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

Abstract

We fabricate MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ultrathin films via the hybrid physical-chemical vapor deposition (HPCVD) technique. With the proper background pressure and B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ≈ 2. 0 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> , 2. 3 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> , and 2. 8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 5 K, respectively, which indicate that the films fabricated by HPCVD are well qualified for device applications.