Beryllium doping and silicon amphotericity in (1 1 0) GaAs-based heterostructures: structural and optical properties

The use of beryllium as an acceptor at high doping levels in (1 1 0)GaAs-based heterostructures is found to be deleterious to the structural and optical properties of these epi-layers. This may limit the use of beryllium as a p-type dopant on the (1 1 0) surface. Because silicon is amphoteric on the (1 1 0), it can be used as an alternative p-type dopant, in addition to its traditional role as an n-type dopant. Transmission electron microscopy, optical absorption, and luminescence data indicate that high quality multiple quantum well structures with p-type GaAs buffer layers doped with silicon, rather than beryllium, can be grown.     

Crystal chemistry of silicates with three-layer <Emphasis Type="Italic">TOT</Emphasis> and <Emphasis Type="Italic">HOH</Emphasis> modules of layered,chainlike, and mixed types

In view of new data on the chemical composition and structure of a series of natural layered silicates containing three-layer blocks and their corresponding microporous minerals with related fragments, their structural features; the structural conditionality of their properties; their transformation in the presence of isomorphism, decationation, and hydration; and the problems related to typomorphism and some other questions urgent for modern mineralogy have been considered. The structures are described in terms of threelayer modules of different forms and dimensions that are present in these structures. The composition and design of the general structural base of related series—three-layer modules forming layered or linear radicals—are also discussed. The minerals of these families differ in chemical composition, symmetry, and unit-cell parameters, as well as in the topological features associated with the type of condensation of the Si tetrahedra into layers or ribbons and the degree of condensation of M polyhedra in the central part of blocks: from layers and octahedral ribbons of different widths to chains and isolated polyhedra.     

Macro-segregation, dynamics of interface and stresses in high pressure LEC grown crystals

High dislocation density and strong dopant inhomogeneities have been found in high pressure liquid-encapsulated Czochralski (HPLEC) grown crystals. The origin and underlying mechanisms of these defects are attributed to the complex nature of transport phenomena in the HPLEC system. Our integrated computer model (MASTRAPP) can simulate this process by calculating the flow and heat transfer in both the melt and the gas, and thermal-elastic stress in the crystal. In this work, this model has been further extended to investigate the development of thermal stress in the growing crystal and the redistribution of dopant in the melt. The results for InP growth show complex gas flow and heat transfer pattern in the system. Two large stress spots are predicted by the model, one at the edge of the crystal just above the encapsulant layer and the other in the top corner of the crystal. Although the stress always remains largest at the first location, its value decreases as the crystal grows, due to the enhanced cooling of the crystal. A curved crystal/melt interface is also found to introduce high thermal stresses in its vicinity, which may be dangerous because of a high temperature at the interface and thus a low strength of the crystal. The model also predicts both radial and longitudinal dopant segregation in the growing crystal, and shows that the dopant redistribution in the melt is caused by the complex flow pattern in the melt. This is the first time, that a strong radial dopant segregation has been predicted based on a comprehensive flow model for a HPLEC growth.     

Morphology and Dopant Distribution in Polyacetylene

We report the results of scanning electron microscopic stadies of the morpology and dopant distribution in polyacetylene, (CH)_x. A variety of fibrillar and rod-like morphologies were observed for trans-(CH)_x and (CH)_x doped with iodine and AsF₅. Results show that iodine and atrsenic distributions are uniform across the surface of the films to a resolution of 5000 A, with no significatn difference in the concentration observed on the dull and shiny sides of the film. Hence any inhomogenieties in the dopant distribution must be on a finer scale. The fiberillar morphology was observed through the bulk of a film even after doping with iodine. Swelling of the fibrils is seen upon doping, with larger effects observed in the AsF₅ doped samples.     

Frequency Dependence of The Conductivity and Dielectric Constant of Ch(Ircl6) Y and Ch(I)Y

Abstract

We have measured the ac properties of polyacetylene doped with H2IrCl₆ 6H₂O and I₂ from 10⁵ to 10⁷ Hz at room temperature. We find in both cases that the conductivity is only weakly dependent on frequency, in agreement with previous work.^1,2 However, we find that the dielectric constant of CH(IrCl₆)_y is anomalously large ?10⁴. We suggest that the large dielectric constant can be modeled by an effective medium theory assuming that the CH(IrCl₆)_y is a mixture of conducting and insulating regions. TEM evidence in support of the model will be presented. The results of CH(IrCl₆)_y will be contrasted with CH(I)_y where TEM evidence shows a uniform distribution of dopant in the polyacetylene and the dielectric constant is significantly lower.     

Structures of Complex Crystals of Alkylammonium Salts with Aromatic Molecules

Abstract

The complex crystal structures of dodecyltrimethylammonium chloride (DTAC) with catechol and hydroquinone were analysed by an X-ray diffraction method. Both complexes have isomorphous layered structures. The guest molecules locate between the interdigitated host molecules. Crystal structures are stabilized by mainly hydrogen bonds including water molecules. A cross-section balance between hydrophilic and hydrophobic parts is important for an energetically stable packing. DTAC can form the crystalline complexes with catechol and hydroquinone by both crystallizing from the solution and mixing host and guest powders in a mortar. In addition, DTAC can also make a complex with resorcinol. Powder diffraction pattern indicates that this complex has similar layered structure with complexes of DTAC / catechol and DTAC / hydroquinone. However, it is unstable in atmospheric condition.     

Oxygen and hydrogen profiles and electrical properties of unintentionally doped gallium nitride grown by hydride vapor phase epitaxy

Commercially available hydride vapor phase epitaxy gallium nitride (GaN) is characterized with the aim to correlate the oxygen and hydrogen secondary ion mass spectrometry profiles of a GaN wafer with the electrical properties of the sample. A GaN layer model, including doping profile and mobility, is derived, utilizing electrical (capacitance–voltage, Hall), structural (high resolution X‐ray diffraction) and optical (polarized infrared spectroscopy) methods. Oxygen and hydrogen are easily incorporated during hydride vapor phase epitaxy growth of GaN. Oxygen is an n‐type dopant in GaN, whereas hydrogen may passivate some of the donors. Electrical and optical properties correlate with a low defect concentration top GaN layer and a high defect concentration GaN interlayer.     

Y2O3掺杂ZrO2配合物的分子结构和光谱特性的理论研究

本文应用密度泛函理论指导人们深入理解氧化锆溶胶中分子的结构和光谱性能,探讨了溶胶中锆配合物的结构参数、光谱特性和原子的Mulliken 电荷布局. 理论结果表明掺杂引起粒径尺寸减少是由于掺杂引起氧桥聚合速度减慢和颗粒间吸引力的减弱.此外,振动光谱分析表明锆配位前驱体的有序化和第二相掺杂剂的引入明显减少了单斜氧化锆的特征光谱.理论分析结果很好地符合了实验结果.     

Structural and energetic aspects of the differences between real and predicted polymorphs

In crystal structure prediction simulations based on lattice energy minimization, usually hundreds of structures within a reasonable range of lattice energy and density are found, whereas in practice, it is very rare to find more than a few polymorphs of the same compound. In the work presented here, this discrepancy is investigated from a structural and energetic point of view. 56 crystal structures of 26 polymorphic mono‐ and disubstituted aromatic compounds, extracted from the Cambridge Structural Database, have been analysed with respect to inter‐polymorphic structural similarity. For comparison, potential crystal packing arrangements of the substances have been predicted with molecular mechanics simulations using a generic force field. The predicted structures are analysed with respect to structural features and similarity, and with respect to the number of structures and their lattice energy. It is found that the real polymorphs studied in this work tend to be structurally quite dissimilar with regard to hydrogen bonding and spatial packing of structural motifs, while many of the predicted structures of a given compound are very similar to each other. The results suggest that structure and lattice energy alone cannot explain why so few polymorphs are found in practice compared to the very large numbers predicted in simulations. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Predicting the changes of relaxation dynamics with various modifications of the chemical and physical structures of glass-formers

One merit of the coupling model (CM) is its relevance to experiments as demonstrated by its ability to rationalize, explain, and sometimes predict experimental data, particularly those on dynamics. It also predicts the changes of relaxation dynamics with various modifications of the chemical and physical structures of glass-formers. The modifications considered include application of pressure, nano-confinement, crosslinking, copolymerization, mixing with another glass-former, and various changes including molecular structure, tacticity, and molecular weight. The relaxation dynamics addressed include the secondary Johari–Goldstein β-relaxation, and the primary structural α-relaxation, and also the chain modes in the case of polymers. The changes of these relaxation mechanisms and their relations to each other with modification of chemical and physical structures are predicted and the predictions compare favorably with experimental data.     

Validation of strong magnetic field asymptotic models for dopant transport during semiconductor crystal growth

This paper presents a numerical solution for the unsteady transport of a dopant during the growth of a semiconductor crystal from a melt with an externally applied magnetic field. This solution confirms the results of an asymptotic model. Both solutions show that at every time during the growth of the crystal, the dopant distribution (1) is very nonuniform throughout the melt, and (2) is far from the instantaneous steady state. The present numerical solution for an arbitrary mass Péclet number Pe_m and an arbitrary Hartmann number Ha predicts a dopant distribution in the crystal, which agrees remarkably well with the dopant distribution predicted by the asymptotic solution for Pe_m 1 and Ha 1. The maximum difference between the crystal compositions predicted by these two different approaches is less than 4% for the range of magnetic field strengths considered.     

Structural inhomogeneities and magnetic properties of Co/Cu multilayer films

The polycrystalline Co/Cu multilayer films are prepared by magnetron sputtering onto Si/SiO₂ substrates. The static magnetization and the ferromagnetic resonance (FMR) spectra of these films are investigated. The microscopic cross-sectional structure of the films is examined using transmission electron microscopy. It is demonstrated that the differences in the magnetization curves and the ferromagnetic resonance spectra are associated with the specific structural features governed by the technological parameters of the film sputtering. The possible structural features that arise during the formation of layered structures and are responsible for the appearance of additional lines in the FMR spectra are discussed.     

1,2-Dithiolene complexes of transition metals-structural systematics and physical properties

The crystal structure analyses along with other physical measurements of 1,2-dithiolene complexes have made it possible to derive many interesting properties of these complexes. This review focuses attention mainly on the structural aspects and physical properties of 1,2-dithiolene complexes of transition metals. Brief mention of the structural aspects of complexes such as 1,2-dithiooxalate complexes and dithiosquarate complexes is included. In addition, structural aspects of a few available non-transition metal complexes are briefly discussed. The idea that bulky counterions lead to nonplanar structures has been disproved by the nonplanarity observed in the anion of [(CH₃)₄N]₂[Cu(S₂C₂(CN)₂)₂]. The analysis of the significance of overall charge shows that the ionic effect is prevalent in [M(dithiolene)₂] ^n– systems. Crystal structure analyses of some bis(dithiolene) complexes of transition metals along with conductivity measurements have shown that it is possible to correlate the electrical behavior to the crystal structure, provided the sample crystals are good. The two-dimensional network structure of the mixed valence compound, [(n-C₄H₉)₄N]_0.29 [Ni(S₂C₂S₂CS)₂], proposes the possibility of reaching the ultimate goal (superconductivity) with this type of compounds.     

New Structural Phases of Polymer Battery Anode Materials: Alkali-Metal-Doped Polyacetylene and Polyphenylene

New ordered phases of alkali-metal complexes of polyacetylene and poly(p-phenylene) have been identified by x-ray diffraction studies and by quasi-equilibrium measurements of potential vs. composition during electrochemical reduction with Li+, Na+, and K+ counterions.     

Light Scattering and Contrast in Thermally Addressed Liquid Crystal Displays

The effects of structural order on the light scattering characteristics of liquid crystal display cells containing smetic p-n-octyl-p′-cyanobiphenyl have been investigated. The scattering characteristics of different structures in the cell due to surface treatments are qualitatively analyzed for their effects on the contrast ratio when used in the thermally addressed liquid crystal display (TALC). Of the four structures investigated, i.e., homeotropic, fan-like, uniaxial rods and spherulites, the spherulitic structure has been found to exhibit the scattering profile best suited for high contrast because of the near-zero scattering near the incident beam. Based on the light scattering theory for spherulites, optimization of structural parameters such as size and order of spherulites, and device design such as collecting angle of projection lens for maximum display contrast has been predicted. Methods to improve contrast have also been discussed.     

Extrinsic doping of CdxHg1−xTe— A review

Extrinsic doping by elements which are stable to subsequent processing will become increasingly important in future infra-red device structures based on Cd_xHg_1−xTe. This paper reviews the incorporation and activation of dopants in the most widely used bulk and epitaxial growth methods. Stoichiometry at the growth temperature is shown to be the critical factor affecting dopant activation. Various factors, including stoichiometry, can affect the as-grown electrical properties and the importance of determining the type of conduction in the as-grown state, if successful extrinsic doping is to be accomplished, is stressed. Data on dopant segregation behaviour, in growth from liquids, acceptor ionization energies and carrier lifetimes are also presented and their importance is discussed.     

Investigation of the effect of aluminum doped zinc oxide (ZnO:Al) thin films by spray pyrolysis

ABSTRACT

In this study, aluminum doped zinc oxide thin films were deposited on ordinary glass substrates at the temperature of 425°C by spray pyrolysis technique for various doping concentrations of aluminum ranging from 1 to 5 at.%. The effect of dopant concentration on structural, morphological and optical properties of ZnO:Al thin films was studied. Optical band gap of the films increased with doping percentage and started to decrease from 2 at.% of dopant. The average transmittance for 2at.% ZnO:Al film is significantly increased over 90% in the visible region at 450 nm which is crucial for optoelectronic applications.     

Structure of Doped Polymers

Studies of the synthesis, molecular weight, structure and morphology of polyacetylene (PA) indicate that the average length of linar chain segments between structural defects like crosslinks and chain folds is of the order of 20 units. Several models for the crystal structure of trans PA are given. The influence of the morphology on the mechanism of charge tranport is discussed. The doping is found to be inhomogeneous and the macroscopic conductivity is controlled by the topology of interlamellar contracts.     

A Comparison of the High Resoluton Crystal Structures of the π- Molecuar Complexes of Anthracene and Phenothiazine with Pyromellitic Dianhydride

The π-molecular complexes formed by aromatic molecules with three rings tend to form crystals of high purity and are therefore ideal for solid state spectroscopic and photocurrent studies. We have therefore undertaken an investigation of the detailed structures of a number of such complexes in order to facilitate the interpretation of such data and other physical properties and in order to better understand the large changes in physical properties generated by small changes in composition and molecular packing. The piezoreflection spectra of the anthracene-pyromellitic dianhydride complex (A-PMDA) has been studied by Eckhardt and Merski (1973). The photo-current excitation spectra of A-PMDA was reported by Karl and Ziegler (1975) and more recently the preparation, optical properties, absorption and photoconductivity properties of phenothiazine-pyromellitic dianhydride have also been studied. (Anthonj, Karl, Robertson and Stezowski, 1979).     

Systematic,properties, and structure predictions of new borate materials

New borates with the non‐linear optical properties, obtained and investigated in Moscow State University in the last years, are reviewed. The positions of compounds in systematic were determined including complicate cases using topology and symmetry analysis of anionic radicals with the separation of blocks, typical for borate groups. Such approach revealed structural correlations between borates and silicates and led to prediction of new structures, two of which are realized. The origin of properties is connected with the highly polarizable electron density on lone pair or asymmetric bonds, presented in the structures. Experimental observation electron density distrubution has been made for Pb‐hilgardite, a crystal with the high optical non‐linearity. Different approaches to the structure‐properties relation are discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)