Symmetry and optical transition rule for low-dimensional semiconductor system with spin-orbit interaction and magnetic field

Starting from effective mass Hamiltonian, we systematically investigate the symmetry of low-dimensional structures with spin-orbit interaction and transverse magnetic field. The position-dependent potentials are assumed to be space symmetric, which is ever-present in theory and experiment research. By group theory, we analyze degeneracy in different cases. Spin-orbit interaction makes the transition between Zeeman sub-levels possible, which is originally forbidden within dipole approximation. However, a transition rule given in this paper for the first time shows that the transition between some levels is forbidden for space symmetric potentials. (C) 2009 Elsevier Ltd. All rights reserved.

Spin dynamics in the second subband of a quasi-two-dimensional system studied in a single-barrier heterostructure by time-resolved Kerr rotation

Spin dynamics in the first and second subbands have been examined simultaneously by time resolved Kerr rotation in a single-barrier heterostructure of a 500 nm thick GaAs absorption layer. By scanning the wavelengths of the probe and pump beams towards the short wavelength in the zero magnetic field, the spin coherent time T-2(1)* in the 1st subband E-1 decreases in accordance with the D'yakonov-Perel' (DP) spin decoherence mechanism. Meanwhile, the spin coherence time T-2(2)* in the 2nd subband E-2 remains very low at wavelengths longer than 810 nm, and then is dramatically enhanced afterwards. At 803 nm, T-2(2)* (450 ps) becomes ten times longer than T-2(1)* (50 ps). A new feature has been discovered at the wavelength of 811nm under the bias of -0.3V (807nm under the bias of -0.6V) that the spin coherence times (T-2(1)* and T-2(2)*) and the effective g* factors (vertical bar g*(E-1)vertical bar and vertical bar g*(E-2)vertical bar) all display a sudden change, presumably due to the "resonant" spin exchange coupling between two spin opposite bands. Copyright (C) EPLA, 2008.

Optical and electrical investigation of low dimensional self-assembled InAs quantum dot field effect transistors

Self-assembled InAs QD dot-in-a-well (DWELL) structures were grown on GaAs substrate by MBE system, and heterojunction modulation-doped field effect transistor (MODFET) was fabricated. The optical properties of the samples show that the photoluminescence of InAs/GaAs self-assembled quantum dot (SAQD) is at 1.265 mu m at 300 K. The temperature-dependence of the abnormal redshift of InAs SAQD wavelength with the increasing temperature was observed, which is closely related with the inhomogeneous size distribution of the InAs quantum dot. According to the electrical measurement, high electric field current-voltage characteristic of the MODFET device were obtained. The embedded InAs QD of the samples can be regard as scattering centers to the vicinity of the channel electrons. The transport property of the electrons in GaAs channel will be modulated by the QD due to the Coulomb interaction. It has been proposed that a MODFET embedded with InAs QDs presents a novel type of field effect photon detector.

Low-Dimensional Dynamical Systems for a Wake-Type Shear Flow with Vortex Dislocations

Low-dimensional systems are constructed to investigate dynamics of vortex dislocations in a wake-type shear flow. High-resolution direct numerical simulations are employed to obtain flow snapshots from which the most energetic modes are extracted using proper orthogonal decomposition (POD). The first 10 modes are classified into two groups. One represents the general characteristics of two-dimensional wake-type shear flow, and the other is related to the three-dimensional properties or non-uniform characteristics along the span. Vortex dislocations are generated by these two kinds of coherent structures. The results from the first 20 three-dimensional POD modes show that the low- dimensional systems have captured the basic properties of the wake-type shear flow with vortex dislocation, such as two incommensurable frequencies and their beat frequency.

Photoluminescence of low-dimensional semiconductor structures under pressure

Photoluminescence of some low-dimensional semiconductor structures has been investigated under pressure. The measured pressure coefficients of In0.55Al0.45 As/Al0.5Ga0.5As quantum dots with average diameter of 26, 52 and 62 nm are 82, 94 and 98 meV/GPa, respectively. It indicates that these quantum dots are type-I dots. On the other hand, the measured pressure coefficient for quantum dots with 7 nm in size is -17meV/GPa, indicating the type-II character. The measured pressure coefficient for Mn emission in ZnS:Mn nanoparticles is -34.6meV/GPa, in agreement with the predication of the crystal field theory. However, the DA emission is nearly independent on pressure, indicating that this emission is related to the surface defects in ZnS host. The measured pressure coefficient of Cu emission in ZnS: Cu nanoparticles is 63.2 meV/GPa. It implies that the acceptor level introduced by Cu ions has some character of shallow level. The measured pressure coefficient of Eu emission in ZnS:Eu nanoparticles is 24.1 mev/GPa, in contrast to the predication of the crystal field theory. It may be due to the strong interaction between the excited state of Eu ions and the conduction band of ZnS host.

EFFECT OF IMAGE FORCES ON ELECTRONS CONFINED IN LOW-DIMENSIONAL STRUCTURES UNDER A MAGNETIC-FIELD

We consider the effect of image forces, arising due to a difference in dielectric permeabilities of the well layer and barrier layers, on the energy spectrum of an electron confined in a rectangular potential well under a magnetic field. Depending on the value and the sign of the dielectric mismatch, image forces can localize electrons near the interfaces of the well or in well centre and change the direct intersubband gaps into indirect ones. These effects can be controlled by variation of the magnetic field, offering possibilities for exact tuning of electronic devices.

Synthesis and structure of new low dimensional polymeric haloplumbate(II) complexes [Pb4Br12(C40H40N8)] and [Pb4Cl12(C40H40N8)]center dot H2O

New low dimensional polymeric haloplumbate(II) complexes of the dication of (4,4'-bis(imidazolyl-ylmethyl)biphenyl) were synthesised and their crystal structures determined. Complex 1, [Pb4Br12(C40H40N8)], has cis-edge-shared, octahedral, lead bromide double chains. In compound 2 [Pb4Cl12(C40H40N8)]center dot H2O, the inorganic chains are corner-shared, square pyramidal chains of lead chloride. In both compounds the organic ammoniums form regular layers that alternate with the inorganic chains.

Control of self-organized low-dimensional morphology in Poly(styrene-b-4vinylpyridine)/polystyrene blend thin films

The surface morphologies of poly(styrene-b-4vinylpyridine) (PS-b-P4VP) diblock copolymer and homopolystyrene (hPS) binary blend thin films were investigated by atomic force microscopy as a function of total volume fraction of PS (phi(PS)) in the mixture. It was found that when hPS was added into symmetric PS-b-P4VP diblock copolymers, the surface morphology of this diblock copolymer was changed to a certain degree. With phi(PS) increasing at first, hPS was solubilized into the corresponding domains of block copolymer and formed cylinders. Moreover, the more solubilized the hPS, the more cylinders exist. However, when the limit was reached, excessive hPS tended to separate from the domains independently instead of solubilizing into the corresponding domains any longer, that is, a macrophase separation occurred. A model describing transitions of these morphologies with an increase in phi(PS) is proposed. The effect of composition on the phase morphology of blend films when graphite is used as a substrate is also investigated.

TEMPERATURE-DEPENDENCE OF THE LOW-FIELD MOBILITY OF MINIBAND CONDUCTION IN SUPERLATTICES

The existing interpretation of the T-1 temperature dependence of the low-field miniband conduction is derived from certain concepts of conventional band theory for band structures resulting from spatial periodicities commensurable with the dimensionalities of the system. It is pointed out that such concepts do not apply to the case of miniband conduction, where we are dealing with band structures resulting from a one-dimensional periodicity in a three-dimensional system. It is shown that in the case of miniband conduction, the current carriers are distributed continuously over all energies in a sub-band, but only those with energies within the width of the miniband contribute to the current. The T-1 temperature dependence of the low-field mobility is due to the depletion of these current-carrying carriers with the rise of temperature.

Distinct two dimensional lateral ordering of self-assembled quantum dots

We report a quantum dot (QD) ensemble structure in which the in-plane arrangements of the dots are in a hexagonal way while the dots are also vertically aligned. Such a distinct lateral ordering of QDs is achieved on a planar GaAs(l 0 0) rather than on a prepatterned substrate by strain-mediated multilayer vertical stacking of the QDs. The analysis indicates that the strain energy of the lateral island-island interaction is minimum for arrangement of the hexagonal ordering. The ordered dots demonstrate strong photoluminescence (PL) emission at room temperature (RT) and the full width at half maximum of PL peak at RT is only 50 meV. (C) 2007 Elsevier B.V. All rights reserved.

An ultra-low power CMOS random number generator

This paper proposes an ultra-low power CMOS random number generator (RING), which is based on an oscillator-sampling architecture. The noisy oscillator consists of a dual-drain MOS transistor, a noise generator and a voltage control oscillator. The dual-drain MOS transistor can bring extra-noise to the drain current or the output voltage so that the jitter of the oscillator is much larger than the normal oscillator. The frequency division ratio of the high-frequency sampling oscillator and the noisy oscillator is small. The RNG has been fabricated in a 0.35 mu m CMOS process. It can produce good quality bit streams without any post-processing. The bit rate of this RNG could be as high as 100 kbps. It has a typical ultra-low power dissipation of 0.91 mu W. This novel circuit is a promising unit for low power system and communication applications. (c) 2007 Elsevier Ltd. All rights reserved.

Temperature dependence of surface quantum dots grown under frequent growth interruption

We grow In-GaAs quantum dot (QD) at low growth rate with 70 times insertion of growth interruption in MBE system. It is found that because of the extreme growth condition, QDs exhibit a thick wetting layer, large QD height value and special surface morphology which is attributed to the In segregation effect. Temperature dependence of photoluminescence measurement shows that this kind of QDs has a good thermal stability which is explained in terms of a "group coupling" model put forward by us. (C) 2007 Elsevier B.V. All rights reserved.

Fabrication of ultra-low density and long-wavelength emission InAs quantum dots

By optimizing the molecular beam epitaxy growth conditions of self-organized InAs/GaAs quantum dots (QDs), we obtained an ultra-low density system of InAs QDs (4 x 10(6)cm(-2)). Photoluminescence (PL) spectroscopy reveals the emission wavelength at room temperature to be longer than 1300 nm with a GaAs capping layer. (c) 2007 Elsevier B.V. All rights reserved.

Influence of nitridation time on the morphology of GaN nanorods synthesized by nitriding Ga2O3/ZnO films

Gallium nitride (GaN) nanorods were synthesized by nitriding Ga2O3/ZnO films which were deposited in turn on Si (111) substrates using radio frequency (RF) magnetron sputtering system. In the nitridation process, ZnO was reduced to Zn and Zn sublimated at 950 degrees C. Ga2O3 was reduced to Ga2O and Ga2O reacted with NH3 to synthesize GaN nanorods with the assistance of the sublimation of Zn. The morphology and structure of the nanorods were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The composition of GaN nanorods was studied by Fourier-transform infrared spectrophotometer (FTIR). The synthesized nanorods is hexagonal wurtzite structured. Nitridation time of the samples has an evident influence on the morphology of GaN nanorods synthesized by this method. (c) 2006 Elsevier B.V. All rights reserved.

Temperature dependence of surface quantum dots grown under frequent growth interruption

We grow InGaAs quantum dot (QD) at low growth rate with 70 times insertion of growth interruption in MBE system. It is found that because of the extreme growth condition, QDs exhibit a thick wetting layer, large QD height value and special surface morphology which is attributed to the enhanced adatom surface diffusion and In-segregation effect. Temperature dependence of photoluminescence measurement from surface QD shows that this kind of QD has good thermal stability which is explained in terms of the presence of surface oxide. The special distribution of QD may also play a role in this thermal character. (c) 2006 Elsevier B.V. All rights reserved.