The effects of MS₂ (M = Mo or W) substrates on electronic properties under electric fields in germanene-based field-effect transistors

Germanene has attracted significant attention due to its novel electronic properties and strong spin-coupling effect. However, the tiny band gap of the germanene dramatically limits its application in field-effect transistors (FETs). Inspired by the utilization of the substrates and electric fields to adjust the band gaps of two-dimensional materials, we investigated the fundamental mechanism of electric fields on the atomic structures and electronic properties of germanene supported by MS2 (M = Mo or W) substrates through first-principles calculation. The results show that the substrates can induce a symmetry breaking in the germanene sublattice via van der Waals interaction, leading to a sizable band gap at the Dirac point. In addition, the band gaps of the germanene/MS2 heterostructures can be effectively modulated by applying an external electric field. Under suitable electric fields, the considerable band gap values of C-Mo germanene/MoS2 and TGeL-W germanene/WS2 configurations can open the maximum band gaps with 263 and 247 meV, which satisfy the requirements of FETs at room temperature. Meanwhile, the evolutions of charge transfers under electric fields were explored to illustrate how electric fields and substrates promote the electronic properties of germanene. More interestingly, a Schottky-Ohmic transition can occur when a specific electric field is imposed on the germanene/MS2 heterostructures. Note that the hole and electron carrier mobilities of germanene/MS2 heterostructures are still significantly preserved, showing some superior electronic performances than some heterostructures. The results provide a critical theoretical guide for improving the electronic properties of germanene, and demonstrate the designed germanene/MS2 heterostructures with the tunable band gaps and higher carrier mobilities as germanene-based FETs.