We report direct pump-probe measurements of the excited-state lifetime of a carrier bound to a shallow impurity in a semiconductor. The dynamics of intra-acceptor level scattering were studied for the cases of uniformly doped GaAs:Be and also a δ-doped AlAs/GaAs multiple-quantum-well sample. The experimental technique used to measure the scattering rate was a balanced pump-probe system using a free-electron laser as a source of intense far-infrared picosecond pulses. The hole relaxation time from the 2p state to 1s ground state of the acceptor was measured as a function of temperature for the two main absorption lines. It was found that in the bulk these transitions have a lifetime of the order of 350 ps independent of temperature up to the thermalization temperature of the acceptor. Studies of the 15-nm δ-doped GaAs quantum well in AlAs barriers show that in this quantum-confined system, where the transition energies are governed by the well width, the lifetime of the excited state is much shorter, of the order of 80 ps. We suggest that the relaxation process is by acoustic phonon emission and that the effect of the superlattice periodicity is to relax the strict k conservation rules of the bulk increasing the acoustic phonon emission rate.
|Physical Review B - Condensed Matter and Materials Physics
|Early online date
|30 Mar 2001
|Published - 15 Apr 2001