Growth Of Cu₂O Films By Mocular Beam Epitaxy

Cuprous oxide (Cu2O) has been considered having a promising prospect of applications in the fields of photovoltaic cells, photocatalysis, resistive random access memory, thin-film transistors, as well as the best material for observing Bose-Einstein condensation of excitons. Currently, most researches focus on the application of Cu2O-based solar cells and water splitting because of its abundant reserves, nontoxicity and low cost. However, the highest ever-reported conversion efficiency (6.1%) is far from the theoretical limitation of 20%. In order to synthesize high-quality and low resistance Cu2O films, it is necessary to study the growth behavior of Cu2O films. Therefore, we choose magnesium oxide (MgO) and silicon (Si) as substrates to perform the growth of Cu2O by using radio-frequency plasma assisted molecular beam epitaxy (rf-MBE). The growth process and properties of the films were investigated by reflection high-energy electron diffraction (RHEED), high-resolution X-ray diffraction (HRXRD), high-resolution transmission electron microscope (HRTEM), atomic force microscope (AFM), scanning electron microscope (SEM) and photoluminescence (PL) and some other measurements.Firstly,{100} facets were introduced on MgO (110) substrate surface by homoepitaxial growth, and Cu2O (113) film was fabricated on the faceted MgO (110) wafer. Through careful analysis of the abnormal, complex RHEED patterns, we speculated the orientation relationship between Cu2O film and MgO (110) substrate, and the existance of 180° rotation domains, which were further confirmed by the results of HRXRD and HRTEM. The continuous Cu2O (113) film exhibits unique surface morphology and good electrical properties, which may be caused by the efficient suppression of island formation due to the amounts of misfit dislocations in the Cu2O/MgO interface.Cu2O has a large mismatch with Si, thus we choose BeO as interlayer and diffusion barrier. We studied the influence of different growth temperatures on the Cu2O buffer layers. A low temperature will lead to poly-crystallization of Gu2O films, while under high temperature BeO will get a phase transition problem and Cu2O will change to CuO further. Thus, we adopted double buffer layer and single-oriented Cu2O (111) films were fabricated. Meanwhile, low temperature oxidation of Cu (111) film pre-deposited on Si was performed and no Cu2O signals were found in XRD measurement. All samples exhibited high hole mobilities of several hundreds cm2/Vs, which may be attributed to the formation of Cu3Si.