GaN nanostructures by hot dense and extremely non-equilibrium plasma and their characterizations

High temperature, high density, and extremely non-equilibrium argon plasma produced in modified dense plasma focus device is used to generate ions from pellet of gallium nitride (GaN) fixed on the top of anode and deposited on glass, quartz, and silicon substrates. Atomic force microscope (AFM) study on glass substrate shows nearly spherical nanostructures. AFM and scanning electron microscope studies on quartz substrate show nearly spherical nanostructures and on silicon substrate show comet-like nanostructures. Energy-dispersive X-ray scattering does not show any peak of impurities and there is deficiency of nitrogen in GaN. X-ray diffraction spectra show amorphous nature on glass substrate, whereas nanocrystalline GaN is observed on quartz and silicon substrates. Photoluminescence spectra show peaks of band edge emission, red luminescence, blue luminescence, and UV luminescence bands. Raman study on silicon substrate show peaks of E2 (high), A1, and E1 modes.     

Preparation of nanostructured silicon oxide and silicon nitride films using nanotechnologies

The synthesis of silicon oxide and silicon nitride surface nanostructures is used as an example to demonstrate the possibility of using correlation relationships with inductive parametric constants for quantitatively assessing the reactivity of surface functional groups and predicting the conditions of surface reactions.     

Nanostructured zinc oxide films synthesized by successive chemical solution deposition for gas sensor applications

Nanostructured ZnO thin films have been deposited using a successive chemical solution deposition method. The structural, morphological, electrical and sensing properties of the films were studied for different concentrations of Al-dopant and were analyzed as a function of rapid photothermal processing temperatures. The films were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron and micro-Raman spectroscopy. Electrical and gas sensitivity measurements were conducted as well. The average grain size is 240 and 224 Å for undoped ZnO and Al-doped ZnO films, respectively. We demonstrate that rapid photothermal processing is an efficient method for improving the quality of nanostructured ZnO films. Nanostructured ZnO films doped with Al showed a higher sensitivity to carbon dioxide than undoped ZnO films. The correlations between material compositions, microstructures of the films and the properties of the gas sensors are discussed.     

Hybrid porous nanotube crystal networks for nanostructured device applications

A set of new porous materials, namely zeolite nanocage schwarzite-like crystals with the elements of both nanotubes and fullerenes in the structure is proposed as a result of ab initio and density-functional theory calculations. Twelve new Extradiamond phases of boron nitride, carbon, silicon and silicon carbide are calculated as three different hybridized crystals. The details of recently synthesized Explosion-BN (E-BN) phase are highlighted for the first time with electronic structure and vibrational frequency analysis. E-BN is supposed to be sp ²/sp ³-hybridized FAU-zeolite structure with calculated unit cell of 12.177 Å and a band gap of 3.2 eV. Calculated IR bands for E-BN₁₂₀ cluster and observed experimentally E-BN absorption spectrum are well-correlated with appropriate IR spectra of FAU-zeolite. Armchair and zig-zag nanotubes are classified as (n,n,k) and (n,0,k), respectively, where k is the number of hexagons along the nanotube axis. Novel materials are proposed as (n,m,k)-FTC, where FTC stands for framework type code. We also indicate the possibility of creation of filled hybrid networks of different segment lengths, radii and compositions for thermoelectric and novel device applications.     

Multi-layer deposition of conformal, transparent, conducting oxide films for device applications

Conformal, high conductivity thin films of indium tin oxide (ITO) have been deposited utilizing a relatively low temperature, ambient, multi-layer dip coating process. Using standard alcohol solutions of indium chloride and stannic chloride to form ultra-thin layers, consecutive multi-layer deposition of ITO onto telecommunications grade glass fibers (as a model system) showed excellent control over grain morphology in the film as determined by electron microscopy, and a linear relationship between thickness and overall fiber conductivity. Ray tracing coupled with a transfer matrix formalism was used to numerically simulate the effects of film thickness on the optical waveguiding nature of the conducting layer. The simulations, carried out for a generic film morphology, show that a significant fraction of the optical energy coupled into the fiber face, is transmitted into the film at the thicknesses studied. These results were then used to estimate an upper limit of optical power transmission provided for the generic system. From this a comparison between the optical performance of sputter deposited and multi-layer conducting oxide films in a device configuration could be made. Organic photovoltaic devices, using both sputter deposited and multi-layer conductors on optical grade fibers, were fabricated and tested. Both compared favorably to the numerical simulations, suggesting that the overall, long range performance between the multi-layer deposited and sputtered films are comparable as cathodes for such conformal devices.     

Ferroelectric films and their device applications

Recent reseach on ferroelectric films is discussed, with emphasis on preparative problems, optimization of film structure and of electrical properties and application in devices. Rapid progress has been made in each of these research areas and this fact, together with the emergence of certain new materials such as polyvinylidene fluoride (PVF₂), is increasing the variety of possible applications to solid state devices. Typical examples considered here include microwave capacitors, thermistor bolometers, pyroelectrics, piezoelectric transducers, memories and optical display devices. The present state of the art for these applications and materials development needed for further progress are reviewed.     

Deposition of nanostructured photocatalytic zinc ferrite films using solution precursor plasma spraying

Deposition of pure spinel phase, photocatalytic zinc ferrite films on SS-304 substrates by solution precursor plasma spraying (SPPS) has been demonstrated for the first time. Deposition parameters such as precursor solution pH, concentration, film thickness, plasma power and gun-substrate distance were found to control physico-chemical properties of the film, with respect to their crystallinity, phase purity, and morphology. Alkaline precursor conditions (7 < pH ≤ 10) were found to favor oxide film formation. The nanostructured films produced under optimized conditions, with 500 mM solution at pH ～ 8.0, yielded pure cubic phase ZnFe₂O₄ film. Very high/low precursor concentrations yielded mixed phase, less adherent, and highly inhomogeneous thin films. Desired spinel phase was achieved in as-deposited condition under appropriately controlled spray conditions and exhibited a band gap of ～1.9 eV. The highly porous nature of the films favored its photocatalytic performance as indicated by methylene blue de-coloration under solar radiation. These immobilized films display good potential for visible light photocatalytic applications.     

Novel approach to growth of precipitate-free, high-quality oxide thin films suitable for device applications

To eliminate precipitates-segregates that can easily occur on the thin film surfaces of the multicomponent materials for electronics, a new approach is proposed, consisting of the following aspects: first, on the substrates, artificial steps of predefined height and width are produced, and second, films are grown on such substrates. The width of the step is taken equal to the ‘double of the migration length’ of the atomic species depositing on the substrate. In these conditions, precipitates migrate and gather at the step edges where the free energy is lowest and the resulting totally precipitate-free surface of the film on the step is suitable for device applications or integration purposes. The method has several other important advantages and they are discussed in the text.Using this new approach we present successful fabrication of a mesa structure showing intrinsic Josephson effect. We have used thin films of Bi-2212/Bi-2223 superstructure grown by MOCVD on (001) SrTiO₃ single crystal substrates with artificial steps of about 20 μm width.     

Layer-by-layer nanostructured hybrid films of polyaniline and vanadium oxide

Supramolecular structures of polyaniline (PANI) and vanadium oxide (V2O5) have been assembled via the electrostatic layer-by-layer (ELBL) technique. Strong ionic interactions and H-bonding impart unique features to the ELBL films, which are distinct from cast films obtained with the same materials. The interactions were manifested in UV-vis and Fourier transform infrared spectroscopy data. They are enhanced by the intimate contact between the components, as the films are molecularly thin, with 25 A per PANI/V2O5 bilayer.     

Properties of a-Si:H films grown using hot wire-ECR plasma techniques

We report on the growth and properties of a-Si:H films and nin layers prepared using combination of hot wire and ECR-plasma growth techniques. The films were prepared using both W and Ta hot wire filaments. A distinguishing feature of the reactor was the large spacing, 11 cm, of the filament from the substrate, thereby avoiding over-heating of the substrate. Films were grown at pressures from 2 to 50 mT, and the corresponding optical and electronic properties of the films were measured. The temperature of the substrate was varied between 225 and 350 °C. It was found that the growth rates do not follow the maximum at a pressure-distance (pd) product 15 mTorr cm postulated by Molenbroek et al.'s model. [J. Appl. Phys. 82, 1909 (1997)]. It was also discovered that the properties of the hot wire films depend upon the pd product, and that the H bonding and electronic properties depend critically upon the growth rate, and on the substrate temperature. The properties of the hot wire films bear a remarkable similarity to the films deposited using expanding thermal plasma (ETP) techniques at similar temperatures. When the films were subjected to low power He plasma, the properties improved dramatically. It was also found that H ions are more efficient at etching a growing film than H radicals alone. The results show that the H bonding and electronic properties of a-Si:H films are determined primarily by the efficiency of H extraction, and that low energy ions have a useful role to play in this process.     

Graphene oxide thin films for flexible nonvolatile memory applications

There has been strong demand for novel nonvolatile memory technology for low-cost, large-area, and low-power flexible electronics applications. Resistive memories based on metal oxide thin films have been extensively studied for application as next-generation nonvolatile memory devices. However, although the metal oxide based resistive memories have several advantages, such as good scalability, low-power consumption, and fast switching speed, their application to large-area flexible substrates has been limited due to their material characteristics and necessity of a high-temperature fabrication process. As a promising nonvolatile memory technology for large-area flexible applications, we present a graphene oxide based memory that can be easily fabricated using a room temperature spin-casting method on flexible substrates and has reliable memory performance in terms of retention and endurance. The microscopic origin of the bipolar resistive switching behavior was elucidated and is attributed to rupture and formation of conducting filaments at the top amorphous interface layer formed between the graphene oxide film and the top Al metal electrode, via high-resolution transmission electron microscopy and in situ X-ray photoemission spectroscopy. This work provides an important step for developing understanding of the fundamental physics of bipolar resistive switching in graphene oxide films, for the application to future flexible electronics.     

Transparent amorphous zinc oxide thin films for NLO applications

This review focuses on the growth and optical properties of amorphous zinc oxide (ZnO) thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz and glass substrates at temperature equal to 350 K. The amorphous nature of the films was verified by X-ray diffraction. Atomic Force Microscopy was used to evaluate the surface morphology of the films. The optical characteristics of amorphous thin films have been investigated in the spectral range 190–1100 nm. Measurement of the polarized optical properties was shows a high transmissivity (80–99%) and low absorptivity (<5%) in the visible and near infrared regions at different angles of incidence. Linear optical properties were investigated by classic and Time-Resolved Photoluminescence (TRPL) measurements. Photoluminescence spectrum exhibits a strong ultraviolet emission while the visible emission is very weak. An innovative TRPL technique has enabled the measurement of the photoluminescence decay time as a function of temperature. TRPL measurements reveal a multiexponential decay behavior typical for amorphous thin films. Second and third harmonic generation measurements were performed by means of the rotational Maker fringe technique using Nd:YAG laser at 1064 nm in picosecond regime for investigations of the nonlinear optical properties. The obtained values of second and third order nonlinear susceptibilities were found to be high enough for the potential applications in the optical switching devices based on refractive index changes. Presented spectra confirm high structural and optical quality of the investigated zinc oxide thin films.     

Optical properties and photovoltaic device applications of InSe films

Films were formed by alternately evaporating InSe and Selenium from separate evaporation sources onto glass substrates kept at temperatures below 150°C with subsequent thermal annealing.The effects of the additional selenium on the properties of the films were investigated by structural investigations and photoconductivity measurements. It is found from the results of the present measurements that compositional changes occur in the films with increasing selenium/InSe ratio and that single-phase films containing only InSe are obtained at a certain critical ratio.The refractive indices and absorption coefficients of amorphous and crystalline InSe films in the spectral range 0.6–2.5 μm are determined by spectrophotometric reflectance and transmittance measurements. The minimum optical energy gap obtained from measurements of the absorption spectrum for the crystalline films is close to the value for single crystals.A heterostructure diode using crystalline InSe films as “window layers” was prepared and the fundamental photovoltaic properties were investigated. An absolute quantum efficiency of about 18% is obtained in the wavelength range 1.1–1.25 μm.     

Growth and characterization of transparent conducting nanostructured zinc indium oxide thin films

Transparent conductive oxide (TCO) films have been widely used in various applications, such as for transparent electrodes in flat-panel displays, and in solar cells, optoelectronic devices, touch panels and IR reflectors. Among these, tin doped zinc oxide (ZTO) and indium doped zinc oxide (ZIO) have attracted considerable attention. Particularly, IZO thin film is the best candidate for high-quality transparent conducting electrodes in OLEDs and flexible displays. In this work zinc indium oxide (ZIO) thin films were deposited on glass substrate with varying concentration (ZnO:In₂O₃ — 100:0, 90:10, 70:30 and 50:50 wt.%) at room temperature by flash evaporation technique. These deposited ZIO films were annealed in vacuum to study the thermal stability and to see the effects on the physical properties. The XRF spectra revealed the presence of zinc and indium with varying concentration in ZIO thin films, while the surface composition and oxidation state were analyzed by X-ray photoelectron spectroscopy. The core level spectra were deconvoluted to see the effect of chemical changes, while the valance band spectra manifest the electronic transitions. The surface morphology studies of the films using atomic force microscopy (AFM) revealed the formation of nanostructured ZIO thin films. The optical band gap was also found to be decreased for both types of films with increasing concentration of In₂O₃.     

Room-temperature formation of low refractive index silicon oxide films using atmospheric-pressure plasma

This study aims to apply atmospheric-pressure (AP) plasma to the fabrication of single-layer anti-reflection (AR) coatings with porous silicon oxide. 150 MHz very high-frequency (VHF) excitation of AP plasma permits to enhance the chemical reactions both in the gas phase and on the film-growing surface, increasing deposition rate significantly. Silicon oxide films were prepared from silane (SiH4) and carbon dioxide (CO2) dual sources diluted with helium. The microstructure and refractive index of the films were studied using infrared absorption and ellipsometry as a function of VHF power density. It was shown that significant increase in deposition rate at room temperature prevented the formation of a dense SiO2 network, decreasing refractive index of the resulting film effectively. As a result, a porous silicon oxide film, which had the lowest refractive index of 1.24 at 632.8 nm, was obtained with a very high deposition rate of 235 nm/s. The reflectance and transmittance spectra showed that the low refractive index film functioned as a quarter-wave AR coating of a glass plate.     

Porous manganese oxide synthesized through organic-electrolyte templates and their catalytic applications

We report a facile approach to the preparation of porous manganese oxide materials by the organic-electrolyte templates based on strategy. The final products are thoroughly characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and Brunauer–Emmett–Teller (BET) techniques. The results reveal that porosity (pore size and distribution, surface area) of these manganese oxides has strong relationship with the templates used, which implies a simple way to obtain a series of porous materials. By comparing the catalytic effects of these manganese oxides in oxidation of indene and benzyl alcohol, we find that the pore size and distribution are also crucial to the catalytic properties of these porous materials.     

Optimizing indium tin oxide thin films with bipolar d.c.-pulsed magnetron sputtering for electrochromic device applications

In this paper, indium tin oxide (ITO) films were prepared by bipolar d.c.-pulsed magnetron sputtering in a mixture of argon and oxygen onto unheated glass substrates. A target of ITO with 10 weight percent (wt %) tin was used. The influences of ratios of t^– _on/t⁺ _on (negative pulse-on time/positive pulse-on time) on the optical, electrical, and structural properties of ITO films have been investigated. The correlations between the deposition parameters and the film properties were discussed. An optimal condition based on reactive bipolar d.c.-pulsed sputtering for obtaining high transmittance, low resistivity, and low surface roughness of ITO films with high deposition rate is suggested. Then, ITO films grown at room temperature by bipolar d.c.-pulsed sputtering were used to form electrochromic devices of WO₃. Better electrochromic performances were found in comparison to those measured with commercially available ITO films on glass substrates.     

Surface treatments of indium tin oxide films by using high density plasma

We investigated the effects of various surface treatments on the work function and chemical composition of an indium tin oxide (ITO) surface. Ultraviolet photoelectron spectroscopy (UPS) was used to measure the work function of ITO. X-ray photoelectron spectroscopy (XPS) was used to study the electron structures of ITO surface. We performed surface treatments on ITO using O₂ plasma and HCl solution. Our UPS/XPS analysis indicates increases in the work functions by O₂ plasma treatments. It is known that the Fermi energy level is controlled by the donor concentration, and thus the Fermi energy level is shifted toward the valence band minimum.     

Ion-assisted deposition of moisture-stable hafnium oxide films for ultraviolet applications

A design-of-experiments statistical approach was taken to determine the optimum ion gun operating parameters for the deposition of moisture-stable, low-absorbing hafnium oxide films by ion-assisted electron-beam evaporation. Factors identified as affecting the quality of hafnia films were chamber pressure, deposition rate, ion gun source gas composition, and ion gun current. Both oxygen and argon were used as source gases. High and low levels of the factors were chosen on the basis of our experience with the operating range of the system, and we made a series of 24 runs with all possible combinations of these factors. From a statistical analysis of the data, we find that the best films are obtained with a 1:1 mixture of argon and oxygen, 3-3.5 x 10(-4) Torr chamber pressure, 0.3-nm/s deposition rate, and 0.5-A ion gun current. X-ray diffraction measurements show that the ion-assisted films exhibit a partial monoclinic crystalline structure, whereas the unassisted films are amorphous.