Paper TF-ThP32
Characterization of Metal Oxide Electrical Properties for Band Engineered Catalysis

Thursday, October 23, 2008, 6:00 pm, Room Hall D

There is good reason to believe that the properties of semiconducting metal oxide catalysts can be improved when designed according to the principles of microelectronic devices. Since oxide semiconductors support space charge, it is possible for surface electronic properties to couple to bulk electronic properties. As a consequence, catalyst surface reactivity can be modified via electronic “band engineering,” wherein the electronic properties of the underlying bulk alter the oxidation state of active sites at the free surface or modify the electric field in the space charge region adjacent to the surface. For example, hydroxyl group acidity on the TiO₂ surface can be tuned via the electron richness of the semiconductor, which can be manipulated via controlled doping. Current metal oxide catalyst synthesis methods do not permit doping with the requisite control for the band engineering approach, partly due to problems with measuring majority carrier type and concentration. Such determinations are difficult for metal oxides because the contacts employed for four-point-probe measurements need to obey Ohm’s Law but, in practice, behave as diodes. The present work describes new methodology to solve this metrology problem, using TiO₂ as an example metal oxide. The approach involves synthesis of a thin film of the semiconductor on a silicon substrate by chemical vapor deposition or atomic layer deposition. A Schottky diode structure is then fabricated on the film to obtain the doping concentration from high frequency C-V measurements. For TiO₂, this approach has been implemented with aluminum contacts to the TiO₂ and InGa eutectic alloy contacts to the underlying silicon. The novelty of the structure lies in its compatibility with oxide semiconductors having widely varying doping levels, and the use of easily applied electrical contacts. Oxide thickness, uniformity, and crystal structure can be precisely tailored to suit the subsequent choice of reaction chemistry. A wide variety of fabrication issues have been characterized, including surface and interface preparation, contact metal type, and method of contact deposition. Detailed I-V measurements confirm diode-like behavior that is free from spurious artifacts and amenable to standard Mott-Schottky analysis. Values for the depletion width, barrier height, and series resistance are reported for the example case of TiO₂ synthesized from titanium tetraisopropoxide and O₂.