The electronic and optical response of Bernal stacked bilayer graphene with geometry modulation and gate voltage are studied. The broken symmetry in sublattices, one dimensional periodicity ...perpendicular to the domain wall and out-of-plane axis introduces substantial changes of wavefunctions, such as gapless topological protected states, standing waves with bonding and anti-bonding characteristics, rich structures in density of states and optical spectra. The wavefunctions present well-behaved standing waves in pure system and complicated node structures in geometry-modulated system. The optical absorption spectra show forbidden optical excitation channels, prominent asymmetric absorption peaks, and dramatic variations in absorption structures. These results provide that the geometry-modulated structure with tunable gate voltage could be used for electronic and optical manipulation in future graphene-based devices.
Down‐scaling of transistor size in the lateral dimensions must be accompanied by a corresponding reduction in the channel thickness to ensure sufficient gate control to turn off the transistor. ...However, the carrier mobility of 3D bulk semiconductors degrades rapidly as the body thickness thins down due to more pronounced surface scattering. Two‐dimensional‐layered materials with perfect surface structures present a unique opportunity as they naturally have atomically thin and smooth layers while maintaining high carrier mobility. To benefit from continuous scaling, the performance of the scaled 2D transistors needs to outperform Si technology nowadays. There are already quite a few reviews discussing on the material property of potential 2D materials. It is believed that rigorous analysis based on industrial perspectives is needed. Herein, an analysis on channel material selection is presented and arguments on the four selected 2D semiconductors are provided, which can possibly meet the needs of future transistors, including WS2, SnSe, PtSe2, and InSe. The challenges and recent related research progresses for each material are also discussed.
To continue to produce tiny transistors without sacrificing device performance, new materials with perfect surface structures are needed. Two‐dimensional‐layered materials with extremely flat surfaces offer great potentials to further scale down the size of the transistor. The perspective on channel material selection is presented. Challenges of suggested potential material candidates are discussed.
A moiré superlattice formed in twisted van der Waals bilayers has emerged as a new tuning knob for creating new electronic states in two-dimensional materials. Excitonic properties can also be ...altered drastically due to the presence of moiré potential. However, quantifying the moiré potential for excitons is nontrivial. By creating a large ensemble of MoSe2/MoS2 heterobilayers with a systematic variation of twist angles, we map out the minibands of interlayer and intralayer excitons as a function of twist angles, from which we determine the moiré potential for excitons. Surprisingly, the moiré potential depth for intralayer excitons is up to ∼130 meV, comparable to that for interlayer excitons. This result is markedly different from theoretical calculations based on density functional theory, which show an order of magnitude smaller moiré potential for intralayer excitons. The remarkably deep intralayer moiré potential is understood within the framework of structural reconstruction within the moiré unit cell.
Recent technology development of logic devices based on 2-D semiconductors such as MoS2, WS2, and WSe2 has triggered great excitement, paving the way to practical applications. Making low-resistance ...p-type contacts to 2-D semiconductors remains a critical challenge. The key to addressing this challenge is to find high-work function metallic materials which also introduce minimal metal-induced gap states (MIGSs) at the metal/semiconductor interface. In this work, we perform a systematic computational screening of novel metallic materials and their heterojunctions with monolayer WSe2 based on ab initio density functional theory and quantum device simulations. Two contact strategies, van der Waals (vdW) metallic contact and bulk semimetallic contact, are identified as promising solutions to achieving Schottky-barrier-free and low-contact-resistance p-type contacts for WSe2 p-type field-effect transistor (pFETs). Good candidates of p-type contact materials are found based on our screening criteria, including 1H-NbS2, 1H-TaS2, and 1T-TiS2 in the vdW metal category, as well as Co3Sn2S2 and TaP in the bulk semimetal category. Simulations of these new p-type contact materials suggest reduced MIGS, less Fermi-level pinning effect, negligible Schottky barrier height and small contact resistance (down to Formula Omitted).
Semiconducting single‐walled carbon nanotube (CNT) is a promising candidate as a channel material for advanced logic transistors, attributed to the ultra‐thin 1‐nm cylindrical geometry, high ...mobility, and high carrier injection velocity. However, the presence of undesired CNT bundles in the CNT arrays for wafer‐scale device fabrication, even when utilizing the state‐of‐the‐art dimension‐limited self‐alignment (DLSA) method, poses challenges. These CNT bundles degrade the transistor gate's efficiency in controlling the flow of charge carriers in the CNT channel, leading to pronounced device‐to‐device variability. Here, a novel method is introduced to alleviate bundling in CNT arrays assembled via DLSA, by involving small molecule additive to screen the attractive van der Waals force between neighboring CNTs during the DLSA process, resulting in over 50% reduction in CNT bundling. Furthermore, a pioneering methodology for quantifying CNT bundles is presented and employed experimentally to assess bundles in dense CNT arrays assembled by DLSA using transmission electron microscopy. Both experimental data and molecular dynamics simulation reveal that CNT bundling originates from van der Waals attraction between CNTs, and the disturbed liquid‐liquid interface by accumulating excess polar molecules. These findings illuminate new pathways for realizing dense, bundle‐free CNT arrays.
Single‐walled carbon nanotubes (CNTs) offer great potential for advanced transistors. Yet, undesired CNT bundles pose challenges. This method reduces 50% of bundling by using small molecule additives to screen van der Waals attraction during assembly. The degree of CNT bundles is quantified by transmission electron microscopy. Experiment and simulation reveal that bundling originates from inter‐tube attraction and mixed liquid‐liquid interface.
Area-selective atomic layer deposition (AS-ALD) has attracted attention due to the process demand for semiconductor device scaling. Here, we propose the “atomic layer nucleation engineering (ALNE)” ...technique, an inhibitor-free AS-ALD of an oxide (Al2O3) and a nitride (AlN) with nearly 100% selectivity between the dielectric (SiO2) and the metal (Pt). The key is to add a radio-frequency substrate bias after precursor exposure and purge in each ALD cycle, where the energy from the ignited plasma selectively removes the precursors on the metal owing to the relatively lower binding energy compared to those on the dielectric, thereby inhibiting the film growth on the metal. This critical step enables the AS-ALD without selectivity loss up to 100 ALD cycles, leading to significant thickness differences of ∼14.9 and ∼8.7 nm for Al2O3 and AlN between the dielectric and metal surfaces. The realization of AS-ALD of Al2O3 and AlN by ALNE is also confirmed on the Pt/SiO2 patterned substrate. The ALNE offers a novel concept and approach to achieve high-selectivity AS-ALD, which is critical to further extension of Moore’s law.
The successful synthesis of a two‐dimensional monolayer lateral heterostructure with multijunctions WS2/WS2(1−x) Se2x/WS2 is presented in article number 1703680 by Keji Lai, Chih‐Kang Shih, and ...co‐workers. Using light‐assisted microwave impedance microscopy, the multijunctions demonstrate functionality, which enhances local photoconductivity by two orders of magnitude over pure WS2.
We develop a novel metal contact approach using an antimony (Sb)-platinum (Pt) bilayer to mitigate Fermi-level pinning in 2D transition metal dichalcogenide channels. This strategy allows for control ...over the transport polarity in monolayer WSe2 devices. By adjustment of the Sb interfacial layer thickness from 10 to 30 nm, the effective work function of the contact/WSe2 interface can be tuned from 4.42 eV (p-type) to 4.19 eV (n-type), enabling selectable n-/p-FET operation in enhancement mode. The shift in effective work function is linked to Sb-Se bond formation and an emerging n-doping effect. This work demonstrates high-performance n- and p-FETs with a single WSe2 channel through Sb-Pt contact modulation. After oxide encapsulation, the maximum current density at |VD| = 1 V reaches 170 μA/μm for p-FET and 165 μA/μm for n-FET. This approach shows promise for cost-effective CMOS transistor applications using a single channel material and metal contact scheme.We develop a novel metal contact approach using an antimony (Sb)-platinum (Pt) bilayer to mitigate Fermi-level pinning in 2D transition metal dichalcogenide channels. This strategy allows for control over the transport polarity in monolayer WSe2 devices. By adjustment of the Sb interfacial layer thickness from 10 to 30 nm, the effective work function of the contact/WSe2 interface can be tuned from 4.42 eV (p-type) to 4.19 eV (n-type), enabling selectable n-/p-FET operation in enhancement mode. The shift in effective work function is linked to Sb-Se bond formation and an emerging n-doping effect. This work demonstrates high-performance n- and p-FETs with a single WSe2 channel through Sb-Pt contact modulation. After oxide encapsulation, the maximum current density at |VD| = 1 V reaches 170 μA/μm for p-FET and 165 μA/μm for n-FET. This approach shows promise for cost-effective CMOS transistor applications using a single channel material and metal contact scheme.
A moiré superlattice formed in twisted van der Waals bilayers has emerged as a new tuning knob for creating new electronic states in two-dimensional materials. Excitonic properties can also be ...altered drastically due to the presence of moiré potential. However, quantifying the moiré potential for excitons is nontrivial. By creating a large ensemble of MoSe
/MoS
heterobilayers with a systematic variation of twist angles, we map out the minibands of interlayer and intralayer excitons as a function of twist angles, from which we determine the moiré potential for excitons. Surprisingly, the moiré potential depth for intralayer excitons is up to ∼130 meV, comparable to that for interlayer excitons. This result is markedly different from theoretical calculations based on density functional theory, which show an order of magnitude smaller moiré potential for intralayer excitons. The remarkably deep intralayer moiré potential is understood within the framework of structural reconstruction within the moiré unit cell.
Area-selective atomic layer deposition (AS-ALD) is gaining widespread attention due to the urgent demand for a self-aligned and “bottom-to-top” fabrication process in advanced semiconductor ...technology. In this study, an innovative concept of the “atomic layer nucleation engineering (ALNE)” and “surface recovery (SR)” techniques is proposed to realize AS-ALD of Al2O3 between metal (W) and dielectric (SiO2) without the involvement of inhibitors. The ALNE treatment is utilized to selectively remove the weakly adsorbed precursors on the metal surface, and the SR process can eliminate the oxidized layer on the metal surface caused by the exposure of oxidants in the ALD process. Accordingly, the AS-ALD with ∼100% selectivity is achieved up to 100 ALD cycles with a considerable difference in an Al2O3 thickness of ∼11.2 nm between the SiO2 and W surfaces. The accomplishment of AS-ALD is also demonstrated on the SiO2/W patterned substrates with the feature size scaling from 75 μm to ∼10 nm. Hence, the inhibitor-free AS-ALD implemented by the ALNE and SR techniques is a critical breakthrough for the further progress of Moore’s law.
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