Amorphous Silica Substrates Research Articles

Magnetic stability of oxygen defects on the SiO2 surface

The magnetic stability of E′ centers and the peroxy radical on the surface of α-quartz is investigated with first-principles calculations to understand their role in magnetic flux noise in superconducting qubits (SQs) and superconducting quantum interference devices (SQUIDs) fabricated on amorphous silica substrates. Paramagnetic E′ centers are common in both stoichiometric and oxygen deficient silica and quartz, and we calculate that they are more common on the surface than the bulk. However, we find the surface defects are magnetically stable in their paramagnetic ground state and thus will not contribute to 1/f noise through fluctuation at millikelvin temperatures.

A New "Quasi-Dynamic" Method for Determining the Hamaker Constant of Solids Using an Atomic Force Microscope.

In order to minimize the effects of surface roughness and deformation, a new method for estimating the Hamaker constant, A, of solids using the approach-to-contact regime of an atomic force microscope (AFM) is presented. First, a previous "jump-into-contact" quasi-static method for determining A from AFM measurements is analyzed and then extended to include various AFM tip-surface force models of interest. Then, to test the efficacy of the "jump-into-contact" method, a dynamic model of the AFM tip motion is developed. For finite AFM cantilever-surface approach speeds, a true "jump" point, or limit of stability, is found not to appear, and the quasi-static model fails to represent the dynamic tip behavior at close tip-surface separations. Hence, a new "quasi-dynamic" method for estimating A is proposed that uses the dynamically well-defined deflection at which the tip and surface first come into contact, dc, instead of the dynamically ill-defined "jump" point. With the new method, an apparent Hamaker constant, Aapp, is calculated from dc and a corresponding quasi-static-based equation. Since Aapp depends on the cantilever's approach speed, vc, and the AFM's sampling resolution, δ, a double extrapolation procedure is used to determine Aapp in the quasi-static (vc → 0) and continuous sampling (δ → 0) limits, thereby recovering the "true" value of A. The accuracy of the new method is validated using simulated AFM data. To enable the experimental implementation of this method, a new dimensionless parameter τ is introduced to guide cantilever selection and the AFM operating conditions. The value of τ quantifies how close a given cantilever is to its quasi-static limit for a chosen cantilever-surface approach speed. For sufficiently small values of τ (i.e., a cantilever that effectively behaves "quasi-statically"), simulated data indicate that Aapp will be within ∼3% or less of the inputted value of the Hamaker constant. This implies that Hamaker constants can be reliably estimated using a single measurement taken with an appropriately chosen cantilever and a slow, yet practical, approach speed (with no extrapolation required). This result is confirmed by the very good agreement found between the experimental AFM results obtained using this new method and previously reported predictions of A for amorphous silica, polystyrene, and α-Al2O3 substrates obtained using the Lifshitz method.

Reactive wetting of amorphous silica by molten Al–Mg alloys and their interfacial structures

The reactive wetting of amorphous silica substrates by molten Al–Mg alloys over a wide composition range was studied using a dispensed sessile drop method in a flowing Ar atmosphere. The effects of the nominal Mg concentration and temperature on the wetting and interfacial microstructures were discussed. The initial contact angle for pure Al on the SiO2 surface was 115° while that for pure Mg was 35° at 1073K. For the Al–Mg alloy drop, it decreased with increasing nominal Mg concentration. The reaction zone was characterized by layered structures, whose formation was primarily controlled by the variation in the alloy concentration due to the evaporation of Mg and the interfacial reaction from the viewpoint of thermodynamics as well as by the penetration or diffusion of Mg, Al and Si from the viewpoint of kinetics. In addition, the effects of the reaction and the evaporation of Mg on the movement of the triple line were examined. The spreading of the Al–Mg alloy on the SiO2 surface was mainly attributed to the formation of Mg2Si at the interface and the recession of the triple line to the diminishing Mg concentration in the alloy.

Computational synthesis, structure, and glass transition of (1,4) Cis-polyisoprene-based nanocomposite by multiscale modeling

The main goal of the present study was to produce insights from the atomistic modeling into the structural changes in elastomer-based polymer nanocomposites caused by the surface modifications of the filler particles, and by the crosslinking of the participating polymer matrix. The fully atomistic molecular-dynamics computer simulations of crosslinked (1,4) cis-polyisoprene (PI) films capped by amorphous silica substrates was set-up, in the presence of realistic coupling and covering agents. The PI film stratified structure has been studied in the proximity of the corresponding glass transition, by varying the degree of crosslinking and the PI film thickness. Some monomer ordering induced by the pristine (bare) silica disappeared almost completely in films with modified surfaces. The average monomer density increased with degree of crosslinking. As compared with PI bulk, the glass-transition temperature was slightly larger for highly crosslinked PI films with bare silica and surfaces with coupling agents, and increased with increasing confinement.

The Structure and Properties of Amorphous Indium Oxide.

A series of In2O3 thin films, ranging from X-ray diffraction amorphous to highly crystalline, were grown on amorphous silica substrates using pulsed laser deposition by varying the film growth temperature. The amorphous-to-crystalline transition and the structure of amorphous In2O3 were investigated by grazing angle X-ray diffraction (GIXRD), Hall transport measurement, high resolution transmission electron microscopy (HRTEM), electron diffraction, extended X-ray absorption fine structure (EXAFS), and ab initio molecular dynamics (MD) liquid-quench simulation. On the basis of excellent agreement between the EXAFS and MD results, a model of the amorphous oxide structure as a network of InOx polyhedra was constructed. Mechanisms for the transport properties observed in the crystalline, amorphous-to-crystalline, and amorphous deposition regions are presented, highlighting a unique structure–property relationship.

Influence of dosing sequence and film thickness on structure and resistivity of Al-ZnO films grown by atomic layer deposition

Aluminum-doped zinc oxide (AZO) films were deposited onto amorphous silica substrates using an atomic layer deposition process with diethyl zinc (DEZ), trimethyl aluminum (TMA), and deionized water at 200 °C. Three different Al doping sequences were used at a ZnO:Al ratio of 11:1 within the films. A minimum film resistivity of 1.6 × 10−3 Ω cm was produced using sequential dosing of DEZ, TMA, DEZ, followed by H2O for the Al doping step. This “ZAZW” sequence yielded an AZO film resistivity that is independent of film thickness, crystallographic texture, and grain size, as determined by high resolution x-ray diffraction (XRD). A pseudo-Voigt analysis method yields values for grain sizes that are smaller than those calculated using other XRD methods. Anisotropic grain sizes or variations in crystallographic texture have minimal influence on film resistivity, which suggests that factors other than film texture, such as intragrain scattering, may be important in influencing film resistivity.

Molecular-dynamics simulations of thin polyisoprene films confined between amorphous silica substrates

Constant temperature-constant pressure (NpT) molecular-dynamics computer simulations have been carried out for the united-atom model of a non-crosslinked (1,4) cis-polyisoprene (PI) melt confined between two amorphous, fully coordinated silica surfaces. The Lennard-Jones 12-6 potential was implemented to describe the polymer-silica interactions. The thickness H of the produced PI-silica film has been varied in a wide range, 1 < H/R(g) < 8, where R(g) is the individual PI chain radius of gyration measured under the imposed confinement. After a thorough equilibration, the PI film stratified structure and polymer segmental dynamics have been studied. The chain structure in the middle of the films resembles that in a corresponding bulk, but the polymer-density profile shows a pronounced ordering of the polymer segments in the vicinity of silica surfaces; this ordering disappears toward the film middles. Tremendous slowing down of the polymer segmental dynamics has been observed in the film surface layers, with the segmental relaxation more than 150 times slower as compared to that in a PI bulk. This effect increases with decreasing the polymer-film thickness. The segmental relaxation in the PI film middles shows additional relaxation process which is absent in a PI bulk. Even though there are fast relaxation processes in the film middle, its overall relaxation is slower as compared to that in a bulk sample. The interpretation of the results in terms of polymer glassy bridges has been discussed.

Optical response of oxygen deficient La0.7Sr0.3MnO3 thin films deposited by pulsed laser deposition

The optical response of 200nm thick La0.7Sr0.3MnO(3−δ) films, deposited by pulsed laser deposition on amorphous silica substrates heated at nearly 600°C, under different oxygen pressures (0.1Pa, 0.5Pa, 1Pa, 5Pa and 10Pa), is reported. The effects of the oxygen non-stoichiometry are investigated at room temperature dealing with the absorption coefficient and the Tauc's plot method rather than conventional optical conductivity. The absorption curves are evaluated by an algorithm able to realistically describe the behavior of thin films without exploiting numerical extrapolations or simplified theoretical models or ab-initio calculations. Optical features, tunable by the growth oxygen pressure, are discussed based on the known theoretical and experimental scenario.

Magnetic characteristics of nanocrystalline GaMnN films deposited by reactive sputtering

The magnetic characteristics of Ga1−xMnxN nanocrystalline films (x = 0.08 and x = 0.18), grown by reactive sputtering onto amorphous silica substrates (a-SiO2), are shown. Further than the dominant paramagnetic-like behaviour, both field- and temperature-dependent magnetization curves presented some particular features indicating the presence of secondary magnetic phases. A simple and qualitative analysis based on the Brillouin function assisted the interpretation of these secondary magnetic contributions, which were tentatively attributed to antiferromagnetic and ferromagnetic phases.

Three-dimensional structure of Au nanoparticles supported on amorphous silica and carbon substrates

Scanning Transmission Electron Microscope (STEM) has been employed to study the three-dimensional structure of gold (Au) nanoparticles deposited by means of thermal evaporation in high vacuum on amorphous silica (a-SiO2) and amorphous carbon (a-C) supports. By performing quantitative analysis on the evolution of the high angle annular dark field (HAADF) images, we studied the influence of the nature and the temperature of support on the growth mode of gold nanoparticles.

ZnO-based ultra-violet light emitting diodes and nanostructures fabricated by atomic layer deposition

We have investigated ZnO-based light-emitting diodes (LEDs) fabricated by atomic layer deposition (ALD), demonstrating that ALD is one of the noteworthy techniques to prepare high-quality ZnO required for ultraviolet (UV) photonic devices. Here, we review our recent investigations on different ZnO-based heterojunction LEDs such as n-ZnO/p-GaN LEDS, n-ZnO:Al/ZnO nanodots-SiO2 composite/p-GaN LEDS, n-ZnO/ZnO nanodots-SiO2 composite/p-AlGaN LEDs, n-ZnO:Al/i-ZnO/p-SiC(4H) LEDs, and also on ZnO-based nanostructures including ZnO quantum dots embedded in SiO2 nanoparticle layer, ZnO nanopillars on sapphire substrates, Al-doped ZnO films on sapphire substrate and highly (0 0 0 1)-oriented ZnO films on amorphous glass substrate. The latest investigation also demonstrated p-type ZnO:P films prepared on amorphous silica substrates, which allow us to fabricate ZnO-based homojunction LEDs. These devices and structures were studied by x-ray diffraction and various analytical electron microscopy observations as well as electric and electro-optical measurements.

The origin of electrical property deterioration with increasing Mg concentration in ZnMgO:Ga

Transparent conductive Ga-doped Zn1−xMgxO (ZnMgO:Ga) films were epitaxially grown via Pulsed Laser Deposition on sapphire by optimizing the substrate temperature and other parameters of deposition. Zn0.68Mg0.31Ga0.01O/sapphire films deposited at 400°C have a Hall mobility (μ) of 9.2±0.5cm2V−1s−1 and a free electron density (n) of 1.79×1020±0.06×1020cm−3, yielding an electrical conductivity (σ)=262±22S/cm. Zn0.90Mg0.09Ga0.01O/sapphire films, deposited under the same growth conditions, have similar crystalline quality, but significantly better electrical properties (σ=1450±10S/cm, μ=24.5±2.5cm2V−1s−1, n=3.81×1020±0.20×1020cm−3). This comparison provides evidence of electrical property deterioration in doped ZnMgO bulk material with increasing Mg content, independent of crystalline quality. Electrical properties of ZnMgO:Ga are further deteriorated by the decrease of the crystalline quality. Polycrystalline Zn0.90Mg0.09Ga0.01O/a-SiO2 samples deposited under identical conditions on amorphous silica substrates had both inferior crystal quality and inferior transport properties (μ=2.5±0.2cm2V−1s−1, n=2.04×1020±0.20×1020cm−3, σ=80±8S/cm) compared to their epitaxial counterparts. Overall, the results of this study indicate that both bulk material properties and crystalline quality influence the electrical properties of single-phase ZnMgO:Ga thin films.

MeV Au2+ ions induced surface patterning in silica

This paper reports the formation of self-organised surface morphological features on silica irradiated with MeV energy gold ions. Amorphous silica substrates were irradiated with 1.8MeV gold ions at normal incidence at room temperature to various doses in the range of 5×1016ions/cm2 to 2×1017ions/cm2. The formation of a periodic surface pattern with a wavelength of 1.35μm was observed at an irradiation dose of 1×1017ions/cm2. The observation of surface segregation of gold at around the same dose suggests possible role of surface stresses caused by the incorporation of metal atoms in the formation of the observed periodic surface morphology.

Highly (100)-oriented Ce 1 − xFe xO 2 − δ solid solution films prepared by laser chemical vapor deposition

Ce 1 − x Fe x O 2 − δ solid solution films were prepared on amorphous silica substrates by laser chemical vapor deposition using metal dipivaloylmethanate precursors and a semiconductor InGaAlAs (808 nm in wavelength) laser. X-ray diffraction revealed the formation of single Ce 1 − x Fe x O 2 − δ phase at x ≤ 0.15, while CeO 2 and Fe 2O 3 phases were found for higher Fe content. Highly (100)-oriented Ce 1 − x Fe x O 2 − δ ( x = 0.02) films were obtained at laser power, P L = 50–200 W and deposition temperature, T dep = 800–1063 K. Lotgering factor (200) was calculated to be above 0.8 for films prepared at P L = 50–150 W. X-ray photoelectron spectroscopy revealed the presence of Fe 3+, Ce 4+ and Ce 3+ on solid solution films. Cross-sectional transmission electron microscope images disclosed a film columnar feather-like structure with a large number of nano-scale interspaces. Deposition rates were 2 or 3 orders of magnitude higher than those reported for conventional metal organic chemical vapor deposition of CeO 2.

Structural characterization of ultrathin Cr-doped ITO layers deposited by double-target pulsed laser ablation

In this paper we report on the growth and structural characterization of very thin (20 nm) Cr-doped ITO films, deposited at room temperature by double-target pulsed laser ablation on amorphous silica substrates. The role of Cr atoms in the ITO matrix is carefully investigated with increasing doping content by transmission electron microscopy (TEM). Selected-area electron diffraction, conventional bright field and dark field as well as high-resolution TEM analyses, and energy dispersive x-ray spectroscopy demonstrate that (i) crystallization features occur despite the low growth temperature and small thickness, (ii) no chromium or chromium oxide secondary phases are detectable, regardless of the film doping levels, (iii) the films crystallize as crystalline flakes forming large-angle grain boundaries; (iv) the observed flakes consist of crystalline planes with local bending of the crystal lattice. Thickness and compositional information about the films are obtained by Rutherford back-scattering spectrometry. Results are discussed by considering the combined effects of growth temperature, smaller ionic radius of the Cr cation compared with the trivalent In ion, doping level, film thickness, the double-target doping technique and peculiarities of the pulsed laser deposition method.

Microscopic Perspective on the Adsorption Isotherm of a Heterogeneous Surface

Adsorption of dissolved molecules onto solid surfaces can be extremely sensitive to the atomic-scale properties of the solute and surface, causing difficulties for the design of fluidic systems in industrial, medical and technological applications. In this communication, we show that the Langmuir isotherm for adsorption of a small molecule to a realistic, heterogeneous surface can be predicted from atomic structures of the molecule and surface through molecular dynamics (MD) simulations. We highlight the method by studying the adsorption of dimethyl-methylphosphonate (DMMP) to amorphous silica substrates and show that subtle differences in the atomic-scale surface properties can have drastic effects on the Langmuir isotherm. The sensitivity of the method presented is sufficient to permit the optimization of fluidic devices and to determine fundamental design rules for controlling adsorption at the nanoscale.

Columnar microstructure of nanocrystalline Ga 1− xMn xN films deposited by reactive sputtering

Ga 1− x Mn x N (0≤ x≤0.18) films grown onto amorphous silica substrate by reactive sputtering are characterised by high resolution transmission electron microscopy, energy dispersive spectroscopy, and energy filtered transmission electron microscopy. The electron transmission images and the electron diffraction patterns evidence the presence, at the substrate–film interface, of a few tens of nm thick intermediate layer with a high density of non-oriented nanocrystals (NCs). This intermediate layer represents the nucleation site for the subsequent growth of a compact Ga 1− x Mn x N columnar nanostructure, whose thickness (600–900 nm) is only limited by the deposition time. The columnar region shows a fibre texture with the c axis of the wurtzite nanocrystals corresponding to the column axis, both disposed perpendicular to the film surface. The thickness of the initial NC-rich layer and the coalescence of the nanocolumns are found to have a systematic dependence on the Mn concentration. No evidence of Mn segregation or of Mn rich phases is observed even for the samples with the highest Mn concentration. The correlation between the observed film microstructure and the reactive sputtering deposition parameters is discussed.

La0.7Sr0.3MnO3 thin films deposited by pulsed laser ablation for spintronic applications

AbstractAmong spintronic materials, mixed‐valence manganite La 0.7Sr0.3MnO3 (LSMO) is widely investigated due to its half‐metal nature. LSMO thin films were grown by pulsed laser deposition (PLD) onto amorphous silica substrates heated at nearly 600 °C. An ArF excimer laser was chosen to induce ablation due to its more energetic photons compared to the other quoted excimer laser sources. Different oxygen pressures were considered in order to study the influence of oxygen on the LSMO optical and electrical properties. In this respect, the visible transparency percentage of the deposited films is found good enough for spin‐OLED applications. The absorption coefficient shows an absorption band tunable as a function of the oxygen content. Its energetic location and evolution with the oxygen content demonstrate it originates from radiative transitions between the spin‐majority bands separated by the Jahn–Teller distortion. All of this lets relate the deposition oxygen pressure to the Mn3+ ion content in each film and interpret electrical data. The 200 and 100 nm thick samples exhibit weak metallic transport behavior at room temperature with a resistivity of 4.8 and 6.9 Ω cm, respectively. Concerning the resistivity response versus temperature, the measured low metal–insulator transition temperature (150 K) is related to the sample structural features as involved by the depositions. Two different transport mechanisms describe the conductivity regime of the deposited samples, namely the small polaron variable range hopping (VRH) and the Arrhenius law.

High quality ZnO and Ga:ZnO thin films grown onto crystalline Si (1 0 0) by RF magnetron sputtering

Undoped and 2% Ga-doped ZnO films have been deposited by RF magnetron sputtering onto single crystal Si (1 0 0) substrates equivalent to the commercial Si solar cells. The same films were also grown on amorphous silica substrates to complete their characterization. The films have been characterized by X-ray diffraction, electrical and optical measurements, X-ray photoelectron spectroscopy, Raman microspectroscopy and scanning and high-resolution transmission electron microscopy. Films present a very good quality crystalline wurtzite structure with the c-axis perpendicular to the substrate, with continuity of the (0 0 0 2) planes along the whole film, as shown by transmission electron microscopy. The doped sample shows an increase of two orders of magnitude of the electrical conductivity, an optical transmittance bigger than 85% along the visible spectrum, a diminution of the grain size in the direction parallel to the substrate and a lower surface roughness. The Ga-cations act only as substitutional impurities, they are homogeneously distributed in the whole film, maintaining the wurtzite structure and increasing the carrier density. The formation of any spurious phase or segregation of Ga 2O 3 clusters that can act as carrier traps can be discarded. The characterization results allow us to conclude that the doped film has improved electrical and optical properties with respect to the undoped one. Therefore, the Ga-doped films are very suitable candidates as transparent conducting electrodes for solar cells, displays and other photoelectronic devices.

P-Type ZnO:P Films Fabricated by Atomic Layer Deposition and Thermal Processing

P-type phosphorus-doped ZnO (ZnO:P) films were fabricated by atomic layer deposition of ZnO upon the amorphous silica substrates, which were prepared by coating the spin-on dopant consisted of P2O5 and SiO2 on silicon wafers. Post-deposition thermal treatments were carried out to diffuse the phosphorus dopants into ZnO and to activate the phosphorus-related acceptor states. The ZnO:P films exhibited p-type conductivity with an average hole concentration of 1.05 × 1017 cm−3. Significant spectral peaks associated with the acceptor states appeared in the low-temperature photoluminescence spectra of the p-type ZnO:P films. Optically-pumped stimulated emission around 393 nm was observed at room temperature, thereby indicating a good optical quality of the p-type ZnO:P film.