Effect Of Hydrogen Dilution Research Articles

Hydrogen Dilution Effect on the Crystallinity of Silicon Films Grow by Hot Wire Cell Method

AbstractA new process, the Hot Wire Cell method, was developed and successfully used to grow polycrystalline silicon thin films at a low temperature and high growth rate. In the Hot Wire Cell method, reactant gases are decomposed as a result of reacting with a heated tungsten filament placed near to a substrate and polycrystalline silicon films can be deposited at a growth rate of 1.2nm/s without hydrogen dilution and 0.9nm/s with the use hydrogen dilution. The film crystallinity changed from amorphous to polycrystalline due to the addition of hydrogen, thus hydrogen dilution was effective for improving film crystallinity. Furthermore, we obtained (220) oriented polycrystalline silicon thin films with a 90% crystal fraction by the use of hydrogen dilution. These results showed that the Hot Wire Cell method is promising for the deposition of device-grade polycrystalline silicon films for photovoltaic applications.

Device-quality polycrystalline silicon films deposited at low process temperatures by hot-wire chemical vapor deposition

Device-grade undoped hydrogenated polycrystalline silicon thin films have been developed from a gas mixture of silane and hydrogen using a hot-wire chemical vapor deposition (HW-CVD) method, optimizing the deposition parameters. Proper design of the HW-CVD reactor helps to deposit a uniform quality of film over a large area (100 cm 2) with a two filament configuration. Extensive studies have been made of the effects of hydrogen dilution (4–60), substrate temperature (180–400°C) and filament temperature (1500–1700°C) on the film growth. Atomic force micrographs give a quantitative estimate of roughness for these films. UV-visible ellipsometry analyses confirm their compactness and crystallinity while X-ray diffraction patterns allow for the determination of the crystallite sizes (up to 400 Å). Using a hydrogen dilution of 60, a substrate temperature of 300°C and a filament temperature of 1500°C, a dark conductivity of 2.5×10 −5 S/cm and its activation energy of 0.45 eV have been obtained. For these films, the Hall mobility attains 10 cm 2/V s. With these deposition parameters, the intrinsic layer of complete p–i–n HW-CVD solar cells has been realized. These cells, deposited on TCO coated Corning glass substrates, exhibit 1.8% conversion efficiency under 100 mW/cm 2 irradiation.

Investigation of Hydrogenated Amorphous Silicon Germanium Fabricated under High Hydrogen Dilution and Low Deposition Temperature Conditions for Stable Solar Cells

The effects of hydrogen dilution of up to 54:1 (=H2:SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) were investigated while keeping the optical gap (Eopt) constant. It was found that deterioration of the film properties of a-SiGe:H due to a decrease in substrate temperature (Ts) can be compensated by the high hydrogen dilution method. As Ts decreases from 230°C to 180°C, the high photoconductivity [∼1×10-5 (Ω·cm)-1] and low silicon dihydride content (∼1 at.%) of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties deteriorate with the conventional low hydrogen dilution ratio of 2.5:1. Probably, hydrogen radicals supply the energy required for the surface reaction during a-SiGe:H deposition which is lost when Ts is decreased. This tendency is useful for solar cell fabrication, especially for superstrate-type a-Si/a-SiGe tandem solar cells, because the decrease in the deposition temperature of a-SiGe:H for the bottom photovoltaic layer can reduce damage to the underlying layers caused by a high deposition temperature. As a result of applying this technique to the fabrication process of an a-Si/a-SiGe stacked solar cell submodule (area: 1200 cm2), the world's highest stabilized efficiency of 9.5% (light-soaked and measured at JQA) was achieved.

Effect of hydrogen dilution on the remote plasma enhanced chemical vapor deposition of chlorinated SiO2 films

Chlorinated silicon dioxide films have been prepared at low temperatures (200 °C) and high deposition rates (900–1700 Å/min) by remote plasma enhanced chemical vapor deposition using mixtures of SiCl4, O2, Ar, and H2 under various hydrogen flow rate conditions. It was found that films deposited without hydrogen grow with the highest deposition rate, however they exhibit poor properties such as high etch rate, low density, and low refractive index. These oxides are also chemically unstable and easily hydrolyzable upon exposure to ambient moisture. It is assumed that the low chlorine content (detected by Rutherford backscattering) in these samples is due to the hydration of the weak Si–Cl bonds existing in the as-deposited films, which generates desorption of HCl and forms the Si–OH bonds observed in the corresponding infrared spectra. The addition of hydrogen to the process reduces the deposition rate but improves the properties and stability of the films by reducing the amount of chlorine incorporated during growth. At hydrogen flow rates moderately higher than the SiCl4 flow, dense and stable chlorinated oxides with properties close to stoichiometric SiO2 are obtained. The composition results indicate that in order to obtain these types of oxides the chlorine concentration should be limited to a value around 2.6 at. %.

Influence of excitation frequency, temperature, and hydrogen dilution on the stability of plasma enhanced chemical vapor deposited a-Si:H

The first comparative study of dc, rf, and very high frequency (VHF) excitation for the plasma enhanced chemical vapor deposition of intrinsic layers of hydrogenated amorphous silicon (a-Si:H) is presented. The effects of hydrogen dilution on film stability are emphasized. Growth rates at comparable plasma power are presented for substrate temperatures between 100 and 300 °C and for various H2 dilution ratios. The optical band gap, H content, and electronic transport properties in the light-soaked state were measured. H2 dilution strongly reduces the growth rate for all techniques. The growth rate for the highest H2 dilution ratio is higher for VHF (∼4 Å/s) than for dc (∼3 Å/s) or rf (0.5–1 Å/s) excitation. In all three cases, increasing the substrate temperature reduces the optical gap and the H content CH. Raising the substrate temperature slightly enhances stability. H2 dilution increases the optical gap for all three techniques. The H content of rf- and VHF-deposited samples increases with increasing H2 dilution ratio, while in dc deposition it produces an initial drop of the H content, followed by an increase. In all three cases, H2 dilution improves the electronic transport properties of the material by roughly a factor of 2. The gain in stability is most pronounced for relatively small dilution; in the case of dc deposition, too strong a dilution even has an adverse effect on stability. We rule out a universal relation between hydrogen content of the a-Si:H films and their stability against light soaking. Enhanced stability may be obtained for films which incorporate either more or less hydrogen than standard a-Si:H.

The effect of hydrogen dilution on the structure of a-C:H

Two a-C:H samples were prepared using a fast-atom deposition system from acetylene and an acetylene/hydrogen gas mixture. Their structure was investigated using neutron and x-ray diffraction and infrared spectroscopy measurements. Compositional analysis shows that a 1:1 mixture results in a change from to a-, i.e. has a very small effect on the composition. The diffraction data also show that the addition of hydrogen to the precursor gas has no significant effect on the average bond distances and angles but shows a small change in the H-C-H and C-C-H correlations between the two samples. However, the infrared data show that there are significant changes in the bonding of hydrogen within the sample - changes which do not affect the average network structure. We observe a decrease in the amount of and groups, and an increase in the fraction of and CH groups, with the formation of a second CH bonding environment in the hydrogen-diluted sample. Therefore, in addition to providing useful structural information on these a-C:H samples, this set of experiments illustrates very well the complementary nature of the data from diffraction and spectroscopic techniques.

The effect of hydrogen dilution on the microstructure and stability of a-Si:H films prepared by different techniques

In this study, we present in situ kinetic and spectroscopic ellipsometry investigations concerning the growth and microstructure of hydrogen (H)-diluted hot-wire (HW) deposited amorphous hydrogenated silicon (a-Si:H). A comparison of the results obtained for HW-films with those for films prepared by the conventional plasma enhanced chemical vapour deposition (PECVD) reveals that H-dilution has a completely different effect in both cases. While the density of PECVD material increases with increasing dilution ratio, the opposite behaviour is measured for the HW-material. In agreement with our earlier results obtained for amorphous hydrogenated thin films deposited in pure silane which directly relate the stability of these films to the corresponding microstructure (density, surface roughness and hydrogen bonding configuration), we explain the decrease in stability of the H-diluted HW-films with increasing dilution ratio by an appropriate change in film growth and the resulting microstructure.

Effects of very high hydrogen dilution at low temperature on hydrogenated amorphous silicon germanium

The effects of hydrogen dilution of up to 54:1 (=H2: SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) were investigated at substrate temperatures <200°C. The photoconductivity (>10−5 Ω−1 cm−1 and silicon dihydride content (<2 at.%) of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties deteriorate with our conventional low hydrogen dilution conditions at a substrate temperature range <200°C. And this high-quality a-SiGe:H film was applied to the bottom photovoltaic layer of a glass superstrate type a-Si/a-SiGe tandem solar cell submodule (30 cm×40 cm), and a stabilized efficiency of 9.5% (light-soaked and measured at Japan Quality Assurance organization (JQA)) was achieved.

Optical, structural and electrical properties of device-quality hydrogenated amorphous silicon-nitrogen films deposited by plasma-enhanced chemical vapour deposition

Abstract High-electronic quality hydrogenated amorphous silicon-nitrogen (a-Si1-xNx: H) films with an energy gap in the range 1.9-2.7eV have been deposited by plasma-enhanced chemical vapour deposition in silane-ammonia gas mixtures at two different gas residence times and in hydrogen-diluted silane-ammonia gas mixtures. Compositional, structural, electrical and optical properties have been investigated. For the first time the effects of hydrogen dilution of SiH4 + NH3 gas mixtures on the a-Si1-xNx: H network is reported. We have observed that hydrogen dilution decreases hydrogen incorporation and increases nitrogen incorporation, promoting a higher connectivity of the a-Si1-xNx :H network. All films show good electronic properties, comparable with or superior to those of amorphous silicon-carbon films, which are improved in films deposited from hydrogen-diluted gas mixtures.

Comparison of VHF, RF and Dc Plasma Excitation for a-Si:H Deposition with Hydrogen Dilution

ABSTRACTA comparative study of DC, RF and VHF excitation for the plasma enhanced chemical vapor deposition (PECVD) of intrinsic layers of a-Si:H is presented, with special emphasis on the effects of hydrogen dilution. Growth rates at comparable plasma power, for substrate temperatures between 100°C and 300°C and for various H2 dilution ratios are presented, along with optical bandgap, H content, and electronic transport properties in the light-soaked state.H2 strongly reduces the growth rate for all techniques. The growth rate for the highest H2 dilution ratio is approximately four times higher for VHF than for DC or RF excitation. In all three cases increasing the substrate temperature reduces the optical gap and the H content CH. Raising the substrate temperature slightly enhances the stability of undiluted films. H2 dilution increases the optical gap for all three techniques. The H content of RF- and VHF-deposited samples increases with increasing H2 dilution ratio, while in DC deposition it produces an initial drop of the H content, followed by an increase.In all three cases, H2 dilution improves the electronic transport properties of the material by roughly a factor of two. The gain in stability is most pronounced for relatively small dilution; in the case of DC deposition, too strong a dilution even has an adverse effect on stability.

Performance of PEM Fuel Cells Operating on Synthetic Reformate: Experimental and Modeling Studies

Experimental results and modeling simulating PEM Fuel Cell operation on reformed gasoline are reported. These studies cover observed and calculated effects of hydrogen dilution at the level expected from gasoline reforming and the extent of catalyst poisoning by CO under such anode operation conditions. At flow rates corresponding to less than 80% hydrogen utilization, modest losses in cell power density are observed experimentally from hydrogen dilution alone. Modeling suggests that the performance loss caused by hydrogen dilution stems primarily from combined effects of limited protonic conductivity and gas permeability within the thin film anode catalyst layer. Effects due to limited transport through the gas diffusion backing are expected to be relatively minor. Under the experimental conditions employed, little evidence was found for deleterious effects caused by intemal generation of CO from CO 2 . Finally, we demonstrate complete tolerance to 100 ppm CO in a 40% H 2 inlet stream at stoichiometric flow of 1.3, using air injection into the anode.

Effect of hydrogen dilution on the structure of amorphous silicon alloys

We investigate why high levels of hydrogen dilution of the process gas lead to enhanced light soaking stability of amorphous silicon (a-Si) alloy solar cells by studying the microstructural properties of the material using high-resolution transmission electron microscopy (TEM) and Raman spectroscopy. The TEM results show that a-Si alloy (with or without hydrogen dilution) is a heterogeneous mixture of amorphous network and linear-like objects that show evidence of order along their length. The volume fraction of these ordered regions increases with increasing hydrogen dilution.

Amorphous and microcrystalline silicon films obtained by hot-wire chemical vapour deposition using high filament temperatures between 1900 and 2500°C

Abstract The effects of hydrogen dilution and substrate temperature on the optical, transport and structural properties of silicon thin films deposited by hot-wire chemical vapour deposition using filament temperatures between 1900 and 2500°C are reported. Amorphous silicon films with a Tauc bandgap of 1·65eV, a photoconductivity-to-dark-conductivity ratio above 105, a deep defect density of 1016 cm−3, an Urbach energy of 55 meV and a structure factor R ≍ 0·2 were prepared with deposition rates up to 40 A s−1 for hydrogen dilutions below 80%. The best properties were obtained for undiluted films deposited at a substrate temperature of 220°C. For hydrogen dilutions above 80%, microcrystalline films were obtained with low growth rates (below 3 A s−1) for all substrate temperatures studied (between 100 and 400°C) regardless of the filament temperature. The Raman spectra show a high crystalline fraction and a small grain size. In this filament temperature regime, the growth mechanism and film properties are c...

Effects of high hydrogen dilution at low temperature on the film properties of hydrogenated amorphous silicon germanium

The effects of hydrogen dilution of up to 54:1 (=H2:SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) was investigated at a low substrate temperature, while keeping the optical gap (Eopt) constant. It was found that deterioration of the film properties, when substrate temperature decreases, can be compensated by the high hydrogen dilution method. As the substrate temperature decreases from 230 to 180 °C, the high photoconductivity, high photosensitivity, and low silicon dihydride content of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties becomes worse with conventional low hydrogen dilution ratios. Probably, hydrogen radicals substitute for the surface reaction energy lost by decreasing the temperature. Besides, a-SiGe:H films deposited under higher hydrogen dilution have more germanium and less hydrogen content than those of the conventional films, despite having the same Eopt. One possible explanation for why Eopt can be kept constant is the suppression of the formation of Ge–Ge bonds at the growing surface by the energy supplied by the hydrogen radicals.

Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP H 2/P Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar+ and H2 which lead to ArH+ and H 2 + ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H+, H 2 + , and H 3 + ) or excited species H(n=2) with H2 producing H atoms.

Impact of Hydrogen Dilution on the Properties and Light Induced Changes of A-SI:H Based Materials and Solar Cells

ABSTRACTStudies have been carried out on a-Si:H materials and corresponding solar cells fabricated with and without hydrogen dilution of silane by rf PECVD. The effect of hydrogen dilution on the growth kinetics and microstructures and their dependence on the substrate temperature have been studied. Hydrogen diluted a-Si:H materials and solar cells exhibit improved properties and higher stability to light induced changes. Distinct differences are found in the electron mobility lifetime (μτ) products and subgap absorption over a wide range of generation rates. Striking differences are also found in the kinetics of light induced degradation in both the materials and their corresponding solar cells. Direct correlations are presented between the degradation kinetics of p(a-SiC:H)/i(a-Si:H)/n(μc-Si) solar cells and those of thin film materials constituting the i-layers.

The Effects of Hydrogen Dilution on VOC in a-Si:H Pin Solar Cells

ABSTRACTWe study the effects of hydrogen dilution on the open circuit voltage of a-Si:H pin solar cells fabricated by rf glow discharge growth. We keep the p and n layers the same and only vary the i layer properties. A normal a-Si:H i layer, an H-diluted i layer, and a thin H-diluted layer inserted between p and normal i layer are selected for this study. We measure the JV characteristics and the internal electric field distribution using a transient-null-current technique both in annealed and light soaked states. We find that hydrogen dilution does stabilize the Voc either in a bulk H-diluted i layer or in a thin layer between p and normal i layer after 100 hours AMI sun light soaking. From dark IV measurement, both H-diluted cells show little change in current at voltage near Voc before and after light soaking; while the normal a-Si:H cell does show a noticeable change. Also the internal field measurements find a stronger electric field starting from p and i interface for both H-diluted cells compared to the normal a-Si:H cell. Furthermore, there are no measurable changes in the field profiles after 100 hour AMI light-soaking for both H-diluted and normal a-Si cells. All these suggest that hydrogen dilution increases the field strength near p and i interface, which is the key that leads to a more stable Voc of H-diluted cells.

Diagnostics of plasma for metal - organic chemical vapour deposition of Cu and fabrication of Cu thin films using the plasma

Effects of hydrogen dilution on properties of Cu films deposited using the metal - organic plasma chemical vapour deposition method were examined. On increasing the dilution from 70 to 88%, the Cu concentration and grain size of the films increase from about 75 to 99% and from about 10 nm to above 100 nm respectively; also their resistivity decreases concomitantly to about . Films of high quality are obtained in the high dilution range above 80% , for which the emission intensity of H is high, suggesting that hydrogen atoms may play an important role in eliminating impurities from the films. The density of Cu atoms in the plasma measured by a light-absorption method is of the order of , which is lower by two orders of magnitude than the density necessary to explain the deposition rate of typical in our experiments.

Effect of hydrogen dilution on the deposition of carbon-rich a-SiC:H films by the electron cyclotron resonance method

The deposition of hydrogenated amorphous silicon carbide (a-SiC:H) films from a mixture of silane, acetylene and hydrogen gas using the electron cyclotron resonance chemical vapour deposition (ECR-CVD) process is reported. The variation in the deposition and film characteristics such as the deposition rate, optical band gap and IR absorption as a function of the hydrogen dilution is investigated. The deposition rate increases to a maximum value of about 250 Å min − at a hydrogen dilution ratio of about 20 (hydrogen flow (sccm)/acetylene + silane flow (sccm)) and decreases in response to a further increase in the hydrogen dilution. There is no strong dependence of the optical band gap on the hydrogen dilution within the dilution range investigated (10–60) and the optical band gap calculated from the E 04 method varied marginally from about 2.85 to 3.17 eV. The room temperature photoluminescence (PL) peak energy and intensity showed a prominent shift to a maximum value of about 2.17 eV corresponding to maximum PL intensity at a moderate hydrogen dilution of about 30. The PL intensity showed a strong dependence on the hydrogen dilution variation.

The effect of hydrogen dilution on the aminosilane plasma regime used to deposit nitrogen-rich amorphous silicon nitride

The first hydrogen dilution study of ammonia/silane plasmas tuned to an aminosilane plasma regime is reported. The use of formal statistical experimental design techniques have enabled us to determine the effects that hydrogen dilution level, rf power density and substrate growth temperature have on various film properties. Hydrogen dilution of the ammonia/silane plasma reduced the amino (NH 2) content of the deposited nitride films while also driving the intrinsic film stress to compressive values. Increasing the substrate temperature also reduced the amino concentration, but drove the film stress tensile. It was therefore possible to control the film stress and produce nitride films that contained only NH and SiN bonds (with no detectable NH 2 or SiH bonds) by choosing a suitable combination of H 2 dilution level and growth temperature. Exodiffusion experiments on such ‘bond optimised’ films revealed only one hydrogen evolution peak at temperatures in excess of 900°C, with no ammonia exodiffusion detected.