QtUCP-A program for determining unit-cell parameters in electron diffraction experiments using double-tilt and rotation-tilt holders

A computer program, QtUCP, has been developed based on several well-established algorithms using GCC 4.0 and Qt (R) 4.0 (Open Source Edition) under Debian GNU/Linux 4.0r0. it can determine the unit-cell parameters from an electron diffraction tilt series obtained from both double-tilt and rotation-tilt holders. In this approach, two or more primitive cells of the reciprocal lattice are determined from experimental data, in the meantime, the measurement errors of the tilt angles are checked and minimized. Subsequently, the derived primitive cells are converted into the reduced form and then transformed into the reduced direct primitive cell. Finally all the patterns are indexed and the least-squares refinement is employed to obtain the optimized results of the lattice parameters. Finally, two examples are given to show the application of the program, one is based on the experiment, the other is from the simulation. (C) 2008 Elsevier B.V. All rights reserved.

Transfer property of electron pulse in a femto-second electron diffraction system

An ultra-fast electron diffraction system has been designed. The static and dynamic characters of an electron pulse with 150 fs temporal dispersion are studied during its transmission in the whole ultra-fast electron diffraction system, including the size of the electron spot, temporal dispersion, distribution of azimuths and elevation angles. The initial status of the photoelectrons are put down by Monte Carlo method, both the two dimensional and three dimensional electric fields are calculated by finite difference method and the magnetic flux are. calculated by finite element method.

MANY-BEAM SIMULATIONS AND OBSERVATIONS OF LARGE-ANGLE CONVERGENT-BEAM ELECTRON-DIFFRACTION IMAGING OF CRYSTAL DEFECTS

Many-beam dynamical simulations and observations have been made for large-angle convergent-beam electron diffraction (LACBED) imaging of crystal defects, such as stacking faults and dislocations. The simulations are based on a general matrix formulation of dynamical electron diffraction theory by Peng and Whelan, and the results are compared with experimental LACBED images of stacking faults and dislocations of Si angle crystals. Excellent agreement is achieved.

CONVERGENT-BEAM ELECTRON-DIFFRACTION STUDY OF GE0.5SI0.5/SI STRAINED-LAYER SUPERLATTICES GROWN BY MOLECULAR-BEAM EPITAXY

Side bands due to purely composition and combined composition-strain modulation in plan-view specimens of a nominally Ge0.5Si0.5(5nm)/Si(25nm) superlattice have been obtained by large-angle convergent-beam electron diffraction. The intensities of the side bands have been calculated from a periodic tension-compression model of the superlattice bilayer using the kinematical theory of electron diffraction. Accurate values of elastic strains in the bilayer and of the Ge content can be obtained in this way.

Electron diffraction and photoemission studies of clean and water-covered Si(5,5,12) and Si(112) surfaces

The structure of silicon surfaces in the orientation range (113)-(5,5,12)-(337)-(112) has been investigated using high resolution LEED and photoemission both on a spherical and on flat samples. We find that Si(5,5,12) [5.3 degrees from (113) and 0.7 degrees from (937)] is the only stable orientation between (113) and (111) and confirm the result of Baski et al. [Science 269, 1556 (1995)] that it has a 2 x 1 superstructure with a very large unit cell of 7.68 x 53.5 Angstrom(2). Adsorption measurements of water on Si(5,5,12) yield a mobile precursor kinetics with two kinds of regions saturating at 0.25 and 0.15 ML which are related to adsorption on different sites. Using these results, a modified structure model is proposed. Surfaces between (113) and (5,5,12) separate into facets of these two orientations; between (5,5,12) and (112), they separate into (5,5,12) and (111) facets. (337) facets in this range may be considered as defective (5,5,12) facets.

ELECTRON-DIFFRACTION STUDY ON MONOLAYER LB FILMS OF LIQUID-CRYSTALLINE POLYACRYLATE CONTAINING PARA-NITRO AZOBENZENE

Monolayers of liquid-crystalline polyacrylate containing para-nitro azobenzene (HP6) on the water subphase were characterized by the surface pressure (pi)-area per monomer unit (A) isotherm and were successfully transferred onto glass substrates by the vertical lifting method. The monolayer Langmuis-Blodgett (LB) films transferred at different surface pressures were studied by electron diffraction. The thickness of the monolayer LB film was measured by the transmission electron microscopy folding method. The results of the electron diffraction of the monolayer LB films of HP6 showed that a two-dimensional arrangement exists in the transferred films. According to the results of the pi-A isotherm, electron diffraction and the measured thickness of the monolayer LB film, a molecular arrangement model of HP6 on the water subphase was proposed. The ordered monolayer formation of HP6 showed it to be promising as a second-order non-linear optical material.

Nucleation of diamond by pure carbon ion bombardment - a transmission electron microscopy study

A cross-sectional high-resolution transmission electron microscopy (HRTEM) study of a film deposited by a 1 keV mass-selected carbon ion beam onto silicon held at 800 degrees C is presented. Initially, a graphitic film with its basal planes perpendicular to the substrate is evolving. The precipitation of nanodiamond crystallites in upper layers is confirmed by HRTEM, selected area electron diffraction, and electron energy loss spectroscopy. The nucleation of diamond on graphitic edges as predicted by Lambrecht [W. R. L. Lambrecht, C. H. Lee, B. Segall, J. C. Angus, Z. Li, and M. Sunkara, Nature, 364 607 (1993)] is experimentally confirmed. The results are discussed in terms of our recent subplantation-based diamond nucleation model. (c) 2005 American Institute of Physics.

Investigation of periodicity fluctuations in strained (GaNAs)(1)(GaAs)(m) superlattices by the kinematical simulation of x-ray diffraction

Periodicity fluctuations of layer thickness and composition in a superlattice not only decrease the intensity, they also broaden the width of the satellite peaks in the x-ray diffraction pattern. In this letter, we develop a method that is dependent on the width of satellite peaks to assess periodicity fluctuations of a superlattice quickly. A linear relation of the magnitude of fluctuations, peak width and peak order has been derived from x-ray diffraction kinematical theory. By means of this method, periodicity fluctuations in strained (GaNAs)(1)(GaAs)(m) superlattices grown on GaAs substrates by molecular beam epitaxy have been studied. Distinct satellite peaks indicate that the superlattices are of high quality. The N composition of 0.25 and its fluctuation of 20% in a strained GaNxAs1-x monolayer are obtained from simulations of the measured diffraction pattern. The x-ray simulations and in situ observation results of reflection high-energy electron diffraction are in good agreement. (C) 1999 American Institute of Physics. [S0003-6951(99)00828-1].

ESTIMATION OF THE STRAIN STATE OF GEXSI1-X/SI STRAINED-LAYER SUPERLATTICES BY DOUBLE-CRYSTAL X-RAY-DIFFRACTION

20-period strained-layer superlattices of nominal composition and width Ge0.2Si0.8 (5 nm)/Si(25 nm) and Ge0.5Si0.5 (5 nm)/Si(25 nm) were studied by double-crystal X-ray diffraction. The Ge content x was determined by computer simulation of the diffraction features from the superlattice. This method is shown to be independent of the relaxation of the superlattice. Alternatively, x can be obtained from the measured difference DELTAa/a in lattice spacing perpendicular to the growth plane. It is sensitive to the relaxation. Comparing the results obtained in these two different ways, information about the relaxation of the superlattices can be obtained.

KINEMATICAL STUDY OF GEXSI1-X/SI STRAINED-LAYER SUPERLATTICE BY A DOUBLE CRYSTAL X-RAY-DIFFRACTION METHOD

Two samples of nominal 20-period Ge0.20Si0.80(5 nm)/Si(25 nm) and Ge0.5Si0.5(5 nm)/Si(25 nm) strained-layer superlattices (SLSs) were studied by the double-crystal X-ray diffraction method. It is convenient to define the perpendicular strains relative to the average crystal. Computer simulations of the rocking curves were performed using a kinematical step model. An excellent agreement between the measured and simulated satellite patterns is achieved. The dependence of the sensitivity of the rocking curves to the structural parameters of the SLS, such as the alloying concentration x and the layer thicknesses and the L component of the reflection g = (HKL), are clearly demonstrated.

MATRIX DESCRIPTION OF DYNAMIC HOLZ DIFFRACTION TESTED ON THE STRAINED LAYER SUPERLATTICE SI/GESI

A matrix formulation has been developed and applied to simulate large-angle convergent-beam electron diffraction (LACBED) patterns from the Si/GexSi1-x strained layer superlattice (SLS). Good quantitative agreement has been achieved between experimental and simulated patterns. By utilizing dynamical HOLZ line patterns, we demonstrate that an accuracy of better than 0.1% can be achieved in the determination of the averaged lattice constant of a SLS, and the averaged number of layers of atoms within one period of SLS can be determined up to a single monolayer.

Effect of electron radiation on crystal structure transition of trans-1, 4-polybutadiene

trans-1,4-Polybutadiene (PTBD) was synthesized by rare earth catalyst system, The effect of electron radiation on phase transition from monoclinic phase to hexagonal phase was observed by TEM, Electron diffraction patterns of monoclinic phase, hexagonal phase and two coexistent phases were recorded, The mechanism of phase transition was also discussed in this paper.

High resolution electron microscopy study of single chain single crystals of gutta percha

Single chain single crystals (SCSC) of gutta percha (GP) were prepared by a dilute-solution spraying method. Electron diffraction (ED) patterns revealed that the single chain single crystal was of a new crystalline modification, the delta form. The images of SCSC of GP obtained with a high resolution electron microscope (HREM) showed a two dimensional periodic structure. Most of the images consisted of lattice fringes derived from the (001) zone. This is the first time that the single chain single crystal images of GP have been observed at a molecular level. Micrographs were image processed using optical filtering methods to improve the signal-to-noise ratio, and were compared with computer-generated simulations of the images. From the viewpoint of the defects seen in high resolution images, the crystal formation and melting processes are discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.

Influence of AlN Buffer Thickness on GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia

Hexagonal GaN is grown on a Si(111) substrate with AlN as a buffer layer by gas source molecular beam epitaxy (GSMBE) with ammonia. The thickness of AlN buffer is changed from 9 to 72 nm. When the thickness of AlN buffer is 36 nm, the surface morphology and crystal quality of GaN is optimal. The in-situ reflection high energy electron diffraction (RHEED) reveals that the transition to a two-dimensional growth mode of AlN is the key to the quality of GaN. However, the thickness of AlN buffer is not so critical to the residual in-plane tensile stress in GaN grown on Si(111) by GSMBE for AlN thickness between 9 to 72 nm.

Fabrication and excitation-power-density-dependent micro-photoluminescence of hexagonal nanopillars with a single InGaAs/GaAs quantum well

Hexagonal nanopillars with a single InGaAs/GaAs quantum well (QW) were fabricated on a GaAs (111) B substrate by selective-area metal-organic vapor phase epitaxy. The standard deviations in diameter and height of the nanopillars are about 2% and 5%, respectively. Zincblende structure and rotation twins were identified in both the GaAs and the InGaAs layers by electron diffraction. The excitation-power-density-dependent micro-photoluminescence (mu-PL) of the nanopillars was measured at 4.2, 50, 100 and 150 K. It was shown that, with increasing excitation power density, the mu-PL peak's positions shift to a higher energy, and their intensity and width increase, which were rationalized using a model that includes the effects of piezoelectricity, photon-screening and band-filling. It was also revealed that the rotation twins significantly reduce the diffusion length of the carriers in the nanopillars, compared to that in the regular semiconductors.