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CAMP Seminar: Opportunities in narrow band transition metal oxides

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Roman Engel-Herbert, The Pennsylvania State University
12 April 2016 from 3:30 PM to 4:30 PM
339 Davey Laboratory
Contact Name
Jie Shan
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Transition metal oxides, although ubiquitous and highly functional, have barely made it into the application space of electronic and photonic devices. Their electronic and optical properties are dominated by partially filled d-orbitals which form a narrow band with high carrier effective masses and high carrier concentration, exhibiting strong electron correlation effects. One major roadblock in utilizing these effects to augment or even replace existing semiconductor based technology is their inferior material quality when synthesized in thin film form. While the structural perfection is high, evidenced by atomically abrupt interfaces, well defined heterostructures, and the demonstration of artificially layered oxide compounds grown with an impressive degree of control using physical vapor deposition techniques, such as pulsed laser deposition (PLD) or molecular beam epitaxy (MBE), the level of perfection needed to minimize unintentional extrinsic defect concentration does not meet the stringent requirements for their use in electronic and photonic devices.

In this talk I will discuss the application of an epitaxial thin film synthesis technique, hybrid molecular beam epitaxy, for the growth of perovskite vanadate compounds. This combinatorial approach of conventional MBE and chemical beam epitaxy (CBE) has been applied to the growth of SrVO3 and LaVO3, a correlated metal and a Mott insulator, as well as to the growth of high quality VO2 thin films. For all three cases the intrinsic material quality and its dependence on growth conditions will be discussed and compared to single crystal bulk standards. It will be shown that wafer scale growth of functional oxide thin films of ‘electronic grade’ quality is possible using this technique. The specific example of a transparent conductor is given to illustrate how transition metal oxides offer new design strategies beyond conventional semiconductors with band structures derived from s and p-orbitals. It will be shown that the high carrier effective mass, originating from the small band width of the conduction band derived from d-orbitals, is key to strike a new balance between the mutually exclusive demands of a high optical transparency and high electrical conductivity with metal-like carrier concentration. With a figure of merit comparable to the industry standard of the transparent conductor tin-doped indium oxide (ITO) at much thinner film thicknesses and lower cost of raw material the huge potential of this class of materials is exemplified.