A brief summary of physics and characterisation of metastable defects in semiconductors is presented. Although the emphasis is put on III-V semiconductors, for which the main features of DX-like centres and EL2 in GaAs are discussed, some generalisation for other materials is offered. The recent findings in the theory of these centres, namely the relation of metastability to the sp~3 reversible sp~2 bonding instability (substitutional reversible interstitial impurity motion), are positively verified in a set of experiments probing the local symmetry and counting the number of electrons in the metastable states.
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