Properties of $\hbox {MgB}_{2}$ Ultrathin Films Fabricated on MgO (111) Substrate by Hybrid Physical–Chemical Vapor Deposition

We fabricate MgB₂ ultrathin films via the hybrid physical-chemical vapor deposition (HPCVD) technique. With the proper background pressure and B₂H₆ flow rate, we fabricate a series of ultrathin films ranging from 10 to 40 nm. These films grow on the MgO (111) substrate and are all c-axis epitaxial. The ultrathin films show good connectivity, very high T_c(0) ≈ 35-38 K, and very low residual resistivity ρ(42 K) ≈ 1. 8-20. 3 μΩ·cm. With the increase in the film thickness, the critical transition temperature increases, and the residual resistivity decreases. In addition, the 10-, 20-, and 40-nm films show extremely high critical current density, i.e., J_c ≈ 2. 0 ×10⁷, 2. 3 ×10⁷, and 2. 8×10⁷ A/cm² at 5 K, respectively, which indicate that the films fabricated by HPCVD are well qualified for device applications.