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1. Mechanical Properties and Strain Transfer Behavior of Molybdenum Ditelluride (MoTe2) Thin Films

   https://doi.org/10.1115/1.4051306  

   Chowdhury, Shoieb Ahmed; Inzani, Katherine; Peña, Tara; Dey, Aditya; Wu, Stephen M.; Griffin, Sinéad M.; Askari, Hesam (January 2022, Journal of Engineering Materials and Technology)

   null (Ed.)

   Abstract Transition metal dichalcogenides (TMDs) offer superior properties over conventional materials in many areas such as in electronic devices. In recent years, TMDs have been shown to display a phase switching mechanism under the application of external mechanical strain, making them exciting candidates for phase change transistors. Molybdenum ditelluride (MoTe2) is one such material that has been engineered as a strain-based phase change transistor. In this work, we explore various aspects of the mechanical properties of this material by a suite of computational and experimental approaches. First, we present parameterization of an interatomic potential for modeling monolayer as well as multilayered MoTe2 films. For generating the empirical potential parameter set, we fit results from density functional theory calculations using a random search algorithm known as particle swarm optimization. The potential closely predicts structural properties, elastic constants, and vibrational frequencies of MoTe2 indicating a reliable fit. Our simulated mechanical response matches earlier larger scale experimental nanoindentation results with excellent prediction of fracture points. Simulation of uniaxial tensile deformation by molecular dynamics shows the complete non-linear stress-strain response up to failure. Mechanical behavior, including failure properties, exhibits directional anisotropy due to the variation of bond alignments with crystal orientation. Furthermore, we show the deterioration of mechanical properties with increasing temperature. Finally, we present computational and experimental evidence of an extended c-axis strain transfer length in MoTe2 compared to TMDs with smaller chalcogen atoms. 

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2. Bandgap opening in MoTe2 thin flakes induced by surface oxidation

   https://doi.org/https://doi.org/10.1007/s11467-020-0952-x  

   Yuan Gan, Jiyuan Liang (April 2020, Frontiers of Physics)

   Recently, the layered transition metal dichalcogenide 1T′-MoTe2 has generated considerable interest due to their superconducting and non-trivial topological properties. Here, we present a systematic study on 1T′-MoTe2 single-crystal and exfoliated thin-flakes by means of electrical transport, scanning tunnelling microscope (STM) measurements and band structure calculations. For a bulk sample, it exhibits large magneto-resistance (MR) and Shubnikov–de Hass oscillations in ρxx and a series of Hall plateaus in ρxy at low temperatures. Meanwhile, the MoTe2 thin films were intensively investigated with thickness dependence. For samples, without encapsulation, an apparent transition from the intrinsic metallic to insulating state is observed by reducing thickness. In such thin films, we also observed a suppression of the MR and weak anti-localization (WAL) effects. We attributed these effects to disorders originated from the extrinsic surface chemical reaction, which is consistent with the density functional theory (DFT) calculations and in-situ STM results. In contrast to samples without encapsulated protection, we discovered an interesting superconducting transition for those samples with hexagonal Boron Nitride (h-BN) film protection. Our results indicate that the metallic or superconducting behavior is its intrinsic state, and the insulating behavior is likely caused by surface oxidation in few layer 1T’-MoTe2 flakes. 

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3. Vertical Organic Tunnel Field-Effect Transistors

   Liu, Shiyi; Tietze, Max; Al-Shadeedi, Akram; Kaphle, Vikash; Keum, Changmin; Lussem, Bjorn (July 2019, ACS applied electronic materials)

   Doping organic semiconductors has become a key technology to increase the performance of organic light-emitting diodes, solar cells, or field-effect transistors (OFETs). However, doping can be used not only to optimize these devices but also to enable new design principles as well. Here, a novel type of OFET is reported—the vertical organic tunnel field-effect transistor. Based on heterogeneously doped drain and source contacts, charge carriers are injected from an n-doped source electrode into the channel by Zener tunneling and are transported toward a p-doped drain electrode. The working mechanism of these transistors is discussed with the help of a tunnel model that takes energetic broadening of transport states in organic semiconductors and roughness of organic layers into account. The proposed device principle opens new ways to optimize OFETs. It is shown that the Zener junction included between the source and drain of the vertical organic tunnel field-effect transistors suppresses short channel effects and improves the saturation of vertical OFETs. 

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4. Vertical GaN Fin JFET: A Power Device with Short Circuit Robustness at Avalanche Breakdown Voltage

   https://doi.org/10.1109/IRPS48227.2022.9764569  

   Zhang, R.; Liu, J.; Li, Q.; Pidaparthi, S.; Edwards, A.; Drowley, C.; Zhang, Y. (March 2022, Vertical GaN Fin JFET: A Power Device with Short Circuit Robustness at Avalanche Breakdown Voltage)

   GaN high-electron-mobility transistors (HEMTs) are known to have no avalanche capability and insufficient short-circuit robustness. Recently, breakthrough avalanche and short-circuit capabilities have been experimentally demonstrated in a vertical GaN fin-channel junction-gate field-effect transistor (Fin-JFET), which shows a good promise for using GaN devices in automotive powertrains and electric grids. In particular, GaN Fin-JFETs demonstrated good short-circuit capability at avalanche breakdown voltage (BV AVA ), with a failure-to-open-circuit (FTO) signature. This work presents a comprehensive device physics-based study of the GaN Fin-JFET under short-circuit conditions, particularly at a bus voltage close to BV AVA . Mixed-mode electrothermal TCAD simulations were performed to understand the carrier dynamics, electric field distributions, and temperature profiles in the Fin-JFET under short-circuit and avalanche conditions. The results provide important physical references to understand the unique robustness of the vertical GaN Fin-JFET under the concurrence of short-circuit and avalanche as well as its desirable FTO signature. 

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5. Single and multi-fin normally-off Ga2O3 vertical transistors with a breakdown voltage over 2.6 kV

   W. Li1, K. Nomoto1 (January 2019, IEDM19-273)

   We demonstrate record-high performance in normally-off single and multi-fin b-Ga2O3 vertical power transistors. The effective channel mobility is significantly improved up to ~130 cm2/V·s with a post-deposition annealing process. With a fin-channel width of 0.15 μm, true normallyoff operation is achieved with a threshold voltage of >1.5 V; a record-high breakdown voltage of 2.66 kV (at Vgs=0 V) and a specific on-resistance of 25.2 mW·cm2 are obtained in multifin devices, corresponding to a Baliga’s figure-of-merit of 280 MW/cm2, which is the highest among all Ga2O3 transistors. Devices with (100)-like fin-channel sidewalls exhibit the lowest interface trapped charge density and a significantly higher current than other fin orientations. These findings offer important insights on the development of Ga2O3 MOSFETs and show great promise of Ga2O3 vertical power devices. 

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