Dipolar modulation of surface states in GaN via molecular ionization energy

Gallium nitride (GaN) surface states play a pivotal role in determining device performance and stability. Here, we show that adsorption of molecules with varying ionization energies modulates GaN surface states, as revealed by surface photovoltage and X-ray photoelectron spectroscopy. We find a linear relationship between adsorbate ionization energy and surface Fermi level, indicating that the Ga-polar surface is not intrinsically pinned but is tunable via charge transfer at the native oxide interface. Removal of this oxide suppresses molecular effects, confirming the active state at the GaN/native oxide interface. The low work function values of adsorbed layers suggest robust, ambient-stable dipolar structures, offering a possible new route for negative electron affinity (NEA) activation. Our findings provide a framework for targeted GaN surface engineering in electronic devices.