Defect theory: elusive state-of-the-art

Defects play essential roles in many materials, especially semiconductors. Today’s most advanced electronics involve defect engineering at the atomic level. The need to control the behavior of nearly every atom in the active area of a device implies a high degree of understanding of how defects behave and interact in a host crystal. This is achieved by a combination of microscopic experimental and theoretical methods. The theory of defects in silicon has evolved tremendously in the past half century and has become quantitative in many respects. This review gives an overview of the evolution of theory and a discussion of ongoing developments.Abraham Lincoln once said, “It has been my experience that folks who have no vices have very few virtues.” His wit and wisdom apply equally well to defects in materials. By ‘defect’, I mean a native defect or an impurity. The words ‘defect’ and ‘impurity’ have an intrinsically negative connotation, but defects are often desirable. They affect or even control the optical, mechanical, and electrical properties of materials1. The color of diamond is determined by N or B impurities. Its thermal conductivity depends strongly upon its isotopic purity2,3. Less than 1% of C transforms Fe into hard steel. As for semiconductors, virtually all their electrical and optical properties are determined by the type and concentration of defects they contain. The story of Si (and other semiconductors) is the story of defect control and defect engineering. Defects are not annoying details that must be eliminated. Instead, some are critically useful, while others render a device inoperable4. Defect-free Si is pretty and shiny, but useless until the right defect is made to behave in the right way at the right place.