Growth,characterization and fabrication of n-WSe2 PEC solar cells

Single crystals of n-WSe₂ have been grown by the chemical vapour transport (CVT) technique, using Br₂, TeCl₄, and SeCl₄ as transporters. Crystal lattice parameters have been determined for all the crystals grown above with an X-ray diffractometer. The composition of the above crystals were examined by energy dispersive analysis of X-rays (EDAX). Optical and some electrical transport measurements are carried out to judge the semiconducting nature of the as-grown crystals. Photoelectrochemical (PEC) solar cells were frabricated by using these crystals as photoelectrodes and platinum grid as counter electrode in an aqueous iodine/iodide solution. Some of the PEC cell parameters were determined. The results thus obtained have been discussed in detail.     

Surface morphology studies on sublimation grown GaN by atomic force microscopy

Bulk single crystals and selectively grown gallium nitride (GaN) have been obtained using the sublimation technique. Crystals of size about 2–3 mm in length and 0.8–1.0 mm in width have been grown successfully. Atomic force microscopy has been employed to analyze the surface morphology of the as-grown samples to understand the growth mechanism. In free standing bulk GaN single crystals, two-dimensional growth is dominated by step growth mechanism. However, in the selective growth of GaN by sublimation, spiral growth originating from screw dislocation dominates.     

Stoichiometric dependence of electrical characteristics in WSSe single crystals grown by direct and chemical vapour transport techniques

Single crystals of tungsten sulphoselenide (WSSe) have been grown by both direct and chemical vapour transport techniques. The crystals have been subjected to Hall effect and resistivity measurements for their electrical characterization. The observed differences in the electrical resistivity, type of conduction, and activation energy have been attributed to the stoichiometric differences between the crystals grown by the two techniques. The crystals grown by chemical vapour transport technique with iodine as the transport agent have been found to be more stoichiometric.     

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。     

A model for the growth of anomalous polytype structures in vapour grown SiC

A number of polytype structures observed in vapour grown SiC crystals have a unit-cell which is an integral multiple of the unit-cell of the basic 6H, 15R or 4H structure. The growth of such anomalous structures cannot be understood in terms of spiral growth round a single screw dislocation in a basic matrix. However many of these polytype crystals display a single growth spiral on their (0001) face indicating that they have resulted from spiral growth round a single screw dislocation. It is shown that this anomaly can be resolved if the basic matrix is assumed to contain stacking faults near the surface at the time of the origin of the screw dislocation ledge. This possibility, overlooked in the earlier deduction of polytype structures, must be taken into consideration since vapour grown SiC crystals frequently contain a high concentration of random stacking faults, producing continuous streaks on their X-ray diffraction photographs. The most probable fault configurations that can occur in 6H, 15R and 4H structures of SiC have been deduced from a calculation of their stacking fault energy. These fault configurations are then considered to lie at different distances from the surface at the time of the origin of a screw dislocation ledge. Such a faulted ledge gives rise to polytype structures during subsequent spiral growth even if the screw dislocation has an integral Burgers vector. The most probable series of polytype structures that can result from such a faulted matrix model are deduced. It is shown that nearly all the polytype structures of SiC hitherto regarded as anomalous (such as 36H, 54H, 66H, 45R, 90R etc.) are among the expected structures and there is no need to postulate a complicated configuration of cooperating dislocations to account for their growth.     

Nearly perfect crystal growth of III–V compounds by the temperature difference method under controlled vapour pressure

Nearly perfect crystals of III–V compounds are grown by a new LPE method which can produce homogeneous and high-quality crystals with controlled stoichiometry. Homogeneous epitaxial growth is performed at fixed temperature with a temperature difference in a melt, under controlled vapour pressure of the group V elements. In the experiment, the fluctuation of growing temperature is ascertained to produce a number of new defects and the effect of controlled vapour pressure is investigated. It has been ascertained that an optimum vapour pressure, which is the function of growing temperature, can suppress the generation of “rooty faults” to a minimum density. There is close agreement between the optimum vapour pressure in the heat treatment of crystals to reduce the generation density of lattice defects and the optimum vapour pressure to obtain defect-free crystals in the process of crystal growth. The epitaxial layers prepared under optimum vapour pressure have the highest mobility and the lowest additional carrier concentration. The lattice constant of the epitaxially grown crystal under controlled vapour pressure also shows a minimum value at optimum pressure. This also shows that the controlled vapour pressure is important in lattice fitting of epitaxial layers. The epitaxial layers grown by the temperature difference method (TDM) under optimum control of the vapour pressure (CVP) are almost as perfect crystallographically as measured by X-ray topography. The experiments are performed for GaP and GaAs.     

Observation and interpretation of eccentric growth spirals

Eccentric growth spirals were observed on the (0001) face of SiC crystals synthesized by the Lely method at the Toshiba Central Research Laboratories. Originating from one screw dislocation, mono-molecular spiral layers (step height 15 Å, 6H polytype), show increasingly wider step separation in the direction towards the inner portion of the furnance where temperature is lower, and hence supersaturation is higher. Contrary to this, step separations become increasingly narrower approaching the base of the crystal which is more close to the wall of the furnace. This eccentricity is interpreted as due to a supersaturation gradient over the surface. In addition, an observed eccentric growth spiral with a hollow core will be presented and interpreted.     

Studies on macrospiral observed in flux-grown YAG single crystals

Presence of spirals with unit cell dimension step height has been established over a large variety of materials. But often spirals with large step height are also being reported. In this paper such a macrospiral observed over the barium based flux grown YAG crystals is reported. Nature of this depressed spiral and growth conditions are discussed. A possible formation mechanism is suggested for the development of spiral with large step height of giant step, which is termed as macrospiral.     

Morphological features of synthetic crystals of As2Se3

Scanning electron microscope (SEM) studies have been carried out on vapour grown arsenic triselenide platelet crystals and whiskers grown under argon and normal air ambients. The crystal morphology and microstructure were found similar in both types of crystals. While investigating the microstructure the layered structure of these crystals could be discerned under SEM. Layers with average thickness of 2–2.5 μm could be distinctly observed for the first time in As₂Se₃ crystals. This might be an important observation.     

Analysis of Fluid Flow and Heat Transfer in a Gas Phase Crystal Growth Furnace

Steady-state simulations of fluid flow and temperature field are presented for an equipment that is used to grow Zinc Selenide single crystals from the gaseous phases via physical (PVT) or chemical vapour transport (CVT). Due to the horizontal arrangement of the air-filled furnace pipe calculating the natural convection in the air requires a 3D (three-dimensional) treatment of the problem. The simulations have been done by applying the commercial finite-element package FIDAP. The Navier-Stokes equation is solved with the Boussinesq approximation. The heat transfer analysis comprises also internal radiation wall-to-wall exchange. Due to the presence of the ampoule in the pipe, the development of vortices with higher velocities is restrained, so that the maximum velocity is roughly 1/4 of that in the case without an ampoule.     

Crystal growth and morphology of hindered phenol AO-60

Crystal growth of a hindered phenol compound, tetrakis [methylene-3-(3-5-ditert-butyl-4-hydroxy phenyl) propionyloxy] methane (trade name AO-60), was successfully recorded by optical microscopy (OM) equipped with a hot stage. The morphology of AO-60 crystals, grown at 100 °C from amorphous state, appeared in the form of tetragonal-sloped step growth. Further study using scanning electron microscopy (SEM) and atomic force microscopy (AFM) experimentally demonstrated that AO-60 crystals had a hopper-like morphology, which had occurred rarely in the condition of organic compound crystals but predominantly rather to inorganic compound crystals in the reported literatures. The morphological features observed on the crystal surfaces suggested step growth and hopper growth mechanism. Besides, a raising around the edge of the AO-60 hopper crystal was also experimentally characterized for the first time.     

Growth and microstructural studies of some indium chalcopyrite semiconductors

Single crystals of ternary chalcopyrite semiconductors such as CuInS₂, CuInSe₂, CuInTe₂, and CuInSSe, a pseudo-ternary semiconductor, have been grown by Chemical Vapour Transport (CVT) technique using iodine as the transporting agent. Microstructural analysis was carried out with the help of Scanning Electron Microscope (SEM) and optical microscope for the grown single crystals.     

Growth of USb2 single crystals and their structural perfection

USb₂ single crystals were grown by three methods; I — Chemical vapour transport with iodine as transporting reagent, II — Crystallization from U Sb liquid solution, III — Crystallization from U Sb Sn liquid solution. The morphology of growth and results of X-ray topography examination of crystal surface are given.     

On the Surface Quality and Micromorphology of HgBrI Single Crystals

Mercuric Bromoiodide (HgBrI) is the only compound in the HgI₂—HgBr₂ system of solid solutions and it is a candidate material for solid state nuclear radiation detector applications, operating at room temperature. In the present paper we report on the surface morphology of the crystals, grown from the vapour phase, using optical microscopy and optical spectroscopy. This examination aims at the determination of the factors that influence the surface quality of the crystals.     

Distinction between topography of prism faces of natural and cultured quartz crystals

Results of detailed microtopographical studies on prism faces of a large number of natural and hydrothermally grown quartz crystals are described, illustrated and discussed. In the light of these results distinction between prism faces of the two varieties of quartz is assessed. Horizontal striations, parallelogram shaped and low angle tetrahedral growth pyramids are reported as characteristic features on prism faces of natural quartz crystals, whereas hexagonal and square shaped pyramids are shown to be characteristic features on prism faces of cultured quartz crystals. Evidence of independent growth on prism faces of both the varieties of quartz crystals are given. A wide variety of growth forms, like oriented snail shaped hillocks, pentagonal elevations, rectangular spiral patterns, screw ledges, missile-like structures, influence of impurities on advancing growth fronts, microdiscs patterns and etch patterns due to chemical etching are explained.     

Influence of gel inclusions on the mechanical behaviour of Gypsum

In a single test tube system, gypsum crystals grown in alcoholic gel comprise of interpenetration twinned crystals at the interface towards the gel, whereas untwinned tabular crystals are observed at greater depths inside the gel. By indentation of the crystals with a microhardness tool and recording the indentation impressions by means of a scanning electron microscope (SEM) and a transmission electron microscope (TEM) it is found that the hardness of the tabular crystals is of the same order of magnitude as those of natural crystals of gypsum, whereas interpenetration twinned crystals show a remarkable increase in hardness. On studying the morphology of cleavage faces and on carrying out energy dispersive X-ray analysis (EDXA), it is concluded that the increase in hardness of interpenetration twinned crystals with respect to tabular and natural crystals may be due to the role of the inclusion of gel particles during the growth. The implications are discussed.     

On crystal growth of ammonium and potassium dichromate. A study of the shubnikov effect

Growth and equilibrium forms of ammonium (A2/a) and potassium dichromate (P 1 ) have been calculated by means of the Fourier transform method of crystal morphology. Twin laws of potassium dichromate and heteroepitaxial intergrowth of both substances can be explained by partitions of space. According to Shubnikov the (001) planes of potassium dichromate crystals grown in aqueous solutions are always smooth, the parallel opposite planes, however, roughened. Wagner observed that this effect only occurs at temperatures shortly above or below 25 °C. The present investigation has shown that roughening of (001 ) appears at 25 °C after addition of potassium chromate to the potassium dichromate solutions. This means that the one-sided intergrowth can be explained by a stereospecific adsorption of polychromates. Solution additions like potassium permanganate and Chromotrope 2B generate roughenings on both {001} faces. X-Ray diffraction investigations can be interpreted in such a way that the intergrown plates are mirror symmetric oriented to two planes being normal to the a- and b-axes. This is in accordance with repeated formation of intergrown plates.     

Growth and Morphology of Hollow Bi2Te3 Whiskers by Physical Vapour Deposition Method

Hollow crystals of bismuth telluride have been grown by the physical vapour deposition (PVD) method. Whisker crystals as long as 10 mm have been grown and were identified by X-ray analysis. The possible mechanism for the growth of hollow whiskers has also been suggested.     

Stoichiometric single crystal growth of AgGaS2 by iodine transport method and characterization

High quality single crystals of ternary AgGaS₂ (AGS) semiconductor with chalcopyrite structure have been grown by chemical vapor transport (CVT) technique using iodine as a transporting agent at different growth zone temperatures. The powder X‐ray diffraction and single crystal X‐ray diffraction studies indicate that the as‐grown AGS crystals belong to the tetragonal (chalcopyrite) system with (112) plane as the dominant peak. The full width at half maximum (FWHM) of the X‐ray rocking curve for the as‐grown AGS single crystal is 5 arcsec. The energy dispersive X‐ray analysis (EDAX) and optical transmission spectra of as‐grown AGS single crystals grown at different conditions show the almost same composition and band gap (2.65 eV). Photoluminescence (PL) spectra of as‐grown AGS single crystals show prominent band edge emission at 2.61 eV. The resistivity of the as‐grown AGS single crystal has been measured. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Chemically assisted vapour transport for bulk ZnO crystal growth

A chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10⁻³ to 1 atm) and temperatures (800–1200 °C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46 mm in diameter and 8 mm in thickness, with growth rates up to 400 μm/h. These values are compatible with an industrial production rate. N type ZnO crystals (μ=182 cm²/(V s) and n=7 10¹⁵ cm⁻³) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10–30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] <10⁺¹⁵ cm⁻³). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented.