60 research outputs found
Three-dimensional SiGe/Si heterostructures: Switching the dislocation sign by substrate under-etching
Monolithic growth of ultra-thin Ge nanowires on Si(001)
Self-assembled Ge wires with a height of only 3 unit cells and a length of up
to 2 micrometers were grown on Si(001) by means of a catalyst-free method based
on molecular beam epitaxy. The wires grow horizontally along either the [100]
or the [010] direction. On atomically flat surfaces, they exhibit a highly
uniform, triangular cross section. A simple thermodynamic model accounts for
the existence of a preferential base width for longitudinal expansion, in
quantitative agreement with the experimental findings. Despite the absence of
intentional doping, first transistor-type devices made from single wires show
low-resistive electrical contacts and single hole transport at sub-Kelvin
temperatures. In view of their exceptionally small and self-defined cross
section, these Ge wires hold promise for the realization of hole systems with
exotic properties and provide a new development route for silicon-based
nanoelectronics.Comment: 23 pages, 5 figure
Kinetic Control of Morphology and Composition in Ge/GeSn Core/Shell Nanowires
The growth of Sn-rich group-IV semiconductors at the nanoscale provides new
paths for understanding the fundamental properties of metastable GeSn alloys.
Here, we demonstrate the effect of the growth conditions on the morphology and
composition of Ge/GeSn core/shell nanowires by correlating the experimental
observations with a theoretical interpretation based on a multi-scale approach.
We show that the cross-sectional morphology of Ge/GeSn core/shell nanowires
changes from hexagonal to dodecagonal upon increasing the supply of the Sn
precursor. This transformation strongly influences the Sn distribution as a
higher Sn content is measured under the {112} growth front. Ab-initio DFT
calculations provide an atomic-scale explanation by showing that Sn
incorporation is favored at the {112} surfaces, where the Ge bonds are
tensile-strained. A phase-field continuum model was developed to reproduce the
morphological transformation and the Sn distribution within the wire, shedding
light on the complex growth mechanism and unveiling the relation between
segregation and faceting. The tunability of the photoluminescence emission with
the change in composition and morphology of the GeSn shell highlights the
potential of the core/shell nanowire system for opto-electronic devices
operating at mid-infrared wavelengths
Dislocation-free SiGe/Si heterostructures
Ge vertical heterostructures grown on deeply-patterned Si(001) were first obtained in 2012 (C.V. Falub et al., Science 2012, 335, 1330–1334), immediately capturing attention due to the appealing possibility of growing micron-sized Ge crystals largely free of thermal stress and hosting dislocations only in a small fraction of their volume. Since then, considerable progress has been made in terms of extending the technique to several other systems, and of developing further strategies to lower the dislocation density. In this review, we shall mainly focus on the latter aspect, discussing in detail 100% dislocation-free, micron-sized vertical heterostructures obtained by exploiting compositional grading in the epitaxial crystals. Furthermore, we shall also analyze the role played by the shape of the pre-patterned substrate in directly influencing the dislocation distribution
Faceting of Si and Ge crystals grown on deeply patterned Si substrates in the kinetic regime: phase-field modelling and experiments
The development of three-dimensional architectures in semiconductor technology is paving the way to new device concepts for various applications, from quantum computing to single photon avalanche detectors. In most cases, such structures are achievable only under far-from-equilibrium growth conditions. Controlling the shape and morphology of the growing structures, to meet the strict requirements for an application, is far more complex than in close-to-equilibrium cases. The development of predictive simulation tools can be essential to guide the experiments. A versatile phase-field model for kinetic crystal growth is presented and applied to the prototypical case of Ge/Si vertical microcrystals grown on deeply patterned Si substrates. These structures, under development for innovative optoelectronic applications, are characterized by a complex three-dimensional set of facets essentially driven by facet competition. First, the parameters describing the kinetics on the surface of Si and Ge are fitted on a small set of experimental results. To this goal, Si vertical microcrystals have been grown, while for Ge the fitting parameters have been obtained from data from the literature. Once calibrated, the predictive capabilities of the model are demonstrated and exploited for investigating new pattern geometries and crystal morphologies, offering a guideline for the design of new 3D heterostructures. The reported methodology is intended to be a general approach for investigating faceted growth under far-from-equilibrium conditions
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