10. Appendix

Appendix C:
Recent Development
A4.1 A Prototypical Deep Center in N-Type
Zincblende-Type Semiconductors: The DX Center
A4.1.1 Introduction
So far the hydrogenic impurities have been very attractive from the viewpoint
of understanding their properties. By applying the effective mass theory we
have been able to explain the properties of a large family of impurities using
only the physical properties of the host lattice, without regard to the chemical
nature of the impurities. The relevant physical properties of the host are
its dielectric constant and the effective mass parameters of the nearest band
extrema. We have defined deep centers as defects whose properties cannot be
understood within the effective mass theory. We expect, therefore, their properties
to be sensitive to their chemical and physical nature, such as their ionic
radii and/or electronegativities. It has been relatively difficult to explain the
properties of deep centers in terms of those of the host lattice alone. The vacancy
without lattice relaxation and the isovalent impurities discussed in 4.3.2
and 4.3.3 are two exceptions we have encountered so far. The utility of the
vacancy model is unfortunately reduced by its neglect of lattice relaxation. In
this addition we shall present a class of deep centers known as the DX centers
which was first mentioned in 4.2.2. This family of defects is technologically
important because it has strong effects on the electrical properties of the
host crystal. It has interesting features found in other deep centers, such as
large lattice relaxation, strong electron-phonon coupling, and the existence of
metastable excited states. It is also an interesting example of a many-body effect
known as negative-U, already mentioned in 4.3.
The outline of this addition is as follows. We shall start with some historical
background on how the DX centers were discovered and in so doing
summarize also their important features. This is followed by a simple qualitative
description of the theoretical model of the DX center first proposed by
Chadi and Chang. This model has successfully explained many of the characteristics
of the DX centers. It has also made predictions about properties
which were subsequently verified experimentally. One important prediction of
the Chadi and Chang model (to be abbreviated as the CCM) is that the DX
centers have a negative correlation energy U. We conclude by discussing the
experimental results which have confirmed this prediction.