Strong Photoluminescence Emission from an Excited-Subband Exciton State in a GaAs/AlxGa1−xAs Triple Quantum Well with Different Well Thicknesses

We have studied the photoluminescence (PL) properties of a GaAs/AlxGa1−xAs quantum well (QW) system consisting of three QW’s with different thicknesses, in which the widest one (7.8 nm) has an excited-subband exciton state located slightly below the barrier band edge. Using above-barrier excitation with a power density of about 10 W/cm at 13 K, a strong PL emission peak appears on the high-energy side of the three PL peaks originating from the ground-state exciton transitions of the three QW’s. This high-energy PL peak with an intensity comparable to the one of the other three peaks is located near an excited-subband (n=2) exciton state of the widest QW. By investigating the position dependence of the PL spectra across a 2-inch wafer, which exhibits a decreasing Al mole fraction x from the center to the edge, the PL intensity of the high-energy peak can be correlated with the one of the ground-state in the widest QW. This correlation indicates that this high-energy PL peak is most likely related to the n=2 exciton state of the widest QW because of the energy alignment of the excited subband state relative to the barrier band edge. PACS: 78.66.Fd, 71.35.-y, 73.21.Fg.