Interdiffusion studies on high-T_c superconducting YBa₂Cu₃O_7−δ thin films on Si(111) with a NiSi₂/ZrO₂ buffer layer

Interdiffusion studies on high-Tc superconducting YBa2Cu3O7−δ thin films with thicknesses in the range of 2000–3000 Å, on a Si(111) substrate with a buffer layer have been performed. The buffer layer consists of a 400 Å thick epitaxial NiSi2 layer, covered with 1200 Å of polycrystalline ZrO2. YBa2Cu3O7−δ films were prepared using laser ablation. The YBa2Cu3O7−δ films on the Si/NiSi2/ZrO2 substrates are of good quality; their critical temperatures Tc,zero and Tc,onset have typical values of 85 and 89 K, respectively. The critical current density jc at 77 K equaled 4 × 104 A/cm2. With X-ray diffraction analysis (XRD), only c-axis orientation has been observed. The interdiffusion studies, using Rutherford backscattering spectrometry (RBS) and scanning Auger microscopy (SAM) show that the ZrO2 buffer layer prevents severe Si diffusion to the YBa2Cu3O7−δ layer, the Si concentration in the ZrO2 layer must be below the detectability limit of 1 at%, but Si diffusion along grain boundaries cannot be excluded completely. During short deposition times (t ≈ 5 min) no severe interface reactions occur. The interfaces are sharp and well defined. However, during long deposition times (t > 30 min), some Cu diffuses from the YBa2Cu3O7−δ layer to the interface between the ZrO2 layer and the NiSi2 layer. Also indications for the formation of BaZrO3 at the interface between the YBa2Cu3O7−δ layer and the ZrO2 layer have been found. Finally, Ni diffusion into the Si substrate and Ni segregation to the surface of the ZrO2 layer may be expected. From the results we may conclude that, when using laser ablation, it is well possible to grow polycrystalline, c-axis-oriented high-Tc superconducting YBa2Cu3O7−δ thin films on a Si(111) substrate with a NiSi2/ZrO2 buffer layer.