Time-resolved Raman studies of the photoexcited electron-hole plasma in InP

Both lateral and perpendicular transport properties of the photoexcited electron-hole plasma in n-type InP have been studied by the time-resolved Raman scattering technique with ≃30 μm and 0.1 μm spatial resolution, respectively, and on a picosecond time scale. The plasma density ranging from 1×${10}^{16}$ to 2×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$ was deduced from fitting of the Raman spectra with the plasmon–LO-phonon scattering theory which took into account the contributions from free holes. In contrast to the experimental results of Young and Wan who found that the ordinary diffusion equation was sufficient to fit their transient plasma density-time profiles in semi-insulating InP, our experimental results have shown that perpendicular transport (i.e., expansion into the bulk crystal) of the plasma in n-type InP can be very well described by a modified diffusion equation including the effect of drifting away from the surface based on a hydrodynamic model. The transient plasma density-time profiles were studied at T=300 K and for an initial injection plasma density n≃2×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$. The plasma has been found to expand laterally at velocity v≃5×${10}^4$ cm/s and perpendicularly into the crystal at a velocity $v_p$≃1.5×${10}^5$ cm/s.