Stable Al₂O₃ Encapsulation of MoS₂‐FETs Enabled by CVD Grown h‐BN

Abstract

Molybdenum disulfide (MoS2) has great potential as a two‐dimensional semiconductor for electronic and optoelectronic application, but its high sensitivity to environmental adsorbents and charge transfer from neighboring dielectrics can lead to device variability and instability. Aluminum oxide (Al2O3) is widely used as an encapsulation layer in (opto)‐electronics, but it leads to detrimental charge transfer n‐doping to MoS2. Here, this work reports a scalable encapsulation approach for MoS2 field‐effect transistors (FETs) where hexagonal boron nitride (h‐BN) monolayers are employed as a barrier layer in‐between each of the Al2O3 and MoS2 interfaces. These devices exhibit a significant reduction of charge transfer, when compared to structures without h‐BN. This benefit of h‐BN in the gate stack is confirmed by ab initio density functional theory calculations. In addition, the devices with h‐BN layers show very low hysteresis even under ambient operating conditions.

Potential of Transition Metal Dichalcogenide Transistors for Flexible Electronics Applications

“…Here the field benefits from the experience made with graphene and there are suitable processes available to perform stable encapsulation of TMDCs. [ 9 ] However, these processes may not yet be compatible with industrial production flows and therefore further research is needed. In integrated circuits, it is essential that the threshold voltages of transistors can be defined to suitable values.…”

“…A detailed explanation of these figures-of-merit is given in Table 1. In addition, there are [9] On the upper left corner a schematic of a field effect transistor using back-gate geometry is displayed, as it is often used for TMDC based FETs. S is the source electrode, D is the drain electrode, G is the gate electrode, and CH refers to the TMDC based channel, e.g.…”

“…The key figures-of-merit, such as threshold voltage V T , subthreshold swing SS, and I On /I Off current ratio are marked. b) Output characteristic of this MoS 2 -based NMOS-FET [9]. On the upper left corner a schematic of a field effect transistor using back-gate geometry is displayed, as it is often used for TMDC based FETs.…”

Molybdenum Disulfide Transistors Bearing All-2D-Material Interfaces: Device Performance Optimization and Influences of Interfaces and Passivation Layers

“…Because mono-layer MoS 2 and WS 2 are direct bandgap semiconductors and can emit light, their applications in electronic and optical devices, such as light-emitting devices, have been extensively reported in the literature. , Although MoS 2 transistors have shown good device performances, such as carrier mobilities compatible with those of silicon, mechanical flexibility, and high ON/OFF ratios, − the electrical transport properties of MoS 2 devices are still strongly affected by interfacial charged impurities and rough SiO 2 surfaces because 2D materials have thin bodies. − Over time, this can degrade material performance and reduce device reliability. To solve this problem, high-bandgap h-BN has previously been adopted as a buffer layer between SiO 2 and MoS 2 layers or as a passivation layer on top of the MoS 2 channel to improve the performance and reliability of MoS 2 -based devices. − Studies have revealed that when a 2D-material buffer/passivation layer is inserted between the 2D-material channel and dielectric layer, the influence of the non-2D-material interface can be substantially reduced. However, high growth temperatures of ∼1000 °C have become the predominant bottleneck for practical applications of h-BN .…”

High‐Performance Monolayer MoS2 Field‐Effect Transistors on Cyclic Olefin Copolymer‐Passivated SiO2 Gate Dielectric

“…A more recent and potentially scalable attempt was to grow h‐BN by chemical vapor deposition (CVD) on atomic layer deposition (ALD) grown Al 2 O 3 . [ 20 ] However, FETs still exhibited rather large subthreshold swings of around S = 250 mV decade −1 suggesting that the trap densities of the high‐ k oxide with h‐BN stack are rather large.…”