Abstract
Abstract Currently, the nature of self-assembly of three-dimensional epitaxial islands or quantum dots (QDs) in a lattice-mismatched heteroepitaxial growth system, such as InAs/GaAs(001) and Ge/Si(001) as fabricated by molecular beam epitaxy (MBE), is still puzzling. The purpose of this article is to discuss how the self-assembly of InAs QDs in MBE InAs/GaAs(001) should be properly understood in atomic scale. First, the conventional kinetic theories that have traditionally been used to interpret QD self-assembly in heteroepitaxial growth with a significant lattice mismatch are reviewed briefly by examining the literature of the past two decades. Second, based on their own experimental data, the authors point out that InAs QD self-assembly can proceed in distinctly different kinetic ways depending on the growth conditions and so cannot be framed within a universal kinetic theory, and, furthermore, that the process may be transient, or the time required for a QD to grow to maturity may be significantly short, which is obviously inconsistent with conventional kinetic theories. Third, the authors point out that, in all of these conventional theories, two well-established experimental observations have been overlooked: i) A large number of “floating” indium atoms are present on the growing surface in MBE InAs/GaAs(001); ii) an elastically strained InAs film on the GaAs(001) substrate should be mechanically unstable. These two well-established experimental facts may be highly relevant and should be taken into account in interpreting InAs QD formation. Finally, the authors speculate that the formation of an InAs QD is more likely to be a collective event involving a large number of both indium and arsenic atoms simultaneously or, alternatively, a morphological/structural transformation in which a single atomic InAs sheet is transformed into a three-dimensional InAs island, accompanied by the rehybridization from the sp 2-bonded to sp 3-bonded atomic configuration of both indium and arsenic elements in the heteroepitaxial growth system.
Highlights
Since the first half of the 20th century, the epitaxial growth technique has been developed to fabricate atomically flat thin films on a solid substrate; it involves depositing atoms or molecules from ballistic particle beams, the vapor phase, or the liquid phase on a solid surface
InAs quantum dots (QDs) fabricated on the GaAs(001) substrate by molecular beam epitaxy (MBE) may be used in mid- and far-infrared detectors, terahertz emitters [4, 5], vertical-external-cavity surface-emitting lasers, and single-photon emitters
In the above two kinetic theories for QD formation, on the basis of the physical concepts associated with the ATGS surface morphology instability and the firstorder phase transformation, it has to be assumed that an unstable or metastable elastically strained flat thin film must form before QD formation
Summary
Since the first half of the 20th century, the epitaxial growth technique has been developed to fabricate atomically flat thin films on a solid substrate (see, for example, Ref. [1]); it involves depositing atoms or molecules from ballistic particle beams, the vapor phase, or the liquid phase on a solid surface. In the last two decades, an increasing number of three-dimensional (3D) nanostructures [e.g., clusters, wires, and quantum dots (QDs)] have been experimentally observed to selfassemble themselves when a few atomic layers of a metal or semiconductor are epitaxially deposited onto a planar substrate. Much of both the energetics and kinetics of the mechanism underlying the spontaneous formation or self-assembly of these surface nanostructures remains a puzzle.
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