Pressure-induced resonance Raman scattering in Ga_1−xIn_xAs/Ga_1−yAl_yAs strained quantum-well structures

Abstract

Pressure‐tuned resonance Raman scattering (RRS) experiments in Ga_1−xIn_xAs/Ga_1−yAl_yAs strained‐layer quantum‐well samples with x=0.15 and y=0.15, on a GaAs substrate, are reported. The photoluminescence (PL), 2LO‐, and LO‐phonon Raman‐scattered intensities were followed as a function of pressure using the diamond cell. In backscattering geometry and with (100) samples, three sharply defined 2LO‐phonon resonances are observed, at 2.1, 3.8, and 5.2 GPa, with 647.1‐nm excitation (ℏω_L=1.916 eV), and these are identified to be from the AlGaAs, GaAs, and InGaAs layers, respectively. At the above pressures, the 2LO peak of the layer falls right on top of the PL peak of the corresponding layer, revealing that the resonance occurs when E₀=ℏω_S (2LO). For the LO‐phonon intensity, maxima are observed at 2.8 and 4.7 GPa, respectively, from the AlGaAs layer and GaAs substrate. While the LO‐resonance profile (pressure versus phonon intensity) for the AlGaAs layer is narrow, the profile of the GaAs substrate layer is highly asymmetric. The InGaAs LO‐resonance is totally masked by the strong GaAs LO phonon. The pressure dependence of the E0 gap, determined from PL data, reveals that the resonance condition for 2LO is E₀=ℏω_S (2LO) and, for LO, E₀=ℏω_L. Results with samples of different Al content and ℏω_L are consistent with the above conditions. A brief discussion of the theory of the RRS cross section is given, and it is suggested that the observed 2LO‐resonance scattering is dominated by Frölich interaction, and that it could be a double resonance. The asymmetry in the LO resonance of the GaAs substrate, we believe, is due to the pressure‐induced transparency, and this is a problem when dealing with pressure‐tuned RRS, in multiple‐quantum‐well structure with GaAs or AlGaAs substrates. The usefulness of the 2LO resonance in locating the E0 gap is indicated, and the indistinguishability between luminescence and Raman scattering at the center of the 2LO resonance is pointed out. The need to study photoluminescence and Raman scattering in pressure‐tuned RRS experiments is emphasized.