Light in Memristive Atomic Scale Junction - Memristors go Photonics

. Memristive devices are an emerging new type of devices operating at the scale of a few or even single atoms. They largely exploited for emulating the electrical function of synapses and are thus currently investigated for performing in-memory and neuromorphic computing. In this contribution, we report the observation of a novel feature in these devices. We show that memristors can also emit photons during their activity. We identiﬁed three mechanisms producing photons with vastly di ﬀ erent properties. The crossover between emission regimes depends on the history of the memristor and its operating conductance. Our results suggests that this new generation of memristor pave the way for multidimensional neural networks using both electrons and photons as information carrier.


Introduction
Electronic components integrating nanometer scale gap in their design were also crucial to the advent of novel form of computing.Memristors for instance are programmable voltage-dependant resistive devices deployed nowadays in cognitive hardware systems such as artificial neural networks, neuromorphic and reservoir computing [? ].Memristive operation relies on resistance switching triggered by the electroformation and disruption of conductive pathways within a nanometer-scale dielectric gap [? ].Charge transport occurs by an electro-chemical reduction of metal ions aggregating to conductive filaments [? ], or by migration of mobile defects, such as oxygen vacancies [? ] and nanoclusters [? ].In this report, we introduce an atomic scale memristive device capable of emitting photons during resistive switching, superseding thus the need for an external optical source.Our device features the compact footprint of transistors and compatibility with the emerging memristive technology.

Device fabrication and characterization
The devices are typically constituted of two in-plane 70 nm-thick silver electrodes thermally deposited on a 3 nm thin Cr adhesion layer on top of a glass coverslip.The electrodes are terminated by a tapered section forming a 90 • angle and are separated by a gap of ranging from 10 to 60 nm.The structures are realized by electron beam lithography complemented by metal deposition and a liftoff process.* e-mail: alexandre.bouhelier@u-bourgogne.frThe first emission mechanism is based upon the creation of optically-active defect centers induced by the structural changes from resistive switching of the memristor.The atomic rearrangements facilitates the creation of silicon-rich regions within the SiOx matrix.Our investigations suggest that the emission stems from these electroluminescent Si-rich defects[?].The blinking dynamics is studied by measuring the photon autocorrelation g (2) (τ) as pictured in Fig. ??.The red curve is a fit to the data with an anomalous diffusion model representing a continuous time random walk where electron hop between disordered traps having a continuous distribution of escape times.

Inelastic electron tunneling
When the cycling the memristor, the operation changes from a volatile to a non-volatile state.In this regime, light is no longer emitted by charge injection in defects centers, but by inelastic electron tunneling whereby an electron injected in the gap has a small probability to loose its energy by exciting optical modes.Under this circumstance, the correlative dependence between photons emitted and current variation changes.

Overbias emission
During the crossover between volatile to non-volatile type switching, we observe a transient response with the occurrence of fluctuating current.Figure ??(a) shows sequences of 1.24 V voltage pulses.At this voltage, any emission mechanism promoted by a single electron process would be emitted in the nearly blind spectral region of the detector.The optical activity suggests that the memristive device releases photons in an overbias emission regime [? ].Opposite to single electron process, overbias light emission is the manifestation of a radiatively decaying of hot electron distribution produced when electrical power is lost in a system with characteristic dimensions smaller than the electron mean-free path.In this picture, the logarithm of the photon count rate is expected to be a function of the parameter 1 √ IV b .Figure ??(b) confirms this effect.

Conclusion
We report that resistive switching in filament-type memristive junction may be accompanied by light emission.We show in our experiments that three mechanisms are at play depending on the nature and the dynamics of the switching.The new memristive photon source discussed here features an atomic-sized footprint and a straightforward and scalable fabrication process.As the emitted photons are associated with a resistive state change, our findings can be exploited in optical memristive neural networks to identify weight changes from the corresponding memristor.

Figure 1 .
Figure 1.Photon autocorrelation with the fitted anomalous diffusion model

Figure 2 .
Figure 2. (a) Extract of a cycling sequence showing the photon counts and the current for a pulse train of 1.6 V, 400 ms period and 200 ms duration.Series of normalized spectra emitted by the memristive gap upon application of different pulse amplitudes Vb = 1.6 V (top), Vb = 1.8 V (middle), and Vb = 2 V (bottom).The vertical flags are the marking the quantum limit given by hν = eV b .

Figure
Figure ??(a) displays a voltage pulse sequence where current shows a stable level and photons are detected at every pulse [? ].In this quantum tunneling process, the high energy side of the emitted spectrum is limited by the quantum limit hν < eV b .Figure ??(b)(a) shows a series of spectra emitted by the device for three different pulse amplitudes.The vertical flags mark the kinetic energy of the electrons as supplied by the operating voltage.

Figure 3 .
Figure 3. (a) Examples of driving pulses for which the device in an overbias regime.The kinetic energy of the electron ( 1.24 eV) is smaller than the photon energy (>1.2 eV).(b) Linear dependence of the photoncount (log scale) with 1 √ IV b expected from radiatively decaying hot electron gas.