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Tuning the Stability of Electronic Defects in Semiconducting Oxides

Figures a,b.c. Visualization of a spin density isosurface shown in yellow of (a) a free electron, (b) a small polaron, and (c) calculated self-trapping Gibbs free energy of a small polaron in SrTiO3 as a model system. Blue (large),
gray (medium), and magenta (small) balls represent Sr, Ti, and O, respectively.

Figure a. Predominance map of electronic defects as a function of temperature and pressure in cubic SrTiO3 based on self-trapping Gibbs free energy calculated by using the quasiharmonic approximation and density functional theory. The green and red indicate small polaron and free electron predominance zones, respectively. The black dashed line represents the experimental boundary between the cubic and tetragonal phases of SrTiO3. Cubic SrTiO3 is stable above the boundary.

Profs. Caroline Ross and Krystyn Van Vliet

Intellectual Merit:
Functional semiconducting oxides are an attractive group of materials for energy and information applications. They are the key enabler for several important technologies, including solid oxide fuel cells, thermochemical fuel production as well as novel memory devices such as red-ox based memristive systems.

MIT MRSEC researchers have demonstrated that the combined action of temperature and mechanical stress can tune the relative stability of electronic defects in semiconducting oxides. Combining density functional theory and the quasiharmonic approximation, they were ableĀ  to predict the effect of temperature and pressure on charged defects in semiconductors. By applying this approach to strontium titanate, they elucidated the rich thermodynamics underlying the free energy landscape of free electrons and small polarons.

Broader Impacts:
The ability to predict temperature and pressure effects in semiconducting oxides will guide the design of optimal conditions to promote desirable forms of electronic defects in electronic or electrochemical applications. For example, at a given temperature, mechanical stress can be tuned to promote free electrons in photo-electrodes to enhance electronic conductivity. Alternatively, stress can be altered to stabilize small polarons and decelerate electronic conduction for design of corrosion-resistant coatings.