Optimization of Electrical Properties of MoS₂ Field‐Effect Transistors by Dipole Layer Coulombic Interaction With Trap States

Abstract

The large negative threshold voltage is one of major electrical factors hindering potential applications of MoS 2 transistors in low-power circuit systems. Here, a strategy by forming an electric dipole layer on the surface of MoS 2 is presented to optimize the threshold voltage in monolayer polycrystalline MoS 2 field-effect transistors. The electric dipole in an inert nonconjugated polymer, perfluoropolyether (PFPE), can interact with trapped electrons in the MoS 2 active layer and transfer the traps from shallow into deep states, which remarkably improves the threshold voltage. Otherwise, ab initio calculation and molecular dynamics simulation are employed to investigate the transport properties of MoS 2 with PFPE. The calculated results imply that localized electrons are dominantly affected by PFPE, while free electrons are irrelevant. This method with dipole layer provides a pathway to implementation of high-performance MoS 2 transistors for future electronic applications.Molybdenum disulfide (MoS 2 ) is a kind of two-dimensional (2D) transition metal dichalcogenides (TMDs) that has been widely studied as a potential material for various electronic applications. [1,2] Especially, when thinning MoS 2 from bulk to atomic monolayer, a transition would occur with band gap increasing from 1.2 to 1.8 eV, crossing from indirect to direct one, which makes the 2D monolayer material promising for optoelectronics applications. [2][3][4][5][6] Due to the benefit immunity to short-channel effect and a large degree of electrostatic control in the electrical conductivity, monolayer MoS 2 has attracted considerable attention and researches in field effect transistors (FETs) applications. [7][8][9][10][11][12][13][14][15][16][17] However, large negative threshold voltage (V TH ), induced by MoS 2 intrinsic defects and interface donor states, would result in extensive power consumption and impede practical applications of MoS 2 FETs. [18][19][20] Literatures have reported charge transfer methods via coating organic molecules on surface of MoS 2 surface and organic seimiconductor for the purpose of tuning the V TH of FET. [21][22][23] In this letter, we report an alternative strategy to tune the V TH of MoS 2 FET by coating a layer of polar polymer on the surface of MoS 2 FET, in which, the dipole electric field of the polymer would convert the gap shallow traps into deep traps. Polymer material PFPE is selected as source layer of the dipoles in this research, for the polar group C þ F-(CF 3 ) À could offer strong local dipole moments. [24] With utilizing the Coulombic interaction between localized charge and electric dipole, the MoS 2 's electronic structures especially for the localized states could be effectively modulated, negative V TH is tuned to around 0. In addition, based on the theoretical methods, the transport properties of electrons in MoS 2 with PFPE are investigated, with results implying that localized electrons would be dominantly affected by electric dipole of PFPE, as compared with free electrons.Processing ...

Hidden Vacancy Benefit in Monolayer 2D Semiconductors

“…[ 12 ] The first increase and then decrease of the subthreshold slope may be related to the increase of the SV density and the change of defect type from the isolated SVs to the clustered SVs, respectively. [ 5b,13 ] Lastly, we note the H 2 ‐annealed high‐mobility FET is stable for up to a few months (Figure S4a, Supporting Information), different from pristine 2D transistors. Moreover, healing SVs would also greatly reduce the electrical performance of the monolayer MoS 2 (Figure S4c, Supporting Information).…”

High-Photosensitivity WSe2 Phototransistor with Electrically Self-Isolated C8-BTBT as a Light Absorption Layer

“…Recently, two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS 2 , MoSe 2 , WS 2 , and WSe 2 , have attracted tremendous attention as next-generation semiconductors due to their fascinating electrical and optical properties. − The electrical properties of 2D TMD-based field effect transistors can be effectively and nondestructively tuned by coating various organic molecules on their surface due to surface charge-transfer doping or formation of heterostructures. − However, many previous reports found that this may result in an increase of both ON and OFF current, severely limiting the signal-to-noise ratio (SNR) when applied to photosensing applications (i.e., phototransistors). − An example of such an increase in OFF current after coating organic molecules on a 2D TMD-based transistor is shown in Figure S1. Therefore, the effective application of organic coatings on TMD-based phototransistors dictates that it is necessary to limit potential OFF current increases and act as a light absorption layer to improve the photosensitivity (i.e., SNR) of TMD-based phototransistors.…”

Comparative study on extraction methods of threshold voltage for thin‐film transistors

“…Threshold voltage (V th ) is a key parameter for thin-film transistors (TFTs), which is widely used to characterize the performance of TFTs, such as bias stress effect and light injury. 1,2 However, differing from metal-oxide semiconductor field-effect transistor (MOSFET), the materials of TFTs are usually ploy-crystalline and amorphous structure, which leads to the different definition and extract methods of V th . Generally, the value of V th extracted by using different methods is various.…”

“…For conventional transistors operating in linear mode (V d < V g -V th ), the threshold voltage (V th ) can be obtained by 2…”