Initial generation of hot LO phonons by photoexcited hot carriers in GaAs and ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As alloys studied by picosecond Raman spectroscopy

The initial generation of hot LO phonons by photoexcited hot carriers is studied with picosecond Raman spectroscopy in GaAs and a series of ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As samples with 0<x<0.4. A rapid decrease in the occupation numbers of the GaAs-like and AlAs-like LO-phonon modes is observed as x is increased. This decrease cannot be explained if the electrons excited from the light- and heavy-hole bands were the primary source of generating the hot phonons. It is shown that most Raman-active hot LO phonons are initially generated by the photoexcited electrons originating from the split-off band, when photon energies of 2.33 eV and pulse durations of 1.5 ps are used. We have used a model assuming the instantaneous thermalization of electrons in the Γ valley which are photoexcited from the split-off hole band. Our experimental results are in good agreement with this calculation.