Fabrication and characterization of self-assembled and self-aligned SOI-based single-electron transistors

Abstract

We present a novel technique to fabricate single-electron transistors (SETs) with silicon quantum dots (QDs) as
conducting islands making use of a combination of self-assembly and self-alignment effects (for an overview of the
fabrication process, see Fig. 1). Starting from an ultra-thin silicon-on-insulator (SOI) substrate we employ aminosilane
molecules as an adhesion agent to self-assemble gold colloidal particles in a sub-monolayer [1]. These particles are then
used as an etch mask for a CF₄ reactive ion etch (RIE) process in which the silicon layer is removed everywhere except
below the gold colloids, yielding silicon QDs on a SiO₂ insulating layer. A metal wire together with symmetric side gate
electrodes is patterned by electron beam lithography (EBL) onto the QD-covered sample, and a nanometer-sized gap is
created in these wires by a controlled electromigration process [2]. Self-alignment of the evolving nano-electrodes with
respect to the QDs is achieved, because the metal layer is locally dilated by the QDs resulting in a locally higher current
density. Therefore the metal wires will preferentially break at the positions of the QDs. To obtain tunneling contacts the
native oxide layer covering the silicon QDs is used as a tunneling barrier. Its thickness can be adjusted in a controlled
manner by self-limiting thermal oxidation [3] to obtain an accurate tunneling resistance.
The devices are electrically characterized at liquid helium temperature by applying a source-drain voltage and
measuring the current. The I(V)-curves (Fig. 2) show clear Coulomb blockade behavior and Coulomb staircase features.
When the source-drain voltage is kept at a constant value and the gate voltage is varied, conductance oscillations
become visible. By collecting I_SD(V_SD)-traces for different gate voltages and calculating their numerical derivatives a so-
called stability diagram is obtained, exhibiting Coulomb diamonds which are typical for SETs.

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