Photoluminescence study of symmetric, coupled, double In xGa 1−xAs-GaAs quantum well structures

Abstract

Detailed photoluminescence (PL) and PL excitation (PLE) measurements are reported for narrow, pseudomorphic, nominally symmetric, coupled In ∼0.08Ga ∼0.92As-GaAs double quantum well (QW) structures. Well widths from 4 to 14 monolayers (MLs) and “coupling” barrier widths from 20 to 120 Å are examined ( 1 ML = 2.8635 A ̊ ). Free-exciton and subband edge transitions for both light- and heavy-holes are observed and are associated with symmetric and antisymmetric combinations of isolated OW wave functions. The difference between the subband energy and the exciton energy gives the exciton binding energy. The subband energy was confirmed by magneto-optical studies involving field dependent PLE measurements from which Landau oscillations were observed. From the Landau level fan diagram the subband energy was obtained. Exciton binding energies deduced from these measurements were found to be greater for symmetric than for antisymmetric heavy-hole free-excitons (HHFEs). For example, the exciton binding energy E B for two 14ML wide In 0.085Ga 0.915As QWs separated by a 120 Å GaAs “coupling” barrier is found to be E B (S-HHFE) ≈ 8.3 meV for the symmetric HHFE (S-HHFE) and E B (A-HHFE) ≈ 6.8 meV for the antisymmetric HHFE (A-HHFE). That E B (S-HHFE) > E B (A-HHFE) is expected since S-HHFEs are more confined than A-HHFEs: heavy-holes and electrons associated with symmetric combinations of isolated QW wave functions lie lower in energy than heavy-holes and electrons associated with antisymmetric combinations of isolated QW wave functions. These results are compared with transition energy calculations based on a four band k.p formalism, and reasonably good agreement is found. Additionally, fine structure is observed for the 14 ML double QW sample in both the symmetric and antisymmetric HHFE transitions and is explained on the basis of variations in well width and/or indium content.