Composition depth profiles of superconducting MgB₂ thin films determined by ion beam analysis methods

Thin superconducting MgB₂ films were grown on randomly oriented sapphire, glassy carbon and silicon (111) substrates by sequential e-beam evaporation of boron and magnesium followed by an in situ annealing at 630 or 700 °C. The best zero resistance T_co values were 26 K for MgB₂/Si film annealed at 630 °C and, similarly, 26 K for MgB₂/Al₂O₃ film annealed at 700 °C. The structure and phase composition of synthesized films were investigated by X-ray diffraction and the surface morphology by SEM. An ion beam analysis (IBA) technique using a 2550 keV ³He⁺ beam has been used to obtain the MgB₂ elemental composition depth profiles and film thicknesses. To examine the elemental composition, a combination of analyses of the ³He⁺ backscattered and nuclear reaction regions of the same particle energy spectrum has been performed. The IBA results show that Mg and B concentration profiles are rather inhomogeneous inside the films. An important result is also the detection of small concentrations of O and C in all the MgB₂ films. The oxidation of the films could be in the form of MgO and/or B₂O₃ and the presence of C is probably due to the ambient contamination. The IBA shows that Mg and B diffuses rather deeply into the glassy carbon and sapphire substrates. In the case of Si(111) substrate, the Mg₂Si phase is created on the film/substrate interface consuming Mg from the film surface.