Introduction

The word ‘flaw’ was introduced by Shockley and Last [5.1.1] ‘for the purpose of distinguishing between imperfections with multiple possibilities for charge condition and ordinary donors and acceptors’ following the early work on the statistics of these centers described in [5.1.2].

In fact it is a convenient term which encompasses both chemical and physical defects and it is sometimes used in this generalized sense [5.1.3]. We shall here use the term defect and use an amended form of Blakemore's classification scheme (Fig. 5.1.1). This is not done to give a systematic discussion of these defect classes (this requires a major book), but to indicate the variety of possibilities. (An As atom sitting on a Ga site (AsGa) in GaAs is a typical anti-site defect.) There is a further classification [5.1.4] which depends on the value of

Δz = valency of the impurity – valency of the host

On this basis what is often called an isoelectronic defect, defined by Δz = 0, is more logically named isovalent, since the total number of electrons on the atom involved to which the term ‘isoelectronic’ refers is often of minor relevance.