Conductance impedance effects in atomically thin semiconductors

Abstract

The rapidly unfolding innovation based on two-dimensional (2D) electronic materials is paving the way to numerous exciting applications in electronics, photovoltaics and sensing. Atomically thin transition metal dichalcogenides are an exciting family of 2D materials that can complement state-of-the-art devices through added functionality (e.g. mechanical flexibility). For optimal device performance, the complete picture of the charge transport mechanisms through the device, i.e. charge injection, transport and retention, needs to be fully understood. This work presents experimental evidence for the formation of Schottky barriers as low as 10 meV between MoTe₂ and metal electrodes at cryogenic temperatures. By varying the electrode work functions, it is demonstrated that Fermi level pinning due to metal induced gap states at the interfaces occurs at 0.14 eV above the valence band maximum. This is the first experimental observation of thermionic emission at temperatures as low as 40 K, which is unexpected since tunnelling usually is the dominant effect at these temperatures. However, as the formed barrier is uniquely broad and shallow, and as the thermal width of the Fermi-Dirac distribution is comparable with these measured barrier heights, thermionic emission is found to be the dominant mechanism. A further investigation reveals a novel transport effect, dubbed threshold voltage transient effect, which is manifested in a hysteresis in the transfer curves. By probing the transient currents measured during cycles of pulses through the gate electrode, it is possible to distinguish between charge trapping that occurs at the semiconductor/dielectric interface, and that which happens at the semiconductor/metal interface, showing that the time-dependent change in threshold voltage is the dominant effect on observed hysteretic behaviour. This new method is then extended into a spectroscopic technique, which allows the density of trap states to be mapped within the band gap of these atomically thin semiconductors. Finally, the threshold voltage transient method is adapted to investigate the changes in charge dynamics with variations in temperature.