Formation of multiple quantum dots in ZnO heterostructures

TL;DR

The paper demonstrates the formation of triple quantum dots in a ZnO heterostructure 2DEG and uses RF reflectometry to map charge stability diagrams, enabling observation of few-electron states and tunable interdot coupling. By adjusting gate voltages, the authors control the interdot coupling and observe electrostatic gaps at dot crossovers, indicating strong capacitive interactions among the dots. They also report the quantum cellular automata (QCA) effect, where multiple electrons move simultaneously due to Coulomb interactions, a phenomenon unique to three or more dots. The results establish ZnO nanostructures as a controllable platform for multi-quantum-dot systems with potential for scalable spin qubits and exploration of fundamental quantum dynamics.

Abstract

In recent years, advancements in semiconductor manufacturing technology have enabled the formation of high-quality, high-mobility two-dimensional electron gases in zinc oxide (ZnO) heterostructures, making the electrostatic formation of quantum dots possible. ZnO, with its low natural abundance of isotopes possessing nuclear spin and its direct bandgap, is considered a potentially suitable material for quantum bit applications. In this study, we achieve the formation of triple quantum dots and the realization of a few-electron state in ZnO heterostructure devices. We also confirm that by varying the gate voltage between the quantum dots, it is possible to control the interdot spacing. Additionally, we observe a tunneling phenomenon called a quantum cellular automata effect, where multiple electrons move simultaneously, which is not seen in single or double quantum dots, due to Coulomb interactions. Our results demonstrate that ZnO nanostructures have reached a level where they can function as controllable multiple quantum dot systems.

Figures (4)

• Figure 1: (a) A cross-sectional schematic view of the ZnO heterostructure device used in the experiment. (b) Scanning electron microscope image of the device. The positions of the formed quantum dots are also drawn. (c) Charge stability diagram of double quantum dots. The number of electrons in each quantum dot is denoted as (n$_{1}$ n$_{2}$). (d) Charge stability diagram of triple quantum dots. The number of electrons in each quantum dot is shown as (n$_{1}$ n$_{2}$ n$_{3}$).
• Figure 2: An enlarged view around the (111) regime in the charge stability diagram of the triple quantum dots in Fig. \ref{['fig1']}(d).
• Figure 3: Tuning of the interdot coupling between QD1 and QD2; charge stability diagrams showing the voltage applied to T1 at different levels. (a) $V_{\rm{T1}} = -2.08$ V. (b) $V_{\rm{T1}} = -2.06$ V. (c) $V_{\rm{T1}} = -2.04$ V. (d) $V_{\rm{T1}} = -2.02$ V. (e) $V_{\rm{T1}} = -2.00$ V. (f) The change in the size of the energy gap of level crossing in response to changes in T1.
• Figure 4: Charge stability diagram showing the QCA effect.