Origin of the two-dimensional electron gas at the CdO (100) surface

Synchrotron-radiation angle-resolved and core-level photoemission spectroscopy are used together to investigate the origin of the two-dimensional (2D) 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 angle-resolved photoemission spectroscopy, 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.