A study of Y–Ba–Cu–O/Si interfaces by x-ray photoelectron spectroscopy

Thin films (∼300 Å) of the superconductor YBa2Cu3O7−y were deposited on a Si 〈100〉 substrate by molecular beam epitaxy. The sample was then cleaved in two pieces and one of these was annealed at 860 °C for 3 h. X-ray photoelectron spectroscopy in conjunction with sputter depth-profiling was utilized to study the interfaces in the unannealed and the annealed samples. Such a comparative study minimizes problems inherent to sputtering. The Y 3d, Ba 3d, Cu 2p, O 1s, and Si 2p regions were investigated. In the unannealed sample, Si diffuses into the constituents of the superconductor near the interface. Oxygen is found to be depleted from the CuO planes in the superconductor thin film with the formation of silicon suboxide. Also, the interface is narrow. In the annealed sample, Si diffuses extensively into the constituents of the superconductor. The Cu–O bonds are broken with the formation of SiO2. This SiO2 is detected on the surface of the sample. At a depth of 450 Å in the sample, Ba–O bonds are also broken partially and some Si suboxide is formed. Because of the extensive interdiffusion, the interface in this sample is broad. It is found that in both cases, Cu–O bonds are broken with oxygen used mainly in the formation of silicon oxides. The study shows that significant interdiffusion occurs at Si/superconductor interface during the high temperature anneal necessary to form the superconductor phase. It is concluded that oxidation of Si at the expense of copper and not the diffusion of Si into the superconductor is responsible for the degradation of performance of the superconductor thin film. Consequently, a suitable diffusion barrier which allows elevated temperature during deposition or annealing stages would be required in the fabrication of superconductor/Si devices.