On the Impact and Growth of Plasma-enhanced Atomic Layer Deposition High- Dielectrics on 2D Crystals
Essential to enabling a new generation of electronics, currently built on metal-oxide field-effect transistors (MOSFETs), is the development of novel electronic materials. Conventional materials, such as silicon in MOSFETs, are reaching the physical limits to which they can be scaled down without incurring deleterious side effects. Among the many promising alternative channel materials for MOSFETs are two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs) and black phosphorus (BP). 2D field-effect transistors (FETs) benefit from the atomic thinness of these materials, allowing them to be aggressively scaled down while avoiding short channel effects (SCEs). Their atomic thinness also means they are physically malleable and have the potential of being integrated directly onto flexible substrates. While 2D crystals show such promise for next generation electronics, some of their intrinsic properties have proven a great hindrance to their implementation. The surface of each 2D crystal is typically an inert basil plane, completely free from dangling bonds. While ideal from a carrier transport perspective, such defect-free surfaces present a considerable challenge for establishing ultrathin, high-quality dielectrics to 2D materials. Since ultrathin dielectrics are an essential aspect of future 2D FETs, this challenge has been a persistent bottleneck for the field.
In this work, the use of plasma-enhanced atomic layer deposition (PEALD) was studied for its ability to enable the nucleation and growth of ultrathin, high-k dielectrics onto various 2D crystal surfaces. It was discovered that the PEALD process can provide significant improvement in the uniform nucleation of films compared to thermal atomic layer deposition (ALD). Demonstration of a top-gate 2D FET with a molybdenum disulfide (MoS2) channel is provided, with the thinnest gate dielectric realized to date at ~3 nm. Further experimental realization of ultrathin PEALD high- dielectrics on MoS2 and BP is presented. Each PEALD dielectric was compared to traditional thermal ALD and integrated into either a top-gate or back-gate FET configuration. Detailed electrical characterization was carried out, revealing that PEALD HfO2 is the most favorable gate-dielectric/passivation layer for both multilayer MoS2 and BP, yielding an enhancement in the back-gate 2D FET properties for both materials. Additionally, the impact of the plasma in the PEALD HfO2 process on MoS2 was examined; revealing that the plasma-induced damage that enables film growth is primarily within the topmost layer – while significant damage occurs to monolayers, less damage is incurred in bilayer and trilayer samples. These findings provide a robust approach for depositing ultra-thin high- dielectrics onto multilayer 2D crystals, with key insights into how a PEALD plasma process impacts the crystal structure and properties of the 2D lattice.
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