Growth of single-crystalline GaN layers in a horizontal reactor by chloride epitaxy

Chloride epitaxy of GaN layers in a horizontal reactor is studied numerically. The steady 3D fluxes of the gas mixture in the reactor are simulated with allowance for heterogeneous reactions on the substrate (growth of epitaxial GaN layers) and on the reactor walls (growth of a polycrystalline GaN deposit). Experimental data on the growth rate distribution for polycrystalline and epitaxial GaN layers are explained. It is shown that, if the diameter of the reactor is not large enough, the growth of the deposit on the walls makes the GaN growth rate distribution over the substrate more nonuniform due to the parasitic diffusion of reagents from the gas phase to the reactor walls.     

Effect of nanoporous GaN templates with different pore diameters on the subsequent thick GaN layers by HVPE

A set of GaN films were overgrown by hydride vapor phase epitaxy (HVPE) on nanoporous GaN templates with different pore diameters. These samples have various properties as seen from the measurements of X-ray diffraction (XRD) and photoluminescence (PL). Cross-sectional observations under a scanning electron microscopy (SEM) reveal that the overgrowth mechanism and process are strongly related to the dimension of nanopores, indicating that an optimum diameter exists for the properties of subsequent HVPE–GaN layers. When the diameters of nanopores are less than the optimum value, the pores on top of GaN templates can be left, and the properties of HVPE–GaN films show significant improvement. In contrast, the pores are almost stuffed with HVPE–GaN films, which obviously limit the improvement degree of HVPE–GaN films.     

On the comparative assessment of ratcheting-induced dislocation density in 42CrMo4 steel by X-ray diffraction profile analysis and hardness measurement

The phenomenon of ratcheting occurs under the influence of non-zero mean stress during cyclic loading; it singificantly reduces the low cycle fatigue life of engineering structures. The present investigation deals with a detailed comparison on the estimation of dislocation densities in the 42CrMo4 steel induced by ratcheting using two different methods, i.e. X-ray diffraction (XRD) profile analysis and hardness. The dislocation densities in the ratcheted specimens were assessed using XRD profile analysis following the modified Williamson–Hall method as well as hardness measurements following the modified Nix and Gao model. The results showed that dislocation density increased in the ratcheted specimens as compared to the unratcheted ones and increase in accumulation of ratcheting strain was correlated with the increase in dislocation densities in the ratcheted specimens. It was established that both hardness and X-ray diffraction profile analysis methods can very effectively be used to assess the dislocation densities in the ratcheted specimens.     

Evolution of a dislocation structure in the surface layers of single crystals during high-temperature annealing

The dislocation structure of KCl crystals annealed in a forevacuum has been shown to display a nonmonotonic variation of the density of dislocations in the direction from the surface. Our experiments and estimates showed that the observed redistribution of dislocations near the surface is due to the diffusion of impurities from the atmosphere and the generation of dislocations in the diffusion zone.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 48–52, September, 1987.     

用同步辐射X射线衍射技术分析GaN/Si外延膜的结构与应变

选用有AlN和AlGaN缓冲层的GaN/Si作为测试样品,采用同步辐射X射线衍射(SRXRD)技术对样品外延膜(GaN)的几何结构、晶格常量及其应变进行了分析.结果表明,同步辐射X射线衍射实验可以作为一种有效的技术手段,测试固体结构及应变.     

Defects in the GaAs and InGaAs layers grown by low-temperature molecular-beam epitaxy

The problem of defect formation in the GaAs and InGaAs layers grown by low-temperature molecular-beam epitaxy is discussed. The effect of growth conditions (temperature and flux ratio between the elements of groups III and V) on the morphology of growth surface, internal structure, type, and concentration of electrically-and optically active defects is analyzed. A comparison is made between the defect formation processes occuring during the epitaxial growth and post-growth annealing of the layers. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 63–72, December, 2006.     

Threading dislocation density comparison between GaN grown on the patterned and conventional sapphire substrate by high resolution X-ray diffraction

GaN epifilms are grown on the patterned sapphire substrates (PSS) (0001) and the conventional sapphire substrates (CSS) (0001) by metal-organic chemical vapor deposition (MOCVD) using a novel two-step growth. High resolution X-ray diffraction (HR-XRD) is used to investigate the threading dislocation (TD) density of the GaN epifilms. The TD density is calculated from the ω-scans full width at half maximum (FWHM) results of HR-XRD. The edge dislocation destiny of GaN grown on the PSS is 2.7×108 cm-2, which is less than on the CSS. This is confirmed by the results of atomic force microscopy (AFM) measurement. The lower TD destiny indicates that the crystalline quality of the GaN epifilms grown on the PSS is improved compared to GaN epifilms grown on the CSS. The residual strains of GaN grown on the PSS and CSS are compared by Raman Scattering spectra. It is clearly seen that the residual strain in the GaN grown on PSS is lower than on the CSS.     

Thermal analysis of GaN laser diodes in a package structure

Using the finite-element method,the thermal resistances of GaN laser diode devices in a TO 56 package for both epi-up configuration and epi-down configuration are calculated.The effects of various parameters on the thermal characteristics are analysed,and the thicknesses of the AlN submount for both epi-up configuration and epi-down configuration are optimized.The obtained result provides a reference for the parameter selection of the package materials.     

On the use of an external reference sample in the X-ray diffraction analysis of epitaxial layers

The differences in the high-resolution X-ray patterns for measurement of the crystal-lattice constant with and without the use of an external reference sample are discussed. The calculation procedures in the measurement of the lattice constant are compared. The results of the measurements of a Si(111) test crystal using a Bruker D8 Discover diffractometer are presented, and the examples of the use of this crystal as an external reference are described.     

Numerical analysis of bulk conical waves in anisotropic cylinders; application to stiffness tensor measurement

A point source-point receiver technique, based on laser generation and laser detection of acoustic waves, allows determination of mechanical properties of an anisotropic cylinder. The nature of the material and the geometry of the sample give a dispersive behaviour to the diffracted waves and make the acoustic signature difficult to interpret. To overpass the intricacies, wave fronts (conical waves in the volume and helical waves on the surface) are synthesized from signals provided by scanning the primitive line of the cylinder with a laser point source. In order to distinguish between direct bulk conical waves and other contributions in the acoustic response, some considerations on line surface waves and on reflected bulk conical waves are supplied. The identification of the stiffness tensor components, based on the inversion of the bulk waves phase velocities, is applied to signals simulated for a composite material.     

GaN及其Ga空位的电子结构

用团簇埋入自洽计算法对宽禁带半导体GaN的电子结构进行了自旋极化的、全电子、全势场的从头计算,得到了与实验值符合的GaN晶体禁带宽度以及价带中N2p带、N2s带和Ga3d带之间的相对位置.在此基础上Ga空位计算(无晶格畸变)显示,Ga空位周围的费米面显著高于正常GaN晶格的费米面.因此Ga空位周围N原子的处于费米面上的2p电子很易被激发成正常晶格处的传导电子 关键词： GaN 电子结构 团簇埋入自洽计算     

Effect of surface and molecular structure on the dielectric behaviour in thick layers of cyanobiphenyl liquid crystals

The static dielectric studies of hexoxy heptoxy and octoxy cyanobiphenyl liquid crystals (6OCB, 7OCB and 8OCB) and the euteric nematic mixture of 6OCB/7OCB indicate that molecular alignment can be induced by the steel electrod surface of a dielectric thick cell with an electrode gap of about 1 mm. Whereas the permittivity values of alkyl cyanobiphenyls (6CB, 7CB and 8CB) in the nematic phase indicate imperfect alignment of the liquid crystal molecules. The good parallel molecular alignment of alkoxy cyanobiphenyls was to existence of oxygen atom in their molecular structures. These observations demonstrated a sensitive relationship between the molecular alignment, molecular structure and surface effect. Analysis of the dielectric data re-confirmed that the dipole components of the alkoxy cyanobiphenyls along the long molecular axes have a strong tendency for anti-parallel alignment which increases with increasing nematic order, whereas the dipole components perpendicular to the long molecular axes indicate a tendency for parallel alignment.     

氮化镓材料中深能级中心光离化谱测试与分析

设计了一种可用于测试氮化镓材料深能级中心光离化截面的光谱测试方法,该方法建立在使用PID技术控制氮化镓样品中光电流变化为恒定值的基础上,结合光电流测试、霍尔效应、光强度测试等实验手段给出深能级中心光离化截面的实际值。使用该方法得到的光离化截面测试误差同光电探测器对不同入射光子的响应能力有关,光离化截面测试误差随入射光子能量增加不断增大,在入射光子能量较高的情况下,光离化截面测试误差约为8%。对氮化镓材料深能级中心光离化谱分析发现,即使在入射光子能量小于深能级同导带之间能量差2.85eV的情况下,深能级中心仍能在一定程度上吸收该能量的入射光子,表明深能级中心缺陷同周围晶格产生一定程度的耦合。     

Characterizing the structure of protein layers adsorbed onto functionalized surfaces by means of in-situ X-ray reflectivity

In a biological cell, proteins face a highly complex environment comprising crowding and confinement effects as well as interactions with interfaces, cosolvents, and other biomolecules. In this study, the X-ray reflectivity technique has been used for the in-situ characterization of adsorbed protein layers at solid-liquid interfaces. The adsorption of bovine serum albumin at the hydrophobic polystyrene-water interface has been investigated in the presence and in the absence of salts. The data indicate that enhanced adsorption occurs at high salt concentrations.     

Coherent X-ray radiation generated by a relativistic electron in an artificial periodic structure

A theory of coherent X-ray radiation from a relativistic electron crossing an artificial periodic layered structure in the Laue scattering geometry is constructed. The expressions describing the spectral-angular radiation parameters are obtained. It is shown that the radiation yield in such a medium may substantially exceed the radiation yield in a crystal under analogous conditions.     

Wavelength dispersive X-ray analysis and cathodoluminescence techniques for monitoring the chemical removal of AlInN on an N-face GaN surface

This paper shows the selective etching process of an AlInN sacrificial layer, lattice-matched to GaN, on N-face GaN by an aqueous solution of 1,2-diaminoethane. Using the wavelength dispersive X-ray (WDX) spectrometers on an electron probe micro-analyser, together with an optical spectrometer and silicon CCD array added to the light microscope, and sharing the same focus as the electron microscope, cathodoluminescence spectra are collected from exactly the same spot as sampled by the WDX spectrometers. This technique allows the compositional properties of the etched AlInN layer and the optical properties of the semiconductor layers underlying the sacrificial layer to be scrutinised, verifying the etching selectivity and the efficiency of the process.     

Raman tensor analysis of hexagonal polyoxymethylene and its application to study the molecular arrangement in highly crystalline electrospun nanofibers

The orientation dependence in space of Raman‐active vibrations in the hexagonal structure of polyoxymethylene (POM) is discussed in terms of Raman tensor elements as intrinsic physical parameters of the lattice. The variation of polarized intensity for the A₁ and the E₁ vibrational modes with respect to the POM molecular orientation is systematically studied, from both theoretical and experimental viewpoints, according to the symmetry assignments of each vibrational mode. A set of working equations including the Raman selection rules associated with the A₁ and the E₁ modes and the orientation distribution function are explicitly formulated and validated by means of a least‐square fitting procedure on experimental data. In addition, an approach based on the introduction of orientation distribution functions is applied to quantitatively assess and compare on a statistical base the molecular orientation of two different types of electrospun POM nanofibers. Copyright © 2012 John Wiley & Sons, Ltd.     

Determination of the structure of ordered adsorbed layers by analysis of LEED spectra

Recent work on the structure of ordered overlayers of chalcogens on nickel is reviewed, and discussed as a convincing example of the use of LEED intensity spectra for surface structure determination. Guidelines are suggested for successful application of LEED to surface structure, and the special suitability of the muffin-tin potential for the LEED calculation is brought out. The model of the complex crystal potential used for the calculation, and the systematic determination of the four parameters of the model from the data for clean Ni are described. The application to chalcogen overlayers is illustrated by the case of c(2 × 2)S and c(2 × 2)O on Ni(001) surfaces, and the correspondence between experiment and theory is exhibited for the intensity versus energy spectra of $\text{1}\text{2}\text{1}\text{2}$ beams. The correspondence is close and detailed; the complex shapes of broad features are reproduced and the average of the magnitude of the deviation between peaks in measured and calculated spectra is less than 2 eV, whereas the accidental correspondence of two spectra is shown to fluctuate around 4 eV. The observed values of the bond lengths of O, S, Se, Te on Ni are shown to be plausible on the basis of the covalent radii and the expected deviations from them. Results for other surfaces and other structures are tabulated. In all cases but one [O/Ni(110)] the chalcogen occupies sites that the next layer of Ni atoms would use.