Fermi level tunability of a novel 2D crystal: Tin Diselenide (SnSe2)

Tin Diselenide (SnSe₂) is a two-dimensional layered crystal commonly found in octahedral coordination (1T phase). It has been reported to have a high electron affinity of around 5.1 eV and a bandgap of 1 eV [1-2], which can form staggered band alignment with tungsten diselenide (WSe₂) in Thin-TFETs [3]. However, its lack of gate modulation remains a mystery [4]. In this work, we investigate the Fermi level tunability of SnSe₂ by counter doping using a polymer electrolyte, PEO:CsClO₄. This counter doping technique increases the on/off ratio of SnSe₂ field effect transistor (FET) from 2 times to 50 times, a record high value. Meanwhile, a device model of SnSe₂ FET with ion doping and subgap density of states (DOS) has been proposed to fit the experimental data. The extracted effective number of acceptor-like subgap states is as high as 4.16 × 10¹⁹ cm^-3 (in comparison with near 5 × 10¹⁷ cm^-3 extracted for amorphous thin-film transistors [5]). This can explain the weak Fermi level tunability of SnSe₂ and direct future material development towards TFETs.