Resistive random access memory (RRAM) devices with analog resistive switching are expected to be beneficial for neuromorphic applications, and consecutive voltage sweeps or pulses can be applied to change the device conductance and behave synaptic characteristics. In this paper, RRAM devices with a reverse stacking order of 6-nm-thick HfO x and 2-nm-thick AlO x dielectric films were fabricated. The device with TiN/Ti/AlO x /HfO x /TiN stacked layers exhibited digital resistive switching, while the other device with TiN/Ti/HfO x /AlO x /TiN stacked layers could demonstrate synaptic characteristics that were analog set and reset processes under consecutive positive and negative voltage sweeps or a train of potentiation and depression pulses. Moreover, this device could also implement synaptic learning rules, spike-timing-dependent plasticity (STDP). Varying temperature measurements and linear fittings of the measured data were conducted to analyze current conduction mechanisms. As a result, the variation of resistive switching behavior between these two devices is attributed to the varying effectiveness of the oxygen scavenging ability of the Ti layer when put into contact with either AlO x or HfO x . Moreover, AlO x functioned as a diffusion limiting layer (DLL) in the device with TiN/Ti/HfO x /AlO x /TiN stacked layers, and gradual modulation of the production and annihilation of oxygen vacancies is the cause of synaptic characteristics.INDEX TERMS Resistive random access memory (RRAM), bilayered dielectric films, synaptic characteristics, diffusion limiting layer (DLL), conductive filament (CF), spike-timing-dependent plasticity (STDP).

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Post-metal annealing (PMA) has been adopted to reduce the operation voltages of HfOx-based resistive random access memory (RRAM) devices, especially the forming voltage (VForming). TiN/Ti/HfOx/TiN stack structures were fabricated and annealed via rapid thermal annealing (RTA) and furnace annealing (FA) to investigate the annealing effects. The result of X-ray photoelectron spectroscopy (XPS) analysis indicates that the distribution of oxygen towards the interposing Ti layer increases after the annealing process, which facilitates the formation of conductive filaments in the dielectric layer. As a result, VForming can be decreased from 4.60 to 3.24 and 3.36 V via RTA for 30 s at 400 °C and via FA for 60 min at 300 °C, respectively, as compared with that without PMA. However, the VForming of the device annealed via FA for 60 min at 400 °C was higher than that at 300 °C. This turn-around phenomenon of the forming voltages of RRAM devices annealed via FA was found. It was attributed to the conversion of the interposing Ti layer into a highly resistive TiO2 film.

Ecological and evolutionary processes linking adaptation to environment are related to species’ range shifts. In this study, we employed amplified-fragment-length-polymorphism-based genome scan methods to identify candidate loci among Zingiber kawagoii populations inhabiting varying environments distributed at low to middle elevations (143–1488 m) in a narrow latitudinal range (between 21.90 and 25.30° N). Here, we show evidence of selection driving the divergence of Z. kawagoii. Twenty-six FST outliers were detected, which were significantly correlated with various environmental variables. The allele frequencies of nine FST outliers were either positively or negatively correlated with the population mean FST. Using several independent approaches, we found environmental variables act in a combinatorial fashion, best explaining outlier genetic variation. Nonetheless, we found that adaptive divergence was affected mostly by annual temperature range, and it is significantly positively correlated with latitude and significantly negatively correlated with the population mean FST. This study addresses a latitudinal pattern of changes in annual temperature range (which ranged from 13.8 °C in the Lanyu population to 18.5 °C in the Wulai population) and emphasizes the pattern of latitudinal population divergence closely linked to the allele frequencies of adaptive loci, acting in a narrow latitudinal range. Our results also indicate environmentally dependent local adaptation for both leading- and trailing-edge populations.

The testing association of environmental variables with genetic and epigenetic variation could be crucial to deciphering the effects of environmental factors playing roles as selective drivers in ecological speciation. Although ecological speciation may occur in closely related species, species boundaries may not be established over a short evolutionary timescale. Here, we investigated the genetic and epigenetic variations using amplified fragment length polymorphism (AFLP) and methylation-sensitive amplification polymorphism (MSAP), respectively, and tested their associations with environmental variables in populations of four closely related species in the R. pseudochrysanthum complex. No distinctive species relationships were found using genetic clustering analyses, neighbor-joining tree, and neighbor-net tree based on the total AFLP variation, which is suggestive of the incomplete lineage sorting of ancestral variation. Nonetheless, strong isolation-by-environment and adaptive divergence were revealed, despite the significant isolation-by-distance. Annual mean temperature, elevation, normalized difference vegetation index, and annual total potential evapotranspiration were found to be the most important environmental variables explaining outlier genetic and epigenetic variations. Our results suggest that the four closely related species of the R. pseudochrysanthum complex share the polymorphism of their ancestor, but reproductive isolation due to ecological speciation can occur if local environmental divergence persists over time.