Electron velocity overshoot in GaAs studied by subpicosecond Raman spectroscopy - carrier density dependence

Electron transient transport in a GaAs-based p-i-n nanostructure under the application of an electric field has been studied by transient Raman spectroscopy at T = 80 K. The non-equilibrium electron distribution function and electron drift velocity were measured at an applied electric field intensity of E = 25 kV/cm and for various injected electron-hole pair densities ranging from 10¹⁷ cm^-3 to 10¹⁹ cm^-3. Our experimental results show that, the electron distribution is very much out of equilibrium for n ≅ 10¹⁷ cm^-3; however, as the electron-hole pair density increases the electron distribution function approaches that of the equilibrium case, in particular, as the carrier density increases to n ≅ 10¹⁹ cm^-3, electron distribution becomes very much in equilibrium and electron drift velocity becomes almost zero. These results have been explained by the effects of momentum randomization and screening of effective electric field by the injected electron-hole pairs.