The origin of DOS tails is fluctuations in the QW composition. Let us consider a QW with of certain (mean!) composition. From the composition, we can calculate the energy gap, Eg, and determine edges of the valence and conduction bands, Ev and Ec (with an accuracy of a reference point). If we have no fluctutations, we have no electron states below Ec and no holes states above Ev. When we have a fluctuation with increase Indium content, the local position of the conductiion band is below Ec. Averaged, we have some density of electron states below Ec and density of hole states above Ev. We assume that they obey the following relations:
electrons: DOS(E-Ec) ~ exp(E), E<0,
holes DOS(E-Ev) ~ exp (-E), E>0
