Role of polarized tip-enhanced Raman spectroscopy in the enhancement of interface optical phonon modes in AlGaN multi-quantum wells

TL;DR

This study demonstrates how polarized tip-enhanced Raman spectroscopy significantly enhances the detection of interface optical phonon modes in AlGaN multi-quantum wells, revealing detailed vibrational properties at the nanoscale.

Contribution

The paper introduces the use of polarized TERS to selectively enhance and observe interface phonon modes in AlGaN MQWs, providing new insights into their vibrational behavior.

Findings

TERS achieves 100-1000x enhancement in Raman signals.

Polarized TERS preferentially enhances E1 symmetry phonon modes.

Finite-difference time-domain simulations confirm electric field confinement.

Abstract

Group III nitride based two-dimensional multi-quantum well (MQW) nanostructures find remarkable applications in the visible to ultraviolet light sources. The interface optical (IFO) phonon modes in a c-axis oriented superlattice of [Al0.35Ga0.65N (~1.75 nm)/Al0.55Ga0.45N (~2nm)]20 MQWs are observed using tip-enhanced Raman spectroscopic (TERS) studies. The near-field studies using TERS probe with an Au spherical nanoparticle of ~ 200 nm diameter were carried out at ambient conditions showing approximately two to three orders of enhancement in the Raman intensities. The interface phonon mode belonging to E1 symmetry [IFO(E1)] vibrating normal to the c-axis of MQWs appeared to be more prominent in the case of TERS measurement compared to that for the other interface phonon mode of A1 symmetry. The confined electric field of the polarized electro-magnetic excitation using TERS probe, parallel to the plane of the interface of MQW, is made responsible for the plasmonic enhancement of IFO(E1) phonon mode. The confinement was verified using finite-difference time-domain simulation.