The role of nitrogen doping on the stability and memory window of resistive state switching in N-doped Ta2O5 deposited by atomic layer deposition is elucidated. Nitrogen incorporation increases the stability of resistive memory states which is attributed to neutralization of electronic defect levels associated with oxygen vacancies. The density functional simulations with the screened exchange hybrid functional approximation show that the incorporation of nitrogen dopant atoms in the oxide network removes the O vacancy midgap defect states, thus nullifying excess defects and eliminating alternative conductive paths. By effectively reducing the density of vacancy-induced defect states through N doping, 3-bit multilevel cell switching is demonstrated, consisting of eight distinctive resistive memory states achieved by either controlling the set current compliance or the maximum voltage during reset. Nitrogen doping has a threefold effect: widening the switching memory window to accommodate the more intermediate states, improving the stability of states, and providing a gradual reset for multi-level cell switching during reset. The N-doped Ta2O5 devices have relatively small set and reset voltages (< 1 V) with reduced variability due to doping.

The effect of fluorine doping on the switching stability of Ta2O5 resistive random access memory devices is investigated. It shows that the dopant serves to increase the memory window and improve the stability of the resistive states due to the neutralization of oxygen vacancies. The ability to alter the current in the low resistance state with set current compliance coupled with large memory window makes multilevel cell switching more favorable. The devices have set and reset voltages of <1 V with improved stability due to the fluorine doping. Density functional modeling shows that the incorporation of fluorine dopant atoms at the two-fold O vacancy site in the oxide network removes the defect state in the mid bandgap, lowering the overall density of defects capable of forming conductive filaments. This reduces the probability of forming alternative conducting paths and hence improves the current stability in the low resistance states. The doped devices exhibit more stable resistive states in both dc and pulsed set and reset cycles. The retention failure time is estimated to be a minimum of 2 years for F-doped devices measured by temperature accelerated and stress voltage accelerated retention failure methods.

SummaryTemporal dynamics of soil nematode community structure at the depth of 0 -30 cm was compared under invasive Ambrosia trifida and native Chenopodium serotinum in an abandoned cropland in Northeast China. The results showed the difference of nematode taxa and dominant genera under A. trifida and C. serotinum during the study period. Acrobeloides and Paratylenchus were found to be dominant genera under both A. trifida and C. serotinum. Helicotylenchus prevailed in soil with C. serotinum, while Macroposthonia was dominant in soil with A. trifida. Nematode taxa was higher under A. trifida than under C. serotinum from June to September. Except in the July, significantly higher numbers of plant-parasites were observed under A. trifida than under C. serotinum during the study period (P < 0.05). Nematode taxa, Simpson index and structure index were found to be sensitive indicators that detected nematode community structural differences under A. trifida and C. serotinum during the study period.

We present a study of room-temperature, ultrafast photoconductivity associated with a strong, subbandgap, resonant absorption around k ¼ 1550 nm in three MBE-grown GaAs epitaxial layers heavily doped with Er at concentrations of %2.9 Â 10 18 (control sample), 4.4 Â 10 20 , and 8.8 Â 10 20 cm À3 , respectively. Transmission-electron microscopy reveals lack of nanoparticles in the control sample, but abundant in the other two samples in the 1.0-to-3.0-nm-diameter range, which is consistent with the previously known results. We measure very high photoelectron (Hall) mobility (2.57 Â 10 3 cm 2 /V-s) and terahertz power (46 lW average) in the 4.4 Â 10 20 sample, but then, an abrupt decay in these properties as well as the dark resistivity is seen as the Er doping is increased just 2 times. The Er doping has little effect on the picosecond-scale, 1550-nm photocarrier lifetime.

This paper reports broadband THz free-space transmission measurements and modeling of indium-tin-oxide (ITO) thin films on p-doped Si substrates. Two such samples having ITO thickness of 50 and 100 nm, and DC sheet conductance 260 and 56 Ω/sq, respectively, were characterized between 0.2 and 1.2 THz using a frequency-domain spectrometer. The 50-nm-film sample displayed very flat transmittance over the 1-THz bandwidth, suggesting it is close to the critical THz sheet conductance that suppresses multi-pass interference in the substrate. An accurate transmission-line-based equivalent circuit is developed to explain the effect, and then used to show that the net reflectivity and absorptivity necessarily oscillate with frequency. This has important implications for the use of thin-film metallic coupling layers on THz components and devices, such as detectors and sources. Consistent with previous reported results, the sheet conductance that best fits the THz transmittance data is roughly 50% higher than the DC values for both samples.