Atomic construction of two-dimensional materials

Speaker: 
Bruce A. Davidson, Physics Department, Temple University, Philadelphia, USA; Oxide MBE Lab, CNR–IOM/TASC National Laboratory, Trieste, Italy
Event Date and Time: 
Thu, 2017-08-03 10:00 - 11:00
Location: 
AMPEL #311
Local Contact: 
Doug Bonn
Intended Audience: 
Graduate
Moore’s law has helped push the search for electronic materials to the twodimensional
limit, where new physical properties and new device physics are being
discovered. For example, single monolayers of transition–metal dichalcogenides (TMDCs)
can show electronic structure different from their bulk counterparts (e.g. direct band gaps,
“valley”–dependent interband transitions, and excitons with high binding energies, to name
a few) that offer promise for novel electronic, optoelectronic and spintronic applications.
Because of the weak van der Waals bonding between adjacent monolayers, coupled
quantum well structures can be made simply by combining two monolayers of different
materials. Alternatively, interfaces can sometimes be considered 2D materials: the interface
between polar and nonpolar insulating oxides can show a 2D conducting layer exhibiting
superconductivity and magnetism that can be modulated by an electric field.
Currently, intense effort world–wide on epitaxy of 2D materials is reminiscent of
the situation in III–V semiconductors 50 years ago when MBE became the enabling
technology for discovery of quantum well and superlattice physics. Our understanding and
exploitation of the new physics of 2D materials depends crucially on improved in situ
control of the structural properties (such as epitaxy, defects, domains, site-specific dopant
incorporation and profiles, and chalcogenide alloying) during growth of monolayers and
heterostructures.
I will present two examples of state-of-the-art growth of 2D materials by MBE.
First, I will explain how we have used reflection high–energy electron diffraction (RHEED)
and atomic absorption spectroscopy (AAS) for the precise control of stoichiometry and
terminating layer in the growth of perovskite oxides. This approach gives superior control
over atomic layer stacking and doping profiles during the growth of interfaces, and has
been exploited to improve the performance of e.g. magnetic tunnel junctions whose
performance is limited by the fragile interfacial magnetism. This RHEED–based growth
approach works universally for all (001) perovskite materials tested so far, and the
phenomenology is identical during pulsed laser deposition (PLD) growth. Second, I will
discuss the state-of-the-art of “van der Waals epitaxy” by MBE of TMDC monolayers and
heterostructures for which, in spite of the vdW bonding to the TMDC layer, the substrate
plays a pivotal and poorly understood role in the growth. I propose that there is much room
for improvement, and point to directions in which RHEED and other in situ diagnostics
may play a crucial role.
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