Bandgap‐Engineered AlGaAs/GaAs Heterostructures for Wavelength‐Selective Dual‐Polarity Photoelectrochemistry

Bandgap engineering provides a powerful route to design light–matter interactions and enable wavelength‐dependent functionalities in semiconductor photoelectrodes. Here, we report a bandgap‐engineered p‐Al 0.4 Ga 0.6 As/n‐GaAs heterostructure that exhibits pronounced wavelength‐selective dual‐polarity photoelectrochemical response. High‐quality epitaxy establishes a well‐defined junction with a built‐in electric field that governs carrier separation across the heterointerface. Under short‐wavelength illumination (λ<614 nm), photon absorption in the AlGaAs layer combined with downward band bending at the p‐AlGaAs/electrolyte interface drives electrons toward the surface, yielding a photocathodic response. In contrast, longer‐wavelength photons (614‐873 nm) penetrate the AlGaAs layer and are absorbed in the GaAs substrate, where photogenerated holes are transported across the heterointerface toward the surface by the built‐in electric field, giving rise to a photoanodic response. Incident photon‐to‐current efficiency spectra and monochromatic illumination experiments provide direct evidence for this band‐ and field‐selective polarity switching. Electrolyte‐dependent studies further reveal that surface chemistry critically governs anodic charge transfer and stability. By elucidating the coupled roles of optical absorption, band alignment, built‐in electric fields, and interfacial surface chemistry, this work establishes a physics‐encoded and designable platform for wavelength‐controlled functional switching, where anodic and cathodic processes are spatially and energetically decoupled, offering new opportunities for optical logic, adaptive photoelectrochemical systems, and spectrally encoded energy conversion.