Hole intersubband relaxation in CdTe/CdMnTe quantum wells

We report a direct measurement of the hole relaxation time in undoped CdTe/CdMnTe quantum wells. The internal piezoelectric field in a (111)-grown structure is screened by e₁h₁ and e₁h₂ excitons with different efficiency. Thus, the temporal evolution of the screened field, as monitored by the e₁h₁ exciton energy in a pump-probe transmission experiment, allows the determination of the e₁h₂ - e₁h₁ relaxation time. We obtained a relaxation time of 15±5 ps for a splitting of 15 meV, smaller than the longitudinal-optical (LO) phonon energy. Deformation-potential scattering is too slow to explain this relaxation time which we attribute to hole-hole scattering and LO-phonon emission by e₁h₂ excitons acquiring enough kinetic energy.