2-D materials for ultra-scaled field-effect transistors: hundred candidates under the ab initio microscope

Thanks to their unique properties single-layer 2-D materials appear as excellent candidates to extend Moore's scaling law beyond the currently manufactured silicon FinFETs. However, the known 2-D semiconducting components, essentially transition metal dichalcogenides, are still far from delivering the expected performance. Based on a recent theoretical study that predicts the existence of more than 1,800 exfoliable 2-D materials, we investigate here the 100 most promising contenders for logic applications. Their "current vs. voltage" characteristics are simulated from first-principles, combining density-functional theory and advanced quantum transport calculations. Both n- and p-type configurations are considered, with gate lengths ranging from 15 down to 5 nm. From this unprecedented collection of electronic materials, we identify 13 compounds with electron and hole currents potentially much higher than in future Si FinFETs. The resulting database widely expands the design space of 2-D transistors and provides original guidelines to the materials and device engineering community

Field-Effect Transistors based on 2-D Materials: a Modeling Perspective

Two-dimensional (2D) materials are particularly attractive to build the channel of next-generation field-effect transistors (FETs) with gate lengths below 10-15 nm. Because the 2D technology has not yet reached the same level of maturity as its Silicon counterpart, device simulation can be of great help to predict the ultimate performance of 2D FETs and provide experimentalists with reliable design guidelines. In this paper, an ab initio modelling approach dedicated to well-known and exotic 2D materials is presented and applied to the simulation of various components, from thermionic to tunnelling transistors based on mono- and multi-layer channels. Moreover, the physics of metal - 2D semiconductor contacts is revealed and the importance of different scattering sources on the mobility of selected 2D materials is discussed. It is expected that modeling frameworks similar to the one described here will not only accompany future developments of 2D devices, but will also enable them

The ab initio microscope: on the performance of 2D materials as future field-effect transistors

In this work, the potential of novel 2D materials for possible application as next generation ultra-scaled field-effect transistor (FET) is evaluated from an atomistic perspective. For this purpose, a first-principles simulation scheme based on density functional theory (DFT) and the non-equilibrium Green’s function (NEGF) formalism is employed. This approach can shed light on the device behavior at the nano-scale, where classical drift-diffusion models reach their limit. A DFT+NEGF simulator allows to investigate the observables of interest such as the charge density and the electric current in different device structures, for example in metal-oxide-semiconductor (MOS) FETs (MOSFETs) or band-to-band tunneling FETs (TFET) controlled by a single or multiple gates.First one-hundred potential 2D contenders for logic applications are examined in a single-gate (SG) MOSFET architecture for both n- and p-type configuration. The full I-V characteristics are simulated at a gate length of 15 nm to determine the potential ON-current at a fixed OFF-current.From this data, we identify 13 compounds that achieve electron and hole currents potentially outperforming those of future silicon FinFETs. The sub-threshold slope (SS) is studied down to a gate length of 5 nm to identify the scalability of all studied 2D materials. To analyze the obtained results, the concepts of transport and density-of-states (DOS) effective mas is generalized and systematically extracted for each 2D material.While these quantities partly explain the device behavior, they are not sufficient as the effect of narrow bands can strongly compromise the FET performance. A novel metric called pass factor is therefore introduced to quantify this phenomenon. Overall it is found that materials with a low transport effective mass, high DOS and a pass factor close to one yield excellent performance. Such materials are often characterized by a strongly asymmetric bandstructure. Black phosphorus (BP), the most promising candidate among all considered belongs to this category. It is used in a second study to explore the influence of a flake misalignment with respect to the source-to-drain direction on the ON-state current. The impact of misalignment is demonstrated using six different transport directions in a single gate MOSFET. Up to a misalignment angle of 20 degrees, the ON-state current remains almost constant. The current reduction does not exceed 30%for angles below 50 degrees before rapidly decreasing to around 60%of its maximum value in the worst-case scenario (90 degrees misalignment). This phenomenon can be explained by inspecting the dependence of the effective mass in transport direction on the misalignment angle. The ON-state current behavior between quasi-ballistic simulations and calculations where phonon- and charged-impurity scattering are present remains qualitatively equal. Consequently, the change in the transport effective mass can explain the observations and a misalignment tolerance of 20 degrees in experiments should be acceptable.In a third study, the potential of 2D materials as TFETs is evaluated. It is demonstrated that the well-known transition metal dichalcogenide(TMD) are not well-suited for TFET applications due to their large bandgap and effective masses a tunneling window is already open in the OFF-state and consequently the desired sub-thermionic SS cannot be achieved. Potential novel single-layer materials with a more favorable effective mass and band gap combination are shown to reach ON-state currents roughly two orders of magnitude higher than all of the TMDs, while also reaching sub-thermionic SS. In a final study, we explore the application of the DFT+NEGF approach to optoelectronic devices, photovoltaic cells in the present case. A dedicated self-energy is implemented for that purpose and the necessary inputs are derived from the ab inito level. A MoS2PIN-junction is then studied as a proof-of-concept structure to verify the implementation of our model. The approach produces physically meaningful results, for example, the Franz-Keldysh effect is captured by our method

Ab initio modelling of photodetectors based on van der Waals heterostructures

Two-dimensional (2-D) materials, especially transition metal dichalcogenides (TMDCs), have attracted great attention due to their unique properties and the vast possibility they offer when combined into van der Waals heterostructures (vdWHs), for example in optoelectronic applications. Although various photodetectors based on TMDC vdWHs have been successfully synthesized, an \textit{ab initio} simulation framework dedicated to such devices is still missing. In this work, we investigate two photodetector designs based on MoSe\textsubscript{2}-WSe\textsubscript{2} vdWHs through a recently developed approach combining density functional theory and quantum transport calculations. Geometries with a partial and total TMDC overlap are considered, with a p-doped (n-doped) left (right) extension and an intrinsic region in the middle. We show that the partial overlap structure supports a non-zero photocurrent, even without a built-in potential, contrary to the full overlap structure

Bulk photovoltaic effect in partial overlap MoSe2-WSe2 van der Waals heterostructures: An ab initio quantum transport study

Two-dimensional (2-D) materials, especially transition metal dichalcogenides (TMDCs), have attracted great attention due to their unique properties and the vast possibility they offer when combined into van der Waals heterostructures (vdWHs), for example in optoelectronic applications. Although various photodetectors based on TMDC vdWHs have been successfully synthesized, an ab initio simulation framework dedicated to such devices is still missing. In this work, we investigate two photodetector designs based on MoSe2-WSe2 vdWHs through a recently developed approach combining density functional theory and quantum transport calculations. Geometries with a partial and total TMDC overlap are considered, with a p-doped (n-doped) left (right) extension and an intrinsic region in the middle. We show that the partial overlap structure supports a non-zero photo-current, even without a p–n junction, and gives rise to the bulk photovoltaic effect (BPVE). The present results provide an easy-to-fabricate guideline for engineering the BPVE in the TMDC vdWHs.ISSN:0038-110

Impact of Orientation Misalignments on Black Phosphorus Ultrascaled Field-Effect Transistors

Two-dimensional materials with strong bandstructure anisotropy such as black phosphorus (BP) have been identified as attractive candidates for logic application due to their potential high carrier velocity and large density-of-states. However, perfectly aligning the source-to-drain axis with the desired crystal orientation remains an experimental challenge. In this letter, we use an advanced quantum transport approach from first-principle to shed light on the influence of orientation misalignments on the performance of BP-based field-effect transistors. Both n -and p -type configurations are investigated for six alignment angles, in the ballistic limit of transport and in the presence of electron-phonon and charged impurity scattering. It is found that up to deviations of 50° from the optimal angle, the ON-state current only decreases by 30%. This behavior is explained by considering a single bandstructure parameter, the effective mass along transport direction.ISSN:0741-3106ISSN:1558-056

Influence of the hBN Dielectric Layers on the Quantum Transport Properties of MoS₂ Transistors

The encapsulation of single-layer 2D materials within hBN has been shown to improve the mobility of these compounds. Nevertheless, the interplay between the semiconductor channel and the surrounding dielectrics is not yet fully understood, especially their electron–phonon interactions. Therefore, here, we present an ab initio study of the coupled electrons and phonon transport properties of MoS2-hBN devices. The characteristics of two transistor configurations are compared to each other: one where hBN is treated as a perfectly insulating, non-vibrating layer and one where it is included in the ab initio domain as MoS2. In both cases, a reduction of the ON-state current by about 50% is observed as compared to the quasi-ballistic limit. Despite the similarity in the current magnitude, explicitly accounting for hBN leads to additional electron–phonon interactions at frequencies corresponding to the breathing mode of the MoS2-hBN system. Moreover, the presence of an hBN layer around the 2D semiconductor affects the Joule-induced temperature distribution within the transistor

Light-matter interactions in van der Waals photodiodes from first-principles

Strong light-matter interactions in van der Waals heterostructures (vdWHs) made of twodimensional (2-D) transition metal dichalcogenides (TMDs) provide a fertile ground for optoelectronic applications. Of particular interest are photo-excited inter-layer electron-hole pairs, where electrons and holes are localized in different monolayers. Here, we present an ab initio quantum transport framework relying on maximally localized Wannier Functions and the Non-equilibrium Green’s Functions to explore light-matter interactions and charge transport in 2-D vdWHs from first-principles. Electron-photon scattering is accurately taken into account through dedicated self-energies. As testbed, the behavior of a MoSe2-WSe2 PIN photodiode is investigated under the influence of a monochromatic electromagnetic signal. Inter-layer electron-hole pair generations are observed even in the absence of phonon-assisted processes. The origin of this phenomenon is identified as the delocalization of one valence band state over both monolayers composing the vdWH.ISSN:1098-0121ISSN:0163-1829ISSN:1550-235XISSN:0556-2805ISSN:2469-9969ISSN:1095-3795ISSN:2469-995

Influence of the hBN Dielectric Layers on the Quantum Transport Properties of MoS2 Transistors

The encapsulation of single-layer 2D materials within hBN has been shown to improve the mobility of these compounds. Nevertheless, the interplay between the semiconductor channel and the surrounding dielectrics is not yet fully understood, especially their electron–phonon interactions. Therefore, here, we present an ab initio study of the coupled electrons and phonon transport properties of MoS2-hBN devices. The characteristics of two transistor configurations are compared to each other: one where hBN is treated as a perfectly insulating, non-vibrating layer and one where it is included in the ab initio domain as MoS2 . In both cases, a reduction of the ON-state current by about 50% is observed as compared to the quasi-ballistic limit. Despite the similarity in the current magnitude, explicitly accounting for hBN leads to additional electron–phonon interactions at frequencies corresponding to the breathing mode of the MoS2-hBN system. Moreover, the presence of an hBN layer around the 2D semiconductor affects the Joule-induced temperature distribution within the transistor.ISSN:1996-194