Similar to GaP, porous GaAs was extensively investigated using the Raman scattering techniques. The lower curve in Figure 4.32 illustrates the micro-Raman spectrum from the bulk GaAs crystal measured under excitation by a 970 nm laser beam. In undoped GaAs the selection rules for backscattering from a (100) surface would allow the observation of the LO phonon (292 cm-1), but not the TO phonon (268 cm-1). However, due to the free carriers in the doped crystal the plasmon (in polar semiconductors) interacts with the LO phonon and the spectrum is dominated by two LO-phonon–plasmon coupled modes (LOPC) L- and L+ instead of the LO phonon. The observed LOPC were fitted to calculated line shape functions using the Lindhard–Mermin approach to the dielectric function . The value n=3.5×1017 cm-3 was obtained, which is in good agreement with the specification data.
It is to be noted that the excitation of the as-grown crystal was done at the wavelength λ=970 nm of the laser radiation (in GaAs Eg=1,43 eV at 300 K, which corresponds to &lambda=867 nm). Under these conditions one can ignore the contribution to the Raman scattering of the surface depleted layer where the LO vibration is decoupled from the plasmon and exhibits a pure ‘unscreened' LO signal [111, 112].
Figure 4.32: Unpolarized micro-Raman spectra of bulk GaAs and of a porous layer. (100)-oriented wafers of n-GaAs cut from LEC-grown ingots doped by silicon were used. The free electron concentration in the as-grown single crystals was n=3x1017 cm-3 at 300 K. The porous layers were produced by anodic etching of GaAs substrates in a 0.2M H2SO4 electrolyte. The anodization process was carried out in darkness for 5 min at a current density of 20 mA/cm2.
On the contrary, the spectrum excited with the 514.5 nm line (the middle curve in Figure 4.32) shows, in addition to the LO-phonon–plasmon mode L+, the ‘unscreened' LO phonon from the depletion layer. One can notice the different appearances of the LO-phonon–plasmon modes in the two lower spectra in Figure 4.32 and in the lower part of Figure 4.33. For instance, the L- mode is not observed under 514.5 nm excitation. These differences are in accordance with calculations performed in the Lindhard-Mermin approach including absorption effects and they prove to be the result of different wavevectors involved in the Raman scattering under excitation by 970 and 514.5 nm laser lines.
Figure 4.33: Deconvolution of the micro-Raman spectra of bulk GaAs and of a porous layer.