Epitaxial growth of cuprous oxide electrodeposited onto semiconductor and metal substrates

It is demonstrated that cuprous oxide (Cu2O) can be electrodeposited epitaxially onto silicon (Si) and indium phosphide (InP) (001) single-crystalline substrates from aqueous solution. Epitaxial electrodeposition under these conditions is remarkable considering the strong driving force for the formation of amorphous native oxide layers on Si and InP substrates. To elucidate the growth mechanisms, the microstructure of the interfaces between the Cu2O layer and the two different substrates was investigated by TEM (transmission electron microscopy) in conjunction with XEDS (X-ray energy-dispersive spectroscopy) and EELS (electron energy-loss spectroscopy). In both heteroepitaxial systems, the Cu2O layers have a unique but non-trivial crystallographic orientation relationship (OR) with the substrate, which can be described as a 45 degrees rotation around the common [001] axis representing the substrate normal. We show that this relationship minimizes the overall misfit between corresponding interatomic spacings of the two adjacent crystals. In apparent contradiction to the unique OR, TEM revealed that in both hetero-systems the Cu2O layer is separated from the substrate by an amorphous interlayer. The thickness of the interlayer typically is a few nanometers. The presence of an amorphous interlayer contrasts with our experimental results on electrodeposited Cu2O on An (001) single-crystal substrates, also included in this article, where TEM shows the Cu2O epilayer in direct contact with the substrate. XEDS and EELS analysis of the chemical composition and bonding at Cu2O/Si and Cu2O/lnP interfaces in the as-grown state as well as after tempering revealed that the interlayer is mainly composed Of SiO2 and InPO4, respectively. Most likely, the observed epitaxial layers on top of an amorphous interlayer evolve by nucleation of epitaxial Cu2O directly on the substrate. While simultaneous oxidation of the substrate leads to the formation of an amorphous layer, the epitaxial nuclei can laterally overgrow the oxide. Consequently, the local composition of the amorphous layer varies with the nature of the substrate.