Elsevier

Solid-State Electronics

Volume 92, February 2014, Pages 24-27
Solid-State Electronics

Resistive switching in lateral junctions with nanometer separated electrodes

https://doi.org/10.1016/j.sse.2013.10.023Get rights and content

Abstract

Resistive switching in lateral tunnel junctions is reported. Nanogap tunnel junctions made of Au/SiO2/Au and Au/TiO2/Au were patterned by electrical-beam-lithography (EBL) and a controlled electromigration process. Depending on the substrate material, different reproducible resistive switching characteristics were observed under vacuum conditions. While for TiO2 substrates bipolar resistive switching was observed, nanogap junctions on SiO2 substrates showed resistive switching characteristics with a negative differential resistance. The role of the substrate material with respect to the resistive switching behavior is discussed in the framework of the electrical breakdown. All experiments were performed under vacuum to suppress parasitic effects due to charged particles in ambient air. Nanogap resistive switching devices are promising candidates for densely integrated memresistive systems such as non-volatile resistive random memories (RRAMs), field programmable arrays (FPGAs), or artificial neural networks (ANNs).

Introduction

Since the 1960s resistive switching phenomena in insulator materials have been studied intensely (for a review the reader is referred to Ref. [1]). In the last decade, resistance switching based nonvolatile storage devices revive considerable interest [2], [3], [4]. Due to their attractive features as low power consumption, fast write and read cycles, low fabrication costs, and scalability to the nanometer range, resistance switching devices are potential candidates for future nonvolatile random access memories [5], [6]. However, the implementation of resistance switching based devices in the market is hindered by several obstacles including reliability, retention, fatigue, and parameter spread. Interesting, even after an in depth study of resistive switching over the last decades the understanding of relevant effects for the resistive switching phenomena on the atomistic and mesoscopic scale is still under debate and various mechanisms have been proposed [7].

In order to explore resistive switching phenomena at the atomistic scale, lateral resistive switching junctions are of interest since those systems enable a direct access to the active part of the device for structural, electrical, and chemical characterization [8], [9], [10], [11]. In particular, to study different device properties at the atomistic scale nanometer separated electrodes (nanogaps) are fundamental building blocks [27]. Several publications reported resistive switching in lateral nanogap tunneling devices on SiO2 substrates [11], [12], [13], [14], [15], [16], [17]. Possible mechanisms explaining these observations are being debated [16]. Atomic migration on the two facing electrodes changing the gap width [12], or breakdown induced filaments through direct Si–Si bond formation [15], [16] may likewise induce resistive switching. At this respect, nanogap systems are used to imaging in situ the formation and evaluation of the conducting filament in a silicon oxide resistive switch [11] and indicated a structural based resistive switching in SiO2x systems.

Here we present a simple and robust strategy to enable a direct view to the active part of resistive switching devices at the atomistic scale. Therefore, we used the method of Ref. [17] to create lateral Au nanogap junctions. In particular, EBL was employed to define nanometer size bridges on oxide substrates and a subsequent controlled electromigration was used to define nanometer sized lateral junctions. Such nanogap junctions served as devices to study the contribution of the substrates material in resistive switching. By using SiO2 and TiO2 as substrate materials, qualitatively different kinds of resistive switching behaviors were observed after electroformation under vacuum conditions. While for junctions on SiO2 resistive switching characteristics with a negative differential resistance was observed according to Refs. [11], [15], nanogap junctions on TiO2 exhibited a bipolar resistive switching behavior.

Section snippets

Experimental

Lateral Au tunnel junctions on SiO2 and on TiO2, as shown in Fig. 1(a), were formed by a controlled electromigration procedure using the method of Ref. [17]. Therefore, 40 nm thick Au films were sputtered on Si substrates coated with a 400 nm thick thermally oxidize SiO2 film, as well as on a 10 nm thick thermally oxidize TiO2 film. Additionally, 2 nm Ti were used as adhesive layer for the Au/SiO2 material system. By referring to the work of Yao et al. we like to point out that the material

Results and discussion

Fig. 2 shows typical tunneling curves observed from Au nanogap junctions on a SiO2 substrate (Fig. 2(a)), as well as on a TiO2 substrate (Fig. 2(b)) right after the electromigration process. In particular, both junctions showed a typical tunneling I–V characteristic, with tunneling resistances of 16 kΩ and 217 kΩ at 100 mV for SiO2 and TiO2 substrates, respectively. The differences in tunneling characteristic in between the two substrate oxide may be attributed to different gap geometries

Conclusion

In conclusion, lateral tunneling junctions with nanometer-scale separated electrodes fabricated on thermally oxide substrates are fabricated. After electromigration those junctions showed resistive switching behavior under vacuum conditions. While on SiO2 films nanogap junctions showed a resistive switching behavior with a negative differential resistance, bipolar resistive switching was observed in nanogap tunneling junctions on TiO2 substrates. Moreover, the impact of the substrate oxide

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