Ultraviolet Stark Effect Optical Modulators Using Nonpolar ZnO/ZnMgO Multiple Quantum Wells

We demonstrate a Stark-effect optical modulator operating in the ultraviolet (UV) wavelength region using nonpolar ZnO/ZnMgO multiple quantum wells (MQWs). The nonpolar a-plane ZnO/ZnMgO MQW wafer was grown on an r-plane sapphire substrate by plasma-assisted molecular beam epitaxy (PA-MBE). The active layer consists of 20 periods of ZnO (4 nm)/ZnMgO (10 nm) MQWs grown on an n-type ZnMgO:Ga contact layer. A UV-transparent conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), was employed as a Schottky window layer formed on the ZnO cap layer by an inkjet printing technique. In electroabsorption (EA) measurements, the excitonic transition energy of the ZnO quantum wells exhibited a red shift of 16 meV under a reverse bias of 30 V due to the quantum-confined Stark effect (QCSE). Optical modulation experiments were performed by applying a reverse bias during optical transmittance measurements. The maximum modulation factor reached 31% at the excitonic absorption peak (3.40 eV, 365 nm) under a reverse bias of 30 V. In the transparent region below the bandgap, the maximum modulation factor was −7% at 3.32 eV (373 nm).