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1.
Sci Rep ; 6: 28860, 2016 07 05.
Article in English | MEDLINE | ID: mdl-27377213

ABSTRACT

Herein we investigate a (001)-oriented GaAs1-xBix/GaAs structure possessing Bi surface droplets capable of catalysing the formation of nanostructures during Bi-rich growth, through the vapour-liquid-solid mechanism. Specifically, self-aligned "nanotracks" are found to exist trailing the Bi droplets on the sample surface. Through cross-sectional high-resolution transmission electron microscopy the nanotracks are revealed to in fact be elevated above surface by the formation of a subsurface planar nanowire, a structure initiated mid-way through the molecular-beam-epitaxy growth and embedded into the epilayer, via epitaxial overgrowth. Electron microscopy studies also yield the morphological, structural, and chemical properties of the nanostructures. Through a combination of Bi determination methods the compositional profile of the film is shown to be graded and inhomogeneous. Furthermore, the coherent and pure zincblende phase property of the film is detailed. Optical characterisation of features on the sample surface is carried out using polarised micro-Raman and micro-photoluminescence spectroscopies. The important light producing properties of the surface nanostructures are investigated through pump intensity-dependent micro-PL measurements, whereby relatively large local inhomogeneities are revealed to exist on the epitaxial surface for important optical parameters. We conclude that such surface effects must be considered when designing and fabricating optical devices based on GaAsBi alloys.

2.
Nanoscale Res Lett ; 11(1): 309, 2016 Dec.
Article in English | MEDLINE | ID: mdl-27342603

ABSTRACT

Electronic and optical properties of InAs/GaAs nanostructures grown by the droplet epitaxy method are studied. Carrier states were determined by k · p theory including effects of strain and In gradient concentration for a model geometry. Wavefunctions are highly localized in the dots. Coulomb and exchange interactions are studied and we found the system is in the strong confinement regime. Microphotoluminescence spectra and lifetimes were calculated and compared with measurements performed on a set of quantum rings in a single sample. Some features of spectra are in good agreement.

3.
Nanotechnology ; 25(24): 245602, 2014 Jun 20.
Article in English | MEDLINE | ID: mdl-24869600

ABSTRACT

We report on the mechanism of strain-influenced quantum well (QW) thickness reduction in GaN/AlN short-period superlattices grown by plasma-assisted molecular beam epitaxy. Density functional theory was used to support the idea of a thermally activated exchange mechanism between Al adatoms and Ga surface atoms that is influenced by the strain state of the GaN QWs. These ab initio calculations support our experimentally observed reduction in QW thickness for different intrinsic strains.

4.
Nanotechnology ; 25(3): 035702, 2014 Jan 24.
Article in English | MEDLINE | ID: mdl-24346504

ABSTRACT

The structural and optical properties of GaAs1-xBix quantum wells (QWs) symmetrically clad by GaAs barriers with and without additional confining AlGaAs layers are studied. It is shown that a GaAs/GaAs1-xBix/GaAs QW with x ~ 4% and well width of ~ 4 nm grown by molecular beam epitaxy demonstrates efficient photoluminescence (PL) that becomes significantly more thermally stable when a cladding AlGaAs layer is added to the QW structure. The PL behavior for temperatures between 10 and 300 K and for excitation intensities varying by seven orders of magnitude can be well described in terms of the dynamics of excitons including carrier capture in the QW layer, thermal emission and diffusion into the cladding barriers. Understanding the role of these processes in the luminescence of dilute GaAs1-xBix QW structures facilitates the creation of highly efficient devices with reduced thermal sensitivity and low threshold current.

5.
Nanotechnology ; 19(50): 505605, 2008 Dec 17.
Article in English | MEDLINE | ID: mdl-19942777

ABSTRACT

Lateral ordering of InGaAs quantum dots on the GaAs (001) surface has been achieved in earlier reports, resembling an anisotropic pattern. In this work, we present a method of breaking the anisotropy of ordered quantum dots (QDs) by changing the growth environment. We show experimentally that using As(2) molecules instead of As(4) as a background flux is efficient in controlling the diffusion of distant Ga adatoms to make it possible to produce isotropic ordering of InGaAs QDs over GaAs (001). The control of the lateral ordering of QDs under As(2) flux has enabled us to improve their optical properties. Our results are consistent with reported experimental and theoretical data for structure and diffusion on the GaAs surface.

6.
Opt Express ; 15(13): 8157-62, 2007 Jun 25.
Article in English | MEDLINE | ID: mdl-19547142

ABSTRACT

GaAs spacer thicknesses are varied to tune the coupling between InGaAs surface quantum dots (QDs) and multilayers of buried QDs. Temperature and excitation intensity dependence of the photoluminescence together with time resolved photoluminescence reveal that coupling between layers of QDs and consequently the optical properties of both the surface and the buried QDs significantly depend on the GaAs spacer. This work provides an experimental method to tune and control the optical performance of surface QDs.

7.
Science ; 311(5761): 636-9, 2006 Feb 03.
Article in English | MEDLINE | ID: mdl-16410487

ABSTRACT

An asymmetric pair of coupled InAs quantum dots is tuned into resonance by applying an electric field so that a single hole forms a coherent molecular wave function. The optical spectrum shows a rich pattern of level anticrossings and crossings that can be understood as a superposition of charge and spin configurations of the two dots. Coulomb interactions shift the molecular resonance of the optically excited state (charged exciton) with respect to the ground state (single charge), enabling light-induced coupling of the quantum dots. This result demonstrates the possibility of optically coupling quantum dots for application in quantum information processing.

8.
Phys Rev Lett ; 95(17): 177403, 2005 Oct 21.
Article in English | MEDLINE | ID: mdl-16383867

ABSTRACT

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric e-e and e-h exchange interactions.

9.
Phys Rev Lett ; 94(4): 047402, 2005 Feb 04.
Article in English | MEDLINE | ID: mdl-15783594

ABSTRACT

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.

10.
Phys Rev Lett ; 93(21): 216402, 2004 Nov 19.
Article in English | MEDLINE | ID: mdl-15601037

ABSTRACT

We report a large and unexpected suppression of the free electron spin-relaxation in lightly doped n-GaAs bulk crystals. The spin-relaxation rate shows a weak mobility dependence and saturates at a level 30 times less than that predicted by the Dyakonov-Perel theory. The dynamics of the spin-orbit field differs substantially from the usual scheme: although all the experimental data can be self-consistently interpreted as a precessional spin-relaxation induced by a random spin-orbit field, the correlation time of this random field, surprisingly, is much shorter than, and is independent of, the momentum relaxation time determined from transport measurements.

11.
Work ; 10(3): 251-9, 1998.
Article in English | MEDLINE | ID: mdl-24441406

ABSTRACT

The article presents four case studies of employed women who have had breast cancer. Specifically, the authors examine respondents' career maintenance concerns in the areas of (a) worksite accessibility, (b) performance of essential job functions, (c) job mastery, and (d) job satisfaction. Post-employment strategies are presented to assist rehabilitation professionals in meeting the on-the-job needs of women with breast cancer.

12.
Child Health Care ; 19(1): 37-42, 1990.
Article in English | MEDLINE | ID: mdl-10106397

ABSTRACT

This study compared mothers', fathers', nurses', and physicians' perceptions of parents' reactions to stressful stimuli when their children were hospitalized. Thirty-six mothers, 14 fathers, 27 nurses, and 23 physicians rated 36 stressful stimuli on an inventory the investigators developed. Analysis of Variance and subsequent multiple comparison tests disclosed numerous differences between parents and health care professionals and between mothers and fathers on the Parental Uncertainty, Annoyance, Child Discomfort, and Negative Emotional States Scales. Further analyses of ratings on individual items from the scales revealed the bases of the differences. Implications for health professionals and directions for further research are discussed.


Subject(s)
Child, Hospitalized/psychology , Fathers/psychology , Mothers/psychology , Stress, Psychological/etiology , Stress, Psychological/prevention & control , Child , Child, Preschool , Evaluation Studies as Topic , Female , Humans , Male , Nurses , Physicians , Social Perception , Surveys and Questionnaires
13.
J Gen Psychol ; 103(1st Half): 41-8, 1980 Jul.
Article in English | MEDLINE | ID: mdl-7411124

ABSTRACT

Four experiments using stimuli which varied in size or texture investigated whether shifts in discrimination occurred when anchors and series stimuli were integral or nonintegral A total of 120 male and female college students judged series stimuli with respect to size. Anchors differed from the series stimuli in size or texture. The participants always observed the anchors visually. The E presented series stimuli either to the visual (intramodal) or haptic (crossmodal) sense. Analysis of variance revealed significant shifts in perceptual judgment for anchors presented both intra- and crossmodally but only when anchors and series stimuli dimensions were integral. Adaptation-level theory predicted significant shifts in judgment and properties associated with the dimensional structure of stimuli accounted for the limits of anchor effectiveness.


Subject(s)
Discrimination Learning , Size Perception , Touch , Female , Humans , Male
14.
Am J Psychol ; 79(3): 458-63, 1966 Sep.
Article in English | MEDLINE | ID: mdl-5968483
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