Radiative recombination of electron-hole pairs spatially separated due to quantum-confined Stark and Franz-Keldish effects in ZnO/Mg0.27Zn0.73O quantum wells
We studied photoluminescence (PL) properties of eighteen samples of wurtzite ZnO/MgxZn1-xO multiple quantum wells (x=0.12 and 0.27) with various well widths (L-w) of 0.7-4.65 nm. Radiative recombination of the electron-hole pairs that are spatially separated due to the quantum-confined Stark (QCS) and Franz-Keldish (QCFK) effects was observed in two thicker samples at 5 K. This PL band is located approximate to 40 meV in energy below the emission band of the localized excitons and approximate to 60 meV below the absorption energy of the free exciton transition. One can not observe such kind of luminescence unless both of the following conditions are accomplished: (1) higher Mg concentration (x=0.27) and (2) L(w)greater than or equal to4.23 nm. These experimental findings do not contradict the following two characteristic features for the QCS and QCFK effects; the magnitude of the electric field due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the L-w, respectively. The coupling strength with longitudinal-optical phonons, which is determined from the relative luminescence intensities of the phonon replicas, is significantly larger than that between the localized excitons and phonons. It is considered that the strong electric field increases the distance between electron and hole charge distributions from that determined by the Coulomb force and leads to the enhancement in the phonon interaction.
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