film resulting in the greatly improved cation stoichiometry. Since oxides are prone to reduce the oxygen content and to alter the cation ratio under such growth conditions, it suggests that the cation and oxygen stoichiometries in complex oxide thin films can be improved by properly optimizing the growth parameters.     

Low temperature deposition of zinc oxide nanoparticles via zinc-rich vapor phase transport and condensation

ZnO nanoparticles as small as 80 nm were successfully synthesized using a modified vapor phase transport (VPT) process at substrate temperatures as low as 222 °C. Particle size distribution and morphology were characterized by scanning electron microscopy and atomic force microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction indicate the synthesis of high quality crystalline ZnO structures. Low temperature (4.2 K) photoluminescence (PL) spectroscopy was used to characterize the optical quality of the nanoparticles. Ultraviolet emission and a nanostructure specific feature at 3.366 eV are strong in the PL spectra. The 3.366 eV feature is observed to predominate the spectrum with decrease in particle size. This size effect corroborates the luminescence as a nanostructure-specific surface related exciton feature as previously speculated in the literature. In addition, self-assembled ZnO mesoparticles (>100 nm) were realized by increasing the growth time. Low growth temperatures of the particles allow for their potential utilization in flexible organic hybrid optoelectronics. However, this work focuses mainly on the modified synthesis and optical characterization of nanoparticles.     

Interface kinetics and the vapour phase mass transport in closed tube CdS:I2-system

Experimental evidence is given for the CdS:I₂ closed-tube CVD system that small deviations of the experimental transport rate from the calculated mass transport rate through the vapour when interface kinetics is assumed not to be rate-limiting, can be enhanced by the presence of mass convection in the This phenomenon which is strictly related to the presence of activational constraints at the solid-vapour interfaces, is shown to be independent on the stoichiometry defect of the CdS charge, but dependent upon the presence of foreign impurities in the charge.     

A theoretical treatment of GaAs growth by vapour phase transport for {001} orientation

The normal growth rate of a {001} face has been theoretically studied; by considering either direct fixation of gallium arsenide molecules, or formation of intermediate surface compounds. From the theory of rate processes, it appears that experimental results can be interpreted by considering the reactions of desorption of the chlorine atoms adsorbed on surface as limiting the growth. A theoretical expression of the normal growth rate based on desorption by hydrogen has been performed. The descending portions of the curves with decreasing substrate temperature or increasing partial pressure of gallium monochloride, appear as due to an increasing coverage of surface with gallium monochloride molecules. Absolute theoretical values agree with experimental published measures, except for the weakest substrate temperatures. This disagreement may be due to the possibility of desorption of two chlorine atoms by gallium monochloride and formation of gallium trichloride molecules.     

Gaseous control of vapour concentration in crystal synthesis from the vapour phase

The gaseous control in the crystal synthesis from the initial components is discussed. The dependences of the vapour concentrations in the growth region on inert gas flows have been calculated. It is shown that the use of additional inert gas flows conducted directly to the entries into the reaction zone makes it possible to change the vapour concentrations in the growth region rapidly and independently one from another, and these concentrations are linear with the main transport gas flows.     

Vapor growth of CdTe laser window materials

Vapor deposition of CdTe from elemental cadmium and tellurium sources was studied as a function of the Cd/Te ratio, the supersaturation and the substrate temperature, in order to achieve optimum growth conditions for CdTe windows with low optical absorptivity in the infrared. A multisubstrate arrangement was designed to enable acquisition of growth data simultaneously on up to six specimens, each exposed to different growth conditions. Polycrystalline blanks up to 15 cm² × 2 mm thick were grown at rates of 0.02-1.5 mm/h with the growth rate exhibiting sensitivity to all of the above variables. Our results show that stoichiometry (as well as free carrier concentrations) can be controlled by adjustments in the Cd/Te ratio and/or the substrate temperature. Similarly, microstructural aspects (e.g., grain and void size) are shown to exhibit strong sensitivity to variations in growth conditions.     

In-situ size control and structural characterization of barium titanate nanocrystal prepared by crystallization of amorphous phase

We examined the influence of nanocrystalline particle size on the cubic-to-tetragonal phase transition of barium titanate (BaTiO₃). In-situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used in combination with electron microscopy to study the evolution of lattice structure and phase-transformation behavior with heat treatment and particle growth for BaTiO₃. The lattice constants were obtained for various polycrystal orientations in the diameter size range 10–90 nm. In-situ XRD results during the crystallization of amorphous 4BaTiO₃-SiO₂ revealed a critical size of ∼25 nm for the cubic-to-tetragonal phase transition of BaTiO₃ at room temperature.     

Structure and Electric Conductivity of Vapor Deposition Products of Cyano Acetylene

Cyanoacetylene can be polymerized from vapor state onto an inactive surface of substrate at a temperature as low as 200°C. The structure and electric conductivity of these cyanoacetylene products have been investigated.     

Growth,structure, and stoichiometry of epitaxial Cu-III-VI2 films on CaF2 substrates

Thin films of CuInTe₂, CuInSe₂, CuInS₂, CuGaSe₂ and CuGaS₂ were deposited onto (111)-and (100)-oriented CaF₂ substrates by flash evaporation technique. Epitaxial growth occurs at substrate temperatures in the range from 750 to 900 K, dependent on the kind of film material. Contrary to the case of GaAs, GaP and Ge substrates there are always polycrystalline parts. The layers were found to be single phase and crystallizing in the chalcopyrite structure only. The composition of the films varied in dependence on the substrate temperature. In the range of about 700 K a nearly stoichiometric composition was found.     

Preparation and characterization of amorphous cadmium–selenium ribbons

Chalcogenide glasses based on the cadmium–selenium system, with the selenium composition varying from 0 to 7.5 wt% have been prepared using melt-quenching method i.e., single-roller quenching technique. The X-ray diffraction (XRD) and selected area electron diffraction (SAD) patterns of the CdSe ribbons indicate that the ribbons are amorphous. The transmission electron microscopy (TEM) studies carried out on these ribbons reveal that the constituents are inhomogeneously distributed in these ribbons. The temperature dependence of the electrical resistivity, ρ and thermoelectric power (TEP) of these ribbons has been studied in the temperature range 30–350 °C. The sudden jump in the values of electrical resistivity at a specific temperature for each case in these ribbons has been correlated with the phase transition i.e., the onset of crystallization in these materials during heating. The crystallization temperature, T_c has been found to be a function of Se content of these ribbons. The phase change in these ribbons as a result of heating does not seem to affect the variation of TEP with temperature. However, the slope of TEP versus temperature curves depends on Se content in these ribbons. The differential scanning calorimetry (DSC) of these ribbons indicates that the supercooled region in these ribbons extends from 50 to 70 °C. The composition CdSe ribbon with 0.5 wt% Se has the highest value of T_c and glass forming ability, K_g = 0.7.     

用热CVD法制备片状碳纳米材料的研究

采用阳极氧化法在草酸溶液中制备了孔径约为100nm的多孔氧化铝(AAO)模板。利用此模板在不负载任何催化剂的条件下,利用热CVD装置制备出片状碳纳米薄膜,并对此片状薄膜做了SEM,TEM显微观察和EDS能谱分析,对其形成条件和生长机理等方面做了探讨分析,为在氧化铝模板上生长包括管状、片状、棒状等形状的碳纳米材料提供了一定的借鉴意义。     

单质直接气相生长ZnMgSe单晶

采用高纯Zn、Mg、Se2单质为原料,以NH4Cl作为反应输运剂,用化学气相输运(CVT)的方法一步成功生长出ZnMgSe单晶。通过XRD、RO-XRD、EDS、紫外可见分光光度计和光致发光(PL)技术研究了ZnMgSe晶体的结构、成份以及光学特性。结果表明:ZnMgSe单晶具有良好的结晶性能,在400～800 nm范围内透过率达到40%～50%,在2.2～2.6 eV范围内存在与深能级电子复合相关的发光带。研究证明由Zn、Mg、Se2单质在输运剂NH4Cl辅助下一步直接合成ZnMgSe单晶是可行的。     

Microstructure optical and electrical studies of cadmium selenide thin films

Nucleation and growth of cadmium selenide thin films are of considerable interest because of their direct effect on the optical and electrical properties of this material. Vacuum-deposited layers on amorphous and crystalline substrates showed a polycrystalline structure. The layers were deposited at a pressure of 10⁻⁴ Pa, with two deposition rates of 12 nm/s and 15 nm/s. The electrical resistivity and the optical absorption were studied for a group of layers on quartz substrate. As well the morphology and the microstructure were investigated by X-ray and electron microscopy. Few activation energies were estimated from the optical absorbance and thermo-resistance.