Direct growth of transition metal dichalcogenides over large areas within the back-end-of-line (BEOL) thermal budget limit of silicon integrated circuits is a significant challenge for 3D heterogeneous integration. At Tyndall, repeatable, rapid, uniform, and continuous MoS2 direct growth films of ~1–10 nm thickness have been achieved by chemical vapour deposition at low temperatures (350–550 °C).
The morphology, material quality and growth rates were found to be substrate dependent, as illustrated in Figure 1 for the growth on silicon-SiO2, sapphire and Alumina coated sapphire.
The production of large area MoS2 direct growth within a BEOL thermal budget is key to the integration of this material to future technology. To illustrate the potential of the grown material a range of device applications including vertical transport memristors, gas sensors and self- switching diodes have been fabricated and tested.
Memristor structures (Au/MoS2/Au), were shown to exhibit switching between the high-low resistive states with a stable memory window of 105 and a retention time > 104 seconds. Although not yet optimised, the switching set and reset voltages in these memristors demonstrate a significant reduction compared to memristors fabricated from pristine, single-crystalline MoS2 at higher temperatures, thereby reducing the energy needed for operation .
Interdigitated electrode-based gas sensors fabricated on MoS2 films were shown to possess excellent selectivity and sub-ppm sensitivity to NO2 gas, with a notable self-recovery at room temperature .
MoS2 in diode structures (MoS2/Al2O3/Hi-Res Si) was also found to behave as a transparent piezoelectric material in the near infrared spectral region and as a strain-induced ferroelectric material with a d33 piezoelectric coefficient of 3–10 pm/V. This enabling the use as a lateral memristor and demonstrating potential as a photodetector in the visible spectrum (responsivities as high as 17 A/W) .