Oxidized Si Substrates Research Articles

Perpendicular Exchange Bias of [Pt/Co]5/IrMn Multilayers on Self-Organized Hexagonally Patterned Nanodots

[Pt/Co] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> /IrMn multilayers were deposited on hexagonally patterned nanodots on the backside of anodized alumina oxide (AAO) templates and thermally oxidized Si substrates. AAO templates with different diameters of about 50, 60, 130 and 400 nm nanodots were used to fabricate the exchange-biased nanostructures and investigate the size effect on the perpendicular exchange bias. With decreasing nanodots diameter, the multilayers exhibited large variations in both exchange bias field H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> and coercivity H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> . Compared to the flat film, H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> of the nanodots samples was significantly enhanced, which can be attributed to the reduction in antiferromagnetic and ferromagnetic domain sizes induced by the curved morphology and domain wall pinning between two adjacent magnetic nanocaps. Unexpectedly, the 50 nm nanodot sample had higher thermal stability than the flat film sample.

Temperature dependence of an optical narrow-bandpass filter at 1.5 μm.

For superconducting nanowire single-photon detectors, we designed and fabricated an on-chip narrow-bandpass filter at the central wavelength of 1.5μm to reduce the influence of blackbody radiation from the fiber. The bandpass filter was prepared on a thermally oxidized Si substrate, and it provided a peak transmittance of 88% at the central passband wavelength and 30dB suppression at the stop bands. We investigate the temperature dependence of the filter from room temperature to 2.9K. The central wavelength of the passband blue-shifted from 1545±2 nm (295K) to 1526±2 nm (2.9K). This effect is explained by the different indices of refraction of Si at different temperatures.

Nano-composite sensors composed of single-walled carbon nanotubes and polyaniline for the detection of a nerve agent simulant gas

This paper reports the response behavior at room temperature of a composite sensor composed of single-walled nanotubes (SWCNTs) and polyaniline, to the nerve agent simulant gas dimethyl-methyl-phosphonate (DMMP), a typical Sarin simulant. The SWCNT–polyaniline composite was synthesized to obtain high-quality composites with good uniformity. The composites were drop-cast onto an oxidized Si substrate patterned with Pd electrodes. SWCNT–polyaniline composite sensors exhibited clear, sharp response curves for DMMP in air at room temperature, even at minuscule concentrations. The response and response time were 27.1% and 5.5s, respectively, at 10ppm DMMP, representing a significant improvement over the pure SWCNT network sensors previously reported. These results indicate that SWCNT–polyaniline composite sensors can be ideal DMMP sensors, operating at room temperature with high response, fast response time, and excellent reproducibility.

Superconductive niobium films coating carbon nanotube fibers

Superconducting niobium (Nb) has been successfully obtained by sputter deposition on carbon nanotube fibers. The transport properties of the niobium coating the fibers are compared to those of niobium thin films deposited on oxidized Si substrates during the same deposition run. For niobium films with thicknesses above 300 nm, the niobium coating the fibers and the thin films show similar normal state and superconducting properties with critical current density, measured at T = 4.2 K, of the order of 105 A cm−2. Thinner niobium layers coating the fibers also show the onset of the superconducting transition in the resistivity versus temperature dependence, but zero resistance is not observed down to T = 1 K. We evidence by scanning electron microscopy (SEM) and current-voltage measurements that the granular structure of the samples is the main reason for the lack of true global superconductivity for thicknesses below 300 nm.

Efficient 1.54-μm emission through Eu2+ sensitization of Er3+ in thin films of Eu2+/Er3+ codoped barium strontium silicate under broad ultraviolet light excitation

Thin films of Eu2+/Er3+ codoped barium strontium silicate were deposited on a thermal oxide Si substrate by magnetron sputtering. Optical properties suggest that after a rapid annealing process, these films can lead to efficient Er3+ emission at 1.54μm with a lifetime of about 7.9ms. Intense 1.54-μm light emission was achieved under broad ultraviolet light excitation through efficient energy transfer from Eu2+ to Er3+. These results indicate that the Eu2+/Er3+ thin films have potential applications as low cost and compact erbium doped waveguide amplifiers pumped by LEDs.

Effect of Tungsten Seed Layer on Perpendicular Magnetic Anisotropy for Co2FeAl Full-Heusler Half-Metal Based Perpendicular Magnetic Tunnel Junctions

Recently, the perpendicular-spin-transfer-torque magnetic-random-access-memory (p-STT MRAM) with full-heusler alloy based perpendicular-magnetic-tunnel-junctions (p-MTJs) is being researched due to unlimited tunnel magnetoresistance (TMR) ratio, high thermal stability and low power consumption [1]. Half metals with Co-based full-heusler alloys have been expected to be optimal material for p-MTJs with high thermal stability and low damping constant (α) of around 0.001. Co2FeAl (CFA) has the full-heusler alloy composition with high Curie temperature (TC ) of 980oC and stable energy band gap on minority-spin conduction band. Furthermore, the hybridization of electron orbitals makes perpendicular magnetic anisotropy (PMA) characteristic at the interface between CFA and MgO [2]. We studied about the p-MTJs using CFA material with PMA characteristic.In our experiments, all stacks were fabricated on 12-inch thermal oxidized Si substrate and the films of stacks were deposited by ultra-vacuum magnetron sputtering system with a pressure of 10– 8 torr at room temperature. After film deposition process, ex-situ annealing was progressed at a temperature ranging from 275oC to 450oC in a vacuum around 10– 6 torr for 1 hr under 3 Tesla out-of-plane magnetic field.To verify the PMA characteristic of CFA p-MTJs, we fabricated the pseudo-spin-valve (P-SV) stack consists of SiO2 / MgO(2.0) / Pt(2.0) / CFA(0.9) / MgO(1.8) / CFA(0.9) / Pt(2.0) where numbers are nominal thickness in nanometers. For the crystal texturing, the stack was ex-situ annealed at Tex = 450oC. Figure 1(a) shows the schematic structure of P-SV and its magnetic moment versus external magnetic field (M-H) curves. The PMA characteristic of CFA was definitely indicated in out-of-plane M-H curve as shown in Fig. 1(a). Nevertheless the conventional-spin-valve (C-SV) with CFA p-MTJs and [Co/Pd]n multilayer based synthetic anti-ferromagnetic coupled (SyAF) layer [3] has low TMR ratio of 1.7% despite the CFA free layer has PMA characteristic as shown in the inset of Fig. 1(b).For understanding this unexpected result, we analyzed high-resolution transmission-electron-microscopy (HR-TEM) image of the PMA multilayer-stack structure of SiO2 / MgO(2.0) / Pt(2.0) / CFA(0.9) / MgO(0.9) / Pt(2.0) annealed at Tex = 450oC as shown in Fig. 2. To obtain high TMR ratio, MgO tunneling barrier system p-MTJs has to satisfy two important conditions. At first, magnetic anisotropy characteristic is required for maximizing resistance difference between parallel state and anti-parallel state. The next thing is body centered cubic (bcc) (100) crystalline linearity for Δ1 coherence tunneling with MgO tunneling barrier. However Pt seed layer was grown as face centered cubic (fcc) (111) orientation and it was transited to CFA during the processes of sputtering and annealing. As a result, this crystalline linearity disturbs Δ1 coherence tunneling and brings the lowering of TMR ratio.It suggests p-MTJs using bcc (100) crystal structure seed layer to realize the good Δ1 coherence tunneling at CFA. Especially, we used W that can be the good assistant for p-MTJs due to the low resistivity and low diffusivity to MgO tunnel barrier. Figure 3 shows the M-H curves of the PMA multilayer-stacks structure of SiO2 / MgO(1.2) / W(3.0) / CFA(1.2) / MgO(1.2) with different ex-situ annealing temperatures. It was obviously obtained that the PMA characteristic was appeared at an ex-situ annealing temperature of higher than 400oC as shown in Fig. 3(d). It is result of that the large thermal energy is required for CFA crystallization. In other words, it is the greatest merit that CFA material has a large thermal endurance to back-end heat budget of semiconductor fabrication process such as a metallization.In our presentation, we present the effects of W seed layer on CFA crystalline linearity affect to TMR ratio. In addition, we will review the magnetic properties of CFA grown on W seed layer using VSM, XRD and HR-TEM, etc.* This work was supported by the IT R&D program of MOTIE/KEIT [No.10043398 and 10041608] and the Brain Korea 21 plus Project in 2014, Korea.Reference[1] W. Wang et al., Phys. Rev. B 81 140402(R) (2010).[2] Z.Wen et al., Appl. Phys. Lett. 98 242507, (2011).[3] G. Hu et al., IEEE Magn. Lett. 4 3000104 (2013).

Superconducting nanowire single photon detector with on-chip bandpass filter

Dark count rate is one of the key parameters limiting the performance of the superconducting nanowire single photon detector (SNSPD). We have designed a multi-layer film bandpass filter that can be integrated onto the SNSPD to suppress the dark counts contributed by the stray light and blackbody radiation of the fiber. The bandpass filter is composed of 16 SiO(2)/Si bilayers deposited onto the backside of a thermally oxidized Si substrate. The substrate shows an excellent bandpass filter effect and provides a high transmittance of 88% at the central wavelength of the pass band, which is the same as that of the bare substrate. The SNSPDs fabricated on the substrate integrated with the bandpass filter show conspicuous wavelength-sensitive detection efficiency. The background dark count rate is reduced by two orders of magnitude to sub-Hz compared with the conventional SNSPD (a few tens of Hz). The detector exhibits a system detection efficiency of 56% at DCR of 1 Hz, with the measured minimal noise equivalent power reaching 2.0 × 10(-19) W/Hz(1/2).

Influence of Nanostructure Geometry on Electronic Properties

Recently, new quantum features have been studied in the area of nanostructured layers. It emerges that properties of nanostructures depend not only on their size but also on their geometry. Particularly, a nanograting (NG) on the surface of the thin layer imposes additional boundary conditions on electron wave function and forbids some quantum states. Density of quantum states reduces. Unlike conventional quantum well, state density per volume, is reduced in the case of NG layer. This leads to changes in electronic properties. Electrons, rejected from forbidden quantum states, have to occupy states with higher energy. In the case of semiconductor layers, electrons rejected from the valence band have to occupy empty quantum states in the conduction band. Such increase in conduction band electron concentration can be termed as geometry-induced doping or G-doping. G-doping is equivalent to donor doping from the point of view of the increase in electron concentration. However, there are no ionized impurities. This preserves charge carrier scattering to the intrinsic semiconductor level and increases carrier mobility with respect to the donor-doped layer. As rejected electrons occupy quantum states with the higher energy, the chemical potential of NG layer increases and becomes NG size dependent. We regard a system composed of NG layer and an additional layer on the top of the NG forming periodic series of p-n junctions. In such system, charge depletion region develops inside the top of NG and its effective height reduces, becoming a rather strong function of temperature T. Consequently, T-dependence of chemical potential magnifies and Seebeck coefficient S increases. Calculations show one order of magnitude increase in the thermoelectric figure of merit ZT relative to bulk material. In the case of metal layers, electrons rejected from forbidden quantum states below Fermi energy, occupy quantum states above Fermi energy. Fermi energy moves up on energy scale and work function (WF) reduces. WF reduction was observed in thin amorphous Au films grown on thermally oxidized Si substrates. WF was measured using Kelvin Probe and PEEM microscope. WF reduction depended on film internal structure and NG sizes. Maximum reduction in WF of 0.56 eV has been obtained.

Magneto-transport properties of oriented Mn2CoAl films sputtered on thermally oxidized Si substrates

Spin gapless semiconductors are interesting family of materials by embracing both magnetism and semiconducting due to their unique band structure. Its potential application in future spintronics requires realization in thin film form. In this Letter, we report fabrication and transport properties of spin gapless Mn2CoAl films prepared on thermally oxidized Si substrates by magnetron sputtering deposition. The films deposited at 673 K are well oriented to (001) direction and display a uniform-crystalline surface. Magnetotransport measurements on the oriented films reveal a semiconducting-like resistivity, small anomalous Hall conductivity, and linear magnetoresistance representative of the transport signatures of spin gapless semiconductors. The magnetic properties of the films have also been investigated and compared to that of bulk Mn2CoAl, showing small discrepancy induced by the composition deviation.

Orientation-controlled nonepitaxial L10 FePt thin films

We report an orientation-controlled FePt thin films grown nonexpitaxially on oxidized Si substrates without using any underlayers. After 700 °C annealing, the FePt single layer with thickness less than 10 nm shows good (001) texture and perpendicular anisotropy. The scanning electron microscope images indicate that these films are composed of isolated islands. For the films thicker than 20 nm, the films become continuous and (111) orientation appears. The transition of film texture relates to the change of morphology of the films. Finally, a theoretical mode based on minimizing surface energy is proposed to explain the texture variation in the films.

A 64-pixel NbTiN superconducting nanowire single-photon detector array for spatially resolved photon detection

We present the characterization of two-dimensionally arranged 64-pixel NbTiN superconducting nanowire single-photon detector (SSPD) array for spatially resolved photon detection. NbTiN films deposited on thermally oxidized Si substrates enabled the high-yield production of high-quality SSPD pixels, and all 64 SSPD pixels showed uniform superconducting characteristics within the small range of 7.19-7.23 K of superconducting transition temperature and 15.8-17.8 μA of superconducting switching current. Furthermore, all of the pixels showed single-photon sensitivity, and 60 of the 64 pixels showed a pulse generation probability higher than 90% after photon absorption. As a result of light irradiation from the single-mode optical fiber at different distances between the fiber tip and the active area, the variations of system detection efficiency (SDE) in each pixel showed reasonable Gaussian distribution to represent the spatial distributions of photon flux intensity.

The electron scattering at grain boundaries in tungsten films

The impact of electron scattering at grain boundaries in polycrystalline tungsten (W) thin films is studied with a unique approach. Two nominally 300nm-thick films, a (110)-oriented epitaxial and a (110)-textured polycrystalline W film, were prepared in identical fabrication conditions except for the substrates, i.e., Al2O3 substrate for the epitaxial film and thermally oxidized Si substrate for the polycrystalline film. The significantly larger dimension of the film thickness than the electron mean free path in W allows the contribution of surface scattering to be negligible, and the additional resistivity for the polycrystalline W film relative to the epitaxial film is attributed to grain boundary scattering. The Mayadas–Shatzkes (MS) grain boundary scattering model is employed for quantitative analyses, with the grain boundary reflection coefficient determined to be 0.42±0.02 and 0.40±0.02 at 293K and 4.2K, respectively, using the measured lateral grain size of 119±7nm. Based on the grain boundary reflection coefficient determined in this study and the surface scattering coefficient (p=0.11) of the Fuchs–Sondheimer model given elsewhere, the resistivity of W lines having line-widths less than 10nm is predicted to be lower than that of Cu lines.

Manipulation of nanoparticles of different shapes inside a scanning electron microscope

In this work polyhedron-like gold and sphere-like silver nanoparticles (NPs) were manipulated on an oxidized Si substrate to study the dependence of the static friction and the contact area on the particle geometry. Measurements were performed inside a scanning electron microscope (SEM) that was equipped with a high-precision XYZ-nanomanipulator. To register the occurring forces a quartz tuning fork (QTF) with a glued sharp probe was used. Contact areas and static friction forces were calculated by using different models and compared with the experimentally measured force. The effect of NP morphology on the nanoscale friction is discussed.

Interplay of the magnetoelastic and shape anisotropy on the evolution of magnetic domain structure of amorphous Nd–Fe–B films

Amorphous Re–Fe–B (Re rare-earth) films are prepared on thermally oxidized Si substrates with Nb buffer layers by dc magnetron sputtering. We observed the evolution of magnetic domain structure in these films under the external magnetic field parallel to the film plane and discussed its origin. The magnetoelastic anisotropy and shape anisotropy can be adjusted by changing the deposition temperature and the thickness of the film. The results showed that the magnetic domain structure for a film with lower sputtering temperature (150°C) and thinner thickness (200nm) has changed significantly with the applied magnetic field and form stripe domains arranged along the direction of the field. However, for the film with higher sputtering temperature (300°C) and thicker thickness (600nm), magnetic domain structure is hardly varied with applied field. We suggest the evolution of magnetic domain structure is determined by the interplay of the magnetoelastic anisotropy and shape anisotropy energy, which will lead to the easy magnetization direction of amorphous Re–Fe–B films inclined to the normal direction instead of perpendicular to the film plane.

Pulsed Laser Deposited Ferromagnetic Chromium Dioxide thin Films for Applications in Spintronics

Stable rutile type tetragonal chromium dioxide (CrO2) thin films have been deposited on lattice-matched layers of TiO2 by KrF excimer laser based pulsed laser deposition (PLD) technique using Cr2O3 target. The TiO2 seed layer was deposited on oxidized Si substrates by the same PLD process followed by annealing at 1100°C for 4h. The lattice-matched interfacial layer is required for the stabilization of Cr (IV) phase in CrO2, since CrO2 behaves as a metastable compound under ambient conditions and readily converts into its stable phase of Cr (III) oxide, Cr2O3. Analyses with X-ray diffraction (XRD), Glancing-angle XRD (GIXRD), Raman spectroscopy and grazing-angle Fourier transform infra-red (FTIR) spectroscopy confirm the presence of tetragonal CrO2 phase in the as-deposited films. Microstructure and surface morphology in the films were studied with field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). Electrical and magnetic characterizations of the films were performed at room temperature. Such type of stable half-metallic CrO2 thin films with low field magnetoresistive switching behaviour are in demand for applications as diverse as spin-FETs, magnetic sensors, and magneto-optical devices.

Effect of Postannealing and Multilayer Structure on Soft Magnetic Properties of FeTaC Thin Film

We report a systematic investigation on room temperature and temperature dependent magnetic properties of [ Fe - Ta - C ( y )/Ta( 1 nm ) ]n = 0-4/FeTaC( y )/substrate] films prepared on thermally oxidized Si substrates at ambient temperature and postannealed at different temperatures (TA) for 30 minutes. All the as-made films exhibit amorphous structure. The stripe domain structure with a large coercivity (HC) observed in FeTaC (200 nm) single layer film due to stress induced during film deposition transforms to in-plane orientation of magnetization with multilayer structure. Postannealing at 200 ° C reduces HC largely and annihilates the strip domains. On further increasing TA, HC increases initially at 300 ° C and reduces slightly for 400 ° C annealed films. Also, the values of HC strongly depend on the number of multilayers. The magnetization reversal process of multilayer films annealed above 250 ° C depicts multistep reversal behavior due to an individual switching of ferromagnetic layer. High temperature thermomagnetization studies display a strong dependence of Curie temperature on TA and multilayer structure. The observed results are discussed on the basis of stress-induced perpendicular anisotropy developed during film deposition, topological coupling at the interface, and magnetostatic interaction between the ferromagnetic layers.

Controlling the Magnetic Properties of Cr-Implanted Co/Pt Multilayer Films Using Ion Irradiation for Planar Patterned Media

We have investigated the modification of the microstructure and magnetic properties of Co/Pt multilayer films with various layer structures using ion beam irradiation. Cr/[Co/Pt] multilayer films prepared by a magnetron sputtering deposition on a surface oxidized Si substrate were irradiated by 30 keV Kr+ ion beam with various ion doses from 1×1014 to 5×1015 ions/cm2. The modifications of magnetic properties and microstructures due to Kr+ ion irradiation depended on Co/Pt layer structures. The as-prepared [Co(0.4 nm)/Pt(0.45 nm)]15 multilayer films were confirmed to exhibit the square shaped M-H loop and the perpendicular easy axis with a saturation magnetization of 620 emu/cc and coercivity of 1.1 kOe. After the irradiation with ion dose of 5×1015 ions/cm2, the easy axis changes from perpendicular to in-plane direction. The coercivity and saturation magnetization decrease down to 2% and 13% of its original values, respectively. The decrease of coercivity and saturation magnetization is due to the implantation of the Cr atom in the top most layers into Co/Pt multilayer films caused by Kr+ ion irradiation, as confirmed from Auger electron spectroscopy (AES) depth profiles.

Microstructure and enhanced H2S sensing properties of Pt-loaded WO3 thin films

Pt-loaded WO3 thin films were deposited on oxidized Si substrates under various discharge gas pressures at room temperature using reactive magnetron sputtering. Structural characterizations by means of X-ray diffraction and field emission scanning electron microscopy show that the films composed of nano-size grains have a monoclinic structure. The films become more porous as the discharge gas pressure increases due to the decrease in film density and the increase in effective surface area. Gas sensing properties demonstrate that the films show reversible response to H2S with various concentrations. Under the same operating conditions the sensor response increases and the operating temperature at which the maximum response is shown decreases with discharge gas pressure. The highest response of 3512 is obtained for a Pt-loaded porous film deposited at 12Pa upon exposure to 5ppm H2S at an operating temperature of 100°C. Such a high response of a Pt-loaded porous film at a low operating temperature is discussed in relation to the Schottky-barrier-limited transport as well as the chemical mechanism of Pt loading.

Kinetics of Ni:C Thin Film Composition Formation at Different Temperatures and Fluxes

In this work analysis considering Ni:C thin films growth on thermaly oxidized Si substrate by proposed kinetic model is presented. Model is built considering experimental results where microstructure evolution as a function of the substrate temperature and metal content of Ni:C nanocomposite films grown by hyperthermal ion deposition is investigated. The proposed kinetic model is based on the rate equations and includes processes of adsorption, surface segregation, diffusion, chemical reactions of constituents. The experimental depth profile curves were fitted by using proposed model. The obtained results show a good agreement with experiment taking into account concentration dependent diffusion. It is shown by modeling that with the increase of substrate temperature the process of nickel surface segregation becomes most important. DOI: http://dx.doi.org/10.5755/j01.ms.19.3.5234

Preparation and gas sensing properties of undoped and Pd-doped TiO2 nanowires

Titanium dioxide (TiO2) nanowires with a rutile structure were prepared by thermal evaporation method using Ti metal powder as a raw material. The major factor that influenced the morphology was the furnace temperature. The uniform nanowires can be prepared at the furnace temperature of 1200°C, which were approximately 50–220nm in diameter and several tens of micrometers in length. Sensors were fabricated by pouring a few drops of product suspended ethanol onto oxidized Si substrates equipped with a pair of interdigitated Pt electrodes and H2 gas sensing properties were investigated. Both undoped and Pd-doped TiO2 nanowire gas sensors showed the highest response at 100°C. Comparing with undoped nanowire sensor, Pd-doped nanowire sensor exhibited much higher response and rapider response/recover characteristic to H2 gas. The results demonstrated that Pd-doped TiO2 nanowires can be used to fabricate high performance H2 sensors.