Carnegie Mellon University

Devices - cleanroom

Devices for non-volatile memory elements

These devices are based on binary oxides such as Ta2O5, HfO2, and TiO2 and store information by having their resistance change due to drift of oxygen ions (or oxygen vacancies) in the electric field. The vacancies form a narrow conducting filament between the metal electrodes. The low resistance state (logical "1") is corresponds to continuous filament while the high resistance is caused by the break in the filament. (logical "0" ) (figure below).

Devices - switching IVMost of device processing including
thin film deposition, lithography, etching,
ion milling, and passivation is done
in our new Nanofab facility.

Devices - 3d crosspoint

Selection devices

Memory arrays are usually arranged in the form of crossbar arrays with memory elements located at the intersections of "bit" lines and "word" lines. A particular bit is accessed by application of a voltage to one bit and one word line. In addition to one cell that is selected, there are many that are half selected (along biased word and bit lines). The READ errors can be prevented by adding a selection device in series with each memory element (figure on right). Skowronski's group investigates threshold switching devices based on oxides and chalcogenides for these applications.

3D XPoint memory announced by Intel in July 2015 and sold under the bard name Optane includes resistive switching devices and threshold switches.

Device simulation

Motion of ions in memory devices is thermally activated, devices to switch need to be heated, which in turn affects the current flow. To understand and properly design the switching devices we use finite element method to solve coupled differential equations describing heat and charge flow. The simulations predict such unusual phenomena as spontaneous current constriction within devices or negative differential resistance in part of device characteristics.

I-V characteristic and temperature distribution in TaOx threshold switching device.


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