Second Harmonic Generation On Excitons In Zno/(Zn,Mg)O Quantum Wells With Built-In Electric Fields

Optical second harmonic generation (SHG) on excitons is studied in polar ZnO/(Zn,Mg)O quantum wells grown on ZnO and sapphire substrates. We observe SHG signal on exciton resonances in a symmetry-forbidden geometry for light propagation along the c axis of the structure. A symmetry analysis allows us to suggest that the signals are induced by a symmetry reduction with respect to ZnO bulk crystals by the well-barrier interfaces in combination with built-in electric fields due to spontaneous and piezoelectric polarization as well as structural defects.