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Inspired by the recently predicted 2D MX2Y6 (M = metal element; X = Si/Ge/Sn; Y = S/Se/Te), we explore the possible applications of alkaline earth metal (using magnesium as example) in this family based on the idea of element replacement and valence electron balance. Herein, we report a new family of 2D quaternary compounds, namely MgMX2Y6 (M = Ti/Zr/Hf; X = Si/Ge; Y = S/Se/Te) monolayers, with superior kinetic, thermodynamic and mechanical stability. In addition, our results indicate that MgMX2Y6 monolayers are all indirect band gap semiconductors with band gap values ranging from 0.870 to 2.500 eV. Moreover, the band edges and optical properties of 2D MgMX2Y6 are suitable for constructing multifunctional optoelectronic devices. Furthermore, for comparison, the mechanical, electronic and optical properties of In2X2Y6 monolayers have been discussed in detail. The success of introducing Mg into the 2D MX2Y6 family indicates that more potential materials, such as Ca- and Sr-based 2D MX2Y6 monolayers, may be discovered in the future. Therefore, this work not only broadens the existing family of 2D semiconductors, but it also provides beneficial results for the future.

The memristor study now generally exhibit threshold voltage characteristics. The memristance changes when the voltage across the memristor is greater than the threshold voltage. Otherwise, the memristance is almost constant. Based on this feature, we design a general-purpose memristor programmable circuit that is simple to operate. In the field of communication and signal processing, programmable analog filters are required, and memristors with multi-valued characteristic are suitable as programmable impedance elements for such circuits. Through simulation and physics experiments, we demonstrate a memristor-based programmable low-pass filter using the designed programmable circuit to realize the dynamic adjustment of circuit parameters.

The memristor is a promising candidate for the next generation non-volatile memory, especially based on HfO2−x, given its compatibility with advanced CMOS technologies. Although various resistive transitions were reported independently, customized binary and multi-level memristors in unified HfO2−x material have not been studied. Here we report Pt/HfO2−x/Ti memristors with double memristive modes, forming-free and low operation voltage, which were tuned by oxidation conditions of HfO2−x films. As O/Hf ratios of HfO2−x films increase, the forming voltages, SET voltages, and Roff/Ron windows increase regularly while their resistive transitions undergo from gradually to sharply in I/V sweep. Two memristors with typical resistive transitions were studied to customize binary and multi-level memristive modes, respectively. For binary mode, high-speed switching with 103 pulses (10 ns) and retention test at 85 °C (>104 s) were achieved. For multi-level mode, the 12-levels stable resistance states were confirmed by ongoing multi-window switching (ranging from 10 ns to 1 μs and completing 10 cycles of each pulse). Our customized binary and multi-level HfO2−x-based memristors show high-speed switching, multi-level storage and excellent stability, which can be separately applied to logic computing and neuromorphic computing, further suitable for in-memory computing chip when deposition atmosphere may be fine-tuned.

In this work we combine experimental data and first principles calculations to investigate the conductive filament (CF) creation in HfO2 based resistive switching memories. First, we propose that the CF in HfO2-based resistive OxRAMs is due to HfOx sub-oxides, possibly assuming a tetragonal symmetry with x≤1.5. Second, we demonstrate that Frenkel-Pair formation is favored by electron injection and by the adoption of oxygen reactive electrodes with low work function, such as Ti. Finally, endurance tests on a memory cell with Ti as the top electrode were performed to assess its cycling stability. Endurance up to 108 cycles with optimized set conditions has been demonstrated.

Oxide based RRAM memories are considered as potential candidates to replace floating gate memory devices [1-2] due to their proven low power operation , high switching speed [3] and compatibility with conventional CMOS back-end-of-line process. The working principle relies on the reversible change of the oxide layer’s resistance between two levels namely HRS (High Resistance State) and LRS (Low Resistance State). This resistance change has been widely attributed to the creation/reconstruction and rupture of a localized or multiple conductive filaments (CF) in the oxide layer. Various companies [4] and research laboratories [5] have adopted the HfOx solution as the active oxide layer that casts the memory effect. However, the process of the CF creation responsible for resistive switching and the oxide layer degradation are yet to be fully explored. In this paper we use first principles calculations to investigate Frenkel-Pair formation (FP) in HfO2 that play an important role in the CF creation. Depending on the electrode nature, its work function and reactivity with oxygen, we bring more insight into the FP formation leading to CF creation and device reliability. We show that oxygen active electrodes with low work function, like Ti, improve device reliability. Finally, endurance tests on a memory cell with Ti as top electrode were performed to assess cycling stability. Endurance up to 108 cycles, with optimized set conditions has been demonstrated. [1] H. Akinaga and H. Shima, Proc. IEEE, vol. 98, p. 2237, 2010.[2] V. Sriraman et al., IPCSIT, vol. 32, p.101, 2012 [3] B. Govoreanu et al., IEDM Tech. Dig., pp. 729-732, 2011 [4] Ch. Walczyk et al., J. Vac. Sci. Technol. B 29, 01AD02 (2011) [5] Y.S.Chen et al., IEDM Tech. Dig,pp. 31.3.1 - 31.3.4,2011.