Analysis of Reduced Built-In Polarization Fields and Electronic Structure of InGaN/GaN Quantum Dot Molecules - Quantum Dot Molecules - page 174

Chapter 6
Analysis of Reduced Built-In Polarization Fields
and Electronic Structure of InGaN/GaN
Quantum Dot Molecules

Stefan Schulz and Eoin P. O'Reilly

Abstract Nitride-based semiconductor materials InN, GaN, AlN, and their alloys
are attracting great attention due to their promising applications in optoelectronic
devices. However, the emission efficiency of c -plane InGaN/GaN quantum wells
(QWs) drops significantly when going to longer wavelengths due to the strong
electrostatic built-in fields in such heterostructures. We present a surface integral
method to show and explain why the polarization potential in an InGaN/GaN
quantum dot (QD) grown along the -direction is strongly reduced compared

to that in a QW of the same height. We show that the sign of the shear strain
piezoelectric coefficient e 15 strongly affects the built-in field and therefore the
electronic structure both of an isolated QD and also of a system of stacked c -
plane InGaN/GaN dots. Based on two different approaches we conclude e 15 <

[ 0001 ]

0, in
agreement with recent independent studies. We then use a tight-binding model and
include strain and polarization fields, to study the electronic structure of InGaN/GaN
quantum dot molecules grown along the c -axis. This analysis is carried out as a
function of the barrier thickness firstly between identical and then between two
non-identical dots. Our results show that the built-in field can be further reduced
in systems of coupled nitride quantum dots, leading to an increased spatial overlap
of electron and hole wave functions compared to an isolated dot. This finding is in
agreement with experimental data reported in the literature and is directly related to
the behavior of the built-in potential outside an isolated dot.

S. Schulz ( )
Tyndall National Institute, Lee Maltings, Cork, Ireland
e-mail: [e-mail]
E.P. O'Reilly
Tyndall National Institute, Lee Maltings, Cork, Ireland
Department of Physics, University College Cork, Cork, Ireland
e-mail: [e-mail]