Transparence and electrical properties of ZnO-based multilayer electrode

Three-layered ZnO/Ag–Ti/ZnO structures were prepared using both the sol-gel technique and DC magnetron sputtering. This study focuses on the electrical and optical properties of the ZnO/Ag–Ti/ZnO multilayers with various thicknesses of the Ag–Ti layer. The ZnO thin film prepared by the sol–gel method was dried at 300°C for 3 minutes, and a fixed thickness of 20 nm was obtained. The thickness of the Ag–Ti thin film was controlled by varying the sputtering time. The Ag–Ti layer substantially reduced the electrical resistivity of the sol–gel-sprayed ZnO thin films. The sheet resistance of the Ag–Ti layer decreased dramatically and then became steady beyond a sputtering time of 60 s. The sputtering time of Ag–Ti thin film deposition was determined to be 60 s, taking into account the optical transmittance. Consequently, the transmittance of the ZnO/Ag–Ti/ZnO multilayer films was 71% at 550 nm and 60% at 350 nm. The sheet resistance was 4.2 Ω/sq.     

Growth of zinc oxide nanostructures

Zinc oxide (ZnO) nanowhiskers have been prepared using a multilayer ZnO(50 nm)/Zn(20 nm)/ZnO(2μm) structure on a polished stainless steel (SS) substrate by high rate magnetron sputtering. The formation of uniformly distributed ZnO nanowhiskers with about 20 nm dia. and 2 to 5 μm length was observed after a postdeposition annealing of the prepared structure at 300–400° C. An array of highlyc-axis oriented ZnO columns (70–300 nm in dia. and up to 10 μm long) were grown on Si substrates by pulsed laser deposition (PLD) at a high pressure (1 Torr), and Raman studies showed the activation of surface phonon modes. The nanosized powder (15–20 nm) and nanoparticle ZnO films on glass substrate were also prepared by a chemical route. Nanowhiskers showed enhanced UV light detection characteristics, and the chemically prepared ZnO nanoparticle films exhibited good sensing properties for alcohol     

Structure and magnetic properties of Zn<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>O nanorods synthesized by a simple sol–gel method using metal acetylacetonate and poly(vinyl alcohol)

Room-temperature ferromagnetism was observed in Zn_0.9Co_0.1O nanorods with diameters and lengths of ∼100–200 nm and ∼200–1000 nm, respectively. Nanorods were synthesized by a simple sol–gel method using metal acetylacetonate powders of Zn and Co and poly(vinyl alcohol) gel. The XRD, FT-IR and SAED analyses indicated that the nanorods calcined at 873–1073 K have the pure ZnO wurtzite structure without any significant change in the structure affected by Co substitution. Optical absorption measurements showed absorption bands indicating the presence of Co²⁺ in substitution of Zn²⁺. The specific magnetization of the nanorods appeared to increase with a decrease in the lattice constant c of the wurtzite unit cell with the highest value being at 873 K calcination temperature. This magnetic behavior is similar to that of Zn_0.9Co_0.1O nanoparticles prepared by polymerizable precursor method. We suggest that this behavior might be related to hexagonal c-axis being favorable direction of magnetization in Co-doped ZnO and the 873 K (energy of 75 meV) being close to the exciton/donor binding energy of ZnO.     

Ti(Cr)缓冲层对用于垂直磁记录材料CoCrTa介质磁特性和微结构的影响

利用直流对靶磁控溅射技术在单晶Si衬底上制备了C/CoCrTa/X (X=Cr，Ti)介质材料.分别采用振动样品磁强计、X射线衍射仪、扫描探针显微镜对样品的磁性、微结构等进行了测试分析.研究发现，Ti缓冲层有利于样品中Co晶粒的易轴垂直于膜面生长.以Ti为缓冲层的样品，颗粒尺寸和表面粗糙度较小，而且磁畴明显，说明以Ti为缓冲层的薄膜样品更适宜做垂直磁记录. 关键词： CoCrTa 垂直磁记录 缓冲层 微结构     

Single pulse near field study on a Co(3 nm)/Cu(6 nm)/Co(20 nm) multilayer structure by using a femtosecond laser

Single pulse near field study on a Co(3 nm)/Cu(6 nm)/Co(20 nm) multilayer structure was experimentally investigated with a laser pulse width of 200 fs at a wavelength of 775 nm. For the near field experiments, we have used polystyrene colloidal particles of 700 nm diameter deposited by spin coating on top of the multilayer structure, as well on top of Co (50 nm) and Cu (50 nm) thin films. The diameter and the morphologies of the holes were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). We have estimated the fluence thresholds values for the near field and discuss their values in respect with the enhancement factor of the intensity of the electromagnetic field due to the use of the colloidal particles. We compare the depths and the widths of the holes obtained at the same peak laser fluence for the Co thin film (50 nm), Cu thin film (50 nm) and Co(3 nm)/Cu(6 nm)/Co(20 nm) multilayer structure. Depending on the laser fluence, the ablation depth can reach the first, the second, or the third layer. Theoretical estimations of the intensity enhancement were done using the finite-difference time-domain (FDTD) by using the RSoft software. This type of a selective distribution of the ablation depth, in the near field regime, of a planar metal/dielectric interface can open new perspective in the excitation of propagating surface plasmons.     

Co/Ti金属磁性多层膜的激光布里渊散射研究(英文)

在室温下利用直流磁控溅射法制备出了具有不同Co层厚度的 [Co(dConm) /Ti(dTinm) ]n 金属磁性多层膜 (其中dCoanddTi是Co和Ti的厚度 ,n薄膜调制周期数 )。在 30 0K下用X射线衍射法 (XRD)和布里渊散射方法研究了Co层厚度分别为dCo=1 ,2 ,2 5 ,3 5nm ,Ti层厚度为dTi=2 5nm的Co/Ti多层膜。XRD的结果所示在Co/Ti多层膜中Co层厚度超过 2 5nm ,其结构类似块材Co的多晶结构。对比之下 ,Co/Ti多层膜中Co层厚度低于 2 5nm ,其XRD衍射峰随Co层厚度减少变的峰形加宽 ,衍射峰的强度减少 ,峰位移动和消失。在Co/Ti多层膜中自旋波的布里渊散射结果表明 :在Co/Ti多层膜中 ,对于dCo>2 5nm磁性呈铁磁性耦合 ,在dCo≤ 2 5nm时 ,其呈反铁磁性耦合。除表面自旋波外 ,在呈反铁磁性耦合的材料中观测到了两个有体特征的自旋波 ,而在呈铁磁耦合的材料中只观测到一个有体特征的自旋波。     

Magneto-optical properties of nanogranulated Co-Ti-O films

The results from studies of the magneto-optical properties of Co-Ti-O nanogranulated films obtained under conditions of solid-state reaction with oxygen exchange in CoO/Ti layered structures are presented. The magneto-optical spectra of the polar Kerr effect in films with a relative volume of the magnetic fraction of 0.38 to 0.64 has a resonance character in the wavelength range of 450–650 nm. The Kerr rotation angle of the films increases by a factor of 3–5, relative to conventional Co films.     

Structure and lattice dynamics of heterostructures based on bismuth ferrite and barium strontium titanate on magnesium oxide substrates

Bismuth ferrite films doped with neodymium on MgO single-crystal substrates with an epitaxial barium strontium titanate thin (1–2 nm) sublayer have been prepared by rf sputtering of ceramic targets at an elevated oxygen partial pressure and at temperatures below the ferroelectric and magnetic transition temperatures. It has been revealed using X-ray diffraction and Raman scattering spectroscopy that, in these bismuth ferrite films, a new phase (not observed in bulk samples) is formed. The symmetry of this phase is monoclinic, the unit cell contains two formula units, and the spontaneous polarization vector deviates from the [111]_cub direction and can have different components along the x, y, and z axes.     

Low-resistance Cr/Al Ohmic contacts to N-polar n-type GaN for high-performance vertical light-emitting diodes

We investigated the electrical properties of Cr(30 nm)/Al(200 nm) contacts to N-polar n-type GaN for high-performance vertical light-emitting diodes and compare them with those of Ti(30 nm)/Al(200 nm) contacts. Before annealing, both the samples show ohmic behaviors with a contact resistivity of 1.9-2.3 × 10⁻⁴ Ωcm². Upon annealing at 250 °C for 1 min in N₂ ambient, the Ti/Al contacts become non-ohmic, while the Cr/Al contacts remain ohmic with a contact resistivity of 1.4 × 10⁻³ Ωcm². Based on X-ray photoemission spectroscopy and secondary ion mass spectrometry results, ohmic formation and degradation mechanisms are briefly described and discussed.     

Probing the effect of nitrogen gas on electrical conduction phenomena of ZnO and Cu-doped ZnO thin films prepared by spray pyrolysis

Zinc oxide (ZnO) and Cu-doped ZnO (CZO) thin films were prepared on borosilicate glass substrates by spray pyrolysis technique. The X-ray diffraction study revealed that Cu doping caused a reduction in crystallite size. AFM study showed an increase in roughness with doping. This is attributed to the aggregation of particles to form clusters. From transmission electron microscopy analysis, the particle size is measured to be in the range 30–65 nm (average particle size 48 nm) for undoped ZnO, whereas it is in the range 24–56 nm (average particle size 40 nm) for CZO film. The electrical resistivity of the thin films was investigated in the presence of air as well as N₂ mixed air at different temperatures in the range 30–270 °C. The change in resistivity properties was explained on the basis of conduction phenomena within the grain along with the grain boundaries as well as Cu- and N₂-induced defect states. The thermal activation energy of ZnO was found to be in the range 0.04–0.7 eV and dependent on Cu doping and N₂ level in air.     

FMR and TEM Studies of Co and Ni Nanoparticles Implanted in the SiO<Subscript>2</Subscript> Matrix

Fused silica plates have been implanted with 40 keV Co⁺ or Ni⁺ ions to high doses in the range of (0.25–1.0) × 10¹⁷ ions/cm², and magnetic properties of the implanted samples have been studied with ferromagnetic resonance (FMR) technique supplemented by transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The high-dose implantation with 3d-ions results in the formation of cobalt and nickel metal nanoparticles in the irradiated subsurface layer of the SiO₂ matrix. Co and Ni nanocrystals with hexagonal close packing and face-centered cubic structures have a spherical shape and the sizes of 4–5 nm (for cobalt) and 6–14 nm (for nickel) in diameter. Room-temperature FMR signals from ensembles of Co and Ni nanoparticles implanted in the SiO₂ matrix exhibit an out-of-plane uniaxial magnetic anisotropy that is typical for thin magnetic films. The dose and temperature dependences of FMR spectra have been analyzed using the Kittel formalism, and the effective magnetization and g-factor values have been obtained for Co- and Ni-implanted samples. Nonsymmetric FMR line shapes have been fitted by a sum of two symmetrical curves. The dependences of the magnetic parameters of each curve on the implantation dose and temperature are presented.     

Influence of drying conditions on the optical and structural properties of sol–gel-derived ZnO nanocrystalline films

Zinc oxide nanothin films were prepared on glass substrate by sol–gel dip-coating method using zinc acetate dihydrate, methanol, and monoethanolamine as precursor, solvent, and stabilizer, respectively. The relationship between drying conditions and the characteristics of ZnO nanocrystalline films (c-axis orientation, grain size, roughness and optical properties) was studied. The films were dried in an oven at different temperatures and by IR radiation. Then, the films were annealed at 500°C in a furnace. The chemical composition, transmission spectra, structure, and morphology of the samples were studied using infrared (IR) and UV–visible spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM), respectively. The XRD results show that the drying conditions affect the orientation of crystallization along the (0 0 2) plane. AFM images show that the thicknesses of the films decrease from 128 to 93 nm by changing the drying conditions. The photoluminescence (PL) of ZnO nanothin films shows the UV emission at near band edge and broad green radiation at about 465 nm wavelength.     

Properties of nano-ZnO/poly(vinyl alcohol)/poly(ethylene oxide) composite thin films

A series of poly(vinyl alcohol)/nano-ZnO composites were prepared by dispersing nano-ZnO in aqueous solutions containing mixtures of the biodegradable polymers poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO), and composite thin films were prepared by casting. The introduction of nano-ZnO into PVA/PEO mixed solutions significantly decreased the resistivity of the solutions. Ultraviolet absorption, thermal behaviour and visco-elastic properties of the thin films were determined as a function of nano-ZnO content up to 15 wt%. Optimum film properties were obtained with 1 wt% nano-ZnO, higher proportions of nano-ZnO resulting in agglomeration of ZnO particles and deterioration in film properties. The Forouhi and Bloomer model was used for the modelling of ZnO thin films.     

Determination of the optimal parameters for the fabrication of ZnO thin films prepared by spray pyrolysis method

In this work, ZnO thin films have been prepared by spray pyrolysis deposition method on the glass substrates. The effect of deposition parameters, such as deposition rate, substrate temperature and solution volume has been studied by X-ray diffraction (XRD) method, UV–Vis–NIR spectroscopy, scanning electron microscopy (SEM), and electrical measurements. The XRD patterns indicate polycrystalline wurtzite structure with preferred direction along (0 0 2) planes. Thin films have transparency around 90% in the visible range. The optical band gap was determined at 3.27 eV which did not change significantly. Evolution of electrical results containing the carriers’ density, sheet resistance and resistivity are in agreement with structural results. All the results suggest the best deposition parameters are: deposition rate, R = 3 ml/min, substrate temperature, T _s = 450°C and thickness of the thin films t = 110–130 nm.     

多层膜［Co85Cr15/Pt］20的磁性、垂直磁记录特性和微结构的关系

采用磁控溅射法制备了性能优良的以Pt为缓冲层的［Co85₈₅Cr15_{15/Pt］2020 多层膜，研究了溅射气压对［Co8585Cr1515/Pt］2020多层膜微结构和磁性的 影响.研究结果表明，Ar溅射气压对［Co8585Cr1515/Pt］2020多层膜的微结构 、垂直磁各向异性和矫顽力有重要的影响 关键词： 溅射气压 多层膜 垂直磁各向异性 有效磁各向异性常数}     

Effect of thermal annealing on the ac impedance of Co/Al2O3/Co/Al2O3/Co double-barrier MTJs

Double-barrier magnetic tunnel junctions (DBMTJs) were prepared from Co(75 nm)/Al₂O₃(2.3 nm)/Co(5 nm)/Al₂O₃(2.3 nm)/Co(50 nm) sputtering pentalayer films. The ac electrical properties of as-deposited DBMTJs and those annealed in a vacuum at 100–350 °C for 30 min were then investigated using a complex impedance spectroscopic technique. The ac impedance responses as a function of annealing temperature were further analyzed based on Maxwell's layered dielectric barrier and Maxwell–Wagner capacitor models after considering the DBMTJs as having double-capacitor-type structures. The effect of thermal annealing on the ac transport behavior of the DBMTJs was interpreted by examining the equivalent electric circuits fitted to Nyquist plots of each different sample. The effects were found to be due to changes in the structural characteristics in both bulk and interface morphologies of Co and Al₂O₃ layers. The structural morphology determined the different ac transport modes that occurred in the DBMTJs.     

Pole density distributions in Co/Cr bilayers and magnetic properties

With regard to sputtered Co (50 nm)/Cr (thickness: T_Cr = 0–600 nm) bilayers on glass substrates, the (110) pole density distributions (PDD) of the bcc Cr layer and the (00·2) PDD of the post-deposited hcp Co layer were quantitatively measured by X-ray diffraction. It was found that with increasing T_Cr, a transition of preferred orientation (PO) occured in the Cr layer, affecting the PO of the Co layer and its magnetic properties.     

Dielectric behaviour of emeraldine base polymer–ZnO nanocomposite film in the low to medium frequency

Emeraldine base (EB) polymer–ZnO nanoparticles composite films has been synthesized by solution casting technique on ITO-coated glass substrate and characterized by XRD, FTIR and TEM for their structure and morphology. Dielectric behaviour of these composite films has been investigated in the very low frequency region to medium frequency region (1 kHz–1 MHz). The dielectric constant of the composite with 30% nanoparticles is almost one-tenth of the pure EB. The dielectric value becomes constant in the frequency region greater than 400 kHz. The change in dielectric behaviour of the composite is explained on the basis of multilayered interface formed between the ZnO nanoparticles and emeraldine chains. Nanoparticles have high energy surface which is responsible for the decrease of free volume for the orientation of polymer chains consequently decrease in dielectric constant of the composite. TEM images shows about 10 nm ZnO particles embedded in the emeraldine matrix. From the XRD data it has been observed that the lattice parameters of ZnO have been modified due to the alignment of polymer chains along the basal planes of the nanoparticles. The shift of N=Q=N and N–B–N vibration bands to higher wave number in IR indicates that interaction between emeraldine chain and nanoparticles which provides stability to emeraldine matrix.     

Polarized neutron reflectometry from the interface of the heterostructures SiO2(Co)/Si and SiO2(Co)/GaAs

Polarized neutron reflectometry is used to investigate SiO₂(Co) granular films (70 at% of Co nanoparticles in SiO₂ matrix) deposited on Si and GaAs substrates. The aim of the study is to compare magnetization depth profiles in two systems: in SiO₂(Co)/GaAs heterostructure which shows at room temperature giant injection magnetoresistance (IMR) with the system SiO₂(Co)/Si which reveals almost no IMR effect. We found that at room temperature and at the same value of external magnetic field mean magnetization in the SiO₂(Co)/GaAs sample is much higher than in the case of SiO₂(Co)/Si. We also demonstrate that magnetic scattering length density, and hence, magnetization profile strongly depends on the substrate. We show that SiO₂(Co)/Si heterostructure is ferromagnetically ordered within the temperature range between 120 and 460 K what could explain a weak IMR.     

MFM and AFM study of thin cobalt films modified by fluorosilane

Polycrystalline cobalt films 100 nm thick were thermally evaporated on oxidized Si(100) substrates. Then 1H, 1H, 2H, 2H perfluorodecyltrichlorosilane (FDTS) films of various thicknesses, in the range of about 2 nm to 30 nm, were grown on cobalt surfaces by vapor phase deposition (VPD). The cobalt films modified by FDTS were investigated using magnetic force microscopy (MFM) and atomic force microscopy (AFM). MFM observation showed that the magnetic structure of the cobalt films modified by FDTS is composed of domains with a considerable component of magnetization perpendicular to the film surface. This in turn indicates that the cobalt films on oxidized Si(100) substrates crystallize in the hexagonal close-packed (HCP) phase and exhibit a texture with the hexagonal axis perpendicular to the film surface. The magnetic domains formed a maze structure. The domain width increased from typically 80–120 nm to 400–500 nm with increasing the thickness of FDTS films from about 2 nm to 30 nm. AFM imaging of the surfaces of FDTS films revealed the presence of an agglomerate morphology. The agglomerates varied in size from typically 30–70 nm to 150–300 nm as the film thickness was increased from about 2 nm to 30 nm.