Complementary metal-oxide-semiconductor technology (CMOS) has allowed following Moore’s law in the last four decades. However, scaling is near to some challenging issues, like heat dissipation and mobility limitations [1]. To further increase the computing capability of logic circuits, alternative approaches are being explored, such as spin-based logic gates, [2] magnetic domain-walls, [3] molecular devices [4] and memristors. In memristors the resistance R can be tuned in response to an applied electrical pulse. Phase change memory (PCM) can be included in this category, thanks to the large number of states with different R and threshold voltage Vt that can be obtained by changing the crystalline fraction fx of the active layer. PCM based memristor is certainly a good candidate for logic application, thanks to its fast switching [5], low programming current [6] and large R window.

In this work, we demonstrate boolean logic operations with PCM memristors as logic gates [7]. To achieve this result, we exploited two fundamental properties of the chalcogenide material Ge2Sb2Te5 (GST), namely conditional switching and additive crystallization. The first property refers to the amorphous GST, displaying threshold switching at threshold voltage Vt. Above Vt, the amorphous phase is highly conductive thanks to the strong electronic and thermal excitation of carriers [8]. Threshold switching allows to compare an externally applied voltage V to Vt, i.e., for V < Vt no threshold switching takes place and the cell remains in its previous state, while for V > Vt the large current flowing in the amorphous phase induces crystallization. Since the Vt is dictated by the state (i.e. the resistance) of the memristor, it is possible to conditionally switch the cell depending on its initial state.

The second property we exploited is additive crystallization, which means that the same memristive state can be obtained with a single or multiple pulses, provided that the total pulse duration is the same. By means of this two functionalities, we were able to perform a logically complete set of boolean operation, such as negation (NOT), negated sum (NOR) and negated product (NAND). In addition, memristive logic offers totally new possibility for logic circuits. First, the non-volatile nature of PCM enables logic-in-memory application. Second, while CMOS logic relies on the gate topology to yield a certain functionality, PCM logic is simply based on memristive properties. This allows to develop reconfigurable circuits where each PCM can provide any logic functionality depending on the initial configuration.

[1] ITRS 2011 release

[2] A. A. Khajetoorians , et al., Science, 332 (2011)

[3] D. A. Allwood, et al., Science, 309 ( 2005)

[4] W. Liang , et al., Nature, 417 ( 2002)

[5] D. Loke, et al., Science 336, 1566 (2012)

[6] F. Xiong, et al., Science 332, 568 (2011)

[7] M. Cassinerio, et al., Adv. Mat, DOI: 10.1002 (2013)[8] D. Ielmini, Phys. Rev. B 78, 035308 (2008)

Nicola Ciocchini

Politecnico di Milano

Appearances