Bismuth nanolines on Si(001) and their influence on mesoscopic surface structure

Experimental studies of Bi heteroepitaxy on Si(001) have recently uncovered a self-organised nanoline motif which has no detectable width dispersion. The Bi lines can be grown with an aspect ratio that is greater than 350 : 1. This paper describes a study of the nanoline geometry and electronic structure using a combination of scanning tunneling microscopy (STM) and ab initio theoretical methods. In particular, the effect that the lines have on Si(001) surface structure at large length scales, l > 100 nm, is studied. It has been found that Bi line growth on surfaces that have regularly spaced single height steps results in a 'preferred' domain orientation.

The electronic properties of Si(001)–Bi(2 × n)

A Bi 2 × n surface net was grown on the Si(001) surface and studied with inverse photoemission, scanning tunnelling microscopy and ab initio and empirical pseudopotential calculations. The experiments demonstrated that Bi adsorption eliminates the dimer related π1* and π2* surface states, produced by correlated dimer buckling, leaving the bulk bandgap clear of unoccupied surface states. Ab initio calculations support this observation and demonstrate that the surface states derived from the formation of symmetric Bi dimers do not penetrate the fundamental bandgap of bulk Si. Since symmetric Bi dimers are an important structural component of the recently discovered Bi nanolines, that self-organize on Si(001) above the Bi desorption temperature, a connection will be made between our findings and the electronic structure of the nanolines.

Reply to comment on 'Bi nanolines on Si(001): Registry with the substrate'

The registry of bismuth dimers, integral components of the bismuth nanoline on Si(001), is examined. In contrast to the currently accepted view, the bismuth dimers are found to be in registry with the two-dimensional lattice created by the silicon dimers. The consequences of this finding are briefly explored.

Doping Dependence of Semiconductor-Metal Transition in InP at High Pressures

The resistivity of selenium-doped n-InP single crystal layers grown by liquid-phase epitaxy with electron concentrations varying from 6.7 x 10$^18$ to 1.8 x 10$^20$ cm$^{-3}$ has been measured as a function of hydrostatic pressure up to 10 GPa. Semiconductor-metal transitions were observed in each case with a change in resistivity by two to three orders of magnitude. The transition pressure p$_c$ decreased monotonically from 7.24 to 5.90 GPa with increasing doping concentration n according to the relation $p_c = p_o [1 - k(n/n_m)^a]$, where n$_m$ is the concentration (per cubic centimetre) of phosphorus donor sites in InP atoms, p$_o$ is the transition pressure at low doping concentrations, k is a constant and $\alpha$ is an exponent found experimentally to be 0.637. The decrease in p$_c$ is considered to be due to increasing internal stress developed at high concentrations of ionized donors. The high-pressure metallic phase had a resistivity (2.02-6.47) x 10$^{-7}$ $\Omega$ cm, with a positive temperature coefficient dependent on doping.

Solvothermal Synthesis of InP Quantum Dots

We report an efficient and fast solvothermal route to prepare highly crystalline monodispersed InP quantum dots. This solvothermal route, not only ensures inert atmosphere, which is strictly required for the synthesis of phase pure InP quantum dots but also allows a reaction temperature as high as 430 degrees C, which is otherwise impossible to achieve using a typical solution chemistry; the higher reaction temperature makes the reaction more facile. This method also has a judicious control over the size of the quantum dots and thus in tuning the bandgap.

Reactivation kinetics of acceptors in hydrogenated InP during unbiased annealing

The reactivation kinetics of passivated Mg acceptors in hydrogenated InP during unbiased annealing of a Schottky diode is reported. The reactivation is found to slow down gradually with annealing time and this phenomenon is attributed to substantial retrapping of H at the acceptor sites. It is found from the concentration profiles and the kinetics data that the reactivation is most likely limited by H2 molecule formation processes for longer annealing times; for shorter annealing times, contributions from in-diffusion of H also become significant. The diffusion of H during the initial period follows an Arrhenius relation with an activation energy for the effective diffusion coefficient of 1.13±0.10 eV. In the H2 formation regime, the reactivation is thermally activated with an activation energy for the annealing parameter of 1.71±0.10 eV. The H2 formation-limited regime of reactivation occurs sooner as the annealing temperature is increased.

Enhancement of uniaxial magnetic anisotropy in Fe thin films grown on GaAs(001) with an MgO underlayer

The understanding and control of anisotropy in Fe films grown on cubic systems such as GaAs and MgO has been of interest from the point of view of applications in devices. We report magnetic anisotropy studies on Fe/GaAs(001) and Fe/MgO/GaAs(001) prepared by pulsed laser deposition. In Fe/GaAs(001), magneto optical Kerr effect (MOKE) measurements revealed a dominant uniaxial anisotropy for Fe thickness less than 20 monolayers (ML) and this was confirmed by ferromagnetic resonance (FMR) studies. Multiple steps in the hysteresis loops were observed for Fe films of thickness 20 and 25 ML. Whereas, in Fe/MgO/GaAs(001), even at 25 ML of Fe, the uniaxial anisotropy remained dominant. The anisotropy constants obtained from FMR spectra have shown that the relative strength of uniaxial anisotropy is higher as compared to the cubic anisotropy constant in the case of Fe/MgO/GaAs(001). (C) 2011 American Institute of Physics. doi:10.1063/1.3556941]

Hydrogen Passivation of Shallow Dopants in InP Studied by Photoluminescence Spectroscopy

The effect of hydrogenation on the photoluminescence (PL) of InP : Mg, InP : Zn and undoped n-InP is presented. An increase in the near band edge pl intensity due to passivation of non-radiative centers was observed in all the samples. A donor - acceptor pair transition was observed before hydrogenation in the InP : Mg sample and after hydrogenation in the InP : Zn sample due to the acceptor deactivation. In n-InP the enhancement of donor bound exciton after hydrogenation points to the absence of donor passivation.

Anisotropic low field magnetoimpedance in (001) oriented La0.7Sr0.3MnO3 thin films

We report the evidence for the anisotropic magnetoimpedance behavior in (001) oriented La0.7Sr0.3MnO3 (LSMO) thin films, in low frequency-low magnetic field regime. (001) oriented LSMO thin films were deposited using pulsed laser deposition and characterized with X-ray diffraction and temperature dependent magnetization studies. In the in-plain configuration, an ac magnetoresistance (MRac) of similar to -0.5% was observed at 1000 Oe, at 100 Hz frequency in these films. The MRac was found to decrease with increase in frequency. We observe increases in MRac at low frequency, indicating major contribution for change of permeability from domain wall motion. At higher frequencies, it decreases due to decrease in transverse permeability, resulting from dampening of domain wall motion. Out-of-plane configuration showed MRac similar to 5.5% at 1000 Oe, at 100 Hz frequency. The MRac turned from positive to negative with increase in frequency in out-of-plane configuration. These changes are attributed to the change in permeability of the film with the frequency and applied magnetic field.

BIMODAL NANOINDENTATION RESPONSE OF THE (001) FACE IN CRYSTALLINE SODIUM SACCHARIN DIHYDRATE

The nanoindentation response of the (001) face of sodium saccharin dihydrate is examined. The structure can be demarcated into regular and irregular regions or domains. The regular domains have solid-like and the irregular ones have liquid-like characteristics. Therefore, these domains impart a microstructure to the crystal. The indent face (001) is prominently developed in this crystal and unambiguously presents the regular and irregular regions to nanoindention. Average values of elastic modulus and hardness show a distinct bimodal mechanical response. Such a response has been observed in the case of intergrown polymorphs of aspirin and felodipine. We examine two possible reasons as to why the responses could be for bimodal in this crystal. The first possibility could be that the two domains correspond to regions of the original dihydrate and a lower hydrate that is obtained by the loss of some water. The second possibility could be that these responses correspond to regular and irregular regions in the structure. Nanoindentation is a very useful technique in the characterization of molecular solids, as a complementary technique to X-ray crystallography, because it samples different length scales.

在InGaAsP/InP双异质结构中的光弹效应及其对侧向光的限制作用

从理论上计算了厚度为110nm的W_(0.95)Ni_(0.05)金属薄膜应变条在InGaAsP/InP双异质结构中形成的应力场分布，及由应力场分布引起的折射率变化。在W_(0.95)Ni_(0.05)金属薄膜应变条半导体中0.2-2μm深度范围内，由应变引起条形波导轴中央的介电常数ε相应增加2.3×10~(-1)-2.2×10~(-2)(2μm应变条宽)和1.2×10~(-1)-4.1×10~(-2)(4μm应变条宽)。同时，测量了由W_(0.95)Ni_(0.05)金属薄膜应变条所形成的InGaAs/InP双异质结光弹效应波导结构导波的近场光模分布。从理论计算和实验结果两方面证实了InGaAsP/InP双异质结光弹效应波导结构对侧向光具有良好的限制作用。

Diário da Constituinte [gravação de vídeo] : [programa n. 001]

Pronunciamento do Presidente da Assembleia Nacional Constituinte (ANC) Deputado Ulysses Guimarães para apresentar o funcionamento da ANC, inaugurar e anunciar o programa Diário da Constituinte como veículo de comunicação e disseminação dos trabalhos dos constituintes. No pronunciamento, o Presidente da ANC também convoca os brasileiros para participar da elaboração da futura constituição do país.