Correlation effects in two-dimensional M X 2 and M A 2 Z 4 ( M = Nb , Ta ; X = S , Se , Te ; A = Si , Ge ; Z = N , P ) cold metals: Implications for device applications

Cold metals, characterized by their distinctive band structures, hold promise for innovative electronic devices such as tunnel diodes with negative differential resistance (NDR) effect and field-effect transistors (FETs) with sub-60 mV/dec subthreshold swing (SS). In this study, we employ the $GW$ approximation and HSE06 hybrid functional to investigate the correlation effects on the electronic band structure of two-dimensional cold metallic materials, specifically focusing on $M{X}₂$ and $M{A}₂{Z}₄$ ($M=\mathrm{Nb}$, $\mathrm{Ta}$; $X=\mathrm{S}$, $\mathrm{Se}$, $\mathrm{Te}$; $A=\mathrm{Si}$, $\mathrm{Ge}$; $Z=\mathrm{N}$, $\mathrm{P}$) compounds in 1H structure. These materials exhibit a unique band structure with an isolated metallic band around the Fermi energy, denoted as ${W}_{\mathrm{m}}$, as well as two energy gaps: the internal gap ${E}_{\mathrm{g}}^{\mathrm{I}}$ below the Fermi level and the external gap ${E}_{\mathrm{g}}^{\mathrm{E}}$ above the Fermi level. These three electronic structure parameters play a decisive role in determining the current-voltage ($I\text{\ensuremath{-}}V$) characteristics of tunnel diodes, the nature of the NDR effect, and the transfer characteristics and SS value of FETs. Our calculations reveal that both $GW$ and HSE06 methods yield consistent electronic structure properties for all studied compounds. We observed a consistent increase in both internal and external band gaps, as well as metallic bandwidths, across all pn-type cold metal systems. Notably, the internal band gap ${E}_{\mathrm{g}}^{\mathrm{I}}$ exhibits the most substantial enhancement, highlighting the sensitivity of these materials to correlation effects. In contrast, the changes in the metallic bandwidth ${W}_{\mathrm{m}}$ and external band gap ${E}_{\mathrm{g}}^{\mathrm{E}}$ are relatively modest. These findings offer valuable insights for designing and optimizing cold metal-based devices. Materials like ${\mathrm{NbSi}}₂{\mathrm{N}}₄, {\mathrm{NbGe}}₂{\mathrm{N}}₄$, and ${\mathrm{TaSi}}₂{\mathrm{N}}₄$ show particular promise for high-performance NDR tunnel diodes and sub-60 mV/dec SS FETs.