Polarization-Gradient Engineering of InGaN Nanorods for Hybrid Mechanical-Optical Energy Harvesting

Integrating mechanical and optical energy harvesting within a single piezoelectric semiconductor platform remains challenging because photoexcited carriers readily screen the piezoelectric potential and suppress nanogenerator output. Here, we realize highly crystalline, polarity-defined N-polar InGaN/GaN nanorods with opposite axial In gradients and correspondingly opposite built-in electrostatic profiles. Depth-sensitive, angle-resolved hard X-ray photoelectron spectroscopy (AR-HAXPES) reveals opposite equilibrium band-bending profiles─surface carrier accumulation in In-increasing nanorods but depletion in In-decreasing ones─showing that polarization grading redistributes as-grown mobile carriers and that only the In-increasing configuration establishes the internal electrostatics needed to suppress carrier screening and preserve the piezoelectric potential during nanogenerator operation. In vertically integrated nanogenerators (VINGs), this asymmetry produces a much larger output from the In-increasing device than from the In-decreasing one, together with more favorable transient behavior and energy-storage performance, and is further amplified under illumination, which enhances the output of the In-increasing device while suppressing that of the In-decreasing one, consistent with opposite photoscreening dynamics set by the grading-defined built-in fields. These results demonstrate hybrid mechanical-optical energy harvesting in direct-band gap piezoelectric semiconductors such as III-nitrides, where light and strain act cooperatively rather than competitively.