Temperature-dependent oxidation behavior of thin copper films and formation of gradient copper oxide layers

Polycrystalline Cu thin films were oxidized at controlled O 2 pressure (20 mbar) and temperature conditions (450, 550, and 650 K). The oxidation process was monitored in situ by probing the integral and spectrally-resolved transmittance of the sample as a function of time. The former quantity gives insights into the kinetics of different oxidation processes that run in parallel at the preset thermodynamic conditions. The spectral data, on the other hand, were analyzed using the transfer-matrix method combined with Bruggeman’s effective-medium approach to determine the changing sample composition during oxidation. The two approaches revealed an oxidation reaction occurring in two phases: a rapid one in which Cu primarily oxidizes to Cu 2 O, and a slow one dominated by CuO formation. Both processes do not follow a layer-by-layer fashion but proceed along the granular structure of the original Cu film. By exploiting the close interplay between experiment and simulation, optimal conditions for the growth of clean Cu 2 O or CuO phases as well as of gradient oxide films were determined, the latter evolving monotonously between the two binary phases. Our approach bears a large potential for synthesizing dielectric materials with tailored properties for nanooptics and photovoltaics.