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TWO principal compounds in the Cu-O system are Cu2O (cuprite or cuprous oxide) and CuO (tenorite or cupric oxide). Cu2O is used as a semiconducting material in optoelectronic solar devices,[1–3] electric conductors,[4] and catalytic species,[5] and CuO is an interesting candidate as a buffer or interdiffusion layer.[6] Copper-based cermets Cu/CuOx can be used as selective coatings because of its good optical properties, good adhesion to metal surfaces, and thermal and chemical stability. It is known that Cu2O (cuprite) and CuO (tenorite) are stable under a standard environment. These two oxides are semiconductors with bandgaps in the visible or near-infrared regions. These materials have several advantages: (1) availability and abundance of the starting materials, (2) nontoxic nature, (3) low production cost, (4) bandgaps lie in an acceptable range for solar energy conversion, and (5) n- and p-type conductivity.[1,7] Copper oxide films have been widely studied for their photoelectrochemical properties,[8] solar–thermal conversion performances,[9] catalytic applications,[10] electrochromic properties,[11] etc. However, a third defined compound is also reported in this binary system: Cu4O3 (paramelaconite).[12] Synthesis of paramelaconite films using conventional elaboration processes such as reactive evaporation[13] and reactive sputtering[14,15] is rarely reported. However, Nancheva et al.[16] deposited mixed Cu4O3 and CuO films by reactive sputtering. Li et al.[17] SHANMUGAN SUBRAMANI, Postdoctoral Research Fellow, and MUTHARASU DEVARAJAN, Associate Professor, are with the Nano Optoelectronics Research Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Minden, Pulau Penang, Malaysia. Contact e-mail: [email protected] Manuscript submitted July 18, 2011. Article published online May 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

formed Cu4O3 films by sputtering a CuO film previously formed by reactive sputtering of a copper target. Medina-Valtierra et al.[18] deposited paramelaconite films on fiberglass by a chemical vapor deposition process and reported only the surface morphology and the visible reflectance of the films. Mansour[19] also observed Cu4O3 along with Cu2O and CuO, when copper (II) acetate was decomposed thermally. von Richthofen et al.[20] formed paramelaconite films by radiofrequency magnetron sputtering ion plating of a copper target in reactive Ar-O2 mixtures. Although the structure and the composition of the films have been widely characterized, no information has been presented about the film properties when a Cu layer deposited over copper oxide (CO) with different stack configurations. In this article, we report the influence of Cu layer on the synthesis of paramelaconite structure as well as CuO structure by direct current (DC) reactive magnetron sputtering processes. The structural parameters such as microstrain, crystallite size, dislocation density, residual stress, or internal stress, as well as optical properties such as absorbance and bandgap, are discussed in this article.