Biomedical Engineering Reference
In-Depth Information
the intermediate layer, which modifies the strain field distribution on the surface and
therefore the size of the upper QDs, or by applying an external perturbation such as
strain or/and an electric field in the growth direction [
11
-
14
].
Although the study of vertical QDMs has permitted the implementation of basic
quantum information devices such as optical quantum gates [
15
], QDMs arranged
in a lateral geometry (in the growth plane) is a more appealing design from the
quantum computation technology point of view. Apart from facilitating the gating
technology, it would easily permit to contact single QDMs one by one in the same
device or to naturally increase the number of coupled QDs in the molecule, thus
forming a more complex quantum network. From a more fundamental point of
view, lateral QDMs would also allow for studying, for the first time, coupling and
hybridization effects in a two-dimensional scenario. Experimentally, lateral cou-
pling of two adjacent QDs has been barely tackled [
16
,
17
]. Difficult to achieve by
pure self-assembling processes, the most successful fabrication approaches to form
lateral QDMs are based on the selective epitaxial growth of QDs on modified surface
morphologies such as mounds or nanoholes [
18
-
22
]. Generally, the use of mounds
takes advantage of its in situ self-assembled formation and consequent high optical
quality of the semiconductor material [
23
-
25
]. However, this approach shows a lim-
ited control in the number of QDs forming the molecule. On the other hand, the main
drawback encountered with the used of etched nanoholes is that the optical quality
of the semiconductor material is often degraded by imperfections introduced during
their fabrication, mainly carried out by ex situ lithographic processes [
26
,
27
].
In this chapter, we will overview two fabrication methods that circumvent the
majority of these growth-engineering problems, allowing for a controlled fabrication
of QDMs with high optical quality.
In the first section, we focus on the in situ droplet epitaxy etching technique
to obtain a template of low-density nanoholes on a GaAs surface through a self-
assembling process [
28
]. Subsequent preferential InAs epitaxial growth into the
nanoholes, when adequate growth conditions are used [
29
,
30
], yields the formation
of low-density vertical QDMs with deliberate emission properties [
31
]. The capabil-
ity and versatility of this fabrication procedure is further demonstrated by showing
the formation of lateral QDMs with high optical quality at the single nanostructure
level [
20
]. Lateral quantum coupling in these QDMs is also demonstrated by micro-
photoluminescence (micro-PL) studies of a single QDM as a function of a laterally
applied electric field [
17
].
In the second section, we present an alternative way to produce lateral ar-
rangements of QDs by ex situ atomic force microscopy local anodic oxidation
(AFM-LAO) nanolithography followed by preferential epitaxial growth [
22
,
32
,
33
].
By means of this approach, not only the number of QDs in a lateral QDMs-like
arrangement can be controlled but also the spatial positioning of the nanostructure
on the substrate with nanometer resolution. The whole optimized fabrication
process is demonstrated to be fully compatible with obtaining high optical quality
nanostructures [
34
].
