Mode-Selective Tip-Enhanced Raman Spectroscopy Reveals Asymmetric Chemical and Carrier-Sensitive Raman Grading in Photovoltaic III–V Tandem Nanowire Junctions

Performance of tandem junction nanowire (NW) solar cells relies on the electronic quality of the tunnel diodes which serve as the separating junctions between subcells. Yet, the relevant interfacial physics occurs below the optical diffraction limit. Here we use hyperspectral tip-enhanced Raman spectroscopy (TERS) to map, with nanoscale precision, axially defined heterojunctions in InP/InGaP tandem junction photovoltaic NWs. By resolving mode-specific phonon and phonon–plasmon responses, we show that the nominally abrupt p/p++ and p++/n++ junctions are compositionally graded. The TOM mode, used as the primary compositional marker, yields transition widths of ∼90–120 nm, whereas other lattice- and carrier-sensitive modes exhibit broader central widths up to ∼190 nm, reflecting their additional sensitivity to alloying, structural disorder, and electronic coupling. In a first-order approximation, rescaling these TERS-derived widths to shorter, device-relevant NWs yields narrower junctions, consistent with independent Kelvin probe force microscopy (KPFM) measurements that qualitatively capture the same trend despite surface and tip convolution effects. Complementary equilibrium band-edge model combined with a simplified one-dimensional Wentzel–Kramers–Brillouin (WKB) calculation, as an illustrative trend, further indicates that such grading is expected to widen the effective tunnel barrier. These findings establish mode-selective hyperspectral TERS as a validated, noninvasive method for quantitative nanoscale mapping of compositional grading in III–V NW junctions, with the carrier-sensitive L1 phonon–plasmon response providing complementary, model-assisted insight into the electronic evolution of the junctions.