Technique for enhancing generation power density of silicon photovoltaic devices

A new type of photovoltaic system with higher generation power density has been studied in detail. The feature of the proposed system is a V-shaped structure with two polycrystalline solar cells. Compared to solar cells in a conventional approach, the V-shaped structure enhances external quantum efficiency and leads to an increase of 24% in power conversion efficiency.

Extremely low density InAs quantum dots realized in situ on (100) GaAs

Extremely low density self-assembled InAs quantum dots are grown by a combination technique of in situ annealing for 2 min and pause of substrate rotation during molecular beam epitaxy. The surface morphology and structural characteristics of the quantum dots are scrutinized by atomic force microscopy and photoluminescence spectra. It is found that the quantum dot size and density increase as the InAs deposition amount rises. Quantum dots with a density between 2.5 x 10(7) cm(-2) and 2.2 x 10(8) cm(-2) are 2-5 nm in height and 18-39 nm in diameter. It is believed that as-grown InAs nanodots may be of important value for future single quantum dot research.

Density matrix Loschmidt echo and quantum phase transitions

We introduce the concept of the Loschmidt echo (LE) to the space of the reduced density matrix of spin and fermionic systems to study the density matrix LEs (DMLEs) of the one-dimensional extended Hubbard model and the transverse field Ising model. Our results show that the DMLEs are remarkably influenced by the criticality of the system, and the method is a convenient way to study quantum phase transitions.

Effect of implantation of nitrogen into SIMOX buried oxide on its fixed positive charge density

In order to obtain greater radiation hardness for SIMOX (separation by implanted oxygen) materials, nitrogen was implanted into SIMOX BOX (buried oxide). However, it has been found by the C-V technique employed in this work that there is an obvious increase of the fixed positive charge density in the nitrogen-implanted BOX with a 150 out thickness and 4 x 10(15) cm(-2) nitrogen implantation dose, compared with that unimplanted with nitrogen. On the other hand, for the BOX layers with a 375 nm thickness and implanted with 2 x 10(15) and 3 x 10(15) cm(-2) nitrogen doses respectively, the increase of the fixed positive charge density induced by implanted nitrogen has not been observed. The post-implantation annealing conditions are identical for all the nitrogen-implanted samples. The increase in fixed positive charge density in the nitrogen-implanted 150 nm BOX is ascribed to the accumulation of implanted nitrogen near the BOX/Si interface due to the post-implantation annealing process according to SIMS results. In addition, it has also been found that the fixed positive charge density in initial BOX is very small. This means SIMOX BOX has a much lower oxide charge density than thermal SiO2 which contains a lot of oxide charges in most cases.

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.

Extremely low density InAs quantum dots with no wetting layer

Extremely low density InAs quantum dots (QDs) are grown by molecular beam droplet epitaxy, The gallium deposition amount is optimized to saturate exactly the excess arsenic atoms present on the GaAs substrate surface during growth, and low density InAs/GaAs QDs (4x10(6) cm(-2)) are formed by depositing 0.65 monolayers (ML) of indium. This is much less than the critical deposition thickness (1.7 ML), which is necessary to form InAs/GaAs QDs with the conventional Stranski-Krastanov growth mode. The narrow photoluminescence line-width of about 24 meV is insensitive to cryostat temperatures from 10 K to 250 K. All measurements indicate that there is no wetting layer connecting the QDs.

Quantum master equation scheme of time-dependent density functional theory to time-dependent transport in nanoelectronic devices

In this work a practical scheme is developed for the first-principles study of time-dependent quantum transport. The basic idea is to combine the transport master equation with the well-known time-dependent density functional theory. The key ingredients of this paper include (i) the partitioning-free initial condition and the consideration of the time-dependent bias voltages which base our treatment on the Runge-Gross existence theorem; (ii) the non-Markovian master equation for the reduced (many-body) central system (i.e., the device); and (iii) the construction of Kohn-Sham master equations for the reduced single-particle density matrix, where a number of auxiliary functions are introduced and their equations of motion (EOMs) are established based on the technique of spectral decomposition. As a result, starting with a well-defined initial state, the time-dependent transport current can be calculated simultaneously along with the propagation of the Kohn-Sham master equation and the EOMs of the auxiliary functions.

Low threshold current density operation of strain-compensated quantum cascade laser

We report the low threshold current density operation of strain-compensated In0.64Ga0.36As/In0.38Al0.62As quantum cascade lasers emitting near 4.94 mu m. By employing an enlarged strain-compensated structure and optimizing the injector doping density, a rather low threshold current density of 0.57 kA/cm(2) at 80K is achieved for an uncoated 20-mu m-wide and 2.5-mm-long laser.

Strain-induced anodization of SiGe/Si multiple layers to form high density SiGe/Si heterogeneous nanorods

Nation Natural Science Foundation of China 50672079 60676027 60837001 60776007; National Basic Research Program of China (973 Program) 2007CB613404; China-MOST International Sci & Tech Cooperation and Exchange 2008DFA51230

Mode softening in charge-density excitations of a two-dimensional electron gas driven by Rashba spin-orbit interaction

The electron density response of a uniform two-dimensional (2D) electron gas is investigated in the presence of a perpendicular magnetic field and Rashba spin-orbit interaction (SOI). It is found that, within the Hartree-Fock approximation, a charge density excitation mode below the cyclotron resonance frequency shows a mode softening behavior, when the spin-orbit coupling strength falls into a certain interval. This mode softening indicates that the ground state of an interacting uniform 2D electron gas may be driven by the Rashba SOI to undergo a phase transition to a nonuniform charge density wave state.

Charge-density and spin-density excitations in a two-dimensional electron gas with Rashba spin-orbit coupling

We study theoretically the charge-density and spin-density excitations in a two-dimensional electron gas in the presence of a perpendicular magnetic field and a Rashba type spin-orbit coupling. The dispersion and the corresponding intensity of excitations in the vicinity of cyclotron resonance frequency are calculated within the framework of random phase approximation. The dependence of excitation dispersion on various system parameters, i.e., the Rashba spin-orbit interaction strength, the electron density, the Zeeman spin splitting, and the Coulomb interaction strength is investigated.

Two opposite gradients of hole density in as-grown and annealed (Ga,Mn)As layers

The depth distribution of the hole density p in 500 nm-thick (Ga,Mn)As layers is investigated. From Raman scattering spectra, it is found that the gradients of p are opposite in the as-grown and annealed layers. At the region around the free surface, with increasing etching depth, p significantly increases in the as-grown layer; however, p decreases distinctly in the annealed layer. Then, in the bulk, p becomes almost homogeneous for both cases. The etching-depth dependence of Curie temperature obtained from magnetic measurements is in agreement with the distribution characterization of p. These results suggest that annealing induces outdiffusion of Mn interstitials towards the free surface, and incomplete outdiffusion during the growth leads to an accumulation of Mn interstitials around the free surface of the as-grown (Ga,Mn)As. (c) 2006 Elsevier B.V. All rights reserved.

High-power and low-threshold-current-density GaAs/AlGaAs quantum cascade lasers

We report on the realization of GaAs/AlGaAs quantum cascade lasers with an emission wavelength of 9.1 mu m above the liquid nitrogen temperature. With optimal current injection window and ridge width of 24 and 60 mu m respectively, a peak output power more than 500 mW is achieved in pulsed mode operation. A low threshold current density J(th) = 2.6 kA/cm(2) gives the devices good lasing characteristics. In a drive frequency of 1 kHz, the laser operates up to 20% duty cycle.

Temperature and power-density-dependent inter-shell energy states in InAs/GaAs quantum dots

We have investigated the evolution of exciton state filling in InAs/GaAs quantum dot (QD) structures as a function of the excitation power density by using rnicro-photoluminescence spectroscopy at different temperatures. In addition to the emission bands of exciton recombination corresponding to the atom-like S, P and D, etc. shells of QDs, it was observed that some extra states V between the S and P shells, and D' between the P and D shells appear in the spectra with increasing number of excitons occupying the QDs at a certain temperature. The emergence of these inter-shell excitonic levels is power density and temperature dependent, which is an experimental demonstration of strong exciton-exciton exchange interaction, state hybridization, and coupling of a multi-exciton system in QDs. (c) 2006 Elsevier B.V. All rights reserved.

Correlation effects for semiconducting single-wall carbon nanotubes: A density matrix renormalization group study

In this paper we report the applicability of the density matrix renormalization group (DMRG) approach to the cylindrical single wall carbon nanotube (SWCN) for the purpose of its correlation effect. By applying the DMRG approach to the t+U+V model, with t and V being the hopping and Coulomb energies between the nearest neighboring sites, respectively, and U the on-site Coulomb energy, we calculate the phase diagram for the SWCN with chiral numbers (n(1)=3, n(2)=2), which reflects the competition between the correlation energy U and V. Within reasonable parameter ranges, we investigate possible correlated ground states, the lowest excitations, and the corresponding correlation functions in which the connection with the excitonic insulator is particularly addressed.