Solutal convection during growth of organic NLO crystals

Effect of solutal and thermal convection plays very important role when a large thermal gradient is applied during crystal growth. To address this problem, we have purified m.nitroaniline (m.NA) and m. dinitrobenzene (m.DNB) and studied crystal growth and effect of growth parameters on the optical quality. Crystals of pure and binary alloy of m.dinitrobenzene and m.nitroaniline were grown by vertical directional solidification method in a two zone transparent furnace. Effect of doping and temperature gradient on the solid-liquid interface morphology and quality of crystal was determined by studying the bulk transparency and nonlinearity.     

Soft organic and metal-organic frameworks with porous architecture: From wheel-and-axle to ladder-and-platform design of host molecules

The family of wheel-and-axle host molecules is reviewed and the approaches to the creation of new host geometries for supramolecular materials based on weaker interactions are discussed. The combination of bulky groups (or platforms) and spacers yields various host geometries including humming-top molecules (Werner complexes, metal dibenzoylmethanates and other bis-chelates, dimeric metal carboxylates and macrocyclic complexes), wheel-and-axle and dumb-bell shaped molecules and their modifications, ladder-and-platform, shish-kebab, multi-decker, stair-case and double-strand ladder-and-platform oligomeric and polymeric structures. The host shapes are compared with cyclic and trifoil (trityl) host types. The use of metal centers, chelating and macrocyclic ligands (β-diketones, carboxylic acids, Schiff bases, porphyrins, phthalocyanines, corroles, annulenes and their analogs), bridging ditopic ligands and other building elements in the engineering of host molecules is illustrated. The role of such factors as size and nuclearity of the platforms, rigidity/flexibility of the spacers and hydrogen bonding is discussed. The relation between porosity and packing efficiency, predisposition to self-inclusion, parallel alignment and interdigitation, dimensionality and secondary assembly of host molecules in the crystal is examined. Clathration and sorption abilities and other important qualities and functionalities of the new host materials are elucidated.     

MOMBE GaAs and AlGaAs for microelectronic devices

This paper reviews the state of the art of GaAs and AlGaAs materials and microelectronic devices grown by MOMBE (metalorganic molecular beam epitaxy) and related techniques. FET, HEMT and HBT devices have been grown using MOMBE and in the case of FETs and HBTs excellent device and MMIC power performance has been obtained. For example, MOMBE HBTs show current gains in excess of 100 and MMIC power HBT circuits are delivering 2 W of power at 8.5 GHz. We will examine device behavior and relate this to properties of the GaAs and AlGaAs materials.     

Progress in Modeling of III-Nitride MOVPE

This review provides an introduction to III-Nitrides MOVPE process modeling and its application to the design and optimization of MOVPE processes. Fundamentals of the MOVPE process with emphasis on transport phenomena are covered. Numerical techniques to obtain solutions for the underlying governing equations are discussed, as well as approaches to describe multi-component diffusion for typical regimes during MOVPE. Properties of common industrial MOVPE reactor types like close spaced showerhead reactors, rotating disk reactors and Planetary Reactors are compared in terms of underlying working principles and generic process parameter dependencies.The main part of the paper is devoted to reviewing gas phase and surface reaction mechanisms during MOVPE. The process design in particular for MOVPE of III-Nitrides is determined by complex gas phase reaction kinetics. Advances in the modeling and predicting of these processes have contributed to understanding and controlling these phenomena in industrial scale MOVPE reactors. Detailed kinetics and simplified surface kinetic approaches describing the incorporation of constituents into multinary solid alloys are compared and a few application cases are presented. Differences in thermodynamic and kinetic properties of multi-layered structures of different compositions such as InGaN, AlGaN can cause enrichment of the adsorbed layer by certain group III atoms (indium in case of InGaN and gallium in case of AlGaN) that translate into specific features of composition profiles along the growth direction.An intrinsic feature of III-nitride materials is epitaxial strain that shows up in different forms during growth and affects both deposition kinetics and material quality. In case of InGaN MOVPE there is a strong interplay between indium content and strain that has direct influence on distribution of material composition in the epitaxial layers and multi-layered structures. Epitaxial strain can relax via different routes such as nucleation and evolution of the extended defects (dislocations), layer cracking and roughening of the surface morphology. Simulation approaches that address coupling of growth kinetics with strain and defect dynamics are discussed and exemplified.     

Thin chalcogenide films prepared by pulsed laser deposition – new amorphous materials applicable in optoelectronics and chemical sensors

Principles, advantages, applications and drawbacks of pulsed laser deposition (PLD) technique for thin films preparation are reviewed. The PLD method is promising for preparation of thin films of complex composition, including rare-earth and Ag-doped chalcogenide films; all components of the target can be evaporated at once. Low volatility and refractory materials can be also deposited. The deposition in vacuum, inert or reactive atmosphere is possible. Results obtained in a study of chalcogenide films are discussed and the current state-of-the-art is reviewed. The composition and structure of PLD films can be different from thermally evaporated films and new materials or materials with new properties applicable in optics and optoelectronics can be prepared. The method can be used for fabrication of different chalcogenide-based sensors and memory materials of complex composition. Photoinduced changes of structure and properties of PLD films and chalcogenides exposed to intense laser pulses are also discussed. The intense laser pulses can change the properties of the materials prepared and can be used for fabrication of chalcogenide-based waveguides, diffractive elements and high-density optical data storage media.     

Photoluminescence characteristics of Mg- and Si-doped GaN thin films grown by MOCVD technique

We have studied the optical, structural and surface morphology of doped and undoped GaN thin films. The p- and n-type thin films have been successfully prepared by low-pressure MOCVD technique by doping with Mg and Si, respectively. The different carrier concentrations were obtained in the GaN thin films by varying dopant concentrations. Photoluminescence (PL) studies were carried to find the defect levels in the doped and undoped GaN thin films at low temperature. In the undoped GaN thin films, a low intensity and broad yellow band peak was observed. The donor–acceptor pair (DAP) emission and its phonon replicas were observed in both the Si or Mg lightly doped GaN thin films. The dominance of the blue and the yellow emissions increased in the PL spectra, as the carrier concentration was increased. The XRD and SEM analyses were employed to study the structural and surface morphology of the films, respectively. Both the doped and the undoped films exhibited hexagonal structure and polycrystalline nature. Mg-doped GaN thin films showed columnar structure whereas Si-doped films exhibited spherical shape grains.     

Fundamentals and engineering of defects

An overview of the important defect types, their origins and interactions during the bulk crystal growth from the melt and selected epitaxial processes is given. The equilibrium and nonequilibrium thermodynamics, kinetics and interaction principles are considered as driving forces of defect generation, incorporation and assembling. Results of modeling and practical in situ control are presented. Strong emphasis is given to semiconductor crystal growth since it is from this class of materials that most has been first learned, the resulting knowledge then having been applied to other classes of material. The treatment starts with melt-structure considerations and zero-dimensional defect types, i.e. native and extrinsic point defects. Their generation and incorporation mechanisms are discussed. Micro- and macro-segregation phenomena – striations and the effect of constitutional supercooling – are added. Dislocations and their patterning are discussed next. The role of high-temperature dislocation dynamics for collective interactions, like cell structuring and bunching, is specified. Additionally, some features of epitaxial dislocation kinetics and engineering are illustrated. Next the grain boundary formation mechanisms, such as dynamic polygonization and interface instabilities, are discussed. The interplay between facets, inhomogeneous dopant incorporations and twinning is shown. Finally, second phase precipitation and inclusion trapping are discussed. The importance of in situ stoichiometry control is underlined. Generally, selected measures of defect engineering are given at the end of each sub-chapter.     

Vapour phase growth of epitaxial silicon carbide layers

After a brief overview of different epitaxial layer growth techniques, the homoepitaxial chemical vapour deposition (CVD) of SiC with a focus on hot-wall CVD is reviewed. Step-controlled epitaxy and site competition epitaxy have been utilized to grow polytype stable layers more than 50 μm in thickness and of high purity and crystalline perfection for power devices. The influence of growth parameters including gas flow, C/Si ratio, growth temperature and pressure on growth rate and layer uniformity in thickness and doping are discussed. Background doping levels as low as 10¹⁴ cm⁻³ have been achieved as well as layers doped over a wide n-type (nitrogen) and p-type (aluminium) range.

Furthermore the status of numerical process simulation is mentioned and SiC substrate preparation is described. In order to get flat and damage free epi-ready surfaces, they are prepared by different methods and characterised by atomic force microscopy and by scanning electron microscope using channelling patterns. For the investigation of defects in SiC high purity CVD layers are grown. The improvement of the quality of bulk crystal substrates by micropipe healing and so-called dislocation stop layers can further decrease the defect density and thus increase the yield and performance of devices. Due to its high growth rate functionality and scope for the use of multi-wafer equipment hot-wall CVD has become a well-established method in SiC-technology and has therefore great industrial potential.     

同质外延生长ZnO单晶结构及光电性能的研究

本文报道了同质外延生长氧化锌(ZnO)单晶在高温氧气气氛退火前后的结构及光电特性。利用化学气相输运(CVT)法生长了红棕色的ZnO单晶,且进行高温氧气气氛退火处理后的ZnO单晶呈现无色透明状。通过X射线衍射仪(XRD)、X射线光电子能谱(XPS)、能谱仪(EDS)和拉曼(Raman)光谱测试分析了高温氧气气氛退火前后的ZnO单晶结构,讨论了退火对单晶内部缺陷类型及结构的影响。XRD测试表明ZnO单晶的生长方向为(002)方向。退火前后ZnO单晶的ω摇摆曲线半高宽分别为59″和31″,表明退火后单晶内缺陷显著减少;XPS和EDS能谱分析了退火前后ZnO单晶的成分和元素价态,结果表明高温氧气气氛处理后单晶内Zn和O元素含量比更接近理论值;通过Raman光谱分析了高温氧气气氛处理前后ZnO单晶的不同拉曼振动模式;通过紫外光谱数据分析,得到了退火前后ZnO单晶的光学禁带宽度分别为3.05 eV和3.2 eV;最后,通过Hall测试分析了高温氧气气氛退火处理前后ZnO单晶的电学性能参数,并深入讨论了退火前后ZnO单晶的低温电输运特性。     

Al粉粒度及其添加量对SiC陶瓷材料结构和性能的影响

以Al粉为烧结助剂,采用无压烧结工艺于1600℃下保温3 h烧制SiC陶瓷材料,研究了不同Al粉粒度及其添加量对SiC陶瓷材料结构和性能的影响.结果表明:Al粉可促进SiC陶瓷材料的烧结和力学性能的提高,同时起抗氧化的作用,且粒度较小的Al粉对其性能提升的幅度较大.当添加4wt;粒度为48μm的Al粉时,SiC陶瓷材料的性能较佳,体积密度和显气孔率分别为2.69 g/cm3和5.8;,显微硬度和抗折强度分别为2790 HV和189 MPa.SiC陶瓷材料烧结性能和力学性能的提高可归因于Al粉的促烧结作用,及其氧化产物Al2 O3和原位生成的少量莫来石分布在SiC颗粒间所起的强化作用.     

Influence of flexible chain,polar substitution and hydrogen bonding on phase stability in Shiff based (4)PyBD(4I)BrA-nOBA series of liquid crystals

Shiff based Linear Hydrogen Bonded Liquid Crystalline homologues, viz., (4)PyBD(4^I)BrA-nOBAs for n = 2,3,4,5,6,7,8,10,11 and 12 are synthesized. ¹H-NMR and ¹³C-NMR studies confirm the formation of HBLC and IR confirms the linear hydrogen bonding. Liquid crystal phase abundance, transition temperatures, enthalpy and order of transition are investigated by Polarized Optical Microscopy textural studies and Differential Scanning Calorimetry. Nematic, Smectic-A, -D, -C, B_hexatic, -B_Crystal, -F, -I and –G phases are identified. Large LC phase variances, viz., NAB_hexFG and NACFG are exhibited by intermediate chain length with n = 5 and 6. A-C transition exhibited unique II-order nature. II-order nature for G-Solid transition explained. Phase diagram drawn with flexible chain length infers INA, and ACF and CIF MultiCritical Points and Lifshitz behaviour. NA TriCiritical Point predicted in [3+7] binary. Influences of chain length, linear HB and end polar(-Br) substitution for LC phase stability are is discussed. Results of Phase stability are discussed in the wake of the body of the data and reports in other LCs. DSC enthalpy suggests for utility of intermediate and higher homologues for device savvy tilted Smectic LC phases.     

Structures of lanthanum and yttrium aluminosilicate glasses determined by X-ray and neutron diffraction

We have measured the structures of lanthanum and yttrium aluminosilicate glasses by X-ray and neutron diffraction and determined the interatomic distances and nearest-neighbor coordination numbers. The results obtained with the two techniques are in good agreement with each other and with recent NMR studies. The Si-O and Al-O coordination numbers are found to be 4 and 4.5, respectively. All the glasses show pronounced intermediate-range order that exhibits a reduced length scale with increasing La or Y content.     

Bowlic and Polar Liquid Crystal Polymers

Bowlic liquid crystals are made up of polar molecules. Both main-chain and side-chain bowlic polymers are possible. Exactly solvable discrete models describing the phases and conformations of these bowlic and other polar liquid crystal polymers are presented. For the ideal one-dimensional case the model is equivalent to the 1D Ising model. Susceptibility and other properties are calculated. Wave propagation and solitons in these polar polymers are discussed. Possible highly conducting and high T_c superconducting liquid crystal polymers are proposed.     

Growth of equiaxed dendritic crystals settling in an undercooled melt,Part 2: Internal solid fraction

Experiments are conducted to measure the internal solid fraction evolution of equiaxed dendritic crystals that are freely growing and settling in an undercooled melt using the transparent model alloy succinonitrile–acetone. The internal solid fraction is determined from the measured settling speed and crystal envelope shape and size. Depending on the melt undercooling and acetone concentration, the internal solid fraction is found to vary between 0.55 and 0.1. In all experiments, the internal solid fraction decreases continually during settling. Based on heat and solute balances, a model is developed for predicting the internal solid fraction evolution under convective conditions. Nusselt and Sherwood number correlations are obtained that allow for the calculation of the thermal and solutal boundary layer thicknesses at the crystal envelope. The measured and predicted internal solid fraction evolutions are found to be in good agreement.     

用数学模型计算生长大尺寸光学晶体加热系统结构 第2部分—在φ4 00mmCaF_2晶体生长中计算辐射-传导热交换的方法和结果(英文)

大尺寸氟化物晶体的生长是基于对晶体炉热交换的实验研究和计算结果 ,在晶体生长过程的不同阶段解决了复杂结构生长容器的边界条件和温场的二维计算任务。我们在这里给出了晶体生长过程中温场设置和转变的具体数据。所有的计算都是根据晶体、熔体 ,容器材料的光学特性与光谱和温度的关系以及它们的热物理值与温度的关系做出的。这些结果包括了迄今有关氟化物晶体生长系统和过程的最精确的数据 ,可用于生长技术工艺的发展以及晶体生长炉和容器的设计。     

Nonlocal Continuum Theory of Liquid Crystals

A nonlocal continuum theory of liquid crystals is constructed to explain and predict the physical behavior of liquid crystals under long range intermolecular forces Balance laws consist of conservation of mass and mocroinertia, balance of momenta and energy. Constitutive equations are given for the equibilibirium and non-equilibirium parts of the stress, couple strees, free energy, entropy an nonlocal body force and couple. Thermodynamic restrinctions and material frame-indifference are studied. The theory is valid for liquid crystals having arbitrary shapes (inertia), Passage is made to the thread-like molecuels and to local theory. Applications are considered to two-dimensional problmes, steady, plane shear flows and disperison of twist waves.     

少铅/无铅钙钛矿太阳能电池研究进展

钙钛矿太阳能电池作为一种新型的低廉高效的光伏材料在近年来备受关注。铅基钙钛矿太阳能电池有光电转换效率高、成本低、易制备等优点,然而由于铅的毒性以及由此带来的环境污染问题在很大程度上限制了其进一步商业化应用。因此人们开发出一系列新型无毒或低毒钙钛矿材料用于制备环境友好的少铅/无铅钙钛矿太阳能电池。本文简明扼要地介绍了钙钛矿材料的结构和形成条件,着重回顾了Sn基钙钛矿太阳能电池的发展历程,对ⅣA族元素Ge,ⅡA族元素Mg、Ca、Sr和Ba,ⅤA族元素Bi和Sb,ⅢA族元素In以及过渡金属元素Cu等取代或部分取代Pb,以及通过调节卤素X与正离子A的组成及配比来调控少铅或无铅钙钛矿材料性能进行了总结,对少铅/无铅钙钛矿太阳能电池器件研究进展进行了综述,并对其未来的发展方向进行了展望。     

基于新元素和新多层结构的MAX相陶瓷材料研究进展

由于独特的层状结构和原子间特殊的化学键合,MAX相陶瓷材料(化学式为M_n+1AX_n)兼具金属和陶瓷材料的优异性能,在很多领域具有广泛的应用前景,自20世纪60年代问世以来就一直备受关注。至今已经发现了100多种MAX相陶瓷材料,其中包括80余种单相以及一系列固溶体。传统的MAX相局限于一定的元素范围和若干M₆X层与单A原子层交替堆垛的结构。最近含有Au、Ir、Cu、Zn等新元素的MAX相材料的成功合成大大丰富了MAX相家族,多A层和多MA层结构MAX相的发现也打开了新型MAX相研究的一扇大门。随着理论计算的发展和实验条件的进步,越来越多的新型MAX相陶瓷材料逐渐出现在人们的视野中。本文综述了基于新元素和新多层结构的MAX相的国内外实验合成和理论研究进展,并指出了后续研究需要克服的问题,最后对新型MAX相的研究方向和发展趋势进行了预测和展望。     

Temporal electric conductivity variations of hydrogenated amorphous silicon due to high energy protons

Electric conductivity variations of undoped, n-type and p-type hydrogenated amorphous silicon (a-Si:H) thin films irradiated with various energy protons are systematically investigated. Dark conductivity (DC) and photoconductivity (PC) of the undoped samples increase at first due to proton irradiation and then decrease dramatically with increasing proton fluence. The increase in DC and PC becomes greater with increased proton energy. However, this increase is metastable and gradually decreases with time at room temperature. Similar results are observed in the n-type a-Si:H, whereas only a monotonic decrease is observed in DC and PC for the p-type samples. The increase of both DC and PC due to proton irradiation is attributed to metastable donor center generation. On further irradiation both the DC and PC decrease by the accumulation of radiation-induced defects, which act as deep traps and compensate carriers. The decrease in DC and PC becomes less pronounced as the proton energy increase and can be fitted along a universal line when the proton fluence is converted into displacement per atom (dpa).     

X-ray emission spectra and electronic structure of amorphous silicon

X-ray K and L emission bands of c-Si and a-Si are reported and compared with available XPS and UPS measurements. The experimental results for a-Si are found to be consistent and in excellent agreement. From comparison of the experimental results with available electron density-of-states calculations based on different structure models of the amorphous state, we conclude that only ST-12 structure and the Polk-Boudreaux model provide results that are compatible with experiment.Furthermore we have studied the high energy L satellite band of c-Si and a-Si.