Electrostatically defined ZnO quantum devices

Semiconductor quantum electronics have been intensively investigated to address fundamental quantum-mechanical problems as well as to realize quantum devices, such as single-electron transistors and spin qubits. Conventionally, GaAs heterostructures have been the primary material platform for these studies, as well-developed fabrication technologies enable highly coherent electron transport and dimensional control through electrostatic gating. More recently, Si heterostructures have been adopted for spin qubits, as the low density of nuclear spins in Si isotopes suppresses the decoherence caused by hyperfine interactions. This review introduces electrostatically defined quantum devices based on ZnO heterostructures, with a particular focus on electrostatically defined quantum dots. Similar to Si, ZnO contains low-density nuclear spins, and high-quality ZnO heterostructures comparable to those of GaAs and Si are currently available. Several key techniques, including the fabrication of a gate insulator and nanoscale gate electrodes, have also been developed to electrostatically confine electrons within a nanoscale region. Consequently, one-dimensional (1D) and zero-dimensional (0D) confinements have recently been achieved in ZnO heterostructures, and typical quantized electrical transport phenomena, such as quantized step structures in the conductance of 1D channels and Coulomb blockade in 0D structures, have been observed. In addition, several unique properties associated with electron correlation have been revealed in ZnO heterostructures. Finally, we remark on the prospects of the electrostatically defined quantum devices using ZnO heterostructures by emphasizing their distinctive properties in comparison with those of conventional semiconductors.