Strain Balanced Epitaxial Stacks of Quantum Dots and Quantum Posts

Abstract

The self assembly of quantum dots by heteroepitaxy of lattice‐mismatched semiconductors is based on elastic energy relaxation, which spontaneously occurs at the growth front when the largest atoms in the crystal cluster together. Because a larger covalent radius is related to weaker bonds, and this is in turn fundamentally related to smaller bandgaps, the formation of quantum dots leads to a confinement potential for electrons and/or holes. This effect has applications ranging from ultralow threshold diode lasers to highly efficient solar cells and usually requires the stacking of multiple quantum dots. The number of layers is limited by the stress accumulated during growth due to the larger covalent radius of the atoms that constitute the quantum dots. This accumulated stress can be relieved by introducing in the epitaxial layers compensating atomic species with a smaller covalent radius, enabling a reduction of spacer layer thickness. In the limit of sub‐nanometer spacer thickness, the quantum dots, which have a tendency to line up vertically, fuse into a quantum post. The current efforts to optimize the properties of strain balanced quantum dot stacks and quantum posts are reviewed.

Understanding the electric field control of the electronic and optical properties of strongly-coupled multi-layered quantum dot molecules

“…Self-assembled quantum dot molecules (QDMs) made up of III-V materials are one of the most popular types of nanostructures used in a variety of devices for optoelectronic [1][2][3][4][5][6][7], photovoltaic [8][9][10][11][12], and quantum information technologies [13,14]. The QDMs typically grow in the form of vertical stacks due to the presence of strain, which stems from the lattice mismatch of the substrate and the QD material.…”

Tuning of polarization sensitivity in closely stacked trilayer InAs/GaAs quantum dots induced by overgrowth dynamics

“…The flexibility in controlling their geometry by means of growth conditions and, in certain cases substrate patterning [2], has stimulated several studies on more complex structures able to add further potentialities to lensshaped or pyramidal nanostructures commonly obtained by the Stranski-Krastanov process. In this context, one relevant physical example are closely stacked quantum dots, either consisting of QD layers separated by a thin GaAs spacer [3][4][5], or without using any GaAs spacer (also known as columnar QDs [6] or quantum posts [7]). In these kinds of nanostructures, the strong compressive biaxial strain component at the center of the typical flat shape dot can be reduced to zero or towards tensile values by increasing the stack height (adding QD layers), thus providing the optical polarization insensitivity desirable for relevant technological applications such as semiconductor optical amplifiers for high speed communication networks [8][9][10].…”

Quantum wells for high-efficiency photovoltaics

“…In a well designed strain balanced structure that alternates compressive and tensile layers (Figure 2b, top) the average stress in the structure is zero and therefore the quality of the crystal is preserved. Such stress can indeed be measured and controlled in situ with a mechano-optical stress sensor technique (MOSS), which has allowed to perfect and better understand the strain-balance process of nanostructures 18,19 .…”