Showing papers on "Potential well published in 1996"

Enhancement of electron-hole exchange interaction in CdSe nanocrystals: A quantum confinement effect

Abstract: Photoluminescence of CdSe nanocrystals in a glass matrix was investigated at low temperature with size-selective excitation. The emission spectrum consists of a line a few meV below the excitation laser energy (denoted the F line) and a two-phonon replica superimposed on a broadband. The energy difference between the excitation energy and the F-line position increases with decreasing nanocrystal size. From the analysis of the time behavior of the luminescence and the degree of linear polarization, we attribute the F line to the recombination of the optically forbidden A exciton. Radiation recombination is made possible through a phonon-assisted virtual transition to the confined B-exciton state. The experimental degree of linear polarization is in good agreement with the theoretical calculations. The value of the electron-hole exchange energy obtained from the energy separation between the excitation energy and the F line is much larger than the bulk value and reaches 24 meV in 30-\AA{}-diam. nanocrystals. The size dependence of the exchange energy is in good agreement with the theoretical prediction in the limit of small nanocrystals. \textcopyright{} 1996 The American Physical Society.

Hydrogenic impurities in a quantum well wire in intense, high-frequency laser fields.

Abstract: Calculation of the binding energy of an axial donor hydrogenic impurity in an ideal, infinite, cylindrical quantum wire placed in an intense, high-frequency laser field is reported. By making use of a nonperturbative theory that ``dresses'' both the potential of the impurity and the confinement potential in the quantum wire, and the variational approach a rapid decrease of the binding energy for different values of the wire radius with increasing field intensity is predicted. \textcopyright{} 1996 The American Physical Society.

On potential energy surfaces and relaxation to the global minimum

Abstract: By analyzing the dynamics of model potential energy surfaces we systematically investigate the processes involved in passing from a high energy state to the global minimum and how the probability of reaching the global minimum depends upon the topography and topology of the potential energy surface (PES). Relaxation to the global minimum is easiest for PES’s consisting of a single funnel (a set of convergent pathways which lead to the global minimum) with low barriers and a significant potential energy gradient towards the global minimum. The presence of additional funnels on the surface can severely reduce the rate of relaxation to the global minimum. Such secondary funnels act most efficiently as kinetic traps when they terminate at a low energy minimum, have a steep potential energy gradient and are wide (i.e., have a large configurational entropy) compared to the primary funnel. Indeed, it is even possible to construct PES’s for which the system relaxes to the minimum at the bottom of a secondary funnel rather than the global minimum and then remains in this metastable state over a long time scale. Our results for these model PES’s are discussed in the context of theoretical and experimental knowledge of the dynamics of proteins, clusters, and glasses.

Observation of lateral confinement effect in Ge quantum wires self‐aligned at step edges on Si(100)

Abstract: Spontaneous formation of step‐edge Ge quantum wires and lateral quantum confinement effect are clearly observed in Si submonolayer‐Ge/Si heterostructures grown on Si(100). By depositing submonolayer equivalent Ge (Q), the Ge atoms are found to line up in a wire‐like fashion at the 〈011〉 step edges as revealed by plan‐view transmission electron microscopy. Photoluminescence (PL) peak energy shift with Q due to lateral quantum confinement of holes is clearly observed, and PL features characteristic of the quasi‐one‐dimensional system are also addressed.

X-ray-diffraction study of size-dependent strain in quantum-wire structures

Abstract: We report a synchrotron x-ray-diffraction study of the strain field in embedded In0.2Ga0.8As/GaAs ~001! quantum wires of widths 50‐250 nm. Our results show a size-dependent orthorhombic lattice deformation in the wires and a linearly strained interfacial region near the wire sidewalls. The measured strain is responsible for an unusual band-gap energy increase that is several times larger than the quantum confinement effect, indicating that strain effects contribute significantly to band-edge energies in this and other quantum structures. @S0163-1829~96!00948-4# Quantum confinement in low-dimensional semiconductor materials has attracted much interest in recent years because of its fundamental connection to quantum and solid-state physics and its potential impact on semiconductor electronic and optoelectronic devices. 1 These materials are categorized

Quantum size effects on the conductivity in porous silicon

Abstract: The quantum confinement effect was examined by measuring dark conductivity and optical transmittance of free-standing porous silicon films of different porosities. With increasing porosity from 40 to 80%, the transmission spectrum shows a blue shift and the activation energy of the conductivity increases from 0.3 to 1.0 eV. Using the activation energies, confinement energies are estimated with respect to the crystallite size, determined from the given porosity, and the results are compared with previous works. Good agreement is obtained.

Weakly correlated exciton pair states in large quantum dots

Abstract: We present a calculation of the two-exciton states in semiconductor quantum dots much larger in size than the exciton Bohr radius, and identify a weakly correlated exciton pair state that has a large oscillator strength, increasing proportionately to the volume of the quantum dot. This state is shown to be responsible for the saturation of the size dependence of the resonant excitonic optical nonlinearity. It also provides a satisfactory understanding of the blueshift of the excited-state absorption in quantum dots. These results and the biexciton binding energy and oscillator strength are in good agreement with reported experimental results on CuCl.

High peak-to-valley current ratio GaAs/InGaAs/InAs double stepped quantum well resonant interband tunneling diodes at room temperature

Abstract: A high ratio of the peak current density to the valley current density of current-voltage characteristic is accomplished for the double stepped quantum well resonant interband tunneling diode (DSQW RITD). Results for good quantum confinement effect and long drift layer with deep quantum well GaAs/In0.59Ga0.41As/InAs DSQW RITD that has a lower valley current density of about 0.98 A/cm2 and a higher peak-to-valley current ratio (PVCR) reached 622 at room temperature than conventionally designed double quantum well resonant interband tunneling diodes (DQW RITDs) are presented. This PVCR value is also the highest value than those of the other resonant tunneling diodes.

Persistent Spectral Hole Burning in Semiconductor Quantum Dots

Abstract: Nanometer-size semiconductor crystals or semiconductor nanocrystals are known as zero-dimensional quantum dots [1–6]. Their optical properties have been characterized by the quantum confinement effect, and the lowest excited states show blue shifts depending on their size. Quantum dots act independently, when they are independently embedded in host materials for the isolation of each dot. Otherwise, the quantum confinement effect is weakened or modified. Quantum dots are sharply different from other low-dimensional quantum structures such as quantum wells and quantum wires because quantum dots are made of as few as 103–106 atoms. A considerable fraction of atoms face the surface or the interface of quantum dots in the surrounding materials. Therefore, it is quite natural to consider that the electronic states of quantum dots should not be treated by themselves, but should be treated together with the real surfaces or interfaces and the surrounding materials. This consideration is correct, in general. However, it is not clearly noted that the electronic states of quantum dots are seriously affected by the surrounding host.

The photoluminescence from ZnS/(ZnSe)1/ZnS heterostructures

Abstract: Using the concept developed previously on the temperature dependence of the radiative lifetime of confined excitons in low dimensional systems, we have investigated the ZnS/(ZnSe) 1 /ZnS single quantum well systems by picosecond optical measurement techniques. It is found that the radiative lifetime of the confined exciton in the 1 monolayer (ML) ZnSe system is independent of temperature below 40 K, a result which is fundamentally different from the behavior of the 5 ML ZnSe system. The 1 ML system thus shows the property of a quantum dot system. In addition, it is observed that quantum wells thicker than 3 ML emit a sharp excitonic luminescence while those thinner than 2 ML emit a broad luminescence. We propose that such behaviour is caused by the lateral quantum confinement effect in ‘quantum slabs’ formed on islands and valleys at the interface. The characteristic decay curve of the 1 ML system is close to that of a stretched exponential. This means that this is a random system since it is composed of quantum slab boxes of random size and shape since the characteristic dimensions and shape of the islands and valleys at the interface are themselves random. The effective lateral size along the interface is estimated as ~ 600 nm 2 for the 1 ML system.

STRUCTURAL, ELECTRONIC STATE CHARACTERIZATION AND PROPERTIES OF COATED TiO2 NANOPARTICLES

Abstract: In this paper the microemulsion preparation, the morphology and structural characteristics of toil nanoparticles are reported in detail. The quantum confinement effect upon the strong polarizable nanoparticle are described with interfacial polarization, and their linear and nonlinear optical absorp-tion responses are explained with self trapped exciton model.

Quantum Confinement in GaAs Microcrystallites Embedded in SiO 2 Thin Film

Abstract: Semiconductor GaAs microcrystallites were embedded in SiO2 thin films by magnetron rf cosputtering technique. Structures of the thin films were characterized by transmission electron microscopy, x-ray diffraction and x-ray photoelectron microscopy. Average size of microcrystallites, depending on the substrate temperature during deposition, is 3-10 nm. Absorption spectra of the films were measured. Blue shift of absorption edge was observed and discussed according to quantum confinement effect.

Dimensionality effects on strain for lattice-mismatched InAsPInP quantum wires: The relationship between strain and 2D quantum confinement

Abstract: Lateral size effects for optical response of lattice-mismatched InAsP InP quantum wires are investigated. We observed clear size-dependent blue shift in photoluminescence peak energy, which is markedly larger than lattice-matched quantum wires. High magnetic-field (28 T maximum) applied PL measurements clarified that the observed blue shift cannot be explained by the lateral quantum confinement effect. We calculated strain energy in lattice-mismatched quantum wires, and found that the sum of size-dependent strain energy and the lateral quantum confinement effects well explain the observed blue shift.

Perpendicular-electric-field dependence of exciton binding energy studied by continuous-wave photoluminescence.

Abstract: The reduction of exciton binding energy induced by a perpendicular electric field in a stepped quantum well is studied. From continuous-wave photoluminescence spectra at 77 K we have observed an obvious blueshift of the exciton peak due to a spatially direct-to-indirect transition of excitons. A simple method is used to calculate the exciton binding energy while the inhomogeneous broadening is taken into account in a simple manner. The calculated result reproduces remarkably well the experimental observation.

The optical transition in porous Si: The effects of quantum confinement, surface states and hydrogen passivation

Abstract: We present a theoretical study of two infinite wires of Si with a different lateral size The analysis is based on the linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA) We consider free, partially and totally H-covered [001] Si quantum wires with rectangular cross-section The results of this investigation prove the quantum wire nature of porous Si and interpret many of its physical features In particular we show that a) as expected quantum confinement originates the opening of the LDA gap; b) the gap opening effect is asymmetric: 1/3 of the widening is in the valence band, while 2/3 in the conduction band; c) the near band gap states originate from Si atoms located at the center of the wire; d) the confinement is enhanced in the case of free surfaces; e) the imaginary part of the dielectric function shows a low-energy side structure strongly anisotropic, identified as responsible of the luminescence transition; f) the presence of dangling bonds destroys the luminescence properties; g) in spite of feature c), all Si atoms are collectively involved in the luminescence transition; h) the shift detected by the Si L2, 3VV Auger signal is due to H-interaction effect and is not a measure of the quantum confinement effect; i) the Si atoms probed by the Si L2, 3VV Auger are bonded with H and H2