Top-gated single-electron transistor in germanium nanowires

Germanium nanowires (GeNWs) of the group IV semiconductors could be one of the attractive candidates for electron-spin based quantum devices because of their long electron-spin coherence time. Besides, Ge has an advantage over Si in terms of the larger quantum effects due to the smaller effective mass. Single-electron transistors (SETs) are basic building blocks of such devices. To define the spin configuration in the dot, it is necessary to reach a few-electron regime or an even-odd regime where the single spin is realized for the odd number of electrons in the dot. So far, we have developed processes to fabricate SETs using n-type monocrystalline GeNWs with a back gate, and succeeded in observing the even-odd effect [1]. In this work, we have developed fabrication processes of the top-gate SETs to enhance the gating efficiency, and succeeded in reaching a few-electron regime.