Query: Native high-k oxides for 2D transistors

Candidate papers:

abstract: The two dimensional semiconductor Bi2O2Se can be oxidized to create an atomically thin layer of Bi2SeO5 that can be used as the insulator in scaled field effect transistors.
author_names: Yu Yu Illarionov

Theresia Knobloch

Tibor Grasser
title: Native high k oxides for 2D transistors
year: 2020

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: HfSe2 is a layered semiconductor relevant for two dimensional (2D) field effect transistors (FETs) with recent reports of a bulk band gap comparable to Silicon (Eg 1.1 eV) [1,2] and oxidation into the high K insulator HfO2 [1,3] However, extreme environmental sensitivity has prevented device measurements in samples below bulk 20 nm) thickness [3] Here, we present the first systematic study of HfSe2 devices, including joint computational and spectroscopic elucidation of its electronic band structure, characterization of ambient degradation, and transport measurements down to carefully encapsulated trilayers. Transistors fabricated in inert atmospheres and capped with AlOx are long term air stable, with comparable performance to other 2D dichalcogenide semiconductors (Ion/Ioff 106, current densities ~30 mA/mm) but offering native integration with high K HfOx dielectrics.
author_names: Michal J Mleczko

Chaofan Zhang

Hye Ryoung Lee

Hsueh-Hui Kuo

Blanka Magyari-Kope

Zhi-Xun Shen

Robert G Moore

Ian R Fisher

Yoshio Nishi

Eric Pop
title: Atomically thin HfSe2 transistors with native metal oxides
venue: 2016 74th Annual Device Research Conference (DRC)
year: 2016

abstract: Abstract Energy band alignments at two dimensional tungsten disulfide (WS2)/high k dielectric oxides interfaces have been studied by high resolution x ray photoelectron spectroscopy (XPS) Our XPS results reveal a Type I heterojunction for 2D WS2/Al2O3, WS2/Y2O3, and WS2/ZrO2 heterojunctions. A valence band offset (VBO) of 2.79 eV and a conduction band offset (CBO) of 3.56 eV were obtained at monolayer WS2/Al2O3 interface, while the VBO and CBO at monolayer WS2/Y2O3 (ZrO2) interface are measured to be 1.98 eV (2.21 eV) and 1.37 eV (1.28 eV) respectively. When increasing the WS2 thickness to its bulk limit, the small split up energy of VBO indicates the existence of interfacial states and the pinning of the Fermi level of WS2 films at the hetero interfaces. Our XPS results demonstrate the nature of WS2/dielectric interfaces and suggest that high k dielectric oxides (Al2O3, Y2O3, and ZrO2) could serve as the gate dielectrics for WS2 based field effect transistors in term of suppressing the gate leakage current.
author_names: Changjie Zhou

Huili Zhu

Shaobin Pan

Tongchang Zheng

Xiaojing Huang

Qiubao Lin

Weifeng Yang
title: Determination of band alignments at 2D tungsten disulfide/high k dielectric oxides interfaces by x ray photoelectron spectroscopy
year: 2020

abstract: The integration of high k gate dielectrics with two dimensional (2D) semiconducting channel materials is essential for high performance and low power electronics. However, the conformal deposition of a uniform high k dielectric with sub 1 nm equivalent oxide thickness (EOT) and high interface quality on high mobility 2D semiconductors is still challenging. Here we report a facile approach to synthesize a uniform high k (er 22) amorphous native oxide Bi2SeOx on the high mobility 2D semiconducting Bi2O2Se using O2 plasma at room temperature. The conformal native oxide can directly serve as gate dielectrics with EOT of ~0.9 nm, while the original properties of underlying 2D Bi2O2Se is preserved. Furthermore, high resolution area selective oxidation of Bi2O2Se is achieved to fabricate discrete electronic components. This facile integration of a high mobility 2D semiconductor and its high k native oxide holds high promise for next generation nanoelectronics.
author_names: Teng Tu

Yichi Zhang

Tianran Li

Jia Yu

Lingmei Liu

Jinxiong Wu

Congwei Tan

Jilin Tang

Yan Liang

Congcong Zhang

Yumin Dai

Yu Han

K Lai

Hailin Peng
title: Uniform high k amorphous native oxide synthesized by oxygen plasma for top gated transistors.
venue: Nano letters
year: 2020

author_names: Brian A Holler

Kyle Crowley

Halyna Volkova

Marie-Helene Berger

Xuan P A Gao
title: 2D Semiconductor Transistors using Layered van der Waals Oxide MoO 3 as High K Gate Dielectric
year: 2020

abstract: 2D material compatible, writable, and high k multifunctional oxide facilitates next generation flexible van der Waals electronics. Similar to silicon based semiconductor devices, van der Waals heterostructures require integration with high k oxides. Here, we demonstrate a method to embed and pattern a multifunctional few nanometer thick high k oxide within various van der Waals devices without degrading the properties of the neighboring two dimensional materials. This transformation allows for the creation of several fundamental nanoelectronic and optoelectronic devices, including flexible Schottky barrier field effect transistors, dual gated graphene transistors, and vertical light emitting/detecting tunneling transistors. Furthermore, upon dielectric breakdown, electrically conductive filaments are formed. This filamentation process can be used to electrically contact encapsulated conductive materials. Careful control of the filamentation process also allows for reversible switching memories. This nondestructive embedding of a high k oxide within complex van der Waals heterostructures could play an important role in future flexible multifunctional van der Waals devices.
author_names: Namphung Peimyoo

Matthew D Barnes

Jake D Mehew

Adolfo De Sanctis

Iddo Amit

Janire Escolar

Konstantinos Anastasiou

Aidan P Rooney

Sarah J Haigh

Saverio Russo

Monica Felicia Craciun

Freddie Withers
title: Laser writable high k dielectric for van der Waals nanoelectronics
venue: Science Advances
year: 2019

abstract: We perform first principles calculations to investigate the electronic structure of native defects in various oxide dielectrics to address the impact of defects on GaN/oxide metal oxide semiconductor (MOS) devices. We calculate defect formation energies in Al2O3, HfO2, and LaAlO3, as a function of the chemical potentials and of the Fermi level position in the band gap. By aligning the conduction band and valence band edges of these oxides to those of GaN, the role of native defect as charge trap or fixed charge centers in GaN based MOS devices is examined. We find that oxygen vacancies in the oxide dielectrics result in charge state transition levels near the GaN conduction band edge. They can therefore introduce border traps and/or leakage current through the gate dielectric in the n GaN/oxide MOS devices such as MOS field effect transistors. While the transition levels for other defects are well away from the GaN conduction band edge, these defects are stable in non neutral charge states and thus act as sources of fixed charge in MOS devices.
author_names: Minseok Choi

John L Lyons

Anderson Janotti

Chris G van de Walle
title: Impact of native defects in high k dielectric oxides on GaN/oxide metal oxide semiconductor devices
year: 2013

abstract: We investigate the operation of FETs with a high K channel material, SrTiO3, (K=300) The transistors show low leakage, high capacitance operation with a sub nm equivalent oxide thickness, in line with expectations. In depletion however, the gate source capacitance appears to have an unusual 1/3power dependence on the device length and width. This awkward scaling behaviour is analyzed in detail in this paper and possible consequences for SrTiO3 devices and related 2D material transistors are discussed. It is argued to relate to the high permittivity channel. This high permittivity is further experimentally shown to result in strong short channel effects in $10 \mu \mathrm{m} $long FETs, in spite of the highly scaled equivalent oxide thickness, when the operation temperature is lowered to 4.2 K.
author_names: Alexandra M Smink

Maurits J de Jong

H Hilgenkamp

Wilfred G van der Wiel

Jurriaan Schmitz
title: Anomalous Scaling of Parasitic Capacitance in FETs with a High K Channel Material
venue: 2020 IEEE 33rd International Conference on Microelectronic Test Structures (ICMTS)
year: 2020

abstract: Fabrication of ultrascaled 2D field effect transistors (FETs) requires 2D semiconductors with sizable bandgaps and high carrier mobilities as well as competitive insulators to separate the channel from the gate. However, so far the focus has been put on channel materials, while insulators suitable for ultrascaled 2D FETs have not been identified. The most common choice is oxides known from Si technologies, e.g. SiO2, Al2O3, and HfO2. However, despite the respectable performance of some devices, the interfaces of these oxides with 2D channels are of poor quality and contain numerous defects which degrade their performance and reliability. On the other hand, while 2D insulators such as hBN form well defined van der Waals interfaces with 2D channels, the poor dielectric properties of hBN make it unsuitable for scaling. As an alternative, we have recently suggested the use of calcium fluoride (CaF2) as an insulator for 2D FETs [1] Few nanometers thin CaF2 layers can be grown on Si(111) by molecular beam epitaxy (MBE) which forms an F terminated inert surface with no dangling bonds [2] This results in a quasi van der Waals interface with 2D materials (Fig.1a) similar to those known from hBN. Furthermore, due to its good dielectric properties, the tunnel currents through CaF2 are lower than for most high k oxides with equal EOT, not to mention SiO2 and hBN [1] The gate currents of our CVD grown MoS2 FETs with epitaxial CaF2 insulators of record small thickness of only about 2 nm (EOT less than 1nm) are small compared to the drain current (Fig.1c) Thus, already in the first bare channel prototypes we achieve competitive on/off current ratios of up to 10 and SS down to 90 mV/dec (Fig.2a) At the same time, the hysteresis in our devices is even smaller than in Al2O3 encapsulated SiO2(25nm)/MoS2 FETs [3] (Fig.2b) In Fig.2c we compare the normalized hysteresis widths for various technologies where only CaF2 gives comparable results to Si/high k FETs. In addition, we analyzed the reliability of our devices and found it to be significantly improved compared to available 2D FETs [4] In summary, we demonstrated the feasibility of FETs with CaF2 insulators and found that the virtually defect free nature of CaF2 leads to good performance, small hysteresis, and excellent reliability.
author_names: Yu Yu Illarionov

Alexander G Banshchikov

Dmitry K Polyushkin

Stefan Wachter

Mikhail I Vexler

Nikolai S Sokolov

T Mueller

Tibor Grasser
title: CaF2 Insulators for Ultrascaled 2D Field Effect Transistors
year: 2019