  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.
