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Resistive Switching Memory

Computer architecture is going to change in the coming decade because today’s architecture has severe limitations in energy efficiency and latency for memory access [1].The adoption of Flash in the memory hierarchy (albeit on a separate chip from the processor) inspired the exploration of computing architectures that capitalize on the salient features of Flash: non-volatility and high density. At the same time, new types of non-volatile memory have emerged that can easily be integrated on-chip with the microprocessor cores because they use a different set of materials and require different device fabrication technologies from Flash. Some of them can be programmed and read quickly; others can have very high data storage density. Importantly, all of these memories are free from the limitations of Flash (low endurance, need for high voltage supply, slow write speed and cumbersome erase procedure) and DRAM – both Flash and DRAM require high temperature processes for device fabrication and is thus difficult to integrate with logic monolithically in 3D. I will give an overview [2] of the “new” memory technologies that are being explored currently in industry and in academia: spin-transfer torque magnetic memory, resistive switching metal oxide memory [3], conductive bridge memory, phase change memory [4]. I will go over the fabrication process, essential device characteristics, and potential applications. To facilitate a connection with circuit designers, a compact model for RRAM has been developed and made available to the public. I will describe our efforts to explore device size scaling below 10 nm as well as 3D stacking of RRAM and the use of nanomaterials such as graphene in RRAM and PCM devices.Work supported in part by STARnet SONIC, IARPA, SRC/GRC, NSF, and member companies of the Stanford Non-Volatile Memory Technology Research Initiative (NMTRI) and Stanford SystemX Alliance.

References:
[1] M. M. Sabry Aly et al., IEEE Computer, p. 24 (2015)
[2] H.-S. P. Wong, S. Salahuddin, Nature Nanotechnology, p. 191 (2015)
[3] H.-S. P. Wong et al., Proc. IEEE, p. 1951 (2012)
[4] H.-S. P. Wong et al., Proc. IEEE, p. 2201 (2010)

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Probing Local Oxygen Deficiency and Resistance Switching Phenomena in Amorphous Titanium Oxide Films using Liquid Electrolyte Contacts

Replacing the metal top electrode of a RRAM device with a liquid electrolyte contact is a useful approach to study conductivity switching and its relation to local oxygen oxygen deficiency in the active layer. In this study, local variations in electrical conductivity in amorphous TiO₂ films prepared by atomic layer deposition (ALD) are examined using conductive atomic force microscopy. The local TiO₂ conductivity is correlated with changes in oxygen stoichiometry revealed by electron energy loss spectroscopy performed in the electron microscope and by oxygen isotope tracer exchange. Both highly resistive ionic liquids and highly conductive electrolyte solutions are used as liquid top contacts to the TiO₂ layers. In this way, the effects of field-driven electromigration of oxygen ions in and out of the TiO₂ film can be distinguished from local accumulation of oxygen vacancies via Joule heating at high programming currents. We find that gating using a resistive ionic liquid contact at room temperature and positive gate bias produces an oxygen deficient ALD-TiO₂ film with enhanced electronic conductivity, demonstrating the ease with which field-driven oxygen vacancy formation occurs in these thin amorphous layers. However, conductive filaments form only when the TiO₂ is programmed using a low-resistivity electrolyte top contact. This is consistent with the decisive importance of programming current in localization of the point defects required for filamentary conduction.

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Memory and Computing Systems Based on Resistive Switching Devices: Merging Electronics with Ionics

Exponential growth of the semiconductor industry, historically driven by transistor scaling, is now facing fundamental challenges. RRAM, with naturally merges electronics with ionics, offers potential for future memory and computing applications. I will discuss our efforts on the development and optimization of RRAM devices and integrated systems, including techniques of controlling the dynamic ionic migration processes and associated modeling efforts. Detailed TEM studies have been carried out to capture the ionic migration process and verify the switching mechanism. Functional high-density crossbar arrays have been integrated directly on top of CMOS circuits using a back-end-of-line (BEOL) process, enabling hybrid non-volatile memory and reconfigurable circuit applications. Properly tuned devices also exhibit incremental conductance changes that make them well suited for hardware-based, bio-inspired neuromorphic logic systems. Prototype neuromorphic circuits based on RRAM arrays have been shown to be able to perform tasks such as pattern recognition in an unsupervised fashion for intelligent sensing, analysis and other data-intensive applications.

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Hexagonal Boron Nitride Based RRAM

In this seminar I will present our recent progress on 2D (layered) insulators, with special emphasis on hexagonal boron nitride (h-BN). By combining device level and nanoscale experiments with theoretical modeling we observe that h-BN is much more reliable than HfO₂ as dielectric when subjected to electrical fields, which may be very useful to enlarge the lifetime of logic electronic devices. The dielectric breakdown process of h-BN doesn't follow the well-known percolation model established for 3D dielectrics because the speed of defect formation is anisotropic, due to the different atomic interactions in-plane (covalent) and out-of-plane (van der Waals). More interestingly, h-BN shows the exciting capability of tuning its electrical resistance when subjected to specific electrical stresses, a phenomenon called resistive switching (RS) that can be used to fabricate cutting-edge NVMs, namely resistive random access memories (RRAM). I will present an entire family of h-BN based RRAM devices with forming-free bipolar and threshold operation, low switching voltages down to 0.4V, high current on/off ratios up to 10⁶, long retention times above 10 hours and striking low variability. The switching mechanism, which is based on the simultaneous generation of B vacancies and penetration of metallic ions from adjacent electrodes, will be discussed. I will also show how to tune the properties of h-BN based RRAM devices by following three different approaches: i) h-BN thickness modification, ii) h-BN grain size modification and iii) graphene interfacial electrode insertion. I will make special emphasis on the use of scalable methods for the fabrication of the devices.

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Monolayer Non-volatile Devices

This talk will focus on emerging atomic materials, notably, non-volatile phenomena in the monolayer limit of layered materials including transitional metal dichalcogenides and hexagonal boron nitride. The unexpected observation defies conventional thinking of the thickness limit of non-volatile effect. Results will be presented to elucidate a mechanism how materials as thin as 0.33nm can afford non-volatile resistance switching of many orders of magnitude without suffering from noticeable leakage while providing good retention and endurance. The talk will conclude with perspective on pressing scientific questions and application development.