ABSTRACT
Bridgman) and liquid-phase growth (e.g. LPE) methods were replaced by vapor-phase growth techniques (Molecular Beam Epitaxy, or MBE, and Metal Organic Chemical Vapor Deposition, or MOCVD), new surprising effects were discovered, such as spontaneous long-range atomic ordering in III-V alloys, in the form of monolayer superlattices [4-6] and localization effects in nitride alloys [7-11]. Both effects contradicted the virtual zincblende view of these alloys. Their mysterious appearance in vapor-phase grown samples were initially thought to result from equally mysterious “non-equilibrium structures,” afforded by MBE and MOCVD. Theoretical investigation of this new type of phenomenology revealed, surprisingly, that both the structural anomalies (ordering) and the electronic anomaly (localization) resulted from surface thermodynamics. In other words, the existence of epitaxial strain and surface reconstruction during growth (absent in liquid-phase growth, or melt-growth) stabilizes a new subsurface structure with non-zincblende ordering and, at the same time, increases the solubility of the alloyed elements, leading to the formation of clusters with their attendant wavefunction localization. In what follows, I will describe the basic ideas leading to this new physical picture of isovalent semiconductor alloys, as deviating from the “virtual zincblende” picture.
