Biomedical Engineering Reference
In-Depth Information
Chapter 5
Size-dependent Electronic and Polarization
Properties of Multi-Layer InAs Quantum
Dot Molecules
Muhammad Usman
Abstract
In this chapter, we analyze the polarization response of multi-layer
quantum dot molecules (QDMs) containing up to nine vertically stacked quantum
dot layers by carrying out a systematic set of multi-million atom simulations.
The atomistic modeling and simulations allow us to include correct symmetry
properties in the calculations of the electronic and optical spectra: a factor critical to
explain the experimental evidence. The values of the degree of polarization (DOP)
calculated from our model based on the geometry parameters directly extracted
from the experimental TEM images follow the trends of the recently published
experimental data. We also present detailed physical insight of the fundamental
underlying physics by examining strain profiles, band edges diagrams, and wave
function plots. Multi-directional calculations of the DOP reveal a unique property
of the InAs QDMs that the TE response in the plane perpendicular to the growth
direction is highly anisotropic. Therefore we propose that a single value of the
DOP is not sufficient to fully characterize the polarization response. We explain this
anisotropy of the TE modes in terms of the orientation of the hole wave functions
that align along the [ 110] direction. Our results provide a new insight that the
isotropic polarization response measured in the experimental PL spectra is due to
two factors: (i) TM
[
001
]
mode increases due to enhanced intermixing of HH and
LH bands and (ii) TE
[
110
]
mode reduces significantly due to the hole wave function
alignments along the [ 110] direction. This is in contrast to general notion that only
an increase in the TM
[
001
]
mode is responsible for the isotropic polarization. We
also present polarization response as a function of various geometry configurations
of the quantum dot layers to provide a guide to experimentalists for the design of
optical devices based on multi-layer QDMs.
M. Usman (
)
Tyndall National Institute, Lee Maltings, Dyke Parade, Cork, Ireland
e-mail:
usman@alumni.purdue.edu
