High Dilution Ratio Research Articles

Kinetic roughening and mound surface growth in microcrystalline silicon thin films

AbstractThe roughening evolution of the hydrogenated microcrys‐talline silicon (μc‐Si:H) film by plasma enhanced chemical vapor deposition (PECVD) and hot wire chemical vapor deposition (HWCVD) were investigated using atomic force microscopy (AFM). It is found that the kinetic roughening of the μc‐Si:H film surface can not be described by the traditional dynamic scaling approach. The appearance of characteristic peaks in the power spectral density (PSD) curves, calculated from the AFM images, indicates the mounded growth characteristic of μc‐Si:H film. The correlation between the mounded surface growth of μc‐Si:H thin films and the shadowing effect has been studied. It is found that high hydrogen dilution ratio in the early growth stage retards the formation of the mound structure (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Solar thermal reduction of ZnO and SnO 2: Characterization of the recombination reaction with O 2

This study addresses the high-temperature solar thermal reduction step of the ZnO/Zn and novel SnO 2/SnO thermochemical cycles for H 2 production. A new method was specifically developed to identify the kinetics of the reduced species recombination with O 2. Experimental tests were performed in a solar vessel subjected to high-flux solar irradiation to investigate the influence of pressure, neutral gas flow-rate, and reduction rate on the solar step yield. As a result, the benefic effect of high dilution ratio or reduced pressure operation was pointed out, which thereby eliminates the need for a complex quenching device that may induce significant material losses by condensation of the vapors released by the reaction. In addition, the first detailed kinetic analysis of the recombination reaction with O 2 (reverse oxidation of SnO and Zn) was proposed. The kinetic parameters (global reaction order and activation energy) were determined by using an original inverse method combining a non-isothermal plug-flow reactor model and ex-situ powders characterization. A global reaction order of about 1.5 corresponding to the reaction stoichiometry was numerically identified for both SnO and Zn recombination, and activation energies were estimated to be 42±4 kJ/mol for SnO and 35±3 kJ/mol for Zn.

Effect of Krypton Addition on Electron Cyclotron Resonance-Radio Frequency Hybrid Oxygen Plasma for Patterning Diamond Surfaces

Electron cyclotron resonance radio frequency (ECR-rf) hybrid krypton-diluted oxygen plasmas were used to pattern the surfaces of diamond films with the assistance of a physical mask, while optical emission spectroscopy was employed to characterize the plasma. It was found that with krypton dilution the etching rate decreased, and also the aspect ratios of nanotips formed in micro-holes were significantly modified. The oxygen atomic densities were estimated by oxygen atom optical emission and argon actinometry. Under a microwave power of 300 W and rf bias of −300 V, the absolute density of ground-state oxygen atoms decreased from 1.3×1012 cm−3 to 1.4×1011 cm−3 as the krypton dilution ratio increased to 80%, accompanied by the decrease in the plasma excitation temperature. It is concluded that oxygen atoms play a dominant role in diamond etching. The relative variations in the horizontal and vertical etching rates induced by the addition of krypton are attributed to the observations of thicker nanotips at a high krypton dilution ratio.

Improving viability of cryopreserved honey bee ( Apis mellifera L.) sperm with selected diluents, cryoprotectants, and semen dilution ratios

This is the first study where the systematic application of theories and techniques used in mammalian sperm cryopreservation have been applied to honey bee ( Apis mellifera L.) semen as a means to improve postthaw viability of cryopreserved sperm. Six newly designed diluents, three cryoprotectants (dimethyl sulfoxide, DMA, glycerol), and five diluent:semen ratios (1:1, 3:1, 6:1, 9:1, and 12:1) were tested. In addition, the sperm freezing tolerance of three honey bee strains was evaluated. Specific protocols were designed to control semen freezing and thawing rates. Sperm motility was assessed visually, whereas sperm viability was assessed using SYBR-14 and propidium iodide fluorescent stains. Diluent treatments did not affect fresh (nonfrozen) sperm viability yet affected fresh sperm motility (P < 0.05). Based on these assessments, two diluents were chosen and used in all successive cryopreservation experiments. Using the selected diluents, semen was collected at various diluent:semen ratios, along with one of the three cryoprotectants. Semen collected at high dilution ratios, using a hypotonic antioxidant diluent containing catalase, in combination with dimethyl sulfoxide, provided higher postthaw sperm viability than that of all other combinations tested (68.3 ± 5.4%; P < 0.05). Using this combination of dilution ratio, diluent, and cryoprotectant, there were no differences among honey bee strains for postthaw sperm viability (P = 0.805). Nevertheless, these new semen dilution and freezing methods improved postthaw viability of sperm to levels that could theoretically sustain worker populations in colonies, thus providing potential for further optimization of cryopreservation techniques for the genetic preservation and improvement of honey bee genotypes.

Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint

Relatively the high reflectivity of copper to CO 2 laser led to the difficulty in joining copper to steel using laser welding. In this paper, a new method was proposed to complete the copper–steel laser butt welding. The scarf joint geometry was used, i.e., the sides of the copper and steel were in obtuse and acute angles, respectively. During the welding process, the laser beam was fixed on the steel side and the dilution ratio of copper to steel was controlled by properly selecting the deviation of the laser beam. The offset of laser beam depended on the scarf angle between the copper and steel, the thickness of plate and the processing parameters used in the laser welding. The microstructure near the interface between Cu plate and the intermixing zone was investigated. Experimental results showed that for the welded joint with high dilution ratio of copper, there was a transition zone with numerous filler particles near the interface. However, if the dilution ratio of copper is low, the transition zone is only generated near the upper side of the interface. At the lower side of the interface, the turbulent bursting behavior in the welding pool led to the penetration of liquid metal into Cu. The welded joint with lower dilution ratio of copper in the fusion zone exhibited higher tensile strength. On the bases of the microstructural evaluation at the interface of the welded joint, a physical model was proposed to describe the formation mechanism of the dissimilar joint with low dilution ratio of copper.

Structure and Optical Characterization of Nanocrystalline Silicon Thin Films for Solar Cells

AbstractEffects of very high frequency- plasma enhanced chemical vapor deposition (VHF-PECVD) using diluted ultrapure silane at higher dilution ratio (R&gt;30) on microstructures and optical characteristics of hydrogenated nanocrystalline silicon (nc-Si:H) film were studied. Nanocrystalline silicon films were prepared by at RF power ranging from 50 to 300 W. It was found that the transition from amorphous phase to nanocrystalline phase occurred between 100 W and 150 W. The nucleation mechanism toward nc-Si:H near the transition point of amorphous phase was discussed based on transmission electron microscopy with atomic scale. Further, it is suggested from UV-visible spectroscopy that nc-Si:H films with the best optical properties would be obtained near the transition point from the amorphous phase to the crystalline phase.

Rapid, low-temperature synthesis of nc-Si in high-density, non-equilibrium plasmas: enabling nanocrystallinity at very low hydrogen dilution

Nanocrystalline silicon thin films were deposited on single-crystal silicon and glass substrates simultaneously by inductively coupled plasma-assisted chemical vapor deposition from the reactive silane reactant gas diluted with hydrogen at a substrate temperature of 200 °C. The effect of hydrogen dilution ratio X (X is defined as the flow rate ratio of hydrogen to silane gas), ranging from 1 to 20, on the structural and optical properties of the deposited films, is extensively investigated by Raman spectroscopy, X-ray diffraction, Fourier transform infrared absorption spectroscopy, UV/VIS spectroscopy, and scanning electron microscopy. Our experimental results reveal that, with the increase of the hydrogen dilution ratio X, the deposition rate Rd and hydrogen content CH are reduced while the crystalline fraction Fc, mean grain size δ and optical bandgap ETauc are increased. In comparison with other plasma enhanced chemical vapor deposition methods of nanocrystalline silicon films where a very high hydrogen dilution ratio X is routinely required (e.g. X > 16), we have achieved nanocrystalline silicon films at a very low hydrogen dilution ratio of 1, featuring a high deposition rate of 1.57 nm/s, a high crystalline fraction of 67.1%, a very low hydrogen content of 4.4 at.%, an optical bandgap of 1.89 eV, and an almost vertically aligned columnar structure with a mean grain size of approximately 19 nm. We have also shown that a sufficient amount of atomic hydrogen on the growth surface essential for the formation of nanocrystalline silicon is obtained through highly-effective dissociation of silane and hydrogen molecules in the high-density inductively coupled plasmas.

Gas Phase Conditions for Obtaining Device Quality Amorphous Silicon at Low Temperature and High Deposition Rate

AbstractThe aim of this paper is to find a parameter space for deposition of amorphous silicon films at low substrate temperature by VHF PECVD process for application in solar cell fabrication on cheap plastics. Our studies show that at lower substrate temperature, keeping the pressure constant, the ion energy flux reaching the growth surface decreases, which we partly attribute to increasing gas phase collisions arising from an increase in gas density. The role of hydrogen is two fold: (1) higher hydrogen dilution increases the ion energy and restores it to its required value at low temperatures; (2) a normal to dusty plasma transition occurs at lower hydrogen to silane flow ratio and this transition regime shifts to higher dilution ratios for lower substrate temperatures. Thus the role of high hydrogen dilution at low temperature is to avoid the dusty regime. Thus the role of high hydrogen dilution at low temperature is to avoid the dusty regime. The ion energy flux at low substrate temperature can also be restored to the value obtained at high substrate temperature, without increasing hydrogen dilution, by simply lowering the chamber pressure or increasing the delivered plasma power, though the IEDFs in these cases differ substantially from the IEDF at high temperature conditions. We propose that a low pressure or high power in combination with a modest hydrogen dilution (high enough to avoid dusty regime) will deliver silicon films at low temperature without sacrificing deposition rate.

Gas doping ratio effects on p-type hydrogenated nanocrystalline silicon thin films grown by hot-wire chemical vapor deposition

Hydrogenated nanocrystalline silicon (nc-Si:H) grown by hot-wire chemical vapor deposition (HWCVD) has recently drawn significant attention in the area of thin-film large area optoelectronics due to possibility of high deposition rate. We report on the effects of diborane (B 2H 6) doping ratio on the microstructural and optoelectrical properties of the p-type nc-Si:H thin films grown by HWCVD at low substrate temperature of 200 °C and with high hydrogen dilution ratio of 98.8%. An attempt has been made to elucidate the boron doping mechanism of the p-type nc-Si:H thin films deposited by HWCVD and the correlation between the B 2H 6 doping ratio, crystalline volume fraction, optical band gap and dark conductivity.

Investigation of plasma parameters in 915 MHz ECR plasma with SiH 4/H 2 mixtures

The plasma parameters in 915 MHz ECR plasma with SiH 4/H 2 mixtures were investigated using a heated Langmuir probe where extremely high dilution ratio of H 2 to SiH 4 was used for preparing microcrystalline silicon thin films. As the incident microwave power was increased, the electron temperature ( T e ) decreased from 7 eV to 2–3 eV and the electron density ( n e ) increased from 0.5 × 10 11 cm − 3 to 1.3 × 10 11 cm − 3 , that is, low T e ECR plasma with high n e was realized using 915 MHz microwaves. As a result of the film deposition, it was found that there is a correlation between the T e and crystallinity of the microcrystalline silicon. Furthermore, it was shown that high deposition rate can be realized by increasing the gas flow rate.

Influence of hydrogen dilution on the growth of nanocrystalline silicon carbide films by low-frequency inductively coupled plasma chemical vapor deposition

Nanocrystalline silicon carbide (nc-SiC) films are prepared by low-frequency inductively coupled plasma chemical vapor deposition from feedstock gases silane and methane diluted with hydrogen at a substrate temperature of 500 °C. The effect of different hydrogen dilution ratios X [hydrogen flow (sccm) / silane + methane flow (sccm)] on the growth of nc-SiC films is investigated by X-ray diffraction, scanning electron microscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). At a low hydrogen dilution ratio X, cubic silicon carbide is the main crystal phase; whereas at a high hydrogen dilution ratio X, hexagonal silicon carbide is the main crystal phase. The SiC crystal phase transformation may be explained by the different surface mobility of reactive Si-based and C-based radicals deposited at different hydrogen dilution ratios X. The FTIR and XPS analyses show that the Si–C bonds are the main bonds in the films and elemental composition of SiC is nearly stoichiometric with almost equal share of silicon and carbon atoms.

DEVELOPMENT OF HIGHLY STABLE HYDROGENATED AMORPHOUS SILICON FILMS FOR APPLICATION IN SOLAR CELLS

Highly stable hydrogenated amorphous silicon (a- Si:H ) films were developed by very high frequency plasma enhanced chemical vapor deposition (VHF PECVD). Their electrical and structural properties were studied. The films were applied as i-layers for p-i-n solar cells. The stability of intrinsic films as well as solar cells was studied. Results suggest that a- Si:H films prepared at high hydrogen dilution ratio (R) and low plasma power (PW) have low hydrogen content (CH) and small microstructure factor (RH) and show high stability against light illumination. The device with i-layer prepared at PW=5 W and R=10 shows a high stability with degradation in fill factor and efficiency of 3.23% and 11.64%, respectively, over 1000 hours illumination. However, the device with i-layer prepared at higher plasma power (PW=25 W ) and lower hydrogen dilution ratio (R=5) was much less stable. The stability of the devices is directly related to the stability of the intrinsic materials.

Characterization of nanocrystalline silicon carbide films

Amorphous silicon carbide films were obtained by plasma enhanced chemical vapor deposition (PECVD) technique using a gas mixture of silane, methane, and hydrogen with a high excitation frequency and a high hydrogen dilution ratio. The high temperature annealing behavior of the amorphous silicon carbide films was studied by annealing at 1373K for 1h in nitrogen atmosphere. A very thin Au film was deposited on part of the films to investigate the metal induced crystallization effect. Well aligned nanotubes were found on the silicon carbide films covered by a thin gold layer after the high temperature annealing by atomic force microscopy. Further study is necessary to identify the nature of the nanotubes and elucidate their growth mechanism.

Etching characteristics of high-k dielectric HfO2 thin films in inductively coupled fluorocarbon plasmas

Inductively coupled fluorocarbon (CF4∕Ar and C4F8∕Ar) plasmas were used to etch HfO2, which is a promising high-dielectric-constant material for the gate of complementary metal-oxide-semiconductor devices. The etch rates of HfO2 in CF4∕Ar plasmas exceeded those in C4F8∕Ar plasmas. The tendency for etch rates to become higher in fluorine-rich (high F∕C ratio) conditions indicates that HfO2 can be chemically etched by fluorine-containing species. In C4F8∕Ar plasmas with a high Ar dilution ratio, the etch rate of HfO2 increased with increasing bias power. The etch rate of Si, however, decreasd with bias power, suggesting that the deposition of carbon-containing species increased with increasing the power and inhibited the etching of Si. The HfO2∕Si selectivity monotonically increased with increasing power, then became more than 5 at the highest tested bias power. The carbon-containing species to inhibit etching of Si play an important role in enhancing the HfO2∕Si selectivity in C4F8∕Ar plasmas.

Carbon aerogels, cryogels and xerogels: Influence of the drying method on the textural properties of porous carbon materials

Carbon materials with tailored texture can be obtained from drying and pyrolysis of resorcinol–formaldehyde gels. The pore texture of both dried and pyrolyzed material depends on the drying process. Several more or less expensive methods (supercritical drying, freeze-drying, evaporative drying) were tested in order to determine which process is the most suitable for the synthesis of a porous carbon with a definite texture. Supercritical drying leads to the highest pore volume and the widest texture range, but residual surface tensions and shrinkage are not avoided when the pore size is small or when the material density is low; this hampers to fix both the pore volume and the pore size easily. Monoliths are very difficult to obtain by freeze-drying, and the appearance of huge channels due to ice crystal growth at high dilution ratio hinders the fabrication of low density materials. Moreover, gels with small pores do not remain frozen throughout drying, which leads to surface tensions and shrinkage. Although generally replaced by more complicated techniques, evaporative drying is suitable when dense carbons are needed or when the only selection criterion is the pore size: all pore sizes are reachable, but this parameter is in this case strongly correlated to the pore volume.

Characterization of silicon carbide thin films and their use in colour sensor

A series of hydrogenated amorphous silicon carbide (a-Si 1− x C x :H) films were prepared by plasma-enhanced chemical vapour deposition (PECVD) using a gas mixture of silane, methane, and hydrogen as the reactive source. The previous results show that a high excitation frequency, together with a high hydrogen dilution ratio of the reactive gases, allow an easier incorporation of the carbon atoms into the silicon-rich a-Si 1− x C x :H film, widen the valence controllability. The data show that films with optical gaps ranging from about 1.9 to 3.6 eV could be produced. In this work the influence of the hydrogen dilution ratio of the reactive gases on the a-Si 1− x C x :H film properties was investigated. The microstuctural and photoelectronic properties of the silicon carbide films were characterized by Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), and FT-IR spectrometry. The results show that a higher hydrogen dilution ratio enhances the incorporation of silicon atoms in the amorphous carbon matrix for carbon-rich a-Si 1− x C x :H films. One pin structure was prepared by using the a-Si 1− x C x :H film as the intrinsic layer. The light spectral response shows that this structure fits the requirement for the top junction of colour sensor.

Laboratory and Field Test of a Sampling Method for Characterization of Combustion Aerosols at High Temperatures

The objective of this study was to design and experimentally examine a sampling method for high-temperature aerosols from biomass combustion, in which nucleation and condensation from fly ash forming vapors is controlled. The sampling method includes a high-temperature probe in which the hot gas is diluted and then cooled. Laboratory results from sampling a model aerosol with known concentrations of SiO2 particles and KCl vapor showed that when using a high dilution ratio, the KCl vapor was effectively separated from the aerosol by deposition onto the probe walls. When a lower dilution ratio was used, the KCl vapor generated a distinct nucleation mode when cooled in the probe. The sampling method was also used for sampling flue gas from a circulating fluidized bed boiler fired with forest residues. The results suggest that the major fractions of Ca, K, S, and Zn were present as particles at 780°C, whereas most of the Cl and Pb were present as gases. The field results are consistent with results previously reported and indicate that the method can be used for efficient and precise characterization of high-temperature combustion aerosols containing inorganic vapors.

The effect of hydrogen dilution on the field emission from hydrogenated amorphous carbon films

The microstructures and field emission characteristics of hydrogenated amorphous carbon films prepared by using different hydrogen dilution ratio were investigated. It was found that a very low threshold electric field emission could be achieved for samples with moderated hydrogen dilution ratio. However, the field emission characteristics became worse for samples with high hydrogen dilution ratio. The change of field emission can be attributed to the change of electronic structures due to the hydrogen dilution in addition to the increase of the hydrogen surface termination.

Structural and optical properties of four-hexagonal polytype nanocrystalline silicon carbide films deposited by plasma enhanced chemical vapor deposition technique

Four-hexagonal polytype films of nanocrystalline silicon carbide (4H-nc-SiC) were deposited by plasma enhanced chemical vapor deposition method with more than 3×10 4 W m −2 threshold of power density, high hydrogen dilution ratio, and bias pretreatment. The source gases were silane, methane and hydrogen. Our work showed that under conditions similar to those used for the growth of μc-SiC—except a higher power densities over a threshold, a bigger bias pretreatment on substrates, and a moderate bias deposition—nc-SiC films could indeed be achieved. The Raman spectra and transmission electron microscopy diffraction patterns demonstrated that the as-grown films from the H 2–CH 4–SiH 4 plasma consist of amorphous network and phase-pure crystalline silicon carbide which has the 4H polytype structure. The microcolumnar 4H-SiC nanocrystallites of a mean size of approximately 1.6×10 −8 m in diameter are encapsulated by amorphous SiC networks. The photoluminescence spectra of 4H-SiC at room temperature, peaking at 8.10×10 −7 m using a wavelength of 5.145×10 −7 m of argon ion laser, were obtained at room temperature; the luminescence mechanism is thought to be related to transitions in the energy band gap which could be ascribed to the surface states and defects in the structure of 4H-SiC nanocrystalline in these films due to its small size. The as-grown films showed an optical transmittance of 89% at 6.58×10 −7 m. This higher transmittance is believed to be from the small size and amorphous matrix.

Formation of highly uniform silicon nanoparticles in high density silane plasmas

We have investigated silicon particles formed in an inductively coupled plasma (ICP) system and have measured their structural properties by electron microscopy. ICP silane plasmas are shown to be an interesting and versatile source of nanoparticles. Three regimes are mapped out: a regime of no observable particle growth at the lowest pressures, a regime of polydisperse and agglomerated particles at the highest pressures, and a regime yielding highly monodisperse particles at intermediate pressures. Our results suggest that an ICP allows this unique production of monodisperse nanoparticles at high concentrations. The particle size can be controlled from 15 to 200 nm and is determined by the plasma on time. For short plasma times, particle growth appears to be due primarily to coagulation, while in the second phase particle growth is attributed primarily to surface deposition. For H2 dilutions less than 92%, the growth rate is almost independent of H2 partial pressure. Transmission electron micrographs, however, indicate that the addition of hydrogen decreases the particle density. At higher dilution ratios, polycrystalline particles are obtained. Under all other conditions the particles are amorphous.