Unipolar resistive switching characteristics in Co3O4 films

Unipolar resistive switching behavior has been investigated in Pt/Co₃O₄/Pt stacks. The resistance ratio of the high- and low- resistance states is over 5 × 10³. The “ON/OFF” operation of the memory cells can be repeated more than 200 times at room temperature. The resistance of the two states can be kept for more than 16 h without showing degradation. The temperature dependence of the resistance shows a metallic behavior at the low-resistance state, but a semiconductor-behavior at the high-resistance state. The mechanism responsible for the observed unipolar resistive switching behavior has been discussed.     

Optical properties of the c-axis oriented LiNbO3 thin film

C-axis oriented Lithium Niobate (LiNbO₃) thin films have been deposited onto epitaxially matched (001) sapphire substrate using pulsed laser deposition technique. Structural and optical properties of the thin films have been studied using the X-ray diffraction (XRD) and UV-Visible spectroscopy respectively. Raman spectroscopy has been used to study the optical phonon modes and defects in the c-axis oriented LiNbO₃ thin films. XRD analysis indicates the presence of stress in the as-grown LiNbO₃ thin films and is attributed to the small lattice mismatch between LiNbO₃ and sapphire. Refractive index (n = 2.13 at 640 nm) of the (006) LiNbO₃ thin films was found to be slightly lower from the corresponding bulk value (n = 2.28). Various factors responsible for the deviation in the refractive index of (006) LiNbO₃ thin films from the corresponding bulk value are discussed and the deviation is mainly attributed to the lattice contraction due to the presence of stress in deposited film.     

Resistive switching characteristics of multilayered (HfO2/Al2O3)n n = 19 thin film

A transparent resistive random access memory used as Indium Tin Oxide (ITO) electrode, ITO/HfO₂/Al₂O₃/…/HfO₂/Al₂O₃/ITO capacitor structure is fabricated on glass substrate by atomic layer deposition. The unipolar resistive switching characteristics can be performed by applying the positive- or negative-bias through top electrode, however, the differences of switching and stability in the two different operations can be observed. The diversities of electrical property are attributed to different oxide/ITO interface materials, which influence the current flow of the injected electrons.     

Microstructure and ferroelectric properties of epitaxial BaTiO3/SrRuO3/SrTiO3 (001) films grown by pulsed laser deposition under high oxygen pressure

BaTiO₃ films were epitaxially grown on SrTiO₃ (001) substrates buffered with SrRuO₃ films as bottom electrode by pulsed laser deposition under high oxygen pressure of 30 Pa. The quality of the BaTiO₃/SrRuO₃/SrTiO₃ multilayer films was analyzed by means of X-ray diffraction, atomic force microscopy and transmission electron microscopy. BaTiO₃ films were found to be highly c-axis-oriented tetragonal phase with c/a = 1.002. The dielectric constant first increased with increasing temperature, and showed a peak at the Curie temperature of about 356 K. The films had well-saturated hysteresis loops with a remnant polarization of 7.3 μC/cm² and a coercive field of 29.5 kV/cm at room temperature.     

Effect of oxygen pressure on the structural and optical properties of ZnO thin films on Si (111) by PLD

ZnO thin films were grown on Si (111) substrates by pulsed laser deposition (PLD) at various oxygen pressures in order to investigate the structural and optical properties of the films. The optical properties of the films were studied by photoluminescence spectra using a 325 nm He-Cd laser. The structural and morphological properties of the films were investigated by XRD and AFM measurements, respectively. The results suggest that films grown at 20 Pa and 50 Pa have excellent UV emission and high-quality crystallinity. The research of PL spectra indicates that UV emission is due to excitonic combination, the green band is due to the replacing of Zn in the crystal lattice for O and the blue band is due to the O vacancies.     

Reproducible unipolar resistive switching behaviors in the metal-deficient CoOx thin film

Pt/CoO_x/Pt tri-layers exhibited reproducible and stable unipolar switching under a dc sweeping voltage. In order to investigate the role of oxygen reduction in the metal-deficient CoO_x layer, resistive switching of the post-annealed CoO_x thin film was compared with those of the as-deposited CoO_x thin film. X-ray photoemission spectroscopy showed larger reproducible resistance switching and decreasing of current level in the post-annealed CoO_x thin film. This may be explained by a reduction in oxygen stoichiometry without phase transformation of the CoO_x. In addition, stable switching in post-annealed CoO_x layer is considered to originate from the decrease of the Co vacancies in local Co₃O₄ region partially distributed in the whole CoO_x layer, not in the dominant CoO.     

Mechanisms of current conduction in Pt/BaTiO3/Pt resistive switching cell

The 80-nm-thickness BaTiO₃ (BT) thin film was prepared on the Pt/Ti/SiO₂/Si substrate by the RF magnetron sputtering technique. The Pt/BT/Pt/Ti/SiO₂/Si structure was investigated using X-ray diffraction and scanning electron microscopy. The current-voltage characteristic measurements were performed. The bipolar resistive switching behavior was found in the Pt/BT/Pt cell. The current-voltage curves were well fitted in different voltage regions at the high resistance state (HRS) and the low resistance state (LRS), respectively. The conduction mechanisms are concluded to be Ohmic conduction and Schottky emission at the LRS, while space-charge-limited conduction and Poole-Frenkel emission at the HRS. The electroforming and switching processes were explained in terms of the valence change mechanism, in which oxygen vacancies play a key role in forming conducting paths.     

Unipolar resistive switching behavior of BiFeO3 thin films prepared by chemical solution deposition

Bismuth ferrite (BiFeO₃, BFO) thin films were spin-coated on Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. The ferroelectric BFO films annealed at 500 °C and 550 °C were found to possess unipolar resistive switching behaviors. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) of the unipolar resistance switching is about 10³ for the ferroelectric BFO films. The conduction mechanisms are concluded to be space charge-limited conduction for the initial state and Ohmic conduction for the LRS. As for the HRS, the Poole-Frenkel emission fits well in the whole voltage region. Traps composed of oxygen vacancies are considered to play a key role in forming conducting paths. The relaxation time of electronic carriers is much shorter than that of ionic oxygen vacancies; therefore, the resistance switching is considered more probably due to carrier injection and emission through the Poole-Frenkel model after forming.     

Optical rib waveguide based on epitaxial Ba0.7Sr0.3TiO3 thin film grown on MgO

Barium strontium titanate (Ba_0.7Sr_0.3TiO₃, abbreviated as BST) thin films were grown on MgO (001) single crystal substrates using pulsed laser deposition. A heteroepitaxial growth relationship of <001> _BST // <001> _MgO was confirmed by X-ray diffraction. The bidimensional structure of a rib waveguide was designed using the effective index method. A BST/MgO rib waveguide was fabricated using photolithography and reactive ion etching techniques. A single-mode near-field output pattern was observed using an end-fire coupling method at a wavelength of 1550 nm, which is consistent with our numerical calculation.     

Preparation and characterization of novel CuClSe2 semiconductor thin film

CuClSe₂ was synthesized by solid-state reaction between copper chloride and selenium at 300 °C. CuClSe₂ thin film was prepared on a glass substrate by pulsed laser deposition (PLD) method. XRD (X-ray diffraction) analysis revealed that the CuClSe₂ thin film has a preferred surface orientation parallel to (006). The transmittance and reflectance spectra of the film indicated that the compound is an indirect band gap material; the energy band gap is about 1.45 eV; its absorption coefficients are in the range of 10⁴-10⁵ cm^− 1 when the wavelength is shorter than 720 nm. The melting point of CuClSe₂ is about 328 °C. These results show that CuClSe₂ is a potential absorber layer material applied in solar cells.     

Effects of ambient oxygen pressures and substrate temperatures in pulsed laser deposited Zn0.85Li0.15O thin films

Li doped zinc oxide Zn_1−xLi_xO (x = 0.15) thin films were grown by using the pulsed laser deposition method. The depositions were done onto Pt(111)/Ti/SiO₂/Si(100) substrate set at temperatures ranging from 300 °C to 700 °C, with varying the ambient O₂ pressure range of 3-20 mTorr. The effects of substrate temperatures and ambient O₂ pressures on the surface morphology and structural properties of the Zn_0.85Li_0.15O thin films were investigated by using the scanning probe microscopy and X-ray diffraction spectra, respectively. Also the chemical structures of the films were investigated by observing the X-ray photoelectron spectra of the core and shallower levels. We observed the deep blue PL emissions centered at about 390 nm (3.20 eV) from the Zn_0.85Li_0.15O thin films. It was investigated with respect to the ambient O₂ pressures during the deposition. It is considered that the deep blue PL emission in the Zn_0.85Li_0.15O thin film may be related to the incorporation of oxygen vacancies.     

Optical properties and aging of gasochromic WO3

The limits of WO₃ as an optical gas sensor were derived by establishing the change in optical constants induced by 2% H₂ in Ar. Using Langmuir's adsorption equation, it was found that at low H₂ concentrations a high sensitivity is predicted, but the coloration could saturate at 57.9% of the material's maximum ion adsorption. The air poisoning problem observed in these devices was shown to be remedied by coating with a permeable polydimethylsiloxane layer, thus eliminating common atmospheric gases as the possible poisoning agents.     

When Pt meets BaTiO3: Interaction between metal nanoparticles and oxide thin films

With the help of a focused ion beam, we can control the location of BaTiO₃ (BTO) grains during pulsed laser deposition by the use of metal nanoparticles seeds as templates. The metal nanoparticles act as nucleation sites on which to grow the BTO grains. Combined with recent work where control of BTO grain orientation was successfully demonstrated [Thin Solid Films, 518, 5806 (2010)], we can widely use the method to deposit various oxide nanoparticles in particular locations with controlled size and orientation. Such an approach could be useful for nanotechnology and device fabrication.     

Effects of periodicity and oxygen partial pressure on the crystallinity and dielectric property of artificial SrTiO3/BaTiO3 superlattices integrated on Si substrates by pulsed laser deposition method

Epitaxial SrTiO₃(STO)/BaTiO₃(BTO) artificial superlattices have been grown on TiN buffered Si (001) substrates by pulsed laser deposition method and the effects of stacking periodicity and processing oxygen partial pressure on their crystallinity and dielectric properties were studied. The crystal orientation, epitaxy nature, and microstructure of STO/BTO superlattices were investigated using X-ray diffraction and transmission electron microscopy. The TiN buffer layer and superlattice thin films were grown with cube-on-cube epitaxial orientation relationship of [110](001)_films∣∣[110](001)_TiN∣∣[110](001)_Si. The c-axis lattice parameter of the STO/BTO superlattice decreased from 0.412 nm to 0.406 nm with increasing oxygen partial pressure and the dielectric constants, measured at the frequency of 100 kHz at room temperature, of the superlattices with 2 nm/2 nm periodicity increased from 312 at 1 × 10^− 5 Torr to 596 at 1 × 10^− 3 Torr. The dielectric constants of superlattices grown at oxygen partial pressure of 1 × 10^− 3 Torr increased from 264 to 678 with decreasing periodicity of the superlattices from 10 nm/10 nm to 1 nm/1 nm.     

Comparison study of microstructure, chemical composition and optical properties between resistive heating and electron beam evaporated LaF3 thin films

LaF₃ thin films were deposited by electron beam (EB) and resistive heating (RH) evaporation, respectively. Properties such as microstructure, chemical composition, surface morphology and optical constants of the LaF₃ thin films were characterized by measurements of X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and spectrophotometer, then comparison was made between this two deposition methods. It's found that the microstructure properties of the LaF₃ films deposited by these two methods were different, and slight content of oxyfluoride films was formed during deposition according to the result of chemical composition analysis. The microstructure of LaF₃ bulk materials after interaction with electron beam and resistive heating was also characterized to analyze how the two deposition processes affect the formation of LaF₃ thin films and their microstructure properties. When it was for the laser resistance of the films, although the EB evaporated LaF₃ thin films occupied lower absorption and optical loss than those of the RH films, they showed slightly smaller laser induced damage thresholds at 355 nm, which was thought to be related to their much more rougher surface and higher tensile stress.     

Pulsed laser deposition of La0.67Ca0.33MnO3 thin films on LiNbO3 and surface acoustic wave studies

Surface Acoustic Waves on piezoelectric substrates can be used to investigate the dynamic conductivity of thin films in a non-contact and very sensitive way, especially at low conductivities. Here, we report on such surface acoustic wave studies to characterize thin manganite film like La_0.67Ca_0.33MnO₃, exhibiting a Jan Teller effect with a strong electron phonon interaction and a metal insulator transition at high temperatures.

We report on the deposition of La_0.67Ca_0.33MnO₃ on piezoelectric substrates (LiNbO₃ in different crystal cuts, employing a pulsed laser deposition technique. The structural quality of the thin films are examined by X-Ray Diffraction, Scanning Electron Microscope and Energy Dispersive X-ray spectroscopy. For the electrical characterization, we employ the surface acoustic wave technique, accompanied by conventional direct current resistance measurements for comparison.     

Growth of CuInO2 thin film using highly dense Cu2O-In2O3 composite targets

It is reported that CuInO₂ single phase is hardly obtainable by using the general mixed oxide synthesis route [M. Shimode, M. Sasaki, K. Mukaida, J. Solid State Chem. 151(1) (2000) 16.; L. Liu, K. Bai, H. Gong, P. Wu, Phys. Rev. B 72(12) (2005) 1252041.; H. Hahnn, C. Lorent, Z. Anorg. Allg. Chem. 279 (1955) 281.]. Therefore, the manufacturing of highly dense single phase targets for CuInO₂ thin film growing is still challenging. In this study, a Cu₂O-In₂O₃ composite target instead of a CuInO₂ single phase target was fabricated by using the general mixed oxide synthesis route. Thin films of CuInO₂ were grown on sapphire substrates by a pulsed laser deposition (PLD) process under various conditions, and c-axis oriented single phase CuInO₂ thin films were grown successfully when oxygen partial pressure was controlled.     

Unipolar resistive switching behavior in Dy2O3 films for nonvolatile memory applications

In this article, we report the forming-free resistive switching behavior of a Ru/Dy₂O₃/TaN memory device incorporating a Dy₂O₃ thin film fabricated entirely through processing at room temperature. We used X-ray diffraction, secondary ion mass spectrometry, and X-ray photoelectron spectroscopy to investigate the structural and chemical features of the Dy₂O₃ film. The dominant conduction mechanisms in the low- and high-resistance states were ohmic behavior and Poole-Frenkel emission, respectively. The Ru/Dy₂O₃/TaN memory device exhibited a high resistance ratio and provided nondestructive readout and reliable data retention. This memory device has a great potential for application in nonvolatile resistive switching memory.     

Dependence of magnetic properties on the Fe ion in Ba-doped BiFeO3 multiferroic films

Ba-doped BiFeO₃ thin films were prepared on Pt/TiO₂/SiO₂/Si (100) substrate at various oxygen pressures by pulsed laser deposition. The influences of oxygen pressure on the crystalline structures, magnetic and electrical properties of these films were studied. Studies showed that the oxygen vacancy content and accordingly the leakage current density of the films decreased with the increase of the oxygen pressure, and the concentration of the Fe²⁺ increased with the decrease of the oxygen pressure. Strong ferromagnetism was observed in these films. The magnetization of the films increased with the decrease of the oxygen pressure and the largest remnant magnetization of 2.46 × 10⁴ A/m was obtained at an oxygen pressure of 0.7 Pa. This enhancement of the magnetization can be attributed to the increase of the Fe²⁺ concentration which may be related closely to the increase of the oxygen vacancy in the films.     

Oxygen pressure dependent VO2 crystal film preparation and the interfacial epitaxial growth study

High quality VO₂ crystal films have been prepared on sapphire substrates by pulsed laser deposition method and the effects of oxygen pressure on the crystal phase structure are investigated. Results indicate that the phases and microstructures of VO₂ films are strongly sensitive to oxygen pressure. High oxygen pressure tends to form coarse B-VO₂ nanocrystals while low pressure favors a flat M1-VO₂ film epitaxial growth. X-ray diffraction φ-scan patterns confirm the [020] epitaxial growth orientation of the M1-VO₂ film and the in-plane lattice epitaxial relationship at the interface is also examined. Raman spectra indicate that M1-VO₂ phase has much stronger Raman scattering modes than B-VO₂, and the clear phonon modes further confirm the idea stoichiometry of VO₂ crystal film. Infrared transmittance spectra as the function of temperature are recorded and the results show that M1-VO₂ crystal films undergo a distinct infrared transmittance variation across metal insulator transition boundary, while B-VO₂ exhibits negligible thermochromic switching properties in the temperature range concerned. The pronounced phase transition behavior of the M1-VO₂ crystal film makes it a promising candidate for optical filter/switch and smart window applications in the future.