Size and shape evolution of self-assembled coherent InAs/GaAs quantum dots influenced by seed layer

The size and shape Evolution of self-assembled InAs quantum dots (QDs) influenced by 2.0-ML InAs seed layer has been systematically investigated for 2.0, 2.5, and 2.9-ML deposition on GaAs(1 0 0) substrate. Based on comparisons with the evolution of InAs islands on single layer samples at late growth stage, the bimodal size distribution of InAs islands at 2.5-ML InAs coverage and the formation of larger InAs quantum dots at 2.9-ML deposition have been observed on the second InAs layer. The further cross-sectional transmission electron microscopy measurement indicates the larger InAs QDs: at 2.9-ML deposition on the second layer are free of dislocation. In addition, the interpretations for the size and shape evolution of InAs/GaAs QDs on the second layer will be presented. (C) 2001 Elsevier Science B.V. All lights reserved.

A density-functional study of small titanium clusters

The atomic structures and electronic properties of small Ti-N (N=2-10) clusters have been studied by using the density-functional theory with a local spin density approximation. We find that the inner-shells (3s3p) of the titanium atom plays an important role in the formation of the small clusters. We have obtained the ground state of titanium clusters, Ti-7 is found to be a magic cluster, which is in good agreement with the experimental results. Starting with Ti-8 cluster some features of the electronic structure of the titanium bulk have been developed. The ionization potentials and magnetic moments for these small titanium clusters are also presented. (C) 2000 American Institute of Physics. [S0021- 9606(00)30544-X].

Effects of seed layer on the realization of larger self-assembled coherent InAs/GaAs quantum dots

The size and shape evolution of self-assembled InAs quantum dots (QDs) influenced by 2.0 ML InAs seed layer has been systematically investigated for 2.0, 2.5, and 2.9 ML deposition on GaAs(100) substrate. Based on comparisons with the formation of large incoherent InAs islands on single-layer samples at late growth stage, the larger coherent InAs quantum dots at 2.9 ML deposition has been observed on the second InAs layer. A simple model analysis accounting for the surface strain distribution influenced by buried islands gives a stronger increment of critical QD diameter for dislocation nucleation on the second layer in comparison with the single-layer samples. Additionally, the inhibition of dislocation nucleation in InGaAs/GaAs large islands can also be explained by our theoretical results. (C) 2000 American Institute of Physics. [S0021-8979(00)08922-2].

High-density InAs nanowires realized in situ on (100) InP

High-density InAs nanowires embedded in an In0.52Al0.48As matrix are fabricated in situ by molecular beam epitaxy on (100) InP. The average cross section of the nanowires is 4.5 x 10 nm(2). The linear density is as high as 70 wires/mu m. The spatial alignment of the multilayer arrays exhibit strong anticorrelation in the growth direction. Large polarization anisotropic effect is observed in polarized photoluminescence measurements. (C) 1999 American Institute of Physics. [S0003-6951(99)04134-0].

Desorption and Ripening of Low Density InAs Quantum Dots

In this paper, combining low deposition rate with proper growth temperature, we have developed a way to prepare very low-density quantum dots (QDs) suited for the study of single OD properties without resorting to submicron lithography. Experiment results demonstrate that InAs desorption is significant during growing the low density QDs. Ripening of InAs QDs is clearly observed during the post-growth annealing. Photoluminescence spectroscopy reveals that the emission wavelength of low density InAs QDs arrives at 1332.4 nm with a GaAs capping layer.

Edge dislocation of b=[001]/2 in the InAs nanostructure on InP(001)

The self-organized InAs/In0.52Al0.48As nanostructure were grown on InP (001) using molecular beam epitaxy (MBE). The nanostructure has been studied using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The edge dislocations with the Burgers vector b = ([001]/2) and extending along the [$(110) over bar $] direction are observed. The results show that in the region near an edge dislocation, no InAs wires were formed, while in the regions free of dislocation, wire-like nanostructures were formed. The mechanisms for the formation of the [001]/2 edge dislocations were discussed.

Monte Carlo simulation of the modulated effect induced by the dislocation to the quantum dot growth

Performing an event-based continuous kinetic Monte Carlo simulation, we investigate the modulated effect induced by the dislocation on the substrate to the growth of semiconductor quantum dots (QDs). The relative positions between the QDs and the dislocations are studied. The stress effects to the growth of the QDs are considered in simulation. The simulation results are compared with the experiment and the agreement between them indicates that this simulation is useful to study the growth mode and the atomic kinetics during the growth of the semiconductor QDs. (c) 2006 Elsevier Ltd. All rights reserved.

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.

A V-shaped module technique for promoting generation photocurrent density of silicon solar cells

A V-shaped solar cell module consists of two tilted mono-crystalline cells [J. Li, China Patent No. 200410007708.6 (March, 2004)]. The angle included between the two tilted cells is 90 degrees. The two cells were fabricated by using polished silicon wafers. The scheme of both-side polished wafers has been proposed to reduce optical loss. Compared to solar cells in a planar way, the V-shaped structure enhances external quantum efficiency and leads to an increase of 15% in generation photocurrent density. The following three kinds of trapped photons are suggested to contribute to the increase: (1) infrared photons converted from visible photons due to a transformation mechanism, (2) photons reflected from top contact metal, and (3) a residual reflection which can not be eliminated by an antireflection coating.

Low threshold current density, low resistance oxide-confined VCSEL fabricated by a dielectric-free approach

We present the fabrication process and experimental results of 850-nm oxide-confined vertical cavity surface emitting lasers (VCSELs) fabricated by using dielectric-free approach. The threshold current of 0.4 mA, which corresponds to the threshold current density of 0.5 kA/cm(2), differential resistance of 76 Omega, and maximum output power of more than 5 mW are achieved for the dielectric-free VCSEL with a square oxide aperture size of 9 mu m at room temperature (RT). L-I-V characteristics of the dielectric-free VCSEL are compared with those of conventional VCSEL with the similar aperture size, which indicates the way to realize low-cost, low-power consumption VCSELs with extremely simple process. Preliminary study of the temperature-dependent L-I characteristics and modulation response of the dielectric-free VCSEL are also presented.

Broadband external cavity tunable quantum dot lasers with low injection current density

Broadband grating-coupled external cavity laser, based on InAs/GaAs quantum dots, is achieved. The device has a wavelength tuning range from 1141.6 nm to 1251.7 nm under a low continuous-wave injection current density (458 A/cm(2)). The tunable bandwidth covers consecutively the light emissions from both the ground state and the 1st excited state of quantum dots. The effects of cavity length and antireflection facet coating on device performance are studied. It is shown that antireflection facet coating expands the tuning bandwidth up to similar to 150 nm, accompanied by an evident increase in threshold current density, which is attributed to the reduced interaction between the light field and the quantum dots in the active region of the device.

Temperature dependent photoluminescence of an In(Ga)As/GaAs quantum dot system with different areal density

We have systematically studied the temperature dependent photoluminescence of a self-assembled In(Ga)As/GaAs quantum dot (QD) system with different areal densities from similar to 10(9) to similar to 10(11) cm(-2). Different carrier channels are revealed experimentally and confirmed theoretically via a modified carrier equation model considering a new carrier transfer channel, i.e. continuum states ( CS). The wetting layer is demonstrated to be the carrier quenching channel for the low-density QDs but the carrier transfer channel for the high-density QDs. In particular, for the InGaAs/GaAs QDs with a medium density of similar to 10(10) cm(-2), the CS is verified to be an additional carrier transfer channel in the low temperature regime of 10-60 K, which is studied in detail via our models. The possible carrier channels that act on different temperature regimes are further discussed, and it is demonstrated that density is not a crucial factor in determining the carrier lateral coupling strength.