260 research outputs found
Control and characterization of ordering in GaInP
Journal ArticleGae,,In,,P layers have been grown by organometallic vapor phase epitaxy on GaAs substrates with [llO]-oriented grooves on the surface that have an important effect on the formation of Cu-Pt ordered structures during growth. In this work, the groove shape is demonstrated to be critically important. For the optimum groove shape, single domains of the (ill) and (lil) variants of the Cu-Pt ordered structure are formed on the two sides of the groove. Shallow grooves produce large domains on each side of the groove containing small domains of the other variant. For deep grooves, only a single variant is formed on each side of the groove, but the domains are small. For substrates with deep grooves on a GaAs substrate misoriented by Y, every groove contains large regions of highly ordered and completely disordered material separated by a few micrometers. This allows a direct determination of the effect of ordering on the band gap of the material using cathodoluminescence spectroscopy, allowing the first direct demonstration that ordering reduces the energy band gap of a III/V alloy
Effects of substrate misorientation and growth rate on ordering in GaInP
Journal ArticleEpitaxial layers of Ga,Tn,_,P with 1=0.52 have been grown by-organometallic vapor-phase apitaxy on GaAs substrates misoriented from the (001) plane in the [ 1 IO] direction by angles 6,) of O", 3", 6", and 9". For each substrate orientation growth rates rg of 1, 2, and 4 pm/h have been used. The ordering was characterized using transmission electron diffraction (TED), dark-field imaging, and photoluminescence. The (110) cross-sectional images show domains of the Cu-Pt structure separated by antiphase boundaries (APBs). The domain size and shape and the degree of order are found to be strongly affected by both the substrate misorientation and the growth rate. For example, lateral domain dimensions range from 50 A for layers grown with rg=4 pm/h and a,?,= 0" to 25(JO A for rg= 1 pm/h and 8, =9". The APBs generally propagate from the substrate/ cpilayer interface to the top surface at an angle to the (001) plane that increases dramatically as the angle of misorientation increases. The angle is nearly independent of growth rate. From the supempot intensities in the TED patterns, the degree of order appears to be a maximum for 8,=5". Judging from the reduction in photoluminescence peak energy caused by ordering, the maximum degree of order appears to occur at ,I?+,-4"
Kinetically controlled order/disorder structure in GaInP
Journal ArticleA Ga0.52In0.58p order/disorder heterostructure having a band-gap energy difference exceeding 160 meV has been grown by organometallic vapor phase epitaxy. The two layers were grown on a nominally (OOl)-oriented GaAs substrate misoriented by 3° toward the [110] direction in the lattice. The disordered layer was grown first, at a temperature of 740 °C. The temperature was then reduced to 620 °C for the growth of the second, highly ordered, layer. X-ray diffraction shows that the two layers have the same composition and are both lattice matched to the GaAs substrate. Transmission electron diffraction patterns indicate that the first layer is completely disordered and that the second layer is highly ordered with only one variant. A low density of antiphase boundaries is observed in the dark field transmission electron microscope image of the top (ordered) layer. High resolution images demonstrate that the interface is abrupt with no dislocations or other defects. Photoluminescence measured at 10 K shows two sharp and distinct peaks at 1.998 and 1.835 eV for high excitation intensities. The peak separation is even larger at lower excitation intensities. The two peaks come from the disordered and ordered materials, respectively. The peak separation represents the largest energy difference between ordered and disordered material reported to date. This large energy difference, much larger than kT at room temperature, may make such heterostructures useful for photonic devices such as light emitting diodes and lasers
Atomic force microscopy study of ordered GaInP
Journal ArticleExamines the nature of the steps on the surface of gallium indium phosphide lattice layers matched to gallium arsenide substrates using atomic force microscopy. Temperatures of organometallic vapor phase epitaxy used; Relation of height of steps with misorientation angle; Link of supersteps with the degree of order and the microstructure of ordered domains
Effect of step structure on ordering in GaInP
Journal ArticleExamines the effect of step structure on ordering in gallium indium phosphite (GaInP) using atomic force microscopy. Coverage of the surface by islands several monolayers in height with elongated direction; Formation of the edges of the islands; Role of the observations in explaining the nature of the order twin boundaries in ordered GaInP
Impurity Energy Level Within The Haldane Gap
An impurity bond in a periodic 1D antiferromagnetic, spin 1 chain with
exchange is considered. Using the numerical density matrix renormalization
group method, we find an impurity energy level in the Haldane gap,
corresponding to a bound state near the impurity bond. When the level
changes gradually from the edge of the Haldane gap to the ground state energy
as the deviation changes from 0 to 1. It seems that there is
no threshold. Yet, there is a threshold when . The impurity level
appears only when the deviation is greater than ,
which is near 0.3 in our calculation.Comment: Latex file,9 pages uuencoded compressed postscript including 4
figure
New exact solution of Dirac-Coulomb equation with exact boundary condition
It usually writes the boundary condition of the wave equation in the Coulomb
field as a rough form without considering the size of the atomic nucleus. The
rough expression brings on that the solutions of the Klein-Gordon equation and
the Dirac equation with the Coulomb potential are divergent at the origin of
the coordinates, also the virtual energies, when the nuclear charges number Z >
137, meaning the original solutions do not satisfy the conditions for
determining solution. Any divergences of the wave functions also imply that the
probability density of the meson or the electron would rapidly increase when
they are closing to the atomic nucleus. What it predicts is not a truth that
the atom in ground state would rapidly collapse to the neutron-like. We
consider that the atomic nucleus has definite radius and write the exact
boundary condition for the hydrogen and hydrogen-like atom, then newly solve
the radial Dirac-Coulomb equation and obtain a new exact solution without any
mathematical and physical difficulties. Unexpectedly, the K value constructed
by Dirac is naturally written in the barrier width or the equivalent radius of
the atomic nucleus in solving the Dirac equation with the exact boundary
condition, and it is independent of the quantum energy. Without any divergent
wave function and the virtual energies, we obtain a new formula of the energy
levels that is different from the Dirac formula of the energy levels in the
Coulomb field.Comment: 12 pages,no figure
Magnetic-field and temperature dependence of the energy gap in InN nanobelt
We present tunneling measurements on an InN nanobelt which shows signatures of superconductivity. Superconducting transition takes place at temperature of 1.3K and the critical magnetic field is measured to be about 5.5kGs. The energy gap extrapolated to absolute temperature is about 110 mu eV. As the magnetic field is decreased to cross the critical magnetic field, the device shows a huge zero-bias magnetoresistance ratio of about 400%. This is attributed to the suppression of quasiparticle subgap tunneling in the presence of superconductivity. The measured magnetic-field and temperature dependence of the superconducting gap agree well with the reported dependences for conventional metallic superconductors. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.3691830
Coherent electron-phonon coupling and polaron-like transport in molecular wires
We present a technique to calculate the transport properties through
one-dimensional models of molecular wires. The calculations include inelastic
electron scattering due to electron-lattice interaction. The coupling between
the electron and the lattice is crucial to determine the transport properties
in one-dimensional systems subject to Peierls transition since it drives the
transition itself. The electron-phonon coupling is treated as a quantum
coherent process, in the sense that no random dephasing due to electron-phonon
interactions is introduced in the scattering wave functions. We show that
charge carrier injection, even in the tunneling regime, induces lattice
distortions localized around the tunneling electron. The transport in the
molecular wire is due to polaron-like propagation. We show typical examples of
the lattice distortions induced by charge injection into the wire. In the
tunneling regime, the electron transmission is strongly enhanced in comparison
with the case of elastic scattering through the undistorted molecular wire. We
also show that although lattice fluctuations modify the electron transmission
through the wire, the modifications are qualitatively different from those
obtained by the quantum electron-phonon inelastic scattering technique. Our
results should hold in principle for other one-dimensional atomic-scale wires
subject to Peierls transitions.Comment: 21 pages, 8 figures, accepted for publication in Phys. Rev. B (to
appear march 2001
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