Physical and electrical properties of band-engineered SiO2/(TiO2) x (SiO2)1−x stacks for nonvolatile memory applications

In our study, the physical properties of (TiO₂) _x (SiO₂)_1?x , including band-gap, band-offset, and thermal stability and the electrical properties of band-engineered SiO₂/(TiO₂) _x (SiO₂)_1?x tunnel barrier stacks, including the tunneling current and charge-trapping characteristics for applications to nonvolatile memory devices were investigated. It was observed that the band-gap and band-offset of (TiO₂) _x (SiO₂)_1?x can be controlled by adjustment in the composition of the (TiO₂) _x (SiO₂)_1?x films. Ti-silicate film with TiO₂:SiO₂ cycle ratio of 1:5 was maintained in an amorphous phase, even after annealing at 950 °C. The tunneling current of the band-engineered SiO₂/(TiO₂) _x (SiO₂)_1?x stacked tunnel barrier was larger than that of a single SiO₂ barrier under a higher external bias, while the tunneling current of a SiO₂/(TiO₂) _x (SiO₂)_1?x stacked tunnel barrier under a lower external bias was smaller. Charge-trapping tests showed that the voltage shift for SiO₂/(TiO₂) _x (SiO₂)_1?x is slightly larger than that for single SiO₂.     

Electric field effect on superconducting YBa2Cu3O7−δ films

Experimental evidence for a significant electric field effect in thin superconducting films of YBa₂Cu₃O_7– is presented. MISFET-type structures have been developed which allow the application of electric fields larger than 4×10⁶ V/cm across insulating SrTiO₃ barriers into thin cpitaxial YBa₂Cu₃O_7– channel layers. The results demonstrate that with these structures the electrical resistivity aboveT _c (R=0) and the density of free carriers in the YBa₂Cu₃O_7– films can be modified by 1–2% with gate voltages smaller than 50 V.     

Mössbauer effect study of the pseudo-binary system (Ti1−x Nb x )Fe2

Mössbauer spectroscopy and X-ray diffraction measurements have been done on (Ti_1–x Nb _x )Fe₂ compounds in order to investigate the effect of Nb on the magnetic properties of TiFe₂. The experimental results show that Nb enters the lattice by filling Ti sites, thereby forming a continuous phase over the whole range of Nb concentrations. The Mössbauer spectra at 80 K fitted with a magnetic hyperfine field distribution show a continuous decrease of the average magnetic hyperfine field with increasing Nb concentration, as well as several different magnetic configurations forx0.3.     

Diffusion and point defects in TiO2−x

The self-diffusion of ⁴⁴Ti has been measured both parallel to and perpendicular to the c axis in rutile single crystals by a serial-sectioning technique as a function of temperature (1000–1500°C) and oxygen partial pressure (10^?14 ? 1 atm). The oxygen-partial-pressure dependence of. D¹_Ti indicates that cation selfdiffusion occurs by an interstitial-type mechanism and that both trivalent and tetravalent interstitial titanium ions may contribute to cation self-diffusion. At p_o2 = 1.50 × 10^?7 atm where impurity-induced defects are unimportant,

$\text{D}{}^1{}_{\mathrm{<mtext>Ti</mtext>}}\text{(∥c)=}\text{6.50}\text{+1.33}\text{?1.11}\text{exp}\text{?}\text{(66.11±0.56}\text{kcal}\text{mole}\text{RT}\text{cm}{}^2\text{S}$

and

$\text{D}{}^1{}_{\mathrm{<mtext>Ti</mtext>}}\text{(⊥c)=}\text{4.55}\text{+1.78}\text{?1.28}\text{exp}\text{?}\text{(64.08±0.99)}\text{kcal}\text{mole}\text{RT}\text{cm}{}^2\text{S}\text{.}$

In the intrinsic region, the ratio D¹_Ti (⊥c)/D¹_Ti(∥c) was found to increase from 1.2 to 1.6 as the temperature decreased from 1500 to 1000°C. Computations based upon the defect model of Kofstad (involving the atomic defects Ti^..._iTi^...._iand V^.._o), of Marucco etal. (Ti^...._i and V^.._o), and of Blumenthal etal. (Ti^..._i and Ti^...._i) are compared with the experimental data on deviation from stoichiometry, electrical conductivity, cation self-diffusion and chemical diffusion in TiO_2?x. These comparisons provide values of the defect concentrations, cation-defect diffusivities, electron mobility and reasonable values of the correlation factor for cation diffusion by the interstitialcy mechanism. Only the model of Kofstad is inconsistent with the data.     

A study of Ba x Mg1 − x F2 optical films

Ba _x Me_{1 ? x} F₂ binary fluoride films (“Me” denotes calcium or magnesium fluoride) are studied. A method of processing the reflection and transmission spectra is proposed to determine the optical constants. The dispersion dependences of the refractive indices and absorption coefficients of films in the range of 1.3–12 μm are found. Dispersion in films in the regions of additional absorption bands, which are absent in single crystals, is observed for the first time. It is shown that the films of binary fluorides have a higher packing density, a lower absorption, and better operating characteristics than do films of pure fluorides. The films are promising for application as optical interference coatings in the mid-IR spectral region.     

Calcium doping effects in57Fe : La2−x Ca x CuO4−y oxides

La_2–x Ca _x Cu_0.99Fe_0.01O_4–y samples with 0.00 x 0.30 were studied by X-ray diffraction and Mössbauer spectroscopy after heat treatment under different atmospheres (air, oxygen and helium). The variation of the relative population and the hyperfine interactions of the sites as well as the change in the lattice parameters were obtained.On leave from National Center for Scientific Research, PO Box 6990, Havana, Cuba.     

Surface effect on the reverse current of Al x Ga1−x Sbp−n structures

The effect of an invertedp-region along the free surface ofn-Al _x Ga_1−x Sb on the reverse current ofp−n structures from the given solid solution is analyzed. Expressions which describe “collection” of the inverted layer current on the cylindrical surface of ann-region are discussed. The contribution of the near-surface and bulk components to the reverse current ofp−n structures with a semi-infiniten-region is estimated. For structures with a two-layern-region of finite thickness we have calculated the dependence of the near-surface current on the voltage across thep−n structure, the thickness of then-region, and its composition and doping level. We have compared the calculated current-voltage characteristics with experiment using a Al_0.15Ga_0.85Sbp−n structure as an example. Tomsk State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 34–40, January, 1999.     

Negatron effects in CdSe1 − x Te x and ZnS1−x Se x films

Various negatron effects in films of alloys of II–VI compounds deposited from solutions as a function of the deposition mode and heat treatment are studied. It is found that the negative photocapacitance effect, which was first discovered in ZnS_1?x Se _x films, and the slowly relaxing negative photoelectric effects, which are caused by the transition of electrons located in a nanoscale surface layer from the shallow energy levels of trapping centers to deeper levels with a lower polarizability and by the presence of nanoscale clusters in these materials, which play the role of a “reservoir” for minority charge carriers, occur according to a single mechanism. A model to explain the basic laws of negative photoconductivity in CdSe_{1 ? x} Te _x films deposited from a solution is proposed. Negative residual conductivity is explained in terms of double-barrier relief model, while negative differential photoconductivity is attributed to the presence of nanoscale electric domains.     

Effect of Ca doping level on the laser-induced voltages in tilted La1−x Ca x MnO3 (0.1 ≤ x ≤ 0.7) thin films

Tilted La_1?x Ca _x MnO₃ (0.1 ≤ x ≤ 0.7) thin films have been grown on vicinal cut LaAlO₃ (100) substrate by pulsed laser deposition. The laser-induced voltage effect was studied at room temperature with the KrF excimer laser using as the thermal source. The relationships between Ca doping level and voltage signal, response time and anisotropy Seebeck coefficient were established. The voltage signal and anisotropy Seebeck coefficient increase at first with increasing Ca doping level, reach a maximum at the same Ca content around x = 0.5, and then decrease. The respond time decreases with the Ca concentration increasing, and changes very little after x = 0.5. The figure of merit F _m was also the largest at this doping level, indicating a potential good performance of the photodetector devices. The variation of intrinsic structural and transport anisotropy induced by the change of Ca concentration has been proposed to account for the different LIV effects observed in LCMO thin films.     

A reassessment of chemical diffusivity in TiO2−x

Chemical diffusivity has been measured in rutile single crystals, at 1100°C and perpendicularly to the c-axis. Reequilibration kinetics are considerably slowed down by a surface reaction in CO/CO₂ mixtures while this effect vanishes in pure oxygen atmospheres. The chemical diffusion coefficient D̃ was found to be nearly independent of x in TiO_2−x.     

Hall effect of noncrystalline Zr−Cu films

Quench condensed Zr _x Cu_1–x films offer a wide concentration range (0.1x0.9) for measurements of the Hall coefficientR _H of amorphous Zr–Cu alloys.R _H changes sign as a function of composition, from negative to positive, as the Zr concentration is increased. The sign change is observed in a narrow concentration range at aboutx=0.22, without any peculiarity of the conductivity at this crossover concentration. The Hall coefficient of the unannealed Zr-rich films is nearly independent of temperature. However, both, the values ofR _H and of the temperature dependence ofR _H change with a heat treatment well below the crystallization temperature. This is most obvious for the films with a composition close to the cross-over concentration.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Effects of laser irradiation on optical properties of a-Se100−x Te x thin films

The effect of laser irradiation on the optical properties of thermally evaporated Se_100?x Te _x (x=8, 12, 16) chalcogenide thin films has been studied. The result shows that the irradiation causes a shift in the optical gap. The results have been analyzed on the basis of laser irradiation-induced defects in the film. The width of the tail of localized state in the band gap has been evaluated using the Urbach edge method. As the irradiation time increases, the values of the optical energy gap for all compositions decrease, while tail energy width increases. It is also observed that the optical energy gap decreases with increasing Te content in the alloy. These changes are a consequence of an increment in disorder produced by laser irradiation in the amorphous structure of thin film.     

Pb1−x Fe x films prepared by vapor condensation

ZF, LF and TF SR experiments with antiferromagnetic (AF) ceramical samples La_2–x Sr _x CuO_4–y have been performed in the temperature range 10–300 K. Zero field muon spin polarization functions obtained below the Neel temperature clearly show a nonzero initial precession phase-–0.35 rad. We propose an explanation based on existence of the dynamical magnetic fields on the muon.We thank Drs. A.G. Chistov and A.M. Brjazkalo from RSC Kurchatov Institute for the preparation the #2 La₂CuO_4–y sample.     

Structure and dielectric properties of Bi6 − x Sr x Ti2 − x Nb2 + x O18 (x = 0–2) solid solutions

The structural and electrophysical characteristics of a series of solid solutions of layered perovs-kite-like oxides Bi_{6 ? x} Sr _x Ti_{2 ? x} Nb_{2 + x} O₁₈ (x = 0, 0.25, 0.5, 1.0, 1.5, 2.0) have been studied. The temperature dependences of the relative permittivity ?/?₀(T) and dielectric loss tangent tanδ have been measured. The dependences of the maximum of the permittivity ?/?₀, Curie temperature T _C, lattice parameters, and the unit cell volume on x have been obtained. The structural parameter a, which corresponds to the polar direction, and the value of the orthorhombic distortion of the unit cell have been found to demonstrate noticeable negative deviations from the Vegard’s law. It has been established that the variations of the orthorhombic distortion correlate with the variations of the permittivity maximum; however, they do not markedly influence the Curie temperature that varies linearly over entire range of changes in x.     

Zr-induced structural changes in Hf1−x Zr x O2 high-k thin films

The local symmetries around the Hf sites in thin films of Hf_1?x Zr _x O₂/Si(100) were probed using grazing incidence extended X-ray absorption fine-structure spectroscopy (EXAFS). The effects of the Zr incorporation on the local crystal symmetries were investigated using Hf L₃ EXAFS at the Beamline X23A2 of the Brookhaven National Laboratory. The Zr ratios in the various films were set to between 0.0 and 1.0. Significant changes in the local environment were observed for x = 0.25 or greater values. For x = 0.0, the film local structure around Hf sites remain in the equilibrium monoclinic phase as referenced from our previous studies on HfO₂ thin films on Si(100). When Zr is introduced, tetragonal symmetry around the Hf atom appears and becomes dominant at x = 0.63. Using the EXAFS theoretical simulations and non-linear least-square fit results, the fractions of the monoclinic versus tetragonal phases were identified in each film.     

Control of tensile stress on inducing formation and tunability of (100) oriented Pb x Sr1−x TiO3 thin films

(100) Oriented Pb _x Sr_1?x TiO₃ (PSTO) thin films are prepared on indium tin oxide (ITO)/glass substrates by sol–gel technique while inserting doped PbTiO₃ (PTO)-inducing layer in between. The effect of tensile stress in PSTO on tunability and (100) orientation of the thin films was investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscope, and atomic force microscope, respectively. Results show that PSTO thin film deposited on doped PTO has (100) oriented structure while it is randomly oriented when deposited directly on the ITO/glass substrate. Lattice mismatch between PSTO and PTO appears, in which the in-plane lattice constant c is 0.3922–0.3924 nm in the former and 4.02–4.07 nm in the latter, respectively, contributing tensile stress in the PSTO due to different lattice constants between them. The stress in the PSTO thin film is 3.04, 3.15, 3.59 and 4.47 GPa when the doped PTOs are Fe–PTO, Tb–PTO, Co–PTO and Zn–PTO, respectively. The orientation degrees of PSTO thin films are from 89.63, 90.31, 91.92 to 93.29 % with increasing stress of PSTO on Fe–PTO, Tb–PTO, Co–PTO and Zn–PTO, respectively. Tunabilities of the well-oriented PSTO thin films increase in ascending order of 63 < 65 < 69 < 73 % when induced by oriented PTO layers of Fe–PTO, Tb–PTO, Co–PTO and Zn–PTO, respectively, which is in accordance with the degree of (100) orientation appearing in the thin films. The high tunability appears in the PSTO thin film while high (100) orientation is derived from the tensile stress. It is much higher than that of randomly oriented PSTO thin film.