Two-dimensional (2D) materials have attracted significant attention because of their outstanding electrical, mechanical, and optical characteristics. Because all of the conducting (graphene), semiconducting (molybdenum disulfide, MoS 2 ), and insulating (hexagonal boron nitride, h-BN) components can be constructed from 2D materials, thin-film transistors based on 2D materials (2D TFTs) have been developed. However, scaling-up is necessary for these technologies to go beyond their initial implementation using the mechanical exfoliation method. Furthermore, it would be beneficial to find a method to realize high flexibility and/or transparency to their full potential. In this study, large-scale, flexible, and transparent 2D TFTs are developed and demonstrated as a backplane in active-matrix organic light-emitting diodes (AMOLEDs). With the optical chemical vapor deposition of the 2D materials, flexible (bending radius < 1 mm) and transparent (transmittance > 70%) TFTs with high electrical performances (mobility ≈ 10 cm 2 V −1 s −1 , on/off current ratio > 10 6 ) can be achieved. Furthermore, 2D TFTs are integrated into OLEDs by connecting the source electrode of the TFT to the anode of the OLED via a single graphene film, thus demonstrating pixel-by-pixel driving through a 2D TFT array in an active-matrix configuration.
This work demonstrates a high-performance and hysteresis-free field-effect transistor based on two-dimensional (2D) semiconductors featuring a van der Waals heterostructure, MoS2 channel, and GaS gate insulator. The transistor exhibits a subthreshold swing of 63 mV/dec, an on/off ratio over 106 within a gate voltage of 0.4 V, and peak mobility of 83 cm2/(V s) at room temperature. The low-frequency noise characteristics were investigated and described by the Hooge mobility fluctuation model. The results suggest that the van der Waals heterostructure of 2D semiconductors can produce a high-performing interface without dangling bonds and defects caused by lattice mismatch. Furthermore, a logic inverter and a NAND gate are demonstrated, with an inverter voltage gain of 14.5, which is higher than previously reported by MoS2-based transistors with oxide dielectrics. Therefore, this transistor based on van der Waals heterostructure exhibits considerable potential in digital logic applications with low-power integrated circuits.
promising channel materials for nextgeneration thin-film transistors (TFTs) because of their outstanding characteristics, such as high mobility, on/off current ratio, and saturation velocity. [1][2][3][4] Moreover, MoS 2 TFTs can potentially be used as future scaled devices because of their atomically thin nature to overcome shortchannel effects that hinder future scaling of metal-oxide-semiconductor field-effect transistors using bulk-type materials. [4][5][6] From the viewpoint of organic lightemitting diodes (OLED), MoS 2 TFTs are expected to be applied as driving transistors based on their superior performance and their potential use as flexible displays, as compared to relatively brittle materials, such as amorphous silicon and oxide semiconductors. [7][8][9] However, the device characteristics of MoS 2 TFTs heavily rely on not only the contact between MoS 2 and metal, [10] but also the interface between MoS 2 and the gate dielectric because of the atomically thin nature. Previous studies have revealed that carrier mobility, saturation velocity, hysteresis, and thermal stability of MoS 2 TFTs depend on the gate dielectric, and a suitable dielectric is required for the optimized device performance and the proper application. [11] Previous studies have reported that high-κ dielectric layer boosts the carrier mobility of MoS 2 TFTs by the charge screening effect. [12] Therefore, several high-κ dielectric layers, such as Al 2 O 3 and HfO 2 , were applied, but high-κ dielectrics inherently have functional groups including hydroxyl groups (OH − ) at the surface that are critical to the device performance.The gate dielectrics of MoS 2 TFTs are normally deposited by atomic layer deposition (ALD), which produces uniform large-scale films with highly controllable thickness at relatively low temperatures. ALD produces a wide range of dielectrics for MoS 2 TFTs based on various precursors and reactants. Recently, polymer dielectrics, such as poly(1,3,5-trimethyl-1,3,5trivinyl cyclotrisiloxane) (pV3D3), based on initiated chemical vapor deposition (iCVD) have emerged. They exhibit high uniformity, purity, low leakage, high chemical stability, and large band-gap with low-κ. [13] Therefore, iCVD-grown polymer dielectrics are used in various applications such as gate dielectrics of TMDs-based transistors, tunneling dielectrics of flash memory cells, flexible devices, and neuromorphic computing systems. [14][15][16][17]
As two-dimensional (2D) materials have a large surface to volume ratio, the stability of thin film transistors (TFTs) is likely to be lowered with air exposure. Therefore, we study the positive bias temperature instability (PBTI) of chemical vapor deposition (CVD) grown molybdenum disulfide (MoS 2 ) TFTs before and after deposition of a passivation layer. The results of the PBTI study demonstrate that the fabricated devices adjust to the stretched-exponential model, which shows a threshold voltage shift attributed to the charge trapping mechanism. However, by depositing the passivation layer (Al 2 O 3 ) that physically blocks the charge transfer process with O 2 and H 2 O adsorbed to the surface of the MoS 2 channel, the threshold voltage shifted reduces from 10 V to 7.4 V under stress condition. The quantitative value of tau (τ ), one of the fitting parameters of the stretched-exponential model, also decreases from 6453 s to 5153 s, resulting in improved device stability.
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