Role of nickel in the phase change from nanocrystalline Cu2O to CuO sputtered films and the formation of a metastable phase of Cu4O3

Abstract

Copper oxide thin films doped with nickel were prepared by sputtering. The ratio of elements to the silicon substrate was measured by energy- dispersive X-ray spectroscopy. By the change in the power from 20 W to 50 W for sputtering nickel, the content of nickel intercalated in the copper-oxide lattice increased from 2.13 wt% to 7 wt%, respectively. The base sample and 2.13 wt% Ni sample exhibited the cubic Cu2O phase, while the increase in the nickel content to 3.54 wt% and 6.21 wt% led to the change in the tetragonal metastable structure of the Cu4O3 phase. The high doping of Ni at 7 wt% in the copper-oxide lattice transformed the metastable phase to stable monoclinic CuO. The lattice strain and dislocation density of the prepared samples depended on the nickel content. The lattice strain of the 6.21 wt% nickel sample ( ̴ 7.90 ×10−3) was greater than those of the stable phases of Cu2O (~2.5 × 10−3) and CuO (~3.65 × 10−3). The copper-oxide thin film nanostructure was confirmed by atomic-resolution transmission electron microscopy. With the change in the nickel content, the average grain size of the prepared films ranged between 16.57 nm and 20.76 nm. The films roughness was correlated to the nickel content and phase change in the copper-oxide lattice. The optical properties were analyzed and calculated. The optical band gap of the films were affected by the phase change and the nickel content, which decreased from 2.535 eV (Cu2O) to 2.090 eV (CuO).