Microstructure of superconducting films fabricated by high-fluence Ga implantation in Si

The feasibility of embedding extrinsic superconducting nanolayers in commercial (100) silicon due to Ga precipitation is presented. To be far beyond the solid solubility limit of 0.1 at.% a high Ga fluence of 4x10¹⁶cm^-2 is introduced in silicon by the ion-implantation technique. This leads to 100 nm thick amorphous silicon layers with a Ga peak concentration of 16 at.%. Subsequent recrystallization and Ga precipitation is initiated via rapid thermal annealing (RTA) for 60 seconds at temperatures of 500 – 800°C. A 30 nm sputter deposited SiO₂ cover layer is used to protect the silicon surface during implantation and prevent Ga out-diffusion during annealing. It was shown that optimized annealing conditions (600 – 700°C) lead to superconducting layers with critical temperatures of 7 K and in plane critical fields up to 14 T [1]. Details of the layer microstructure investigations using of RBS/C and TEM as well as depth dependent XPS will be presented. The presented structural investigations reveal poly-crystalline silicon layers and show a strong Ga enrichment at the Si/SiO₂ interface. Even if no crystalline Ga clusters were detected it is shown that the superconductivity arises due to a high density of amorphous Ga-rich precipitates at the Si/SiO₂ interface. Since all involved processing steps are fully compatible with standard microelectronic technology and high criti-cal current densities of more than 2 kA/cm² are reached, the proposed material system may implicate a high potential for future microelectronic applications.
[1] Skrotzki R. et al., Appl. Phys. Lett. 97 (2010) 192505