Thermal oxidation of copper over a broad temperature range: Towards the formation of cupric oxide (CuO)

P-type semiconducting cupric oxide (CuO) was formed by thermal oxidation of copper (Cu) sheets. Oxidation variables such as temperature, oxidation time, and oxidation atmosphere were controlled with the aim of form the single phase of CuO. Structural, morphological, and electrical properties of copper oxide were measured with x-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), and Hall effect techniques, respectively. Oxidation at temperatures lower than 200 °C showed that a thin layer of copper oxide, mainly of cuprous oxide (Cu₂O), is grown on the surface of Cu. Oxidation at 300 °C promoted growth of a passivating oxide layer composed by CuO and Cu₂O, even when Cu is oxidized during 24 h. In the range of temperatures from 400 to 700 °C, copper oxide layer was fragile and showed poor adherence to the Cu surface. Further Cu oxidation at higher temperatures promoted complete oxidation of the Cu sheets. However, the bulk was found to be composed by a mixture of Cu₂O and CuO. Oxidation at 1000 °C for 24 h in flow of air or for 8 h in ambient of oxygen allowed formation of pure and high cristallinity CuO. Carrier concentration and mobility charge carriers of CuO estimated from Hall measurements for the CuO sample oxidized in oxygen atmosphere were found to be 7.64 ×10¹² cm^-3 and 1.1164 ×10² cm² V^-1 s^-1, respectively.