Large Film Thickness Research Articles

Rapid Chemical Bath Depositions and Properties of SnS Films

Tin sulfide films were deposited from an aqueous solution of tin dichloride, thiacetamide, and disodium citrate. X-ray diffraction, atomic force microscopy, ultraviolet–visible spectrophotometry, and electrical measurement were used to characterize the films. The films directly deposited at 70 and 80 °C for 1 and 2 h showed thicknesses of 70–400 and 120–650 nm, respectively. But the deposition at 40 °C for 1 h (nucleation step) made the films subsequently deposited at 60–80 °C denser and thicker (∼1600–7500 nm) than directly deposited films. Average transmittances of the films in UV–visible range decrease with increase in the thickness and are only in range of 0.33–2.3% for the films deposited with the nucleation step. The band gap energies of the films estimated from the transmittance spectra are in the range of ∼1.37–1.55 eV. Moreover, the films are p-type conductive. The electrical resistance and carrier mobility are in the ranges of 0.3–1.4 × 105 Ω cm and 0.77–1.56 cm2 (V s)−1, respectively. The films deposited with the nucleation step show lower resistance and higher carrier mobility than those deposited without nucleation because of their denser film microstructure and larger film thickness.

Near-field radiative heat transfer between metamaterial thin films

We investigate near-field radiative heat transfer between two thin films made of metamaterials. The impact of film thickness on magnetic and electric surface polaritons (ESPs) is analyzed. It is found that the strength as well as the location of magnetic resonance does not change with film thickness until the film behaves as semi-infinite for the dielectric function chosen in this study. When the film is thinner than vacuum gap, both electric and magnetic polaritons contribute evenly to near-field radiative heat transfer. At larger film thicknesses, ESPs dominate heat transfer due to excitation of a larger number of modes. Results obtained from this study will facilitate applications of metamaterials as thin-film coatings for energy systems.

Effects of operating conditions and the reed rigidity on the opening delay of a discharge reed valve in compressors

On the opening delay of a discharge reed valve for compressors caused by the oil film in the clearance between the reed and the valve seat, effects of design conditions on the opening delay are analyzed through the experiment and the numerical simulation. Investigated are three kinds of conditions, the time rate of the bore pressure (compression speed), the oil viscosity, and the thickness of the reed. The experiment shows that the over-compression by the delay increases with increasing each of three condition values. The simulation reproduces qualitatively the effect of the bore pressure rate on the delay. The smaller bore pressure rate makes, under a certain bore pressure, the larger film thickness or the larger cavitation bubble diameter that results in the oil film rupture and the valve opening under the smaller bore pressure. The simulation also reproduces qualitatively the effect of the reed thickness. The larger reed thickness with the larger bending rigidity leads to the smaller reed deformation, the smaller oil film thickness, and the longer delay. On the oil film viscosity, the simulation fails in reproducing the change in the delay. The fail is due to the change in the oil film thickness of the simulation being smaller than that of the experiment. The experiment and simulation results indicate that, on all three conditions, the change in the increase of the oil film thickness leads to the change in the valve opening time and thereby the over-compression.

Epitaxial growth of Si1 − xGex alloys and Ge on Si(100) by electron-cyclotron-resonance Ar plasma chemical vapor deposition without substrate heating

By using electron-cyclotron-resonance (ECR) Ar-plasma chemical vapor deposition (CVD) without substrate heating, the epitaxial growth process of Si1−xGex alloy and Ge films deposited directly on dilute-HF-treated Si(100) was investigated. From the reflection high energy electron diffraction patterns of the deposited Si1−xGex alloy (x=0.50, 0.75) and Ge films on Si(100), it is confirmed that epitaxial growth can be realized without substrate heating, and that crystallinity degradation at larger film thickness is observed. The X-ray diffraction peak of the epitaxial films reveals the existence of large compressive strain, which is induced by lattice matching with the Si(100) substrate at smaller film thicknesses, as well as strain relaxation behavior at larger film thicknesses. The Ge fraction of Si1−xGex thin film is in good agreement with the normalized GeH4 partial pressure. The Si1−xGex deposition rate increases with an increase of GeH4 partial pressure. The GeH4 partial pressure dependence of partial deposition rates [(Si or Ge fraction)×(Si1−xGex thickness)/(deposition time)] shows that the Si partial deposition rate is slightly enhanced by the existence of Ge. From these results, it is proposed that the ECR-plasma CVD process can be utilized for Ge fraction control in highly-strained heterostructure formation of group IV semiconductors.

Influences of solution subcooling, wall superheat and porous-layer coating on heat transfer in a horizontal-tube, falling-film heat exchanger

An experimental study was conducted to investigate the effects of solution subcooling and wall superheat on both sensible and evaporative heat transfer in a falling-film heat exchanger using both plain and porous-layer coated tubes. By varying the subcooling and wall superheat, it was seen that the portions of sensible and evaporative heat transfers could be controlled. Since the falling-film heat exchanger is limited by the coupled relationship between flooding of the upstream tubes and partial dry-out of the bottommost tubes, the ability to control this evaporation-to-sensible heat transfer ratio is important in system optimization. It was shown that sensible heat transfer can be promoted for the upstream tubes while evaporation heat transfer can be promoted for the downstream tubes. Since sensible heat transfer is enhanced by higher solution Reynolds number and therefore larger film thickness, flooding of the upstream tube rows is no longer detrimental.

Highly conductive and transparent aluminum-doped zinc oxide thin films deposited on polyethylene terephthalate substrates by pulsed laser deposition

Highly transparent and conductive aluminum-doped zinc oxide thin films were deposited on low-cost flexible polyethylene terephthalate substrates at room temperature using pulsed laser deposition and the effects of oxygen pressure and film thickness on film properties were investigated. It was found that grain sizes play a greater role only at smaller film thicknesses in affecting carrier mobility. Resistivity changes at larger film thickness can be caused by near surface/interface depletion that affected both mobility and carrier concentration. The inherent film transparency did not change and any reduction in the film transmittance is likely related to a thickness dependent attenuation effect. This means that different transparent conducting oxides should each possess an optimum film thickness, whereby optimized zinc oxide is typically about 100nm. A low resistivity of ~6.6×10−4Ωcm with a high normalized transparency index of >0.9 for a 110±10nm thick room-temperature deposited film was obtained, representing one of the best results obtained to date.

Multi-layered mesoporous TiO2thin films with large pores and highly crystalline frameworks for efficient photoelectrochemical conversion

Mesoporous thin films with various compositions are unique architectures for photoelectrochemical (PEC) solar cells. In this paper, we report the synthesis of highly ordered, multi-layered, continuous mesoporous TiO2 thin films with uniform large pores, crystalline walls and tunable film thickness, via a ligand-assisted evaporation induced self assembly (EISA) method. A Ti(acetylacetone) precursor and a diblock copolymer PEO-b-PS are employed for the controlled assembly of the TiO2/template mesostructure, followed by a two-step pyrolysis that generates carbon residue as an intermediate protection layer to support the TiO2 framework and mesostructures during the crystallization. Other transition metal ion dopants (such as Cr, Ni and Co) can be facilely incorporated into the TiO2 frameworks by co-assembly of these metal acetylacetone precursors during the EISA process. The obtained TiO2 thin film possesses an ordered monoclinic mesostructure distorted from a (110)-oriented primitive cubic structure, uniform and tunable large pores of 10–30 nm, a large surface area of ∼100 m2 g−1 and a high crystallinity anatase wall. The film thickness can be well controlled from 150 nm to several microns to tune the absorption, with the capability of generating free-standing film morphologies. Furthermore, this designed architecture allows for effective post-deposition of other small-bandgap semiconductor nanomaterials inside the large, open and interconnecting mesopores, leading to significantly improved solar absorption and photoconversion. As a proof-of-concept, we demonstrate that the photoanodes made of 4.75 μm thick mesoporous TiO2 film with deposited cadmium sulfide quantum dots exhibit excellent performance in PEC water splitting, with an optimized photocurrent density of 6.03 mA cm−2 and a photoconversion efficiency of 3.9%. These multi-layered mesoporous TiO2-based thin films can serve as a unique architecture for PEC and other solar energy conversion and utilization.

Optimization of polymer photovoltaic cells with bulk heterojunction layers hundreds of nanometers thick: modifying the morphology and cathode interface

In polymer photovoltaic (PV) cell, it is desirable to use a relatively thick polymer semiconductor film in order to maximize the light absorption, and to achieve better controllability and reproducibility of the film in manufacturing processes. However, the low fill factor due to restricted charge transport and extraction at large film thickness serially limits the performance of the polymer PV cell. In this work, we investigate the factors that can impact the device performances as film thickness is increased. We also introduce ways to help alleviate these problems in thick BHJ PVs. Our measurement results, based on the space-charge limited-current (SCLC) model and the photo-induced carrier extraction by linearly increasing voltage (photo-CELIV) method, show that the thicker BHJ devices have relatively low electron mobility compared with hole mobility, which directly correlates with high contact resistance at the top cathode interface that prevents efficient transport of photo-generated electrons. Specifically, we found that the newly introduced ESSENCIAL fabrication process helps improve the blend donor and acceptor domain morphologies; and adding an ultrathin C60 layer at the cathode interface helps improve the surface morphology and significantly reduce the contact resistance. The effects of the added thin C60 layer on PV cells were further studied by examining several important diode characteristics. Our results proved that this layer not only decreases the contact resistance at the cathode but also improves the hole-blocking, thereby providing significantly suppressed recombination at the cathode interface. Consequently, the fabricated PV devices optimized in morphology and interface show significantly improved internal quantum efficiency (IQE) as compared with the thermally annealed conventional PV cells, leading to 5.11% PCE from a P3HT:PCBM blend system. The modifications to the fabrication of BHJ PV cells described in this work allow for photoactive layers to be hundreds of nanometers thick for efficient light absorption and better controllability.

Effect of solid content on performance of conductive silver paste for crystalline silicon solar cells

Abstract The effect of solid content of conductive silver paste on the film thickness, microstructure of screen-printed front contact, and electrical performance of crystalline silicon solar cells was investigated. Three different solid content pastes were tested. It is found that the solid content of silver paste strongly affects the film thickness, microstructure of the thick film, and electrical performance of the cells. The paste with 84 wt.% solid content has higher viscosity and density than the ones with 82 wt.% and 83 wt.%. After sintering, the front Ag electrode thick film prepared using the paste with 84 wt.% solid content has a larger film thickness and the corresponding cell exhibits higher photoelectric conversion efficiency than the ones with 82 wt.% and 83 wt.% under the same conditions. This indicates that increasing film thickness can improve the conversion efficiency.

Effect of doping on polarization profiles and switching in semiconducting ferroelectric thin films

This paper proposes a theory to describe the polarization and switching behavior of ferroelectrics that are also wide-gap semiconductors. The salient feature of our theory is that it does not make any a priori assumption about either the space charge distribution or the polarization profile. The theory is used to study a metal-ferroelectric-metal capacitor configuration, where the ferroelectric is n-type doped. The main result of our work is a phase diagram as a function of doping level and thickness that shows different phases, namely, films with polarization profiles that resemble that of undoped classical ferroelectrics, paraelectric, and a new head-to-tail domain structure. We have identified a critical doping level, which depends on the energy barrier in the Landau energy and the built-in potential, which is decided by the electronic structures of both the film and the electrodes. When the doping level is below this critical value, the behavior of the films is almost classical. We see a depleted region, which extends through the film when the film thickness is very small, but is confined to two boundary layers near the electrodes for large film thickness. When the doping level is higher than the critical value, the behavior is classical for only very thin films. Thicker films at this doping level are forced into a tail-to-tail configuration with three depletion layers, lose their ferroelectricity, and may thus be described as nonlinear dielectric or paraelectric. For films which are doped below the critical level, we show that the field required for switching starts out at the classical coercive field for very thin films, but gradually decreases.

Kinetics and driving forces of abnormal grain growth in thin Cu films

The abnormal growth of individual (100) oriented grains is monitored by the in situ electron backscatter diffraction technique for more than 24h at three different annealing temperatures (90°C, 104°C and 118°C) in 1–5μm thick Cu films on polyimide substrates. The (100) grain growth velocity increases with higher film thickness and annealing temperature, as suggested by an earlier model by Thompson and Carel. As a result, the final (100) texture fraction becomes more dominant for higher annealing temperatures and larger film thicknesses. The Thompson–Carel model, however, predicts that the (111) grains will preferably grow at temperatures up to 118°C. Our calculations of the driving forces revealed that in addition to minimization of the strain energy (due to the thermal mismatch between film and substrate) and of the surface energy, the energy stored in the dislocations plays a decisive role in grain growth. Our observations can be understood by the notion that initially available (100) grain nuclei start to grow very rapidly, due to dislocation annihilation, and thus “overrun” the (111) grains in size.

Extended Exciton Diffusion in Rubrene Single-Crystalline Organic Solar Cells

ABSTRACTSingle-crystalline organic solar cells were investigated. Rubrene single crystals made by train sublimation method were used for the active layer of the solar cells. Typical solar cell characteristics and external quantum efficiency (EQE) were observed with the film thickness of several micrometers. In spite of their large film thickness, the EQE spectra showed no screening effect, which means that absorbed photons efficiently converted to electric charges. This can be attributed to the extended exciton diffusion due to uniform and trap free characteristic of rubrene single crystal.

Magnetic and Transport Properties of As-grown and Annealed GaMnAs Thick Layers

We studied post-growth annealing of GaMnAs thin films with large film thickness t up to 100 nm. Our results show that for films of up to t = 100 nm comparable values of Curie temperature TC and saturation magnetization Ms can be obtained as for t = 25 nm. Hence, the out-diffusion based annealing process is not thickness limited. Furthermore, anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) measurements of as-grown and annealed samples are carried out to characterize the magnetic anisotropies of the different films.

Impact of film thickness of organic semiconductor on off-state current of organic thin film transistors

Impact of film thickness on off-state current of bottom contact organic thin film transistor has been investigated using two dimensional numerical simulations and analytical model. Off-state current of the device reduces by six orders of magnitude as film thickness is scaled from 45 nm to 10 nm, with rate of reduction in off-state current being slow first and then significantly higher. An analytical model for off-state current has been developed to offer an insight into above results of off-state current, and the model predictions are found in good agreement with reported experimental results. The developed model is especially important for the device with smaller film thickness as at such film thicknesses, space charge limited current model is inadequate to explain off-state current of such devices. A horizon for scaling device channel length through a reduction in film thickness only has been explored using an analytical model and simulation results. Off-state current of a shorter channel length (L) device can be comparable to a relatively longer channel length (i.e., L + δL) device if the fractional reduction in film thickness becomes equal to square of the fractional reduction in channel length. Following such reduction in film thickness successively for a number of steps, an expression for film thickness corresponding to the device with a desired value of channel length has been developed and verified with simulation results. Although the device with larger film thickness has a problem of poor subthreshold performance, it, in general, has advantage of better mobility. To alleviate this problem of the device with larger film thickness, a stack contact device has been proposed. An investigation of its subthreshold performance shows that its off-state current can be significantly lower as compared to conventional contact device.

Effect of Molecular Weight on the Adsorption Characteristics of Lignosulfonates

Sequentially adsorbed multilayers composed of sodium lignosulfonate (SL) and poly(diallyldimethylammonium chloride) (PDAC) were built up by a layer-by-layer self-assembly technique to investigate the adsorption characteristics of SL microgels for sensitivity improvement. The effect of molecular weight on the adsorption characteristics was investigated by light scattering, UV-visible spectroscopy, ellipsometer, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements. Results show that the self-assembly of SL microgels and PDAC is not mainly driven by electrostatic interaction but by cation-π interaction and hydrophobic interaction. The Coulomb interaction in SL microgels causes a "slow mode" peak in dynamic light scattering measurement, but it can be completely suppressed when SL is dissolved in 1.2 mol/L NaCl solutions due to charge-screening. The SL in 1.2 mol/L NaCl solutions can be well adsorbed on a PDAC surface, indicating that the electrostatic interaction is not the main driving force. The presence of cation-π interaction between SL and PDAC is detected by a stoichiometric ratio of SL to PDAC in SL/PDAC self-assembled films. During SL/PDAC self-assembly, hydrophobic interaction plays an important role in determining the adsorption rate, film thickness, surface morphology, and surface properties. SL with a higher M(w) has a stronger hydrophobic ability in water, so it exhibits a slower adsorption rate, a larger film thickness, and a higher film roughness. SL with different M(w) has almost the same wettability because of a larger hydrophobic effect that increases contact angle and a higher film roughness that decreases contact angle. In this case, the opposite effects cancel each other to make no difference in wettability, but it still reflects the role of the hydrophobic effect in surface properties.

Liquid Film Features Analysis on Mechanical Seals with Micro-Pore Face by CFD

The six kinds of different models were simulated by FLUENT software under certain circumstances. As a result, their pressure distributions and liquid film opening force with different thickness were obtained. The relationship between liquid film thickness and opening force was obtained by least square fitting. And then the relationship between stiffness and liquid film thickness was calculated and analyzed. The result shows that: seal opening force and liquid film stiffness decrease as the liquid film thickness increase, this simulation results is identical with the theoretical data. In larger film thickness range, the opening force is larger, and so was the liquid film stiffness, which provide the basis for seal optimization design and the stable operation.

Electronic versus Lattice Match for Metal-Semiconductor Epitaxial Growth: Pb on Ge(111)

Lattice match is important for epitaxial growth. We show that a competing mechanism, electronic match, can dominate at small film thicknesses for metal-semiconductor systems, where quantum confinement and symmetry requirements may favor a different growth pattern. For Pb(111) on Ge(111), an accidental lattice match leads to a √3 × √3 configuration involving a 30° in-plane rotation at large film thicknesses, but it gives way to an incommensurate (1 × 1) configuration at small film thickness. The transformation follows an approximately inverse-film-thickness dependence with superimposed bilayer oscillations.

Evolution of proton order during ice-film growth: An analysis of island shapes

Scanning tunneling microscopy reveals that two-dimensional islands equilibrated on the surface of ice multilayers change their shape with increasing film thickness from triangular to hexagonal. We propose that this shape change is caused by a transition from polarized ice at the Pt substrate toward proton disorder (i.e., vanishing net polarization) at larger film thicknesses. A kinetic model estimates how quickly the substrate-induced proton order decays via introduction of protonic defects during growth.

Numerical simulation on vapor absorption by wavy lithium bromide aqueous solution films

Numerical simulation has been made on heat and mass transfer of vapor absorption by wavy lithium bromide aqueous solution films. The velocity fields and interface positions are obtained by VOF model. Solitary waves are generated by periodically disturbed inflow boundary. Based on these, the temperature and concentration fields are obtained with a stationary interface shape. The effect of solitary waves on the heat and mass transfer across the film is investigated. It is shown that due to the mixing of circulation and stretch of large film thickness, the gradient of concentration and absorption rate decrease for solitary wave region. The region of capillary waves shows a significant amount of absorption enhancement. The percentage of absorption for the different regions is quantified.

Effect of walking patterns on the elastohydrodynamic lubrication of metal-on-metal total hip replacements

The profiles of forces and kinematics can significantly affect the results of both hip simulators and computer simulations of hip implants, in terms of lubrication and wear. Most of the related studies have employed the simplified load and motion profiles to simulate walking. The purpose of this study is to investigate the lubrication of a typical metal-on-metal (MOM) total hip implant, under different loading and motions of walking; which included three patterns for hip simulators (Leeds Mk I, Leeds ProSim, and the ISO standard) and a combined walking-stopping-start-up-walking pattern. A general ball-in-socket lubrication model was solved and full numerical solutions of the elastohydrodynamic lubrication were presented. The multi-grid method was adopted to solve the Reynolds equation. A combined multi-level multi-integration and fast Fourier transform technique was used to obtain the elastic deformation of bearing surfaces. Complex three-dimensional loading and motion were found to result in large variations of the film thickness during one walking cycle, particularly for the physiological case considered in the Leeds Mk I simulator. Under such a physiological pattern, the squeeze film effect was generally greater than the other two hip simulator patterns, particularly during the swing phase, resulting in larger film thickness. The squeeze film effect was encouraged by the translation of the lubricating contact area in the entrainment direction. Small variations in the film thickness and friction torque were observed between the Leeds ProSim and ISO patterns. Intermittent breaks during steady walking were unlikely to cause complete depletion of lubrication in the MOM hip implants. At 5 s after stopping, the lubrication regime was shifted from full film lubrication to mixed lubrication. A lubricant pocket was developed quickly and the film profile finally achieved an equilibrium condition.