Growth far from equilibrium: Examples from III-V semiconductors

Abstract

The development of new applications has driven the field of materials design and synthesis to investigate materials that are not thermodynamically stable phases. Materials which are not thermodynamically stable can be synthesized and used in many applications. These materials are kinetically stabilized during use. The formation of such metastable materials requires both an understanding of the associated thermochemistry and the key surface transport processes present during growth. Phase separation is most easily accomplished at the growth surface during synthesis where mass transport is most rapid. These surface transport processes are sensitive to the surface stoichiometry, reconstruction, and chemistry as well as the growth temperature. The formation of new metastable semiconducting alloys with compositions deep within a compositional miscibility gap serves as model systems for the understanding of the surface chemical and physical processes controlling their formation. The GaAs1−yBiy system is used here to elucidate the role of surface chemistry in the formation of a homogeneous metastable composition during the chemical vapor deposition of the alloy system.

The thermodynamic scale of inorganic crystalline metastability

“…However, even though this brings about greater thermodynamic risk for phase separation, long-range chemical separation is diffusion-limited, which can be a kinetic barrier that enables the persistence of highly metastable (>70 meV/atom) multinary compounds. Indeed, there are emerging examples that the formation of low-dimensional crystals from multicomponent precursors under diffusion-limited conditions can result in novel crystalline phases that are metastable with respect to phase separation ( 4 , 40 – 42 ). In contrast, polymorphic phase transformations occur under constant local composition and thus lack this kinetic barrier of chemical separation, which may rationalize why the energy scale of metastability for phase-separating compounds is larger in magnitude than that for polymorphic compounds.…”

Exotic forms of silicon

“…Those small differences complicate the synthesis of pure-phase materials far from equilibrium-even ones that are metastable under ambient conditions. 8 That's the case, for example, for many potential Si clathrates, cage-like structures that form around guest atoms or molecules. The impurity guests turn out to be hard to remove after the cage has formed.…”

“…Another is to construct a clathrate Si lattice with an electron-acceptor element, such as boron or aluminum, to perfectly compensate for the interstitial Na electron donors-the compensation is precisely controlled by the chemistry. Theorists have suggested the compound K 8 Al 8 Si 38 for that effort. 6 But to date, none of those dopant-reduction efforts has succeeded experimentally.…”

Utilizing TiO2 amorphous precursors for polymorph selection: An in situ TEM study of phase formation and kinetics

“…Metastable materials open avenues to increased functionality and performance beyond classical equilibrium crystalline materials, only if we can target and stabilize them . This is often observed in polymorphic material systems where stoichiometrically identical, but structurally dissimilar materials can have diverse functionality.…”