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.

Interfacial Analysis of Anatase TiO2 in KOH Solution by Molecular Dynamics Simulations and Photoelectrochemical Experiments

“…38 During K + and H 2 O coadsorption, a potassium hydroxide film grows, inducing a reduction of the work function, which was recorded by Irkha. 39 However, OH − does not form a surface characteristic adsorption layer shown in the inset. It is explained that TiO 2 is an n-type semiconductor and the surface states are available for electron trapping, causing a negatively charged surface.…”

“…In the bulk region, the number densities of K + tend to be zero because the cations are mostly located at the surface, which is also observed in the literature . During K + and H 2 O coadsorption, a potassium hydroxide film grows, inducing a reduction of the work function, which was recorded by Irkha . However, OH – does not form a surface characteristic adsorption layer shown in the inset.…”

Processing Strategies for High-Performance Schottky Contacts on n-Type Oxide Semiconductors: Insights from In2O3

“…The Pt 4f intensity increases during progressive Pt deposition (Figure b), while the In 2 O 3 -induced signals are attenuated by the Pt adlayer. This effect is utilized to calculate the Pt thickness based on the analysis of different Pt and In core-level peak area ratios as adapted from the procedure used for alkali-based layers on GaN surfaces . The calculations implement a two-layer model, the cross sections and asymmetry factors by Yeh and Lindau, and the energy- and material-dependent electron inelastic mean free paths (IMFPs) of the materials from the predictive formula of Tanuma, Powell, and Penn …”

“…This effect is utilized to calculate the Pt thickness based on the analysis of different Pt and In core-level peak area ratios as adapted from the procedure used for alkali-based layers on GaN surfaces. 54 The calculations implement a two-layer model, 55 the cross sections and asymmetry factors by Yeh and Lindau, 56 and the energy-and material-dependent electron inelastic mean free paths (IMFPs) of the materials from the predictive formula of Tanuma, Powell, and Penn. 57 The shift of the In 3d 5/2 core-level binding energy (BE) and the spectral features of the VB are indications of changes in the interface band alignment.…”

Integration of Multijunction Absorbers and Catalysts for Efficient Solar‐Driven Artificial Leaf Structures: A Physical and Materials Science Perspective

“…Candidate materials for the passivation layers are usually specifically adjusted metal oxides (TiO 2 , [130] HfO 2 , [131] Ga 2 O 3 , [132] BaSnO 3 [133] ) and group III binary, ternary, and doped nitrides, [134] which can be deposited in a highly controlled, layer-by-layer fashion using atomic layer deposition or molecular beam epitaxy. For example, the interaction of hydrogen, oxygen, and water (also in the presence of alkali atoms) has been investigated (see, e.g., [135][136][137][138][139] ).…”