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

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 [Al_0.35Ga_0.65N (~1.75 nm)/Al_0.55Ga_0.45N (~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.