Stark effect in GaN/AlN nanowire heterostructures: Influence of strain relaxation and surface states

TL;DR

This study models the quantum Stark effect in GaN/AlN nanowire heterostructures, highlighting how strain relaxation and surface states influence electric fields, charge distribution, and electronic properties, with theoretical results aligning with experimental data.

Contribution

It provides a self-consistent modeling approach that includes surface states and strain effects to better understand the Stark effect in nanowire heterostructures.

Findings

Electric fields bend energy bands and transfer charges to an electron gas.

Surface charges screen the electric field, reducing the Stark effect.

Strain relaxation weakens piezoelectric polarization, affecting electronic properties.

Abstract

We model the quantum confined Stark effect in AlN/GaN/AlN heterostructures grown on top of [0001]-oriented GaN nanowires. The pyro- and piezoelectric field are computed in a self-consistent approach, making no assumption about the pinning of the Fermi level, but including an explicit distribution of surface states which can act as a source or trap of carriers. We show that the pyro- and piezoelectric field bends the conduction and valence bands of GaN and AlN and transfers charges from the top surface of the nanowire to an electron gas below the heterostructure. As a consequence, the Fermi level is likely pinned near the valence band of AlN at the top surface. The electron gas and surface charges screen the electric field, thereby reducing the Stark effect. The efficient strain relaxation further weakens the piezoelectric polarization. We compute the electronic properties of the heterostructures with a sp3d5s* tight-binding model, and compare the theoretical predictions with the available experimental data.