Interface-related in-plane optical anisotropy of quantum wells studied by reflectance-difference spectroscopy

The in-plane optical anisotropy of three groups of GaAs/AlGaAs quantum well structures has been studied by reflectance-difference spectroscopy (RDS). For GaAs/Al0.36Ga0.64As single QW structures, it is found that the optical anisotropy increases quickly as the well width is decreased. For an Al0.02Ga0.98As/AlAs multiple QW with a well width of 20nm, the optical anisotropy is observed not only for the transitions between ground states but also for those between the excited states with transition index n up to 5. An increase of the anisotropy with the transition energy, or equivalently the transition index n, is clearly observed. The detailed analysis shows that the observed anisotropy arises from the interface asymmetry of QWs, which is introduced by atomic segregation or anisotropic interface roughness formed during the growth of the structures. More, when the 1 ML InAs is inserted at one interface of GaAs/AlGaAs QW, the optical anisotropy of the QW can be increased by a factor of 8 due to the enhanced asymmetry of the QW. These results demonstrate clearly that the RDS is a sensitive and powerful tool for the characterization of semiconductor interfaces.

Surface morphology control of strained InAs/GaAs(331)A films: From nanowires to island-pit pairs

We have studied the effect of molecular beam epitaxy growth conditions on the surface morphology of strained InAs/GaAs(331)A films. Our results reveal that InAs nanowires aligned along the [1 (1) over bar0] direction are formed under As-rich conditions, which is explained by the effect of anisotropic buffer layer surface roughing. Under In-rich conditions, however, the surface morphology of the InAs layers is characterized by a feature of island-pit pairs. In this case, cooperative nucleation of islands and pits can lower the activation barrier for domain growth. These results suggest that the surface morphology of strained InAs layers is highly controllable. (C) 2005 American Institute of Physics.

Dependence of elastic strain field on the self-organized ordering of quantum dot superlattices

A systematic investigation of the strain distribution of self-organized, lens-shaped quantum dot in the case of growth direction on (001) substrate was presented. The three-dimensional finite element analysis for an array of dots was used for the strain calculation. The dependence of the strain energy density distribution on the thickness of the capping layer was investigated in detail when the elastic characteristics of the matrix material were anisotropic. It is shown that the elastic anisotropic greatly influences the stress, strain, and strain energy density in the quantum dot structures. The anisotropic ratio of the matrix material and the combination with different thicknesses of the capping layer, may lead to different strain energy density minimum locations on the capping layer surface, which can result in various vertical ordering phenomena for the next layer of quantum dots, i.e. partial alignment, random alignment, and complete alignment.

SWAP operation in the two-qubit Heisenberg XXZ model: Effects of anisotropy and magnetic field

In this paper we study the SWAP operation in a two-qubit anisotropic XXZ model in the presence of an inhomogeneous magnetic field. We establish the range of anisotropic parameter lambda within which the SWAP operation is feasible. The SWAP errors caused by the inhomogeneous field are evaluated.

Inductively coupled plasma etching of two-dimensional InP/InGaAsP-based photonic crystal

Inductively coupled plasma (ICP) etching of InP in Cl-2/BCl3 gas mixtures is studied in order to achieve low-damage and high-anisotropy etching of two-dimensional InP/InGaAsP photonic crystal. The etching mechanisms are discussed and the effect of plasma heating on wafer during etching is analyzed. It is shown that the balance between the undercut originating from plasma heating and the redeposition of sputtering on the side-wall is crucial for highly anisotropic etching, and the balance point moves toward lower bias when the ICP power is increased. High aspect-ratio etching at the DC bias of 203 V is obtained. Eventually, photonic crystal structure with nearly 90 degrees side-wall is achieved at low DC bias after optimization of the gas mixture.

Polarized Raman scattering studies of nonpolar a-plane GaN films grown on r-plane sapphire substrates by MOCVD

Nonpolar (1120) a-plane GaN thin films were grown on r-plane (1102) sapphire substrates by low-pressure metal organic chemical vapor deposition (MOCVD). The stress characteristics of the a-plane GaN films were investigated by means of polarized Raman scattering spectra in backscattering configurations. The experimental results show that there are strong anisotropic in-plane stresses within the epitaxial a-plane GaN films by calculating the corresponding stress tensors. The temperature dependence of Raman scattering spectra was studied in the range from 100 K to 550 K. The measurements reveal that the Raman phonon frequencies decrease with increasing temperature. The temperature at which nonpolar a-plane GaN films are strain free is discussed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Tunable Fano resonance in photonic crystal slabs

A three dimensional analysis of a special class of anisotropic materials is presented. We introduce an extension of the Scattering Matrix Method (SMM) to investigate the behavior of anisotropic Photonic Crystal Slabs (PhCS) subject to external radiation. We show how the Fano effect can play a fundamental role in the realization of tunable optical devices. Moreover, we show how to utilize electron injection, electric field and temperature as parameters to control the Fano resonance shift in both isotropic and anisotropic materials as Si and Potassium Titanium Oxide Phosphate (KTP). We will see that because Fano modes are sensitive and controllable, a broad range of applications can be considered. (c) 2006 Optical Society of America

Synthesis and structures of morphology-controlled ZnO nano- and microcrystals

Zinc oxide flower-like bunches were directly synthesized on indium-doped tin oxide (ITO) glass substrates through a simple chemical bath deposition process. By adjusting precursor concentration, other morphologies ( spindles and rods) were also obtained. All of them are hexagonal and single crystalline in nature and grow along the [ 0001] crystallographic direction. The possible growth mechanisms for these nano- and microcrystals were proposed. It was revealed that both the inherent highly anisotropic structure of ZnO and the precursor concentration play crucial roles in determining final morphologies of the products. In addition, vibrational properties of ZnO crystals with different morphologies were investigated by Raman spectroscopy.

Rashba spin splitting in biased semiconductor quantum wells

Rashba spin splitting (RSS) in biased semiconductor quantum wells is investigated theoretically based on eight-band k center dot p theory. We find that at large wave vectors, RSS is both nonmonotonic and anisotropic as a function of in-plane wave vector, in contrast to the widely used isotropic linear model. We derive an analytical expression for RSS, which can qualitatively reproduce such nonmonotonic behavior at large wave vectors. We also investigate numerically the dependence of RSS on the various band parameters and find that RSS increases with decreasing band gap and subband index, increasing valence band offset, external electric field, and well width. All these dependences can be qualitatively described by our analytical model.

Interface-related in-plane optical anisotropy in GaAs/AlxGa1-xAs single-quantum-well structures studied by reflectance difference spectroscopy

The in-plane optical anisotropies of a series of GaAs/AlxGa1-xAs single-quantum-well structures have been observed at room temperature by reflectance difference spectroscopy. The measured degree of polarization of the excitonic transitions is inversely proportional to the well width. Numerical calculations based on the envelope function approximation incorporating the effect of C-2v-interface symmetry have been performed to analyze the origin of the optical anisotropy. Good agreement with the experimental data is obtained when the optical anisotropy is attributed to anisotropic-interface structures. The fitted interface potential parameters are consistent with predicted values.

Influence of strain on annealing effects of In(Ga)As quantum dots

Post-growth rapid thermal annealing has been performed with In(Ga)As quantum dots (QDs) at different strain statuses. It is confirmed that the strain-enhanced interdiffusion decreases the inhomogeneous size distribution. The preferential lateral interdiffusion of QDs during annealing was observed. we attribute it to the naturally anisotropic strain distribution in/around the dots and the saturation of strain difference between the base boundary and the top of the dots. There exist strain-enhanced mechanism and vacancy diffusion enhanced mechanism during the annealing. As to which one dominates the QD interdiffusion depends on the thickness of capping layer and the annealing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

Determination of the values of hole-mixing coefficients due to interface and electric field in GaAs/AlxGa1-xAs superlattices

Numerical calculations within the envelope function framework have been performed to analyze the relations between the magnitude of in-plane optical anisotropy and the values of the additional hole-mixing coefficients due to interface and electric field in (001) symmetric GaAs/AlxGa1-xAs superlattices for light propagating along the [001] direction. It is found that the heavy- and light-hole states are mixed independently by interface and electric field. The numeric results demonstrate that the line shape of the in-plane anisotropic spectrum is determined by the ratio of the two hole-mixing coefficients. Theoretical analysis shows that with the help of simple calculation of the anisotropy at k=0, reliable values of the hole-mixing coefficients can be determined by reflectance-difference spectroscopy (IDS) technique, demanding no tedious fitting of experimental curves. The in-plane optical anisotropy measured by RDS provides a new method of getting the information on buried interfaces through the Value of the hole-mixing coefficient due to interface.

Polishing-related optical anisotropy of semi-insulating GaAs studied by reflectance difference spectroscopy

The strong in-plane optical anisotropy of (001) semi-insulating GaAs, which comes from the submicron region under the surface, has been observed by reflectance difference spectroscopy. The optical anisotropy can be explained by the anisotropic strain that is introduced by the asymmetric distribution of 60 degrees dislocations during surface polishing. The simulated spectra reproduce the line shape of the experimental ones. The simulations show that the anisotropic strain is typically about 2.3x10(-4). (C) 2000 American Institute of Physics. [S0021-8979(00)01315-3].

Optical-phonon modes and Frohlich potential in quantum dots

By extending the microscopic dipole model on optical-phonon modes as applied in quantum wells and quantum wires, to rectangular quantum dots (QD), optical phonon modes and their accompanying Frohlich potentials in QD are calculated and classified. When the bulk phonon dispersion is ignored, the optical phonon modes in QD can be clearly divided into the confined LO- and TO-bulk-like modes and the extended interface-like modes. Among the interface-like modes, a special attention is given to the corner modes, whose anisotropic behavior is depicted in the long wavelength limit. Based on the numerical results, a set of analytical formula are proposed to approximately describe the bulk-like modes, for which both the optical displacements and Frohlich potentials vanish at the interfaces. (C) 2000 Elsevier Science Ltd. All rights reserved.

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].