Sustained Sub-60 mV/decade Switching via the Negative Capacitance Effect in MoS2 Transistors.

It has been shown that a ferroelectric material integrated into the gate stack of a transistor can create an effective negative capacitance (NC) that allows the device to overcome "Boltzmann tyranny". While this switching below the thermal limit has been observed with Si-based NC field-effect transistors (NC-FETs), the adaptation to 2D materials would enable a device that is scalable in operating voltage as well as size. In this work, we demonstrate sustained sub-60 mV/dec switching, with a minimum subthreshold swing (SS) of 6.07 mV/dec (average of 8.03 mV/dec over 4 orders of magnitude in drain current), by incorporating hafnium zirconium oxide (HfZrO2 or HZO) ferroelectric into the gate stack of a MoS2 2D-FET. By first fabricating and characterizing metal-ferroelectric-metal capacitors, the MoS2 is able to be transferred directly on top and characterized with both a standard and a negative capacitance gate stack. The 2D NC-FET exhibited marked enhancement in low-voltage switching behavior compared to the 2D-FET on the same MoS2 channel, reducing the SS by 2 orders of magnitude. A maximum internal voltage gain of ∼28× was realized with ∼12 nm thick HZO. Several unique dependencies were observed, including threshold voltage (Vth) shifts in the 2D NC-FET (compared to the 2D-FET) that correlate with source/drain overlap capacitance and changes in HZO (ferroelectric) and HfO2 (dielectric) thicknesses. Remarkable sub-60 mV/dec switching was obtained from 2D NC-FETs of various sizes and gate stack thicknesses, demonstrating great potential for enabling size- and voltage-scalable transistors.

Uniform Growth of Sub-5-Nanometer High-κ Dielectrics on MoS2 Using Plasma-Enhanced Atomic Layer Deposition.

Regardless of the application, MoS2 requires encapsulation or passivation with a high-quality dielectric, whether as an integral aspect of the device (as with top-gated field-effect transistors (FETs)) or for protection from ambient conditions. However, the chemically inert surface of MoS2 prevents uniform growth of a dielectric film using atomic layer deposition (ALD)-the most controlled synthesis technique. In this work, we show that a plasma-enhanced ALD (PEALD) process, compared to traditional thermal ALD, substantially improves nucleation on MoS2 without hampering its electrical performance, and enables uniform growth of high-κ dielectrics to sub-5 nm thicknesses. Substrate-gated MoS2 FETs were studied before/after ALD and PEALD of Al2O3 and HfO2, indicating the impact of various growth conditions on MoS2 properties, with PEALD of HfO2 proving to be most favorable. Top-gated FETs with high-κ films as thin as ∼3.5 nm yielded robust performance with low leakage current and strong gate control. Mechanisms for the dramatic nucleation improvement and impact of PEALD on the MoS2 crystal structure were explored by X-ray photoelectron spectroscopy (XPS). In addition to providing a detailed analysis of the benefits of PEALD versus ALD on MoS2, this work reveals a straightforward approach for realizing ultrathin films of device-quality high-κ dielectrics on 2D crystals without the use of additional nucleation layers or damage to the electrical performance.

Poly(oligo(ethylene glycol) methyl ether methacrylate) Brushes on High-κ Metal Oxide Dielectric Surfaces for Bioelectrical Environments.

Advances in electronics and life sciences have generated interest in "lab-on-a-chip" systems utilizing complementary metal oxide semiconductor (CMOS) circuitry for low-power, portable, and cost-effective biosensing platforms. Here, we present a simple and reliable approach for coating "high-κ" metal oxide dielectric materials with "non-fouling" (protein- and cell-resistant) poly(oligo(ethylene glycol) methyl ether methacrylate (POEGMA) polymer brushes as biointerfacial coatings to improve their relevance for biosensing applications utilizing advanced electronic components. By using a surface-initiated "grafting from" strategy, POEGMA films were reliably grown on each material, as confirmed by ellipsometric measurements and X-ray photoelectron spectroscopy (XPS) analysis. The electrical behavior of these POEGMA films was also studied to determine the potential impact on surrounding electronic devices, yielding information on relative permittivity and breakdown field for POEGMA in both dry and hydrated states. We show that the incorporation of POEGMA coatings significantly reduced levels of nonspecific protein adsorption compared to uncoated high-κ dielectric oxide surfaces as shown by protein resistance assays. These attributes, combined with the robust dielectric properties of POEGMA brushes on high-κ surfaces open the way to incorporate this protein and cell resistant polymer interface into CMOS devices for biomolecular detection in a complex liquid milieu.

Control of radiative processes using tunable plasmonic nanopatch antennas.

The radiative processes associated with fluorophores and other radiating systems can be profoundly modified by their interaction with nanoplasmonic structures. Extreme electromagnetic environments can be created in plasmonic nanostructures or nanocavities, such as within the nanoscale gap region between two plasmonic nanoparticles, where the illuminating optical fields and the density of radiating modes are dramatically enhanced relative to vacuum. Unraveling the various mechanisms present in such coupled systems, and their impact on spontaneous emission and other radiative phenomena, however, requires a suitably reliable and precise means of tuning the plasmon resonance of the nanostructure while simultaneously preserving the electromagnetic characteristics of the enhancement region. Here, we achieve this control using a plasmonic platform consisting of colloidally synthesized nanocubes electromagnetically coupled to a metallic film. Each nanocube resembles a nanoscale patch antenna (or nanopatch) whose plasmon resonance can be changed independent of its local field enhancement. By varying the size of the nanopatch, we tune the plasmonic resonance by ∼ 200 nm, encompassing the excitation, absorption, and emission spectra corresponding to Cy5 fluorophores embedded within the gap region between nanopatch and film. By sweeping the plasmon resonance but keeping the field enhancements roughly fixed, we demonstrate fluorescence enhancements exceeding a factor of 30,000 with detector-limited enhancements of the spontaneous emission rate by a factor of 74. The experiments are supported by finite-element simulations that reveal design rules for optimized fluorescence enhancement or large Purcell factors.

Plasmonic waveguide modes of film-coupled metallic nanocubes.

A metallic nanoparticle positioned over a metal film offers great advantages as a highly controllable system relevant for probing field-enhancement and other plasmonic effects. Because the size and shape of the gap between the nanoparticle and film can be controlled to subnanometer precision using relatively simple, bottom-up fabrication approaches, the film-coupled nanoparticle geometry has recently been applied to enhancing optical fields, accessing the quantum regime of plasmonics, and the design of surfaces with controlled reflectance. In the present work, we examine the plasmon modes associated with a silver nanocube positioned above a silver or gold film, separated by an organic, dielectric spacer layer. The film-coupled nanocube is of particular interest due to the formation of waveguide cavity-like modes between the nanocube and film. These modes impart distinctive scattering characteristics to the system that can be used in the creation of controlled reflectance surfaces and other applications. We perform both experimental spectroscopy and numerical simulations of individual nanocubes positioned over a metal film, finding excellent agreement between experiment and simulation. The waveguide mode description serves as a starting point to explain the optical properties observed.