Artificial leaves based on III−V semiconductor tandem devices currently offer the highest solar-to-fuel conversion efficiencies [1-3]. However, the practical use of these materials in photoelectrochemical (PEC) cells is strongly limited by their chemical degradation under operating conditions, particularly in acidic electrolytes that are preferred for efficient hydrogen evolution reaction (HER) kinetics. Therefore, they require suitable protection strategies that combine chemical and electronic passivation with good charge carrier transport. Atomic layer deposited (ALD) TiO₂ protection layers have been widely applied for this purpose, protecting the III‑V absorbers and their charge carrier selective contacts from corrosion while enabling charge carrier transport to the surface. Although TiO₂ can suppress corrosion, long-term operational stability under HER conditions remains a critical challenge. Degradation is commonly initiated by catalyst detachment, followed by electrolyte permeation through structural defects and pinholes into the passivation layer and III-V layers underneath, leading to progressive dissolution of the layers [4].

To overcome corrosion and degradation, we investigated gallium nitride (GaN) prepared via plasma-enhanced-ALD (PE-ALD) as an alternative protective passivation layer for III–V photoelectrodes. GaN forms chemically robust and oxygen-free interfaces with the underlying III-V layers, enabling efficient charge carrier transport [5-6]. We compared the stability and interfacial chemistry of PE-ALD-grown GaN with ALD-grown TiO₂ on AlInP/III-V stack layer/Ge photocathodes. As the HER catalyst, a 2 nm Pt layer was deposited on all photocathodes. The stability of the photocathodes for unassisted water splitting was evaluated in 1.0 M HClO₄ solution under AM1.5G illumination. The measurements were carried out at 0 V vs. counter electrode using stepwise photoelectrochemical measurements to monitor degradation processes. Changes in surface chemistry and material dissolution were analyzed by X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry. Morphological changes were examined using scanning electron microscopy and atomic force microscopy (AFM). Photocathodes employing TiO₂ protection exhibited rapid performance decay during operation, with devices losing functionality within an hour. Post-characterization analyses showed that this degradation was initiated by early catalyst detachment, followed by progressive breakdown of the passivation layer and corrosion of the III-V layers. AFM measurements showed an increased surface roughness and the formation of nanoscale pits in the TiO₂ layers, indicating localized defects that likely act as initiation sites for electrolyte penetration and degradation. In contrast, GaN-coated photocathodes demonstrated significantly improved stability under identical conditions. AFM analysis of as-prepared samples indicates that the GaN layers form a smoother and closed surface in contrast to TiO₂, suggesting less catalyst detachment and a significantly reduced density of nanoscale defects and pinholes.

In addition, the interfacial chemistry and electronic structure at the GaN/III–V and TiO₂/III–V heterointerface were analyzed. Photoemission spectroscopy showed that TiO₂ deposition preserved a chemically complex AlInPO_x interface composed of mixed III–O–P and oxy(hydroxide) bonding motifs, accompanied by mid-gap valence band features attributed to defect states such as Ti³⁺ species and residual hydroxides. These interfacial states indicate an unfavorable band alignment that can hinder efficient charge carrier transport. In contrast, plasma-enhanced ALD growth of GaN resulted in a cleaner interface. The N₂ plasma pretreatment removed hydroxide species and resulted in the formation of P–N and P–O–N bonding configurations while maintaining the underlying phosphate network.

Overall, the enhanced performance is attributed to the chemically favorable GaN interface and the suppression of oxygen-related interfacial degradation pathways. Our results highlight the limitations of TiO₂ protection layers for III–V PEC photocathodes in acidic environments and demonstrate that GaN is a promising alternative to TiO₂ for improving the long-term stability of III-V-based artificial leaves in acidic PEC environments.