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The strain effect on the hole mobility is investigated by bulk Si field-effect transistor, substrate-strained Si devices. The effective hole mass, which increases at small strain, then decreases at large strain, is responsible for the abnormal hole mobility behavior.

Charge trapping characteristics of high-relative permittivity (high-?) HfO2films with Al2O3as a blocking oxide in p-Si/SiO2/HfO2/Al2O3/metal memory structures have been investigated. All high-? films have been grown by atomic layer deposition. A transmission electron microscope image shows that the HfO2film is polycrystalline, while the Al2O3film is partially crystalline after a high temperature annealing treatment at 1000 °C for 10 s in N2ambient. A well-behaved counter-clockwise capacitance-voltage hysteresis has been observed for all memory capacitors. A large memory window of ?7.4 V and a high charge trapping density of ?1.1 × 1013cm?2have been observed for high-? HfO2charge trapping memory capacitors. The memory window and charge trapping density can be increased with increasing thickness of the HfO2film. The charge loss can be decreased using a thick trapping layer or thick tunnelling oxide. A high work function metal gate electrode shows low charge loss and large memory window after 10 years of retention. High-? HfO2memory devices with high-? Al2O3as a blocking oxide and a high work function metal gate can be used in future high-density non-volatile memory device applications. [ABSTRACT FROM AUTHOR]

Characteristics of strained-germanium (Ge) p- and n-channel field effect transistors directly on Si (1?1?1) substrates have been investigated. A strained-Ge layer with a thickness of ?4 nm has been grown on the relaxed Si/Si (1?1?1) substrate by ultra-high-vacuum chemical vapour deposition. To improve the oxide/strained-Ge interface, a thin Si-cap layer with a thickness of 3 nm has been grown on the strained-Ge layer. After the device process, 1 nm thickness of Si-cap layer remains on the strained-Ge layer. Thicknesses of all epitaxial layers have been measured by transmission electron microscopy. Raman spectroscopy measurement on the Si-cap/strained-Ge layer shows that the strained-Ge layer has a compressive strain of ?1.25%. A hole confinement shoulder on the capacitance-voltage curve at the accumulation region has been observed due to carrier confinement at the Si-cap/strained-Ge hetero-interface. A metal-oxide-semiconductor (MOS) structure on the strained-Ge layer shows a moderate interface trap charge density of ?2.8 × 1011cm?2eV?1. Strained-Ge p- and n-channel field effect transistors show low off-state leakage currents of ?3.8 × 10?13A µm?1and ?6.5 × 10?13A µm?1, respectively. Drive currents of strained-Ge p- and n-channel field effect transistors are enhanced by ?100% and ?40%, respectively, as compared with bulk Si (1?1?1) transistors. Peak hole and electron mobility of strained-Ge (1?1?1) field effect transistors at the low effective field are found to be ?110% and ?30% enhancement, respectively, as compared with bulk Si (1?1?1) transistors, due to high hole and electron mobility enhancement factor as well as strain-induced lower conduction mass in the strained-Ge channel. [ABSTRACT FROM AUTHOR]

Abstract  Very exciting and promising results from recent developments in group-IV alloy heterostructures (viz., SiGe, SiGeC, SiC, GeC and strained-Si) have led to the belief that SiGe-based devices will open up an entirely new dimension to the future of VLSI/ULSI technology. The growth of ultrathin dielectric films on a strained group-IV alloy layer is a challenging task. As metal-oxide-semiconductor devices are being aggressively scaled down, high permittivity dielectrics are being widely investigated as alternative gate insulating layers in advanced MOS devices. The present paper reviews the recent results of different gate and high-k dielectrics on group-IV alloy layers for scaled CMOS devices, high-mobility pure-Ge channel devices and nanocrystal floating gate memories. [ABSTRACT FROM AUTHOR]

Ultra-thin (<10 nm) gate oxides have been grown directly on tensile-strained Si0.993 C0.007 layers at a low temperature using microwave O2-plasma. The changes in gate voltage (ΔVg), flat-band voltage (VFB), oxide charge density (Qox/q) an interface state density (Dit) have been studied using a metal-oxide-semiconductor structure over the temperature range of 77–450 K. Inversion capacitance increases with temperature above 400 K, leading to a transition from high-frequency to low-frequency characteristics. The dominant types of charges in the oxide are found to be strongly temperature dependent. It is found that charge-trapping properties under Fowler–Nordheim (F–N) constant-current stressing are significantly improved with increasing temperature. [ABSTRACT FROM AUTHOR]

The charge storage characteristics of the high-k HfAlO nanocrystal memory capacitors prepared by atomic layer deposition in an n-Si/SiO2/HfAlO/Al2O3/Pt structure have been investigated after high-temperature annealing treatment. The high-resolution transmission electron microscopy image shows that the diameter of high-k HfAlO nanocrystal is<2 nm. The high-k HfAlO nanocrystals have been also confirmed by x-ray photoelectron spectroscopy measurement. Due to the formation of high-k HfAlO nanocrystals with the high-temperature ([?]900 °C) annealing treatment, a large hysteresis memory window of 3.7 V at a sweeping gate voltage <10 V is observed as compared to that of the as-deposited memory capacitors. A hysteresis memory window of [?]1.7 V with a small sweeping gate voltage of ± 5 V is also observed. A small equivalent oxide thickness (EOT) of 5.6 nm is obtained due to the high-k memory structure design. A significant memory window of DV[?]0.7 V at 20 °C and DV[?]0.6 V at 85 °C is observed after 104 s of retention time, due to the charge confinement in the high-k HfAlO nanocrystals. [ABSTRACT FROM AUTHOR]

Popularity of herbal drugs has always been in high demand, but recently it has been increasing all over the world, especially in India, because of the lower range of adverse health effects as compared to synthetic or man-made drugs. Not only this but their cost-effectiveness and easy availability to the poor people and the masses, particularly in developing countries, are major causes for their demand. But there lies a huge problem during the process of plant collection that affects their medicinal properties to certain degrees. This is caused by heavy metal toxicity in soil in different locations of the Indian subcontinent. This was correlated with their potential to cause health damage. Exposure of humans to heavy metals includes diverse pathways from food to water to consumption and inhalation of polluted air to permanent damage to exposed skin and even by occupational exposure at workplaces. As we can understand, the main mechanisms of heavy metal toxicity include the production of free radicals to affect the host by oxidative stress, damaging biological molecules such as enzymes, proteins, lipids, and even nucleic acids and finally damaging DNA which is the fastest way to carcinogenesis and in addition, neurotoxicity. Therefore, in this paper, we have researched how the plants/herbs are affected due to heavy metal deposition in their habitat and how it can lead to serious clinical complications., (© 2022. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)

Sarcosine prostate cancer biomarker with the low concentration of 1 pM has been detected by controlling oxygen from 1 to 15 sccm in a NiO x membrane on chemically etched vertical Si nanowires (SiNWs) in an electrolyte-insulator-nanowire (EIN) structure. The vertical Si nanowires with approximately 17 μm length and polycrystalline NiO x membrane are observed by both field-emission scanning electron microscope (FE-SEM) and high-resolution transmission electron microscope (HRTEM) images, respectively. The optimized NiO x membrane with oxygen content of 4 sccm on planar SiO x /Si substrate shows good pH sensitivity of approximately 50 mV/pH, low hysteresis of 3.4 mV, and low drift rate of 2.4 mV/h as compared to other oxygen content membranes of 1, 10, and 15 sccm. Further, uric acid with the concentration of 0.1 μM is detected directly by using the optimized NiO x membrane. In addition, repeatable H 2 O 2 sensing with the low concentration of 10 pM as well as prostate cancer biomarker is detected, which is owing to the reduction-oxidation phenomena of the NiO x membranes. The sensing mechanism is owing to the Ni 2+ /Ni 3+ oxidation states of the NiO x membrane, which is confirmed by X-ray photoelectron spectroscopy. The optimized NiO x membrane on vertical Si nanowire in the EIN structure shows a good drift rate of 3.84 mV/h and sarcosine detection with improvement of approximately 1000 times as compared to the planar Si in an electrolyte-insulator-semiconductor (EIS) structure. This sensor paves a way to detect early-stage diagnosis of prostate cancer rapidly in the near future.

The Cu migration is controlled by using an optimized AlO x interfacial layer, and effects on resistive switching performance, artificial synapse, and human saliva detection in an amorphous-oxygenated-carbon (a-CO x )-based CBRAM platform have been investigated for the first time. The 4 nm-thick AlO x layer in the Cu/AlO x /a-CO x /TiN x O y /TiN structure shows consecutive >2000 DC switching, tight distribution of SET/RESET voltages, a long program/erase (P/E) endurance of >10 9 cycles at a low operation current of 300 μA, and artificial synaptic characteristics under a small pulse width of 100 ns. After a P/E endurance of >10 8 cycles, the Cu migration is observed by both ex situ high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy mapping images. Furthermore, the optimized Cu/AlO x /a-CO x /TiN x O y /TiN CBRAM detects glucose with a low concentration of 1 pM, and real-time measurement of human saliva with a small sample volume of 1 μL is also detected repeatedly in vitro. This is owing to oxidation-reduction of Cu electrode, and the switching mechanism is explored. Therefore, this CBRAM device is beneficial for future artificial intelligence application., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Controlled resistive switching by using an optimized 2 nm thick MoS 2 interfacial layer and the role of top electrodes (TEs) on ascorbic acid (AA) sensing in a TaO x -based resistive random access memory (RRAM) platform have been investigated for the first time. Both the high-resolution transmission electron microscopy (HRTEM) image and depth profile from energy dispersive X-ray spectroscopy confirm the presence of each layer in IrO x /Al 2 O 3 /TaO x /MoS 2 /TiN structure. The pristine device including the IrO x TE with the 2 nm thick interfacial layer shows the highest uniform rectifying direct current endurance >1000 cycles and a large rectifying ratio >3.2 × 10 4 , and a high nonlinearity factor >700 is obtained, greater than that of Pt and Ru TEs. After formation, this IrO x device produces bipolar resistive switching characteristics and a long program/erase (P/E) endurance >10 7 cycles at a low operation current of <50 μA with small pulse width of 100 ns. The stressed device shows a reduced Al 2 O 3 /TaO x interface from the HRTEM image, which is owing to O 2- ions' migration toward TiN electrode. By adjusting the RESET voltage and current level, consecutive >100 complementary resistive switching as well as long P/E endurance of >10 6 cycles are obtained. Schottky barrier height modulation at a low field is observed owing to reduction-oxidation of the TE, which is evidenced through reversible AA detection. At a higher field, Fowler-Nordheim tunneling and hopping conduction are observed. Ascorbic acid detection with a low concentration of 1 pM by using a porous IrO x /Al 2 O 3 /TaO x /MoS 2 /TiN RRAM device directly is an additional novelty of this work, which will be useful in future for early diagnosis of scurvy.

The resistive switching characteristics of a scalable IrO x /Al 2 O 3 /W cross-point structure and its mechanism for pH/H 2 O 2 sensing along with glucose detection have been investigated for the first time. Porous IrO x and Ir 3+ /Ir 4+ oxidation states are observed via high-resolution transmission electron microscope, field-emission scanning electron spectroscopy, and X-ray photo-electron spectroscopy. The 20 nm-thick IrO x devices in sidewall contact show consecutive long dc cycles at a low current compliance (CC) of 10 μA, multi-level operation with CC varying from 10 μA to 100 μA, and long program/erase endurance of >10 9 cycles with 100 ns pulse width. IrO x with a thickness of 2 nm in the IrO x /Al 2 O 3 /SiO 2 /p-Si structure has shown super-Nernstian pH sensitivity of 115 mV per pH, and detection of H 2 O 2 over the range of 1-100 nM is also achieved owing to the porous and reduction-oxidation (redox) characteristics of the IrO x membrane, whereas a pure Al 2 O 3 /SiO 2 membrane does not show H 2 O 2 sensing. A simulation based on Schottky, hopping, and Fowler-Nordheim tunneling conduction, and a redox reaction, is proposed. The experimental I-V curve matches very well with simulation. The resistive switching mechanism is owing to O 2- ion migration, and the redox reaction of Ir 3+ /Ir 4+ at the IrO x /Al 2 O 3 interface through H 2 O 2 sensing as well as Schottky barrier height modulation is responsible. Glucose at a low concentration of 10 pM is detected using a completely new process in the IrO x /Al 2 O 3 /W cross-point structure. Therefore, this cross-point memory shows a method for low cost, scalable, memory with low current, multi-level operation, which will be useful for future highly dense three-dimensional (3D) memory and as a bio-sensor for the future diagnosis of human diseases.

Formation-free multi-level resistive switching characteristics by using 10 nm-thick polycrystalline GeO x film in a simple W/GeO x /W structure and understanding of switching mechanism through redox reaction in H 2 O 2 /sarcosine sensing (or changing Ge°/Ge 4+ oxidation states under external bias) have been reported for the first time. Oxidation states of Ge 0 /Ge 4+ are confirmed by both XPS and H 2 O 2 sensing of GeO x membrane in electrolyte-insulator-semiconductor structure. Highly repeatable 1000 dc cycles and stable program/erase (P/E) endurance of >10 6 cycles at a small pulse width of 100 ns are achieved at a low operation current of 0.1 µA. The thickness of GeO x layer is found to be increased to 12.5 nm with the reduction of polycrystalline grain size of <7 nm after P/E of 10 6 cycles, which is observed by high-resolution TEM. The switching mechanism is explored through redox reaction in GeO x membrane by sensing 1 nM H 2 O 2 , which is owing to the change of oxidation states from Ge 0 to Ge 4+ because of the enhanced O 2- ions migration in memory device under external bias. In addition, sarcosine as a prostate cancer biomarker with low concentration of 50 pM to 10 µM is also detected.

Negative voltage modulated multi-level resistive switching with quantum conductance during staircase-type RESET and its transport characteristics in Cr/BaTiO x /TiN structure have been investigated for the first time. The as-deposited amorphous BaTiO x film has been confirmed by high-resolution transmission electron microscopy. X-ray photo-electron spectroscopy shows different oxidation states of Ba in the switching material, which is responsible for tunable more than 10 resistance states by varying negative stop voltage owing to slow decay value of RESET slope (217.39 mV/decade). Quantum conductance phenomenon has been observed in staircase RESET cycle of the memory devices. By inspecting the oxidation states of Ba + and Ba 2+ through measuring H 2 O 2 with a low concentration of 1 nM in electrolyte/BaTiO x /SiO 2 /p-Si structure, the switching mechanism of each HRS level as well as the multi-level phenomenon has been explained by gradual dissolution of oxygen vacancy filament. Along with negative stop voltage modulated multi-level, current compliance dependent multi-level has also been demonstrated and resistance ratio up to 2000 has been achieved even for a thin (<5 nm) switching material. By considering oxidation-reduction of the conducting filaments, the current-voltage switching curve has been simulated as well. Hence, multi-level resistive switching of Cr/BaTiO x /TiN structure implies the promising applications in high dense, multistate non-volatile memories in near future.

A 15-nm-thick GdO x membrane in an electrolyte-insulator-semiconductor (EIS) structure shows a higher pH sensitivity of 54.2 mV/pH and enzyme-free hydrogen peroxide (H2O2) detection than those of the bare SiO2 and 3-nm-thick GdO x membranes for the first time. Polycrystalline grain and higher Gd content of the thicker GdO x films are confirmed by transmission electron microscopy (TEM) and X-ray photo-electron spectroscopy (XPS), respectively. In a thicker GdO x membrane, polycrystalline grain has lower energy gap and Gd(2+) oxidation states lead to change Gd(3+) states in the presence of H2O2, which are confirmed by electron energy loss spectroscopy (EELS). The oxidation/reduction (redox) properties of thicker GdO x membrane with higher Gd content are responsible for detecting H2O2 whereas both bare SiO2 and thinner GdO x membranes do not show sensing. A low detection limit of 1 μM is obtained due to strong catalytic activity of Gd. The reference voltage shift increases with increase of the H2O2 concentration from 1 to 200 μM owing to more generation of Gd(3+) ions, and the H2O2 sensing mechanism has been explained as well.

Post-metal annealing temperature-dependent forming-free resistive switching memory characteristics, Fowler-Nordheim (F-N) tunneling at low resistance state, and after reset using a new W/WO3/WOx/W structure have been investigated for the first time. Transmission electron microscope image shows a polycrystalline WO3/WOx layer in a device with a size of 150 × 150 nm(2). The composition of WO3/WOx is confirmed by X-ray photo-electron spectroscopy. Non-linear bipolar resistive switching characteristics have been simulated using space-charge limited current (SCLC) conduction at low voltage, F-N tunneling at higher voltage regions, and hopping conduction during reset, which is well fitted with experimental current-voltage characteristics. The barrier height at the WOx/W interface for the devices annealed at 500 °C is lower than those of the as-deposited and annealed at 400 °C (0.63 vs. 1.03 eV). An oxygen-vacant conducting filament with a diameter of ~34 nm is formed/ruptured into the WO3/WOx bilayer owing to oxygen ion migration under external bias as well as barrier height changes for high-resistance to low-resistance states. In addition, the switching mechanism including the easy method has been explored through the current-voltage simulation. The devices annealed at 500 °C have a lower operation voltage, lower barrier height, and higher non-linearity factor, which are beneficial for selector-less crossbar memory arrays.

It is known that conductive-bridge resistive-random-access-memory (CBRAM) device is very important for future high-density nonvolatile memory as well as logic application. Even though the CBRAM devices using different materials, structures, and switching performance have been reported in Nanoscale Res. Lett., 2015, however, optical switching characteristics by using thermally grown Ge0.2Se0.8 film in Cu/GeSex/W structure are reported for the first time in this study. The Cu/GeSex/W memory devices have low current compliances (CCs) ranging from 1 nA to 500 μA with low voltage of ±1.2 V, high resistance ratio of approximately 10(3), stable endurance of >200 cycles, and good data retention of >7 × 10(3) s at 85 °C. Multi-steps of RESET phenomena and evolution of Cu filaments' shape under CCs ranging from 1 nA to 500 μA have been discussed. Under external white-light illumination with an intensity of 2.68 mW/cm(2) (wavelength ranges from 390 to 700 nm), memory device shows optical switching with long read pulse endurance of >10(5) cycles. This CBRAM device has optically programmed and electrically erased, which can open up a new area of research field for future application.

The performances of conductive-bridging random access memory (CBRAM) have been reviewed for different switching materials such as chalcogenides, oxides, and bilayers in different structures. The structure consists of an inert electrode and one oxidized electrode of copper (Cu) or silver (Ag). The switching mechanism is the formation/dissolution of a metallic filament in the switching materials under external bias. However, the growth dynamics of the metallic filament in different switching materials are still debated. All CBRAM devices are switching under an operation current of 0.1 μA to 1 mA, and an operation voltage of ±2 V is also needed. The device can reach a low current of 5 pA; however, current compliance-dependent reliability is a challenging issue. Although a chalcogenide-based material has opportunity to have better endurance as compared to an oxide-based material, data retention and integration with the complementary metal-oxide-semiconductor (CMOS) process are also issues. Devices with bilayer switching materials show better resistive switching characteristics as compared to those with a single switching layer, especially a program/erase endurance of >10(5) cycles with a high speed of few nanoseconds. Multi-level cell operation is possible, but the stability of the high resistance state is also an important reliability concern. These devices show a good data retention of >10(5) s at >85°C. However, more study is needed to achieve a 10-year guarantee of data retention for non-volatile memory application. The crossbar memory is benefited for high density with low power operation. Some CBRAM devices as a chip have been reported for proto-typical production. This review shows that operation current should be optimized for few microamperes with a maintaining speed of few nanoseconds, which will have challenges and also opportunities for three-dimensional (3D) architecture.

The resistive switching memory characteristics of 100 randomly measured devices were observed by reducing device size in a Cr/CrO x /TiO x /TiN structure for the first time. Transmission electron microscope image confirmed a via-hole size of 0.4 µm. A 3-nm-thick amorphous TiO x with 4-nm-thick polycrystalline CrO x layer was observed. A small 0.4-µm device shows reversible resistive switching at a current compliance of 300 µA as compared to other larger size devices (1-8 µm) owing to reduction of leakage current through the TiO x layer. Good device-to-device uniformity with a yield of >85 % has been clarified by weibull distribution owing to higher slope/shape factor. The switching mechanism is based on oxygen vacancy migration from the CrO x layer and filament formation/rupture in the TiO x layer. Long read pulse endurance of >10 5 cycles, good data retention of 6 h, and a program/erase speed of 1 µs pulse width have been obtained.

Impact of the device size and thickness of Al2O3 film on the Cu pillars and resistive switching memory characteristics of the Al/Cu/Al2O3/TiN structures have been investigated for the first time. The memory device size and thickness of Al2O3 of 18 nm are observed by transmission electron microscope image. The 20-nm-thick Al2O3 films have been used for the Cu pillar formation (i.e., stronger Cu filaments) in the Al/Cu/Al2O3/TiN structures, which can be used for three-dimensional (3D) cross-point architecture as reported previously Nanoscale Res. Lett.9:366, 2014. Fifty randomly picked devices with sizes ranging from 8 × 8 to 0.4 × 0.4 μm(2) have been measured. The 8-μm devices show 100% yield of Cu pillars, whereas only 74% successful is observed for the 0.4-μm devices, because smaller size devices have higher Joule heating effect and larger size devices show long read endurance of 10(5) cycles at a high read voltage of -1.5 V. On the other hand, the resistive switching memory characteristics of the 0.4-μm devices with a 2-nm-thick Al2O3 film show superior as compared to those of both the larger device sizes and thicker (10 nm) Al2O3 film, owing to higher Cu diffusion rate for the larger size and thicker Al2O3 film. In consequence, higher device-to-device uniformity of 88% and lower average RESET current of approximately 328 μA are observed for the 0.4-μm devices with a 2-nm-thick Al2O3 film. Data retention capability of our memory device of >48 h makes it a promising one for future nanoscale nonvolatile application. This conductive bridging resistive random access memory (CBRAM) device is forming free at a current compliance (CC) of 30 μA (even at a lowest CC of 0.1 μA) and operation voltage of ±3 V at a high resistance ratio of >10(4).

Resistive random access memory (RRAM) characteristics using a new Cr/GdOx/TiN structure with different device sizes ranging from 0.4 × 0.4 to 8 × 8 μm(2) have been reported in this study. Polycrystalline GdOx film with a thickness of 17 nm and a small via-hole size of 0.4 μm are observed by a transmission electron microscope (TEM) image. All elements and GdOx film are confirmed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses. Repeatable resistive switching characteristics at a current compliance (CC) of 300 μA and low operating voltage of ±4 V are observed. The switching mechanism is based on the oxygen vacancy filament formation/rupture through GdOx grain boundaries under external bias. After measuring 50 RRAM devices randomly, the 8-μm devices exhibit superior resistive switching characteristics than those of the 0.4-μm devices owing to higher recombination rate of oxygen with remaining conducting filament in the GdOx film as well as larger interface area, even with a thinner GdOx film of 9 nm. The GdOx film thickness dependence RRAM characteristics have been discussed also. Memory device shows repeatable 100 switching cycles, good device-to-device uniformity with a switching yield of approximately 80%, long read endurance of >10(5) cycles, and good data retention of >3 × 10(4) s at a CC of 300 μA.

A novel idea by using copper (Cu) pillar is proposed in this study, which can replace the through-silicon-vias (TSV) technique in future three-dimensional (3D) architecture. The Cu pillar formation under external bias in an Al/Cu/Al2O3/TiN structure is simple and low cost. The Cu pillar is formed in the Al2O3 film under a small operation voltage of <5 V and a high-current-carrying conductor of >70 mA is obtained. More than 100 devices have shown tight distribution of the Cu pillars in Al2O3 film for high current compliance (CC) of 70 mA. Robust read pulse endurances of >10(6) cycles are observed with read voltages of -1, 1, and 4 V. However, read endurance is failed with read voltages of -1.5, -2, and -4 V. By decreasing negative read voltage, the read endurance is getting worst, which is owing to ruptured Cu pillar. Surface roughness and TiO x N y on TiN bottom electrode are observed by atomic force microscope and transmission electron microscope, respectively. The Al/Cu/Al2O3/TiN memory device shows good bipolar resistive switching behavior at a CC of 500 μA under small operating voltage of ±1 V and good data retention characteristics of >10(3) s with acceptable resistance ratio of >10 is also obtained. This suggests that high-current operation will help to form Cu pillar and lower-current operation will have bipolar resistive switching memory. Therefore, this new Cu/Al2O3/TiN structure will be benefited for 3D architecture in the future.

Self-compliance resistive random access memory (RRAM) characteristics using a W/TaO x /TiN structure are reported for the first time. A high-resolution transmission electron microscope (HRTEM) image shows an amorphous TaO x layer with a thickness of 7 nm. A thin layer of TiO x N y with a thickness of 3 nm is formed at the TaO x /TiN interface, owing to the oxygen accumulation nature of Ti. This memory device shows 100 consecutive switching cycles with excellent uniformity, 100 randomly picked device-to-device good uniformity, and program/erase endurance of >10(3) cycles. It is observed that the 0.6-μm devices show better switching uniformity as compared to the 4-μm devices, which is due to the thinner tungsten (W) electrode as well as higher series resistance. The oxygen-rich TaO x layer at the W/TaO x interface also plays an important role in getting self-compliance resistive switching phenomena and non-linear current-voltage (I-V) curve at low resistance state (LRS). Switching mechanism is attributed to the formation and rupture of oxygen vacancy conducting path in the TaO x switching material. The memory device also exhibits long read endurance of >10(6) cycles. It is found that after 400,000 cycles, the high resistance state (HRS) is decreased, which may be due to some defects creation (or oxygen moves away) by frequent stress on the switching material. Good data retention of >10(4) s is also obtained.

Time-dependent pH sensing phenomena of the core-shell CdSe/ZnS quantum dot (QD) sensors in EIS (electrolyte insulator semiconductor) structure have been investigated for the first time. The quantum dots are immobilized by chaperonin GroEL protein, which are observed by both atomic force microscope and scanning electron microscope. The diameter of one QD is approximately 6.5 nm. The QDs are not oxidized over a long time and core-shell CdSe/ZnS are confirmed by X-ray photon spectroscopy. The sensors are studied for sensing of hydrogen ions concentration in different buffer solutions at broad pH range of 2 to 12. The QD sensors show improved sensitivity (38 to 55 mV/pH) as compared to bare SiO2 sensor (36 to 23 mV/pH) with time period of 0 to 24 months, owing to the reduction of defects in the QDs. Therefore, the differential sensitivity of the QD sensors with respect to the bare SiO2 sensors is improved from 2 to 32 mV/pH for the time period of 0 to 24 months. After 24 months, the sensitivity of the QD sensors is close to ideal Nernstian response with good linearity of 99.96%. Stability and repeatability of the QD sensors show low drift (10 mV for 10 cycles) as well as small hysteresis characteristics (<10 mV). This QD sensor is very useful for future human disease diagnostics.

Enhanced resistive memory characteristics with 10,000 consecutive direct current switching cycles, long read pulse endurance of >10(5) cycles, and good data retention of >10(4) s with a good resistance ratio of >10(2) at 85°C are obtained using a Ti nanolayer to form a W/TiO x /TaO x /W structure under a low current operation of 80 μA, while few switching cycles are observed for W/TaO x /W structure under a higher current compliance >300 μA. The low resistance state decreases with increasing current compliances from 10 to 100 μA, and the device could be operated at a low RESET current of 23 μA. A small device size of 150 × 150 nm(2) is observed by transmission electron microscopy. The presence of oxygen-deficient TaO x nanofilament in a W/TiO x /TaO x /W structure after switching is investigated by Auger electron spectroscopy. Oxygen ion (negative charge) migration is found to lead to filament formation/rupture, and it is controlled by Ti nanolayer at the W/TaO x interface. Conducting nanofilament diameter is estimated to be 3 nm by a new method, indicating a high memory density of approximately equal to 100 Tbit/in.(2).

Enhanced resistive memory characteristics with 10,000 consecutive direct current switching cycles, long read pulse endurance of >105 cycles, and good data retention of >104 s with a good resistance ratio of >102 at 85°C are obtained using a Ti nanolayer to form a W/TiOx/TaOx/W structure under a low current operation of 80 μA, while few switching cycles are observed for W/TaOx/W structure under a higher current compliance >300 μA. The low resistance state decreases with increasing current compliances from 10 to 100 μA, and the device could be operated at a low RESET current of 23 μA. A small device size of 150 × 150 nm2 is observed by transmission electron microscopy. The presence of oxygen-deficient TaOx nanofilament in a W/TiOx/TaOx/W structure after switching is investigated by Auger electron spectroscopy. Oxygen ion (negative charge) migration is found to lead to filament formation/rupture, and it is controlled by Ti nanolayer at the W/TaOx interface. Conducting nanofilament diameter is estimated to be 3 nm by a new method, indicating a high memory density of approximately equal to 100 Tbit/in.2.

Enhanced resistive switching phenomena of IrOx/GdOx/W cross-point memory devices have been observed as compared to the via-hole devices. The as-deposited Gd2O3 films with a thickness of approximately 15 nm show polycrystalline that is observed using high-resolution transmission electron microscope. Via-hole memory device shows bipolar resistive switching phenomena with a large formation voltage of -6.4 V and high operation current of >1 mA, while the cross-point memory device shows also bipolar resistive switching with low-voltage format of +2 V and self-compliance operation current of <300 μA. Switching mechanism is based on the formation and rupture of conducting filament at the IrOx/GdOx interface, owing to oxygen ion migration. The oxygen-rich GdOx layer formation at the IrOx/GdOx interface will also help control the resistive switching characteristics. This cross-point memory device has also Repeatable 100 DC switching cycles, narrow distribution of LRS/HRS, excellent pulse endurance of >10,000 in every cycle, and good data retention of >104 s. This memory device has great potential for future nanoscale high-density non-volatile memory applications.

Resistive switching properties of a self-compliance resistive random access memory device in cross-point architecture with a simple stack structure of Ir/TaOx/W have been investigated. A transmission electron microscope and atomic force microscope were used to observe the film properties and morphology of the stack. The device has shown excellent switching cycle uniformity with a small operation of ±2.5 V and a resistance ratio of >100. The device requires neither any frorming-process nor current compliance limit for repeatable operation in contrast to conventional resistive random access memory devices. The effect of bottom electrode morphology and surface roughness is also studied. The improvement is due to the enhanced electric field at the nanotips in the bottom electrode and the defective TaOx switching layer which enable controlled filament formation/rupture. The device area dependence of the low resistance state indicates multifilament formation. The device has shown a robust alternating current endurance of >105 cycles and a data retention of >104 s.

Comparison of resistive switching memory characteristics using copper (Cu) and aluminum (Al) electrodes on GeOx/W cross-points has been reported under low current compliances (CCs) of 1 nA to 50 μA. The cross-point memory devices are observed by high-resolution transmission electron microscopy (HRTEM). Improved memory characteristics are observed for the Cu/GeOx/W structures as compared to the Al/GeOx/W cross-points owing to AlOx formation at the Al/GeOx interface. The RESET current increases with the increase of the CCs varying from 1 nA to 50 μA for the Cu electrode devices, while the RESET current is high (>1 mA) and independent of CCs varying from 1 nA to 500 μA for the Al electrode devices. An extra formation voltage is needed for the Al/GeOx/W devices, while a low operation voltage of ±2 V is needed for the Cu/GeOx/W cross-point devices. Repeatable bipolar resistive switching characteristics of the Cu/GeOx/W cross-point memory devices are observed with CC varying from 1 nA to 50 μA, and unipolar resistive switching is observed with CC >100 μA. High resistance ratios of 102 to 104 for the bipolar mode (CCs of 1 nA to 50 μA) and approximately 108 for the unipolar mode are obtained for the Cu/GeOx/W cross-points. In addition, repeatable switching cycles and data retention of 103 s are observed under a low current of 1 nA for future low-power, high-density, nonvolatile, nanoscale memory applications.

Resistive switching memories (RRAMs) are attractive for replacement of conventional flash in the future. Although different switching materials have been reported; however, low-current operated devices (<100 μA) are necessary for productive RRAM applications. Therefore, TaOx is one of the prospective switching materials because of two stable phases of TaO2 and Ta2O5, which can also control the stable low- and high-resistance states. Long program/erase endurance and data retention at high temperature under low-current operation are also reported in published literature. So far, bilayered TaOx with inert electrodes (Pt and/or Ir) or single layer TaOx with semi-reactive electrodes (W and Ti/W or Ta/Pt) is proposed for real RRAM applications. It is found that the memory characteristics at current compliance (CC) of 80 μA is acceptable for real application; however, data are becoming worst at CC of 10 μA. Therefore, it is very challenging to reduce the operation current (few microampere) of the RRAM devices. This study investigates the switching mode, mechanism, and performance of low-current operated TaOx-based devices as compared to other RRAM devices. This topical review will not only help for application of TaOx-based nanoscale RRAM devices but also encourage researcher to overcome the challenges in the future production.

Improved switching characteristics were obtained from high-κ oxides AlOx, GdOx, HfOx, and TaOx in IrOx/high-κx/W structures because of a layer that formed at the IrOx/high-κx interface under external positive bias. The surface roughness and morphology of the bottom electrode in these devices were observed by atomic force microscopy. Device size was investigated using high-resolution transmission electron microscopy. More than 100 repeatable consecutive switching cycles were observed for positive-formatted memory devices compared with that of the negative-formatted devices (only five unstable cycles) because it contained an electrically formed interfacial layer that controlled 'SET/RESET' current overshoot. This phenomenon was independent of the switching material in the device. The electrically formed oxygen-rich interfacial layer at the IrOx/high-κx interface improved switching in both via-hole and cross-point structures. The switching mechanism was attributed to filamentary conduction and oxygen ion migration. Using the positive-formatted design approach, cross-point memory in an IrOx/AlOx/W structure was fabricated. This cross-point memory exhibited forming-free, uniform switching for >1,000 consecutive dc cycles with a small voltage/current operation of ±2 V/200 μA and high yield of >95% switchable with a large resistance ratio of >100. These properties make this cross-point memory particularly promising for high-density applications. Furthermore, this memory device also showed multilevel capability with a switching current as low as 10 μA and a RESET current of 137 μA, good pulse read endurance of each level (>105 cycles), and data retention of >104 s at a low current compliance of 50 μA at 85°C. Our improvement of the switching characteristics of this resistive memory device will aid in the design of memory stacks for practical applications.

The resistive switching memory of Ge nanowires (NWs) in an IrOx/Al2O3/Ge NWs/SiO2/p-Si structure is investigated. Ge NWs with an average diameter of approximately 100 nm are grown by the vapor-liquid-solid technique. The core-shell structure of the Ge/GeOx NWs is confirmed by both scanning electron microscopy and high-resolution transmission electron microscopy. Defects in the Ge/GeOx NWs are observed by X-ray photoelectron spectroscopy. Broad photoluminescence spectra from 10 to 300 K are observed because of defects in the Ge/GeOx NWs, which are also useful for nanoscale resistive switching memory. The resistive switching mechanism in an IrOx/GeOx/W structure involves migration of oxygen ions under external bias, which is also confirmed by real-time observation of the surface of the device. The porous IrOx top electrode readily allows the evolved O2 gas to escape from the device. The annealed device has a low operating voltage (<4 V), low RESET current (approximately 22 μA), large resistance ratio (>103), long pulse read endurance of >105 cycles, and good data retention of >104 s. Its performance is better than that of the as-deposited device because the GeOx film in the annealed device contains more oxygen vacancies. Under SET operation, Ge/GeOx nanofilaments (or NWs) form in the GeOx film. The diameter of the conducting nanofilament is approximately 40 nm, which is calculated using a new method.

We demonstrate enhanced repeatable nanoscale bipolar resistive switching memory characteristics in Al/Cu/Ge0.5Se0.5/W, as compared with Al/Cu/Ge0.2Se0.8/W structures, including stable AC endurance (>105 cycles), larger average SET voltage (approximately 0.6 V), excellent data retention (>105 s) at 85°C, and a high resistance ratio (>104) with a current compliance of 8 μA and a small operation voltage of ±1.5 V. A small device size of 150 × 150 nm2 and a Cu nanofilament with a small diameter of 30 nm are both observed by high-resolution transmission electron microscope in the SET state. The GexSe1 - x solid electrolyte compositions are confirmed by both energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The switching mechanism relies on the smaller barrier heights for holes rather than for electrons; the positively charged Cuz+ ions (i.e., holes) migrate through the defects in the GexSe1 - x solid electrolytes during SET/RESET operations. Hence, the Cu nanofilament starts to grow at the Ge0.5Se0.5/W interface, and starts to dissolve at the Cu/Ge0.5Se0.5 interface, as illustrated in the energy band diagrams. Owing to both the higher barrier for hole injection at the Cu/Ge0.5Se0.5 interface than at the Cu/Ge0.2Se0.8 interface and greater thermal stability, the resistive switching memory characteristics of the Al/Cu/Ge0.5Se0.5/W are improved relative to the Al/Cu/Ge0.2Se0.8/W devices. The Al/Cu/Ge0.5Se0.5/W memory device can also be operated with a low current compliance of 1 nA, and hence, a low SET/RESET power of 0.61 nW/6.4 pW is achieved. In addition, a large memory size of 1,300 Pbit/in2 is achieved with a small nanofilament diameter of 0.25 Å for a small current compliance of 1 nA.

Excellent resistive switching memory characteristics were demonstrated for an Al/Cu/Ti/TaOx/W structure with a Ti nanolayer at the Cu/TaOx interface under low voltage operation of ± 1.5 V and a range of current compliances (CCs) from 0.1 to 500 μA. Oxygen accumulation at the Ti nanolayer and formation of a defective high-κ TaOx film were confirmed by high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photo-electron spectroscopy. The resistive switching memory characteristics of the Al/Cu/Ti/TaOx/W structure, such as HRS/LRS (approximately 104), stable switching cycle stability (>106) and multi-level operation, were improved compared with those of Al/Cu/TaOx/W devices. These results were attributed to the control of Cu migration/dissolution by the insertion of a Ti nanolayer at the Cu/TaOx interface. In contrast, CuOx formation at the Cu/TaOx interface was observed in an Al/Cu/TaOx/W structure, which hindered dissolution of the Cu filament and resulted in a small resistance ratio of approximately 10 at a CC of 500 μA. A high charge-trapping density of 6.9 × 1016 /cm2 was observed in the Al/Cu/Ti/TaOx/W structure from capacitance-voltage hysteresis characteristics, indicating the migration of Cu ions through defect sites. The switching mechanism was successfully explained for structures with and without the Ti nanolayer. By using a new approach, the nanoscale diameter of Cu filament decreased from 10.4 to 0.17 nm as the CC decreased from 500 to 0.1 μA, resulting in a large memory size of 7.6 T to 28 Pbit/sq in. Extrapolated 10-year data retention of the Ti nanolayer device was also obtained. The findings of this study will not only improve resistive switching memory performance but also aid future design of nanoscale nonvolatile memory.

Improved resistive switching memory characteristics by controlling the formation polarity in an IrOx/Al2O3/IrOx-ND/Al2O3/WOx/W structure have been investigated. High density of 1 × 1013/cm2 and small size of 1.3 nm in diameter of the IrOx nano-dots (NDs) have been observed by high-resolution transmission electron microscopy. The IrOx-NDs, Al2O3, and WOx layers are confirmed by X-ray photo-electron spectroscopy. Capacitance-voltage hysteresis characteristics show higher charge-trapping density in the IrOx-ND memory as compared to the pure Al2O3 devices. This suggests that the IrOx-ND device has more defect sites than that of the pure Al2O3 devices. Stable resistive switching characteristics under positive formation polarity on the IrOx electrode are observed, and the conducting filament is controlled by oxygen ion migration toward the Al2O3/IrOx top electrode interface. The switching mechanism is explained schematically based on our resistive switching parameters. The resistive switching random access memory (ReRAM) devices under positive formation polarity have an applicable resistance ratio of > 10 after extrapolation of 10 years data retention at 85°C and a long read endurance of 105 cycles. A large memory size of > 60 Tbit/sq in. can be realized in future for ReRAM device application. This study is not only important for improving the resistive switching memory performance but also help design other nanoscale high-density nonvolatile memory in future.