T. Y. Tan
14. DEFECTS IN CRYSTALS
Any deviation from a perfect and pure crystal that is infinitely large in size constitutes a kind
of defects in the crystal. Many important properties of crystals are influenced or even determined
by the types and concentrations of various defects present in the crystal. To name a few exam-
ples: diffusion, kinetics of phase transformations, electrical conductivity of semiconductors,
color of ionic crystals, mechanical strength of metals, etc. The detailed behavior of the defects
and their influences on materials' properties are quite complicated, and, often, different defects
interact with each other, leading to more complexities.
Our main interest in studying defects is to gain an understanding of their behavior for pur-
pose of applying the knowledge to control the properties of the materials via controlling the de-
fect generation. This is not always possible. More often than not, we must simply learn to utilize
the material within the limitations set by the nature of the defects. An important approach for
studying the defects is to apply the method of statistical thermodynamics, pioneered by Wagner
and Schottky. Using this approach we are able to distinguish the important types of defect in a
crystal and the interactions among them.
Types of Point Defects
A most important group of defects consists of point defect species. They are responsible for
diffusion, kinetics of phase transformations, electrical conductivity of semiconductors, color in
ionic crystals, etc. phenomena to occur. It is convenient to categorize point defects into two
groups: simple or primary defects, and complex or secondary defects. Simple or primary defects
consist of (i) single atomic defects; and (ii) electronic defects, i.e., electrons and holes in semi-
conductors. Complex defects are a small number of simple defects associated together, which we
will not be concerned with.
Simple point defects in a crystal are consisted of three kinds: (i) vacancies,
, which are lat-
tice sites unoccupied by atoms; (ii) self-interstitials,
, which are the crystal self-atoms occupying
interstitial sites; and (iii) foreign atoms, either substitutional or interstitial. Vacancies and self-
interstitials are called native point defects or intrinsic point defects.
For elemental crystals including metals, e.g., Al, Cu. W, etc., and semiconductors, e.g., Ge
and Si, etc., two native point defect species exist:
, see Fig. 14.1.