MORE

Scientific background

ReRAMs represent a large class of emerging non-volatile memory concepts based on a resistor as a memory element that can be programmed in a high and low conductive state, using electrical pulses. ReRAMs could encompass the limitation of the current technologies and target the implementation of 3D high-density and low power memory arrays for mass storage beyond the 20 nm technology node [1-4]. In order to fulfil this goal, challenging material issues still have to be solved, and a better understanding/control of the switching mechanisms down to the nano-scale level must be achieved.

Oxide-based ReRAMs devices exploit the functionality of Metal/transition metal binary Oxides/Metal (MOM) heterostructures, where the binary oxide is sandwiched between two metal electrodes [1-4]. After an initial electro-forming step, which induces a current-limited electric breakdown in the pristine sample, the memory element can be reversibly switched between a low resistance set (ON) state and a high resistance reset (OFF) state. The MOM-based ReRAM system is attractive since it is very simple, low cost, low power and offers high scalability potential to achieve high-density memory array.

Despite the encouraging data reported in the literature, several aspects such as switching mechanisms, role of electrodes and interfaces, and a proper correlation between electrical and physical properties, are still under investigation. Indeed, the selection of materials for the MOM heterostructure memory element represents a challenging task, since it is necessary:

to optimize the electrical properties (in terms of programming currents and voltages, resistance windows, endurance and reliability), by screening different materials for the heterostructures
to obtain a better understanding/control of the switching mechanisms down to the nano-scale level
to demonstrate the scalability of the proposed concept, also fabricating nanoscale heterostructures exhibiting controlled switching properties.

The MORE project will focus on the synthesis and characterization of thin film- and of nanowire MOM heterostructures with emphasis on material selection, interfaces of the heterostructure, control of switching parameters (programming currents and voltages, resistance window) and switching mechanisms, down to the nanoscale level. Moreover, so far, only few reports deal with resistive switching effects in one dimensional (1D) structures, such as nanowires and nanowires heterostructures [5,6],which could constitute the building blocks of future high-density architectures, and could also represent a powerful method to address switching properties at the nanoscale levels.

Ordered arrays of metal-oxides and MOM heterostructures can be fabricated using as a template a porous anodic aluminium oxide (AAO) membrane [5], combined with electrodeposition and selective oxidation into the nanopores [5,7]. Since the AAO membrane [8] have hexagonally ordered porous structures with pore diameters from 10 to 200 nm and pore density ranging from 1010 to 1012 cm-2, they are suitable for investigating high-density and scalability of resistive elements. Despite the potentialities of this approach for the fabrication of 1D-MOM heterostructures and the increasing interest in this subject, up to date only a few and recent studies [5,9] have been published especially in the field of ReRAM. Moreover, the latter studies focuses more on the characterization of single nanowire after dissolution of the alumina template. The idea of the MORE project is to characterize large array of MOM nanowires embedded in a matrix (as in real application) and to address the resistive switching properties of each of them.

References

[1] R. Waser, R. Dittmann, G. Staikov, K. Szot, Adv. Mater. 21, 2632 (2009); R. Waser et al., Nature Materials 6, 833 (2007)
[2] S.-E Ahn et al., Adv. Mater. 20, 924 (2008)
[3] A. Sawa, Materials Today 11(6), 28 (2008)
[4] S. Spiga et al., Microelectr. Eng. 85, 2414 (2008)
[5] I. Kim et al., Appl. Phys. Lett. 93, 033503 (2008)
[6] M.-J. Lee et al., Nano Letters 9, 1476 (2009)
[7] X. Zhao et al., Appl. Phys.Lett. 93, 152107 (2008)
[8] V.Stasi et al., Photonics and Nanostructures - Fundamentals and Applications 5 ,136-139 (2007)
[9] E. D. Herderick and al., Appl. Phys. Lett. 95, 203505 (2009)