Behavior of electron gas in selectively doped GaAs/AlxGa1−xAs multiple-quantum-well structures by Raman scattering

Abstract

Two-dimensional (2D) and three-dimensional (3D) behavior of electron gas (plasmons) in selectively doped GaAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As multiple-quantum-well (MQW) structures have been studied in detail with the use of Raman scattering, utilizing coupled plasmon–LO-phonon modes as an indication of the dimensionality of the plasmons. Raman scattering was measured at room temperature for two types of the MQW structures in which each well is either doped or modulation doped with electrons. Two Raman modes are observed besides the LO-phonon peaks in the spectra, and the modes are assigned to the intersubband quasi-2D plasmons coupled with LO phonons due to the dependence of the peak frequencies on the well thickness and the electron density. Charge transfer from the ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As barriers into the wells in the modulation-doped MQW structure is established by comparing the Raman spectrum of the MQW structure with that of the thick ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As reference layer with the same Al content and doping density as those of barriers. Also, transition from quasi-2D to 3D character of the plasmons with decreasing barrier thickness is discussed on the shifts of the frequencies of the coupled intersubband-plasmon–LO-phonon modes to those of the coupled 3D-plasmon–LO-phonon modes.