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Bilayer insulator tunnel barriers for graphene-based vertical hot-electron transistors
KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.ORCID iD: 0000-0003-1234-6060
KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.ORCID iD: 0000-0003-0333-376X
KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
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2015 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 7, no 30, 13096-13104 p.Article in journal (Refereed) Published
Abstract [en]

Vertical graphene-based device concepts that rely on quantum mechanical tunneling are intensely being discussed in the literature for applications in electronics and optoelectronics. In this work, the carrier transport mechanisms in semiconductor-insulator-graphene (SIG) capacitors are investigated with respect to their suitability as electron emitters in vertical graphene base transistors (GBTs). Several dielectric materials as tunnel barriers are compared, including dielectric double layers. Using bilayer dielectrics, we experimentally demonstrate significant improvements in the electron injection current by promoting Fowler-Nordheim tunneling (FNT) and step tunneling (ST) while suppressing defect mediated carrier transport. High injected tunneling current densities approaching 103 A cm(-2) (limited by series resistance), and excellent current-voltage nonlinearity and asymmetry are achieved using a 1 nm thick high quality dielectric, thulium silicate (TmSiO), as the first insulator layer, and titanium dioxide (TiO2) as a high electron affinity second layer insulator. We also confirm the feasibility and effectiveness of our approach in a full GBT structure which shows dramatic improvement in the collector on-state current density with respect to the previously reported GBTs. The device design and the fabrication scheme have been selected with future CMOS process compatibility in mind. This work proposes a bilayer tunnel barrier approach as a promising candidate to be used in high performance vertical graphene-based tunneling devices.

Place, publisher, year, edition, pages
2015. Vol. 7, no 30, 13096-13104 p.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
URN: urn:nbn:se:kth:diva-172640DOI: 10.1039/c5nr03002aISI: 000358615200036PubMedID: 26176739ScopusID: 2-s2.0-84937928928OAI: diva2:849110
Swedish Research CouncilEU, European Research Council, 307311

QC 20150827

Available from: 2015-08-27 Created: 2015-08-27 Last updated: 2016-05-03Bibliographically approved
In thesis
1. Graphene Hot-electron Transistors
Open this publication in new window or tab >>Graphene Hot-electron Transistors
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Graphene base transistors (GBTs) have been, recently, proposed to overcome the intrinsic limitations of the graphene field effect transistors (GFETs) and exploit the graphene unique properties in high frequency (HF) applications. These devices utilize single layer graphene as the base material in the vertical hot-electron transistors. In an optimized GBT, the ultimate thinness of the graphene-base and its high conductivity, potentially, enable HF performance up to the THz region.  This thesis presents an experimental investigation on the GBTs as well as integration process developments for the fabrication of graphene-based devices.

In this work, a full device fabrication and graphene integration process were designed with high CMOS compatibility considerations. To this aim, basic process modules, such as graphene transfer, deposition of materials on graphene, and formation of tunnel barriers, were developed and optimized. A PDMS-supporting graphene transfer process were introduced to facilitate the wet/dry wafer-scale transfer from metal substrate onto an arbitrarily substrate. In addition, dielectric deposition on graphene using atomic layer deposition (ALD) was investigated. These dielectric layers, mainly, served as the base-collector insulators in the fabricated GBTs. Moreover, the integration of silicon (Si) on the graphene surface was studied.

Using the developed fabrication process, the first proof of concept devices were demonstrated. These devices utilized 5 nm-thick silicon oxide (SiO2) and about 20 nm-thick aluminum oxide (Al2O3) as the emitter-base insulator (EBI) and base-collector insulator (BCI). The direct current (DC) functionality of these devices exhibited >104 on/off current ratios and a current transfer ratio of about 6%. The performance of these devices was limited by the non-optimized barrier parameters and device manufacturing technology.

The possibility to improve and optimize the GBT performance was demonstrated by applying different barrier optimization approaches. Comparing to the proof of concept devices, several orders of magnitude higher injection current density was achieved using a bilayer dielectric tunnel barrier. Utilizing the novel TmSiO/TiO2 (1 nm/6 nm) dielectric stack, this tunnel barrier prevents defect mediated tunneling and, simultaneously, promotes the Fowler-Nordheim tunneling (FNT) and step tunneling (ST). Furthermore, it was shown that Si/graphene Schottky junction can significantly improve the current gain by reducing the electron backscattering at the base-collector barrier. In this thesis, a maximum current transfer ratio of about 35% has been achieved.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. xviii, 81 p.
TRITA-ICT, 2016:08
Graphene, hot-electron transistors, graphene base transistors, GBT, cross-plane carrier transport, tunneling, ballistic transport, heterojunction transistors, graphene integration, graphene transfer
National Category
Engineering and Technology
Research subject
Information and Communication Technology
urn:nbn:se:kth:diva-186044 (URN)ISBN 978-91-7595-932-0 (ISBN)
Public defence
2016-05-26, SAL C, Electrum 229, Kista, 10:00 (English)
EU, FP7, Seventh Framework Programme, 317839EU, European Research Council, 228229

QC 20160503

Available from: 2016-05-03 Created: 2016-04-29 Last updated: 2016-05-05Bibliographically approved

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