Stephanie Bohaichuk2 Suhas Kumar1 Gregory Pitner2 Connor McClellan2 Jaewoo Jeong3 Mahesh Samant3 Stuart Parkin3 H.S. Philip Wong2 R. Stanley Williams1 Eric Pop2

2, Stanford University, Stanford, California, United States
1, HP Labs, Palo Alto, California, United States
3, IBM Almaden Research Center, San Jose, California, United States

The emergence of data-driven computing has prompted a search for systems based on new computational elements. One novel paradigm is the use of spikes, or neuron-like action potentials, to communicate and compute. However, implementing such a neuromorphic system in physical hardware requires compact devices that can produce controllable, low-energy, and fast spiking.
One approach is to exploit instabilities in the transient dynamics of VO2, which produce periodic spiking when used in a relaxation oscillator, corresponding to periodic heating and cooling across its Mott metal-insulator transition. Speeding up these devices requires aggressive shrinking to minimize both the electrical and thermal time constants of these oscillations. To effectively scale such VO2 devices to sub-10 nm widths, in previous work we used metallic carbon nanotubes (CNTs) with ~1 nm diameter as nanoscale heaters in physical contact with an electrically parallel thin film of VO2 [1].
In this work we demonstrate that a single CNT-VO2 device forms a Pearson-Anson relaxation oscillator when driven by a DC source, generating neuron-like periodic spiking. The highly localized Joule heating of the CNT acts to trigger and confine the insulator-metal transition in the VO2 to nanoscale dimensions, greatly reducing the effective thermal mass of the device. This reduction of thermal time constant allows the device to oscillate very rapidly across its Mott transition. Compared to a VO2-only device, the CNT-VO2 devices showed a reduction in quasi-static switching voltage accompanied by the following dramatic changes in its dynamical behavior: (a) an increase in the spiking frequency by over 3 orders of magnitude, (b) a decrease in the spike’s transient duration by 3-4 orders of magnitude, and (c) a decrease in pulse energy by 2 orders of magnitude. We also constructed a compact model that could quantitatively reproduce these observations, providing insights into the thermal oscillation effects induced by the CNT.
We further characterized the tunability and scalability of our CNT-VO2 devices. We found that the spiking frequency of a single device can be tuned by nearly an order of magnitude by adjusting the DC bias conditions. Our results also showed that the spike width and energy decrease with length, with our shortest 300 nm devices showing sub-20 ns spike widths.
In summary, we demonstrated that the addition of a CNT nanoscale heater results in a significant improvement in the dynamical behavior of VO2-based oscillators. This provides an accessible path to scaling electronic devices by thermally engineering and localizing the dynamics of an otherwise bulk transition mechanism.
[1] S. Bohaichuk, M. Muñoz Rojo, G. Pitner, C. McClellan, F. Lian, J. Li, J. Jeong, M. Samant, S. S. P. Parkin, H.-S. P. Wong, E. Pop, Device Research Conference, IEEE, June 2018. DOI: 10.1109/DRC.2018.8442223