Local Aspects of Hydrogen-Induced Metallization of the ZnO(10$\mathbf{\overline{1}}$0) Surface

TL;DR

This study investigates how hydrogen adsorption affects the electronic structure of the ZnO(10-1-0) surface, revealing a coverage-dependent transition from surface metallization to localized effects, with implications for energy level alignment.

Contribution

It combines experimental photoemission data with DFT calculations to provide a detailed microscopic understanding of hydrogen-induced modifications on ZnO surfaces, highlighting localized effects and coverage dependence.

Findings

Hydrogen initially causes surface metallization via charge accumulation.

Increased hydrogen coverage reverses adsorption energies, reducing metallization.

Localized surface potential changes are confirmed by sub-surface excitons.

Abstract

This study combines surface-sensitive photoemission experiments with density functional theory (DFT) to give a microscopic description of H adsorption-induced modifications of the ZnO(10${\overline{1}}$0) surface electronic structure. We find a complex adsorption behavior caused by a strong coverage dependence of the H adsorption energies: Initially, O--H bond formation is energetically favorable and H acting as an electron donor leads to the formation of a charge accumulation layer and to surface metallization. The increase of the number of O--H bonds leads to a reversal in adsorption energies such that Zn--H bonds become favored at sites close to existing O--H bonds, which results in a gradual extenuation of the metallization. The corresponding surface potential changes are localized within a few nanometers both laterally and normal to the surface. This localized character is experimentally corroborated by using sub-surface bound excitons at the ZnO(10${\overline{1}}$0) surface as a local probe. The pronounced and comparably localized effect of small amounts of hydrogen at this surface strongly suggests metallic character of ZnO surfaces under technologically relevant conditions and may, thus, be of high importance for energy level alignment at ZnO-based junctions in general.