Multi-scale ordering of self-assembled InAs/GaAs(001) quantum dots

SOWOS: an open-source program for the three-dimensional Wulff construction

A Fortran90 program for the determination of the Wulff construction, starting solely from the directions of the bounding facets (defined by the user), is presented.SOWOSstands for solid of Wulff open source, and the program is distributed freely with no charge to the user, being readily available to the community for immediate use. Its simple algorithm (which will be explained) allows the determination of complex solids with hundreds of facets in just seconds on any machine, requiring only a small amount of memory. It is able to determine even the smallest facets and shortest edges and to distinguish almost adjacent vertices. The output files give a complete range of information about the structure: the coordinates of the vertices and the facets common to them, the extension of the facets and bounding vertices, and the length of the edges and extreme vertices. These details enable the reconstruction of the shape in any other (commercial) software for further processing. Visualization is straightforwardviathe free programgnuplot. A feature for the creation of cubic crystal atomistic models of the resultant solids is included. The program may be a useful tool for crystallography, nanostructures and any other field where crystal facets are involved.

Optical properties of as-grown and annealed InAs quantum dots on InGaAs cross-hatch patterns

InAs quantum dots (QDs) grown on InGaAs cross-hatch pattern (CHP) by molecular beam epitaxy are characterized by photoluminescence (PL) at 20 K. In contrast to QDs grown on flat GaAs substrates, those grown on CHPs exhibit rich optical features which comprise as many as five ground-state emissions from [1-10]- and [110]-aligned QDs, two wetting layers (WLs), and the CHP. When subject to in situ annealing at 700°C, the PL signals rapidly degrades due to the deterioration of the CHP which sets the upper limit of overgrowth temperature. Ex situ hydrogen annealing at a much lower temperature of 350°C, however, results in an overall PL intensity increase with a significant narrowing and a small blueshift of the high-energy WL emission due to hydrogen bonding which neutralizes defects and relieves associated strains.

Size and density control of In droplets at near room temperatures

We report on the ability to control the size and density of In droplets on GaAs(100) substrates at near room temperatures using solid source molecular beam epitaxy. We specifically demonstrate the height, diameter and density control of In droplets as functions of substrate temperature (T(sub)) and monolayer (ML) coverage. For a range of density (approximately 10(9)-10(10) cm(-2)), the growth window is revealed to be between 20 and 70 degrees C. For a fixed ML coverage, the size and density of droplets can be controlled by controlling the T(sub). For a fixed T(sub), by controlling the ML coverage, droplet size and density can be controlled. Even at near room temperatures (20-70 degrees C), In atoms are extremely sensitive to surface diffusion and this enables the control of the size and density of droplets. This study provides an aid to understanding the formation of In droplets at near room temperatures and can find applications in the formation of quantum structures and/or nanostructures based on droplet epitaxy.

Site-controlled lateral arrangements of InAs quantum dots grown on GaAs(001) patterned substrates by atomic force microscopy local oxidation nanolithography

In this work, we present a fabrication process that combines atomic force microscopy (AFM) local oxidation nanolithography and molecular beam epitaxy (MBE) growth techniques in order to control both the nucleation site and number of InAs quantum dots (QDs) inside different motifs printed on GaAs(001) substrates. We find that the presence of B-type slopes (As terminated) inside the pattern motifs is the main parameter for controlling the selectivity of the pattern for InAs growth. We demonstrate that either single InAs QDs or multiple InAs QDs in a lateral arrangement (LQDAs) can be obtained, with a precise control in their position and QD number, simply by varying the fabricated oxide length along the [110] direction.

Energy transfer within ultralow density twin InAs quantum dots grown by droplet epitaxy

Ultralow density (approximately 10(6)/cm(2)) of twin InAs quantum dot (QD) hybrid structure was grown by a droplet epitaxy technique. The photoluminescence (PL) from ensemble and individual twin InAs QD structures showed a bimodal behavior and an energy transfer between the well-separated (approximately 190 nm) twin QDs, which was supposedly due to the special wetting ring that built the channel for exciton transfer. This research demonstrates a novel approach to fabricate lateral InAs QD pairs as the candidate for a laterally coupled QD molecule.

Theory of spontaneous emission of quantum dots in the linear regime

We develop a fully quantum-mechanical theory for the interaction of light and electron-hole excitations in semiconductor quantum dots. Our theoretical analysis results in an expression for the photoluminescence intensity of quantum dots in the linear regime. Taking into account the single-particle Hamiltonian, the free-photon Hamiltonian, the electron-hole interaction Hamiltonian, and the interaction of carriers with light, and applying the Heisenberg equation of motion to the photon number expectation values, to the carrier distribution functions and to the correlation term between the photon generation (destruction) and electron-hole pair, we obtain a set of luminescence equations. Under quasi-equilibrium conditions, these equations become a closed-set of equations. We solve them analytically, in the linear regime, and we find an approximate solution of the incoherent photoluminescence intensity. The validity of the theoretical analysis is tested by investigating the emission spectra in the high-temperature regime, interpreting the experimental findings for the emission spectra of a lens-shaped In(0.5)Ga(0.5)As self-assembled quantum dot. Our theoretical predictions for the interlevel spacing as well as for the dephasing time caused by electron-longitudinal optical phonon interactions are in good agreement with the experimental results.

Strategies for controlled placement of nanoscale building blocks

The capability of placing individual nanoscale building blocks on exact substrate locations in a controlled manner is one of the key requirements to realize future electronic, optical, and magnetic devices and sensors that are composed of such blocks. This article reviews some important advances in the strategies for controlled placement of nanoscale building blocks. In particular, we will overview template assisted placement that utilizes physical, molecular, or electrostatic templates, DNA-programmed assembly, placement using dielectrophoresis, approaches for non-close-packed assembly of spherical particles, and recent development of focused placement schemes including electrostatic funneling, focused placement via molecular gradient patterns, electrodynamic focusing of charged aerosols, and others.

Observation of change in critical thickness of In droplet formation on GaAs(100)

We present a study on the formation of In droplets on GaAs(100) substrates as functions of substrate temperature and monolayer (ML) deposition by using molecular beam epitaxy (MBE) and atomic force microscopy (AFM). We specifically reveal the change in critical thickness of In deposition to form In droplets at different substrate temperatures. At a relatively high substrate temperature, the critical thickness of In droplets becomes relatively thinner as the amount of As atoms on the surface decreases. The control of the size and density of In droplets is also systematically discussed. This study provides an aid in understanding the formation of In droplets and thus can find applications in the formation of quantum structures and/or nanostructures based on droplet epitaxy.