Band nonparabolicity effects in semiconductor quantum wells

We propose an empirical two-band model for heterostructures which provides a consistent energy-dependent effective-mass characterization of nonparabolicity in quantum wells. We show that it predicts several surprising results. Nonparabolicity has a very small effect on the lowest subband edge regardless of the well width and hence the energy of the state. Nonparabolicity causes a raising of the lowest subband edge rather than the expected lowering. Nonparabolicity causes a lowering of subband edge energies for higher subbands and the effect becomes substantial for the higherst subband edges. We show that a large nonparabolicity lowering of a subband edge requires the state to have both a high-energy and a high occupancy probability in the well, i.e., not in the barriers.