Characteristics of the crystalline and luminescence properties of a-plane GaN films grown on γ-LiAlO_2(302) substrates

A-plane GaN films are deposited on(302) γ-LiAlO 2 substrates by metalorganic chemical vapor deposition(MOCVD) . The X-ray diffraction(XRD) results indicate that the in-plane orientation relationship between GaN and LAO substrates is [010] LAO [0001] GaN and [203] LAO [1100] GaN with 0.03% and 2.85% lattice mismatch,respectively. Raman scattering results indicate that the strain in the films decreases along with the increase in the thickness of the films. In addition to the band edge emission at 3.42 eV,defects-related luminescence at 3.35 eV is observed in the photoluminescence(PL) spectra. The cathodoluminescence(CL) spectra indicate that the 3.35-eV emission is related to the V pits.     

The influence of AlN/GaN superlattice intermediate layer on the properties of GaN grown on Si（111） substrates

AlN/GaN superlattice buffer is inserted between GaN epitaxial layer and Si substrate before epitaxial growth of GaN layer. High-quality and crack-free GaN epitaxial layers can be obtained by inserting AlN/GaN superlattice buffer layer. The influence of AlN/GaN superlattice buffer layer on the properties of GaN films are investigated in this paper. One of the important roles of the superlattice is to release tensile strain between Si substrate and epilayer. Raman spectra show a substantial decrease of in-plane tensile strain in GaN layers by using AlN/GaN superlattice buffer layer. Moreover, TEM cross-sectional images show that the densities of both screw and edge dislocations are significantly reduced. The GaN films grown on Si with the superlattice buffer also have better surface morphology and optical properties.     

Defects in GaN Films Grown on Si（111） Substrates by Metal-Organic Chemical Vapour Deposition

We report the transmission-electron microscopy study of the defects in wurtzitic GaN films grown on Si(111) substrates with AlN buffer layers by the metal-organic chemical vapour deposition method. The In0.1 Ga0.9N/GaN multiple quantum well (MQW) reduced the dislocation density by obstructing the mixed and screw dislocations passing through the MQW. No evident reduction of the edge dislocations density by the MQW was observed. It was found that dislocations with screw component can be located at the boundaries of sub-grains slightly in-plane misoriented.     

Electrical properties of MOCVD-grown GaN on Si （111） substrates with low-temperature AIN interlayers

The electrical properties of the structure of GaN grown on an Si （111） substrate with low-temperature （LT） A1N interlayers by metal-organic chemical-vapour deposition are investigated. An abnormal P-type conduction is observed in our GaN-on-Si structure by Hall effect measurement, which is mainly due to the A1 atom diffusing into the Si substrate and acting as an acceptor dopant. Meanwhile, a constant n-type conduction channel is observed in LT-A1N, which causes a conduction-type conversion at low temperature （50 K） and may further influence the electrical behavior of this structure.     

Growth and photoluminescence properties of inclined ZnO and ZnCoO thin films on SrTiO_3（110） substrates

ZnO thin film growth prefers different orientations on the etched and unetched SrTiO 3（STO）（110） substrates.Inclined ZnO and cobalt-doped ZnO（ZnCoO） thin films are grown on unetched STO（110） substrates using oxygen plasma assisted molecular beam epitaxy,with the c-axis 42 inclined from the normal STO（110） surface.The growth geometries are ZnCoO[100]//STO[110] and ZnCoO[111]//STO[001].The low temperature photoluminescence spectra of the inclined ZnO and ZnCoO films are dominated by D 0 X emissions associated with A 0 X emissions,and the characteristic emissions for the 2 E（2G）→ 4A2（4F） transition of Co 2＋ dopants and the relevant phonon-participated emissions are observed in the ZnCoO film,indicating the incorporation of Co 2＋ ions at the lattice positions of the Zn 2＋ ions.The c-axis inclined ZnCoO film shows ferromagnetic properties at room temperature.     

The effect of single A1GaN interlayer on the structural properties of GaN epilayers grown on Si （111） substrates

High-quality and nearly crack-free GaN epitaxial layer was obtained by inserting a single AlGaN interlayer between GaN epilayer and high-temperature AlN buffer layer on Si (111) substrate by metalorganic chemical vapor deposition. This paper investigates the effect of AlGaN interlayer on the structural properties of the resulting GaN epilayer. It confirms from the optical microscopy and Raman scattering spectroscopy that the AlGaN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer and preventing the formation of cracks. X-ray diffraction and transmission electron microscopy analysis reveal that a significant reduction in both screw and edge threading dislocations is achieved in GaN epilayer by the insertion of AlGaN interlayer. The process of threading dislocation reduction in both AlGaN interlayer and GaN epilayer is demonstrated.     

Laser-induced voltage effects in CaaCoaO9 thin films on tilted LaAlO3（001） substrates grown by chemical solution deposition

Laser-induced voltage effects in c-axis oriented Ca3Co4O9 thin films have been studied with samples fabricated on 10°tilted LaAIO3 （001） substrates by a simple chemical solution deposition method. An open-circuit voltage with a rise time of about 10 ns and full width at half maximum of about 28 ns is detected when the film surface is irradiated by a 308-nm laser pulse with a duration of 25 ns. Besides, opemcircuit voltage signals are also observed when the film surface is irradiated separately by the laser pulses of 532 nm and 1064 nm. The results indicate that Ca3Co4O9 thin films have a great potential application in the wide range photodetctor from the ultraviolet to near infrared regions.     

Effects of AlGaN/AlN Stacked Interlayers on GaN Growth on Si （111）

We report the growth of high quality and crack-free GaN film on Si （111） substrate using Al0.2Ga0.8N/AlN stacked interlayers. Compared with the previously used single AlN interlayer, the AlGaN/AlN stacked interlayers can more effectively reduce the tensile stress inside the GaN layer. The cross-sectional TEM image reveals the bending and annihilation of threading dislocations （TDs） in the overgrown GaN film which leads to a decrease of TD density.     

Near Band-Edge and Mid-Gap Photoluminescence of a ZnO Thin Film Grown on a （001） silicon Substrate

A nominally undoped wurtzite ZnO thin film of highly c-axis orientation was successfully grown on (001) silicon by metal-organic chemical vapour deposition, and its photoluminescence was measured as a function of excitation intensity at room temperature. The ZnO sample exhibited a strong near band-edge (NBE) line at 379.48nm (3.267eV) and a weak broad green band around-510 nm (2.43eV), showing a linear and sublinear excitation dependence of the luminescence intensity, respectively. No discernible intensity dependence of lineshape and emission peak was found for the NBE line. On the other hand, the peak energy of the green luminescence was found to increase nearly logarithmically with the increasing excitation intensity. The above results clearly indicate that in the ZnO epilayer, the NBE line was due to an excitonic spontaneous emission, while the mid-gap green luminescence can be assigned to the tunnel-assisted donor-acceptor pair (DAP) radiative recombination.Moreover, we obtained an energy depth β-11.74 meV for the potential wells due to the fluctuating distribution of the unintentional impurities/defects responsible for the tunnel-assisted DAP emission.     

Theoretical studies on particle shape classification based on simultaneous small forward angle light scattering and aerodynamic sizing

Particle shape contributes to understanding the physical and chemical processes of the atmosphere and better ascertaining the origins and chemical compositions of the particles. The particle shape can be classified by the aspect ratio,which can be estimated through the asymmetry factor measured with angularly resolved light scattering. An experimental method of obtaining the asymmetry factor based on simultaneous small forward angle light scattering and aerodynamic size measurements is described briefly. The near forward scattering intensity signals of three detectors in the azimuthal angles at 120?offset are calculated using the methods of T-matrix and discrete dipole approximation. Prolate spheroid particles with different aspect ratios are used as the shape models with the assumption that the symmetry axis is parallel to the flow axis and perpendicular to the incident light. The relations between the asymmetry factor and the optical size and aerodynamic size at various equivalent sizes, refractive indices, and mass densities are discussed in this paper. The numerically calculated results indicate that an elongated particle may be classified at diameter larger than 1.0 μm, and may not be distinguished from a sphere at diameter less than 0.5 μm. It is estimated that the lowest detected aspect ratio is around 1.5:1 in consideration of the experimental errors.     

衍射散射式颗粒粒度测量法的研究新进展

介绍了衍射散射式颗粒粒度测量法的基本光路结构和理论模型,讨论了决定其性能优劣的重要指标--测量下限,对影响测量下限向小粒径范围延伸的参数进行了分析.继而介绍了近年来国内外主要粒度仪品牌在光学结构和散射理论模型方面所做的改进,阐述了它们的工作原理和性能特点.最后对衍射散射式颗粒粒度测量法的发展前景做出了展望,从修正理论模...     

Raman scattering from small particle size polycrystalline silicon

Raman scattering measurements are reported on polycrystalline silicon films prepared in a hydrogen plasma at temperatures between 70 and 400°C. The spectra show several features which are correlated with X-ray diffraction measurements and assigned to crystalline and amorphous-like components.     

用于前向光散射颗粒粒度测量的基函数Tikhonov正则化算法

以前向光散射颗粒测试技术中的反演问题为研究对象,将颗粒粒径分布函数表示为一系列B-spline函数的线性组合以降低反演问题的病态性。模拟计算与实验表明:采用基函数形式的Tikhonov正则化方法可减少传统Tikhonov正则化反演带来的振荡,使其更加光顺。     

从散射谱反演颗粒尺寸分布的测试方法改进

针对从散射谱反演颗粒尺寸分布测量中,由于衍射近似要求近前向取值而带来的反演噪声问题,提出了一种改进的方法.在Chin-Shifrin(C-S)积分变换反演中,插入一种调节函数使得噪声基本消失,又不至于影响反演谱的分布峰位置.对理想单分散颗粒群的模拟效果说明了该法的可行性.对以线阵CCD为接受器件的实验测量和反演结果显示...     

Simultaneous measurement of moving particle size and velocity

An optical method that makes it possible to measure the size and velocity of particles in flows and mixtures independently and simultaneously is considered. Measurements are taken by analyzing a moving diffraction pattern using a multielement photodetector and a special signal processing algorithm. The shape of photocurrent pulses arising when the particle crosses a light beam is determined. Examples of applying this method to particles with internal optical inhomogeneities are given in comparison with other methods, such as the method of low-angle scattering.     

Impact of particle shape on refractive-index dependence of scattering in resonance domain

The impact of particle shape on how scattering, in particular the asymmetry parameter g, depends on the refractive index m is studied. Light scattering simulations for spheres and 16 different spheroids with varying refractive indices and sizes are carried out using an exact T-matrix method to establish how m affects scattering for different shapes. In addition to single shapes, shape distributions of spheroids are used to mimic scattering properties of ensembles of irregularly shaped particles. The results show that Δg resulting from Δm are both size and shape dependent, and spheroids are not universally less or more sensitive to m than the corresponding spheres are. While shape distributions of spheroids show much weaker and much more consistent m-dependence of g than the spheres of the same size, an integration over a size distribution (SD) largely eliminates these differences. Thus, the use of spheres for estimating Δg resulting from Δm for a collection of nonspherical particles appears to be safe except for very narrow SDs. The actual g-values tend to be, however, considerably and consistently in error.     

光散射法测量颗粒尺寸、浓度的实验研究

为了能够准确快速地求解出微米量级颗粒系的尺寸和浓度,设计了一套基于Fraunhofer衍射,以线阵CCD为接收器件的实验颗粒测量装置.采用Shifrin积分变换方法,分析了给定样品颗粒的粒径分布、峰值、平均值和体积浓度.实验结果表明,与传统的Swithenbank方法采用环形光电管阵列为探测器接收衍射光强来反演颗粒分布方法相比,该方法不需要知道颗粒粒径上下限,各粒径区间间隔等预知信息,而且对粒径、浓度的实验测量值与理论值相差较小,样品峰值粒径为9.849 8 μm,与给定峰值的相对误差为3.432%,具有较高的测试准确度和较好的测试效果.     

Energy and spatial redistributions of the particle flux resulting from its interaction with the scattering center

Energy and intensity distributions of both projectiles and recoil atoms on the plane behind the scattering center at finite distance are calculated for different projectile-to-target mass ratio and different projectile energies. The projectile energy and intensity have been found to be double-valued or three-valued functions of the distance from the collision epicenter. At the same time, the recoil energy and intensity have proved to single-valued and double-valued functions of the distance from the collision epicenter for parallel and divergent projectile fluxes, respectively.