Showing papers on "Thin film published in 1993"

Fabrication and Magnetic Properties of Arrays of Metallic Nanowires

Abstract: Arrays of ferromagnetic nickel and cobalt nanowires have been fabricated by electrochemical deposition of the metals into templates with nanometer-sized pores prepared by nuclear track etching. These systems display distinctive characteristics because of their one-dimensional microstructure. The preferred magnetization direction is perpendicular to the film plane. Enhanced coercivities as high as 680 oersteds and remnant magnetization up to 90 percent have also been observed.

Thin‐film CdS/CdTe solar cell with 15.8% efficiency

Abstract: This letter describes the fabrication and characteristics of high‐efficiency thin‐film CdS/CdTe heterojunction solar cells. CdS films have been prepared by chemical bath deposition and p‐CdTe films have been deposited by close‐spaced sublimation. A CdS/CdTe solar cell of greater than 1 cm2 area with an AM1.5 efficiency of 15.8% is reported.

Experimental Realization of the Covalent Solid Carbon Nitride

Abstract: Pulsed laser ablation of graphite targets combined with an intense, atomic nitrogen source has been used to prepare C-N thin film materials. The average nitrogen content in the films was systematically varied by controlling atomic nitrogen flux. Rutherford backscattering measurements show that up to 40 percent nitrogen can be incorporated on average into these solids under the present reaction conditions. Photoelectron spectroscopy further indicates that carbon and nitrogen form an unpolarized covalent bond in these C-N materials. Qualitative tests indicate that the C-N solids are thermally robust and hard. In addition, strong electron diffraction is observed from crystallites within the films. Notably, analysis of these diffraction data show that the only viable structure for the C-N crystallites is that of β-C3N4, a material predicted theoretically to exhibit superhardness. The experimental synthesis of this new C-N material offers exciting prospects for both basic research and engineering applications.

Modeling and Experimental Diagnostics in Polymer Electrolyte Fuel Cells

Abstract: This paper presents a fit between model and experiment for well-humidified polymer electrolyte fuel cells operated to maximum current density with a range of cathode gas compositions. The model considers, in detail, losses caused by: (1) interfacial kinetics at the Pt/ionomer interface, (2) gas-transport and ionic-conductivity limitations in the catalyst layer and (3) gas-transport limitations in the cathode backing. The authors` experimental data were collected with cells that utilized thin-film catalyst layers bonded directly to the membrane, and a separate catalyst-free hydrophobic backing layer. This structure allows a clearer resolution of the processes taking place in each of these distinguishable parts of the cathode. In their final comparison of model predictions with the experimental data, they stress the simultaneous fit of a family of complete polarization curves obtained for gas compositions ranging from 5 atm O{sub 2} to a mixture of 5% O{sub 2} in N{sub 2}, employing in each case the same model parameters for interfacial kinetics, catalyst-layer transport, and backing-layer transport. This approach allowed them to evaluate losses in the cathode backing and in the cathode catalyst layer, and thus identify the improvements required to enhance the performance of air cathodes in polymer electrolyte fuel cells. Finally, theymore » show that effects of graded depletion in oxygen along the gas flow channel can be accurately modeled using a uniform effective oxygen concentration in the flow channel, equal to the average of inlet and exit concentrations. This approach has enabled simplified and accurate consideration of oxygen utilization effects.« less

Atomic force microscopy and surface-enhanced Raman spectroscopy. I. Ag island films and Ag film over polymer nanosphere surfaces supported on glass

Abstract: The surface roughness and nanometer scale structure of Ag films used for surface‐enhanced Raman scattering (SERS) are characterized using atomic force microscopy (AFM). Two important types of thin film based SERS‐active surface have been examined in this study: (1) Ag island films (AgIF’s) on smooth, insulating substrates and (2) thick Ag films evaporated over both preroughened and smooth substrates. AFM is demonstrated to be capable of quantitatively defining the three‐dimensional (3D) structure of these roughened surfaces. The effects of mass thickness, dm, and thermal annealing on the nanostructure of AgIF’s are studied in detail. Particle size histograms are calculated from the AFM images for both ‘‘as‐deposited’’ and annealed IF’s with dm=1.8 and 3.5 nm. Quantitative measurements of the SERS enhancement factor (EF) are coupled with the AFM data and interpreted within the framework of the electromagnetic theory of SERS. AFM images for thick evaporated Ag films over a monolayer of polymer nanospheres (AgFON) shows the clear presence of ‘‘random substructure roughness’’ reducing their utility as controlled roughness surfaces. Similar roughness structures are observed for thick evaporated Ag films on smooth, insulating substrates. Nevertheless, AgFON surfaces are demonstrated to be among the most strongly enhancing thin film based surfaces ever studied with EF’s comparable to those found for electrochemically roughened surfaces. Applications of FON surfaces to ultrahigh sensitivity SERS, anti‐Stokes detected SERS, and surface‐enhanced hyper‐Raman spectroscopy (SEHRS) are reported.

ZnO/CdS/CuInSe2 thin‐film solar cells with improved performance

Abstract: An important milestone in the development of photovoltaic thin‐film solar cells is the achievement of 15% conversion efficiency. This letter describes the highest efficiency single junction thin‐film cell reported to date. An active area efficiency of 14.8% is obtained with the cell structure n‐ZnO/n‐CdS/p‐CuInSe2 deposited on a soda‐lime glass substrate. The current achievements are due to improved properties of the CuInSe2 layer and the heterojunctions compared to previously reported results. The rate and substrate temperature profiles used during the coevaporation process yield a relatively large‐grained material with very strong 〈112〉 orientation and low porosity. This results in reduced recombination rates, hence higher open circuit voltage and fill factor.

Internal structure of layer-by-layer adsorbed polyelectrolyte films : a neutron and X-ray reflectivity study

Abstract: Ultrathin polymer films were physisorbed to surface-modified Si wafers by electrostatic deposition of polyelectrolytes from aqueous solutions. For the first it has been demonstrated that the preparation procedure, which involves repeated dipping of the substrate into solutions of polycations and polyanions in an alternating sequence, leads to the deposition of continuous molecular layers that form a polymer film with a well-defined supramolecular structure

Silicide formation and silicide‐mediated crystallization of nickel‐implanted amorphous silicon thin films

Abstract: The nucleation and growth of isolated nickel disilicide precipitates in Ni‐implanted amorphous Si thin films and the subsequent low‐temperature silicide‐mediated crystallization of Si was studied using in situ transmission electron microscopy. Analysis of the spatial distribution of the NiSi2 precipitates strongly suggested the occurrence of site saturation during nucleation. NiSi2 precipitates were observed in situ to migrate through the amorphous Si thin films leaving a trail of crystalline Si at temperatures as low as ∼484 °C. Initially, a thin region of epitaxial Si formed on {111} faces of the octahedral NiSi2 precipitates with a coherent interface which was shown by high‐resolution electron microscopy to be Type A. Migration of the NiSi2 precipitates led to the growth of needles of Si which were parallel to 〈111〉 directions. The growth rate of the crystalline Si was limited by diffusion through the NiSi2 precipitates, and an effective diffusivity was determined at 507 and 660 °C. A mechanism for the enhanced growth rate of crystalline Si is proposed.

Effects of strain energy on the preferred orientation of TiN thin films

Abstract: The effects of strain energy on the preferred orientation of TiN thin films were investigated. In the TiN film deposited by plasma‐enhanced chemical‐vapor deposition with a power of 50 W, the overall energy of the film mainly depended on the surface energy because its strain energy was relatively small. The preferred orientation of the film corresponded to the plane with the lowest surface energy, i.e., (200). However, in the TiN film deposited by rf sputtering with a power of 200 W, the overall energy of the film was largely controlled by strain energy due to its large strain energy, and its growth orientation corresponded to the plane with the lowest strain energy, i.e., (111). Furthermore, the preferred orientation of the TiN film was changed from (200) to (111) with the film thickness. It is considered that this phenomenon is due to the increase of strain energy with its thickness.

Heteroepitaxial wurtzite and zinc‐blende structure GaN grown by reactive‐ion molecular‐beam epitaxy: Growth kinetics, microstructure, and properties

Abstract: Reactive‐ion molecular‐beam epitaxy has been used to grow epitaxial hexagonal‐structure α‐GaN on Al2O3(0001) and Al2O3(0112) substrates and metastable zinc‐blende‐structure β‐GaN on MgO(001) under the following conditions: growth temperature Ts=450–800 °C; incident N+2/Ga flux ratio JN+2/JGa=1–5; and N+2 kinetic energy EN+2=35–90 eV The surface structure of the α‐GaN films was (1×1), with an ≊3% contraction in the in‐plane lattice constant for films grown on Al2O3(0001), while the β‐GaN films exhibited a 90°‐rotated two‐domain (4×1) reconstruction Using a combination of in situ reflection high‐energy electron diffraction, double‐crystal x‐ray diffraction, and cross‐sectional transmission electron microscopy, the film/substrate epitaxial relationships were determined to be: (0001)GaN∥ (0001)Al2O3 with [2110]GaN∥[1100]Al2O3 and [1100]GaN∥[1210]Al2O3, (2110)GaN∥(0112)Al2O3 with [0001]GaN∥[0111]Al2O3 and [0110]GaN∥[2110]Al2O3, and (001)GaN∥(001)MgO with [001]GaN∥[001]MgOFilms with the lowest e

Quantitative measurement of residual biaxial stress by Raman spectroscopy in diamond grown on a Ti alloy by chemical vapor deposition

Abstract: Raman spectroscopy is used to study residual stress in diamond grown on Ti--6Al--4V by chemical vapor deposition. A general model is developed to use Raman spectroscopy to measure biaxial stress in polycrystalline, diamond-structure films. The as-grown film has 7.1 GPa of residual compressive stress, consistent with the difference in thermal-expansion coefficients between the diamond film and the substrate. Examination of the Raman spectra of the film in the vicinity of Brale indentations reveals that residual stresses in the film of up to approximately 17 GPa can be accommodated in the film before delamination occurs.

Mechanism of Chemical Bath Deposition of Cadmium Sulfide Thin Films in the Ammonia‐Thiourea System In Situ Kinetic Study and Modelization

Abstract: The mechanism of chemical bath deposition of cadmium sulfide thin films from the ammonia‐thiourea system is studied in situ by means of quartz crystal microbalance technique (QCM). The influence of reaction parameters (concentration of reactants, pH, anions, temperature, stirring rate) is determined. The growth is thermally activated with an activation energy of about 85 kJ/mol, which probably corresponds to a chemical step related to the decomposition of thiourea. The results are well interpreted by assuming an atom‐by‐atom growth mechanism. A model is presented, which fits most of the experimental results quantitatively. It involves two or three rate‐limiting surface steps, the formation of taking place via a surface complex between thiourea and cadmium hydroxide. Analytical expressions are given, allowing prediction of the rate under various conditions in this system.

Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices

Abstract: Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin film, microstructures (heterojunctions, quantum wells, superlattices, quantum dots), surfaces/interfaces and actual device structures in addition to semiconductor growth/processing. Furthermore, the influence of external perturbations such as temperature, electric fields, hydrostatic pressure, uniaxial stress, etc. can be investigated. This optical technique utilizes a very general principle of experimental physics, in which a periodically applied perturbation (either to the sample or probe) leads to sharp, derivative-like spectral features in the optical response of the system. Because of the richness of the derivative-like spectra, the information in the lineshape fits, room temperature performance and relative simplicity of operation this method is becoming increasingly more important as a tool to study these materials and structures. This article will review developments in the field during the last decade.

Atomic layer epitaxy (ALE) apparatus for growing thin films of elemental semiconductors

Abstract: An apparatus for and a method of growing thin films of the elemental semiductors (group IVB), i.e., silicon, germanium, tin, lead, and, especially diamond, using modified atomic layer epitaxial (ALE) growth techniques are disclosed. In addition, stoichiometric and non-stoichiometric compounds of the group IVB elements are also grown by a variation of the method according to the present invention. The ALE growth of diamond thin films is carried-out, inter alia, by exposing a plurality of diamond or like substrates alternately to a halocarbon reactant gas, e.g., carbon tetrafluoride (CF 4 ), and a hydrocarbon reactant gas, e.g., methane (CH 4 ), at substrate temperatures between 300 and 650 Celsius. A stepping motor device portion of the apparatus is controlled by a programmable controller portion such that the surfaces of the plurality of substrates are given exposures of at least 10 15 molecules/cm 2 of each of the reactant gases. The chemical reaction time to complete the growth of an individual atom layer is approximately 25×10 -6 second.

Transparent conductive electrodes for electrochromic devices: A review

Abstract: This paper reviews the optical and electrical performance of thin films that are useful as transparent electrodes in electrochromic devices. The properties of certain heavily doped wide-bandgap semiconductor oxides (especially In2O3:Sn) and of certain coinage metal films are discussed.

Molecular design for nonpolymeric organic dye glasses with thermal stability : relations between thermodynamic parameters and amorphous properties

Abstract: The molecular structures of low-molecular-weight organic compounds and their amorphous properties have been investigated to obtain a design rule for uniform amorphous films with high thermal stability. The glass transition temperature (Te/K), maximum crystal-growth velocity (MCV/m s -1 ), and maximum crystal-growth temperature (T c,max /K) are key parameters for characterizing the amorphous properties of organic materials. Some quantitative relations between these parameters and thermodynamic parameters were examined from both theoretical and experimental viewpoints

Fundamental mechanisms of interfacial friction. 2. Stick-slip friction of spherical and chain molecules

Abstract: The friction and rheological properties of molecularly thin films of hexadecane between two shearing mica surfaces were measured under different conditions of load or applied pressure, film thickness, sliding velocity, temperature, and time or previous history. The aim was to investigate the tribological and rheological properties of ultrathin liquid films composed of chain molecules and to compare these properties with those of spherical molecules. The differences were found to be more quantitative than qualitative, and the results therefore provide insights into the general properties of liquids in thin films. The results show that during steady-state sliding between two solid crystalline surfaces, a liquid film may be considered to be in a one-phase or two-phase dynamic regime. In the two-phase regime, the film undergoes periodic [open quotes]stick-slip[close quotes] transitions between a solidlike and a liquidlike state. However, especially in the case of chain molecules such as hexadecane, the liquidlike state is very different from the bulk liquid. The frequency of transitions between the two dynamic states determines the static and dynamic friction forces and other dynamic properties of a shearing film. The effects produced by changing the load, temperature, sliding velocity, stopping time, etc., were found to be highly correlated. 49more » refs., 21 figs., 3 tabs.« less

Process for forming a thin metal film by chemical vapor deposition

Abstract: When a thin metal film is formed on a substrate at a constant substrate temperature by chemical vapor deposition while alternately and discontinuously introducing a raw material gas and a reducing gas onto the substrate, reducing the raw material gas with the reducing gas on the substrate, thereby conducting chemical vapor deposition, and repeating the chemical vapor deposition to obtain a desired film thickness, a thin metal film having a good surface flatness without any current leakage can be obtained without etching the substrate wafer, and when the reducing gas is excited to excited species by an exciting means at the introduction of the reducing gas and the excited species is used be reduce the raw material gas, a lower substrate temperature can be used and chemical vapor deposition process time can be made shorter than without using the exciting means.

Magnetron sputter deposition with high levels of metal ionization

Abstract: A new deposition technique has been developed which combines conventional magnetron sputter deposition with a rf inductively coupled plasma (RFI). The RFI plasma is located in the region between the magnetron cathode and the sample position, and is set up by a metal coil immersed in the plasma. A large fraction of the metal atoms sputtered from the magnetron cathode are ionized in the RFI plasma. By placing a negative bias on the sample, metal ions are then accelerated across the sample sheath and deposited at normal incidence. Results from a gridded energy analyzer configured with a microbalance collector and located at the sample position indicate the level of ionization is low at a few mTorr and rises to ≳80% at pressures in the 25–35 mTorr range. Optical measurements of metal ion and neutral emission lines show scaling of the relative ionization to higher discharge powers. Significant cooling of the plasma electron temperature is observed when high concentrations of metal atoms were sputtered into the...

Conductivity and magnetoresistance in magnetic granular films (invited)

Abstract: The transport properties in magnetic granular films are modeled by considering the spin‐dependent impurity scattering within the granules and the interface roughness scattering at the boundaries of the granules The magnetoresistance for these films is derived by using the formalism developed for layered structures with currents perpendicular to the plane of the layers and which is applicable to random systems With this model, various features of the magnetoresistance observed in recent experiments can be explained and the optimal choice of parameters to maximize the magnetoresistance can be determined

Layer-by-layer adsorbed films of polyelectrolytes, proteins or DNA

Abstract: During the last three years the authors have developed a new technique which allows to construct ultrathin multilayer assemblies by consecutive adsorption of anionic and cationic polyelectrolytes out of aqueous solution. The technique allows the easy construction of functional multicomponent films that can be deposited on surfaces with any given topology. They show excellent thermal stability and the total film thickness can be adjusted within a few {angstrom}s. Physisorbed films can be assembled with a linearly increasing thickness up to at least 100 layers. The films are transparent and exhibit a homogenous interference color in white light. The average thickness of individual layers can be adjusted with a precision of 0.5 {angstrom}. Film deposition can also be carried out using natural polyelectrolytes such as DNA or by biospecific binding to proteins such as strepavidin.

CuInS2 based thin film solar cell with 10.2% efficiency

Abstract: Efficient solar energy conversion with CuInS2 thin films is reported. The copper‐rich p‐type absorber is prepared by thermal coevaporation. A copper to indium ratio between 1.0 and 1.8 can be tolerated with small (≤10%) solar‐to‐electrical conversion losses. Copper excess phases (CuS) are removed chemically. The cell structure glass/Mo/p‐CuInS2/n‐CdS/n+‐ZnO/Al delivers 10.2% at simulated AM 1.5 conditions. The device properties are discussed based on its energy band diagram.