Direct Assessment of Defective Regions in Monolayer MoS2 Field-Effect Transistors through In Situ Scanning Probe Microscopy Measurements.

Implementing two-dimensional materials in field-effect transistors (FETs) offers the opportunity to continue the scaling trend in the complementary metal-oxide-semiconductor technology roadmap. Presently, the search for electrically active defects, in terms of both their density of energy states and their spatial distribution, has turned out to be of paramount importance in synthetic transition metal dichalcogenides layers, as they are suspected of severely inhibiting these devices from achieving their highest performance. Although advanced microscopy tools have allowed the direct detection of physical defects such as grain boundaries and point defects, their implementation at the device scale to assess the active defect distribution and their impact on field-induced channel charge modulation and current transport is strictly restrained. Therefore, it becomes critical to directly probe the gate modulation effect on the carrier population at the nanoscale of an FET channel, with the objective to establish a direct correlation with the device characteristics. Here, we have investigated the active channel in a monolayer MoS2 FET through in situ scanning probe microscopy, namely, Kelvin probe force microscopy and scanning capacitance microscopy, to directly identify active defect sites and to improve our understanding of the contribution of grain boundaries, bilayer islands, and defective grain domains to channel conductance.

Unraveling the impact of nano-scaling on silicon field-effect transistors for the detection of single-molecules.

Electrolyte-gated silicon field-effect transistors (FETs) capable of detecting single molecules could enable high-throughput molecular sensing chips to advance, for example, genomics or proteomics. For solid-gated silicon FETs it is well-known that nano-scaled devices become sensitive to single elementary charges near the silicon-oxide interface. However, in electrolyte-gated FETs, electrolyte screening strongly reduces sensitivity to charges near the gate oxide. The question arises whether nano-scaling electrolyte-gated FETs can entail a sufficiently large signal-to-noise ratio (SNR) for the detection of single molecules. We enhanced a technology computer-aided design tool with electrolyte screening models to calculate the impact of the FET geometry on the single-molecule signal and FET noise. Our continuum FET model shows that a sufficiently large single-molecule SNR is only obtained when nano-scaling all FET channel dimensions. Moreover, we show that the expected scaling trend of the single-molecule SNR breaks down and no longer results in improvements for geometries approaching the decananometer size. This is the characteristic size of the FET channel region modulated by a typical molecule. For gate lengths below 50 nm, the overlap of the modulated region with the highly conductive junctions leads to saturation of the SNR. For cross-sections below 10-30 nm, SNR degrades due to the overlap of the modulated region with the convex FET corners where a larger local gate capacitance reduces charge sensitivity. In our study, assuming a commercial solid-state FET noise amplitude, we find that a suspended nanowire FET architecture with 35 nm length and 5 × 10 nm2 cross-section results in the highest SNR of about 10 for a 15-base DNA oligo in a 15 mM electrolyte. In contrast with typical silicon nanowire FET sensors which possess micron-scale gate lengths, we find it to be key that all channel dimensions are scaled down to the decananometer range.

Impact of device scaling on the electrical properties of MoS2 field-effect transistors.

Two-dimensional semiconducting materials are considered as ideal candidates for ultimate device scaling. However, a systematic study on the performance and variability impact of scaling the different device dimensions is still lacking. Here we investigate the scaling behavior across 1300 devices fabricated on large-area grown MoS2 material with channel length down to 30 nm, contact length down to 13 nm and capacitive effective oxide thickness (CET) down to 1.9 nm. These devices show best-in-class performance with transconductance of 185 µS/µm and a minimum subthreshold swing (SS) of 86 mV/dec. We find that scaling the top-contact length has no impact on the contact resistance and electrostatics of three monolayers MoS2 transistors, because edge injection is dominant. Further, we identify that SS degradation occurs at short channel length and can be mitigated by reducing the CET and lowering the Schottky barrier height. Finally, using a power performance area (PPA) analysis, we present a roadmap of material improvements to make 2D devices competitive with silicon gate-all-around devices.

The Significance of Nonlinear Screening and the pH Interference Mechanism in Field-Effect Transistor Molecular Sensors.

Electrolyte screening is well known for its detrimental impact on the sensitivity of liquid-gated field-effect transistor (FET) molecular sensors and is mostly described by the linearized Debye-Hückel model. However, charged and pH-sensitive FET sensing surfaces can limit the FET molecular sensitivity beyond the Debye-Hückel screening formalism. Pre-existing surface charges can lead to the breakdown of Debye-Hückel screening and induce enhanced nonlinear Poisson-Boltzmann screening. Moreover, the charging of the pH-sensitive surface groups interferes with biomolecule sensing resulting in a pH interference mechanism. With analytical equations and TCAD simulations, we highlight that the Debye-Hückel approximation can underestimate screening and overestimate FET molecular sensitivity by more than an order of magnitude. Screening strengthens significantly beyond Debye-Hückel in the proximity of even moderately charged surfaces and biomolecule charge densities (≥1 × 1012 q/cm2). We experimentally show the strong impact of both nonlinear screening and the pH interference effect on charge-based biomolecular sensing using a model system based on the covalent binding of single-stranded DNA on silicon FET sensors. The DNA signal increases from 24 mV at pH 7 to 96 mV at pH 3 in 1.5 mM PBS for a DNA density of 7 × 1012 DNA/cm2. Our model quantitatively explains the signal's pH dependence with roughly equal nonlinear screening and pH interference contributions. This work shows the importance of reducing the net charge and the pH sensitivity of the sensing surface to improve molecular sensing. Therefore, tailoring the gate dielectric and functional layer of FET sensors is a promising route to strong silicon FET molecular sensitivity boosts.

Understanding ambipolar transport in MoS2 field effect transistors: the substrate is the key.

2D materials offer a pathway for further scaling of CMOS technology. However, for this to become a reality, both n-MOS and p-MOS should be realized, ideally with the same (standard) material. In the specific case of MoS2 field effect transistors (FETs), ambipolar transport is seldom reported, primarily due to the phenomenon of Fermi level pinning (FLP). In this study we identify the possible sources of FLP in MoS2 FETs and resolve them individually. A novel contact transfer technique is used to transfer contacts on top of MoS2 flake devices that results in a significant increase in the hole branch of the transfer characteristics as compared to conventionally fabricated contacts. We hypothesize that the pinning not only comes from the contact-MoS2 interface, but also from the MoS2-substrate interface. We confirm this by shifting to an hBN substrate which leads to a 10 fold increase in the hole current compared to the SiO2 substrate. Furthermore, we analyse MoS2 FETs of different channel thickness on three different substrates, SiO2, hBN and Al2O3, by correlating the p-branch I ON/I OFF to the position of oxide defect band in these substrates. FLP from the oxide is reduced in the case of Al2O3 which enables us to observe ambipolar transport in a bilayer MoS2 FET. These results highlight that MoS2 is indeed an ambipolar material, and the absence of ambipolar transport in MoS2 FETs is strongly correlated to its dielectric environment and processing conditions.

Reconfigurable submicrometer spin-wave majority gate with electrical transducers.

Spin waves are excitations in ferromagnetic media that have been proposed as information carriers in hybrid spintronic devices with much lower operation power than conventional charge-based electronics. Their wave nature can be exploited in majority gates by using interference for computation. However, a scalable spin-wave majority gate that can be cointegrated alongside conventional electronics is still lacking. Here, we demonstrate a submicrometer inline spin-wave majority gate with fan-out. Time-resolved imaging of the magnetization dynamics by scanning transmission x-ray microscopy illustrates the device operation. All-electrical spin-wave spectroscopy further demonstrates majority gates with submicrometer dimensions, reconfigurable input and output ports, and frequency-division multiplexing. Challenges for hybrid spintronic computing systems based on spin-wave majority gates are discussed.

On the van der Waals Epitaxy of Homo-/Heterostructures of Transition Metal Dichalcogenides.

Layered materials held together by weak van der Waals (vdW) interactions are a promising class of materials in the field of nanotechnology. Besides the potential for single layers, stacking of various vdW layers becomes even more promising since unique properties can hence be precisely engineered. The synthesis of stacked vdW layers, however, remains to date, hardly understood. Therefore, in this work, the vdW epitaxy of transition metal dichalcogenides (TMDs) on single-crystalline TMD templates is investigated in depth. It is demonstrated that the role of lattice mismatch is insignificant. More importantly is the role of surface energy, calculated using density functional theory, which plays an essential role in the activation energy for adatom diffusion, hence nucleation density. This in turn correlates with defect density since the stacking sequence in vdW epitaxy is generally poorly controlled. Moreover, the vapor pressure of the transition metal is also found to correlate with adatom diffusion. Consequently, the proposed study enables important and new insight in the vdW epitaxy of multilayer 2D homo-/heterostructures.

Peculiar alignment and strain of 2D WSe2 grown by van der Waals epitaxy on reconstructed sapphire surfaces.

The increasing scientific and industry interest in 2D MX2 materials within the field of nanotechnology has made the single crystalline integration of large area van der Waals (vdW) layers on commercial substrates an important topic. The c-plane oriented (3D crystal) sapphire surface is believed to be an interesting substrate candidate for this challenging 2D/3D integration. Despite the many attempts that have been made, the yet incomplete understanding of vdW epitaxy still results in synthetic material that shows a crystallinity far too low compared to natural crystals that can be exfoliated onto commercial substrates. Thanks to its atomic control and in situ analysis possibilities, molecular beam epitaxy (MBE) offers a potential solution and an appropriate method to enable a more in-depth understanding of this peculiar 2D/3D hetero-epitaxy. Here, we report on how various sapphire surface reconstructions, that are obtained by thermal annealing of the as-received substrates, influence the vdW epitaxy of the MBE-grown WSe2 monolayers (MLs). The surface chemistry and the interatomic arrangement of the reconstructed sapphire surfaces are shown to control the preferential in-plane epitaxial alignment of the stoichiometric WSe2 crystals. In addition, it is demonstrated that the reconstructions also affect the in-plane lattice parameter and thus the in-plane strain of the 2D vdW-bonded MLs. Hence, the results obtained in this work shine more light on the peculiar concept of vdW epitaxy, especially relevant for 2D materials integration on large-scale 3D crystal commercial substrates.

Microwave Characterization of Ba-Substituted PZT and ZnO Thin Films.

The microwave dielectric properties of (Ba0.1Pb0.9)(Zr0.52Ti0.48)O3 (BPZT) and ZnO thin films with thicknesses below were investigated. No significant dielectric relaxation was observed for both BPZT and ZnO up to 30 GHz. The intrinsic dielectric constant of BPZT was as high as 980 at 30 GHz. The absence of strong dielectric dispersion and loss peaks in the studied frequency range can be linked to the small grain diameters in these ultrathin films.

Electrically Driven Unidirectional Optical Nanoantennas.

Directional antennas revolutionized modern day telecommunication by enabling precise beaming of radio and microwave signals with minimal loss of energy. Similarly, directional optical nanoantennas are expected to pave the way toward on-chip wireless communication and information processing. Currently, on-chip integration of such antennas is hampered by their multielement design or the requirement of complicated excitation schemes. Here, we experimentally demonstrate electrical driving of in-plane tunneling nanoantennas to achieve broadband unidirectional emission of light. Far-field interference, as a result of the spectral overlap between the dipolar emission of the tunnel junction and the fundamental quadrupole-like resonance of the nanoantenna, gives rise to a directional radiation pattern. By tuning this overlap using the applied voltage, we record directivities as high as 5 dB. In addition to electrical tunability, we also demonstrate passive tunability of the directivity using the antenna geometry. These fully configurable electrically driven nanoantennas provide a simple way to direct optical energy on-chip using an extremely small device footprint.

From the metal to the channel: a study of carrier injection through the metal/2D MoS2 interface.

Despite the fact that two-dimensional MoS2 films continue to be of interest for novel device concepts and beyond silicon technologies, there is still a lack of understanding on the carrier injection at metal/MoS2 interface and effective mitigation of the contact resistance. In this work, we develop a semi-classical model to identify the main mechanisms and trajectories for carrier injection at MoS2 contacts. The proposed model successfully captures the experimentally observed contact behavior and the overall electrical behavior of MoS2 field effect transistors. Using this model, we evaluate the injection trajectories for different MoS2 thicknesses and bias conditions. We find for multilayer (>2) MoS2, the contribution of injection at the contact edge and injection under the contact increase with lateral and perpendicular fields, respectively. Furthermore, we identify that the carriers are predominantly injected at the edge of the contact metal for monolayer and bilayer MoS2. Following these insights, we have found that the transmission line model could significantly overestimate the transfer length and hence the contact resistivity for monolayer and bilayer MoS2. Finally, we evaluate different contact strategies to improve the contact resistance considering the limiting injection trajectory.

Material-Device-Circuit Co-optimization of 2D Material based FETs for Ultra-Scaled Technology Nodes.

Two-dimensional (2D) material based FETs are being considered for future technology nodes and high performance logic applications. However, a comprehensive assessment of 2D material based FETs has been lacking for high performance logic applications considering appropriate system level figure-of-merits (FOMs) e.g. delay, and energy-delay product. In this paper, we present guidelines for 2D material based FETs to meet sub-10 nm high performance logic requirements focusing on material requirement, device design, energy-delay optimization for the first time. We show the need for 2D materials with smaller effective mass in the transport direction and anisotropicity to meet the performance requirement for future technology nodes. We present novel device designs with one such 2D material (monolayer black-phosphorus) to keep Moore's alive for the HP logic in sub-5 nm gate length regime. With these device proposals we show that below 5 nm gate lengths 2D electrostatistics arising from gate stack design becomes more of a challenge than direct source-to-drain tunneling for 2D material-based FETs. Therefore, it is challenging to meet both delay and energy-delay requirement in sub-5 nm gate length regime without scaling both supply voltage (V DD ) and effective-oxide-thickness (EOT) below 0.5 V and 0.5 nm respectively.

Highly efficient and stable MoS2 FETs with reversible n-doping using a dehydrated poly(vinyl-alcohol) coating.

Despite rapid progress in 2D molybdenum disulfide (MoS2) research in recent years, MoS2 field-effect transistors (FETs) still suffer from a high metal-to-MoS2 contact resistance and low intrinsic mobility, which are major hindrances to their future application. We report an efficient technique to dope thin-film MoS2 FETs using a poly(vinyl-alcohol) (PVA) polymeric coating. This results in a reduction of the contact resistance by up to 30% as well as a reduction in the channel resistance to 20 kΩ sq-1. Using a dehydration process, we were able to effectively control the surface interactions between MoS2 and the more electropositive hydroxyl groups (-OH) of PVA, which provided a controllable and yet reversible increase in the charge carrier density to a value of 8.0 × 1012 cm-2. The non-covalent, thus non-destructive, PVA doping of MoS2 increases the carrier concentration without degrading the mobility, which shows a monotonic increase while enhancing the doping effect. The PVA doping technique is then exploited to create heavily doped access regions to the intrinsic MoS2 channel, which yields 200% increase of the ON-state source-drain current. This establishes PVA doping as an effective approach to enhance the transport properties of MoS2 FETs for a variety of applications.

Density and Capture Cross-Section of Interface Traps in GeSnO2 and GeO2 Grown on Heteroepitaxial GeSn.

An imperative factor in adapting GeSn as the channel material in CMOS technology, is the gate-oxide stack. The performance of GeSn transistors is degraded due to the high density of traps at the oxide-semiconductor interface. Several oxide-gate stacks have been pursued, and a midgap Dit obtained using the ac conductance method, is found in literature. However, a detailed signature of oxide traps like capture cross-section, donor/acceptor behavior and profile in the bandgap, is not yet available. We investigate the transition region between stoichiometric insulators and strained GeSn epitaxially grown on virtual Ge substrates. Al2O3 is used as high-κ oxide and either Ge1-xSnxO2 or GeO2 as interfacial layer oxide. The interface trap density (Dit) profile in the lower half of the bandgap is measured using deep level transient spectroscopy, and the importance of this technique for small bandgap materials like GeSn, is explained. Our results provide evidence for two conclusions. First, an interface traps density of 1.7 × 10(13) cm(-2)eV(-1) close to the valence band edge (Ev + 0.024 eV) and a capture cross-section (σp) of 1.7 × 10(-18) cm(2) is revealed for GeSnO2. These traps are associated with donor states. Second, it is shown that interfacial layer passivation of GeSn using GeO2 reduces the Dit by 1 order of magnitude (2.6 × 10(12) cm(-2)eV(-1)), in comparison to GeSnO2. The results are cross-verified using conductance method and saturation photovoltage technique. The Dit difference is associated with the presence of oxidized (Sn(4+)) and elemental Sn in the interfacial layer oxide.

Graphene oxide monolayers as atomically thin seeding layers for atomic layer deposition of metal oxides.

Graphene oxide (GO) was explored as an atomically-thin transferable seed layer for the atomic layer deposition (ALD) of dielectric materials on any substrate of choice. This approach does not require specific chemical groups on the target surface to initiate ALD. This establishes GO as a unique interface which enables the growth of dielectric materials on a wide range of substrate materials and opens up numerous prospects for applications. In this work, a mild oxygen plasma treatment was used to oxidize graphene monolayers with well-controlled and tunable density of epoxide functional groups. This was confirmed by synchrotron-radiation photoelectron spectroscopy. In addition, density functional theory calculations were carried out on representative epoxidized graphene monolayer models to correlate the capacitive properties of GO with its electronic structure. Capacitance-voltage measurements showed that the capacitive behavior of Al2O3/GO depends on the oxidation level of GO. Finally, GO was successfully used as an ALD seed layer for the deposition of Al2O3 on chemically inert single layer graphene, resulting in high performance top-gated field-effect transistors.

Chemically enhanced double-gate bilayer graphene field-effect transistor with neutral channel for logic applications.

In this article, we present the simulation, fabrication, and characterization of a novel bilayer graphene field-effect transistor exhibiting electron mobility up to ~1600 cm(2) V(-1) s(-1), a room temperature I on/I off ≈ 60, and the lowest total charge (~10(11) cm(-2)) reported to date. This is achieved by combined electrostatic and chemical doping of bilayer graphene, which enables one to switch off the device at zero top-gate voltage. Using density functional theory and atomistic simulations, we obtain physical insight into the impact of chemical and electrostatic doping on bandgap opening of bilayer graphene and the effect of metal contacts on the operation of the device. Our results represent a step forward in the use of bilayer graphene for high-performance logic devices in the beyond-complementary metal-oxide-semiconductor (CMOS) technology paradigm.

Measuring the electrical resistivity and contact resistance of vertical carbon nanotube bundles for application as interconnects.

Carbon nanotubes (CNT) are known to be materials with potential for manufacturing sub-20 nm high aspect ratio vertical interconnects in future microchips. In order to be successful with respect to contending against established tungsten or copper based interconnects, though, CNT must fulfil their promise of also providing low electrical resistance in integrated structures using scalable integration processes fully compatible with silicon technology. Hence, carefully engineered growth and integration solutions are required before we can fully exploit their potentialities. This work tackles the problem of optimizing a CNT integration process from the electrical perspective. The technique of measuring the CNT resistance as a function of the CNT length is here extended to CNT integrated in vertical contacts. This allows extracting the linear resistivity and the contact resistance of the CNT, two parameters to our knowledge never reported separately for vertical CNT contacts and which are of utmost importance, as they respectively measure the quality of the CNT and that of their metal contacts. The technique proposed allows electrically distinguishing the impact of each processing step individually on the CNT resistivity and the CNT contact resistance. Hence it constitutes a powerful technique for optimizing the process and developing CNT contacts of superior quality. This can be of relevant technological importance not only for interconnects but also for all those applications that rely on the electrical properties of CNT grown with a catalytic chemical vapor deposition method at low temperature.

Bandgap opening in oxygen plasma-treated graphene.

We report a change in the semimetallic nature of single-layer graphene after exposure to oxygen plasma. The resulting transition from semimetallic to semiconducting behavior appears to depend on the duration of the exposure to the plasma treatment. The observation is confirmed by electrical, photoluminescence and Raman spectroscopy measurements. We explain the opening of a bandgap in graphene in terms of functionalization of its pristine lattice with oxygen atoms. Ab initio calculations show more details about the interaction between carbon and oxygen atoms and the consequences on the optoelectronic properties, that is, on the extent of the bandgap opening upon increased functionalisation density.