On the Origin of the Two-Dimensional Electron Gas at the CdO (100) Surface

TL;DR

This study investigates the origin of the two-dimensional electron gas at CdO (100) surfaces, revealing that surface adsorbates donate electrons and that hydrogen doping can reversibly modulate surface electron density.

Contribution

It provides experimental evidence linking surface adsorbates to electron donation and demonstrates reversible hydrogen doping effects on the surface electron gas.

Findings

Surface adsorbates donate electrons to the surface state.

Cleaning the surface reduces the 2D electron density.

Hydrogen doping increases the surface electron density.

Abstract

Synchrotron-radiation angle-resolved and core-level photoemission spectroscopy are used together to investigate the origin of the two-dimensional electron gas on the surface of single crystal CdO (100) films. A reduction in the two-dimensional electron density of the surface state is observed under the synchrotron beam during ARPES, which is shown to be accompanied by a concomitant reduction in the surface-adsorbed species (monitored through the O 1s core level signal). This shows that surface adsorbates donate electrons into the surface accumulation layer. When the surface is cleaned, the surface conduction band state empties. A surface doped with atomic H is also studied. Here, interstitial H increases the two-dimensional electron density at the surface. This demonstrates that reversible donor doping is possible. The surface band bending profiles, 2D electron densities, and effective masses are calculated from subband dispersion simulations.