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Atomic Layer Deposition of Metal Oxide Nanolaminates for Nonvolatile Resistive Switching Memory  Device (MOX-SWITCH)

Transition metal oxide-based resistance-switching RAM (ReRAM) has attracted considerable attention as a potential candidate for next generation NV-RAM technology because of its low operation power, fast read and write access, high retention and a quite simple capacitor-like metal-oxide-metal (MOM) device structures. Atomic layer deposition (ALD) offers nearly pinhole free, conformal metal oxide thin films with good thickness control which are required for next-generation memory devices. Here ALD of VOx and TaOx thin films followed by their post-growth treatments is studied for potential applications in resistive switching devices. It is shown that  MOM capacitor structures fabricated from amorphous VOx films showed I-V characteristics interesting for the RS applications. [1,2,3]

Cooperation Model:This collaborative project integrates material synthesis, device fabrication and function testing. The precursors of different compositions and forms for the aimed metal oxide thin films have been synthesized and deposited using an ALD reactor (BENEQ TFS 200) at the University of Cologne (UoC) (Fig. 1a). The further device construction and its RS electrical measurements are done in Forschungszentrum Jülich (FZJ). Nanomaterial and device characterizations have been performed either at the UoC or FZJ. An interdisciplinary platform for integration of nanomaterials into functional devices has been made possible, in which UoC alone is currently not able to achieve without the expertise of  the FZJ.

Progress and Outcome: As grown VOx films are amorphous and possess smooth surfaces with an average roughness of 0.17 nm (Fig.1b). The RS device is composed of a bottom electrode of Pt(30 nm)/Ti(5 nm), a 15 nm amorphous VOx thin film and a top electrode Ti(5 nm)/Pt(25 nm) (Fig 1c). Depending on the electroforming conditions, bipolar-type memory switching behavior with a resistance ratio ROFF/RON > 103 is obtained (Fig. 1d). Positive voltage driven forming of about 1.3 V brings the devices into a low resistance state (LRS), which can be reset to a high resistance state (HRS) by applying a negative voltage. After a higher electroforming voltage of 4-5 V, a combination of bipolar-type memory and threshold switching is observed, with a stable endurance in 100 cycles (Fig. 1e, f). The threshold switching is attractive for its highly non-linear I-V characteristic and it is attributed to the temperature-induced insulator to metal transition (IMT) in vanadium dioxide.

Fig.1 Amorphous VOx thin film and its based RS devices behave memory-type bipolar switching as well as threshold switching.

In order to overcome the problem of surface roughening of the crystalline VOx film after post-depostion annealing (Fig. 2a) and to control selectively the oxidation states, an amorphous VOx layer (a-VOx) has been deposited over a crystalline VOx layer (c-VOx), forming a homogeneous VOx bilayer structured based RS device (Fig 2b). Various annealing conditions have been studied, and the bilayer-VOx thin film surfaces show much improved roughness with a mean value of about 0.19 nm (Fig. 2c, d, e). Extremely low switching currents of less than 1 µA are observed in these devices, most probably due to electrochemical and redox based reactions within VOx films.


 Fig. 2 RS device which is based on a crystalline VOx layer covered by an amorphous VOx layer.

Tantalum oxide Ta2O5 is considered to be one of the most promising switching materials. Synthesized [Ta(OiPr)5] has been successfully deposited into homogeneous Ta2O5 thin films by ALD.

Publication: T. Singh, S. Wang, N. Aslam, H. Zhang, S. Hoffmann-Eifert, S. Mathur, Chem. Vap. Deposition, 2014, 20, 291–297

 [1] R. Waser, R. Dittmann, G. Staikov, K. Szot, Adv. Mater. 2009, 21, 2632–2663.

[2] B. J. Choi, D. S. Jeong, S. K. Kim, C. Rohde, S. Choi, J. H. Oh, H. J. Kim, C. S. Hwang, K. Szot, R. Waser, B. Reichenberg, S. Tiedke, J. Appl. Phys. 2005, 98, 033715.

[3] M. D. Pickett, R. S. Williams. Nanotechnology, 2012, 23, 215202.