Unscrambling for Subgap Density-of-States in Multilayered MoS2 Field Effect Transistors under DC Bias Stress via Optical Charge-Pumping Capacitance-Voltage Spectroscopy

Herein, we quantitatively analyze the evolution of the subgap density of states (DOSs) for multilayered molybdenum disulfide (m-MoS
₂
) field effect transistors (FETs) with bilayered SiN
_x
/SiO
_x
gate dielectrics under positive bias stress (PBS) and negative bias stress (NBS) by using optical charge-pumping capacitance-voltage spectroscopy. To decouple external gas ambient effects on device instability, hydrophobic fluoropolymers (cyclized transparent optical polymer; CYTOP) are employed for m-MoS
₂
FETs followed by the evaluation of subgap-DOSs in the devices for the respective PBS (or NBS). Through extraction of subgap-DOSs and their deconvolution with an analytical model of acceptor (or donor)-like states, it is shown that the device instability is closely correlated with state transitions of DOSs, corresponding to shallow (or midgap) levels for monosulfur vacancy (V
_S
) (or disulfur vacancy ( V
_S2
)). Moreover, after PBS, the initial states of V
_S
(0) (or H
_S2
(0)) transit toward V
_S
(-1) (or H
_S2
(-1)) via electron trapping, whereas the transition toward V
_S
(0) and H
_S2
(+1) during NBS is assessed from the initial V
_S
(-1) and H
_S2
(0) states. Furthermore, technology computer-aided design (TCAD) simulation based on the extracted DOSs properly replicates the measured I-V characteristics of m-MoS
₂
FETs with (and without) CYTOP encapsulation. In this study, subgap-DOS characterization via optical charge pumping and validation of the quantitative evolution of subgap-DOSs suggest that this platform can be potentially beneficial for an in-depth understanding of the origins of device instability for transition metal dichalcogenide (MX
₂
)-based FETs, where M is a transition metal and X is a chalcogenide.