A low Schottky barrier height (SBH) at source/drain contact is essential for achieving high drive current in atomic layer MoS 2 channel based field-effect transistors. Approaches such as choosing metals with appropriate work functions and chemical doping are employed previously to improve the carrier injection from the contact electrodes to the channel and to mitigate the SBH between the MoS 2 and metal. Recent experiments demonstrate significant SBH reduction when graphene layer is inserted between metal slab (Ti and Ni) and MoS 2 . However, the physical or chemical origin of this phenomenon is not yet clearly understood. In this work, density functional theory (DFT) simulations are performed, employing pseudopotentials with very high basis sets to get insights of the charge transfer between metal and monolayer MoS 2 through the inserted graphene layer. Our atomistic simulations on 16 different interfaces involving five different metals (Ti, Ag, Ru, Au and Pt) reveal that: (i) such a decrease in SBH is not consistent among various metals, rather an increase in SBH is observed in case of Au and Pt (ii) unlike MoS 2 -metal interface, the projected dispersion of MoS 2 remains preserved in any MoS 2 -graphene-metal system with shift in the bands on the energy axis. (iii) a proper choice of metal (e.g., Ru) may exhibit ohmic nature in a graphene inserted MoS 2 -metal contact. These understandings would provide a direction in developing high performance transistors involving hetero atomic layers as contact electrodes.
Using first-principles density functional theoretical analysis, we predict coexisting ferroelectric and semi-metallic states in two-dimensional monolayer of h-NbN subjected to electric field and in-plane strain ( ). At strains close to =4.85%, where its out-of-plane spontaneous polarization changes sign without inverting the structure, we demonstrate a hysteretic response of its structure and polarization to electric field, and uncover a three-state (P=±Po, 0) switching during which h-NbN passes through Dirac semi-metallic states. With first-principles evidence for a combination of electronic and phononic ferroelectricity, we present a simple model that captures the energetics of coupled electronic and structural polarization, and show that electronic ferroelectricity arises in a material which is highly polarizable (small bandgap) and exhibits a large electron-phonon coupling leading to anomalous dynamical charges. These insights will guide search for electronic ferroelectrics, and our results on 2D h-NbN will stimulate development of piezo-field effect transistors and devices based on the multi-level logic.
Atomically thin layered black phosphorous (BP) has recently appeared as an alternative to the transitional metal di chalcogenides for future channel material in a MOS transistor due to its lower carrier effective mass. Investigation of the electronic property of source/drain contact involving metal and two-dimensional material is essential as it impacts the transistor performance. In this paper we perform a systematic and rigorous study to evaluate the Ohmic nature of the side-contact formed by the monolayer BP (mBP) and metals (gold, titanium and palladium), which are commonly used in experiments. Employing the Density Functional Theory (DFT), we analyse the potential barrier, charge transfer and atomic orbital overlap at the metal-mBP interface in an optimized structure to understand how efficiently carriers could be injected from metal contact to the mBP channel. Our analysis shows that gold forms a Schottky contact with a higher tunnel barrier at the interface in comparison to the titanium and palladium. mBP contact with palladium is found to be purely Ohmic, where as titanium contact demonstrates an intermediate behaviour.
In this paper we show the effect of electron-phonon scattering on the performance of monolayer (1L) MoS2 and WSe2 channel based n-MOSFETs. Electronic properties of the channel materials are evaluated using the local density approximation (LDA) in density functional theory (DFT). For phonon dispersion we employ the small displacement / frozen phonon calculations in DFT. Thereafter using the non-equilibrium Green’s function (NEGF) formalism, we study the effect of electron-phonon scattering and the contribution of various phonon modes on the performance of such devices. It is found that the performance of the WSe2 device is less impacted by phonon scattering, showing a ballisticity of 83% for 1L-WSe2 FET for channel length of 10 nm. Though 1L-MoS2 FET of similar dimension shows a lesser ballisticity of 75%. Also in the presence of scattering there exist a a 21–36% increase in the intrinsic delay time (τ) and a 10–18% reduction in peak transconductance (gm) for WSe2 and MoS2 devices respectively.
Investigation of a TMD-metal interface is essential for the effective functioning of monolayer TMD based field effect transistors (FETs). In this work, we employ Density Functional Theory (DFT) calculations to analyze the modulation of the electronic structure of monolayer WS 2 with chlorine doping and the relative changes in the contact properties when interfaced with gold and palladium. We initially examine the atomic and electronic structures of pure and doped monolayer WS 2 supercell and explore the formation of mid gap states with band splitting near the conduction band edge. Further we analyze the contact nature of the pure supercell with Au and Pd. We find that while Au is physiosorped and forms n-type contact, Pd is chemisorped and forms p-type contact with a higher valence electron density. Next, we study the interface formed between the Cl-doped supercell and metals and observe a reduction in the Schottky barrier height (SBH) in comparison to the pure supercell. This reduction found is higher for Pd in comparison to Au which is further validated by examining the charge transfer occurring at the interface. Our study confirms that Cl doping is an efficient mechanism to reduce the n-SBH for both Au and Pd which form different types of contact with WS 2 .After the fabrication of FET using monolayer MoS 2 , 1 2D layered transition metal dichalcogenides(TMDs) have garnered enormous attention in the electron devices community. Apart from MoS 2 , other TMDs as such WS 2 2,3 , WSe 2 4 , MoSe 2 5 and MoTe 2 6 are also explored as channel material for FET's. In the absence of efficient doping techniques, these transistors exhibit SBH at source/drain contact which leads to low ON current. Tremendous efforts are dedicated to reduce the contact resistance at TMD-metal interface by employing different techniques both theoretically and experimentally. [7][8][9][10][11][12] However, most of these efforts are focused towards MoS 2 and WSe 2 and a minimum study is devoted to other TMD'smetal contact interfaces. A study using ballistic MOSFET model reveals that WS 2 outperforms all other TMD's 13 . Experimental reports of WS 2 device fabrication 2 and Cl doping technique for reducing WS 2 -metal contact resistance are also reported 14 . However, theoretical investigations of WS 2 metal contact interface using first principles is still lacking in the literature. Since first principles are extensively used to analyze the graphene-metal 15,16 , MoS 2 -metal 17 and WSe 2 -metal 18 , it is believed that it will efficiently describe the contact nature with other TMD's as well. For WS 2 , chlorine doping, which is done by replacing sulfur atoms is the first ever method demonstrated experimentally to reduce the WS 2 -contact resistance, exhibiting both high drain current and field-effect mobility. 14 In this work we employ density function theory (DFT) to study the electronic structure of the interface between WS 2 and one physiosorped (Au) and one chemisorped (Pd) metal. We then examine the effect of chlorine doping by substituti...
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