THE REFRACTIVE-INDEX ENHANCEMENT AT E(G) IN NARROW-GAP SEMICONDUCTORS- COMPARISON BETWEEN THE INTERBAND ABSORPTION-EDGE AND THE OSCILLATORMODEL

A comparison is made between the refractive index enhancement near to the M(0)-type absorption edge in direct narrow-gap semiconductors and the dispersive structure of a Lorentz oscillator. Phenomenologically, both absorptive structures can be described by three parameters, and t he analogy between both models concerning n(E) is discussed. Examples for the predicted dependences are given. With increasing energy gap th e n(E) enhancement becomes related to discrete excitonic bands rather than to free-to-free transitions, thereby undergoing a spectral shift from energies above gap to energies below gap.