Tunneling in BP-MoS$_{\mathrm{2}}$ heterostructure*

Tunnel field effect transistor (TFET) is considered to be a leading option for achieving SS \textless 60 mV/dec. In this work, black phosphorus (BP) and molybdenum disulfide (MoS$_{\mathrm{2}})$ heterojunction devices are fabricated. We find that thin BP flake and MoS$_{\mathrm{2}}$ form normal p-n junctions, tunneling phenomena can be observed when BP thickness increases to certain level. PEO:CsClO$_{\mathrm{4}}$ is applied on the surface of the device together with a side gate electrode patterned together with source and drain electrodes. The Fermi level of MoS$_{\mathrm{2}}$ on top of BP layer can be modulated by the side gating, and this enables to vary the MoS$_{\mathrm{2}}$-BP tunnel diode property from off-state to on-state. Since tunneling is the working mechanism of MoS$_{\mathrm{2}}$-BP junction, and PEO:CsClO$_{\mathrm{4\thinspace }}$possesses ultra high dielectric constant and small equivalent oxide thickness (EOT), a low SS of 55 mV/dec is obtained from MoS$_{\mathrm{2}}$-BP TFET.