Ram Krishna Ghosh

Atomically thin resonant tunnel diodes built from synthetic van der Waals heterostructures

Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2–WSe2–graphene and WSe2–MoS2–graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.

Atomically thin heterostructures based on single-layer tungsten diselenide and graphene

Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. In order to engineer pristine layers and their interfaces, epitaxial growth of such heterostructures is required. We report the direct growth of crystalline, monolayer tungsten diselenide (WSe2) on epitaxial graphene (EG) grown from silicon carbide. Raman spectroscopy, photoluminescence, and scanning tunneling microscopy confirm high-quality WSe2 monolayers, whereas transmission electron microscopy shows an atomically sharp interface, and low energy electron diffraction confirms near perfect orientation between WSe2 and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrier to carrier transport beyond the expected WSe2/EG band offset exists due to the interlayer gap, which is supported by theoretical local density of states (LDOS) calculations using self-consistent density functional theory (DFT) and nonequilibrium Greens function (NEGF).

Performance benchmarking of p-type In 0.65 Ga 0.35 As/GaAs 0.4 Sb 0.6 and Ge/Ge 0.93 Sn 0.07 hetero-junction tunnel FETs

We experimentally demonstrate and benchmark the performance of p-channel TFETs (PTFETs) comparing Group III-V (In<inf>0.65</inf>Ga<inf>0.35</inf>As/GaAs<inf>0.4</inf>SW<inf>0.6</inf>) against Group IV (Ge/Ge<inf>0.93</inf>Sn<inf>0.07</inf>) semiconductor hetero-junctions. This is enabled via gate stack engineering with extremely scaled dielectrics achieving the highest accumulation capacitance density (≥3μF/cm²) on both GaAs<inf>0.4</inf>Sb<inf>0.6</inf> and Ge<inf>0.88</inf>Sn<inf>0.12</inf> channels, respectively. Temperature and electric field dependent I-V measurements coupled with first-principles density functional theory (DFT) based band-structure calculations and analytical modeling based on modified Shockley-Read-Hall formalism, are used to quantify contributions to carrier transport from band-to-band tunneling and trap-assisted tunneling (TAT). GeSn based PTFETs are found to outperform In<inf>0.65</inf>Ga<inf>0.35</inf>As/GaAs<inf>0.4</inf>Sb<inf>0.6</inf> PTFETs benefiting from band-gap engineering (higher I<inf>on</inf>) and reduced phonon assisted TAT current (lower D<inf>it</inf>).

Ag/HfO 2 based threshold switch with extreme non-linearity for unipolar cross-point memory and steep-slope phase-FETs

We demonstrate a novel Ag/HfO2 based threshold switch (TS) with a selectivity∼107, a high ON-state current (Ion) of 100 μA, and ∼10pA leakage current. The thresholding characteristics of the TS result from electrically triggered spontaneous formation and rupture of an Ag filament which acts an interstitial dopant in the HfO2 insulating matrix. Further, we harness the extreme non-linearity of the TS in (1) Selectors for Phase Change Memory (PCM) based cross-point memory. We show through array level simulations of a 1024kb memory, a read margin of 28% and write margin of 32% for a leakage power of <25μW (V/3 scheme); (2) A steep-slope sub-kT/q Phase-FET, experimentally demonstrating a switching-slope (SS) of 3mV/decade (over 5 orders of Ids), and >10x Ion improvement over the conventional FET (at iso-Ioff) at T=90C (50x at T=25C); making this a promising TS for both emerging memory, and steep-slope transistor applications.

Hubbard Gap Modulation in Vanadium Dioxide Nanoscale Tunnel Junctions

We locally investigate the electronic transport through individual tunnel junctions containing a 10 nm thin film of vanadium dioxide (VO2) across its thermally induced phase transition. The insulator-to-metal phase transition in the VO2 film collapses the Hubbard gap (experimentally determined to be 0.4 ± 0.07 V), leading to several orders of magnitude change in tunnel conductance. We quantitatively evaluate underlying transport mechanisms via theoretical quantum mechanical transport calculations which show excellent agreement with the experimental results.

Heterojunction resonant tunneling diode at the atomic limit

In this work, we present atomically thin resonant tunnel diode, based on vertically stacked heterostructures by combining graphene with layered transition-metal dichalocogenides (TMDs) such as molybdenum disulfide (MoS2), and tungsten diselenide (WSe2). Density functional theory (DFT) coupled with non-equilibrium Greens function (NEGF) transport calculation shows resonant tunnelling in heterolayer TMD and graphene (i.e. MoS2-WSe2-Gr) system with a prominent negative differential resistance (NDR) characteristic. However, homolayer TMD-graphene stack (i.e. bilayer WSe2-Gr) does not show any NDR in its I-V characteristics.

Electrically triggered insulator-to-metal phase transition in two-dimensional (2D) heterostructures

We evaluate the heterogeneous integration of the layered correlated electron material, 1T-TaS2, on semiconducting 2H-MoS2 for the realization of an all two-dimensional insulator-to-metal (IMT) phase transition device. First principles calculations investigate the band structure of the resulting heterostructure and confirm the existence of a charge density wave (CDW)-based bandgap. 1T-TaS2 films are synthesized via powder vapor deposition on monolayer MoS2 substrates and shown to exhibit CDW induced IMT phase transitions. Both Raman and electrical measurements display reversible commensurate to nearly commensurate CDW IMT phase transitions. Finally, a phase transition transistor device is demonstrated that harnesses the electrically triggered abrupt IMT in 1T-TaS2 and semiconducting properties of 2H-MoS2.We evaluate the heterogeneous integration of the layered correlated electron material, 1T-TaS2, on semiconducting 2H-MoS2 for the realization of an all two-dimensional insulator-to-metal (IMT) phase transition device. First principles calculations investigate the band structure of the resulting heterostructure and confirm the existence of a charge density wave (CDW)-based bandgap. 1T-TaS2 films are synthesized via powder vapor deposition on monolayer MoS2 substrates and shown to exhibit CDW induced IMT phase transitions. Both Raman and electrical measurements display reversible commensurate to nearly commensurate CDW IMT phase transitions. Finally, a phase transition transistor device is demonstrated that harnesses the electrically triggered abrupt IMT in 1T-TaS2 and semiconducting properties of 2H-MoS2.

Investigation of electrically gate-all-around hexagonal nanowire FET (HexFET) architecture for 5 nm node logic and SRAM applications

This work investigates, in detail, the electrically gate-all-around (eGAA) Hexagonal NW FET (HexFET) which combines the high current drive of FinFETs with the excellent electrostatic robustness of conventional Gate-All-Around Nanowire (GAA NW) FETs. We evaluate HexFET as a potential successor to FinFET for 5nm node logic and SRAM applications using first principles atomistic-based modeling, calibrated 3D numerical device simulations, and circuit-level benchmarking. From this, we conclude that the eGAA HexFET architecture offers superior performance to both FinFET and GAA NW FET for 5nm node applications.

Orbitronics — Harnessing metal insulator phase transition in 1T-MoSe 2

In this work, we reveal a remarkable orbital ordering texture associated with a Peierls distortion that leads to metal-insulator phase transition (MIT) in 1T molybdenum diselenide (MoSe2). The metallic conductivity in this MIT is realized by an isotropically distributed Mo t2g orbitals whereas splitting of both t2g and eg orbitals near the Fermi level opens a bandgap in distorted semiconducting phase. Our results also indicate that a stacking of two orbitally ordered layers allows triggering of phase transition by external perturbations such as, vertical pressure and vertical electric field.