Biomedical Engineering Reference
In-Depth Information
for growing them is via the Stranski-Krastanow (SK) growth mode where lattice
mismatch between the growing epilayer and the substrate is relaxed by the formation
of a thin wetting layer (WL) and a three-dimensional (3D) QD structure whose
size, density, and uniformity are dictated by growth conditions and are a subject
of intensive investigations during the past decade [
2
,
3
]. Another route for growing
QDs follows a two-step, droplet epitaxial (DE) process where, typically, group-III
metallic droplets are first grown on a III-V substrate and subsequently crystallized
by a group-V element at low temperatures to preserve the original droplet geometry
[
4
]. Both routes yield good quality QDs whose properties can be engineered. Optical
properties in particular are tunable as they are sensitive to QD geometry; the latter
dictates the degree of zero-dimensionality or quantum confinements. For InGaAs
QDs, the effective mass for holes is much greater than those for electrons and, as a
result, holes are confined within the QDs whereas extended states of electrons exist
outside the QDs [
5
]. This offers the possibility of controlling the overlapping or
coupling electron wavefunctions between two nearby QDs and is one of the main
forces that drive the MBE and MOCVD communities towards the growth of QD
molecules (QDMs).
Quantum coupling between or among constituent QDs in a QDM can occur
vertically, in the growth direction, or laterally, in the growth plane [
6
]. Vertical
geometry allows precise control of tunnel barrier thickness and it is this advantage
that is critical to the demonstrated quantum coupling [
7
] and state entanglements
[
8
] which form the foundation of quantum computation [
9
]. Unfortunately for the
vertical geometry, access to the barrier, key to coupling control, is limited to top
and bottom contacts. Lateral geometry, on the other hand, lacks precise control over
barrier thickness but allows better electrostatic control of the tunnel barriers using
top- and/or side gates which can be readily fabricated using well-established planar
technology.
This chapter begins by giving a general overview of the growth procedures used
to form lateral QDMs where coupling occurs in the growth plane. This includes a
specific growth procedure central to this chapter called partial-cap and regrowth
process. The optical properties of the resulting QDMs are then explained and
compared with typical QDs as well as QDMs obtained from other techniques.
The chapter ends by discussing stacked QDM structures in the form of bi-layers
and demonstrating their broadband characteristics which are potentially useful for
devices such as solar cells and superluminescent diodes.
3.2
Growth of Lateral QDMs
Lateral QDM is a term used to broadly describe ensembles of QDs that, by design,
are closely connected in the horizontal direction or the growth plane. Many growth
procedures have demonstrated successful formation of lateral QDMs with high
crystalline quality; yet, there is still no consensus as to which procedure would
yield QDMs with characteristics that best match requirements for quantum coupling
