Effects of Potassium Adsorption and Potassium–Water Coadsorption on the Chemical and Electronic Properties of n-Type GaN(0001) Surfaces

Abstract

The interaction of n-type GaN(0001) surfaces with potassium and water is investigated using photoelectron spectroscopy, with special focus on adsorbate–substrate charge-transfer processes and water dissociation. Potassium atoms adsorb at the surface, forming a distinct surface dipole layer. For very low K coverage, the attached ionized K adsorbates result in a drop of the work function and the released electrons induce a reduction of the initial upward band bending. After stabilization of both quantities in the sub-monolayer regime, a reverse effect is observed for higher K coverage up to one monolayer (ML), exceeding the upward band bending of the clean surface. If the K-covered surface is exposed to water, hydroxyl groups are formed, whereas during long K and H2O coadsorption, a potassium hydroxide film grows. In both cases, a further reduction of the work function and an abrupt change in the surface depletion layer is recorded. For the coadsorption, initially an electron accumulation layer forms at the surface, approaching flat band conditions for higher KOH thickness. Overall, the surface band bending can be drastically modified in the range between +0.5 and −0.6 eV. These observations clearly show that the electron density at the GaN(0001) surface can be reversibly tuned by alkali-based adsorbates. Different reactions are observed, which are directly linked to the charge-transfer processes and chemical reactions induced by the K 4s electrons.