Unscrambling for Subgap Density-of-States in Multilayered MoS₂Field Effect Transistors under DC Bias Stress via Optical Charge-Pumping Capacitance-Voltage Spectroscopy

Abstract

Herein, we quantitatively analyze the evolution of the subgap density of states (DOSs) for multilayered molybdenum disulfide (m-MoS2) field effect transistors (FETs) with bilayered SiNx/SiOx 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-MoS2 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 (VS) (or disulfur vacancy (VS2)). Moreover, after PBS, the initial states of VS (0) (or HS2 (0)) transit toward VS (−1) (or HS2 (−1)) via electron trapping, whereas the transition toward VS (0) and HS2 (+1) during NBS is assessed from the initial VS (−1) and HS2 (0) states. Furthermore, technology computer-aided design (TCAD) simulation based on the extracted DOSs properly replicates the measured I-V characteristics of m-MoS2 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 (MX2)-based FETs, where M is a transition metal and X is a chalcogenide.INDEX TERMS MoS2, field effect transistor, density of state (DOS), bias stress instability, TCAD simulation

Visible Light Illumination Effects on Instability of MoS2 Thin‐Film Transistors for Optical Sensor Application

“…This is attributed to hole trapping in the bulk traps in insulators or in border traps located between gate‐insulators and MoS 2 channel layers. [ 14,15 ] Furthermore, effects on the V th shift associated with E‐field strength and polarity need to be examined to identify whether the amplitude of bias stress, which is relevant to the stretched‐exponential equation, [ 19,20 ] is closely linked with device instability or not. Figure 2b shows evolution of the V th shift to confirm the effects of E‐field strength on device instability from either positive or negative polarity, in the bias range from ±2 to ±4 MV cm −1 .…”

“…Thus, it is highly expected that the large portion of electron‐hole pairs can be trapped into hole traps, which are largely located as the above of valance band. [ 13,15 ] Based on the aforementioned scenarios, the trapping of holes into hole traps can occur more frequently than trapping of electrons into electron traps, which possibly leads to V th shift toward negative direction. Moreover, the magnitude on V th shift can be more significant as wavelength decreases owing to large absorbance of light at 450 nm.…”

“…In parallel, it is worthy to note that the measurement of Figure 2a,c was performed in ambient condition with bare MoS 2 TFTs because electrical stability of MoS 2 is well known to be easily affected by water/oxygen molecules. [ 10,13,15,27 ] Therefore, adsorbed molecules on the MoS 2 flakes could be regarded as one of key factors in the threshold voltage shift of the devices. However, first, most of the adsorbed water/oxygen molecules on the MoS 2 flakes were removed by high‐temperature annealing process (≈400 °C), resulting small hysteresis gap as shown in Figure 1c.…”

“…Until now, device reliability of MoS 2 thin‐film transistors (TFTs) has been actively addressed in terms of DC/pulsed‐mode stress and defect characterization. [ 10–15 ] However, to achieve device stability and extendibility toward circuit‐level applications based on MoS 2 layers, optical device stability under targeted light wavelengths should be investigated. The aim is to lead the way toward achieving core requirements of reliable circuit‐level performance and functionality.…”