Diffraction contrast imaging of hydrogen passivation of silicon using helium microscopy

Hydrogen is a key element in the transition to a sustainable energy future, playing a vital role in clean fuel technologies, energy storage, and semiconductor engineering. Yet despite its significance, hydrogen remains challenging to image directly, due to its low mass and weak interaction with conventional microscopy probes. In this study, we present the first direct spatially resolved image of hydrogen-passivated surfaces across large (mm) lateral length scales, obtained using scanning helium microscopy (SHeM), an emerging and exclusively surface-sensitive imaging technology based on neutral and extremely low-energy helium atom beams. Using HF:NH4F-treated Si(111) surfaces as a model system, we directly observe a strong contrast between hydrogen-passivated and non-passivated surfaces as well as localized heterogeneities within the nominally homogeneous hydrogen layer. These spatial features are further probed using temperature programmed desorption and helium diffraction measurements, confirming surface diffraction from the hydrogen-stabilized surface lattice, rather than topography or absorption, as the origin of the observed contrast. Our work establishes SHeM as a powerful tool for light-element surface imaging, with broad implications for hydrogen-related behaviors in many fields.