Real-time effect of electron beam on MoS ₂ field-effect transistors

Abstract

Irradiation of MoS₂ field-effect transistors (FETs) fabricated on Si/SiO₂ substrates with electron beams (e-beams) below 30 keV creates electron-hole pairs (EHP) in the SiO₂, which increase the interface trap density (N_it ) and change the current path in the channel, resulting in performance changes. In situ measurements of the electrical characteristics of the FET performed using a nano-probe system mounted inside a scanning electron microscope show that e-beam irradiation enables both multilayer and monolayer MoS₂ channels act as conductors. The e-beams mostly penetrate the channel owing to their large kinetic energy, while the EHPs formed in the SiO₂ layer can contribute to the conductance by flowing into the MoS₂ channel or inducing the gate bias effect. The analysis of the device parameters in the initial state and the vent-evacuation state after e-beam irradiation can clarify the effect of the interplay between the e-beam-induced EHPs and ambient adsorbates on the carrier behavior, which depends on the thickness of the MoS₂ layer. DC and low frequency noise analysis reveals that the e-beam-induced EHPs increase N_it from 10⁹-10¹⁰ to 10¹¹ cm^-2 eV^-1 in both monolayer and multilayer devices, while the interfacial Coulomb scattering parameter α_SC increases by three times in the monolayer and decreases to one-tenth of its original value in the multilayer. In other words, an MoS₂ layer with a thickness of ∼30 nm is less sensitive to adsorbates by surface screening. Thus, the carrier mobility in the monolayer device decreases from 45.7 to 40 cm² V^-1 s^-1, while in the 30 nm-thick multilayer device, it increases from 4.9 to 5.6 cm² V^-1 s^-1. This is further evidenced by simulations of the distribution of interface traps and channel carriers in the MoS₂ FET before and after e-beam irradiation, demonstrating that Coulomb scattering decreases as the effective channel moves away from the interface.