Optical evidence of a quantum well channel in low temperature molecular beam epitaxy grown Ga(AsBi)/GaAs nanostructure.

A Ga(AsBi) quantum well (QW) with Bi content reaching 6% and well width of 11 nm embedded in GaAs is grown by molecular beam epitaxy at low temperature and studied by means of high-resolution x-ray diffraction, photoluminescence (PL), and time-resolved PL. It is shown that for this growth regime, the QW is coherently strained to the substrate with a low dislocation density. The low temperature PL demonstrates a comparatively narrow excitonic linewidth of ∼ 40 meV. For high excitation density distinct QW excited states evolve in the emission spectra. The origins of peculiar PL dependences on temperature and excitation density are interpreted in terms of intra-well optical transitions.

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.

InAs quantum dot clusters grown on GaAs droplet templates: surface morphologies and optical properties.

GaAs nano-mounds formed by droplet epitaxy are used as templates for growth of self-assembled InAs quantum dot clusters (QDCs). These QDCs are found to contain an average of thirteen dots per cluster, of which there are two families of different sized quantum dots. Excitation intensity-dependent photoluminescence (PL) demonstrates that there is no lateral coupling between the two different size quantum dots. Lateral transfer of carriers is observed between different size quantum dots due to thermal activation as seen in their different temperature-dependent optical behaviors.

Tuning the optical performance of surface quantum dots in InGaAs/GaAs hybrid structures.

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.

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.

Various Quantum- and Nano-Structures by III-V Droplet Epitaxy on GaAs Substrates.

We report on various self-assembled In(Ga)As nanostructures by droplet epitaxy on GaAs substrates using molecular beam epitaxy. Depending on the growth condition and index of surfaces, various nanostructures can be fabricated: quantum dots (QDs), ring-like and holed-triangular nanostructures. At near room temperatures, by limiting surface diffusion of adatoms, the size of In droplets suitable for quantum confinement can be fabricated and thus InAs QDs are demonstrated on GaAs (100) surface. On the other hand, at relatively higher substrate temperatures, by enhancing the surface migrations of In adatoms, super lower density of InGaAs ring-shaped nanostructures can be fabricated on GaAs (100). Under an identical growth condition, holed-triangular InGaAs nanostructures can be fabricated on GaAs type-A surfaces, while ring-shaped nanostructures are formed on GaAs (100). The formation mechanism of various nanostructures can be understood in terms of intermixing, surface diffusion, and surface reconstruction.

Composite droplets: evolution of InGa and AlGa alloys on GaAs(100).

We present a comparative study for the evolution of utilizing indium gallium (InGa) and aluminum gallium (AlGa) alloys fabricated on GaAs(100) by means of simultaneous and sequential droplet formation. The composite alloys reported using the sequential approach lack the ability to precisely determine the final alloy composition as well as consistency in the density of the droplets. Further, the composition of the InGa alloy is not uniform, as seen by the size distribution using an atomic force microscope (AFM). Although this approach may be acceptable for materials with similar surface kinetics, as in the case of AlGa, it is not acceptable for InGa. This investigation reveals that the simultaneous approach for fabricating composite alloys is the optimum approach for producing InGa alloys with better control on composition for plasmonic applications such as plasmonic waveguides.

Structural Evolution During Formation and Filling of Self-patterned Nanoholes on GaAs (100) Surfaces.

Nanohole formation on an AlAs/GaAs superlattice gives insight to both the "drilling" effect of Ga droplets on AlAs as compared to GaAs and the hole-filling process. The shape and depth of the nanoholes formed on GaAs (100) substrates has been studied by the cross-section transmission electron microscopy. The Ga droplets "drill" through the AlAs layer at a much slower rate than through GaAs due to differences in activation energy. Refill of the nanohole results in elongated GaAs mounds along the [01-1] direction. As a result of capillarity-induced diffusion, GaAs favors growth inside the nanoholes, which provides the possibility to fabricate GaAs and AlAs nanostructures.

Controlling planar and vertical ordering in three-dimensional (In,Ga)As quantum dot lattices by GaAs surface orientation.

Anisotropic surface diffusion and strain are used to explain the formation of three-dimensional (In,Ga)As quantum dot lattices. The diffusion characteristics of the surface, coupled with the elastic anisotropy of the matrix, provides an excellent opportunity to influence the dot positions. In particular, quantum dots that are laterally organized into long chains or chessboard two-dimensional arrays vertically organized with strict vertical ordering or vertical ordering that is inclined to the sample surface normal are accurately predicted and observed.