Impact of Arsenic Species on Self-Assembly of Triangular and Hexagonal Tensile-Strained GaAs(111)A Quantum Dots

We use dimeric arsenic (As2) or tetrameric arsenic (As4) during molecular beam epitaxy to manipulate the structural and optical properties of GaAs(111)A tensile-strained quantum dots (TSQDs). Choice of arsenic species affects nucleation and growth behavior during TSQD self-assembly. Previously, epitaxial GaAs(111)A TSQDs have been grown with As4, producing TSQDs with a triangular base, and \u27A-step\u27 edges perpendicular to the three 1̅1̅2 directions. We demonstrate that using As2 at low substrate temperature also results in triangular GaAs(111)A TSQDs, but with \u27B-step\u27 edges perpendicular to the three 112̅ directions. We can therefore invert the crystallographic orientation of these triangular nanostructures, simply by switching between As4 and As2. At higher substrate temperatures, GaAs(111)A TSQDs grown under As2 develop with a hexagonal base. Compared with triangular dots, the higher symmetry of hexagonal TSQDs may reduce fine-structure splitting on this (111) surface, a requirement for robust photon entanglement. Regardless of shape, GaAs(111)A TSQDs grown under As2 exhibit superior optical quality

Self-Assembly of (111)-Oriented Tensile-Strained Quantum Dots by Molecular Beam Epitaxy

The authors report on a comprehensive study of the growth of coherently strained GaAs quantum dots (QDs) on (111) surfaces via the Stranski–Krastanov (SK) self-assembly mechanism. Recent reports indicate that the long-standing challenges, whereby the SK growth mechanism could not be used to synthesize QDs on (111) surfaces, or QDs under tensile strain, have been overcome. However, a systematic study of the SK growth of (111)-oriented, tensile-strained QDs (TSQDs) as a function of molecular beam epitaxy growth parameters is still needed. Here, the authors explore the effects of deposition amount, substrate temperature, growth rate, and V/III flux ratio on the SK-driven self-assembly of GaAs(111)A TSQDs. The authors highlight aspects of TSQD SK self-assembly on (111) surfaces that appear to differ from the SK growth of traditional compressively strained QDs on (100) surfaces. The unique properties of (111) QDs and tensile-strained QDs mean that they are of interest for various research areas. The results discussed here offer a practical guide for tailoring the size, shape, density, uniformity, and photon emission wavelength and intensity of (111) TSQDs for future applications

Self-assembly of (111)-oriented tensile-strained quantum dots by molecular beam epitaxy

The authors report on a comprehensive study of the growth of coherently strained GaAs quantum dots (QDs) on (111) surfaces via the Stranski–Krastanov (SK) self-assembly mechanism. Recent reports indicate that the long-standing challenges, whereby the SK growth mechanism could not be used to synthesize QDs on (111) surfaces, or QDs under tensile strain, have been overcome. However, a systematic study of the SK growth of (111)-oriented, tensile-strained QDs (TSQDs) as a function of molecular beam epitaxy growth parameters is still needed. Here, the authors explore the effects of deposition amount, substrate temperature, growth rate, and V/III flux ratio on the SK-driven self-assembly of GaAs(111)A TSQDs. The authors highlight aspects of TSQD SK self-assembly on (111) surfaces that appear to differ from the SK growth of traditional compressively strained QDs on (100) surfaces. The unique properties of (111) QDs and tensile-strained QDs mean that they are of interest for various research areas. The results discussed here offer a practical guide for tailoring the size, shape, density, uniformity, and photon emission wavelength and intensity of (111) TSQDs for future applications

Researching the use of force: The background to the international project

This article provides the background to an international project on use of force by the police that was carried out in eight countries. Force is often considered to be the defining characteristic of policing and much research has been conducted on the determinants, prevalence and control of the use of force, particularly in the United States. However, little work has looked at police officers’ own views on the use of force, in particular the way in which they justify it. Using a hypothetical encounter developed for this project, researchers in each country conducted focus groups with police officers in which they were encouraged to talk about the use of force. The results show interesting similarities and differences across countries and demonstrate the value of using this kind of research focus and methodology

Structural Comparison of Human Mammalian Ste20-Like Kinases

BACKGROUND: The serine/threonine mammalian Ste-20 like kinases (MSTs) are key regulators of apoptosis, cellular proliferation as well as polarization. Deregulation of MSTs has been associated with disease progression in prostate and colorectal cancer. The four human MSTs are regulated differently by C-terminal regions flanking the catalytic domains. PRINCIPAL FINDINGS: We have determined the crystal structure of kinase domain of MST4 in complex with an ATP-mimetic inhibitor. This is the first structure of an inactive conformation of a member of the MST kinase family. Comparison with active structures of MST3 and MST1 revealed a dimeric association of MST4 suggesting an activation loop exchanged mechanism of MST4 auto-activation. Together with a homology model of MST2 we provide a comparative analysis of the kinase domains for all four members of the human MST family. SIGNIFICANCE: The comparative analysis identified new structural features in the MST ATP binding pocket and has also defined the mechanism for autophosphorylation. Both structural features may be further explored for inhibitors design. ENHANCED VERSION: This article can also be viewed as an enhanced version in which the text of the article is integrated with interactive 3D representations and animated transitions. Please note that a web plugin is required to access this enhanced functionality. Instructions for the installation and use of the web plugin are available in Text S1

Anomalous Stranski-Krastanov Growth of (111)-Oriented Quantum Dots with Tunable Wetting Layer Thickness

Driven by tensile strain, GaAs quantum dots (QDs) self-assemble on In0.52Al0.48As(111)A surfaces lattice-matched to InP substrates. In this study, we show that the tensile-strained self-assembly process for these GaAs(111)A QDs unexpectedly deviates from the well-known Stranski-Krastanov (SK) growth mode. Traditionally, QDs formed via the SK growth mode form on top of a flat wetting layer (WL) whose thickness is fixed. The inability to tune WL thickness has inhibited researchers’ attempts to fully control QD-WL interactions in these hybrid 0D-2D quantum systems. In contrast, using microscopy, spectroscopy, and computational modeling, we demonstrate that for GaAs(111)A QDs, we can continually increase WL thickness with increasing GaAs deposition, even after the tensile-strained QDs (TSQDs) have begun to form. This anomalous SK behavior enables simultaneous tuning of both TSQD size and WL thickness. No such departure from the canonical SK growth regime has been reported previously. As such, we can now modify QD-WL interactions, with future benefits that include more precise control of TSQD band structure for infrared optoelectronics and quantum optics applications

Single-Photon Generation from Self-Assembled GaAs/InAlAs(111)A Quantum Dots with Ultrasmall Fine-Structure Splitting

We present a novel semiconductor single-photon source based on tensile-strained (111)-oriented GaAs/InAlAs quantum dots (QDs) exhibiting ultrasmall exciton fine-structure splitting (FSS) of ≤ 8 µeV. Using low-temperature micro-photoluminescence spectroscopy, we identify the biexciton-exciton radiative cascade from individual QDs, which, combined with small FSS, indicates these self-assembled GaAs(111) QDs are excellent candidates for polarization-entangled photon-pair generation

Using Distributed Bragg Reflectors to Improve Photon Collection From Quantum Dots

Quantum dots (QDs) are novel nanostructures that, when excited by electricity or light, emit photons useful for devices such as LEDs, quantum light sources, and solar cells. QDs emit light in all directions, with wavelengths characteristic of QD size and shape. We are interested in studying how these QDs emit light, but at times the light emission is too low in intensity to accurately detect with photoluminescence (PL) spectroscopy. To improve detection, we use molecular beam epitaxy (MBE) to insert a distributed Bragg reflector (DBR) into the sample structure, with the goal of reflecting additional photons into the PL detector. Theoretical modeling shows that changes in DBR thickness alter the reflected light wavelength, allowing us to tune the reflectivity of the DBR to match the emission spectra from GaAs QDs. Initial results show that we have accurately synthesized two DBR structures tuned to wavelengths of 1000 and 1100 nm. These results will be used to optimize the DBR structure and MBE growth conditions to provide maximum reflectivity. This work will give us a deeper understanding of our QDs, which is essential for device integration and continued QD research