Surface-mediated tunable self-assembly of single crystal semimetallic ErSb/GaSb nanocomposite structures

Fermi-Level Pinning in ErAs Nanoparticles Embedded in III-V Semiconductors

Embedding rare-earth monopnictide nanoparticles into III-V semiconductors enables unique optical, electrical, and thermal properties for THz photoconductive switches, tunnel junctions, and thermoelectric devices. Despite the high structural quality and control over growth, particle size (<3 nm), and density, the underlying electronic structure of these nanocomposite materials has only been hypothesized. Structural and electronic properties of ErAs nanoparticles with different shapes and sizes (cubic to spherical, 1.14, 1.71, and 2.28 nm) in AlAs, GaAs, InAs, and their alloys are investigated using first-principles calculations, revealing that spherical nanoparticles have lower formation energies. For the lowest-energy nanoparticles, the Fermi level is pinned near midgap in GaAs and AlAs but resonant in the conduction band in InAs. The Fermi level is shifted down as the particle size increases and is pinned on an absolute energy scale considering the band alignment at AlAs/GaAs/InAs interfaces, offering insights into the rational design of these nanomaterials.

Nucleation Selectivity and Lateral Coalescence of GaAs over Graphene on Ge(111)

We use epitaxial lateral overgrowth (ELO) to produce semimetallic graphene nanostructures embedded in a semiconducting GaAs matrix for potential applications in plasmonics, THz generation and detection, and tunnel junctions in multijunction solar cells. We show that (1) the combination of low sticking coefficient and fast surface diffusion on graphene enhances nucleation selectivity at exposed regions of the substrate and (2) high growth temperatures favor efficient lateral overgrowth, coalescence, and planarization of epitaxial GaAs films over the graphene nanostructures. Our work provides a more complete understanding of ELO using graphene masks, as opposed to more conventional dielectric masks, and enables new types of metal/semiconductor nanocomposites.

Emerging Nontrivial Topology in Ultrathin Films of Rare-Earth Pnictides

Thin films of rare-earth monopnictide (RE-V) semimetals are expected to turn into semiconductors due to quantum confinement effects (QCE), lifting the overlap between electron pockets at Brillouin zone edges (X) and hole pockets at the zone center (Γ). Instead, using LaSb as an example, we find the emergence of the quantum spin Hall (QSH) insulator phase in (001)-oriented films as the thickness is reduced to 7, 5, or 3 monolayers (MLs). This is attributed to a strong QCE on the in-plane electron pockets and the lack of quantum confinement on the out-of-plane pocket projected onto the zone center, resulting in a band inversion. Spin-orbit coupling (SOC) opens a sizable nontrivial gap in the band structure of ultrathin films. Such effect is anticipated to be general in rare-earth monopnictides and may lead to interesting phenomena when coupled with the 4f magnetic moments present in other members of this family of materials.

Free-running Sn precipitates: an efficient phase separation mechanism for metastable Ge1-xSnx epilayers

The revival of interest in Ge_1−xSn_x alloys with x ≥ 10% is mainly owed to the recent demonstration of optical gain in this group-IV heterosystem. Yet, Ge and Sn are immiscible over about 98% of the composition range, which renders epilayers based on this material system inherently metastable. Here, we address the temperature stability of pseudomorphic Ge_1−xSn_x films grown by molecular beam epitaxy. Both the growth temperature dependence and the influence of post-growth annealing steps were investigated. In either case we observe that the decomposition of epilayers with Sn concentrations of around 10% sets in above ≈230 °C, the eutectic temperature of the Ge/Sn system. Time-resolved in-situ annealing experiments in a scanning electron microscope reveal the crucial role of liquid Sn precipitates in this phase separation process. Driven by a gradient of the chemical potential, the Sn droplets move on the surface along preferential crystallographic directions, thereby taking up Sn and Ge from the strained Ge_1−xSn_x layer. While Sn-uptake increases the volume of the melt, single-crystalline Ge becomes re-deposited by a liquid-phase epitaxial process at the trailing edge of the droplet. This process makes phase separation of metastable GeSn layers particularly efficient at rather low temperatures.

Highly tensile-strained Ge/InAlAs nanocomposites

Self-assembled nanocomposites have been extensively investigated due to the novel properties that can emerge when multiple material phases are combined. Growth of epitaxial nanocomposites using lattice-mismatched constituents also enables strain-engineering, which can be used to further enhance material properties. Here, we report self-assembled growth of highly tensile-strained Ge/In_0.52Al_0.48As (InAlAs) nanocomposites by using spontaneous phase separation. Transmission electron microscopy shows a high density of single-crystalline germanium nanostructures coherently embedded in InAlAs without extended defects, and Raman spectroscopy reveals a 3.8% biaxial tensile strain in the germanium nanostructures. We also show that the strain in the germanium nanostructures can be tuned to 5.3% by altering the lattice constant of the matrix material, illustrating the versatility of epitaxial nanocomposites for strain engineering. Photoluminescence and electroluminescence results are then discussed to illustrate the potential for realizing devices based on this nanocomposite material.

Self-ordered heterogeneous nanostructures are of broad interest for both fundamental studies and technological applications. Here authors show that segregation in a multicomponent system during growth can yield highly strained germanium nanowire arrays embedded within a ternary semiconductor matrix.

Self-assembled ErSb nanostructures with optical applications in infrared and terahertz

Plasmonic effects have proven to be very efficient in coupling light to structures much smaller than its wavelength. Efficient coupling is particularly important for the infrared or terahertz (λ ∼ 0.3 mm) region where semiconductor structures and devices may be orders of magnitude smaller than the wavelength and this can be achieved through nanostructures that have a desired plasmonic response. We report and demonstrate a self-assembly method of embedding controllable semimetallic nanostructures in a semiconducting matrix in a ErSb/GaSb material system grown by molecular beam epitaxy. The plasmonic properties of the ErSb/GaSb are characterized and quantified by three polarization-resolved spectroscopy techniques, spanning more than 3 orders of magnitude in frequency from 100 GHz up to 300 THz. Surface plasmons cause the semimetallic nanostructures to resonate near 100 THz (3 μm wavelength), indicating the semimetal as a potential infrared plasmonic material. The highly conductive ErSb nanowires polarize electromagnetic radiation in a broad range from 0.2 up to ∼100 THz, providing a new platform for electromagnetics in the infrared and terahertz frequency ranges.