Peculiarities Of Growth Research Articles

Time of flight mass spectroscopy of recoiled ions studies of surface kinetics and growth peculiarities during gas source molecular beam epitaxy of GaN

High growth rate GaN thin films were successfully grown by gas source molecular beam epitaxy and studied in situ by time of flight mass spectroscopy of recoiled ions (TOF-MSRI) and reflection high energy electron diffraction (RHEED). We show that TOF-MSRI allows for in situ monitoring and control of sapphire surface chemistry and its nitridation. In the latter case, TOF-MSRI is more sensitive to the surface changes during nitridation than RHEED. Using both RHEED and TOF-MSRI, growth of low-temperature GaN buffer layers was monitored, and their recrystallization and island-like nature were demonstrated. A model describing the probable growth mechanism for gas source molecular beam epitaxy of GaN is suggested. The model explains both the chemical dissociation of ammonia at low temperature and the origin of Ga to N TOF-MSRI peak ratio changes for various Ga and ammonia fluxes. High-resolution transmission electron microscopy studies confirm that GaN films grown with a buffer layer have excellent structural quality without any evidence of interfacial defects. Those without a buffer layer are highly defective.

New anisotropic rare earth fluorides BaR2F8 (R=Y, Dy–Lu): growth and characterization

Polymorphism of BaR2F8 compounds is analyzed, and some peculiarities of the crystal growth from the melt are discussed. We report on the orthorhombic BaLu2F8 crystal with ordered structure, a new crystalline material for generating Ln3+ activator ions. Spectroscopic and laser characteristics of Nd3+- and Er3+-doped BaLu2F8 samples are discussed as representative examples. Besides, we pay special attention to monoclinic BaY2F8:Er3+ crystals, as active media for highly efficient CW 3 μm laser-diode pumped lasers.

Silica transfer and β-quartz growth from supercritical aqueous fluids

Methods used to grow β-quartz crystals include, gas and hydrothermal high-pressure vessels (10–12 ml internal volume) and autoclaves (20, 75 and 100 ml internal volume). The crystals were grown on bar-like α-quartz seeds at temperatures up to 900°C and pressures from 20 to 500 MPa, under isothermal and thermal gradient conditions. Pure water and aqueous solutions of NaOH, K 2CO 3, NaCl, NH 4F, AlF 3, HF, Li 3PO 4 were used as solvents. The nutrient was similar quartz bars or amorphous silica. Impurity elements (Fe, Al, P, Ti, Ge etc.) were added to the nutrient in the form of oxides. The details of silica transfer during the crystal growth process, under supercritical conditions, have been analyzed. It was demonstrated for the first time that in pure water and nearly neutral or alkaline fluids, the direction of silica transfer always coincides with the direct temperature gradient. However, in acidic fluoride fluids and pressures below 80–100 MPa, the direction of silica transfer undergoes inversion and is opposite to the direct temperature gradient. This phenomenon is related to the nonmonotonous and relatively strong temperature and density dependence of the HF dissociation constant which affects quartz solubility. The intensity and direction of silica transfer is substantially dependent on temperature, temperature gradient, density and alkalinity of the solutions, as well as being dependent on the fluoride ion concentrations in acidic solutions. It was shown that only faces {10 1 ̄ 0} and {10 1 ̄ 1} , as well as higher indexed pyramidal {h0 h ̄ l} faces are stable under experimental conditions. The growth rates of faces {10 1 ̄ 0} and {10 1 ̄ 1} are about the same (∼0.02 mm day −1). Impurity elements, except for Ge, are scarce (∼0.001%) in the quartz structure. The results obtained provide an explanation for the growth peculiarities of β-quartz crystals in natural environments.

Growth of LaNiO 3 thin films on MgO by flash MOCVD

LaNiO 3 thin films on MgO were grown by flash MOCVD on single crystalline (0 0 1)MgO substrates using La(thd) 3, Ni(acim) 2 or Ni(thd) 2 (acim and thd are anions of acetylacetonimine and dipivaloylmethane, respectively) as volatile precursors. X-ray diffraction study of the films with different thicknesses was used to clarify the growth peculiarities. Two stages of growth were observed. At the beginning (0 0 1) oriented LaNiO 3 layer (pseudocubic indices) grows with uniform in-plane alignment. Next, another orientation, namely (1 1 0), aligned in-plane with (0 0 1) layer appears as auxiliary one. In-plane matching variants are discussed. Consistent explanation of the preferable in-plane orientations observed was achieved with a nearly coincident sites lattices (NCSL) analysis.

Peculiarities of spontaneous-grating formation in light-sensitive films under elliptically polarized laser radiation

The formation of spontaneous gratings in thin waveguide AgCl–Ag photosensitive films was studied under elliptically polarized laser radiation. New peculiarities of the gratings growth were found (i) a deviation in angular position of the wave vectors of the most probable microgratings from π / 2 about the major axis of the polarization ellipse; and (ii) an asymmetry, which depends on the ellipticity sign, in the diffraction and small-angle scattering patterns. The experimental results are explained within the framework of a model that shows the scattering of incident light by small prolate absorptive Ag spheroids oriented across the major axis of the ellipse and arising from the stage of photoinduced dichroism that precedes grating formation. We show that the dependence of the diffraction patterns' asymmetry on the ellipticity sign is related to an instability due to the angular position of the wave vectors of the most probable microgratings. PACS No. 78.65

Indium surface segregation during chemical beam epitaxy of and heterostructures

We report on the investigation of In surface segregation in GaInP/GaAs and GaInAs/GaAs heterostructures grown by chemical beam epitaxy (CBE). Owing to the peculiarities of CBE growth, it is shown that In segregation can be quantitatively evaluated in real-time by using reflection high-energy electron diffraction (RHEED). A segregation coefficient and its variation with the growth temperature is extracted from the RHEED data. It is used to determine the In composition profiles at the interfaces of GaInAs/GaAs quantum well (QW) structures as a function of the growth temperature. A good agreement is found between optical transition energies calculated from these profiles and the experimental photoluminescence (PL) energies. PL energy shifts observed in GaInP/GaAs QW's as a function of the growth temperature are qualitatively explained by In segregation.

Metalorganic chemical vapour deposition of high quality LuBa 2Cu 3O 7 −δ thin films Peculiarities of growth and superconducting properties

Epitaxial LuBa 2Cu 3O 7−δ films were prepared by flash evaporation metalorganic chemical vapour deposition (MOCVD) on LaAlO 3, SrTiO 3 and ZrO 2(Y 2O 3) substrates. The highest T c and j c (77 K, 100 Oe) values achieved were 89 K, 2.7·10 6 A cm −2, 88 K, 1.5·10 6 A cm −2 and 87 K, 1.0·10 6 A cm −2, respectively. An influence of p(O 2) (at the deposition temperature T=800 °C) on structural and superconducting characteristics of LuBa 2Cu 3O 7−δ films was found to be similar to that in the case of YBa 2Cu 3O 7−δ, in spite of the difference in morphology features and j c( T) dependencies. Occurrence of secondary phases inclusion in LuBa 2Cu 3O 7−δ films was analysed by XRD and TEM. The “fully oxygenated” LuBa 2Cu 3O 7 was found to be overdoped: an increase of T c value by 3 K was observed after an annealing of the films at reduced p(O 2).

Modelling of pyrolytic laser-induced deposition

The growth of deposits in pyrolytic laser-induced chemical vapor deposition is modelled using a finite-element technique. Numerical results are compared with experimental observations in various systems. Different peculiarities of growth can be explained.

Fuegian floating tree-ring chronology from subfossil Nothofagus wood

The discovery of subfossil Nothofagus wood buried in several peat bogs of Argentine Tierra del Fuego is reported. A provisional, fragmented-floating tree-ring width chronology covering the last 1400 years has been obtained. Statistical and spectral analysis revealed no significant differences between the present and floating Tierra del Fuego tree-ring chronologies. Some cyclical growth peculiarities found in both modern and floating chronologies are discussed.

Reactive sputtering of Ti target with nonneutralized Ar beam

Ion-beam reactive co-sputtering system with separate metal targets of Pb, La and Ti was used to study peculiarities of film growth under different sputtering energies. Despite the use of a single ion gun to sputter exclusively the Ti target, the deposited films were polluted with foreign elements in the form of a uniform Pb film and drop-like islands of Pb and La. The contamination of uniform film with Pb film could be eliminated by increasing the energy of sputtering ions, whereas the presence of foreign metal islands remained. The roles of both spreading the incident beam and arc-effect are discussed.

MBE growth physics: application to device technology

We present in this review a consistent picture of the current status of epitaxial growth physics, as related to molecular beam epitaxy. After a short introduction in the field we describe the different growth modes and the attributed theoretical models. Among the simulation methods used presently to describe the different growth modes, we selected the Monte-Carlo simulation for a more detailed example. The experimental part of the paper is divided in to two main sections of lattice-matched and lattice-mismatched systems. In the first part, we concentrate on the growth mechanisms on nominally oriented substrates and substrates with vicinal surfaces, which includes the description of the growth of tilted and serpentine superlattices. In the second part we discuss the concept of the critical layer thickness together with island formation and strain-induced effects. Besides these two main topics we report on the application of surface-active species, which provide a new avenue to achieve high-quality man-made microstructures against thermodynamic odds. Within this context self-organisation of epitaxial deposits is discussed. Finally, we describe material-related growth peculiarities, which are usually connected with a specific property of the constituent element, like volatility or extraordinary chemical reactivity. We briefly present the currently known growth characteristics for IV-IV heterostructures, wide-gap III–V and narrow-gap II–VI compounds.

Growth of high temperature β-quartz from supercritical aqueous fluids

Methods have been developed for growing of β-quartz crystals using gas and hydrothermal high-pressure vessels (10–12 ml internal volume), and autoclaves (20, 75, and 100 ml internal volume). The crystals were grown on bar-like α-quartz seeds at temperatures from 580 to 900°C and pressures from 0.5 to 5 kbar under isothermal and thermal gradient conditions. Pure water and aqueous solutions of NaOH, K 2CO 3, NH 4F, AlF 3, HF, Li 3PO 4, etc., were used as solvents. The nutrient was similar quartz bars or amorphous silica. Impurity elements (Fe, Al, P, Ti, Ge, etc.) were added to the nutrient in the form of oxides. It is shown that only the faces {{10 1 0}} and {{10 1 1}}, as well as higher-indexed pyramidal {{ h0 h l}} stable under the experimental conditions used. The growth rates of the {{10 1 0}} and {{10 1 1}} faces were about the same (∼ 0.02 mm/day), giving rise to isometric dipyramidal or prismatic habits. At temperatures above 600°C noticeable growth was observed under the temperature gradient conditions used even from fluids of rather low (0.05 to 0.15 g/cm 3) density. The intensity and direction of silica transfer substantially depended on temperature, temperature gradient, density and the alkalinity of the solutions, as well as on the fluoride ion concentration in acidic solutions. Impurity elements, except for Ge, were low (∼ 0.001%). The results obtained in these experiments can provide an explanation for the growth peculiarities of β-quartz crystals in miarole pegmatites and gas cavities of volcanic rocks.

Peculiarities of tumor growth in pregnant rats

The development of a transplantable tumor (Zajdela ascitic hepatoma) is studied in pregnant rats. Pregnancy is shown to retard tumor growth and influence its course. It prolongs the survival of tumor-bearing rats and induces total regression of the tumor in 15–20% of cases; pregnancy causes the ascitic growth to be transformed into a solid form, and prevents the development of hemorrhagic ascites.

The Study of Whisker Growth by the Monte Carlo Method

The peculiarities of tungsten whisker growth by the diffusion-dislocation and vapour-liquid-solid mechanisms are studied. The model accounts for the basic physical causes of whisker growth in halogen lamps observed in experiments. The simulation by the Monte Carlo technique takes into account local supersaturation fluctuations, caused by certain impurities, their frequency of appearance and their duration. The role of impurities in each of the growth mechanisms is shown and proved by the morphology of whiskers obtained in experiments. [Russian Text Ignored].

Mechanisms of brittle fracture of rock with pre-existing cracks in compression

Fracture of rocks containing a multitude of pre-existing cracks is considered from both theoretical and experimental points of view, paying attention mainly to the underlying mechanisms. The competition between a number of mechanisms in producing tear or shear type fractures is discussed in relation to the properties of the rock and the system of pre-existing cracks on the one hand and the type of loading on the other hand. First, 2-D theoretical models and experimental results aimed at the explanation and description of brittle fracture under compression are considered. Their insufficiency and the necessity to address the 3-D peculiarities of crack growth in rock are shown on the basis of new experimental results on 3-D crack propagation in transparent rock-like brittle materials under uniaxial compression. The results show that in contrast to the 2-D case, a single 3-D crack cannot propagate any appreciable distance and the loading results in dynamic, burst-like failure of the sample. Possible mechanisms of the routinely observed extensive fracture propagation in rock samples (splitting), as well as the possibility of shear (oblique) fracture in uniaxial compression, are discussed in connection with these experiments.

The growth of Li2B4O7 single crystals and investigation of low temperature elastic anomalies

The growth installation and peculiarities of growth Li 2 B 4 O 7 (LBO) crystals by Czochralsky method have been described in this paper. Experimental results of temperature dependences of ultrasonic velocities for various modes and crystallographic directions were measured and some anomaly in this dependence was discovered. Our results are discussed in comparison with some previous data

Characterization of a single SA7-like VA RNA gene in subgroup F adenoviruses.

The virus-associated (VA) RNA gene regions of the human subgroup F adenoviruses (types 40 and 41) were amplified using primers corresponding to flanking open reading frames of human and simian (SA7) adenovirus sequences previously published. The subgroup F adeno-viruses, like human Ad12 (subgroup A) and SA7, were found to have only one VA RNA gene at this locus (map unit 30). The type 40 and type 41 VA RNA genes have primary sequence characteristics in common with other known VA RNA genes, have high cross-identity (93%), and show appreciably higher identity to the single VA RNA gene present in SA7 (77% and 81%, respectively) than to any of the VA RNA genes of Ad2, Ad7 or Ad12 (< 61%). These findings may have implications for explaining the evolution and growth peculiarities of human subgroup F adenoviruses.

Static domain structure of ferroelectric flux-grown KTiOPO4crystals

Abstract It is found that static domain structure in different growth pyramids and crystal cuts of flux-grown KTiOPO4 ferroelectric crystals can be visualized with high resolution and studied in detail by nematic liquid crystal method. The domain structure decorates the peculiarities of the crystal growth and corresponds, in the main, to the layers of growth. Head-to-head and tail-to-tail domain walls exist due to high ionic conductivity of the crystal in the region, of domain formation.

Preparation and Properties of Gallium Phosphide Doped by Rare-Earth Elements

ABSTRACTGaP single crystals have been grown from 5at.% solution of P in Ga with (0.01-0.1)at.% of La, Sm or Gd as dopants. Electric properties, photoconductivity and luminescence of the crystals have been investigated by standard methods.Energy levels of impurities, mobility and concentration of current carriers have been determined in wide temperature region. It was shown that nitrogen (N) and other uncontrolled impurities present in the crystals due to some peculiarities of crystal growth. The intrinsic RE impurity luminescent spectra can be seen only together with spectra of excitons bounded on N traps. The evaluation of quantum eficiency for RE impurity made on the base of luminescence decay measurements gives the value close to 1. The interpretation of GaP:RE luminescence spectra is presented as intrashell transitions activated by bound exciton recombination as well as D-A and D-valency band transitions between some contaminating impurity (donor) and RE element (acceptor).

Basic problems of vertical Bridgman growth of CdTe

Many efforts have been made to grow CdTe bulk crystals with a low defect content but improvements are limited. The best ingots with large extended single-crystal regions can be grown by the vertical Bridgman method. However, fundamental studies about the CdTe growth peculiarities are absent. Our investigations are concentrated on the following problems: (i) the influence of the melt structure, from which an associated state is assumed, on the crystalline quality, (ii) the composition instability in conventional ampoules, (iii) the segregation behaviour of the excess component (normally tellurium), (iv) the axial distribution of inclusions and precipitations, (v) the mass transport in modified ampoules with an additional cadmium source, (vi) the correlation between the vacancy and impurity segregation and (vii) the substrate purity as a function of the axial crystal position.