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Title:AB initio study and design of 2D materials/III-nitrides-based post-CMOS devices using first-principles multiphysics simulation framework
Author(s):Lu, Shang-Chun
Director of Research:Ravaioli, Umberto; Palacios, Tomas; Mohamed, Mohamed Y
Doctoral Committee Chair(s):Ravaioli, Umberto
Doctoral Committee Member(s):Lyding, Joseph W.; van der Zande, Arend
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):2D materials
tunnel FET
density functional theory
NEGF
nanowire FET
Abstract:As logic devices are nearing their physical scaling limit, many new materials and novel device-operation concepts have been proposed lately. Among the most promising candidates, two-dimensional (2D) materials-based tunnel FETs (TFETs) are especially in the spotlight nowadays. In this dissertation, we provide insights/guidelines on designing 2D materials-based TFETs using a first-principles multiphysics framework coupling density functional theory (DFT) and non-equilibrium Green's function (NEGF) method. To start with, we provide a systematic and theoretical DFT study of the electronic properties of a large number of impurities, vacancies, and adatoms in monolayer MoS2, including groups III and IV dopants, as well as magnetic transition metal atoms such as Mn, Fe, Co, V, Nb, and Ta. By using DFT over a 5x5 atomic cell, we identify the most promising element candidates for p-doping of MoS2. Specifically, we found VB group impurity elements, such as Ta, substituting Mo to achieve negative formation energy values with impurity states all sitting at less than 0.1 eV from the valence band maximum (VBM), making them the optimal p-type dopant candidates. Moreover, our 5x5 cell model shows that B, a group III element, can induce impurity states very close to the VBM with low formation energy around 0.2 eV, which has not been reported previously. Among the magnetic impurities, such as Mn, Fe, and Co with 1, 2, and 3 magnetic moments per atom, respectively, Mn has the lowest formation energy, the most localized spin distribution, and the nearest impurity level to the conduction band among those elements. Additionally, our simulation shows the possibility of n-type doping by Mn, due to our 5 x 5 cell model. After gaining detailed knowledge about 2D materials, we proposed vertical hetero- and homojunction TFETs based on multilayer black phosphorus (BP) and transition metal dichalcogenides (TMDCs), including MoS2. Also, we tested our ideas by numerical simulations employing the semiclassical density gradient quantum correction model. It is found that the vertical TFET based on BP can achieve high on-current (>200 μA/μm) and steep subthreshold swing (SS) (average value = 24.6 mV/dec) simultaneously, due to its high mobility, direct narrow band gap, and low dielectric constant. We also found that the on-current in vertical TFETs based on a MoS2/MoSe2 heterojunction is two orders of magnitude higher than the one in MoS2 homojunction TFETs, due to the reduced effective band gap in heterostructures with staggered band alignment. In addition, we present various design considerations and recommendations as well as provide a qualitative comparison with published data. To back up our proposed design ideas for 2D materials-based vertical TFETs with accurate quantum mechanics taken into account, we conduct first-principles simulations coupling DFT and NEGF. In this dissertation, not only monolayer lateral TFETs are simulated, but also new designs for vertical BP-based TFETs are proposed adopting asymmetric layer numbers for the top and bottom layer with undoped source/drain regions. The results show that abrupt turn-on and Ion/Ioff > 1e5 can be sustained when the channel length is down to sub-5 nm. The results are benchmarked against other TFETs based on promising 2D materials homo-/heterostructures; meanwhile, the limitations, as well as guidelines, are presented. On the other hand, we report performances of TFETs based on a newly proposed monolayer WTe2/ZrS2 vertical heterojunction, besides BP TFETs. With 1e-6 μA/μm off-current and a supply voltage of 0.4V, on-current of 75 μA/μm for both n- and p-type FETs is demonstrated. With designs of dielectric and gate configuration, on-current is improved to >150 μA/μm. We also show the device switching mechanism and the scaling limit of the device. Lastly, we investigate the performance of 5 nm gate-length GaN n-type nanowire field-effect transistor (GaN-NW-nFET) of various geometrical shapes as a promising alternative to high-performance Si FETs, complementing 2D-based TFETs, which are aimed for low-power applications. Benchmarking results with simulated Si-NW-nFET reveal over 30% enhancement in GaN drive current in both low standby power (LP-IOFF = 1 nA/μm) and high performance (HP-ION = 100 nA/μm) applications. Further performance enhancement is observed with the use of non-square shape geometries that are akin to GaN's wurtzite crystal structure. Particularly, for Tnw = 2.4 nm, triangular cross-section GaN-NW-nFETs exhibit SS down to 62 mV/dec, excellent drive current, IDSAT > 2e6 μA/μm2, and superior energy-delay product compared to simulated Si-NW-nFETs.
Issue Date:2019-04-17
Type:Text
URI:http://hdl.handle.net/2142/104850
Rights Information:Copyright 2019 Shang-Chun Lu
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05


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