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The atom marks the ultimate scaling limit of Moore’s law, which is why atomic scale devices have attracted significant research interests from the electronics industry. To allow efficient co-integration of electronics and photonics, key components such as photodetectors [1] and modulators [2] should match the footprint of electronic devices.
Here we demonstrate the first atomic-scale plasmonic photodetector where atoms rather than electrons are responsible for the device operation. The concept is based on a so-called electro-chemical metallization (ECM) cell where an atomic-scale conductive filament is partially dissolved through a plasmonic-thermal effect.
To realize this new type of photodetectors, three different disruptive technologies have been combined into one single fabrication process. First, a 3-D photonic technology based on a modified self-aligned approach of local-oxidation of silicon (LOCOS) has been developed for silicon-on-insulator (SOI) substrates. This is an important step as it enables the integration of tip-based atomic-scale plasmonics within a low-loss bus photonic waveguide. Second, vertical 3-D adiabatic plasmonic couplers have been fabricated using two e-beam lithography steps and a lift off process. The resulting metal-insulator-metal (MIM) waveguide that houses the ECM cell consists of a silver and a platinum contact separated by a gap of 20 nanometers. Finally, the atomic scale junction has been realized by electroforming a silver filament inside the ECM cell.
To investigate the operation principle of this photodetector, a 3-D axis-symmetrical finite element method (FEM) model has been implemented that is able to self-consistently simulate the device resistance as a function of the applied voltage and temperature. The electrochemical growth and dissolution of a conductive filament between two electrodes is modeled analogously to the work of Refs. [3] and [4]. The current through the device is approximated as a tunneling current whose dependence on the filament state can be derived from ab initio quantum transport calculations. The microscopic nature of the device is also taken into account by considering an electrical double layer at the metal-insulator interfaces that accurately describes the electrostatic potential distribution within the ECM device. The incorporation of first-principles results [5] allowed us to significantly reduce the number of free parameters.
Two light-matter interaction mechanisms have been identified and investigated, namely the optical force acting on individual filament atoms and the heating through electromagnetic dissipation in the metal. An atomistic study based on real-time time-dependent density-functional theory revealed that the optical forces are not strong enough to move single atoms, which leaves the optically-induced temperature as the main driving force behind the filament dissolution. In this paper we will show through accurate device simulations that this is indeed what is happening: the variation of the temperature at the metal-insulator interfaces strongly affect the electron transfer rates between these two regions, which explains the observed device behavior. Quantitative agreement between simulation and experiments will be demonstrated, thus opening up the possibility of future computer-aided designs of atomic-scale photodetectors.
References
[1] Emboras et al. doi:10.1021/acsnano.8b01811
[2] Emboras et al. doi:10.1021/acs.nanolett.5b04537
[3] Menzel. doi:10.1007/s10825-017-1051-2
[4] Lin et al. doi:10.1109/IEDM.2012.6479107
[5] Ducry et al. doi:10.1109/IEDM.2017.8268324
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