Biomedical Engineering Reference
In-Depth Information
a large decrease in the TE
[
110
]
mode. This topic chapter aims to provide a detailed
overview of our recent work [
4
,
10
] on the study and design of the multi-layer QDMs
for the isotropic polarization properties.
5.1.2
Organization of Chapter
The rest of this chapter is organised as follows: Sect.
5.2.1
documents geometry
parameters of the studied quantum dot nanostructures. We study four QDMs with
increasing number of QD layers to investigate the impact of the size on the
polarization properties. The results for a single QD layer have also been included
for reference. Section
5.2.2
outlines a brief overview of our methodologies to com-
pute strain profiles, electronic spectra, optical transition modes, and piezoelectric
profiles. In Sect.
5.3
, we study electronic structure of the QDMs in detail. First we
present an analysis of hydrostatic and biaxial strain profiles, and their impact on the
HH-LH intermixing in the valence band states with the aid of band edge diagrams.
This is followed by electron and hole wave function plots to show that the strongly
coupled QD layers result in molecular states in the conduction band. The hole states
in the valence band remain localized in the individual quantum dot layers due to
their heavier mass. Section
5.4
provides a detailed investigation of the polarization
properties of the QDMs. Our calculations show that the TM mode increases as a
function of the size of the QDMs due to the enhanced HH-LH intermixing. We
also reveal an interesting property of the QDMs that the in-plane polarization is
highly anisotropic (TE
TE
[
110
]
), and therefore only DOP
[
110
]
can be designed
for the isotropic polarization response. We demonstrate that our calculated values
of the DOP
[
110
]
]
[
110
qualitatively follow the trends of the experimentally
measured values. In Sect.
5.5
of this topic chapter, we provide an analysis of the
variations in the geometry parameters on the polarization properties of the QDM
consisting of nine QD layers with respect to the single QD layer. In Sect.
5.6
,we
provide overall summary and main conclusions of our chapter.
and DOP
[
110
]
5.2
Simulated QD Systems and Methodologies
5.2.1
Simulated QD Systems
In order to investigate the impact of the size of QDMs on their polarization
properties, we study four QDMs labelled as QDM-1, QDM-2, QDM-3, and QDM-
4 consisting of two, three, six, and nine closely-spaced QD layers, respectively.
The schematic diagrams of these QDMs are shown in Fig.
5.1
b-e. Each individual
QD layer (L
i
) consists of a dome-shaped InAs QD with a base diameter of
20 nm and a height of 4.0 nm, lying on top of a 0.5 nm InAs wetting layer. The
separation between the adjacent wetting layers in the QDMs is 4.5 nm. This wetting
