In situ tracking the phase change in metastable copper oxide thin film via thermal oxidation

Phase change (PC) materials, such as metal oxides, possess high fundamental and applied importance for modern memory, sensor, and photonic devices. However, revealing the kinetics and mechanisms of PC and achieving the reversible and irreversible PC regimes are still a challenge. Here, we report on 200 nm films of metastable Cu₄O₃ obtained by magnetron sputtering. In situ structural and optical analyses during air annealing revealed the thermal induced PC from Cu₄O₃ to CuO. Combination of x-ray diffraction and optical transmittance with heating up to 450 °C allowed us to explore the PC rate and then to correlate it with classical nucleation and crystal growth mechanism. We have also revealed two regimes of PC, possessing reversible 12% (1.42–1.25 eV) and irreversible 220% (1.25–2.75 eV) direct bandgap tuning by temperature. The results, thereby, pave the way for controllable transformation of copper oxide thin films to achieve the desired microstructure and its functionality.