Free-electron laser study of the nonlinear magnetophotoconductivity in n-GaAs

The University of California at Santa Barbara free-electron laser was used to investigate the kinetics of electrons bound to shallow donors in n-GaAs by saturation spectroscopy. The resonant photothermal conductivity arising from 1s-2p⁺ shallow donor excitations in a magnetic field was measured at intensities greatly exceeding that of earlier investigations and saturation of bound-to-free photoionization transitions was achieved. The impurity resonance photoconductive signal shows a distinct intensity dependence caused by competing bound-to-free transitions which saturate differently. This permits a more detailed evaluation of the electron recombination kinetics than was previously possible, yielding the ionization probability of the 2p⁺ state, the transition time of electrons from the 2p⁺ level to the gound state, and the recombination time of free carriers.