Energy levels of a spherical quantum dot in a confining potential

Using an exact analytical method, we obtain the ground state and the excited states wave functions as well as the corresponding eigenvalues of a spherical quantum dot in the presence of a confining potential which is a combination of linear, Coulomb and quadratic terms. Next, we investigate the quantum dot energy as the potential coefficients are changed. Our study reveals that considering such a confining potential leads to results which are in good agreement with experimental results.     

Four Electrons in a Coupled Three-Layer Quantum Dot

We investigate four electrons confined in a coupled three-layer quantum dot, by the exact diagonalization method. A vertical magnetic field to the confinement plane is considered. The ground-state electronic structures and angular momentum transitions are investigated. We find that for four-electron Q Ds, the Series of the magic numbers in three-layer QDs are different from those in one-, and two-layer Q Ds. These are connected to the exchange and rotational symmetries of the systems.     

Phonon effect on two coupled quantum dots at finite temperature

The quantum oscillations of population in an asymmetric double quantum dots system coupled to a phonon bath are investigated theoretically. It is shown how the environmental temperature has effect on the system.     

Modified growth of Ge quantum dots using C2H4 mediation by ultra-high vacuum chemical vapor deposition

C₂H₄ mediations were used to modify the Stranski-Krastanow growth mode of Ge dots on Si(0 0 1) at 550 °C by ultra-high vacuum chemical vapor deposition. With appropriate C₂H₄-mediation to modify the Si surface, the elongated Ge hut clusters can be transformed to highly uniform Ge domes with a high Ge composition at the core. These C₂H₄-mediated Ge dots, almost bounded by {1 1 3} facets, have an average diameter and height of 55 and 9 nm, respectively. We propose two major mechanisms to depict the formation of these C₂H₄-mediated Ge dots: (i) an almost hydrogen-passivated Si surface to limit the nucleation sites for dot formation, and (ii) the incorporation of Ge atoms, repelled by the C-rich areas, into the existing Ge dots. This work provides a useful scheme to tune the topography of Ge dots in an UHV/CVD condition for possible optoelectronic applications.     

Investigation of multichannel spatial demultiplexer based on thickness-tapered thin film stacks

A conventional high reflectance (HR) coating sandwiched between two thickness-tapered period thin film stacks is investigated to enhance multichannel spatial dispersion, since phase shift on reflection changes rapidly due to the mismatch induced by the overlaps of different reflective bands. Compared with ordinary Bragg reflector, this structure can be more freely to enhance multichannel spatial shift. The design, fabrication and testing results of this special multichannel filter were presented in the paper, and show the potential to act as a novel demultiplexer.     

Carrier gas effects on the SiGe quantum dots formation

SiGe quantum dots (QDs) grown by ultra-high vacuum chemical vapor deposition using H₂ and He carrier gases are investigated and compared. SiGe QDs using He carrier gas have smaller dot size with a better uniformity in terms of dot height and dot base as compared to the H₂ carrier gas. There is a higher Ge composition and less compressive strain in the SiGe QDs grown in He than in H₂ as measured by Raman spectroscopy. The Ge content is higher for He growth than H₂ growth due to hydrogen induced Si segregation and the lower interdiffusivity caused by the more strain relaxation in the He-grown SiGe dots. The photoluminescence also confirms more compressive strain for H₂ growth than He growth. Hydrogen passivation and Ge-H cluster formation play an important role in the QDs growth.     

Optimization of Metamorphic InGaAs Quantum Wells on GaAs Grown by Molecular Beam Epitaxy

We investigate the molecular beam epitaxy growth of metamorphic InxGal-xAs materials （x up to 0.5） on GaAs substrates systematically. Optimization of structure design and growth parameters is aimed at obtaining smooth surface and high optical qualdty. The optimized structures have an average surface roughness of 0.9-1.8 nm. It is also proven by PL measurements that the optical properties of high indium content （55%） InGaAs quantum wells are improved apparently by defect reduction technique and by introducing Sb as a surfactant. These provide us new ways for growing device quality metamorphic structures on GaAs substrates with long-wavelength emissions.     

Surface modification of exchange-coupled Co/NiOx magnetic bilayer by bias sputtering

We have investigated the effect of bias voltage on sheet resistance, surface roughness and surface coverage of Co/NiO_x magnetic bilayer. In addition, interface topography and corrosion resistance of the Ta/Co/Cu/Co/NiO_x/Si(1 0 0) system have been studied for Co layers deposited at an optimum bias voltage. Atomic force microscopy (AFM) and four point probe sheet resistance (R_s) measurement have been used to determine surface and electrical properties of the sputtered Co layer at different bias voltages ranging from 0 to −80 V. The Co/NiO_x bilayer exhibits a minimum surface roughness and low sheet resistance value with a maximum surface coverage at V_b=−60 V resulted in a slight increase of magnetic resistance and its sensitivity for the Co/Cu/Co/NiO_x/Si(1 0 0) magnetic multilayers, as compared with the same magnetic multilayers containing unbiased Co layers. The presence of Ta protection layer improves the corrosion resistance of the multilayers by three orders of magnitude in a humid environment.     

pH-dependent aggregation and photoluminescence behavior of thiol-capped CdTe quantum dots in aqueous solutions

pH-dependent aggregation of thiol-capped CdTe quantum dots (QDs) in solutions was observed with a confocal microscope. The average size of the QD aggregates increased from 28 nm to 1.4 μm as the pH decreased from 12 to 3. The basic condition improved the dispersion of QDs while the acidic condition caused the detachment of surface ligands, leading to the aggregation of QDs. A PL lifetime of 80 ns was detected for QDs at pH from 12 to 7, while it was shortened to 57 and 34 ns at pH 5 and 3, respectively, due to the formation of surface defects.     

Alkali metal intercalation into SnS2

The intercalation of sodium and potassium into the layered semiconductor SnS₂ has been investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and ion scattering spectroscopy (ISS). After deposition of the alkali metals onto (0001) cleavage planes of SnS₂ in ultra high vacuum (UHV), semiconducting intercalation phases were formed. They seem to be homogeneous and disordered under the given experimental conditions. The valence electrons of the alkali metals are transferred into electronic states of the host lattice, whose valence band density of states changes significantly during intercalation. The underlying changes of the binding properties of the host lattice are discussed. The course of intercalation can be separated into three phases. During an induction period the concentration of the alkali metal on the surface remains very small, the electronic states of the substrate are shifted by band bending. During an intercalation period the topotactic reaction proceeds. After reaching saturation compositions of the intercalation phase at the surface, the alkali metal diffuses into the bulk. Crystal or surface defects seem to have a significant influence on the kinetics of intercalation and on the stoichiometry of the intercalation compounds.     

Disc-Capped ZnO Nanocombs

Nanocombs with a disc cap structure of ZnO have been synthesized on Si substrates by using pure Zinc powaers as the source materials based on a vapour-phase transport process. The morphology and the microstructure are investigated by a scanning electron microscopy and x-ray diffraction. Based on the transmission electron microscopy and selected area electron diffraction analysis, the growth directions of three representative parts, nanoribbon stem, nanorod branch and nanodisc cap of the nanocomb are revealed. The growth mechanism of the disc-capped nanocombs is discussed based on the self-catalyzed vapour-liquid-solid process.     

Thermal stability and diffusion processes in Mo x Si y /Si multilayers studied with high-resolution RBS

Mo _x Si _y /Si multilayers with a period thickness of ∼7.5 nm and bilayers Mo _x Si _y /Si have been fabricated by e⁻-beam evaporation in UHV at a deposition temperature of 150°C [1]. The composition of the as-deposited layer systems and changes in the composition after baking the samples have been studied with high-resolution RBS. For a multilayer with a mixing ratioy/x≃2, no interdiffusion is observed up to a baking temperature of 830°C. For samples with a mixing ratioy/x≃1, diffusion is observed up to a baking temperature of 630°C, resulting in a mixing ratio close toy/x≃2. This mixing ratio remains almost stable up to ∼830°C, and considerable interdiffusion is only observed in those systems where regions with a mixing ratio smaller than 2 still exist. Possible reasons for the high thermal stability of the samples are the lack of a concentration gradient for Si in the system and/or the crystallization of MoSi₂.     

Lithium intercalation in MoO3: A comparison between crystalline and disordered phases

We report properties of lithium-intercalated MoO₃ crystalline and thin-film which are potential cathode materials for high energy density batteries. Discharge and charge reactions of MoO₃ electrodes in a non-aqueous Li⁺-electrolyte have been studied. The kinetically accessible discharge range amounts to 0x1.5 for Li insertion in Li _x MoO₃. Transport parameters such as the Li⁺ chemical diffusion coefficient, thermodynamic factor and ionic conductivity are investigated during the Li⁺ insertion process and discussed with respect to the crystallinity of the cathode material.     

Two dimensional XY type magnetism in intercalated graphite: an elastic and inelastic neutron scattering study

Stage-2 CoCl₂-GIC approximates a two-dimensional easy-plane (XY) ferromagnet on a triangular lattice. It has been found in prior work to order in two steps, with the intermediate phase showing long-ranged ferromagnetic correlations within the intercalate plane, but no correlations between neighboring planes. We have probed the wave vector and temperature dependence of the static and dynamic spin correlations in detail, including measurements of the critical scattering, the quasielastic scattering from vortex diffusion and the spin wave excitations with and without an external magnetic field. Some of the predictions for a Kosterlitz-Thouless type transition are met in this compound, at least qualitatively, including an apparent jump in the spin stiffness at the critical point and the existence of a diffusive central peak in the scattering function possibly originating from vortex autocorrelations. However, there are some inconsistencies between our observations and recent analytical studies as well as Monte Carlo-molecular dynamics simulations of the vortex dynamics that prevent unambigous assignment of the upper critical temperature as a vortex-binding transition.     

Ground state energy of the electron-hole liquid in type-II (GaAs)m/(AlAs)m quantum wells

The ground state energy of quasi-two-dimensional electron-hole liquid (EHL) at zero temperature is calculated for type-II (GaAs)_m/(AlAs)_m (5≤m≤10) quantum wells (QWs). The correlation effects of Coulomb interaction are taken into account by a random phase approximation of Hubbard. Our EHL ground state energy per electron-hole pair is lower than the exciton energy calculated recently for superlattices, so we expected that EHL is more stable state than excitons at high excitation density. It is also demonstrated that the equilibrium density of EHL in type-II GaAs/AlAs QWs is of one order of magnitude larger than that in type-I GaAs/AlAs QWs.     

The simulation of single-crystal growth by molecular beam epitaxy using a kinetic rate-equation model

A computer simulation of Molecular Beam Epitaxial (MBE) growth under typical growth conditions is presented. The method is based on a solution of kinetic rate equations that govern the time dependence of the concentration of islands of varying sizes and differing heights on the top of the surface during the MBE growth. The time dependence for the coverage of each monolayer () is calculated. The mode of the MBE growth is determined by a calculation of the RHEED (Reflection High-Energy Electron Diffraction) intensity. A calculation of the Exposed Coverage (EC) (which is defined as the number of surface atoms in each layer unscreened by other atoms directly above them) and the interface width (IW) (which determines the roughness of the growing surface) serves to confirm the growth mode of the MBE structure. The possible role of these type of calculations in determining the optimized growth conditions for the production of 2D growth in a general materials system is also described.On leave from Department of Microelectronics, Slovak Technical University, Bratislava, Slovakia     

Strain induced ferroelectricity in the SrZrO3 / SrTiO3 superlattice: First principles study

The lattice distortion strain induced ferroelectricity in SrZrO₃/ SrTiO₃ superlattice is studied using first principles density functional theory. Within the tetragonal symmetry , the lattice distortion from the lattice mismatch in the superlattice structure is determined through energy minimization. This kind of lattice distortion leads to the formation of spontaneous polarization in the superlattice, although neither SrZrO₃ nor SrTiO₃ is ferroelectric. From analysis of the relative displacements of the cations and anions, we have found that the SrZrO₃ cell may make a greater contribution to the polarization in the SrZrO₃/ SrTiO₃ superlattice than the SrTiO₃ cell. An extremely large polarization of 42.7 μC/cm² has been predicted.     

Metamorphic InGaAs Quantum Well Laser Diodes at 1.5μm on GaAs Grown by Molecular Beam Epitaxy

We report a 1.5-μm InGaAs/GaAs quantum well laser diode grown by molecular beam epitaxy on InGaAs metamorphic buffers. At 150K, for a 1500×10μm^2 ridge waveguide laser, the lazing wavelength is centred at 1.508 μm and the threshold current density is 667 A/cm^2 under pulsed operation. The pulsed lasers can operate up to 286 K.     

X-ray interface analysis of aperiodic Mo/Si multilayers

We present the non-destructive analysis of aperiodic Mo/Si multilayers by X-ray emission spectroscopy induced by electrons. The Si 3p occupied valence states of the silicon atoms present within these structures are analysed. Because of the great sensitivity of these states to the physico-chemical environment of the Si atoms, it is possible to distinguish the emission from the center of the Si layer (amorphous silicon) to that of the interfacial zones between the Mo and Si layers. Thus, the presence of molybdenum silicides is evidenced in the interfacial zones. It is also shown that the relative proportion of interfacial silicides depends on the deposition conditions.     

Morphology development and oriented growth of single crystalline ZnO nanorod

Single crystalline ZnO nanorods were achieved by the assembly of nanocrystallines in tens of nanometer under hydrothermal conditions with the assistance of surfactant cetyltrimethylammonium bromide (CTAB). The obtained nanorod has rough surface as a result of oriented attachment growth. Transmission electron microscope (TEM) images showed the morphology evolution of the nanorod at different reaction time. Defects were observed and porous structure was left after the assembly of hundreds of nanocrystalline building blocks. Effect of pH condition on the morphology of the nanorod was also investigated.