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

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 [0001]-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 < 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.