Morphology and preferred orientation of Y2O3 film prepared by high-speed laser CVD

Yttria (Y₂O₃) films were prepared at high deposition rates of up to 83 nm/s (300 μm/h) by laser chemical vapor deposition (LCVD) using an Y(dpm)₃ precursor. The effects of deposition conditions, mainly total gas pressure and laser power, on morphology, deposition rate and preferred orientation were studied. Plasma was produced around the substrate over a critical laser power resulting in significant increases in deposition temperature and deposition rate. The high deposition rate (300 μm/h) by LCVD was about 100 to 1000 times as high as those by conventional CVD. The morphology of Y₂O₃ films changed from faceted and columnar structures with high (400) orientation to a columnar structure with high (440) orientation, and finally to a cone-like structure with moderate (440) orientation with increasing total gas pressure (P_tot).     

Numerical study of quantum transport in carbon nanotube transistors

A deeper understanding of quantum effects in nano-electronic devices helps to improve the functionality and to develop new device types. The performance of carbon nanotube (CNT) field-effect transistor is studied using the non-equilibrium Green’s function (NEGF) formalism. The effects of elastic and inelastic scattering and the impact of parameters, such as electron–phonon coupling strength and phonon energy, on the device performance are analyzed.     

On the interpretation of the forbidden spots observed in the electron diffraction patterns of flat Au triangular nanoparticles

In many cases nanostructures present forbidden spots in their electron diffraction patterns when they are observed by transmission electron microscopy (TEM). To interpret their TEM and high resolution transmission electron microscopy (HRTEM) images properly, an understanding of the origin of these spots is necessary. In this work we comment on the origin of the forbidden spots observed in the [111] and [112] electron diffraction patterns of flat gold triangular nanoparticles. The forbidden spots were successfully indexed as corresponding to the first laue Zone (FOLZ) and the HRTEM images presented a contrast produced by the interference of the zero-order Laue zone (ZOLZ) and FOLZ spots. We discuss the use of the forbidden spots in the study of the structure of nanoparticles and show that they are related to the shape and incompleteness of layers in the very thin particles.     

Solving surface structures from normal incidence X-ray standing wave data

A program is provided to determine structural parameters of atoms in or adsorbed on surfaces by refinement of atomistic models towards experimentally determined data generated by the normal incidence X-ray standing wave (NIXSW) technique. The method employs a combination of Differential Evolution Genetic Algorithms and Steepest Descent Line Minimisations to provide a fast, reliable and user friendly tool for experimentalists to interpret complex multidimensional NIXSW data sets.

Program summary

Program title: NIXSW Planewave SolverCatalogue identifier: ADZE_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADZE_v1_0.htmlProgram obtainable from: CPC Program Library, Queen's University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 16 874No. of bytes in distributed program, including test data, etc.: 1 631 874Distribution format: tar.gzProgramming language: Borland C++ Builder 5Computer: Any Windows CompatibleOperating system: Windows 2000 and XPRAM: <10 MBClassification: 7.4Nature of problem: Using NIXSW experimental data to calculate atomic positions of adsorbates.Restrictions: Data from substrates must have cubic, tetragonal or orthorhombic crystal structures i.e. with 90° between conventional cell axes.Running time: Seconds-minutes dependant on the number of plane waves and the number of atomic sites.     

Low temperature photoluminescence and infrared dielectric functions of pulsed laser deposited ZnO thin films on silicon

C-axis oriented ZnO thin films were grown on silicon (100) and (111) substrates by pulsed laser deposition. Low temperature photoluminescence spectra show besides the peaks of free excitons, of defect bound excitons, and of a donor-acceptor pair transition a new doublet at 3.328/3.332 eV. The doublet seems to originate from the columnar textured ZnO film structure. A corresponding structural dependence of the broadening parameter of the infrared dielectric functions was derived from spectroscopic ellipsometry in the spectral range from 380 to 1200 cm^− 1. The wave numbers of the E₁ transverse optical and A₁ longitudinal optical phonon modes of the ZnO films on silicon are determined to be 406 and 573 cm^− 1, respectively. These values are slightly smaller than those of single-crystalline ZnO thin films on sapphire.     

Influence of oxygen pressure on Nd:LuVO4 films grown by pulsed laser deposition

High quality Nd-doped lutecium vanadate thin films on silica glass substrates were fabricated successfully by using a pulsed laser deposition technique. The properties of the samples were characterized by using X-ray diffraction, Rutherford backscattering, atomic force microscopy (AFM), and prism-coupling measurements. The RBS shows that the ratio of Lu/V in the film is 0.991, which is in good agreement with the target composition. X-ray diffraction results show that the degree of crystal orientation along (2 0 0) increases with increasing oxygen pressure up to 20 Pa. The refractive indices of the films determined with dark-mode prism coupling measurements are slight, smaller than that of the bulk crystal. An optimum 20 Pa oxygen pressure, at which the oxygen was leaked into the chamber as the reactive ambient, was determined.     

Effects of oxygen pressure on n-ZnO/p-Si heterojunctions fabricated using pulsed laser deposition

ZnO films were prepared on p-Si substrates using pulsed laser deposition (PLD) and n-ZnO/p-Si heterojunctions were fabricated at different oxygen partial pressures. The effects of oxygen pressures on crystallinity and surface morphology of ZnO films and the I-V characteristics of n-ZnO/p-Si heterojunctions were studied. It was found that the films grown in the oxygen pressure range from 10⁻⁵-10⁻² Torr were all c-axis oriented. The surface morphologies were strongly dependent on the oxygen partial pressure. The current-voltage (I-V) characteristics of the heterojunctions could be classified into two categories depending on the oxygen pressure. At low pressure (10⁻⁵-10⁻⁴ Torr), the I-V curves were similar to those of common p-n junctions. As the oxygen pressure increased to 10⁻³ Torr, the I-V curves changed markedly. Based on the I-V characteristics, an energy band diagram of n-ZnO/p-Si was proposed.     

Stochastic optimization for the calculation of the optimal critical curve from experimental data in a model of the process of regaining balance after perturbation from quiet stance

We demonstrate the successful application of ALOPEX stochastic optimization to the problem of calculating the optimal critical curve in a dynamical systems model of the process of regaining balance after perturbation from quiet stance. Experimental data provide the time series of angles for which the subjects were able to regain balance after an initial perturbation. The optimal critical curve encloses all data points and has a minimum distance from the border points of the data set. We demonstrate the results of the optimization firstly using the traditional cost function of chi-square distance. We then successfully introduce a modified cost function that fits the model to the experimental data by taking into account the specific requirements of the model. By use of the proposed cost function, combined with the efficiency of our optimization method, an optimal critical curve is calculated even in the cases of very asymmetric data sets that lie within the capabilities of the existing model.     

Synthesis,structural, thermal and optical studies of rare earth coordinated complex: Tb(Sal)3Phen

Complexes of salicylic acid (Sal) and 1,10-phenanthroline (Phen) were synthesized coordinated with terbium ion (Tb³⁺) in crystalline phases. The structural characterizations of the lanthanide complex were made using FT-IR, NMR (¹H and ¹³C) and XRD techniques. These measurements confirm the formation of Tb(Sal)₃Phen complex structure. The thermal aspects of the complex were examined using DTA and TGA techniques. An enhancement in luminescence intensity of Tb³⁺ ion bands were observed in Tb(Sal)₃Phen complex as compared to TbCl₃ crystals on 355 nm laser excitation. Enhancement is reported due to the efficient energy transfer process from Sal to Tb³⁺ ions. This is also confirmed by the time resolved photoluminescence spectroscopy with increase in lifetime of Tb³⁺ ions due to encapsulation in Sal/Phen network. Our system in itself can be a deserving candidate for luminescent solar collector material when coupled with solar cells.     

Exchange effects in the screening of a phonon mode in a semiconductor quantum well

We study the screening of the first two polar optical phonon modes in a QW using the rigid wall case. The screened potential was calculated by employing nonlocal dielectric functions obtained in multisubband extensions of (a) the RPA and (b) the Hubbard (H) approximations. The purpose of the calculations is to study the influence of the design parameters, such as the well width and the change in carrier population, in the above mentioned different approximations. The QW is modelled by an infinite potential well and also by the selfconsistent Hartree approximation to consider the modulation doped QW. The main design parameter is found to be the carrier population. Furthermore, it is found that the selfconsistent re-shaping of the electronic potential well—and hence the carrier distribution—affects the resulting screened potential significantly.