Bending the Laws of Thermodynamics for Enhanced Material Design

Wide metastable composition ranges are possible in alloys of semiconductors with different crystal structures.

semiconductors
Using computational methods, scientists showed how to mix manganese oxide and zinc oxide to create an alloy with a “sweet spot” (right, blue diamond) where the elements are uniformly mixed (lower left). When synthesized at a higher temperature (right, red diamond), the same material lacks homogeneity (upper left). Image courtesy of the National Renewable Energy Laboratory

The Science

Whether it is better solar panels or smaller computer chips, new technologies demand improved semiconductors. Today’s standard approach to fabricating semiconductor alloys combines materials with similar crystal structures and often results in materials that are poorly mixed. That is, the materials are susceptible to composition fluctuations. Now, scientists have a new way to create well-mixed semiconductor alloys by combining materials with different crystal structures. The result? They bend the laws of thermodynamics to reduce or eliminate the driving force for such fluctuations. The new alloys can be stable over wide ranges of composition and may have desirable properties.

The Impact

The team’s work shows how to alloy materials with different crystal structures to create new semiconductors. Being able to homogeneously combine two materials opens up much wider “design spaces” for tailoring materials. With this technique, scientists can tailor material properties.

Summary

Structure and composition control the properties of materials. For semiconductors, the historically successful approach for such control has been the isostructural alloying of two “end point” phases with the same crystal structure. However, the ability to synthesize and tune these alloys can be constrained by solubility limits, spinodal decomposition or weak composition dependence of the properties. Researchers have now demonstrated a new approach for such control: the heterostructural alloying of two “end point” phases with different crystal structures. Through a combination of computational calculations and combinatorial thin-film phase-equilibria experiments, the researchers demonstrated that a prototypical alloy (Mn1-xZnxO) exhibits a dramatically widened window within which binodal decomposition is suppressed and spinodal decomposition is impossible. In this new class of alloys, not only is the metastable window for compositionally homogeneous single-phase alloys wider, but properties (e.g., electronic, optoelectronic, piezoelectric, ferroelectric) can change in a highly non-linear or even discontinuous fashion near the critical composition, providing two new routes to materials design.

Source : U.S. Department of Energy