Electronic Properties of Defects

Abstract

This chapter dealt with the study of the electronic properties of defects in semiconductors because electrically active defects play an important role in the operation of many semiconductor devices. Since defects come in many different forms we restricted our discussions to point defects only. These are separated into donors and acceptors and further divided into shallow or hydrogenic centers and deep centers. For shallow centers we introduced the effective mass approximation for calculating their energies and wave functions. Properties of shallow centers were shown to be very similar to those of the hydrogen atom except for effective mass anisotropy and other corrections arising from the host crystal lattice. Hence energy levels of shallow centers are sometimes referred to as hydrogenic levels. Many defect centers cannot be understood within this approximation and they are referred to as deep centers. Properties of deep centers are often determined by potentials, localized within one unit cell, known as central cell corrections. These localized potentials are difficult to handle. We have, therefore, presented only a rather rudimentary Green's functions approach to calculating deep center energies. This approach was applied to explain the chemical trends of deep levels in tetrahedrally bonded semiconductors and to the special case of isovalent (substitutional nitrogen) impurities in GaAsP alloys. A very serious limitation of our approach is the neglect of lattice relaxations, which are often associated with deep centers.