Energy-filtered TEM studies on silicon nanoparticles acting as quantum dots in single electron transistors

The miniaturization of computing devices and the introduction of the internet of things generate an increasing demand for the development of low-power devices. Single electron transistors (SETs) are ideally suited for this demand, because they are promising very low power dissipation devices. For roomtemperature operation of an SET it is necessary to create a single quantum dot (QD) with a diameter below 5 nm exactly positioned between source and drain at a tunnel distance of only a few nanometers. Within the IONS4SET project [1], we aim to achieve this goal by ion irradiation induced Si-SiO₂ mixing and subsequent thermally activated self-assembly of single Si nanocrystals surrounded by a thin SiO₂ layer. This process is illustrated in Fig. 1 by means of simulations results.
Here, we present energy-filtered (EF)TEM studies in order to monitor the influence of process parameters, such as stack geometry, ion fluence for irradiation, annealing temperature and annealing time, on the self-assembly of Si QDs. Fig. 2 shows representative EFTEM micrographs of a Si-SiO₂-Si layer stack imaged using different electron energy-loss (EEL) windows. The Si plasmon-loss filtered images yield thereby the best signal-to-noise for detection of Si nanodots, because the Si plasmon peak is the most intense peak with a relatively small FWHM of 4 eV in the EEL spectrum.
Moreover, since the obtained (raw) EFTEM images provide only qualitative information about the Si concentration in the oxide layer, they cannot give a clear answer if for example the observed contrast corresponds to one or more Si nanodots (NDs) in projection. Therefore, EFTEM images are quantified further by converting them into so-called thickness over mean free path length (MFPL) t/λ_Si maps, in which λ_Si is the MFPL corresponding to the chosen energy range. The experimental t/λ_Si maps are then compared with simulated t/λ_Si maps of a single Si ND. Fig. 3 depicts that our approach enables us not only to detect single Si nanodots (Fig. 3c,e) but also to count them if they are arranged in projection direction of the electron beam (Fig. 3d,f). For these experiments, the layer stacks were irradiated with Ne⁺ ions within an Orion NanoFab (Zeiss). This allows controlled line or point irradiation and ensures Si QD formation within a confined region. In a next step, confined regions will be established by fabricated nanopillars that enhances reproducibility as the volume relevant for the self-assembly of the nanocluster will be better defined.
[1] We thank for financial support within the European Union"s Horizon 2020 research and innovation program under Grant Agreement No 688072 (Project IONS4SET).