MgB$_{2\, }$Ultrathin$_{\, }$Films Fabricated by Hybrid Physical Chemical Vapor Deposition and Subsequent Ion Milling.

Hot electron bolometer (HEB) mixers are a great tool for measuring high-resolution spectroscopy at Terahertz frequencies. MgB$_{2\, }$offers a higher critical temperature (39 K) compared to commonly used Nb and NbN and boasts a shorter intrinsic electron-phonon relaxation time, giving rise to a broader intermediate frequency (IF) bandwidth. We have fabricated high quality ultrathin MgB$_{2\, }$films using hybrid physical-chemical vapor deposition (HPCVD) and employing ion milling to achieve thickness down to 2 nm. The thinnest achieved films show high $T_{c}_{\, }$of 28 K with residual resistivity below 28 \textmu $\Omega $cm and high critical current \textit{Jc}$_{\, }$of 1x10$^{6}$ A/cm$^{2\, }$at 20~K. As a result of the employed low angle ion milling process, the films remain well connected even after being thinned down since the initial thick films offer a better connectivity than as-grown thin films. The established process offers a way to realize MgB$_{2}$ based HEB mixers of extremely low thickness and therefore small local oscillator power requirements and increased IF bandwidth.