Structure and properties of copper oxide thin films
Copper oxide thin films were prepared on glass substrates by radio frequency sputtering of a sintered CuO target in various argon-oxygen atmospheres. Films sputtered in pure argon consisted of both CuO and Cu₂O due to partial dissociation of CuO during deposition. Sputtering in 10% and 50% oxygen afforded films composed entirely of CuO₁₊ₓ. All the sputtered films exhibited extrinsic p-type semiconduction. Both resistivity and thermal activation energy decreased with increasing oxygen content of the sputtering atmosphere. The resistivity of the CuO/Cu₂O films decreased with increasing Cu₂O content. Annealing of the CuO₁₊ₓfilms at 300℃ in air increased their resistivity, activation energy, and visible transmittance, while decreasing the lattice parameters slightly. These changes were attributed to the loss of excess oxygen trapped within the film lattice during sputtering. Annealing of the CuO/Cu₂O films under the same conditions oxidized the Cu₂O to CuO and increased film resistivity to values greater than 10³Ω-cm. A high degree of preferred orientation was observed in the films sputtered in the presence of oxygen. These films were oriented with the (111) planes parallel to the substrate surface and remained so even after annealing in air at 300℃. The (111) planes in CuO are composed of equal concentrations of copper and oxygen atoms and were identified as the most densely packed planes in the lattice. Films consisting of Cu₂O only were prepared by reducing previously sputtered films in a mixture of CO and CO₂ at 300℃. The technique proved to be a convenient means for obtaining Cu₂O films with reproducible properties which were useful in explaining the behavior of the CuO/Cu₂O films.
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