Composition Effects on Ultrafast Optical Properties of Cu_xO_y Thin Films: A Transient Absorption Study

Cu₂O, CuO, and mixed-phase Cu₂O/CuO thin films with different relative compositions were prepared by oxidizing Cu films at temperatures 150–380 °C for a time period ranging from 2 to 24 h, and their ultrafast transient absorption spectra have been characterized to understand the carrier dynamics of the heterostructured Cu₂O/CuO system. The absorption dynamics of a pure p-type Cu₂O sample followed a biexponential decay, with a fast time ∼0.3 ps and a long life >150 ps, while a pure p-type CuO sample showed triexponential decay dynamics, with three time constants, 0.25, 2.5, and >150 ps. For the mixed-phased Cu₂O/CuO thin films, their absorption dynamics all followed the triexponential decay, and the two ultrafast time constants showed strong composition dependence. Possible energy band structures and electron transition processes are proposed to understand both the spectroscopic and dynamics behaviors of these samples.