Photoluminescence properties of tensile-strained GaAsP/GaInP single quantum wells grown by metal organic chemical vapor deposition

Tensile-strained GaAsP/GaInP single quantum well (QW) laser diode (I-D) structures have been grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) and related photoluminescence (PL) properties have been investigated in detail. The samples have the same well thickness of 16 nm but different P compositions in a GaAsP QW. Two peaks in room temperature (RT) PL spectra are observed for samples with a composition larger than 0.10. Temperature and excitation-power-dependent PL spectra have been measured for a sample with it P composition of 0.15. It is found that the two peaks have a 35 meV energy separation independent of temperature and only the low-energy peak exists below 85 K. Additionally, both peak intensities exhibit a monotonous increase as excitation power increases. Analyses indicate that the two peaks arise from the intrinsic-exciton recombination mechanisms of electron-heavy hole (e-hh) and electron-light hole (e-hh). A theoretical calculation based oil model-solid theory, taking, into account the spin-orbit splitting energy, shows good agreement with our experimental results. The temperature dependence of PL intensity ratio is well explained using the spontaneous emission theory for e-hh and e-hh transitions. front which the ratio can be characterized mainly by the energy separation between the fill and Ill states.

Study of valence intersubband absorption in tensile strained Si/SiGe quantum wells

The hole subband structures and effective masses of tensile strained Si/Si1-yGey quantum wells are calculated by using the 6x6 k.p method. The results show that when the tensile strain is induced in the quantum well, the light-hole state becomes the ground state, and the light hole effective masses in the growth direction are strongly reduced while the in-plane effective masses are considerable. Quantitative calculation of the valence intersubband transition between two light hole states in a 7nm tensile strained Si/Si0.55Ge0.45 quantum well grown on a relaxed Si0.5Ge0.5 (100) substrates shows a large absorption coefficient of 8400 cm(-1).

Small SiGe quantum dots obtained by excimer laser annealing

In this paper, we obtain SiGe quantum dots with the diameters and density of 15-20 nm and 1.8 x 10(11) cm(-2), respectively, by 193 nm excimer laser annealing of Si0.77Ge0.23 strained films. Under the excimer laser annealing, only surface atoms diffusion happens. From the detailed statistical information about the size and shape of the quantum dots with different annealing time, it is shown that the as-grown self-assembled quantum dots, especially the {105}-faceted dots, are not stable and disappear before the appearance of the laser-induced quantum dots. Based on the calculation of surface energy and surface chemical potential, we show that the {103}-faceted as-grown self-assembled quantum dots are more heavily strained than the {105}-faceted ones, and the heavy strain in the dot can decrease the surface energy of the dot facets. The formation of the laser-induced quantum dots, which is also with heavy strain, is attributed to kinetic constraint. (c) 2008 Elsevier B.V. All rights reserved.

Electronic energy levels in an asymmetric quantum-dots-in-a-well structure for infrared photodetectors

Theoretical calculation of electronic energy levels of an asymmetric InAs/InGaAS/GaAS quantum-dots-in-a-well (DWELL) structure for infrared photodetectors is performed in the framework of effective-mass envelope-function theory. Our calculated results show that the electronic energy levels in quantum dots (QDs) increase when the asymmetry increases and the ground state energy increases faster than the excited state energies. Furthermore, the results also show that the electronic energy levels in QDs decrease as the size of QDs and the width of quantum well (QW) in the asymmetric DWELL structure increase. Additionally, the effects of asymmetry, the size of QDs and the width of QW on the response peak of asymmetry DWELL photodetectors are also discussed.

Electron transport properties of In0.53Ga0.47As/In0.52Al0.48As quantum wells with two occupied subbands

Magnetotransport properties of two-dimensional electron gas have been investigated for three In0.53Ga0.47As/In0.52Al0.48As quantum well samples having two occupied subbands with different well widths. When the intersubband scattering is considered, we have obtained the subband density, transport scattering time, quantum scattering time and intersubband scattering time, respectively, by analyzing the result of fast Fourier transform of the first derivative of Shubnikov-de Haas oscillations. It is found that the main scattering mechanism is due to small-angle scattering, such as ionized impurity scattering, for the first subband electrons.

Electronic structure in GaAs/AlxGa1-xAs spherical quantum dots

In this paper, how the dots' radius, At concentration and external electric field affect the single electron energy states in GaAs/AlxGa1-xAs spherical quantum dots are discussed in detail. Furthermore, the modification of the energy states is calculated when the difference in effective electron mass in GaAs and AlxGa1-xAs are considered. In addition, both the analytical method and the plane wave method are used in calculation and the results are compared, showing that they are in good agreement with each other. The results and methods can provide useful information for the future research and potential applications of quantum dots.

Electronic structures and mechanical properties of uranium monocarbide from first-principles LDA plus U and GGA plus U calculations

The electronic structure, elastic constants, Poisson's ratio, and phonon dispersion curves of UC have been systematically investigated from the first-principles calculations by the projector-augmented-wave (PAW) method. In order to describe precisely the strong on-site Coulomb repulsion among the localized U 5f electrons, we adopt the local density approximation (LDA) + U and generalized gradient approximation (GGA) + U formalisms for the exchange correlation term. We systematically study how the electronic properties and elastic constants of UC are affected by the different choice of U as well as the exchange-correlation potential. We show that by choosing an appropriate Hubbard U parameter within the GGA + U approach, most of our calculated results are in good agreement with the experimental data. Therefore. the results obtained by the GGA + U with effective Hubbard parameter U chosen around 3 eV for UC are considered to be reasonable. (C) 2009 Elsevier B.V. All rights reserved.

Well-width dependence of in-plane optical anisotropy in (001) GaAs/AlGaAs quantum wells induced by in-plane uniaxial strain and interface asymmetry

The well-width dependence of in-plane optical anisotropy (IPOA) in (001) GaAs/AlxGa1-xAs quantum wells induced by in-plane uniaxial strain and interface asymmetry has been studied comprehensively. Theoretical calculations show that the IPOA induced by in-plane uniaxial strain and interface asymmetry exhibits much different well-width dependence. The strain-induced IPOA is inversely proportional to the energy spacing between heavy- and light-hole subbands, so it increases with the well width. However, the interface-related IPOA is mainly determined by the probability that the heavy- and light-holes appear at the interfaces, so it decreases with the well width. Reflectance difference spectroscopy has been carried out to measure the IPOA of (001) GaAs/AlxGa1-xAs quantum wells with different well widths. Strain- and interface-induced IPOA have been distinguished by using a stress apparatus, and good agreement with the theoretical prediction is obtained. The anisotropic interface potential parameters are also determined. In addition, the energy shift between the interface- and strain-induced 1H1E reflectance difference (RD) structures, and the deviation of the 1L1E RD signal away from the prediction of the calculation model have been discussed.

Energy Levels of Hydrogenic Impurities in InAs Quantum Ring

The distribution of energy levels of the ground state and the low-lying excited states of hydrogenic impurities in InAs quantum ring was investigated by applying the effective mass approximation and the perturbation method. In 2D polar coordinates, the exact solution to the Schrodinger equation was used to calculate the perturbation integral in a parabolic confinement potential. The numerical results show that the energy levels of electron are sensitively dependent on the radius of the quantum ring and a minimum exists on account of the parabolic confinement potential. With decreasing the radius, the energy spacing between energy levels increases. The degenerate energy levels of the first excited state for hydrogenic impurities are not relieved, and when the degenerate energy levels are split and the energy spacing will increase with the increase in the radius. The energy spacing between energy levels of electron is also sensitively dependent on the angular frequency and will increase with the increases in it. The degenerate energy levels of the first excited state are not relieved. The degenerate energy levels of the second excited state are relieved partially. The change in angular frequency will have a profound effect upon the calculation of the energy levels of the ground state and the low-lying excited states of hydrogenic impurities in InAs quantum ring. The conclusions of this paper will provide important guidance to investigating the optical transitions and spectral structures in quantum ring.

Investigation of Electronic Energy Levels in InAs Quantum Dot with Shape of Lens by Using B-Spline Technique

The dependence of the electronic energy levels on the size of quantum dots (QDs) with the shape of spherical lens is studied by using the B-spline technique for the first time. Within the framework of the effective-mass theory, the values of electronic energy levels are obtained as a function of the height, radius and volume of QDs, respectively. When the height or radius of QDs increases, all the electronic energy levels lower, and the separations between the energy levels decrease. For lens-shape QDs, height is the key factor in dominating the energy levels comparing with the effect of radius, especially in dominating the ground-state level. These computational results are compared with that of other theoretical calculation ways. The B-spline technique is proved to be an effective way in calculating the electronic structure in QDs with the shape of spherical lens.

Variational calculations of neutral bound excitons in GaAs quantum-well wires

The binding energy of an exciton bound to a neutral donor (D-0,X) in GaAs quantum-well wires is calculated variationally as a function of the wire width for different positions of the impurity inside the wire by using a two-parameter wavefunction. There is no artificial parameter added in our calculation. The results we have obtained show that the binding energies are closely correlated to the sizes of the wire, the impurity position, and also that their magnitudes are greater than those in the two-dimensional quantum wells compared. In addition, we also calculate the average interparticle distance as a function of the wire width. The results are discussed in detail.

Asymmetric quantum-confined Stark effects of hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots

Quantum-confined Stark effects in GaAs/AlxGa1-xAs self-assembled quantum dots are investigated theoretically in the framework of effective-mass envelope function theory. The electron and hole energy levels and optical transition energies are calculated in the presence of an electric field in different directions. In our calculation, the effect of finite offset, valence-band mixing, the effects due to the different effective masses of electrons and holes in different regions, and the real quantum dot structures are all taken into account. The results show that the electron and hole energy levels and the optical transition energies can cause blueshifts when the electric field is applied along the opposite to the growth direction. Our calculated results are useful for the application of hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots to photoelectric devices. (c) 2005 American Institute of Physics.

Design and analysis of 1.3 mu m GaAs-based quantum dot vertical-cavity surface-emitting lasers

A theoretical study on 1.3 mu m GaAs-based quantum dot vertical-cavity surface-emitting lasers (VCSELs) was made. Investigation of the influence of VCSELs on the optical confinement factors and the optical loss and the calculation of the material gain of the assembled InGaAs/GaAs quantum dots. Analysis of the threshold characteristic was made and the multi-wavelength cavity and multilayer quantum-dot stack structure is found to be more suitable for quantum dot VCSELs.