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Impact of ultrathin Al2O3 interlayers on resistive switching in TiOx thin films deposited by atomic layer deposition
TIME:2017-6-15 8:33:00

TiOx and TiOx/Al2O3/TiOx sandwiched thin films were fabricated by using atomic layer deposition at 250 ¡ãC. As compared to single-layered TiOx, TiOx/Al2O3/TiOx sandwiched thin films exhibit stable resistive switching with a substantially improved OFF/ON ratio and elongated endurance. Essentially, the embedded ultrathin Al2O3 interlayer will suppress the crystallization of amorphous TiOx and thus reduce the channels for current leakage. As a result, the resistive switching properties are substantially enhanced. The TiOx/Al2O3/TiOx sandwiched thin films with the Al2O3 thickness of more than 2 nm show stable unipolar resistive switching, and the rupture and reformation of conductive filaments mainly occur at the top interface between Al2O3 and TiOx layers. A physical model is proposed to understand the resistive switching behaviors.


Although Si-based flash memory devices are still popular, they suffer from low read speed and high-power consumption required for the write operations. They will reach the physical limit in the near future if the memory density is increased continuously. To overcome the shortages of traditional nonvolatile memories, various memory concepts have been exploited.1,2 Great progress has been made in ferroelectric random access memory (FeRAM), magnetoresistive random access memory (MRAM), and resistive random access memory (RRAM), covering niche markets for special applications. However, FeRAM and MRAM exhibit technological and physical problems in the scalability.3,4 RRAM, which can be switched between two or more resistance states by controlling the voltage, has attracted extensive attention owing to its low operation voltage, high density integration, simple construction, and fast switching speed.5,6 Most importantly, it is compatible with conventional complementary metal-oxide-semiconductor technology.1,7¨C9 RRAM becomes one potential candidate for next-generation nonvolatile memories.10 The resistive switching (RS) effect has been observed in various transition metal oxides, such as TiO2,11,12 HfO2,13,14 ZnO,15 and Ta2O5,16 and perovskite oxides like SrZrO3 (Ref. 17) and SrTiO3.1 The formation and rupture of oxygen-vacancy-based filaments between the top and bottom electrodes result in resistive switching.18 However, the mechanism of RRAM is still not well understood, and there are still many problems to be resolved.
TiO2 is one of the most promising binary oxides for RRAM.19,20 Commonly, TiO2 is a typical n-type semiconductor due to the formation of oxygen vacancies (VO2+)18,21¨C24 and exhibits both unipolar resistive switching (URS) and bipolar resistive switching (BRS) behavior.25 RS is generally related to oxygen vacancies, and the transformation characteristics of RS depend on the motion of VO2+.24,26¨C28 Yang et al. stimulated the drift of oxygen vacancies by applying a voltage, and they found that the motion of VO2+ changed the electronic barrier at the Pt/TiO2 interface, resulting in bipolar resistive switching.18 Recently, in situ high-resolution transmission electron microscopy (HRTEM) confirmed that RS switching was related to the formation and disruption of TinO2n−1.27 So, TiO2 itself can exhibit prominent RS properties. However, it is extremely easy for TiO2 to be crystallized.29,30 The grain boundaries (GBs) will become the main channels of carriers,31,32 and the leakage current becomes severe. So, the high-resistance state (HRS) of TiOx based devices is not stable. Many researchers focused on the optimization of TiOx based RRAM cells and obtained much better performance. Hwang et al. prepared HfO2/TiO2/Ru thin films via atomic layer deposition (ALD) and found that the insertion of the HfO2 layer decreased the leakage current and improved the switching reliability significantly.33 Chen et al. modulated the Schottky barrier in the TiOx/Pt/TiOx thin films and observed the rectified behavior as well as good RS performance.34 Apparently, the RS performance might be improved by inserting an appropriate insulating layer.

In this article, the RRAM devices with a configuration of Ag/TiOx/Al2O3/TiOx/p-Si are fabricated, in which the TiOx/Al2O3/TiOx (T/A/T) sandwiched thin films are prepared by ALD. The microstructure and the resistive switching properties are studied. The results show that, upon inserting an ultrathin Al2O3 layer in TiOx thin films, the crystallization of amorphous TiOx can be substantially suppressed and the resistive switching properties can be improved significantly with a high resistance ratio and good uniformity. The physical mechanism is discussed.

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