As-Grown Versus Ion-Milled MgB2 Ultrathin Films for THz Sensor Applications

The exploration of superconductivity in ultrathin films has attracted much attention in the past decades for device applications such as hot electron bolometers (HEBs) and superconducting nanowire single photon detectors (SNSPDs). With a superconducting critical temperature (Tc) of 39 K, which is far above that of other conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, and its faster thermal response, ultrathin MgB 2 films present a promising alternative for the use in HEBs and SSNPDs. In this paper, we present the results on ultrathin MgB2 films grown by hybrid physical-chemical vapor deposition. The films less than 10 nm are difficult to achieve directly because of weak grain connectivity due to incomplete film coverage. To achieve high-quality films less than 10-nm Ar ion milling of thicker films was employed. The ultrathin films fabricated by ion milling show superior quality over as-grown films. The surface roughness of the films was significantly improved and the suppression of Tc from the bulk value is much slower in milled films than in as-grown films. The critical current density of the 5-nm milled film is 1 × 10
⁷
A·cm
^-2
at 4 K, which is the same as that of the 10-nm as-grown film.