The refractive index enhancement at Eg in narrow-gap semiconductors: comparison between the interband absorption edge and the oscillator model

A comparison is made between the refractive index enhancement near the M₀-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 the analogy between both models concerning n(E) is discussed. Examples for the predicted dependencies are given. With increasing energy gap the 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.