Direct Observation of Asymmetric Interfacial Charge Distribution in InGaN/GaN Heterostructures.

Interfacial polarization discontinuities are fundamental to the operation of group-III nitride optoelectronics, yet their microscopic charge distribution is conventionally treated as an idealized mathematical boundary. Here, we report the direct, atomic-scale visualization of polarization-induced electric fields at Ga-polar InGaN/GaN heterointerfaces using four-dimensional scanning transmission electron microscopy. Contrary to the classical approximation of an abrupt sheet charge, our measurements reveal a spatially delocalized negative charge accumulation biased toward the GaN barrier. Complementary electron energy-loss spectroscopy and density functional theory calculations identify the microscopic origin of this asymmetry: the incorporation of indium weakens the local metal-to-nitrogen charge donation and enhances electronic delocalization, thereby reshaping the interfacial electrostatic potential. These findings demonstrate that the polarization charge is an intrinsic, chemically modulated volume property rather than a simple boundary condition, providing critical insights for accurate modeling of carrier confinement in nitride devices.