Cu2O thin films obtained from sol-gel cuo films using a simple argon/dry-air microwave plasma

Abstract

Cu2O thin films were obtained from CuO films using an argon/dry-air plasma treatment (PT) for times less than 30s. CuO films were dip-coated on glass at a withdrawal speed of 8cm/min from a sol-gel precursor solution. The as-prepared CuO samples were air-annealed conventionally at different temperatures, from 300 to 550°C, to fabricate the CuO film-targets. An argon/dry-air PT was later applied to film-targets for 15, 20, 25 and 30s in a modified microwave oven made with simple components. Cu2O mixed with a low amount of Cu was identified by X-ray diffraction in films treated for 20s or less. Films only constituted of Cu2O were produced for most of the samples treated for 25 and 30s. Depending on plasma processing time and CuO film-target annealing temperature, the crystallite size of Cu2O was largely changed (from 6 to 25nm). Bigger Cu2O crystallite sizes were observed for targets with annealing of 400°C. SEM studies showed surface morphology to depend on time of PT: grains grew, formed agglomerates and granules with longer exposure. These last caused a decrease in Cu2O film transparency below 800nm for 30s of treatment, but not for larger wavelengths nor for shorter times. Band gap value for CuO was 1.20eV, and around 2.35eV for Cu2O films below 25s of plasma-treatment. However, 2.16eV was calculated for the biggest crystallite size obtained at 30s. Thickness and resistivity measurements were also performed. A strategic experiment showed that CuO films are reduced to metastable metallic copper with PT. This metallic copper readily oxidizes to Cu2O in open atmosphere. The biggest advantage of this plasma processing lies in the simplicity, short time of treatment and, low cost of the home-made equipment.