Felicia McGuire

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.

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.

Sub-60 mV/decade switching in 2D negative capacitance field-effect transistors with integrated ferroelectric polymer

There is a rising interest in employing the negative capacitance (NC) effect to achieve sub-60 mV/decade (below the thermal limit) switching in field-effect transistors (FETs). The NC effect, which is an effectual amplification of the applied gate potential, is realized by incorporating a ferroelectric material in series with a dielectric in the gate stack of a FET. One of the leading challenges to such NC-FETs is the variable substrate capacitance exhibited in 3D semiconductor channels (bulk, Fin, or nanowire) that minimizes the extent of sub-60 mV/decade switching. In this work, we demonstrate 2D NC-FETs that combine the NC effect with 2D MoS2 channels to extend the steep switching behavior. Using the ferroelectric polymer, poly(vinylidene difluoride-trifluoroethylene) (P(VDF-TrFE)), these 2D NC-FETs are fabricated by modification of top-gated 2D FETs through the integrated addition of P(VDF-TrFE) into the gate stack. The impact of including an interfacial metal between the ferroelectric and dielectric ...

Film-coupled nanoparticles by atomic layer deposition: Comparison with organic spacing layers

Film-coupled nanoparticle systems have proven a reliable platform for exploring the field enhancement associated with sub-nanometer sized gaps between plasmonic nanostructures. In this Letter, we present a side-by-side comparison of the spectral properties of film-coupled plasmon-resonant, gold nanoparticles, with dielectric spacer layers fabricated either using atomic layer deposition or using organic layers (polyelectrolytes or self-assembled monolayers of molecules). In either case, large area, uniform spacer layers with sub-nanometer thicknesses can be accurately deposited, allowing extreme coupling regimes to be probed. The observed spectral shifts of the nanoparticles as a function of spacer layer thickness are similar for the organic and inorganic films and are consistent with numerical calculations taking into account the nonlocal response of the metal.

Realizing ferroelectric Hf0.5Zr0.5O2 with elemental capping layers

Hafnium zirconium oxide (Hf0.5Zr0.5O2 or HZO) thin films show great promise for enabling ferroelectric field-effect transistors (FeFETs) for memory applications and negative capacitance FETs for low-power digital devices. One challenge in the integration of ferroelectric HZO is the need for specific capping layers to yield the most pronounced ferroelectric behavior; to date, superior HZO devices use titanium nitride or tantalum nitride, which limits HZO integration into various device structures. In this work, the authors demonstrate the use of elemental capping layers, including Pt, Ni, and Pd, for driving ferroelectricity in HZO. Different combinations of these capping metals, along with changes in the HZO thickness and annealing conditions, have yielded the optimal conditions for maximizing ferroelectric behavior. A remnant polarization of 19 μC/cm2 and a coercive field strength of 1.07 MV/cm were achieved with the Pt/HZO/Ni stack annealed at 650 °C with a HZO thickness of ∼20 nm. These results bring e...

Using Ar Ion beam exposure to improve contact resistance in MoS 2 FETs

Contact resistance is a dominant factor in the performance of field-effect transistors (FETs) from two-dimensional MoS2. Several techniques have been shown to improve carrier transport at the metal-MoS2 interface, thus lowering the contact resistance. These approaches include the use of molecular doping, different contact materials, phase transformation of MoS2 [6], and adding an interfacial oxide at the contacts. The challenges for the most effective of these techniques are that they generally require additional processing, sometimes involving very high temperatures, or the addition of materials at the metal-MoS2 interface that may lower contact resistance but still yield relatively poor FET performance. In graphene, it has been demonstrated that intentionally damaging the crystal lattice in the contact region using O2 plasma can substantially reduce the contact resistance. In this work, we examine a related contact engineering approach for MoS2 FETs by using an in situ, broad-beam ion source to modify the MoS2 lattice immediately prior to contact metal deposition. The result is a substantial improvement in key performance metrics, including contact resistance and on-Contact resistance is a dominant factor in the performance of field-effect transistors (FETs) from two-dimensional MoS2. Several techniques have been shown to improve carrier transport at the metal-MoS2 interface, thus lowering the contact resistance. These approaches include the use of molecular doping, different contact materials, phase transformation of MoS2, and adding an interfacial oxide at the contacts. The challenges for the most effective of these techniques are that they generally require additional processing, sometimes involving very high temperatures, or the addition of materials at the metal-MoS2 interface that may lower contact resistance but still yield relatively poor FET performance. In graphene, it has been demonstrated that intentionally damaging the crystal lattice in the contact region using O2 plasma can substantially reduce the contact resistance. In this work, we examine a related contact engineering approach for MoS2 FETs by using an in situ, broad-beam ion source to modify the MoS2 lattice immediately prior to contact metal deposition. The result is a substantial improvement in key performance metrics, including contact resistance and on-current.current.

Modifying the Ni-MoS 2 Contact Interface Using a Broad-Beam Ion Source

Charge transport at the contacts is a dominant factor in determining the performance of devices using 2D MoS2. Using a low-energy beam of Ar ions, the interface between Ni and MoS2 was modified to improve the performance in 2D field-effect transistors (FETs). This broad-beam ion source is integrated into an ultrahigh vacuum, physical vapor deposition system that allowed for in situ modification of the MoS2 immediately prior to Ni contact deposition. The contact resistance decreased leading to a corresponding and highly reproducible boost in the on-current by up to four times. Spectroscopic analysis of the ion beam-modified MoS2 suggests that there are generated defects, which supply dangling bonds that improve carrier injection between the Ni metal contact and MoS2. This approach for modifying the Ni-MoS2 interface opens a promising new path for reducing the impact of contacts on MoS2 FET performance.

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.

MoS 2 negative capacitance FETs with CMOS-compatible hafnium zirconium oxide

The attractiveness of negative capacitance field-effect transistors (NC-FETs) stems from their ability to enable a subthreshold swing (SS) below the 60 mV/decade thermal limit at room temperature — a direct effect of the step-up voltage amplifier behavior of the ferroelectric [1]. This effect has been shown to yield sub-60 mV/dec SS in several Si-based NC-FETs [2-4]; however, as Si-based devices become increasingly difficult to scale, it is pertinent to explore alternative materials for NC-FETs that offer scalability in voltage as well as size [5]. One promising alternative channel material is the 2D transition metal dichalcogenide (TMD, such as MoS2), which offer sub-nm thinness and a more stable channel capacitance that, when coupled with the NC-effect, could produce steep switching over a broad range of current. To date, the only demonstration of an NC-FET with a 2D channel used a polymeric ferroelectric, resulting in a lack of stability and CMOS-compatibility despite superb low-voltage switching [6]. In this work, we demonstrate 2D NC-FETs using MoS2 with CMOS-compatible hafnium zirconium oxide (HfZrO2 or HZO) as the ferroelectric to achieve repeatable and sustained sub-60 mV/dec switching.

Tunable plasmonic platform for giant fluorescence enhancement

We demonstrate a colloidally synthesized plasmonic platform for giant fluorescence enhancement and increased spontaneous emission rate of embedded fluorophores. A transition between fluorescence enhancement and quenching is revealed depending on the plasmonic resonance.