Browsing by Author "Soree, Bart"
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Item Figure of Merit for and Identification of Sub-60 mV/Decade DevicesVandenberghe, William G.; Verhulst, Anne S.; Soree, Bart; Magnus, Wim; Groeseneken, Guido; Smets, Quentin; Heyns, Marc; Fischetti, Massimo V.A figure of merit I₆₀ is proposed for sub-60 mV/decade devices as the highest current where the input characteristics exhibit a transition from sub- to super-60 mV/decade behavior. For sub-60 mV/decade devices to be competitive with metal-oxide-semiconductor field-effect devices, I₆₀ has to be in the 1-10 μA/μm range. The best experimental tunnel field-effect transistors (TFETs) in the literature only have an I₆₀ of 6 x 10⁻³ μA/μm but using theoretical simulations, we show that an I₆₀ of up to 10 μA/μm should be attainable. It is proven that the Schottky barrier FET (SBFET) has a 60 mV/decade subthreshold swing limit while combining a SBFET and a TFET does improve performance.Item Modelling topological and magnetic materials for charge and spin-based devices(2022-05-01T05:00:00.000Z) Tiwari, Sabyasachi; Vandenberghe, William; Wong, W. Eric; Fischetti, Massimo V.; Kim, Moon J.; Soree, BartThe imminent halt of Moore’s law and discontinuation of scaling of transistors based on three-dimensional materials, e.g., silicon, has prompted researchers to look for different ma- terials and device systems apart from the conventional ones to form the backbone of the electronics industry of the future. Topological insulators (TIs) open a vast avenue to realize devices with high ON current and low power consumption. TIs are a class of materials with topologically protected edge states which are spin-polarized and robust against impurity scattering. The possibility of spin-polarization in TIs and efficient transfer of spin-current in soft-layered magnets opens another avenue of research for realizing fast memory devices. In this dissertation, first, we model carrier transport through imperfect two-dimensional (2D) TI ribbons. In particular, we investigate the impact of vacancy defects on the carrier trans- port of 2D TIs. We show that carrier transport through the topologically protected edge states is robust against a high percentage of defects (up to 2%), whereas the carrier trans- port through the bulk state is strongly suppressed due to backscattering. We show that the suppression of bulk transport in 2D TIs can be used to design devices using 2D TI ribbons. Next, we develop a computational method to model the magnetic interactions in layered magnetic materials and calculate their critical temperature from the first principles, taking into account both the magnetic anisotropy as well as the out-of-plane interactions. We ap- ply our method on Cr-compounds: CrI3, CrBr3, and CrGeTe3, and FeCl2, and show that our predictions match well with experimental values. Using the same model we next inves- tigate the magnetic order in two-dimensional (2D) transition-metal-dichalcogenide (TMD) monolayers: MoS2, MoSe2, MoTe2, WSe2 , and WS2 substitutionally doped with period-four transition-metals (Ti, V, Cr, Mn, Fe, Co, Ni). We show that five distinct magnetically or- dered states can exist among the 35 distinct TMD-dopant combinations including the non- magnetic (NM), the ferromagnetic (FM) with out-of-plane spin polarization (Z FM), the out-of-plane polarized clustered FMs (clustered Z FM), the in-plane polarized FMs (X–Y FM), and the anti-ferromagnetic (AFM) state. Most remarkably, we find from our study that V-doped MoSe2 and WSe2, and Mn-doped MoS2, are the most suitable candidates for realizing a room-temperature FM at a 16–18% atomic substitution. We then compare three first-principles methods (the MC, the Green’s function, and the RNSW) of calculating the Curie temperature in 2D FMs in the presence of exchange anisotropy, modeled using the Heisenberg model. We find that the Curie temperature obtained from the Green’s function in high-anisotropy regimes is higher compared to MC, whereas the Curie temperature cal- culated using the renormalized spin-waves (RNSW) is lower compared to the MC and the Green’s function for all anisotropies. Finally, we present a theoretical model to simulate spin- dynamics and spin-induced switching in a semiconductor-ferromagnet heterostructure. Our theoretical model combines the non-equilibrium Green’s function method for spin-dependent electron transport and time-quantified Monte-Carlo for simulating magnetization dynamics. We use the adiabatic approximation for combining the electron dynamics and the magne- tization dynamics. We study spin-induced switching in a 2D TI-FM interface. Finally, we show that for a certain range of magnetic exchange parameters (or certain materials), it is possible to change magnetic domains in a 2D FM using spin-torque from TIs, which can be used for designing high-speed memories.Item Phonon-Assisted Tunneling in Direct-Bandgap Semiconductors(Amer Inst Physics, 2019-01-02) Mohammed, Mazharuddin; Verhulst, Anne S.; Verreck, Devin; Van de Put, Maarten L.; Magnus, Wim; Soree, Bart; Groeseneken, Guido; 0000-0001-9179-6443 (Van de Put, ML); Van de Put, Maarten L.In tunnel field-effect transistors, trap-assisted tunneling (TAT) is one of the probable causes for degraded subthreshold swing. The accurate quantum-mechanical (QM) assessment of TAT currents also requires a QM treatment of phonon-assisted tunneling (PAT) currents. Therefore, we present a multi-band PAT current formalism within the framework of the quantum transmitting boundary method. An envelope function approximation is used to construct the electron-phonon coupling terms corresponding to local Frohlich-based phonon-assisted inter-band tunneling in direct-bandgap III-V semiconductors. The PAT current density is studied in up to 100 nm long and 20 nm wide p-n diodes with the 2- and 15-band material description of our formalism. We observe an inefficient electron-phonon coupling across the tunneling junction. We further demonstrate the dependence of PAT currents on the device length, for our non-self-consistent formalism which neglects changes in the electron distribution function caused by the electron-phonon coupling. Finally, we discuss the differences in doping dependence between direct band-to-band tunneling and PAT current. Published under license by AIP Publishing.Item Quantum Mechanical Solver for Confined Heterostructure Tunnel Field-Effect Transistors(Amer Inst Physics) Verreck, Devin; Van de Put, Maarten; Soree, Bart; Verhulst, Anne S.; Magnus, Wim; Vandenberghe, William G.; Collaert, Nadine; Thean, Aaron; Groeseneken, GuidoHeterostructure tunnel field-effect transistors (HTFET) are promising candidates for low-power applications in future technology nodes, as they are predicted to offer high on-currents, combined with a sub-60 mV/dec subthreshold swing. However, the effects of important quantum mechanical phenomena like size confinement at the heterojunction are not well understood, due to the theoretical and computational difficulties in modeling realistic heterostructures. We therefore present a ballistic quantum transport formalism, combining a novel envelope function approach for semiconductor heterostructures with the multiband quantum transmitting boundary method, which we extend to 2D potentials. We demonstrate an implementation of a 2-band version of the formalism and apply it to study confinement in realistic heterostructure diodes and p-n-i-n HTFETs. For the diodes, both transmission probabilities and current densities are found to decrease with stronger confinement. For the p-n-i-n HTFETs, the improved gate control is found to counteract the deterioration due to confinement.Item Tensile Strained Ge Tunnel Field-Effect Transistors: K . P Material Modeling and Numerical Device Simulation(Amer Inst Physics) Kao, Kuo-Hsing; Verhulst, Anne S.; Van de Put, Maarten; Vandenberghe, William G. H.; Soree, Bart; Magnus, Wim; De Meyer, KristinGroup IV based tunnel field-effect transistors generally show lower on-current than III-V based devices because of the weaker phonon-assisted tunneling transitions in the group IV indirect bandgap materials. Direct tunneling in Ge, however, can be enhanced by strain engineering. In this work, we use a 30-band k.p method to calculate the band structure of biaxial tensile strained Ge and then extract the bandgaps and effective masses at Γ and L symmetry points in k-space, from which the parameters for the direct and indirect band-to-band tunneling (BTBT) models are determined. While transitions from the heavy and light hole valence bands to the conduction band edge at the L point are always bridged by phonon scattering, we highlight a new finding that only the light-holelike valence band is strongly coupling to the conduction band at the Γ point even in the presence of strain based on the 30-band k.p analysis. By utilizing a Technology Computer Aided Design simulator equipped with the calculated band-to-band tunneling BTBT models, the electrical characteristics of tensile strained Ge point and line tunneling devices are self-consistently computed considering multiple dynamic nonlocal tunnel paths. The influence of field-induced quantum confinement on the tunneling onset is included. Our simulation predicts that an on-current up to 160 (260) μA/μm can be achieved along with on/off ratio > 10(6) for V-DD - 0.5V by the n-type (p-type) line tunneling device made of 2.5% biaxial tensile strained Ge.