Oxide-based resistive switches are being extensively studied for their potential to close the latency gap between DRAM and FLASH in the computer memory hierarchy. Strontium titanate (SrTiO₃) has been a model material to investigate the switching mechanism in these emerging non-volatile memories [1]. The advantage of SrTiO₃ is its well-known defect chemistry at higher temperatures and various reduction states. Despite the promising transport of defects at high electric field strengths, microstructural changes affecting defect states upon growth of multilayers still remain unclear. The formation and modulation of the Schottky barrier at the SrTiO₃–metal contacts have been extensively studied [2]-[5]. However in thin films there are several other mechanisms creating an asymmetry in the device that go beyond the Schottky barrier model. Through the thin film growth as a stack of various metal-oxide interfaces an asymmetry in defect distribution is introduced on the one hand, but also an environmental asymmetry in the "buried" and "open" electrode interfaces exist on the other hand. These asymmetries transfer into asymmetric I-V curves and switching properties. This of course will play an important role also for memory bit stacking to increase the memory density.

Here we create a model experiment to investigate the asymmetries due to the environment and different electrodes on a 50 nm thick nominally undoped SrTiO₃ thin films. For this, we fabricated two model resistive switches between metallic electrodes (LaNiO₃ and Pt) differing in their work functions and implication on defects formed at the interfaces. We grow the switching oxide either on top of LaNiO₃ or on platinum that form the bottom electrode and fabricate the top electrode from the other material respectively.

The I-V curves of the model resistive devices are in general asymmetric in the positive and negative bias direction due to the above-mentioned asymmetries.

Firstly, we observe independently on the choice of the bottom electrode resistive switching behavior with the same counter-clockwise switching polarity. We link the switching polarity to the microstructural defect density at the interface of SrTiO₃ and the top electrode, which we investigate by transmission electron microscopy.

Secondly, we observe a highly rectifying asymmetric I-V behavior with high current levels in the positive bias direction for both model samples. The similar asymmetry in the I-V characteristics we link to the environmental influence. To investigate the humidity influence on the two model devices further, we perform I-V measurements at 40% and 5% relative humidity. We report, that the transport characteristics are consistent with hydrogen forming a shallow donor level in the switching material in accordance to Ref. [7]. This leads to a lower conduction, higher switching voltage and a larger hysteresis opening of the I-V switching curve under lower humidity levels.

Through the model experiment we contribute to an understanding on the role of defects at interfaces and how to explain humidity influence for SrTiO₃ resistive switching units. These findings have large implications for the fabrication of 3D staked multi-bit memories to increase the computer memory storage density.

References

[1] K. Szot, W. Speier, G. Bihlmayer, and R. Waser, “Switching the electrical resistance of individual dislocations in single-crystalline SrTiO₃,” Nature Materials, 5 (2006), 312320.

[2] J. Robertson, “Band offsets of wide-band-gap oxides and implications for future electronic devices,” Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 18 (2000), 1785.

[3] E. Mikheev, B. D. Hoskins, D. B. Strukov, and S. Stemmer, “Resistive switching and its suppression in Pt/Nb:SrTiO₃ junctions,” Nat Comms, 5 (2014).

[4] F. Messerschmitt, M. Kubicek, S. Schweiger, and J. L. Rupp, “Memristor kinetics and diffusion characteristics for mixed anionic-electronic SrTiO_3-δ bits: The memristor-based Cottrell analysis connecting material to device performance,” Advanced Functional Materials, 24 (2014), 74487460.

[5] M. Kubicek, R. Schmitt, F. Messerschmitt, and J. L. M. Rupp, “Uncovering two competing switching mechanisms for epitaxial and ultrathin Strontium Titanate-based resistive switching bits,” ACS Nano, 9 (2015), 1073710748.

[6] F. Messerschmitt, M. Kubicek, and J. L. M. Rupp, “How does moisture affect the physical property of memristance for anionic-electronic resistive switching memories?” Advanced Functional Materials, 25 (2015), 51175125.

[7] K. Xiong, J. Robertson, S.J. Clark, "Behavior of hydrogen in wide band gap oxides," Journal of Applied Physics, 102 (2007), 083710.