Improved Current Density and Contact Resistance in Bilayer MoSe2 Field Effect Transistors by AlOx Capping.

Atomically thin semiconductors are of interest for future electronics applications, and much attention has been given to monolayer sulfides, such as MoS2, grown by chemical vapor deposition (CVD). However, reports on the electrical properties of CVD-grown selenides, and MoSe2 in particular, are scarce. Here, we compare the electrical properties of monolayer (1L) and bilayer (2L) MoSe2 grown by CVD and capped by sub-stoichiometric AlOx capping. The 2L channels exhibit ~20× lower contact resistance (RC) and ~30× larger current density compared with 1L channels. RC is further reduced by > 5× with AlOx capping, which enables improved transistor current density. Overall, 2L AlOx-capped MoSe2 transistors (with ~ 500 nm channel length) achieve relatively better current density (∼65 μA/μm at VDS=4 V), good Ion/Ioff ratio > 106 and RC ~ 60 kΩ∙µm. The weaker performance of 1L devices is due to their sensitivity to processing and ambient. Our results suggest that 2L (or few layers) are preferable to 1L for improved electronic properties in applications that do not require direct band gap, which is a key finding for future two-dimensional (2D) electronics.