Memristive Devices
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Memristor devices have been postulated as the fourth fundamental circuit
element since 1970 with promising applications in non-linear circuit design,
data storage, and neuromorphic circuitry. The recent developments in device
microfabrication revealed exceptional behavior in thin oxide films, such as
electroluminescence, electron emission and most notably negative differential
resistance. There has been a consensus that the resistive behavior is thermally
driven due to the thermal conductivity of glasses and oxides which increases
with temperature, unlike that of crystal metallic and semiconductor materials.
Consequently, understanding the electrical behavior of memristive devices
requires full thermal characterization in which the main research drives,
in collaboration with the Australian National University, were:
- Identifying the particular mechanism of current conduction in Pt/NbOx based memristors. While some researchers attribute the NDR change to an electric field driven and thermally accelerated Poole-Frenkel conduction, occuring at moderate temperatures of around 500 Kelvin, other researchers attribute the conductivity to a thermally induced phase change in the metal-oxide at temperatures exceeding 1000 K. The high resolution thermal maps allow the observation of the thermal response that accompanies the electrical switching behavior to estimate the switching temperature and thus infer the governing mechanism.
- Based on the observed thermal and electrical operation, a two region conductance model is developed to represent the electrical behavior of NbOx memristive devices. The model predicts the behavior of NbOx devices under different oxide thickness, stoichiometry, and electrode size. This model will be the corner-stone for using the devices in system applications. The model is then expanded to investigate other factors such as electrode material and the loading polarity and conditions.
- The measurement of the thermal properties of the tested transition metal oxides (Niobium oxide and Hafnium oxide) is essential to model any thermal behavior of the corresponding devices. A TDTR method is used to measure the thermal conductivity of sample films of NbOx and HfOx at different temperatures, process conditions, and stoichiometric ratios. The results are then compared to theoretical models for nanoscale thermal transport in amorphous materials.
Investigation of the Thermally-Governed Electrical Characteristics of Memristive Devices and Negative Differential Resistors (NDRs)
Assaad El Helou, Sanjoy Nandi, Shimul Nanth, Rob Elliman, and Peter E. Raad
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Time domain thermoreflectance measurements of thermal conductivity of NbOx and HfO2
Assaad El Helou, Sanjoy Nandi, Shimul Nanth, Rob Elliman, and Peter E. Raad
To complement the simulation results and to understand the thermal characteristics of NbOx devices, the thermal conductivity of thin-film NbOx samples are measured at different temperatures and stoichiometric ratios. The thermal resistance of the different thickness samples are extracted from a curve fitting with the numerical model to extract the thermal properties of the sample stack. The theraml conductivity of NbOx is extracted and plotted at temperatures between 20 and 200°C.
