Passivation Of Oxygen Vacancies Research Articles

The effect of gadolinium doping on the structural, magnetic and photoluminescence properties of electrospun bismuth ferrite nanofibers

Gadolinium (Gd) doped Bismuth ferrite (BFO) nanofibers (Bi1−xGdxFeO3 (x=0.0, 0.05, 0.10, 0.15 and 0.20)) were synthesized via electrospinning. Scanning Electron Microscope (SEM) analysis showed that the diameter of the nanofibers ranged from 150 to 250nm. X-Ray Diffraction (XRD) analysis revealed a structural phase transition with varying ‘x’, the compositions with x≤0.10 have crystal structures with space group R3c, while the compositions with x>0.10 have crystal structures with space group Pnma. Vibrating Sample Magnetometer (VSM) analysis exhibited the weak ferromagnetic nature of the BFO nanofibers. However an increase in the saturated magnetic moment with increase in Gd dopant concentration was observed. The Photoluminescence (PL) spectra of the Bi1−xGdxFeO3 nanofibers show enhanced Near Band Emission (NBE) intensity at x=0.10 due to the passivation of oxygen vacancies by Gd doping.

Effect of SiO<sub>2</sub> and SiO<sub>2</sub>/SiN<sub><italic>x</italic></sub> Passivation on the Stability of Amorphous Indium-Gallium Zinc-Oxide Thin-Film Transistors Under High Humidity

We studied the environmental stability of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with single-layer (SiO2) and bilayer (SiO2/SiN x ) passivation under high-humidity (80%) storage. During the 30 days of investigation, all single-layer passivated TFTs showed negative turn-ON voltage shifts ( $\Delta V_{\mathrm{{\scriptstyle ON}}}$ ), the size of which increased with storing time. The negative $\Delta ~V_{\mathrm{{\scriptstyle ON}}}$ is attributed to donor generation inside the active a-IGZO caused by the diffusion of ambient hydrogen/water molecules passing through the SiO2 passivation layer. The X-ray photoelectron spectroscopy depth profile for the SiO2 passivated structures confirms that the concentration of oxygen vacancies, which is initially larger at the a-IGZO/SiO2 interface, compared with the bulk a-IGZO, decreases after 30 days of storage under high humidity. This can be explained as the passivation of oxygen vacancies by diffused hydrogen. On the other hand, all bilayer passivated TFTs showed good air stability at room temperature and high humidity (80%).

Observation of defect state in highly ordered titanium dioxide nanotube arrays

For the first time, a conductive-substrate induced electro-deposition approach is employed to achieve highly ordered TiO2 nanotube arrays based on an anodic aluminum oxide template. Different from other methods, the morphology and parameters of arrays can be adjusted easily through changing the exposure area of a conductive layer. All these arrays are used as matrixes to explore the defect state emission by photoluminescence (PL) spectra. Interestingly, we find that the emission from blue edge to red edge (∼450 nm–600 nm) are apparently quenched in the ordered nanotube arrays, especially when compared to the PL spectra of nanowire arrays, single nanotube and nanoparticles. This distinct result originates from passivation of oxygen vacancies residing along the tube walls when the tubes are interconnected, which is further evidenced by the observation of PL spectra with crystalline phase and sintering. The passivation of defects suggests valuable charge transport perpendicular to the long axis of the tubes in the ordered arrays. This point is particularly significant to the design of highly efficient devices and the applications in various energy-related fields.

Electrical properties improvement of high-k HfO2 films by combination of C4F8 dual-frequency capacitively coupled plasmas treatment with thermal annealing

Abstract The effect of fluorine incorporation on the electrical properties of HfO 2 gate oxide were investigated, especially on the frequency dispersion, hysteresis and the density of interface states. By treating HfO 2 films using octafluorocyclobutane (C 4 F 8 ) 60 MHz/2 MHz dual-frequency capacitively coupled plasmas, fluorine atoms were incorporated into the HfO 2 films, but thinner C:F films also deposited on the surface of the HfO 2 films. After a following thermal annealing, the C:F films were removed, accompanied the formation of the C C group and Hf F bonds. By optimizing the low frequency (LF) power, the appropriate fluorine incorporation significantly improved the quality of the gate oxide, resulting in excellent electrical properties. At the LF power of 30 W, the smallest Δ V fb , hysteresis and the lowest interface state density were obtained. These improvements were attributed to the passivation of oxygen vacancies and the reduction of defects states density in the gap by forming Hf F bonds.

Improved characteristics for MOHOS memory with oxygen-rich GdO as charge storage layer annealed by NH3

Characteristics of metal–oxide–high-k–oxide–silicon (MOHOS) memories with oxygen-rich or oxygen-deficient GdO as charge storage layer annealed by NH3 or N2 are investigated. Transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction are used to analyze the cross-sectional quality, composition and crystallinity, respectively, of the stacked gate dielectric with a structure of Al/Al2O3/GdO/SiO2/Si. The MOHOS capacitor with oxygen-rich GdO annealed in NH3 exhibits a good trade-off among its memory properties: large memory window (4.8 V at ±12 V, 1 s), high programming speed (2.6 V at ±12 V/100 μs), good endurance and retention properties (window degradation of 5 % after 105 program/erase cycles and charge loss of 18.6 % at 85 °C after 10 years, respectively) due to passivation of oxygen vacancies, generation of deep-level traps in the grain boundaries of the GdO layer and suppression of the interlayer between GdO and SiO2 by the NH3 annealing.

Enhancing mechanism for room temperature ferromagnetism in ZnO films on amine functionalization

In this paper, we have reported the enhancing mechanism for room temperature ferromagnetism (RTFM) in pure and V doped ZnO (Zn0.95V0.05O and Zn0.90V0.10O) films on trioctylamine (amine) functionalization. X-ray diffraction patterns of amine functionalized ZnO films show slight weak reflection as compared to the corresponding as-deposited ZnO films. The presence of N 1s peak at 398.5 eV implies the formation of Zn–N bond on ZnO film surface. The quenching of visible emission on amine functionalization indicates the passivation of oxygen vacancies via Zn–N bonding. Amine functionalized ZnO films exhibit robust RTFM and the origin is discussed based on the experimental results.

Defect evolution on the optical properties of H+-implanted ZnO whiskers

The interplay of intentionally doped H impurity with native defects as well as its influence on the microstructure and optical properties of ZnO whiskers was systematically studied by varying the H+ implantation fluencies from 1 × 1014 to 1 × 1016 ions cm−2. Higher dose of H+ implantation resulted in the serious deterioration of the crystallinity and introduction of more nonradiative VO and Zni defects in the energy gap, as revealed by the X-ray diffraction (XRD), absorption spectra, Raman scattering, photoluminescence and electronic paramagnetic resonance (EPR) measurements. We demonstrate that H passivation of oxygen vacancies is in terms of forming V+0 defects rather than H+0 while deactivation of zinc vacancy related acceptor defects is through the process of hydrogen filling. The V+0 and ionized Zni defects are suggested to give rise to the H+-implantation induced blue band optical absorption.

Photoluminescence and positron annihilation spectroscopic investigation on a H+ irradiated ZnO single crystal

Low temperature photoluminescence and room temperature positron annihilation spectroscopy have been employed to investigate the defects incorporated by 6 MeV H+ ions in a hydrothermally grown ZnO single crystal. Prior to irradiation, the emission from donor bound excitons is at 3.378 eV (10 K). The irradiation creates an intense and narrow emission at 3.368 eV (10 K). The intensity of this peak is nearly four times that of the dominant near band edge peak of the pristine crystal. The characteristic features of the 3.368 eV emission indicate its origin as a ‘hydrogen at oxygen vacancy’ type defect. The positron annihilation lifetime measurement reveals a single component lifetime spectrum for both the unirradiated (164 ± 1 ps) and irradiated crystal (175 ± 1 ps). It reflects the fact that the positron lifetime and intensity of the new irradiation driven defect species are a little higher compared to those in the unirradiated crystal. However, the estimated defect concentration, even considering the high dynamic defect annihilation rate in ZnO, comes out to be ∼4 × 1017 cm−3 (using SRIM software). This is a very high defect concentration compared to the defect sensitivity of positron annihilation spectroscopy. A probable reason is the partial filling of the incorporated vacancies (positron traps), which in ZnO are zinc vacancies. The positron lifetime of ∼175 ps (in irradiated ZnO) is consistent with recent theoretical calculations for partially hydrogen-filled zinc vacancies in ZnO. Passivation of oxygen vacancies by hydrogen is also reflected in the photoluminescence results. A possible reason for such vacancy filling (at both Zn and O sites) due to irradiation has also been discussed.

Threshold shift observed in resistive switching in metal-oxide-semiconductor transistors and the effect of forming gas anneal

The resistive switching mechanism, which is crucial for the operations of resistive random access memory (RRAM) devices, is investigated using HfO2 based MOSFETs. After the SET operation, MOSFETs exhibit a threshold voltage (VT) shift that is found to be closely related to the formation of conductive filaments in the gate oxide. The RESET operation performed through a forming gas anneal treatment is found to have the same effect of applying a reverse polarity gate voltage sweep, as usually done in bipolar switching RRAM devices. After RESET, the gate current and VT measured shift back to their pristine levels, indicating the passivation of oxygen vacancies (forming the conductive path) as the most likely physical mechanism responsible for RRAMs RESET operation. Transmission electron microscopy analysis and physical simulations support these conclusions.

Constant voltage stress induced charge trapping and detrapping characteristics of the Si3N4 uniaxial strained n-channel metal-oxide-semiconductor field-effect-transistor with fluorinated HfO2/SiON gate stack

Charge trapping and detrapping characteristics of Si3N4 contact etch stop layer (SiN CESL) uniaxial strained n-channel metal-oxide-semiconductor field-effect-transistor (nMOSFET) with fluorinated HfO2/SiON gate stack have been investigated for the first time. Smaller threshold voltage shift can consequently obtain for the SiN CESL strained nMOSFET with fluorinated HfO2/SiON gate stack, primarily due to passivation of oxygen vacancies and dangling bonds by either nitrogen or fluorine atoms. However, the SiN CESL strained nMOSFET with fluorinated gate stack inevitably exhibits less charge detrapping ratio, which means greater part of stress-induced charges would remain in the gate stack after nitrogen or fluorine incorporation, respectively.

Morphological evolution of large-scale vertically aligned ZnO nanowires and their photoluminescence properties by hydrogen plasma treatment

ABSTRACTIn this report, large-scale vertically aligned ZnO nanowires, with diameter around 75 nm and length around 2-5 μm, were synthesized on a-plane sapphire by a single step chemical vapor deposition method. The XRD pattern of the as-prepared sample showed a strong ZnO (0002) peak and a weak ZnO (0004) peak that indicate good orientation and high crystal quality of the ZnO nanowires. The sample was then treated by hydrogen plasma, without exhibiting obvious structural damage to the nanowires. The photoluminescence spectra of as-prepared and H2-plasma-treated samples were then examined. A strong green emission peak (centered at 520 nm) was observed in the PL spectrum of as-prepared sample. In sharp contrast, a significant increase of the near-band edge emission (centered at 380 nm) and a strong decrease of the green emission (centered at 520 nm) were found in the PL spectrum of H2-plasma-treated sample. We propose that an efficient passivation of oxygen vacancies by H atoms will cause a drastic decrease of the green emission. More important, it would lead to a significant reduction of surface depletion layer, leading to a great enlargement of total effect area for UV emission. Meanwhile, the significant enhancement of the intensity of UV emission might also attribute to the combined effects of structure-induced waveguide behavior and UV amplified spontaneous emission. It is expected that the enhanced UV emission of vertically aligned ZnO nanowires can be used to improve the performance of UV light emitting devices.

Effects of oxygen plasma treatment on the dielectric properties of Ba0.7Sr0.3TiO3 thin films

Barium strontium titanate (BST) films prepared by RF magnetron sputtering were processed in an O2-plasma treatment system. Experiment results indicate that the leakage current and dielectric loss of the films were effectively reduced after plasma treatment. The possible mechanism may be attributed to the passivation of oxygen vacancies, which act as electric conduction paths of leakage current. The effects of the time, power and O2-plasma pressure of oxygen plasma treatment on the dielectric properties of BST thin films were further investigated and optimized. Compared to as-deposited films, the BST films treated in oxygen plasma for 5min, 200W and 3Torr demonstrated reduced loss tangent around 0.7% and leakage current density of 7.96×10−9A/cm2. However, it was also found that excessive plasma treatment would affect the dielectric properties of BST films negatively.

Effects of additive gases and plasma post-treatment on electrical properties and optical transmittance of ZnO thin films

A zinc oxide (ZnO) thin film as a channel layer in an oxide thin film transistor (TFT) has been characterized by investigating the effects of additive gases (such as hydrogen and oxygen) during growth and plasma treatment (using argon or hydrogen) after growth on its electrical, optical, and structural properties. By decreasing the additive gas ratio of O 2/H 2 or by increasing the treatment time of hydrogen plasma, the electrical resistivities of ZnO films were significantly reduced, and their transmittances and optical bandgap energies were blue-shifted in wavelength. These results were considered to be closely related to the passivation of oxygen vacancies as well as the formation of shallow donors that were induced by the injection of hydrogen in ZnO via gas addition and plasma treatment. In addition, the injection of hydrogen-including additive gas resulted in a decrease in grain size and crystallinity of ZnO films, whereas the plasma treatment hardly affected their crystalline structures.

Scanning tunneling microscopy study of nitrogen incorporated HfO2

The impact of nitrogen incorporation on the physical and electrical characteristics of the HfO2 is examined. X-ray photoelectron spectroscopy shows that nitrogen can be incorporated into the HfO2 via a two-step thermal anneal—first in ultrahigh vacuum (UHV) and subsequently in N2. Following the N2 anneal, scanning tunneling microscopy in UHV reveals a marked reduction in the low-voltage leakage current under gate injection biasing. From band theory and existing first-principles simulation results, one may consistently attribute this improvement to the passivation of oxygen vacancies in the HfO2 by nitrogen. Improvement in the breakdown strength of the HfO2 subjected to ramp-voltage stress (substrate injection) is also observed after the N2 anneal. The local current-voltage curves acquired concurrently during the ramp-voltage stress exhibit “space-charge limited conduction,” which implies that the observed improvement in breakdown strength may be related to a limitation of the current flow through the gate...

Cathodoluminescence study of Te-doped ZnO microstructures grown by a vapour–solid process

Sintering of a mixture of TeO_2 and ZnO powders under an Ar flux leads to the formation of elongated Te-doped ZnO nano- and microstructures on the sample surface. The growth of the structures occurs via a vapour-solid process. Cathodoluminescence (CL) and energy dispersive spectroscopy (EDS) measurements show an inhomogeneous incorporation of Te along the growth axis of the structures. An enhancement of the band edge emission upon Te doping is attributed to the passivation of oxygen vacancies by Te atoms, which reduces the deep energy level-related recombination path.

Oxygen vacancy induced charge trapping and positive bias temperature instability in HfO 2nMOSFET

The charge trapping and positive bias temperature instability (PBTI) are investigated at different post deposition annealing conditions (PDA) in HfO 2 nMOSFET. Pulse based measurements (Pulsed Id-Vg and “Pulse on the fly”) are performed to characterize charge trapping effect. Compared with NH 3 PDA, the NH 3 + O 2 PDA shows significant reduction of charge trap sites in HfO 2, which causes the improvement of device performance and reliability. The significant improvement after additional annealing can be explained by the passivation of oxygen vacancies in HfO 2.

Comparative study of passivation mechanism of oxygen vacancy with fluorine in HfO2 and HfSiO4

It has been experimentally observed that the incorporated fluorine will greatly improve the reliability of high-permittivity gate dielectric based transistors, but the role of fluorine passivation on leakage current change through gate is still a debated issue. The authors performed first-principles calculations to study the role of fluorine in oxygen vacancies in HfO2 and HfSiO4. They found that fluorine behaves completely differently in these two gate dielectrics. Fluorine can passivate the gap states of HfO2 completely but has no effect on the passivation of oxygen vacancies in HfSiO4, which is a possible explanation to the controversy surrounding varying leakage current.

Defect passivation in HfO2 gate oxide by fluorine

The authors have calculated that fluorine substituting for oxygen gives no gap states in HfO2. This accounts for the good passivation of oxygen vacancies by F seen experimentally. Bonding arguments are used to account for why F may be the most effective passivant in ionic oxides such as HfO2, whereas hydrogen is effective in covalent solids. General principles of defect passivation in ionic oxides are discussed. Vacancies are more detrimental in ionic oxides than in SiO2 because they are more likely to be charged in the presence of a gate electrode.

Enhancement of exciton emission from ZnO nanocrystalline films by pulsed laser annealing

Pulsed ArF laser annealing in air and in hydrogen atmosphere improves the optical properties of ZnO nanostructured films. Independently on the ambient atmosphere, laser annealing produces two major effects on the photoluminescence (PL) spectra: first, the efficiency of the exciton PL increases due to the decrease of the number of non-radiative recombination centers; second, the intensity of the defect-related orange band decreases because of the removing of excessive oxygen trapped into the films during deposition. However, annealing in the ambient air also increases the intensity of the green band related to oxygen vacancies. We show that the combination of laser annealing and passivation of oxygen vacancies by hydrogen results in films free of defect-related emission and keeps intact their nanostructural character.