Direct observation of the LO phonon bottleneck in wide GaAs/AlxGa1-xAs quantum wells

We report the direct observation of a bottleneck in electron cooling in wide GaAs quantum wells. Intersubband lifetimes tau and their dependence on intensity, lattice temperature T-L, and well width have been measured using a ps excite-probe technique in wells with subband separation less than the longitudinal optical (LO) phonon energy. Above an electron temperature of about T-e=35 K the lifetime depends on T-e and is determined by LO-phonon emission. Below this bottleneck temperature acoustic phonons dominate the plasma cooling. An energy balance model of these interactions, with no adjustable parameters, gives good agreement with our results. At electron temperatures below 35 K we determine tau=500 and 200 ps for samples of subband energy 19.5 and 26.6 meV, respectively.