Biomedical Engineering Reference
In-Depth Information
RH
Relative humidity
RMS
Root mean square
TEM
Transmission electron microscopy
WL
Wetting layer
1.1
Introduction
A semiconductor quantum dot (QD) is a semiconductor nanostructure where the
motion of electrons and holes are confined in all three spatial directions. A
consequence of such three-dimensional (3D) carrier confinement is that, in analogy
with atoms, QDs show a discrete quantized energy spectrum, which makes them to
be generally referred to as “artificial atoms.” However, semiconductor QDs have
some important advantages with respect to real atoms in terms of applicability
into devices. As they can be obtained in a solid host matrix with their position
permanently defined, they can be further processed and functionalized using well-
developed semiconductor technology. The integration of semiconductor QDs as
active elements in quantum information processing and communication applications
has been extensively studied during the last years. Electrically driven on-demand
sources of single photons or entangled photon pairs have been realized by incorpo-
rating QDs in a device structure [
1
,
2
]. Also, from a more fundamental point of view,
a single QD embedded within an optical micro-resonator has been studied, forming
an excellent playground to investigate cavity quantum electrodynamics phenomena
in the weak and strong coupling regimes [
3
-
5
]. Despite all this progress, to take full
advantage of the use of QDs in such applications or to develop novel functionalities,
a precise control of QDs size, density, and spatial location is necessary.
Particularly interesting is the formation of ordered networks of well-defined
QDs in close proximity. The simplest case is a quantum dot molecule (QDM)
formed by two interacting semiconductor QDs [
6
], which can be considered as a
basic building block for the realization of solid-state quantum computation devices
[
7
]. Arranged in a vertical geometry (in the growth direction), QDMs have been
deeply studied during the last years allowing for the direct observation of quantum-
mechanical coupling and hybridization of energy levels between the QDs. Coupling
in these molecules is mediated by coherent electron or hole tunneling through a thin
intermediate layer, which is normally a few nanometers thick. The most common
fabrication process of vertical QDMs is based on self-assembling processes and,
more specifically, on the formation of two layers of self-assembled QDs separated
by an epitaxial intermediate layer of another material, which acts as the tunneling
potential barrier. The QDs of the second layer form mostly on top of the buried
QDs of the first layer due to a strain relaxation process [
8
-
10
]. A drawback
inherent to this self-assembling growth procedure is the low control of QDs size,
strain, and composition, which makes the emission energy of the QDs forming
the molecule to be generally mismatched and difficult to tune on demand. These
growth inhomogeneities are often solved by either choosing a suitable thickness of
