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Fundamental Zero Energy Gap States in Superlattices

The band structure of a model superlattice, in which the conduction band bottom in the first host semiconductor is located very close to or even below the valence band top in the other host semiconductor, is analyzed as a function of layer thickness and of the conduction band–valence band edge discontinuity. The analysis is carried out in the envelope-function approximation and in Kane's two-band framework. It is shown that with increasing magnitude of the discontinuity (or of layer thickness), the nature of the superlattice changes from a direct gap semiconductor to a direct zero gap semiconductor. Furthermore, it is also shown that in the direct zero energy gap states the valence and conduction bands may touch each other simultaneously over several circumferences in the Brillouin zone of the superlattice. 
 
 
 
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