Transition between quantum confinement and bulklike behavior in polar quantum wells

The properties of III-nitride quantum wells (QWs) are significantly affected by polarization fields. We show that, in the case of wide QWs, there exist two fundamentally different transition types characterized by quantum-confinement effects or bulklike behavior-depending on the carrier density in the structure. At low carrier density, spontaneous and piezoelectric polarization lead to tight confinement and low spatial overlap between the electron and hole wave functions. However, if the carrier density is sufficient for screening of the polarization fields, the wave functions spread out, reach high overlap, and the energy states come close to each other, similar to a bulk semiconductor. Streak camera measurements in the subthreshold regime of nitride laser diodes and numerical simulations show consistent results and let us clearly differentiate between contributions of individual states for low carrier density and the emergence of a high density of energetically close states at high carrier density.