Figure 1: Oxygen vacancies in MgO.

(a) Schematic diagram of the energy positions of oxygen vacancies in the MgO barrier49,50,54,69. See Supplementary Note 1 for further details. PL processes resulting from 3.8 eV optical excitation are also represented. A legend depicts for each localized state the robustness of thermal addressing within specific T ranges. (b) Typical PL spectrum at T=300 K and (c) temperature dependence of the PL spectrum from MgO(2.5 nm) tunnel barriers collected after excitation with 3.8 eV light. Data in b for λ<350 nm has been zeroed.