Energy Relaxation of Photoexcited Hot Electrons in GaAs

The energy relaxation of an initially hot photoexcited free-electron population has been investigated in high-purity GaAs at lattice temperatures $T_L=2\mathrm{} \mathrm{to} 4$ K. The electron energy distribution function $f\left(E\right)\mathrm{}$ was determined directly from the line shape of the conduction-band-acceptor luminescence. Two different experiments—transient and steady state—were performed: (i) the time development of $f\left(E\right)\mathrm{}$ in the electron energy range $0\le E\le 8$ meV was measured with subanosecond time resolution after excitation with short light pulses (∼ 0.2 nsec, $\hslash \omega =1.92\mathrm{}$ eV), and (ii) the steady-state dependence of $f\left(E\right)\mathrm{}$ on cw-excitation-light power was measured. The experimental results are compared with theoretical energy relaxation rates using the known standard electron-phonon scattering mechanisms. In the electron temperature range above ∼ 16 K both experiments indicate higher energy-loss rates than expected theoretically. At lower electron temperatures good agreement with theory is obtained. It is shown that interelectronic collisions effectively randomize the electron energies during the relaxation process even for the low electron densities ($n_e$ as low as 2 × ${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$) achieved in the experiments.