Query: Two-dimensional semiconductors for transistors.

Candidate papers:

abstract: In the quest for higher performance, the dimensions of field effect transistors (FETs) continue to decrease. However, the reduction in size of FETs comprising 3D semiconductors is limited by the rate at which heat, generated from static power, is dissipated. The increase in static power and the leakage of current between the source and drain electrodes that causes this increase, are referred to as short channel effects. In FETs with channels made from 2D semiconductors, leakage current is almost eliminated because all electrons are confined in atomically thin channels and, hence, are uniformly influenced by the gate voltage. In this Review, we provide a mathematical framework to evaluate the performance of FETs and describe the challenges for improving the performances of short channel FETs in relation to the properties of 2D materials, including graphene, transition metal dichalcogenides, phosphorene and silicene. We also describe tunnelling FETs that possess extremely low power switching behaviour and explain how they can be realized using heterostructures of 2D semiconductors. Field effect transistors (FETs) with semiconducting channels made from 2D materials are known to have fewer problems with short channel effects than devices comprising 3D semiconductors. In this Review, a mathematical framework to evaluate the performance of FETs is outlined with a focus on the properties of 2D materials, such as graphene, transition metal dichalcogenides, phosphorene and silicene.
author_names: Manish Chhowalla

Debdeep Jena

Hua Zhang
title: Two dimensional semiconductors for transistors
year: 2016

abstract: As the dimensions of the semiconducting channels in field effect transistors decrease, the contact resistance of the metal semiconductor interface at the source and drain electrodes increases, dominating the performance of devices1 3. Two dimensional (2D) transition metal dichalcogenides such as molybdenum disulfide (MoS2) have been demonstrated to be excellent semiconductors for ultrathin field effect transistors4,5. However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition metal dichalcogenide3,5 9. Recent studies have shown that van der Waals contacts formed by transferred graphene10,11 and metals12 on few layered transition metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three dimensional metal and a monolayer 2D transition metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10 nanometre thick indium metal capped with 100 nanometre thick gold electrodes and monolayer MoS2. Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold capped indium and the MoS2 is atomically sharp with no detectable chemical interaction between the metal and the 2D transition metal dichalcogenide, suggesting van der Waals type bonding between the gold capped indium and monolayer MoS2. The contact resistance of the indium/gold electrodes is 3,000 300 ohm micrometres for monolayer MoS2 and 800 200 ohm micrometres for few layered MoS2. These values are among the lowest observed for three dimensional metal electrodes evaporated onto MoS2, enabling high performance field effect transistors with a mobility of 167 20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220 50 ohm micrometres on ultrathin niobium disulfide (NbS2) and near ideal band offsets, indicative of defect free interfaces, in tungsten disulfide (WS2) and tungsten diselenide (WSe2) contacted with indium alloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors.Ultraclean van der Waals bonds between gold capped indium and a monolayer of the two dimensional transition metal dichalcogenide molybdenum disulfide show desirably low contact resistance at the interface, enabling high performance field effect transistors.
author_names: Yan Alexander Wang

Jong Chan Kim

Ryan J Wu

Jenny Martinez

Xiuju Song

Jieun Yang

Fang Zhao

Andre Mkhoyan

Hu Young Jeong

Manish Chhowalla
title: Van der Waals contacts between three dimensional metals and two dimensional semiconductors
venue: Nature
year: 2019

abstract: Quinoidal oligothiophenes (QOT) as classical n type semiconductors, have been well known for a long time but with non optimal semiconducting properties. We report here the design and selective synthesis of new two dimensional (2D) p expanded quinoidal terthiophenes, 2DQTTs, with proximal (2DQTT i) and distal (2DQTT o) regiochemistry for high performance n channel organic thin film transistors (n OTFTs) featuring high electron mobility, solution processability, and ambient stability. The elegant combination of thieno[3,4 b]thiophene [TT, donor (D) and 5 alkyl 4H thieno[3,4 c]pyrrole 4,6(5H) dione [TPD, acceptor (A) units with relatively large p surface endows these 2DQTTs with distinctive 2D structural characteristics and flat configuration stabilized by weak intramolecular S O/S weak interactions. Furthermore, the A D A D A electronic structure maintains an adequately low LUMO energy level. These 2DQTTs are shown to exhibit outstanding semiconducting properties with electron mobilities of up to 3.0 cm(2) V( 1) s( 1) and on/off ratios of up to 10(6) (2DQTT o) in ambient and solution processed OTFTs. Investigations on thin film morphology reveal that the microstructure of 2DQTTs is highly dependent on the orientation of the fused thiophene subunits, leading to differences in electron mobilities of 1 order of magnitude. X ray diffraction studies in particular reveal increased crystallinity, crystalline coherence, and orientational order in 2DQTT o compared to 2DQTT i, which accounts for the superior electron transport property of 2DQTT o.
author_names: Cheng Zhang

Yaping Zang

Eliot Gann

Christopher R McNeill

Xiaozhang Zhu

Chong-an Di

Daoben Zhu
title: Two dimensional p expanded quinoidal terthiophenes terminated with dicyanomethylenes as n type semiconductors for high performance organic thin film transistors.
venue: Journal of the American Chemical Society
year: 2014

abstract: Atomically thin transition metal dichalcogenides (TMDs) are of interest for next generation electronics and optoelectronics. Here, we demonstrate device ready synthetic tungsten diselenide (WSe2) via metal organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e. TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios ~107) high current density (1 10 mA*mm 1) and good field effect transistor mobility ~30 cm2*V 1*s 1) at room temperature. This work establishes that realization of electronic grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.
author_names: Yu-Chuan Lin

B Jariwala

Brian M Bersch

Ke Xu

Yifan Nie

Baoming Wang

Sarah M Eichfeld

Xiaotian Zhang

Tanushree H Choudhury

Yi Pan

Rafik Addou

Christopher M Smyth

Jun Li

Kehao Zhang

M Aman Haque

Stefan Folsch

Randall M Feenstra

Robert M Wallace

Kyeongjae Cho

Susan K Fullerton-Shirey

Joan M Redwing

Joshua A Robinson
title: Realizing Large Scale, Electronic Grade Two Dimensional Semiconductors.
venue: ACS nano
year: 2018

abstract: Atomically thin two dimensional semiconductors feature silicon like band gaps and native high k metal oxides. The success of silicon as a dominant semiconductor technology has been enabled by its moderate band gap (1.1 eV) permitting low voltage operation at reduced leakage current, and the existence of SiO2 as a high quality "native" insulator. In contrast, other mainstream semiconductors lack stable oxides and must rely on deposited insulators, presenting numerous compatibility challenges. We demonstrate that layered two dimensional (2D) semiconductors HfSe2 and ZrSe2 have band gaps of 0.9 to 1.2 eV (bulk to monolayer) and technologically desirable "high k" native dielectrics HfO2 and ZrO2, respectively. We use spectroscopic and computational studies to elucidate their electronic band structure and then fabricate air stable transistors down to three layer thickness with careful processing and dielectric encapsulation. Electronic measurements reveal promising performance (on/off ratio 106; on current, ~30 mA/mm) with native oxides reducing the effects of interfacial traps. These are the first 2D materials to demonstrate technologically relevant properties of silicon, in addition to unique compatibility with high k dielectrics, and scaling benefits from their atomically thin nature.
author_names: Michal J Mleczko

Chaofan Zhang

Hye Ryoung Lee

Hsueh-Hui Kuo

Blanka Magyari-Kope

Robert G Moore

Zhi-Xun Shen

Ian R Fisher

Yoshio Nishi

Eric Pop
title: HfSe2 and ZrSe2: Two dimensional semiconductors with native high k oxides
venue: Science Advances
year: 2017

abstract: Two dimensional semiconductors could be used to fabricate ultimately scaled field effect transistors and more than Moore nanoelectronic devices. However, these targets cannot be reached without appropriate gate insulators that are scalable to the nanometre range. Typically used oxides such as SiO2, Al2O3 and HfO2 are, however, amorphous when scaled, and 2D hexagonal boron nitride exhibits excessive gate leakage currents. Here, we show that epitaxial calcium fluoride (CaF2) which can form a quasi van der Waals interface with 2D semiconductors, can serve as an ultrathin gate insulator for 2D devices. We fabricate scalable bilayer MoS2 field effect transistors with a crystalline CaF2 insulator of ~2 nm thickness, which corresponds to an equivalent oxide thickness of less than 1 nm. Our devices exhibit low leakage currents and competitive device performance characteristics, including subthreshold swings down to 90 mV dec 1, on/off current ratios up to 107 and a small hysteresis.High performance MoS2 transistors can be created using 2 nm thick CaF2 as a gate insulator, which forms a quasi van der Waals interface with the 2D semiconductor.
author_names: Yu Yu Illarionov

Alexander G Banshchikov

Dmitry K Polyushkin

Stefan Wachter

Theresia Knobloch

Mischa Thesberg

Lukas Mennel

Matthias Paur

Michael Stoger-Pollach

Andreas Steiger-Thirsfeld

Mikhail I Vexler

Michael Waltl

Nikolai S Sokolov

T Mueller

Tibor Grasser
title: Ultrathin calcium fluoride insulators for two dimensional field effect transistors
venue: Nature Electronics
year: 2019

abstract: Two dimensional semiconductors have a number of valuable properties that could be used to create novel electronic devices. However, creating 2D devices with good contacts and stable performance has proved challenging. Here we show that transferred via contacts, made from metal embedded in insulating hexagonal boron nitride and dry transferred onto 2D semiconductors, can be used to create high quality 2D transistors. The approach prevents damage induced by direct metallization and allows full glovebox processing, providing a clean, stable and damage free platform for 2D device fabrication. Using the approach, we create field effect transistors (FETs) from bilayer p type tungsten diselenide (WSe2) that exhibit high hole mobility and low contact resistance. The fabricated devices also exhibit high current and stability for over two months of measurements. Furthermore, the low contact resistance and clean channel allow us to create a nearly ideal top gated p FET with a subthreshold swing of 64 mV per decade at 290 K.Bilayer WSe2 field effect transistors with near ideal device characteristics can be created using transferred via contacts made from metal embedded hexagonal boron nitride.
author_names: Younghun Jung

Min Sup Choi

Ankur Nipane

Abhinandan Borah

Bumho Kim

Amirali Zangiabadi

Takashi Taniguchi

Kenji Watanabe

Won Jong Yoo

James C Hone

James T Teherani
title: Transferred via contacts as a platform for ideal two dimensional transistors
venue: Nature Electronics
year: 2019

abstract: Two dimensional (2D) layered materials hold great promise for various future electronic and optoelectronic devices that traditional semiconductors cannot afford. 2D pnictogen, group VA atomic sheet (including phosphorene, arsenene, antimonene, and bismuthene) is believed to be a competitive candidate for next generation logic devices. This is due to their intriguing physical and chemical properties, such as tunable midrange bandgap and controllable stability. Since the first black phosphorus field effect transistor (FET) demo in 2014, there has been abundant exciting research advancement on the fundamental properties, preparation methods, and related electronic applications of 2D pnictogen. Herein, we review the recent progress in both material and device aspects of 2D pnictogen FETs. This includes a brief survey on the crystal structure, electronic properties and synthesis, or growth experiments. With more device orientation, this review emphasizes experimental fabrication, performance enhancing approaches, and configuration engineering of 2D pnictogen FETs. At the end, this review outlines current challenges and prospects for 2D pnictogen FETs as a potential platform for novel nanoelectronics.
author_names: Wenhan Zhou

Jiayi Chen

Pengxiang Bai

Shiying Guo

Shengli Zhang

Xiufeng Song

Lijuan Tao

Haibo Zeng
title: Two Dimensional Pnictogen for Field Effect Transistors
venue: Research
year: 2019

abstract: Layered two dimensional semiconductors have attracted tremendous attention owing to their demonstrated excellent transistor switching characteristics with a large ratio of on state to off state current, Ion/Ioff. However, the depletion mode nature of the transistors sets a limit on the thickness of the layered semiconductor films primarily determined by a given Ion/Ioff as an acceptable specification. Identifying the optimum thickness range is of significance for material synthesis and device fabrication. Here, we systematically investigate the thickness dependent switching behavior of transistors with a wide thickness range of multilayer MoS2 films. A difference in Ion/Ioff by several orders of magnitude is observed when the film thickness, t, approaches a critical depletion width. The decrease in Ion/Ioff is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm. For t larger than 100 nm, Ion/Ioff approaches unity. Simulation using technical computer aided tools established for silicon technology faithfully reproduces the experimentally determined scaling behavior of Ion/Ioff with t. This excellent agreement confirms that multilayer MoS2 films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate Ion/Ioff. Our findings are helpful in guiding material synthesis and designing advanced field effect transistors based on the layered semiconductors.
author_names: Youwei Zhang

Hui Li

Haomin Wang

H Xie

Ran Liu

Shi-Li Zhang

Zhi-Jun Qiu
title: Thickness Considerations of Two Dimensional Layered Semiconductors for Transistor Applications
venue: Scientific reports
year: 2016

abstract: Two dimensional semiconductors (2DSCs) have attracted considerable attention as atomically thin channel materials for field effect transistors. Each layer in 2DSCs consists of a single or few atom thick, covalently bonded lattice, in which all carriers are confined in their atomically thin channel with superior gate controllability and greatly suppressed OFF state current, in contrast to typical bulk semiconductors plagued by short channel effects and heat generation from static power. Additionally, 2DSCs are free of surface dangling bonds that plague traditional semiconductors, and hence exhibit excellent electronic properties at the limit of single atom thickness. Therefore, 2DSCs can offer significant potential for the ultimate transistor scaling to single atomic body thickness. Earlier studies of graphene transistors have been limited by the zero bandgap and low ON OFF ratio of graphene, and transition metal dichalcogenide (TMDC) devices are typically plagued by insufficient carrier mobility. To this end, considerable efforts have been devoted towards searching for new 2DSCs with optimum electronic properties. Within a relatively short period of time, a large number of 2DSCs have been demonstrated to exhibit unprecedented characteristics or unique functionalities. Here we review the recent efforts and progress in exploring novel 2DSCs beyond graphene and TMDCs for ultra thin body transistors, discussing the merits, limits and prospects of each material.
author_names: Yuan Liu

Xidong Duan

Yu Huang

Xiangfeng Duan
title: Two dimensional transistors beyond graphene and TMDCs.
venue: Chemical Society reviews
year: 2018