Brief history and main research contributions
The Epitaxy and Surface Science Laboratory (ESSL) started its operation in year 2000 and has a 15 years history of exploratory research on materials and devices for advanced CMOS and post CMOS nanoelectronics. Starting as an MBE laboratory, the team raised around 4.4 million Euros from EU competitive projects which allowed building new infrastructure worth of 1 million Euro and supporting a team of 8-10 members on average consisting of postdoctoral research associates and PhD students. The last few years the lab has become a unique facility in Europe for the large area MBE growth of 2D dichalcogenide semiconductors and the surface analysis of low dimensionality materials.
Leadership in EU research
ESSL has demonstrated leadership in EU research by coordinating three big industrially-oriented projects in the area of advanced CMOS and a more recent one on the 2D materials silicene and germanene. ESSL has contributed along with other labs around the word, for the early identification of suitable hafnium based high-k gate dielectric and new high mobility semiconductors (i.e Ge) for next generation CMOS. High-k gate dielectrics have already come in production since year 2007 by leading chip manufacturers, while Ge is in the row for the next generation microprocessors.
The lab has demonstrated scientific excellence receiving a number of grants supporting frontier scientific research such as the prestigious ERC Advanced Grant 2011–SMARTGATE, the Greek program of excellence ARISTEIA–TOP ELECTRONICS, the FET Open project 2D NANOLATTICES and the Marie Curie REACT and ACT projects. These projects allow us to extend our research in the area of emerging 2D materials for nanoelectronics and introduce new fascinating materials such as topological insulators for information and communication technologies.
Current research and progress
Silicene and germanene-the graphene “cousins”
The advent of graphene has ignited an enormous research interest on a number of 2D materials. Among them are silicene and germanene, the Si and Ge analogue of graphene, which do not exist in Nature in free standing form. Important questions need to be answered. Is it possible to engineer silicene and germanene on suitable substrates? Are these materials similar to graphene supporting Dirac Fermions?
The effort in realizing silicene and germanene is not merely a matter of scientific interest or curiosity but it is also driven by potential applications. The continuous lateral scaling of Si nanoelectronic devices and circuits suffers from insufficient electrostatic control which is mitigated mainly by reducing the gate equivalent oxide thickness and the channel thickness. Using silicene, an inherently 2D material consisting of a single layer of Si atoms one can reach the ultimate limit of channel thickness. This is a new Si allotrope with radically different electronic properties compared to Si ultrathin films scaled down from the bulk.
ESSL lab has led the effort searching for silicene and germanene by coordinating the first European consortium 2D NANOLATTICES (www.2dnanolattices.eu) exploring these materials (A. Dimoulas, MEE 131, 68 (2015)). Members of the consortium pioneered research proving for the first time that silicene can be grown on metals, silver in particular, although it loses its identity due to hybridization with the substrate as we have proven at ESSL (D. Tsoutsou, E. Xenogiannopoulou, E. Golias , P. Tsipas, A. Dimoulas, Appl. Phys. Lett. 103, 231604 (2013)). Members of the consortium transferred silicene from the metal substrate onto silicon dioxide and demonstrated the first field effect transistor made of silicene (L. Tao, E. Cinquanta, D. Chiappe, C. Grazianetti, M. Fanciulli, M. Dubey, A. Molle, and D. Akinwande, Nat. Nanotech. 10, 227 (2015)), opening the way for exploitation of this material in nanoelectronics.
Graphene and topological insulators for negative capacitance FETs
The basic idea, funded by the ERC advanced SMARTGATE is to integrate graphene and topological insulators (e.g. Bi2Se3) in the electronic gate of silicon MOS field effect transistors with the aim to take advantage of their quantum capacitance and increase their energy efficiency. We recently fabricated high quality metal insulator semiconductor (MIS) devices with composite graphene-encapsulated gates and provided evidence of negative quantum capacitance contributions due to electron correlation effects, which are postulated by capacitance enhancement peaks in the C-V caharacteristics (P. Tsipas, et al., “Negative Quantum Capacitance Effects in Metal-Insulator-Semiconductor Devices with Composite Graphene-Encapsulated Gates”, Adv. Electron. Mater., in press (2016)). This effect in transistor gates could give a positive feedback and internal gate amplification meaning that lower voltage bias is needed in the input to turn the transistor on, making these devices energy efficient. The graphene-based devices can be regarded as an alternative to negative capacitance FETs previously conceptualized and realized using ferroelectric materials (S. Salahuddin, S. Datta, Nano Lett. (2008), 8, 405-410, M. H. Lee,et al, 2015 IEEE International Electron Devices Meeting (IEDM) Proceedings 15-616 (2015) and K. S. Li ,et al, 2015 IEEE International Electron Devices Meeting (IEDM) Proceedings (2015)). These devices are capable of subthreshold steep slope switching and can be used for low power nanoelectronics.
3D topological insulators like Bi2Se3 have some extraordinary properties: They possess robust metallic states on the surface, coexisting with an insulating bulk. This has created an excitement among physicists during the last decade but topological insulators have not impacted the electrical engineering community yet. Due to surface metallic states in the form of Dirac cone, the quantum capacitance of these materials could be used to make composite gate gates similar to those made out of graphene. Our team has managed to grow high quality Bi2Se3 which is also very thin to comply with the scaling trends of nanoelectronic devices (P. Tsipas et al, ACS Nano 8, 6614 (2014)). MIS type capacitors with both Bi2Se3 and graphene in the gate have been fabricated and electrically characterized revealing signatures of negative quantum capacitance. The work is in progress to verify the reproducibility and the yield of devices showing this effect before advancing to the next step proving steep slope switching in transistor devices.
2D metal diselenide materials and v.d. Waals heterostructures
While graphene is expected to enable diverse applications, the consensus is that, being a semimetal, it will be difficult to contribute in nanoelectronics where semiconductor materials are needed. For this reason, stable 2D semiconductors like metal dichalcogenides are intensively studied at present. Most of the work focuses on small (micron-sized) pieces exfoliated from bulk which is sufficient for research. However, passing from the “lab- to- fab” environment, synthesis of 2D semiconductors on large area substrates is required. Our laboratory has developed high quality epitaxial growth of atomically thin MoSe2, HfSe2 and ZrSe2 and van der Waals heterostructures made of these materials on AlN(0001)/Si substrates using MBE (E. Xenogiannopoulou et al, Nanoscale 7, 7896 (2015), K. E. Aretouli et al APL 106, 143105 (2015), P. Tsipas et al MEE 147, 269-272 (2015)). The films are single crystalline, continuous with flat surface morphology and sharp interfaces on a 2 inch wafer scale. The availability of low cost, large area (300 mm) AlN/Si wafers defines a manufacturable route for future large scale production of 2D semiconductors.
We have further proposed epitaxial 2D metal TaSe2 as a metal contact to 2D semiconductors materials such as MoSe2 and HfSe2 (D. Tsoutsou et al. ACS Appl. Mater. Interf. just accepted Article DOI: 10.1021/acsami.5b09743 (2016)). Our MBE work shows that the structural and chemical compatibility of the 2D metal and semiconductor materials, their workfunction matching and their weak coupling via van der Waals forces create the prospect for clean and low resistivity metal contacts to 2D semiconductors which is a big challenge at present. In addition, the formation of fully epitaxial 2D metal / 2D semiconductor v.d. Waals multilayers could lead to novel “matamaterials” with important new applications in nanoelectronics.
- Inside Front cover in the Adv. Electron. Mater.
Quantum Capacitance: Negative Quantum Capacitance Effects in Metal–Insulator–Semiconductor Devices with Composite Graphene-Encapsulated Gates (Adv. Electron. Mater. 4/2016)
Tsipas, S. A. Giamini, J. Marquez-Velasco, N.Kelaidis, D. Tsoutsou, K. E. Aretouli, E. Xenogiannopoulou, E.K. Evangelou and A. Dimoulas
In article number 1500297, P. Tsipas et al. present evidence that graphene incorporated into the gate of metal-insulator-semiconductor devices enhances the capacitance of these devices above its geometrical value. This is attributed to negative capacitance contributions due to electron correlation effects in graphene. As a consequence, it is expected that less bias at the input will be required to switch the transistors on, resulting in devices with improved energy efficiency.
- Publishing of new book: 2D Materials for Nanoelectronics by Houssa, A. Dimoulas, A. Molle
CRC Press, 13 April 2016
- Offers a holistic view of the latest progress in two-dimensional (2D) materials
- Emphasises nanoelectronic applications such as Schottky barrier transistors, flexible electronics, and 2D topological insulators
- Contains chapters written by leading researchers from around the globe
Major developments in the semiconductor industry are on the horizon through the use of two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, for integrated circuits (ICs). 2D Materials for Nanoelectronics is the first comprehensive treatment of these materials and their applications in nanoelectronic devices.
Comprised of chapters authored by internationally recognised researchers, this book:
- Discusses the use of graphene for high-frequency analog circuits
- Explores logic and photonic applications of molybdenum disulfide (MoS2)
- Addresses novel 2D materials including silicene, germanene, stanene, and phosphorene
- Considers the use of 2D materials for both field-effect transistors (FETs) and logic circuits
- Provides background on the simulation of structural, electronic, and transport properties from first principles
2D Materials for Nanoelectronics presents extensive, state-of-the-art coverage of the fundamental and applied aspects of this exciting field.
List of Publications: 2014-2016
Invited presentations: 2014-2016
SMARTGATE – “Smart Gates for the ‘Green’ Transistor” Advanced Investigator Grant 2011- “IDEAS” Funded by the European Research Council (ERC)–PE7 Systems and Communication Engineering
TOP-ELECTRONICS – “Topological Insulator Smart Gates for “Green” Electronics” Co-financed by Greece and the European Union