Effect of oxygen flow rate on properties of Cu₄O₃ thin films fabricated by radio frequency magnetron sputtering

Cu₄O₃ thin films have been synthesized in an ambient of Ar and O₂ plasma using a pure Cu target by radio frequency magnetron sputtering. The structural, electrical, and optical properties of the films were studied systematically as a function of O₂ gas flow. The study reveals that O₂ flow rate (R_O2 ) during sputtering has major impacts on both the composition and functional properties of the resultant Cu₄O₃ thin films. X-ray diffraction and Raman spectroscopy measurements suggest that the parameter window for the growth of single-phase Cu₄O₃ thin films was very narrow. Oxygen partial pressure of 7.9%–9.1% was required to grow the pure phase of Cu₄O₃. From optical absorption analyses, pure phase Cu₄O₃ films exhibited a direct transition at E_g= 1.52–1.62 eV. All the Cu₄O₃ thin films showed p-type conductivity with resistivities in the order of 102 –103 Ω cm. An increase of R_O2 resulted in the increase of the Hall mobility from 0.01 to 0.25 cm² /V s, which is the highest mobility reported so far for this material. These results indicate clearly that Cu₄O₃ is a semiconductor with a high potential as absorber materials in low-cost thin film photovoltaics.