Ghost: Graphene Heterostructures with ultra-thin films Of wide bandgap SemiconducTors

Joint research project

Project leaders: Filippo Giannazzo, Bela Pecz
Agreement: UNGHERIA - HAS (MTA) - Accademia Ungherese delle Scienze
Call: CNR/HAS (MTA) 2016-2018
Department: Physical sciences and technologies of matter
Thematic area: Physical sciences and technologies of matter
Status of the project: New

Research proposal

Graphene (Gr) researchers received the Nobel Prize in Physics in 2010, while the Nitrides research was awarded by the same prize in 2014. The present proposal would like to take benefit from the outstanding physical properties of both these materials, by investigating the heterostructures of Gr with ultra-thin films of Nitrides (AlN, GaN). These studies will be extended to the possibility of realising Gr heterostructures with other wide bandgap (WBG) semiconductors thin films, such as ZnO. The combination of the above materials promises for novel devices applications by the virtue of their band gap engineering, thermal conductivity, elasticity and piezoelectricity.
Although AlN and ZnO crystals are commonly found in the wurtzite (wz) crystal structure with tetrahedral sp3 coordination of the atoms, theoretical studies and some preliminary experimental investigations indicated that ultra-thin films of these materials can be thermodynamically stable in the flat graphitic structure with planar trigonal sp2 coordination of the atoms. Similarly to h-BN, a h-AlN (h-ZnO) layer may be depicted as a Gr layer, in which the C atoms have been fully substituted by alternating Al (Zn) and N (O) atoms with sp2-hybridized covalent bonding. Theoretical papers predict that sp2 bonding can be preserved not only on a monolayer, but typically up to 10 layers of the deposited h-AlN (or h-ZnO). From the experimental point of view, to date a graphite-like AlN thin film has been experimentally realized on silver substrate.
The Ghost project is aimed to fabricate ultra-thin films of the above mentioned wide bandgap semiconductors (AlN, GaN, ZnO) on silicon carbide (SiC) with epitaxial Gr on the top of SiC. This approach is expected to result in high quality heterostructures directly on a large area semiconductor substrate suitable for electronics/optoelectronics and sensing applications.
Van der Waals heterostructures of 2D materials currently represent one of the hot topics in nanoelectronics. These are mostly obtained by separating 2D films with different bandstructure (semimetallic Gr, semiconducting transition metal dichalchogenides, insulating BN) from bulk crystals and stacking of layers by an elaborate transfer/alignment procedure. The realization of few layers AlN and ZnO with graphitic structure on as Gr/SiC template will provide an alternative scalable route to obtain heterostructures of Gr with semiconducting 2D materials.
The Ghost project is a completely new cooperation between two research groups with complementary expertises: CNR-IMM in Catania, and MFA in Budapest.
The CNR-IMM group is expert in the nanoscale resolution electrical characterization of materials for micro and nanoelectronics by scanning probe microscopy (SPM). Two dimensional mapping of the carrier concentration and resistivity of WBG semiconductors (SiC, GaN and AlGaN/GaN heterostructures) and 2D materials (epitaxial graphene grown on SiC) have been demonstrated by scanning capacitance microscopy (SCM) and conductive atomic force microscopy (CAFM). Advanced processing capabilities of WBG semiconductors and 2D materials are also available for the fabrication of electronic devices test structures (Hall bars, field effect transistors, ...). CNR-IMM is currently involved in Graphene Flagship (WP8 Flexible Electronics).
The MFA group is expert in the growth and structural characterization of WBG semiconductors. MFA recently carried out a successful experiment which resulted in high quality GaN grown on patterned graphene/SiC template (Advanced Materials Interfaces, Vol. 2, January 21, 2015). MFA also published a number of papers on the growth and microstructure of WBG semiconductors.
The two groups are both partners of a recently approved FLAG-ERA proposal (GRIFONE: Graphitic films of group III nitrides and group II oxides: platform for fundamental studies and applications) with the coordination of Linköping University (LiU), Sweden. It is worth mentioning that, due to the exhaustion of funds allocated by MIUR for the FLAG-ERA program, the CNR-IMM group will participate in the GRIFONE project on in kind base, i.e. without receiving financial support.
The Ghost project will take benefit from the participation of CNR-IMM and MFA in the GRIFONE EU initiative. LiU group will provide the templates of epitaxial graphene on SiC. Furthermore, some of the growth experiments of AlN, GaN and ZnO films on the Gr/SiC stacks by the MOCVD technique will be carried out in LiU, in addition to theoretical simulations on the stability and electronic properties of the heterostructures.
The thin layers (AlN and ZnO) grown on Gr/SiC(0001) will be studied by transmission electron microscopy (high resolution and electron diffraction methods) at MFA and by AFM at CNR-IMM in order to understand essential nucleation/deposition phenomena. Cross sectional and plan view specimens will be prepared for TEM by gentle Ar ion milling, in which the accelerating voltage can be decreased down to 50 V, but definitely this process will be finished below 1kV. The uniformity of the layers will be determined, the possible domains and defects will be revealed. Raman spectroscopy will be also applied to characterize Gr defectivity/doping after AlN(ZnO) growth. Graphitic layers are expected for ultrathin AlN and ZnO films and this will be confirmed and compared to the thick layer cases. For these graphitic layers, the bandgap and the conductivity (both thermal and electrical) are expected to be different from those of the thick layers with wz- structure.
CNR-IMM will investigate the local electrical properties (vertical and lateral current conduction, capacitance,..) at nanoscale using the CAFM and SCM techniques. Finally, device test structure will be fabricated to characterize the electrical properties of the thin h-AlN (h-ZnO) films grown on epitaxial Gr as barrier layers for perspective vertical devices or as gate insulators for lateral Gr field effect transistors.

Research goals

- Fabrication and high resolution structural, chemical and electrical characterization of Gr heterostructures with ultra-thin films of wide bandgap semiconductors (AlN, GaN, ZnO).
- Training of young-researchers:
From the Hungarian side, Dr. Ildiko Cora is a young post-doc (Ph.D. awarded in 2014); Erzsebet Dodony is a Ph.D student in the Physics Doctoral School of Roland Eötvös University and carries out her research work in the MFA laboratory; A M.Sc. student (Gergely Ádám) will be also involved.
From the Italian side, Dr. G. Fisichella and Dr. G. Greco are young post-docs (PhD awarded in 2015 and 2013, respectively).
- Joint dissemination activities:
Scientific publications (at least 4) on high IF journals, such as Nature Nanotech., Nanolett., Nanoscale, Advanced Materials & Interfaces, Carbon, Nanotechnology, PRB, APL,...
Participation in specialized international conferences (at least 4), such us Graphene Week, Graphene, ICSCRM/ECSCRM,..
Organization of a workshop and/or proposal of a Symposium for EMRS Spring/Fall meeting 2017 on the topic of "Growth and advanced characterization of 2D materials heterostructures".
Proposal for a special issue or a book.
Organization of dedicated seminars with industries (such as STMicrolectronics) to illustrate the potentialities of the investigated heterostructures for electronics, sensing,..
- Strengthening the cooperation between CNR-IMM and MFA, with the aim of submitting H2020 proposals together with other EU partners.

Last update: 25/04/2024