Layer Scavenging Research Articles

Oxygen vacancy engineering in Si-doped, HfO2 ferroelectric capacitors using Ti oxygen scavenging layers

In this paper, the ferroelectric properties of TiN/Ti/Si:HfO2/TiN stacks are shown to be modulated by the Ti oxygen scavenging layer, leading to improved remanent polarization and wake-up at low switching fields. Hard x-ray photoelectron spectroscopy is used to measure the oxygen vacancy (VO) concentration in Si-doped HfO2 based capacitors. This VO engineered ferroelectric stack is then assessed at the wafer scale within 16 kbit 1T-1C FeRAM arrays integrated into 130 nm CMOS. The introduction of a Ti oxygen scavenging layer at the top interface of Si:HfO2-based BEOL-integrated metal/ferroelectric/metal capacitors enhances their ferroelectric (FE) behavior (+100% remanent polarization 2.Pr), confirming the positive role of modest concentrations of VO in orthorhombic phase crystallization at low thermal budgets. It is statistically demonstrated at 16 kbit array level that VO-rich FE Si:HfO2 facilitates 2.5 V FeRAM array operation with improved memory window. The field cycling dependence of the memory window is correlated with the VO concentration, and an endurance of 108 cycles is measured at the array level.

Filament Formation in TaOx Thin Films for Memristor Device Application: Modeling Electron Energy Loss Spectra and Electron Transport

AbstractAlthough understanding filament formation in oxide‐based memristive devices by theory has emerged, there are still fundamental unanswered questions. Importantly, for practical application of thin films the material in its amorphous state is to be considered, but mostly lacking so far, and details on sub‐stoichiometry are also scarce. To gain insight into the optical and electronic properties of sub‐stoichiometric amorphous tantalum oxide (TaOx), the electron energy loss spectrum (EELS) of model systems is characterized theoretically and electron transport characteristics are analyzed in detail. Calculated blue‐shifts by increasing sub‐stoichiometry explained the measurements, potentially suggesting estimation of oxygen vacancy concentrations through EEL spectra. Electron transport results based on TaOx material models validated by EELS measurements show that oxygen vacancy filamentary paths are initiated at low bias upon increasing sub‐stoichiometry yet noting an interplay with the local amorphous structure. Contact resistances at interfaces of the TaOx switching layer and a tantalum scavenging layer or titanium nitride electrode are quantified, indicating the possibility for either oxygen vacancy‐ or metal cluster‐based conduction mechanisms at the interface. The computational work, combined with experimental characterization for validation, provides a basis for investigating effects of sub‐stoichiometry on filament formation in TaOx thin film memristive devices.

Simultaneously improving the fire safety and mechanical properties of epoxy resin with iron phosphonated grafted polyethylenimine

Endowing epoxy resins with enhanced flame retardancy, toughness and strength as well as glass transition temperature (Tg) simultaneously is a big challenge in academia and industry fields. Herein, a hyperbranched multifunctional additive (PEIP-Fe) was designed, which was prepared through a Mannich condensation reaction of polyethyleneimine and phosphorous acid and followed by combining with ferric. When 3 wt% of PEIP-Fe was incorporated into epoxy resin, the impact strength, tensile strength, flexural strength and Tg were all enhanced due to the multiple toughening mechanisms, good compatibility and the increased cross-linking density. Simultaneously, fire safety such as flame retardancy, smoke suppression and toxicity were all improved. The flame-retardant mechanism was verified in both condensed-phase and gas-phase by catalytic charring and free radical scavenging, forming compact and continuous char layer and releasing phosphorus-containing species respectively. This work provides new thinking for the design of multifunctional and high efficiency additives for epoxy resin.

Oxygen‐Scavenging Effects of Added Ti Layer in the TiN Gate of Metal‐Ferroelectric‐Insulator‐Semiconductor Capacitor with Al‐Doped HfO2 Ferroelectric Film

AbstractTi layer inserted in the TiN gate electrode effectively scavenges the oxygen in the low‐k interfacial SiO2 of the metal–ferroelectric–insulator–semiconductor (MFIS) capacitors with the ferroelectric Al‐doped HfO2 (HAO) thin film. The scavenging effect increases remanent polarization (Pr) and reduces coercive voltage (Vc) and capacitance equivalent thickness (CET) of the HAO films, particularly when the MFIS capacitor is annealed at 800 °C. Additionally, frequency dispersion of capacitance characteristics and interface trap density (Dit) calculations reveal that actively‐triggered oxygen‐scavenging effects also reduce defect or trap‐induced degradation. The Ti‐inserted MFIS structure exhibits fatigue‐free endurance and relatively low leakage current characteristics as compared to a structure without the Ti scavenging layer in high annealing temperature conditions required for crystallization of the HAO ferroelectric films.

14‐2: Student Paper: Enhanced Electrical Characteristics of Low‐Temperature Processed In‐Ga‐Zn‐O Thin‐Film Transistors with Oxygen Scavenging Layer

In this study, the oxygen scavenging layer (OSL) was introduced between channel and gate insulator to improve the electrical characteristics even at 200"C fabrication of amorphous indium‐gallium‐ zinc oxide thin‐film transistors (a‐IGZO TFTs). The OSL is a layer introduced between the channel layer and the gate insulator layer, which is thinly deposited to absorb oxygen in the effective channel of a‐IGZO with annealing process. MgOx was used to make the OSL. The a‐IGZO TFTs with OSL, even if annealed at a low‐temperature of 200°C, exhibit improved electrical characteristics and stability under positive bias temperature stress (PBTS) compared to those without OSL: field‐effect mobility from 8.15 to 16.08 cm2/Vs, subthreshold swing from 0.45 to 0.42 V/decade, on/off current ratio from 3.19 × 108 to 9.53 × 109. A threshold voltage shift under PBTS at 50 °C for 10,000 sec was decreased from + 8.94 V to + 1.98 V. These improvements are attributed to Mg of OSL absorbing oxygen ions in the effective channel layer of a‐IGZO.

Dynamics of the spatial separation of electrons and mobile oxygen vacancies in oxide heterostructures

In the search for an oxide-based 2D electron system with a large concentration of highly mobile electrons, a promising strategy is to introduce electrons through donor doping while spatially separating electrons and donors to prevent scattering. In $\mathrm{SrTi}{\mathrm{O}}_{3}$, this can be achieved by tailoring the oxygen vacancy profile through reduction, e.g., by creating an interface with an oxygen scavenging layer. Through reduction, oxygen atoms are removed close to the interface, leaving behind oxygen vacancies in the $\mathrm{SrTi}{\mathrm{O}}_{3}$ lattice and mobile electrons in the $\mathrm{SrTi}{\mathrm{O}}_{3}$ conduction band. The commonly assumed picture is that the oxygen vacancies then remain confined close to the interface while the electrons leak a few nanometers into the bulk, resulting in an electron-defect separation and a highly mobile, oxide-based 2D electron system. So far it has remained unclear how the confinement and electron-defect separation develop over time. Here, we present transient finite element simulations that consider three driving forces acting on the oxygen vacancy distribution: diffusion due to the concentration gradient, drift due to the intrinsic electric field, and an oxygen vacancy trapping energy that holds oxygen vacancies at the interface. Our simulations show that at room temperature, three distinct regions are formed in $\mathrm{SrTi}{\mathrm{O}}_{3}$ within days: (1) Oxygen vacancies are partially held at the interface due to the oxygen vacancy trapping energy. (2) The accompanying positive space charge causes an oxygen vacancy depletion layer with large electron concentration and high mobility just below the interface. This electron-defect separation, indeed, leads to a highly conductive region. (3) While we are able to describe measured conductivity data with an oxygen vacancy trapping energy of $\ensuremath{-}0.2$ eV, this value does not prevent oxygen vacancy diffusion into the bulk: A diffusion front progresses into the bulk and leads to significant conductivity arising over the first micrometer within a couple of months. An enhanced oxygen vacancy trapping energy of $\ensuremath{-}0.5$ eV or below would suppress this loss of confinement, leading to a static and pronounced electron-defect separation. Consequently, our results highlight the importance of oxygen vacancy redistribution and suggest the trapping energy of oxygen vacancies at the interface as an important design parameter for oxygen-vacancy-based 2D electron systems.

Geographical variation in total phenolics, flavonoids and antioxidant activities of Eucalyptus camaldulensis leaves in Burkina Faso

This study aimed to elucidate the effect of sampling location of Eucalyptus camaldulensis leaves on antioxidant activity, total phenolic content (TPC) and total flavonoid content (TFC). Leaves of E. camaldulensis collected at three different locations in Burkina Faso (north, central, south-west) were subjected to hydrodistillation to obtain a water extract and subsequent ethanol extraction of the solid residue. The extracts were analysed for antioxidant activity using 2,2-diphenyl-1-picryl-hydrazyl (DPPH) radical scavenging, TPC, TFC and high-performance thin layer chromatography (HPTLC) fingerprint. TPC were higher in water extracts and presented considerable variation from 129±9 to 266±4 milligram Gallic Acid Equivalent per gram of extract (mg GAE/g) with the highest value 266±4 mg GAE/g at the south-west site. This aqueous extract also showed high antioxidant activity with EC50 8.3 µg/mL. Furthermore, HPTLC fingerprint of the extracts showed both band intensity and size differences. For TFC, we found a slight differences within both water and ethanolic extracts with the highest value of 38.3 ±4.2 mg QE/g for the south-west ethanolic extract. The noticeable impact of climate on chemical variability of E. camaldulensis leaves needs to be taken into consideration for any further applications based on TPC biological activity of the extracts. Key words: Eucalyptus camaldulensis, total phenolic content, total flavonoid content, antioxidant activity, sample location, chemical variation.

Quantifying the contributions of various emission sources to black carbon and assessment of control strategies in western China

In this study, an air quality model WRF-Chem (Weather Research and Forecasting Chemistry) was used to simulate meteorological conditions and surface black carbon (BC) concentrations in the western China from June 2016 to May 2017, given emissions from various sources in Asia. Comparison between simulations and measurements in western China showed that the model can capture the key spatial and temporal features of meteorological elements and surface BC concentrations. The modeling framework was then used to quantify the relative contributions of different emission sectors to BC concentrations via sensitivity experiments. Our results show that the residential emission sector presented the largest contribution in western China. The second largest contributor for the highly populated mega-cities (HM) region including Sichuan and Guanzhong basins, and for the remote background (RB) region covering the central part of the Tibetan Plateau (TP), was the industrial sector and the transportation sector, respectively. Power plants and open biomass burning sources played minor roles in the regional BC concentration. The seasonality of BC concentrations showed higher values in winter, mainly due to the residential winter heating under conditions of lower precipitation scavenging and poor boundary layer mixing. A further evaluation of emission control strategies shows that a 50% reduction of residential emissions caused annual mean surface BC concentrations in the RB and HM regions to decrease by 36.2% and 36.7%, respectively. In contrast, a 50% reduction in industrial emissions or transportation emissions led to less than 12% decreases in both regions. The 50% reduction of transportation emissions caused BC concentrations to decrease by 9.2% in the RB region, larger than the 5.9% decrease caused by a 50% reduction of industrial emissions. Transportation emissions were responsible for more BC pollution than industrial emissions for the RB region, in contrast to the highly-industrialized HM region. Therefore, more attention should be paid to transportation emissions when designing control strategies for air pollution over the TP. The results from this work provide useful information for local governments to prepare and implement air pollution guidelines in western China.

Ultralow Defect Density at Sub-0.5 nm HfO2/SiGe Interfaces via Selective Oxygen Scavenging.

The superior carrier mobility of SiGe alloys make them a highly desirable channel material in complementary metal-oxide-semiconductor (CMOS) transistors. Passivation of the SiGe surface and the associated minimization of interface defects between SiGe channels and high- k dielectrics continues to be a challenge for fabrication of high-performance SiGe CMOS. A primary source of interface defects is interfacial GeO x. This interfacial oxide can be decomposed using an oxygen-scavenging reactive gate metal, which nearly eliminates the interfacial oxides, thereby decreasing the amount of GeO x at the interface; the remaining ultrathin interlayer is consistent with a SiO x-rich interface. Density functional theory simulations demonstrate that a sub-0.5 nm thick SiO x-rich surface layer can produce an electrically passivated HfO2/SiGe interface. To form this SiO x-rich interlayer, metal gate stack designs including Al/HfO2/SiGe and Pd/Ti/TiN/nanolaminate (NL)/SiGe (NL: HfO2-Al2O3) were investigated. As compared to the control Ni-gated devices, those with Al/HfO2/SiGe gate stacks demonstrated more than an order of magnitude reduction in interface defect density with a sub-0.5 nm SiO x-rich interfacial layer. To further increase the oxide capacitance, the devices were fabricated with a Ti oxygen scavenging layer separated from the HfO2 by a conductive TiN diffusion barrier (remote scavenging). The Pd/Ti/TiN/NL/SiGe structures exhibited significant capacitance enhancement along with a reduction in interface defect density.

Smooth Interfacial Scavenging for Resistive Switching Oxide via the Formation of Highly Uniform Layers of Amorphous TaOx.

We demonstrate that the inclusion of a Ta interfacial layer is a remarkably effective strategy for forming interfacial oxygen defects at metal/oxide junctions. The insertion of an interfacial layer of a reactive metal, that is, a "scavenging" layer, has been recently proposed as a way to create a high concentration of oxygen defects at an interface in redox-based resistive switching devices, and growing interest has been given to the underlying mechanism. Through structural and chemical analyses of Pt/metal/SrTiO3/Pt structures, we reveal that the rate and amount of oxygen scavenging are not directly determined by the formation free energies in the oxidation reactions of the scavenging metal and unveil the important roles of oxygen diffusibility. Active oxygen scavenging and highly uniform oxidation via scavenging are revealed for a Ta interfacial layer with high oxygen diffusibility. In addition, the Ta scavenging layer is shown to exhibit a highly uniform structure and to form a very flat interface with SrTiO3, which are advantageous for the fabrication of a steep metal/oxide contact.

EELS Analysis of Oxygen Scavenging Effect in a Resistive Switching Structure of Pt/Ta/SrTiO3/Pt

ABSTRACTA complex mechanism of interfacial oxygen scavenging is revealed by electron energy-loss spectroscopy (EELS) for a resistive switching oxide of SrTiO3 with a scavenging layer of Ta. When Ta thin layer is inserted at one of the interfaces of Pt/SrTiO3/Pt structure, a large reduction of electrical resistance is induced for the structure, and oxygen defects are introduced at the interfacial part of SrTiO3. In the resistance decrease by voltage applications, simultaneous occurrence of oxidation and reduction of Ta scavenging layer is shown by EELS analyses from the low-loss spectra. The EELS and scanning transmission electron microscopy observations demonstrate that oxygen scavenging by Ta layer is an interfacial phenomenon where the redox reactions occur at the whole part of the interface. In addition, Pt electrode of the structure, which is chemically inert for oxidation, is revealed to have significant effects in the scavenging processes.

Effects of High-Pressure Hydrogen Annealing (HPHA) on Reliability Characteristics of RRAM.

Reliability characteristics (retention and endurance) of RRAM are critical for its practical realization and need to be improved. In this work, we confirmed the trade-off between retention and endurance by using various top electrode thickness conditions. The trade-off between retention and endurance characteristics was mainly due to the different amount of oxygen in scavenging layer (Ta) and the amount of oxygen vacancy in switching layer (HfO2). The amount of the oxygen in scavenging layer (Ta) and the amount of the oxygen vacancy in switching layer (HfO2) will be increased with the increase of Ta thickness. Therefore, the thicker Ta cells have worse retention because the large amount of oxygen in scavenging layer (Ta) can diffuse back into switching layer (HfO2) and recombine with oxygen vacancies in the filament. However, it has longer endurance because the large amount of oxygen vacancy in switching layer (HfO2) can be a source of the filament. Hence, there exists a trade-off relation between retention and endurance under the various Ta thickness conditions. To improve both retention and endurance characteristics, we proposed a new method by using high-pressure hydrogen annealing (HPHA). The thin Ta cells have longer retention and worse endurance because it has small amount of both oxygen in scavenging layer (Ta) and oxygen vacancy in switching layer (HfO2). Therefore, to generate more oxygen vacancies in switching layer (HfO2) maintaining small amount of oxygen in scavenging layer (Ta), we treated the samples by HPHA before Ta deposition. Finally, we obtained both improved retention and endurance characteristics in HfO2 based RRAM devices after high-pressure hydrogen annealing treatment.

Electrical Characteristics of Metal-Oxide-Semiconductor Capacitor with High-κ/Metal Gate Using Oxygen Scavenging Process.

It has been widely accepted that the mismatch of lattice constants between HfO2 and Si generates interface traps at the HfO2-Si interface, which causes the degradation of device performances. For better interface quality, very thin SiO2 film (< 2 nm) has been inserted as an interlayer (IL) between HfO2 and Si despite of the increase of EOT. In order to obtain both the better interface quality and the reduction of EOT, we used Ti metal on HfO2/IL SiO2 stack as a scavenging layer to absorb oxygens in the SiO2 and various annealing conditions were applied to optimize the thickness of the SiO2. As a result, we can effectively shrink the EOT from 3.55 nm to 1.15 nm while maintaining the same physical thickness of gate stacks. Furthermore, the diffusion of oxygen was confirmed by high resolution transmission electron microscopy (HRTEM) and time-of-flight secondary ion mass Spectrometry (SIMS).

Combining a multi deposition multi annealing technique with a scavenging (Ti) to improve the high-k/metal gate stack performance for a gate-last process

ALD HfO2 films fabricated by a novel multi deposition multi annealing (MDMA) technique are investigated, we have included samples both with and without a Ti scavenging layer. As compared to the reference gate stack treated by conventional one-time deposition and annealing (D&A), devices receiving MDMA show a significant reduction in leakage current. Meanwhile, EOT growth is effectively controlled by the Ti scavenging layer. This improvement strongly correlates with the cycle number of D&A (while keeping the total annealing time and total dielectrics thickness the same). Transmission electron microscope and energy-dispersive X-ray spectroscopy analysis suggests that oxygen incorporation into both the high-k film and the interfacial layer is likely to be responsible for the improvement of the device. This novel MDMA is promising for the development of gate stack technology in a gate last integration scheme.

Remote scavenging technology using a Ti/TiN capping layer interposed in a metal/high-k gate stack

High permittivity materials have been required to replace traditional SiO2 as the gate dielectric to extend Moore's law. However, growth of a thin SiO2-like interfacial layer (IL) is almost unavoidable during the deposition or subsequent high temperature annealing. This limits the scaling benefits of incorporating high-k dielectrics into transistors. In this work, a promising approach, in which an O-scavenging metal layer and a barrier layer preventing scavenged metal diffusing into the high-k gate dielectric are used to engineer the thickness of the IL, is reported. Using a Ti scavenging layer and TiN barrier layer on a HfO2 dielectric, the effective removal of the IL and almost no Ti diffusing into the HfO2 have been confirmed by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy.

Remote Scavenging Technology Using Ti/TiN Capping Layer Interposed in a Metal/High-k Gate Stack

High permittivity materials have been required to replace traditional SiO2 to be gate dielectric to extend Moore's law. However, growth of a thin SiO2-like interfacial layer (IL) is almost unavoidable during the deposition or subsequent high temperature anneal. This limits the scaling benefits of incorporating high-k dielectrics into transistors. In this work, a promising approach, in which O-scavenging metal layer and a barrier layer preventing scavenging metal diffusing into high-k gate dielectric are used to engineer the thickness of the IL, is reported. Using a Ti scavenging layer and TiN barrier layer on HfO2 dielectric, the effective removal of interfacial layer (IL) and almost no Ti diffusing into HfO2, have been confirmed by high resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS).

Electrical Properties and Interfacial Structures of High-k/Metal Gate MOSCAP Using Ti/TiN Scavenging Stack between High-κ Dielectric and Metal Gate

High permittivity materials have been required to replace traditional SiO2 to be gate dielectric to extend Moore’s law. However, growth of a thin SiO2-like interfacial layer (IL) is almost unavoidable during the deposition or subsequent high temperature anneal. This limits the scaling benefits of incorporating high-k dielectrics into transistors. A promising approach, in which O-scavenging metal layer and a barrier layer preventing scavenging metal diffusing into high-k gate dielectric are used to engineer the thickness of the IL, is reported in this paper. Using a Ti scavenging layer and TiN barrier layer on HfO2 dielectric, the equivalent oxide thickness (EOT) decreased due to the effect of O-scavenging indicated by the C-V measurement results. From high resolution transmission electron microscopy (HRTEM) pictures and X-ray photoelectron spectroscopy (XPS), the removal of interfacial layer (IL) has been seen clearly.

Impacts of post-metallization annealing on the memory performance of Ti/HfO2-based resistive memory

Impacts of post-metallization annealing (PMA) on bipolar resistance switching of Ti/HfOx stacked films were investigated. A Ti capping film as a scavenging layer with assistance of PMA is used to tune the dielectric strength of the 10-nm-thick HfOx layer. The polycrystalline microstructure of 10-nm-thick HfOx seems immune to the temperature of PMA in this work. The initial resistance and forming voltage in the Ti/HfOx devices mitigate as the increment of the annealing temperature. With enough annealing temperature (>450 °C), the device shows a good on/off ratio, high temperature operation ability and robust endurance (>106 cycles). Through the reaction between Ti and HfOx at 500 °C, the abundant oxygen ions are depleted from the insulator and the left charge-defects building conductive percolative paths in the dielectric layer. The operation-polarity independence of the form-free HfOx device in initial state is demonstrated. The forming-free memory with initial low resistance of 800 Ω at 0.1 V can be operated with stable bipolar resistance switching via initially positive or negative voltage sweep. The formless device with 10 nm thick HfOx also exhibits excellent nonvolatile memory performances, including enough on/off ratio, improved HRS uniformity and good high temperature retention (3 × 104 s at 200 °C). The results of this work suggest that the PMA temperature will affect the memory window and cycling reliability of the Ti/HfOx-based resistive memory. Optimum temperature (450 °C) will improve the memory performance of the Ti/HfOx stacked layer.

Nature of the filament formed in HfO2-based resistive random access memory

The conduction mechanism and the nature of the filament formed in the low-resistive state of TiN/HfO2/TiN resistive random access memory devices with and without the insertion of an Hf anodic interlayer were studied from the temperature dependence of their current–voltage characteristics. These characteristics were analyzed using the percolation theory of conductor networks, allowing to extract the effective resistance of the filament. In the samples without the Hf interlayer the effective resistance decreased with temperature, suggesting a semiconducting nature of the filament. Conversely, the introduction of an Hf interlayer in the structure, likely acting as an oxygen scavenging layer, yielded a filament showing a metallic-like behaviour, i.e. with a significantly reduced temperature dependence of its effective resistance. These results can be explained by taking into account the role of oxygen vacancies created in the HfO2 layer during the filament formation, as these cause a transition from the original insulating material to a semiconducting or metallic sub-stoichiometric composition, depending on their local density.

Technique to improve performance of Al2O3 interpoly dielectric using a La2O3 interface scavenging layer for floating gate memory structures

A technique of scavenging the SiO2 interfacial layer (IL) to improve the electrical performance of Al2O3 as the interpoly dielectric for flash memories has been studied. Scavenging was performed by the reaction of a thin La2O3 layer with the native oxide to form a high-κ lanthanum silicate. Significant improvement in the charge trapping and leakage characteristics were obtained. Transmission electron microscopy analysis was done to corroborate the electrical results. Results show that seven orders of magnitude leakage reduction was achieved by the replacement of the SiO2 IL with a higher-κ dielectric LaSiO at the Si interface.