W. Banerjee

Formation polarity dependent improved resistive switching memory characteristics using nanoscale (1.3 nm) core-shell IrOx nano-dots

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.

Impact of metal nano layer thickness on tunneling oxide and memory performance of core-shell iridium-oxide nanocrystals

The impact of iridium-oxide (IrOx) nano layer thickness on the tunneling oxide and memory performance of IrOx metal nanocrystals in an n-Si/SiO2/Al2O3/IrOx/Al2O3/IrOx structure has been investigated. A thinner (1.5 nm) IrOx nano layer has shown better memory performance than that of a thicker one (2.5 nm). Core-shell IrOx nanocrystals with a small average diameter of 2.4 nm and a high density of ∼2 × 1012/cm2 have been observed by scanning transmission electron microscopy. The IrOx nanocrystals are confirmed by x-ray photoelectron spectroscopy. A large memory window of 3.0 V at a sweeping gate voltage of ±5 V and 7.2 V at a sweeping gate voltage of ± 8 V has been observed for the 1.5 nm-thick IrOx nano layer memory capacitors with a small equivalent oxide thickness of 8 nm. The electrons and holes are trapped in the core and annular regions of the IrOx nanocrystals, respectively, which is explained by Gibbs free energy. High electron and hole-trapping densities are found to be 1.5 × 1013/cm2 and 2 × 1013/...

Impact of electrically formed interfacial layer and improved memory characteristics of IrOx/high-κx/W structures containing AlOx, GdOx, HfOx, and TaOx switching materials

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.

Excellent Uniformity and Multilevel Operation in Formation-Free Low Power Resistive Switching Memory Using IrOx/AlOx/W Cross-Point

Excellent uniformity and multilevel operation in formation-free low-power resistive switching memory fabricated using the IrOx/AlOx/W cross-point structure have been investigated. The thickness of the deposited films has been measured by high-resolution transmission electron microscopy with energy dispersive X-ray spectroscopy for each layer. The cross-point resistive switching memory devices have a tight distribution of SET/RESET voltages and low/high-resistance states as well as switching cycles. A high resistance ratio of >8×102 is obtained. This memory device shows excellent AC endurance of >5×103 cycles, read endurance of >1×105 cycles, and 10-year-data retention at 85 °C at a low power of 55 µW and low-current compliances of 50–200 µA. This study is not only important for cross-point memories but will also help in the design of high-density nanoscale nonvolatile memories in the future.

Temperature-dependent physical and memory characteristics of atomic-layer-deposited RuO x metal nanocrystal capacitors

Physical and memory characteristics of the atomic-layer-deposited RuOx metal nanocrystal capacitors in an n-Si/SiO2/HfO2/ RuOx/Al2O3/Pt structure with different postdeposition annealing temperatures from 850-1000° have been investigated. The RuOx metal nanocrystals with an average diameter of 7nm and a highdensity of 0.7×1012/cm2 are observed by high-resolution transmission electron microscopy after a postdeposition annealing temperature at 1000°. The density of RuOx nanocrystal is decreased (slightly) by increasing the annealing temperatures, due to agglomeration of multiple nanocrystals. The RuO3 nanocrystals and Hf-silicate layer at the SiO2/HfO2 interface are confirmed by X-ray photoelectron spectroscopy. For postdeposition annealing temperature of 1000°, the memory capacitors with a small equivalent oxide thickness of ∼9 nm possess a large hysteresis memory window of >5V at a small sweeping gate voltage of ±5V. A promising memory window under a small sweeping gate voltage of ∼3 V is also observed due to charge trapping in the RuOx metal nanocrystals. The program/erase mechanism is modified Fowler-Nordheim (F-N) tunneling of the electrons and holes from Si substrate. The electrons and holes are trapped in the RuOx nanocrystals. Excellent program/erase endurance of 106 cycles and a large memory window of 4.3 V with a small charge loss of ∼23% at 85° are observed after 10 years of data retention time, due to the deep-level traps in the RuOx nanocrystals. The memory structure is very promising for future nanoscale nonvolatile memory applications.

Improvement of resistive switching memory parameters using IrO x Nanodots in high-κ AlO x Cross-Point

The improvement in resistive switching memory parameters by embedding IrO_x nanodots in IrO_x/AlO_x/IrO_x NDs/AlO_x/W cross-point structure is reported. The fabricated memory devices exhibit MLC operation of both LRS and HRS, excellent read endurance of >;10⁶ times, program/erase endurance of >;10⁵ cycles, robust data retention of >;10⁴s at 125°C with a small operation voltage of ±2V and a low CC of <;200 μA.

Novel IrOx nanodots based capacitive pH sensor

In recent years, EIS structure has drawn major attention among the various semiconductor based biosensors such as ISFET, EIS, and LAPS etc. because of its simplicity in layout, label free detection, easy and cost effective fabrication. Nanoparticles have shown their potential as major diagnostic tool in biological detection because of their unique size dependent electronic, optical and spectroscopic properties due to their small size and comparative high surface area [1]. Very few studies have been reported towards nanoparticles modified EIS structures. Various nanostructure metal oxides have been used as transducers for biosensor such as ZnO, ZrOx, TiOx etc. which have shown their functional biocompatibility, non-toxic and high catalytic efficiency [2]. Nanostructured iridium oxide (IrOx) has attracted more attention due to its promising properties such as chemical stability and good adsorption properties for the biomolecules immobilization. IrOx shows good response to charge variation at interface because it maintains high charge transfer ratio [3]. As iridium oxide nanostructure are compatible with intracellular material, IrOx based sensors has been developed to detect biochemical such as enzymes, antibodies and hybridized DNA. Presence of IrOx nanostructure facilitates the control of the morphology and wettability of the surface which ultimately results in the enhanced sensitivity, single molecule detection and significant reduction in analyte concentration. Recently, W. D. Huang et al. [4] reported the IrOx film as an electrochemical pH sensor [4], however IrOx nanodots (NDs) modified EIS structure as capacitive pH sensor has not been reported yet. This novel IrOx–NDs based pH sensor shows a promising sensing performance with a near-Nernstian response in sensitivity repeatedly and reversibly in between 46.4-52.4 mV/pH in the pH range between 2 and 12 at 25C.

Characteristics of ALD High-k HfAlOx Nanocrystals in Memory Capacitors Annealed at High Temperatures

Major challenges in transistor gate stack materials include the replacement of SiO2 and SiON dielectrics with high-κ dielectric materials, and that of doped polycrystalline Si gate electrodes with metals [1]. A ternary system, such as a high-temperature crystalline system of HfAlOx is another approach to obtain a new high-κ material due to expectations that the ternary system may provide a new function and alleviate disadvantages of the binary one. n-type silicon (100) substrate was cleaned by the standard RCA process. After the Si wafers were cleaned, a tunneling oxide (SiO2) with a thickness of 3 nm was grown by rapid thermal oxide (RTO) process at a substrate temperature of 1000C for 15 s. A thin high-κ HfO2 film with a thickness of >2 nm and a thick high-κ Al2O3 film with a thickness of 15 nm as a blocking oxide were both deposited by ALD. To improve the charge storage characteristics, the post deposition annealing (PDA) treatment was carried out at temperatures of 900C for 1 min in N2 atmosphere. To form the memory capacitors, the high work function platinum (Pt) metal gate electrode (area: 1.12×10 cm) was deposited by sputtering using a shadow mask. The post metal annealing (PMA) treatment was performed at a temperature of 400°C for 5 min in N2 (90%) and H2 (10%) gases.The HfO2 film is shown to be fully polycrystalline grain, while the Al2O3 film shows a partial crystalline grain. After the PDA treatment, the HfO2/Al2O3 nanolaminate layers show double HfO2 charge trapping layers with a HfAlO film as a barrier layer. It is observed that the Al2O3/HfO2/Al2O3 middle layers are mixed together after PDA treatment, resulting in a partially crystalline HfAlO layer. Fig. 1 shows the depth profiles of Al 2p, O 1s and Hf 4f from the Al2O3 through to the SiO2 layers. XPS intensities associated with Hf decrease noticeably at etch cycle 11, while that of Hf increases substantially at etch cycle 7 as the annealing temperature goes up to 900°C. This implies that the phase structure of HfO2 changes to Al+HfO2 at a temperature of about 900°C, which is due to Hf out diffusion. To explain the transformation of HfO2 to Al +HfO2, inference may be made from the oxidation phenomenon of HfO2 alloys. Many reports have explained the preferential oxidation due to larger negative Gibbs free energy [2, 3]. From the perspective of thermodynamics, HfO2 is expected to be spontaneously reduced to Hf, while Al is oxidized to Al2O3. This is in agreement with our experimental observation. The reduction of HfO2 to Hf was observed in the samples annealed at 1000°C. This may be explained by the exponential dependence of reaction kinetics on temperature. The thermodynamic stability of Hf in the Hf–Al–O matrix can also prevent any traces of oxygen from oxidizing Hf during the nanocrystal formation process, resulting in enlargement of the process window. Fig. 2(a) shows the clockwise capacitance-voltage (CV) hysteresis characteristic with different sweeping gate voltages for the as-deposited memory capacitor. The holding time was 3s and delay time was 0.1s during C-V measurement. A small memory window of ΔV≈2.5V was observed only under a large sweeping voltage of ±15V. The memory window at a sweeping gate voltage of ±10V was not observed, and the same situation was true with ±5V and ±2V. This indicates that the trapping sites in the as-deposited memory capacitor were negligible. The calculated equivalent oxide thickness (EOT) value from the C-V hysteresis was found to be 9.3±0.5 nm. Large memory windows of ΔV≈10.0V@=±15V and ΔV≈5.9V@±10V were observed for the annealed memory capacitor [Fig. 2(b)], due to the charge storage in the high-κ HfAlOx nanocrystals. The HfAlOx nanocrystals embedded in HfO2 and Al2O3 matrix efficiently stored the charge, which is responsible for the enhancement of the memory characteristics. A large memory window was observed due to the charge storage in the high-κ HfAlOx nanocrystals. The resultant tunneling oxide thickness is > 5nm which improves the retention. The high-κ Al2O3 and high work function Pt metal gate can enhance erasing speed and eliminate electron back tunneling current. The initial memory window is ΔV~ 2.1V. After 7 days of retention, a significant memory window of ΔV~0.7V and charge loss of 66% were observed at room temperature due to the charge confinement in HfAlOx nanocrystals.

Highly Thermally Stable and Reproducible of ALD RuO 2 Nanocrystal Floating Gate Memory Devices with Large Memory Window and Good Retention

Highly thermally stable (~1000degC) and reproducible of ALD RuO2 nanocrystal floating gate memory devices with a large hysteresis memory window of DeltaV ap 14.6 V under a gate voltage of plusmn10 V have been observed. The memory window of DeltaV ap 4.2 V under a small gate voltage of plusmn3 V is also observed. Both program and erase speeds of DeltaVFB>1 V@100 mus are achieved under Fowler-Nordheim injections. Excellent endurance of DeltaV ap 8.5V, before and after 104 cycles and a large memory window of DeltaV ap 4.9 V after 10 years of retention (9% charge loss at 20degC and -20% charge loss at 85degC) are obtained. The high-performance ALD RuO2 nanocrystal flash memory devices can be operated below 5 V.

Physical and electrical characteristics of atomic layer deposited RuO 2 nanocrystals for nanoscale nonvolatile memory applications

The physical and electrical characteristics of atomic layer deposited RuO₂ nanocrystals embedded in high-¿ HfO₂/Al₂O₃ films in an n-Si/SiO₂/HfO₂/RuO₂/Al₂O₃/Pt memory structure have been investigated. A small size of <10 nm and high-density of ~ 1.6 × 10¹²/cm² for the RuO₂ nanocrystals have been observed by high-resolution transmission electron microscope (HRTEM). The RuO₂ metal nanocrystals and all high-¿ films have been confirmed by x-ray photoelectron spectroscopy (XPS). A large hysteresis memory window of ¿V¿10.8 V at a gate voltage of V_g = ±10 V has been observed for RuO₂ nanocrystal memory capacitors. A hysteresis memory window of ¿V¿2.4 V has also been observed under a small sweeping gate voltage of V_g = ±5 V, due to charge storage in the RuO₂ metal nanocrystals. The RuO₂ metal nanocrystal memory capacitors have a large breakdown voltage of -15V. A low charge loss of 15% is observed after 10 years of retention.