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There is great interest in developing transition-metal oxides such as TiO₂ or SrTiO₃ for semiconductor devices applications.   As with any semiconductor in the early stages of development, one of the main challenges is to control the conductivity by doping and reducing the concentration of native defects and unintentional impurities, thus, minimizing their deleterious effects.  These transition-metal oxides significantly differ from conventional semiconductors (III-V, Si, and Ge) with respect to their electronic structure and electronic transport.  While in conventional semiconductors, electron and holes originated from doping are delocalized and result in high carrier mobility, excess electrons in TiO₂ and SrTiO₃ occupy degenerate conduction bands derived from Ti d states, leading to a variety of interesting physical phenomena; for instance, is often argued that transport in TiO₂ occurs through hopping of small polarons (electronic defects), giving rise to relatively low electron mobility. Here we will present results of density functional theory and hybrid functional for electronic and structural defects in TiO₂ and SrTiO₃.  We will discuss the effects of charge localization on specific sites in TiO₂ and the consequences for optical and transport measurements. Shortcomings of the conventional density functional methods in describing charge localization will also be addressed.

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