Narcís Mestres

Light emission at the E1 and E1+Δ1 gaps in heavily doped p-type Ge and GaAs

Abstract We report the observation in heavily doped p-type germanium (Nh ≥ 1018 cm−3) of two weak light emission bands centered at the energies of the E1 and E1+Δ1 interband gaps (2.22 and 2.42 eV at 80 K). These bands, which are 100% polarized, are found only for excitation with laser frequencies slightly above the gaps. We attribute them to photon scattering by inter-valence-band excitations of the holes associated with the heavy doping. The fact that the emission bands do not shift with the exciting laser frequency is assigned to a strong resonance enhancement of this scattering near the E1 and E1+Δ1 gaps. We have also observed the corresponding light emission at the E1 gap (3.0 eV) in p-type GaAs.

Resonant Raman scattering by plasmons in n-type Ge

Abstract We have measured the Raman efficiency for plasmon scattering in n-type Ge as a function of laser excitation frequency in the range of the E 1 and E 1 +Δ 1 optical gaps (2.1–2.5 eV). Our results can be explained by a mechanism involving the macroscopic electric field that accompanies the plasma oscillation in a manner similar to that responsible for Frohlich-interaction-induced forbidden LO-phonon resonances in polar semiconductors plus some contribution of the standard cdf mechanism for scattering by plasmons.

In-Plane Electronic Excitations in GaAs/GaAlAs Modulation Doped Quantum Wells

We study the in-plane motion of electrons in modulation doped GaAs/AlGaAs multiple quantum wells by resonant Raman scattering. Electrons from donors incorporated in the barriers of these structures drop into the GaAs wells, forming a multilayered 2D electron gas with very high mobility. The long range interlayer Coulomb interaction couples the in-plane motion of electrons in different wells, even if there is no wave function overlap, as in our present samples. The interlayer Coulomb interaction thus leads to a band of N non-degenerate coupled layer plasmon eigenmodes for an electron system with N sheets. Using resonant Raman scattering we measure the dispersion of these discrete layer plasmon eigenmodes and their coupling with intersubband excitations yielding a very detailed instrument for the characterisation of multilayered electron systems. We measure the single particle excitation spectra as a function of k||. We fit the single particle excitation spectra with the Lindhard formula for the dielectric response function of the 2D electron gas and we show that the electron density and the scattering time can thus be determined optically.

Light Scattering by Plasmons in Heavily Doped n-Type Ge and Si

We report Raman scattering by plasmons in n-type silicon and its resonant behavior near the E1 gap. The scattering mechanism seems to be similar to the forbidden scattering by LO-phonons in ionic materials.