Volatile two-dimensional electron gas in ultrathin BaTiO3 films

BaTiO${}_{3}$ (BTO) is one of the most important ferroelectric oxides. Its most common source of uncontrolled doping, the oxygen vacancy, releases free electrons that confine in a two-dimensional electron gas (2DEG) at the surface. This 2DEG can be readily monitored by angle-resolved photoemission, and be used to measure the defect dynamics in ultrathin BTO films. The authors reveal two remarkable properties: first, oxygen defects close to the thin film surface are preferably in a charge state of 2+, which limits charge carrier trapping. Second, these defects migrate even below room temperature, fostering significant surface charge carrier density fluctuations that ultimately quench the 2DEG.