Electronic states of InAs/GaAs quantum ring

In the framework of effective mass envelope function theory, the electronic states of the InAs/GaAs quantum ring are studied. Our model can be used to calculate the electronic states of quantum wells, quantum wires, and quantum dots. In calculations, the effects due to the different effective masses of electrons in rings and out rings are included. The energy levels of the electron are calculated in the different shapes of rings. The results indicate that the inner radius of rings sensitively changes the electronic states. The energy levels of the electron are not sensitively dependent on the outer radius for large rings. If decreasing the inner and outer radii simultaneously, one may increase the energy spacing between energy levels and keep the ground state energy level unchanged. If changing one of two radii (inner or outer radius), the ground state energy level and the energy spacing will change simultaneously. These results are useful for designing and fabricating the double colors detector by intraband and interband translations. The single electron states are useful for studying the electron correlations and the effects of magnetic fields in quantum rings. Our calculated results are consistent with the recent experimental data of nanoscopic semiconductor rings. (C) 2001 American Institute of Physics.

Electronic characteristics of InAs self-assembled quantum dots

Deep-level transient spectroscopy and photoluminescence studies have been carried out on structures containing self-assembled InAs quantum dots formed in GaAs matrices. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the deep-level transient spectroscopy technique. From analysis of deep-level transient spectroscopy measurements it follows that the quantum dots have electron levels 130 meV below the bottom of the GaAs conduction band and heavy-hole levels at 90 meV above the top of the GaAs valence band. Combining with the photoluminescence results, the band structures of InAs and GaAs have been determined. (C) 2000 Elsevier Science B.V. All rights reserved.

Energy structure and fluorescence of Eu2+ in ZnS : Eu nanoparticles

Eu2+-doped ZnS nanoparticles with an average size of around 3 nm were prepared, and an emission band around 530 nm was observed. By heating in air at 150 degrees C, this emission decreased, while the typical sharp line emission of Eu3+ increased. This suggests that the emission around 530 nm is from intraion transition of Eu2+: In bulk ZnS:Eu2+, no intraion transition of Eu2+ was observed because the excited states of Eu2+ are degenerate with the continuum of the ZnS conduction band. We show that the band gap in ZnS:Eu2+ nanoparticles opens up due to quantum confinement, such that the conduction band of ZnS is higher than the first excited state of Eu2+, thus enabling the intraion transition of Eu2+ to occur.

Methods to tune the electronic states of self-organized InAs/GaAs quantum dots

After capping InAs islands with a thin enough GaAs layer, growth interruption has been introduced. Ejected energy of self-organized InAs/GaAs quantum dots has been successfully tuned in a controlled manner by changing the thickness of GaAs capping layer and the time of growth interruption and InAs layer thickness. The photoluminescence (PL) spectra showing the shift of the peak position reveals the tuning of the electronic states of the QD system. Enhanced uniformity of Quantum dots is observed judging from the decrease of full width at half maximum of FL. Injection InAs/GaAs quantum dot lasers have been fabricated and performed on various frequencies. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Electronic characteristics of InAs/GaAs self-assembled quantum dots by deep level transient spectroscopy

A deep level transient spectroscopy technique has been used to determine the emission activation energies and capture barriers for electrons and holes in InAs self-assembled quantum dots embedded in GaAs. The ground electron and hole energies relative to their respective energy band edges of GaAs are 0.13 and 0.09 eV. Measurements show that the capture cross section of quantum dots is thermally activated. The capture barrier of quantum dots for electrons and holes are 0.30 and 0.26 eV, respectively. The results fit well with the results of photoluminescence spectroscopy measurements. (C) 2000 Elsevier Science B.V. All rights reserved.

Electronic structure of quantum spheres with wurtzite structure

The hole effective-mass Hamiltonian for the semiconductors with wurtzite structure is given. The effective-mass parameters are determined by fitting the valence-band structure near the top with that calculated by the empirical pseudopotential method: The energies and corresponding wave functions are calculated with the obtained effective-mass Hamiltonian for the CdSe quantum spheres, and the energies as functions of sphere radius R are given for the zero spin-orbital coupling (SOC) and finite SOC cases. The energies do not vary as 1/R-2 as the general cases, which is caused by the crystal-field splitting energy and the linear terms in the Hamiltonian. It is found that the ground state is not the optically active S state for the R smaller than 30 Angstrom, in agreement with the experimental results and the "dark exciton'' theory. [S0163-1829(99)01040-1].

Radiative transition in delta-doped GaAs superlattices

Radiative transition in delta-doped GaAs superlattices with a weak coupling was investigted at low temperature, The experimental results show that the transitions from both electron ground state and excited state to hole state have been observed, Based on the effective mass approximation theory, the structures of energy band and photoluminescence spectra for the samples used were calculated. Comparing the experiment with theory, a good agreement was abtained.

Evidence of multimodal patterns of self-organized quantum dots

We have used Deep Level Transient Spectroscopy to investigate self-organized InAs/GaAs quantum dots. The existence of different dot families is confirmed by the deconvolution of the spectra in Gaussian components with full width at half maximum of 60-70meV. The strain of quantum dots is responsible for the relaxation of large quantum dots leading to generation of dislocations. (C) 1998 Academic Press.

Ge related defect energy and microcavity effect in GaN epitaxial layer

Using solid-phase regrowth technique, Pd/Ge contact has been made on the GaN layer, and very good ohmic behavior was observed for the contact. The Photoluminescence (PL) spectra for different structures formed by the Pd/Ge contact, GaN layer, sapphire substrate, and mirror were studied, and a defect-assisted transition was found at 450 nm related to Ge impurity. The results show that the microcavity effect strongly influences the PL spectra of the band-gap and defect-assisted transitions.

Electronic structures of InAs self-assembled quantum dots in an axial magnetic field

The electronic structure of an InAs self-assembled quantum dot in the presence of a perpendicular magnetic field is investigated theoretically. The effect of finite offset, valence-band mixing, and strain are taken into account. The hole levels show strong anticrossings. The large strain and strong magnetic field decrease the effect of mixing between heavy hole and light hole. The hole energy levels have in general a weaker field dependence compared with the corresponding uncoupled levels.

Influence of interdot electronic coupling on photoluminescence spectra of self-assembled InAs/GaAs quantum dots

The influence of interdot electronic coupling on photoluminescence (PL) spectra of self-assembled InAs/GaAs quantum dots (QDs) has been systematically investigated combining with the measurement of transmission electron microscopy. The experimentally observed fast red-shift of PL energy and an anomalous reduction of the linewidth with increasing temperature indicate that the QD ensemble can be regarded as a coupled system. The study of multilayer vertically coupled QD structures shows that a red-shift of PL peak energy and a reduction of PL linewidth are expected as the number of QD layers is increased. On the other hand, two layer QDs with different sizes have been grown according to the mechanism of a vertically correlated arrangement. However, only one PL peak related to the large QD ensemble has been observed due to the strong coupling in InAs pairs. A new possible mechanism to reduce the PL linewidth of QD ensemble is also discussed.

Electronic states of a two-dimensional electron system in a lateral superlattice and a perpendicular magnetic field

The electronic state of a two-dimensional electron system (2DES) in the presence of a perpendicular uniform magnetic field and a lateral superlattice (LS) is investigated theoretically. A comparative study is made between a LS induced by a spatial electrostatic potential modulation (referred to as a PMLS) and that induced by a spatial magnetic-field modulation (referred ro asa MMLS). By utilizing a finite-temperature self-consistent Hartree-Fock approximation scheme; the dependence of the electronic state on different system parameters (e.g., the modulation period, the modulation strength, the effective electron-electron interaction strength, the averaged electron density, and the system temperature) is studied in detail. The inclusion of exchange effect is found to bring qualitative changes to the electronic state of a PMLS, leading generally to a nonuniform spin splitting, and consequently the behavior of the electronic state becomes similar to that of a MMLS. The Landau-level coupling is taken into account, and is found to introduce some interesting features not observed before. It is also found that, even in the regime of intermediate modulation strength, the density dependence of the spin splitting of energy levels, either for a PMLS or a MMLS, can be qualitatively understood within the picture of a 2DES in a perpendicular magnetic field with the modulation viewed as a perturbation. [S0163-1829(97)02248-0].

Quantum confinement effect in silicon quantum-well layers

The electronic states and optical transition properties of silicon quantum-well layers embedded by SiO2 layers are studied by the empirical pseudopotential homojunction model. The energy bands, wave functions, and the optical transition matrix elements are obtained for layers of thickness from 1 to 6 nm, and three oriented directions (001), (110), and (111). It is found that for Si layers in the (001) direction the energy gap is pseudodirect, for these in the (111) direction the energy gap is indirect, while for those in the (110) direction the energy gap is pseudodirect or indirect for a thickness smaller or larger than 3 nm, respectively. The optical transition matrix elements are smaller than that of diner transition, and increase with decreasing layer thickness. When the thickness of a layer is smaller than 2 nm, the Si QW layers have larger transition matrix elements. It is caused by mixing of bulk X states with the Gamma(1) state. The calculated results are compared with experimental results.

Monostable-Bistable Transition Logic Element (MOBILE) Model for Single-Electron Transistors

The traditional monostable-bistable transition logic element (MOBILE) structure is usually composed of resonant tunneling diodes (RTD). This letter describes a new type MOBILE structure consisting of single-electron transistors (i.e. SET-MOBILE). The analytical model of single-electron transistors ( SET) has been considered three states (including an excited state) of the discrete quantum energy levels. The simulation results show negative differential conductance (NDC) characteristics in I-DS-V-DS curve. The SET-MOBILE utilizing NDC characteristics can successfully realize the basic logic functions as the RTD-MOBILE.

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51 % and a stabilized efficiency of 8.01% (AM 1.5, 100 mw/cm(2)) at room temperature. (c) 2006 Published by Elsevier B.V.