Resistive Switching Characteristics of Si-Rich Oxides with Embedding Ti Nanodots

Resistive random access memories (ReRAMs) have been intensively studied as potential candidates for the next generation of memory devices due to their simplicity in structure.  Among various switching materials, Si-rich oxide (SiO_x) has been received much attention from the viewpoint of the good compatibility to the current Si-ULSI technology.  So far, we have studied resistance switching behaviors of SiO_x sandwiched with Pt and/or Ni electrodes caused by formation of conducting filament path presumably due to the oxygen deficiency [1, 2].  In addition, from local I-V measurements performed by contacting a single Ni-nanodots (NDs) as a top electrode with a Rh coated-AFM tip, we have confirmed the size scalability of SiO_x as a resistive switching material.  However, to realize low power switching operation, one of the major concerns is the control of the formation of the conductive filament paths in SiO_x. 

In this work, we have expected that NDs can trigger the formation of the conductive filament path in SiO_x, and evaluated an impact of NDs embedding into SiO_xon the switching characteristics.    

MIM diodes consisting of Ni electrodes and SiO_x in which Ti-NDs were embedded were fabricated on SiO₂/Si(100) by electron beam (EB) deposition and surface treatment using a remote plasma of pure H₂ (H₂-RP) as follows:  At first, a ~15 nm-thick Ni layer was formed as a bottom electrode and followed by the EB deposition of a ~1 nm-thick SiO_x layer.  And then, Ti-NDs were formed by a combination of a Ge(20 nm)/Ti(3 nm) stacked layer formation and subsequent H₂-RP exposure without external heating [3], where Ge layer was used as a barrier layer against oxidation of the ultrathin Ti layer.  During the H₂-RP exposure, H₂ gas pressure and VHF power of a 60 MHz generator were maintained at 24 Pa and 500 W, respectively.  After the H₂-RP exposure, a ~8 nm-thick SiO_x layer and then Ni top electrodes were formed  by EB deposition.   We also fabricated Ni/SiO_x/Ni-MIM diodes without NDs as references. 

AFM observations showed the formation of Ti-NDs with an areal density as high as 3.3×10¹¹cm^-2 was taken after H₂-RP exposure.  XPS analysis confirmed that the Ge layer was completely etched away by H₂-RP exposure and surface oxidation of Ti-NDs proceeds during air exposure. 

Resistive switching characteristics of SiO_x with and without NDs were measured from I-V curves in voltage sweep mode (Figs. 1(a) and (b)).  In both cases, a distinct unipolar type resistive-switching was repeatedly observed.  However, SET and RESET voltages were well separated by embedding NDs.  These results indicate that the NDs in SiO_x are likely to act as a trigger for the formation of conductive path in a reproducible fashion.  Also, the resistances in high and low resistance states (HRS and LRS) calculated from the current levels at -0.1 V were summarized as functions of the switching cycle (Fig. 2).    Obviously, for the diodes with NDs, a stable resistive switching was repeatedly observed in comparison with the diodes without NDs.  With embedding NDs, the variations in the resistance of both ON and OFF states became large markedly, and the ON/OFF ratio in resistance was increased from 45 to 150, which indicates that the Ti oxide formed at embedded Ti-NDs surface reduces the current levels through the SiO_x.

In summary, the embedding of Ti-based NDs in SiO_xis very effective to separate the operation voltages between SET and RESET voltages and to increase in the ON/OFF ratio in resistance.

Acknowledgements

This work was supported in part by Young Scientists (A) No. 15H05520 from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by JSPS Core-to-Core Program of International Collaborative Research Center on Atomically Controlled Processing for Ultra large Scale Integration.

References

1) A. Ohta, et al, Jpn. J. Appl. Phys., 52(2012) 06FF02.

2) A. Ohta, et al, IEICE TRANSACTIONS on Electronics, E96-C(2013) 702.

3) K. Makihara, et. al, J. Optoelectronics and Adv. Materials, 12 (2010) 626.