Defect engineering of polycrystalline titanium dioxide synthesized by atomic layer deposition

摘要

There is good reason to believe that the properties of semiconducting metal oxides for catalytic and integrated circuit applications can be improved when designed according to the principles of microelectronic devices. Defect engineering is used extensively in the semiconductor processing industry to produce devices with well-defined physical properties (e.g., electrical conductivity, switching frequency, power consumption). Polycrystalline TiO2 is an attractive material for supported metal catalyst, photocatalyst, memory resistor, and spin-based transistor devices due to its photostability, low cost, and non-toxicity. Nevertheless, methods for characterization and manipulation of carrier concentration, near-surface and near-interface electric fields, and magnetic ordering are lacking.The present work employs ALD as a film synthesis technique to allow precise control over TiO2 microstructure, crystallinity, and composition. It outlines the application of a sound metrological method for the evaluation of charge carrier concentration to metal oxide semiconductors not amenable to standard characterization techniques such as four-point-probe or Hall effect. This science base permits elucidation of complex defect behavior in polycrystalline TiO2, a system comprising native defects such as oxygen vacancies and titanium interstitials as well as grain boundaries and voids, all of which may be electrically active. An emphasis is placed on manipulating these defects to affect charge carrier concentration and decoupling trends observed in the literature as a function of film thickness, illumination, and synthesis method. Subsequently, doping is investigated with an aim at precisely controlling carrier concentration by judicious selection of n-type (Cr, Nb) and p-type (Mn) transition metal species. For the case of undoped TiO2, photoreflectance, a type of modulation spectroscopy, is invoked to examine the degree of fixed charge buildup at buried solid-solid interfaces in metal oxide thin film structures. Lastly, since the method of in-situ doping during ALD is unique in its ability to allow for decoupling of microstructure, defect composition, and doping level, the suitability of Mn-TiO2 deposited in this fashion for novel integrated circuit devices is highlighted.