Coulomb-staircase observed in silicon-nanodisk structures fabricated by low-energy chlorine neutral beams

Abstract

A silicon-nanodisk structure, a nanometer-scale silicon disk on extremely thin SiO2 film, was fabricated by etching a 3.5–4-nm-thick polycrystalline silicon (poly-Si) thin film/1.4–3-nm-thick underlying oxide layer/Si substrate structure with a 7-nm-diameter ferritin iron-core mask and Cl neutral beam etching (NBE). The degree of etching was precisely controlled by detecting its depth using x-ray photoelectron spectroscopy. Cross-sectional scanning transmission electron microscopy (STEM) with elemental mapping by electron energy-loss spectroscopy (EELS) revealed that the underlying oxide layer remained while the Si layer was accurately etched. The STEM-EELS observation also revealed that there was an Si layer about 1–2-nm thick even in the nanodisk, while the nanodisk’s surface region was covered by native oxide. Removing the surface oxide layer prior to the NBE process could decrease the nanodisk diameter. Irradiation by Cl NB of the underlying 1.4-nm-thick SiO2 film increased the thickness of the SiO2 film and drastically decreased the SiO2 leakage current. This worked as self-aligned isolation in the space between the nanodisks when measuring their current-voltage (I−V). This may also be useful for fabricating future quantum-effect devices using nanodisks. Coulomb staircases could be observed by measuring the I−V of nanodisks even at room temperature. These results indicated that the nanodisks fabricated in this research have a precise quantum-effect structure and they attained single-electron properties. This research has great potential for the development of practical and robust fabrication processes for future quantum-effect devices.