SOI-based silicon quantum dots contacted by self-aligned nano-electrodes

Abstract

We present a technique to contact individual silicon quantum dots (QDs) by nano-­electrodes making use of a self-­alignment effect. Starting from an ultra thin silicon on insulator (SOI) substrate we employ self­-assembled gold colloidal particles as an etch mask. These particles are deposited onto the substrate using aminosilane [3­-(2­-aminoethylamino)propyltrimethoxysilane] as an adhesion agent yielding a sub­-monolayer sample coverage. The QDs are then fabricated by applying a CF₄ reactive ion etch (RIE) process to remove the silicon layer everywhere except below the gold colloids. Subsequently, the colloidal mask is removed by a wet chemical etch and 100-200 nm wide metal wires are patterned by electron beam lithography (EBL) onto the QD-­covered samples. A nanometer­-sized gap is created in these wires by a controlled electromigration process. The metal wires will preferentially break at the positions of the QDs, because the metal layer is dilated there resulting in a locally higher current density. This leads to a self­-alignment effect of the evolving nano­-electrodes with respect to the QDs. The native oxide of the silicon QDs is used as a tunneling barrier leading to a single­-electron device. The oxide thickness can be increased in a controlled manner by self­-limiting thermal oxidation to adjust the tunneling resistance. Finally, I(V)­-traces of these devices are collected at liquid helium temperature. They show clear Coulomb blockade behavior as well as Coulomb staircase features.

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