Optical characterization of type-II ZnO/ZnS multiple quantum wells grown by atomic layer deposition

•Synthesis of type-II ZnO/ZnS multiple quantum wells on Si substrates by ALD.•Type-II transition due to staggered band alignment of ZnO and ZnS.•Strong visible wavelength transition is obtained with type-II multiple quantum wells.•Type-II luminescence showed large blue shift with increasing excitation power. ZnO/ZnS multiple-quantum wells (MQWs) were grown on Si substrates by atomic layer deposition, and their optical properties were investigated. Light emission well below the band-to-band transition energies of constituent materials was observed, which was attributed to the type-II band alignment of ZnO/ZnS MQWs. The type-II transition showed a significant blue shift with increasing excitation power, which originated from modified band bending caused by spatial separation of electrons and holes as electrons are confined in the ZnO conduction band quantum well and holes are confined in the ZnS valence band quantum well. The conduction band offset of the ZnO/ZnS heterojunction was determined to be 1.58 eV simulated from the type-II emission of the MQWs with controlled layer thicknesses. The type-II transition provides an important opportunity to utilize visible light emission and absorption for various optical and energy harvesting devices using the ZnO/ZnS material system.