(508f) Characterization of Metal Oxide Electrical Properties for Band Engineered Catalysis

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 metal 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. Alternatively, the direction and magnitude of the near-surface electric field within the space charge region can be adjusted by bulk doping which, in turn, affects the flow of photogenerated charge carriers toward the surface in photocatalysis. Current metal oxide catalyst synthesis methods generally do not permit electrical doping with the requisite control for the band engineering approach, partly because of problems with measuring majority carrier type and concentration. Such determinations are difficult for metal oxides because the contacts employed for four-point-probe or Hall Effect 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 an underlying silicon substrate by methods such as 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 capacitance-voltage measurements. For TiO₂, this approach has been implemented with aluminum contacts to the TiO₂ and indium-gallium 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 metal deposition. Detailed current-voltage 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 (TTIP) and O₂.