PRESSURE-INDUCED GAMMA-X ELECTRON-TRANSFER RATES IN A (GAAS)15/(ALAS)5 SUPERLATTICE

Time-resolved measurements on pressure-induced type-II GAMMA-X electron transfer in a (GaAs)15/(AlAs)5 superlattice have been performed using the femtosecond pump-and-probe technique. In the case of type-II character, the measured transfer times tau-GAMMA-X depend on the energy separation DELTA-GAMMA-X of Gamma and X states as tau-GAMMA-X-1 infinity (DELTA-GAMMA-X)1/2, showing that the transfer process consists of (1) relaxation and quasithermalization of the electrons and holes in the GaAs, and (2) transfer of the electrons to the AlAs layer. Well above the crossover pressure P(c) the scattering rates are independent of the carrier density. Near P(c) = 1.2 GPa, a carrier-induced type-I-type-II crossover leads to a shift of P(c) towards lower pressures, giving P(c) almost-equal-to 0.9 GPa at 1.5 X 10(12) cm-2. This value P(cdyn) is explained in terms of the different band-gap renormalizations of the direct GAMMA and indirect X-point transitions. The renormalization of the X state is found to be twice as large as the GAMMA-state renormalization.