CHEMICAL INTERACTIONS AMONG DEFECTS IN Gc AND Si 575 



VIII. PHENOMENA ASSOCIATED WITH ION PAIRING IN SEMICONDUCTORS 



In this section we shall discuss some of the phenomena which are to 

 be expected in semiconductors when ion pairing takes place. At the time 

 of writing several of these phenomena have been investigated quantita- 

 tively in germanium and casually in silicon. A report on these studies 

 will be given in the later sections of this paper. 



In the meantime it is fitting to inquire into the peculiarities which arise 

 because a semiconducting medium rather than a dielectric liquid is in- 

 volved. The possible means of detecting and measuring ion pairing in 

 semiconductors are numerous, and many of them do not have counter- 

 parts in aqueous solution. This implies that a host of new phenomena are 

 to be expected, many of which are peculiar to semiconductors. 



Some distinctions between semiconductors and liquids are apparent 

 at once. Thus ions are not always mobile in semiconductors at tempera- 

 tures where ion pairing is pronounced. Lithium is exceptional in this 

 respect, being mobile in germanium and silicon down to very low tem- 

 peratures. In fact ion pairing has been observed in germanium containing 

 lithium down to dry ice temperatures, and even below. Another difference 

 is the low dielectric constant of semiconductors as compared with water. 

 I^'urthermore, in semiconductors, charge balance need not be maintained 

 l)y the ions themselves, but may be effected by the presence of holes or 

 electrons. Although charged the latter entities need not be considered in 

 pairing processes since, as particles, they possess effective radii of the 

 order of their thermal wavelengths which may exceed 20 Angstroms at 

 the temperatures involved. At these distances very little coulomb binding 

 energy would be available. Under certain rare conditions the screening 

 effect of these mobile carriers may make some contribution. This may be 

 particularly the case when relaxation processes (to be discussed later) are 

 carried out in poorly compensated specimens of semiconductor, since 

 such processes involve phenomena between ions separated by large dis- 

 tances. 



A very obvious distinction is the fact that ions in a semiconductor 

 Dccupy a lattice, and cannot therefore move through a continuum of 

 positions, as in the case of liquid solutions. Furthermore the lattice may 

 introduce elastic strain energy into the binding energy of a pair. This 

 influence will alter the value of a, the distance of closest approach, when 

 the latter is chosen so as to achieve the best fit between theory and ex- 

 periment. As the extent of pairing is extremely sensitive to the magnitude 

 of a, its measurement provides a useful tool for exploring the state of 

 telrain in the neighborhood of an isolated impurity. We shall demonstrate 



