High crystal quality and purity Cu2O by consecutive in situ annealing and thermal oxidation of Cu under H2 and O2 at elevated temperatures

Cu2O has been obtained via the thermal oxidation of Cu foils with a maximum thickness of 120 mu m under Ar and O2 at 1020 degrees C for 30 min. Care was taken to suppress the oxidation of Cu during the temperature ramp by using H2 prior to admitting O2 at 1020 degrees C. We suggest that the ruby red Cu2O crystals are obtained in this way by strain-driven grain growth and consist of similar to 500 mu m grains that have a cubic crystal structure. More importantly we find that the crystal quality and purity of the Cu2O is improved significantly by pre-annealing the Cu under Ar and H2 at 1000 degrees C and 10 mbar for up to 180 min. The Cu2O exhibited photoluminescence at 6.5 K with a maximum at 2.01 eV corresponding to the direct energy gap in very good agreement with density functional theory calculations of the electronic band structure. No emission due to recombination of bound excitons to copper vacancies VCu and single VO or double charged VO oxygen vacancies was observed consistent with high crystal quality Cu2O. We find that the Cu2O crystals obtained in this way are p-type and the highest mobility measured at room temperature was mu p = 82 cm V-1 s-1 with a carrier density of 7.3 x 10 cm3.