Mutli-level switching in resistive memory devices enables a wide range of computational paradigms, including neuromorphic and cognitive computing. To this end, we have developed a bi-layer tantalum oxide based resistive random access memory device using Hf as the oxygen exchange layer. Multiple, discrete resistance levels were achieved by modulating the RESET pulse width and height, ranging from 2 kΩ to several MΩ. For a fixed pulse height, OFF state resistance was found to increase gradually with the increase in the pulse width, whereas for a fixed pulse width, the increase in the pulse height resulted in drastic changes in resistance. Resistive switching in these devices transitioned from Schottky emission in the OFF state to tunneling based conduction in the ON state, based on I-V curve fitting and temperature dependent current measurements. These devices also demonstrated endurance of more than 108 cycles with a satisfactory Roff/Ron ratio and retention greater than 104 s.

Hardware security has emerged as a field concerned with issues such as integrated circuit (IC) counterfeiting, cloning, piracy, and reverse engineering. Physical unclonable functions (PUF) are hardware security primitives useful for mitigating such issues by providing hardware-specific fingerprints based on intrinsic process variations within individual IC implementations. As technology scaling progresses further into the nanometer region, emerging nanoelectronic technologies, such as memristors or RRAMs (resistive random-access memory), have become interesting options for emerging computing systems. In this article, using a comprehensive temperature dependent model of an HfO x (hafnium-oxide) memristor, based on experimental measurements, we explore the best region of operation for a memristive crossbar PUF (XbarPUF). The design considered also employs XORing and a column shuffling technique to improve reliability and resilience to machine learning attacks. We present a detailed analysis for the noise margin and discuss the scalability of the XbarPUF structure. Finally, we present results for estimates of area, power, and delay alongside security performance metrics to analyze the strengths and weaknesses of the XbarPUF. Our XbarPUF exhibits nearly ideal (near 50%) uniqueness, bit-aliasing and uniformity, good reliability of 90% and up (with 100% being ideal), a very small footprint, and low average power consumption ≈104μW.

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In this work, we investigated the effect of the oxygen exchange layer (OEL) on the resistive switching properties of TaO x based memory cells. It was found that the forming voltage, SET-RESET voltage, R off , R on and retention properties are strongly correlated with the oxygen scavenging ability of the OEL, and the resulting oxygen vacancy formation ability of this layer. Higher forming voltage was observed for OELs having lower electronegativity/lower Gibbs free energy for oxide formation, and devices fabricated with these OELs exhibited an increased memory window, when using similar SET-RESET voltage range.

The recent surge of research on resistive random access memory (ReRAM) devices has resulted in a wealth of different materials and fabrication approaches. In this work, we describe the performance implications of utilizing a reactive ion etch (RIE) based process to fabricate HfO 2 based ReRAM devices, versus a more unconventional shadow mask fabrication approach. The work is the result of an effort to increase device yield and reduce individual device size. Our results show that choice of RIE etch gas (SF 6 versus CF 4 ) is critical for defining the post-etch device profile (cross-section), and for tuning the removal of metal layers used as bottom electrodes in the ReRAM device stack. We have shown that etch conditions leading to a tapered profile for the device stack cause poor electrical performance, likely due to metal re-deposition during etching, and damage to the switching layer. These devices exhibit nonlinear I-V during the low resistive state, but this could be improved to linear behavior once a near-vertical etch profile was achieved. Device stacks with vertical etch profiles also showed an increase in forming voltage, reduced switching variability and increased endurance.