Diffraction of Quantum Dots Reveals Nanoscale Ultrafast Energy Localization

Unlike in bulk materials, energy transport in low-dimensional and nanoscale systems may be governed by a coherent “ballistic” behavior of lattice vibrations, the phonons. If dominant, such behavior would determine the mechanism for transport and relaxation in various energy-conversion applications. In order to study this coherent limit, both the spatial and temporal resolutions must be sufficient for the length-time scales involved. Here, we report observation of the lattice dynamics in nanoscale quantum dots of gallium arsenide using ultrafast electron diffraction. By varying the dot size from <i>h</i> = 11 to 46 nm, the length scale effect was examined, together with the temporal change. When the dot size is smaller than the inelastic phonon mean-free path, the energy remains localized in high-energy acoustic modes that travel coherently within the dot. As the dot size increases, an energy dissipation toward low-energy phonons takes place, and the transport becomes diffusive. Because ultrafast diffraction provides the atomic-scale resolution and a sufficiently high time resolution, other nanostructured materials can be studied similarly to elucidate the nature of dynamical energy localization

Control over the Number Density and Diameter of GaAs Nanowires on Si(111) Mediated by Droplet Epitaxy

We present a novel approach for the growth of GaAs nanowires (NWs) with controllable number density and diameter, which consists of the combination between droplet epitaxy (DE) and self-assisted NW growth. In our method, GaAs islands are initially formed on Si(111) by DE and, subsequently, GaAs NWs are selectively grown on their top facet, which acts as a nucleation site. By DE, we can successfully tailor the number density and diameter of the template of initial GaAs islands and the same degree of control is transferred to the final GaAs NWs. We show how, by a suitable choice of V/III flux ratio, a single NW can be accommodated on top of each GaAs base island. By transmission electron microscopy, as well as cathodo- and photoluminescence spectroscopy, we confirmed the high structural and optical quality of GaAs NWs grown by our method. We believe that this combined approach can be more generally applied to the fabrication of different homo- or heteroepitaxial NWs, nucleated on the top of predefined islands obtained by DE

Growth of Metamorphic InGaAs on GaAs (111)A: Counteracting Lattice Mismatch by Inserting a Thin InAs Interlayer

We have successfully grown high quality In_<i>x</i>Ga_1–<i>x</i>As metamorphic layer on GaAs (111)­A using molecular beam epitaxy. Inserting a thin 3.0–7.1 monolayer (ML) InAs interlayer between the In_0.25Ga_0.75As and GaAs allowed the formation of a nearly lattice-relaxed In_0.25Ga_0.75As with a very flat upper surface. However, when the thickness of the inserted InAs is thinner or thicker than these values, we observed degradation of crystal quality and/or surface morphology. We also revealed this technique to be applicable to the formation of a high quality metamorphic In_<i>x</i>Ga_1–<i>x</i>As layer with a range of In compositions (0.25 ≤ <i>x</i> ≤ 0.78) on GaAs (111)­A. Cross-sectional scanning transmission electron microscope studies revealed that misfit dislocations formed only at the interface of InAs and GaAs, not at the interface of In_0.25Ga_0.75As and InAs. From the dislocation density analysis, it is suggested that the dislocation density was decreased by growing In_0.25Ga_0.75As on InAs, which effectively contribute the strain relaxation of In_0.25Ga_0.75As. The InGaAs/InAlAs quantum wells that were formed on the metamorphic layers exhibit clear photoluminescence emissions up to room temperature

InAs/GaAs Sharply Defined Axial Heterostructures in Self-Assisted Nanowires

We present the fabrication of axial InAs/GaAs nanowire heterostructures on silicon with atomically sharp interfaces by molecular beam epitaxy. Our method exploits the crystallization at low temperature, by As supply, of In droplets deposited on the top of GaAs NWs grown by the self-assisted (self-catalyzed) mode. Extensive characterization based on transmission electron microscopy sets an upper limit for the InAs/GaAs interface thickness within few bilayers (≤1.5 nm). A detailed study of elastic/plastic strain relaxation at the interface is also presented, highlighting the role of nanowire lateral free surfaces