Surface passivated and encapsulated ZnO atomic layers by high-κ ultrathin MgO layers

Atomically transparent vertically aligned ZnO-based van der Waals materials have been developed by surface passivation and encapsulation with atomic layers of MgO using materials by design, and their physical properties have been investigated. The passivation and encapsulation led to a remarkable improvement in the optical and electronic properties. The valence-band offset ΔE_v between MgO and ZnO, ZnO and MgO/ZnO, and ZnO and MgO/ZnO/MgO heterointerfaces is determined to be 0.37 ± 0.02, -0.05 ± 0.02, and -0.11 ± 0.02 eV, respectively, and the conduction-band offset ΔE_c is deduced to be 0.97 ± 0.02, 0.46 ± 0.02, and 0.59 ± 0.02 eV, respectively, indicating straddling type-I in MgO and ZnO, and staggering type-II heterojunction band alignment in ZnO and the various heterostructures. The band-offsets and interfacial charge transfer are used to explain the origin of n-type conductivity in the superlattices. Enhanced optical absorption due to carrier confinement in the layers demonstrates that MgO is an excellent high-κ dielectric gate oxide for encapsulating ZnO-based optoelectronic devices.