Energy bands, definition examples

As a working definition, a surface state can be any electron energy level within the bandgap of the semiconductor located at its surface that is coupled to the semiconductor lattice strongly enough to allow inelastic capture of carriers from the semiconductor bands. Several examples of possible surface states are illustrated in Figure 2. In the next section experimental manifestations of some of these are described. 

The next step is the definition of deep. The choice of quantitative values will again involve some arbitrariness. However, a further complication is that there is at present no universally accepted qualitative criterion. For instance, from the point of view of energy level calculations it is often convenient to define deep states as noneffective-mass-like or as those with a localized potential (see, for example, Bassani and Pastori Parravicini, 1975 Jaros, 1980). However, the disadvantage of this definition of deep is that it includes many isoelectronic states that are very shallow on an energy scale. On the other hand, if one uses an energy criterion, should the states be deep with respect to some fraction of the band gap, with respect to kT, or with respect to some shallow levels In this chapter we shall adopt an energy criterion for deep, and we shall require that our states be deep enough to be important in recombination. The importance of deep levels in recombination under many conditions of practical interest was already realized in the early work of Hall... 

The development of ultrashort laser pulses down to pulse durations of 5 x 10 s has opened access to studies of extremely fast transient phenomena. Examples are the relaxation of electrons in semiconductors after their excitation by a short light pulse. The electrons excited with a definite energy tuo into the conduction band thermalize within 10 s by electron-phonon collisions (Figure 10). With a much longer decay constant, they recombine with holes in the valence band before they can be excited again by the next pulse. Such time-resolved studies give important information on the limiting processes for the maximum speed of computers. 

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