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Preparation and properties of reactively sputtered copper oxide films Drobny, Vladimir F.


The deposition of thin copper oxide films by reactive sputtering has been investigated from both experimental and theoretical standpoints. Experimental work utilized both r.f. and d.c. sputtering in argon/oxygen mixtures. The film properties of [O]/[Cu] compositional ratio, optical constants and resistivity were experimentally determined. The composition of the films was found to change with increasing oxygen partial pressure in the following manner Cu → Cu + Cu₂O →Cu₂O →Cu₂O + CuO → CuO The above trend could also be observed at a given oxygen partial pressure by varying the metal deposition rate. This observation led to the postulate that the film compositional ratio was determined by the relative magnitudes of the fluxes of copper and oxygen atoms incident on the substrate plane. Based on this postulate a comprehensive theory describing film deposition by reactive sputtering was developed. The theory is sufficiently general to apply to any metal - elemental reactive gas system. Good agreement was obtained between the predictions of the theory and the phase composition of copper oxide films deposited over a wide range of experimental conditions. Correlation of theoretical and practical data also allowed determination of the sticking coefficient for oxygen on copper-coated surfaces. The theory also served to guide experiments in producing, in a very reproducible manner, copper oxide films with a wide range of optical and electrical properties. The optical constants of complex refractive index and absorption coefficient were determined for the first time for reactively-sputtered films of Cu + Cu₂O, Cu₂O, Cu₂O + CuO and CuO. The presence of Cu₂O was found to be characterized by a peak in the real part of the refractive index (n = 3.36) occurring at a wavelength of 0.48 μm. Evidence of free carrier absorption and acceptor level - conduction band transitions was obtained for (Cu₂O films of various stochiometry. The resistivity of the Cu₂O films depended strongly on the stochiometry and values as low as 30 ohm cm could be easily obtained. Such low values of resistivity are not readily obtainable by other preparative techniques and allow Cu₂O to be considered as a semiconductor material for a variety of applications. One such application, namely Cu₂O/Si heterojunction solar cells, was investigated in the present work.

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