Reduction Of Oxygen Vacancies Research Articles

Oxygen concentration and its effect on the leakage current in BiFeO3 thin films

Epitaxial c-axis oriented BiFeO3 (BFO) films were fabricated on (001) oriented SrTiO3 substrates by pulsed laser deposition. Nuclear resonance backscattering spectrometry was used to directly measure the oxygen concentration of as-deposited and annealed BFO films. Compared to the ideal stoichiometry of BFO, the as-deposited BFO film shows more than 10% oxygen deficiency. However, postannealing the as-deposited BFO films reduces the oxygen deficiency almost half. The reduced oxygen vacancies in annealed BFO films are believed to be responsible for the different leakage mechanisms and the two orders of magnitude drop in leakage current density.

Improved Uniformity of Resistive Switching Behaviors in HfO[sub 2] Thin Films with Embedded Al Layers

A technical solution is presented to improve the uniformity of HfO 2 -based resistive switching memory by embedding thin Al layers between HfO 2 and electrode layers. Compared with those pure HfO 2 devices, a remarkably improved uniformity of switching parameters such as forming voltages, set voltages, and resistances in high/low states was demonstrated in the HfO 2 devices with embedded Al layers. Al atoms are assumed to diffuse into HfO 2 thin films and are intended to localize oxygen vacancies due to reduced oxygen vacancy formation energy, thus stabilizing the generation of conductive filaments, which helps improve the resistive switching uniformity.

Effect of chemical modification on the structure and dielectric and magnetic properties of multiferroic (1 – x)BiFeO3-xDyFeO3 solid solution

The solid solution of (1-x)BiFeO3-xDyFeO3 was prepared by solid state reactions in the form of ceramics. The effects of chemical modification by means of aliovalent ionic substitution of Ti4+ for Fe3+ on the structure and dielectric properties were investigated. A morphotropic phase boundary bridging the perovskite rhombohedral phase and the orthoferrite orthorhombic phase was identified at x around 0.1. The chemical modification was found to stabilize the perovsite phase. The dielectric performance of the solid solution was improved by the substitution of Ti4+ for Fe3+, which decreased the electric conductivity by reducing oxygen vacancies, as evidenced by the decrease in loss tangent values. Magnetic hysteresis and large saturated magnetization (0.5 μB/f.u.) were realized in 0.92BiFeO3-0.08DyFeO3 with 2% substitution of Ti4+ for Fe3+, which is believed to arise from the disruption of the spiral spin modulation after structural modification, and the interaction between the spins of the Dy3+ and Fe3+ at low temperatures which decouples the antiferromagnetic order between the Fe3+ ions.

Optical properties of molybdenum oxide thin films deposited by chemical vapor transport of MoO3(OH)2

MoO3 thin films are useful for optical devices due to their electrochromic properties. In this study, deposition of MoO2 thin films was successfully accomplished by chemical vapor transport (CVT) of volatile MoO3(OH)2. Subsequently, a MoO3 thin film was obtained by annealing of the deposited MoO2 at 400°C in an O2 atmosphere. As annealing commenced, the optical transmittance of the films increased and their absorbance peaks were broadened and red-shifted due to reduced oxygen vacancy. Thus, the molybdenum oxide thin films can successfully be deposited using the CVT technique.

Surface chemical bonding states and ferroelectricity of Ce-doped BiFeO3 thin films prepared by sol–gel process

Bi1−x Ce x FeO3 (x = 0, 0.05, 0.1, 0.15 and 0.20) (BCFO) thin films were deposited on Pt/TiN/Si3N4/Si substrates by sol–gel technique. Crystal structures, surface chemical compositions and bonding states of BCFO films were investigated by X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Compared to BiFeO3 (BFO) counterparts, the fitted XPS narrow-scan spectra of Bi 4f7/2, Bi 4f5/2, Fe 2p3/2, Fe 2p1/2 and O 1s peaks for Bi0.8Ce0.2FeO3 film shift towards higher binding energy regions by amounts of 0.33, 0.29, 0.43, 0.58 and 0.49 eV, respectively. Dielectric constants and loss tangents of the BCFO (x = 0, 0.1 and 0.2) film capacitors are 159, 131, 116, 0.048, 0.041 and 0.035 at 1 MHz, respectively. Bi0.8Ce0.2FeO3 film has a higher remnant polarization (P r = 2.04 μC/cm2) than that of the BFO (P r = 1.08 μC/cm2) at 388 kV/cm. Leakage current density of the Bi0.8Ce0.2FeO3 capacitor is 1.47 × 10−4 A/cm2 at 388 kV/cm, which is about two orders of magnitude lower than that of the BFO counterpart. Furthermore, Ce cations are feasibly substituted for Bi3+ in the Bi0.8Ce0.2FeO3 matrix, possibly resulting in the enhanced ferroelectric properties for the decreased grain sizes and the reduced oxygen vacancies.

FATIGUE-FREE MULTIFERROIC Sc-DOPED BiFeO3 THIN FILMS PROCESSED BY A CHEMICAL SOLUTION DEPOSITION METHOD

ABSTRACT Sc-modified (0.3 mol%) multiferroic BiFeO3 thin films (BFS) were synthesized on Pt/Ti/SiO2/Si substrates through a chemical deposition method. The effect of Sc doping on the fatigue and retention properties, and polarization switching with magnetic poling of the films were investigated. Through Sc-doping, the film exhibited marked improvement in fatigue behavior at least up to 1011 cycles in comparison with the BFO film. The stable polarization retention property of the BSF film over the range of 1–105 seconds demonstrated that Sc doping in BFO improved the retention characteristics compared to BFO. This supports that Sc addition can reduce oxygen vacancies and charge defects in the BSF system which results in the excellent fatigue and retention properties. Magnetic field poling of 0.8T improved the polarization of the BSF film by about 48% which decreased slowly to ∼10% after 14 days. These results suggest that the BSF film has potential in ferroelectric and magnetic devices applications.

Effect of Rapid Thermal Annealing on the Electrical Characteristics of ZnO Thin-Film Transistors

Thin-film transistors (TFTs) with a bottom-gate configuration were fabricated with an RF magnetron sputtered undoped zinc oxide (ZnO) channel layer and plasma-enhanced chemical vapor deposition (PECVD) grown silicon nitride as a gate dielectric. Postfabrication rapid thermal annealing (RTA) and subsequent nitrous oxide (N2O) plasma treatment were employed to improve the performance of ZnO TFTs in terms of on-current and on/off current ratio. The RTA treatment increases the on-current of the TFT significantly, but it also increases its off-current. The off-current of 2×10-8 A and on/off current ratio of 3×103 obtained after the RTA treatment were improved to 10-10 A and 105, respectively, by the subsequent N2O plasma treatment. The better device performance can be attributed to the reduction of oxygen vacancies at the top region of the channel due to oxygen incorporation from the N2O plasma. X-ray photoelectron spectroscopy (XPS) analysis of the TFT samples showed that the RTA-treated ZnO surface has more oxygen vacancies than as-deposited samples, which results in the increased drain current. The XPS study also showed that the subsequent N2O plasma treatment reduces oxygen vacancies only at the surface of ZnO so that the better off-current and on/off current ratio can be obtained.

The Effect of Nanoscale Nonuniformity of Oxygen Vacancy on Electrical and Reliability Characteristics of $\hbox{HfO}_{2}$ MOSFET Devices

To understand the influence of oxygen vacancies in on the electrical and reliability characteristics, we have investigated area-dependent leakage-current characteristics of HfO2 with large-area device and conducting atomic force microscopy (C-AFM). Unlike with the large-area analysis with typical capacitor and transistor, a clear evidence of oxygen vacancy was observed in nanoscale-area measurement using the C-AFM. Similar observations were made in various postdeposition annealing ambients to investigate the generation and reduction of oxygen vacancy in HfO2 . With optimized postdeposition annealing for oxygen vacancy, significantly reduced charge trapping was observed in HfO2 nMOSFET.

Effect of N[sub 2]O Plasma Treatment on the Performance of ZnO TFTs

Postfabrication rapid thermal annealing (RTA) and subsequent nitrous oxide (N 2 O) plasma treatment improved the performance of zinc oxide (ZnO) thin-film transistors (TFTs) in terms of off current and on/off current ratio by almost 2 orders of magnitude. The off current of 2 × 10 -8 A and on/off current ratio of 3 × 10 3 obtained after RTA were improved to 10 -10 A and 10 5 , respectively, by the subsequent N 2 O plasma treatment. X-ray photoelectron spectroscopy analysis of the TFT samples showed that the RTA-treated ZnO surface had more oxygen vacancies as compared to as-deposited samples, and the oxygen vacancies at the surface of RTA-treated ZnO were reduced by subsequent N 2 O plasma treatment. The reduction of oxygen vacancies at the top region of the ZnO channel is the cause of better off current and on/off current ratio of the TFTs.

Structural and Magneto-transport Properties of Cr-doped In2O3 Diluted Magnetic Semiconductors

AbstractWe report the experimental study of the structural and magnetotransport properties of chromium-doped indium oxide (In2O3:Cr) thin films using x-ray diffractometer, and by measuring the resistivity and Hall effect as a function of temperature in various magnetic fields. The In2O3:Cr diluted magnetic semiconductor thin films were grown under different partial oxygen pressures (Po2) on sapphire substrates using pulsed laser deposition (PLD) technique. Observed expansions in lattice parameter and crystal size in these films with increase in oxygen growth pressure are traceable to the reduction in oxygen vacancies. A redshift of the absorption edges of the samples with increase in oxygen growth pressure is attributed to the significant improvement in crystallinity. The exchange interaction between the electron spins in the conduction band and the spins of the Cr 3d electrons was evident in the anomalous Hall effect (AHE), which persisted up to 300 K. An analysis of the dc electrical transport in the films was carried out using hopping conduction and ionized impurity scattering models.

The influence of the pressure and temperature on the light emission of the ZnO

A new route for the preparation of zinc oxide powder is described along with its characterization. A proteic sol was prepared dissolving zinc nitrate in filtered coconut water. After calcination at 1000 °C, the powder was compressed to 1.3×10 8 Pa and ZnO pellets were obtained. The emission spectra were recorded under UV excitation at 325 and 400 nm. The powder showed no spectroscopic response, whereas one peak around 396 nm was observed for the pressed powder (pellet with no heat treatment). The pellets were then annealed for 24 h at 500, 800 and 1000 °C. In the first case, bands at 396 and 440 nm and a structure of narrow peaks around 480 nm (oxygen vacancies) were observed. Increasing the annealing temperature led to a decrease in the intensity of the emissions at 440 and 480 nm. We propose that the high pressure induces a red-shift in the UV region of the ZnO nanopowder emission peaks to 396 nm. This is an indication that the ZnO nanopowder treated under pressure and sintering temperature exhibits the spectroscopic behavior characteristic of the ZnO single crystal. The disappearance of the 440 and 480 nm lines indicate the reduction of oxygen vacancies. The atomic force micrographs suggest a coalescence thermal point.

Improving Electrical Characteristics of Ta ∕ Ta2O5 ∕ Ta Capacitors Using Low-Temperature Inductively Coupled N2O Plasma Annealing

The electrical characteristics of capacitors are improved by treatments with inductively coupled plasma. A low-temperature and short process was used to reduce the leakage current and improve the reliability. A low leakage current density ( under ), high breakdown field ( at ), and lifetime of over 10 years at is obtained for the capacitor with the inductively coupled plasma treatment. The conduction mechanism of the leakage current in the capacitor is discussed using current-voltage analyses and shows that the leakage current of the capacitor is dominated by Schottky emission. plasma treatment can effectively reduce oxygen vacancies and the surface roughness of the film, inhibiting the conduction of the leakage current.

Changes in surface band bending, surface work function, and sheet resistance of undoped ZnO films due to (NH4)2Sx treatment

In this study, the interaction of undoped ZnO films with (NH4)2Sx treatment have been investigated by x-ray photoelectron spectroscopy, photoluminescence, optical transmittance, and four-point probe. According to the experimental results, we find that the formation of Zn–S bonds and the reduction of oxygen vacancies (i.e., the S occupation of oxygen vacancies) near the ZnO surface might lead to an increase in the upward band bending, resulting in an increase in the sheet resistance and work function of ZnO.

Electrical properties of PLZT thin films formed by photochemical metal-organic deposition with various Zr/Ti ratios

Lanthanum-doped lead zirconate titanate (PLZT) films formed by photochemical metal-organic deposition using photosensitive starting precursors were characterized. The substitution of La for Pb was found to induce the improved ferroelectric properties, especially fatigue property through the reduction of oxygen vacancies with excess electrons. But remnant polarization was reduced, because the doping of La gives the same effect as shift of composition of Zr/Ti ratio in PLZT film from morphotropic phase boundary composition of PZT(Zr/Ti ratio of 56/44). The measurement results of polarization-electric field and capacitance-voltage indicate that the composition of maximum ferroelectric properties of PLZT film shifts toward Ti-rich side between 52/48 and 48/52 of Zr/Ti ratio. A decrease in Zr/Ti ratio induced an improvement in I-V and fatigue properties due to the low defect concentration such as Pb or O vacancies and the inter-grain depletion of grain boundary limited conduction.

Influences of Cr Doping on the Electrical Properties in BiFeO3 Thin Films

Cr-doped BiFeO3 (BFO) thin films were deposited on Pt(200)/TiO2/SiO2/Si(100) substrates by a chemical solution deposition method. The dielectric constant and dissipation factor of the BFO thin films decrease from 165 and 0.054 (undoped) to 100 and 0.02 (3 mol% Cr-doped) at a frequency of 10 kHz as the Cr content increases. The leakage current and ferroelectric properties were improved significantly by means of Cr doping. The leakage current density of 4.1×10−6 A/cm2 for the 3 mol% Cr-doped BFO thin film is about four orders of magnitude lower than that of undoped BFO thin film at an external electric field of 100 kV/cm. The 3 mol% Cr-doped BFO thin film exhibited a well-saturated hysteresis loop with a large remanent polarization (P r) of 61 μC/cm2 at room temperature. The reason for the improved leakage current and ferroelectric properties in Cr-doped BFO thin films can be attributed to the reduced oxygen vacancies in the films by Cr doping. Communicated by Dr. George W. Taylor

Micro-structure and ferroelectric properties of BiFeO3 thin films formed on Pt-coated r-plane sapphire substrates

Magnetoelectric BiFeO3 (BFO) materials exhibit ferroelectric and ferromagnetic properties simultaneously, therefore they have a potential to be applied in magnetic as well as ferroelectric devices. BFO thin films were formed by depositing sol-gel solutions on Pt-coated r-plane sapphire dielectric substrates. We did not observe any secondary phase such as Bi2Fe4O9 on the r-plane sapphire substrates, which is generally observed on Si substrates. We observed small ferroelectric grains of about 0.1 μm on Pt/sapphire structures. The leakage current density in BFO films was found to be decreased dramatically after optimizing process conditions of stoichiometric BFO chemical solution. The leakage current densities were in the range of 10− 7 A/cm2 at room temperature and 10− 9 A/cm2 at 80 K under 0.4 MV/cm applied electric field. The main reason for low leakage current is considered to be reduction of oxygen vacancies due to the presence of exclusive Fe3 + valance state in the films. An applied electric field higher than 0.5 MV/cm was required to pole the BFO films, which made it difficult to obtain the saturated polarization at room temperature. We could measure the saturated remanent polarization in the BFO films at 80 K and the obtained remanent polarization was 100 μC/cm2.

Ni fully GermanoSilicide for gate electrode application in pMOSFETs with HfSiON gate dielectrics

A study on using a novel metal gate-the Ni fully GermanoSilicide (FUGESI)-in pMOSFETs is presented. Using HfSiON high-/spl kappa/ gate dielectrics and comparing to Ni fully Silicide (FUSI) devices, this paper demonstrates that the addition of Ge in poly-Si gate (with Ge/(Si+Ge)/spl sim/50%) results in: 1) an increase of the effective work function by /spl sim/ 210 mV due to Fermi-level unpinning effect; 2) an improved channel interface; 3) a reduced gate leakage; and 4) the superior negative bias temperature instability characteristics. Low-frequency noise measurement reveals a decreased 1/f and generation-recombination noise in FUGESI devices compared to FUSI devices, which is attributed to the reduced oxygen vacancies (V/sub o/)-related defects in the HfSiON dielectrics in FUGESI devices. The reduced V/sub o/-related defects stemming from Ge at FUGESI /HfSiON interface are correlated with the Fermi-level unpinning effect and the improved electrical characteristics observed in FUGESI devices.

Dependence of dc and ac conduction on the pre-sintering temperature in sol–gel derived Ba 0.5Sr 0.5TiO 3 thin films

The effect of pre-sintering temperature on the activation energies of dc and ac conduction in sol–gel spin-coated Ba 0.5Sr 0.5TiO 3 (BST) thin films is reported in this paper. Layer by layer coating of BST sol on Pt/TiN/SiO 2/Si substrates was done with varying pre-sintering temperatures (drying temperature for individual layers). Dominant mechanism of dc conduction was identified as Schottky process whereas ac conduction was governed by trap controlled hopping process. The activation energy for dc conduction decreased from 0.45 to 0.38 eV with increase in the pre-sintering temperature from 400 to 600 °C; whereas, that for ac conduction increased from 0.17 to 0.21 eV. The results have been correlated with grain growth phenomena and reduction of oxygen vacancies in the films with increase in the pre-sintering temperature.

Enhanced piezoelectric and pyroelectric effects in single-phase multiferroic Bi1−xNdxFeO3 (x=0–0.15) ceramics

Single-phase multiferroic Bi1−xNdxFeO3 (x=0–0.15) ceramics are polarized and their piezoelectric and pyroelectric effects are reported. The success in polarizing the ceramics originates from low electrical conductivities controlled predominantly by the Poole-Frenkel conduction mechanism. Limited internal traps due to reduced oxygen vacancies in the ceramics result in sufficiently low leakage current densities of <30mA∕m2 even at a high electric field of 145kV∕cm. The piezoelectric d33 coefficients of the ceramics before and after annealing at 550°C for 4h are measured to be ∼28 and ∼24pC∕N, respectively. These values are higher than those of most major lead-free high-temperature piezoelectric ceramics.

The Effect of Ar ∕ O2 Sputtering Gas on the Phosphorus-Doped p -Type ZnO Thin Films

The influence of gas on phosphorus-doped ZnO thin films deposited by radio frequency magnetron sputtering was studied. As the partial pressure of increased in the sputtering gas, the structure of changed from nanorods to a smooth thin film. The electron concentration of the thin film was also decreased due to the reduction in oxygen vacancies that act as donors. Photoluminescence spectra of thin films also showed a reduction in the intensity of the deep level emission peaks, due to the reduction in native defects related to the oxygen vacancies as the partial pressure was increased. The thin films grown under an gas ratio of showed -type characteristics after a rapid thermal annealing (RTA) activation process. The -type showed a resistivity of , a mobility of , with a hole concentration of .