Epitaxial Growth of Cu₂O on Ag Substrate by Electrodeposition

Abstract

Cuprous oxide (Cu 2 O) is a p-type semiconductor with a direct bandgap of 2.1 eV and high hole mobility at ambient temperature. Recently, the importance of using an epitaxial film to fabricate optoelectronic devices to improve the performances has been pointed out [Mahenderkar et al., Science, 355, 1203–1206 (2017)]. Investigation of epitaxial growth of Cu 2 O on, mostly, MgO substrate by molecular beam epitaxy or pulsed laser deposition has been carried out for a decade. However, these techniques require high-quality substrates and expensive facilities. Electrodeposition, on the other hand, provides another option with ease of operation. However, the substrate has to be electrically conductive and possesses a low lattice mismatch to Cu 2 O. In this study, the epitaxial growth of Cu 2 O on Ag substrate by electrodeposition was studied by a combinatorial substrate approach mainly using the electron backscatter diffraction (EBSD) technique. The effect of substrate orientation on epitaxial growth can thus be explored efficiently. The lattice mismatch between Ag and Cu 2 O is 4.3 %. Prior to the deposition, the annealed, polycrystalline Ag substrate was electropolished and analyzed by EBSD over a marked region of about 500 x 500 μm 2 . Atomic force microscopy was used to characterize the surface morphology of the substrate grains of different orientations. Afterwards, Cu 2 O films were deposited galvanostatically on the substrates in an electrolyte containing copper sulfate and lactic acid. The crystal structure and orientation of the marked area was analyzed by EBSD again after electrodeposition. The orientation distribution of the films and the orientation relationship between the overlaid Cu 2 O and the Ag substrate were thus obtained. X-ray diffraction was also used for phase identification. Results indicated that the substrate grains exhibited similar surface roughness values of approximately 3 nm regardless the orientation. X-ray diffraction reults confirmed that the films deposited at different conditions contained solely the Cu 2 O phase. No diffraction peaks of Cu and CuO were found. At a low current density of 0.06 mA/cm 2 , the film has a rough surface composing of faceted islands of 5-20 mm in size. A portion of the islands exhibited the same orientation as the substrate grain. On the other hand, at a high current density of 0.25 mA/cm 2 , films with smooth surfaces were obtained. The Cu 2 O films all grew epitaxially on the Ag grains in two orientation relationships: a cube-on-cube one and a <100>/45 o one. The origin of the <100>/45 o orientation relationship was further studied and will be explained in detail. The deposited films were also analyzed by photoluminescence at room temperature and showed strong near band-edge emissions at 1.97-2.06 eV.