Total SiH4/H2 pressure effect on microcrystalline silicon thin films growth and structure

The effect of the total SiH₄/H₂ gas pressure (1–10 Torr) on the growth rate, the film crystallinity and the nature of hydrogen bonding of microcrystalline silicon thin films deposited by 13.56 MHz plasma-enhanced chemical vapour deposition (PECVD) was investigated under well-controlled discharge conditions. The deposition rate presents an optimum for 2.5 Torr, which does not follow the trend of silane consumption that increases with pressure and is attributed to an increase in plasma density. The film crystallinity increases with pressure from 1–2.5 Torr and then remains almost the same, whereas the films deposited at 1 Torr are highly stressed. On the other hand, hydrogen bonding is also drastically affected.     

Characterization of undoped μc-SiO:H films prepared from (SiH4+CO2+H2)-plasma in RF glow discharge

Undoped hydrogenated microcrystalline silicon oxygen alloy films (μc-SiO:H) have been prepared from (SiH₄+CO₂+H₂)-plasma in RF glow discharge at a high H₂ dilution, moderately high RF power and substrate temperature. A detailed characterization of the films has been done by electrical, optical as well as structural studies, e.g., IR absorption spectroscopy, Raman scattering and transmission electron microscopy. The presence of a very small amount of oxygen induces the crystallization process, which fails to sustain at a higher oxygen dilution. At higher deposition temperature and in improved μc-network H content reduces, however, O incorporation is favoured. Sharp crystallographic rings in the electron diffraction pattern identify several definite planes of c-Si and no such crystal planes from c-SiO_X is detected.     

Hydrogen plasma induced microcrystallization in layer-by-layer growth scheme

Significant improvement in the microcrystallization in Si:H network has been demonstrated by introducing layer-by-layer (LBL) growth and H-plasma treatment on the stacking layers. During the development of microcrystalline network, the amorphous incubation layer as well as the microcrystalline transition layer thickness has been reduced efficiently by the enhanced reactivity of atomic H from the surface into the bulk through the growth zone, so that the virtual saturation in crystallization is obtained at a significantly low thickness. The growth process becomes more flexible because of the inclusion of additional independent parameter, namely, the time span of H-plasma exposure (t_P) on the growing surface, compared to the conventional process.     

High-pressure condition of SiH4+Ar+H2 plasma for deposition of hydrogenated nanocrystalline silicon film

The characteristics of 13.56-MHz discharged SiH₄+Ar+H₂ plasma at high pressure (2–8 Torr), used for the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films in a capacitively coupled symmetric PECVD system, has been investigated. Plasma parameters such as average electron density, sheath field and bulk field are extracted from equivalent circuit model of the plasma using outputs (current, voltage and phase) of RF V–I probe under different pressure conditions. The conditions of growth in terms of plasma parameters are correlated with properties of the hydrogenated nanocrystalline silicon films characterized by Raman, AFM and dc conductivity. The film deposited at 4 Torr of pressure, where relatively low sheath/bulk field ratio is observed, exhibits high crystallinity and conductivity. The crystalline volume fraction of the films estimated from the Raman spectra is found to vary from 23% to 79%, and the trend of variation is similar to the RF real plasma impedance data.     

Influence of hydrogen dilution on low-temperature polycrystalline silicon formation using RF excitation SiH4/H2 plasma

The effect of hydrogen dilution was investigated on polycrystalline silicon formation using radio frequency excitation SiH₄/ H₂ plasma. The hydrogen dilution reduces the growth rate of the a-Si : H films. The dark conductivity of the a-Si : H films increases with increasing H₂ dilution. The dark conductivity of the poly-Si films formed by recrystallization annealing of the a-Si : H film decrease with H₂ dilution. The grain size, with X-ray diffraction spectroscopy and scanning electron microscopy images of the poly-Si films, is in reverse ratio to the H₂ dilution.     

Synthesis,characterization and hydrogen storage behaviour of AB2 (ZrFe2, Zr(Fe0.75V0.25)2, Zr(Fe0.5V0.5)2 type materials

In this paper, we describe and discuss the synthesis, structural-microstructural and hydrogen storage behaviour of three AB₂ type storage materials namely (a) ZrFe₂, (b) Zr(Fe_0.75V_0.25)₂ and (c) Zr(Fe_0.5V_0.5)₂. These alloys were synthesied by radio frequency induction melting in argon atmosphere. X-ray diffraction and transmission electron microscope have been employed for structural and microstructural characterizations. The XRD study reveals that the lattice constants and the unit cell volume of ZrFe₂, Zr (Fe_0.75V_0.25)₂, Zr(Fe_0.5V_0.5)₂ alloys, which has C14 type hexagonal Laves phase. The Surface morphology and elemental composition of these alloys were investigated by scanning electron microscope and energy dispersive X-ray analysis. The pressure composition isotherms of these alloys were investigated at room temperature and pressure ranges of 0–100 atm respectively, measured through a fully computerized PCI apparatus. As we increase the concentration of V (substituted for Fe), the total hydrogen storage capacities increased up to 1.45 wt%. This capacity is achieved in Zr(Fe_0.5V_0.5)₂ alloy, while the reversible hydrogen storage capacity decreases due to the formation of a stable hydride phase. It has been found that the lattice constants increase with higher vanadium concentration. This is indicating that the majority of vanadium atoms reside in the B-site. The broader X-ray diffraction peaks observed in Zr(Fe_0.5V_0.5)₂ alloy indicates a higher degree of disorder for alloys with the higher V-content. The yet another interesting feature observed in our present study is that the plateau pressure remains well below 1 atm for all the compositions.     

Studies on the structural properties of SiO:H films prepared from (SiH4+CO2+He) plasma in RF-PECVD

A comprehensive report on the structural studies on SiO:H films prepared at high growth rate from He-diluted (SiH₄+CO₂) plasma has been presented and extraction of some intriguing ideas that deserve high relevance for the potential development of nano-crystalline hydrogenated silicon oxide (nc-SiO:H) films has been tried. Poly-hydrogenation has been found as inherent to increasing alloying of the network; however, the bonded H-content reduces linearly with the degree of oxygen incorporation, i.e., the solubility of H in the SiO:H network decreases as the O-content increases in the presence of He. This result happens to be opposite to the conventional H₂-diluted plasma condition and appears to be attractive as well. In addition, He-dilution contributes to a high growth rate of the material. Dynamic interaction of He* in the formation of activated oxygen atoms in the plasma and their efficient mobilization on the surface reaction process at the growing network induces abstraction of H from the SiH_n groups and the terminal H atoms are replaced by bridging O atoms to form the SiO:H network. Abstraction of H from the network being an essential criteria for developing nanocrystallinity and it being inherent to oxygenation in Si network when prepared from He-diluted (SiH₄+CO₂) plasma in PECVD, the process could provide an appropriate pathway for preparing nc-SiO:H structures for solar cells, from such plasma in suitable parametric conditions.     

Microstructural study of hydrogen desorption in 2NaBH4 + MgH2 reactive hydride composite

The desorption mechanism of as-milled 2NaBH₄ + MgH₂ was investigated by volumetric analysis, X-ray diffraction and electron microscopy. Hydrogen desorption was carried out in 0.1 bar hydrogen pressure from room temperature up to 450 °C at a heating rate of 3 °C min⁻¹. Complete dehydrogenation was achieved in two steps releasing 7.84 wt.% hydrogen. Desorption reaction in this system is kinetically restricted and limited by the growth of MgB₂ at the Mg/Na₂B₁₂H₁₂ interface where the intermediate product phases form a barrier to diffusion. During desorption, MgB₂ particles are observed to grow as plates around NaH particles.     

A pre-hydrogen glow method to improve the reproducibility of intrinsic microcrystalline silicon thin film depositions in a single-chamber system

Material property differences are observed in hydrogenated microcrystalline silicon (μc-Si:H) thin films deposited under the same nominal conditions in a single-chamber plasma enhanced chemical vapor deposition system but at different stages of chamber history during prolonged usage. This phenomenon is called system shift, which results from the increase of powder coverage on the surface of the cathode and the coatings on other areas in the chamber. We propose a pre-hydrogen glow method to suppress the system shifting. Experimental results show that this method is very effective to reduce the non-reproducibility in μc-Si:H depositions for prolonged usage of the deposition system. In addition, the μc-Si:H films deposited with the pre-hydrogen glow have an improved structural homogeneity along the film thickness.     

Preparation of highly conductive p-type μc-Si:H window layer using lower concentration of hydrogen in the rf glow discharge plasma

Boron doped p-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by radio-frequency glow discharge method. Highly conductive p-type μc-Si:H films can be obtained even with lower concentration of hydrogen in the rf glow discharge plasma if chamber pressure is low. Effects of increase in hydrogen (H₂) flow rate and chamber pressure have been studied. The structural properties of the films have been studied by X-ray diffractometry. The electrical and optical characterization have been done by dark conductivity, Hall measurements and optical absorption measurements respectively. Film with conductivity 0.1(Ω-cm)⁻¹ with band gap 2.1 eV has been obtained.     

Raman spectroscopy of carbon-coated LiCoPO4 and LiFePO4 olivines

The effect of laser power on the Raman spectra of two carbon-coated nano-powders of LiCoPO₄ and LiFePO₄ olivine cathode materials were investigated. In the ambient atmosphere at a moderate laser power, the phenomenon of the removal of the carbon coating layer from both samples was detected. The olivine structure of LiCoPO₄-C powder therefore remains unchanged during the prolonged exposure to a 4.3 mW laser beam. The mild removal of the carbon layer makes it possible to analyze the details of the LiCoPO₄ structure in air without interference from carbon.LiFePO₄-C powder, together with carbon layer gasification, undergoes oxidative decomposition by the oxygen with the formation of Li₃Fe₂(PO₄)₃ and Fe₂O₃, even at a laser power of 1 mW. Thus, care should be taken when measuring and interpreting the Raman spectra of this material both in air and in an inert atmosphere, as obvious decomposition of the LiFePO₄ olivine structure takes place even at a moderate power of the excitation laser.A comparative study of the stability of these two carbon-coated nano powders under laser beam irradiation and heating was carried out with the use of TGA-mass spectrometry.     

Preparation of highly conductive p-type μc-Si:H window layer using lower concentration of hydrogen in the rf glow discharge plasma

Boron doped p-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by radio-frequency glow discharge method. Highly conductive p-type μc-Si:H films can be obtained even with lower concentration of hydrogen in the rf glow discharge plasma if chamber pressure is low. Effects of increase in hydrogen (H₂) flow rate and chamber pressure have been studied. The structural properties of the films have been studied by X-ray diffractometry. The electrical and optical characterization have been done by dark conductivity, Hall measurements and optical absorption measurements respectively. Film with conductivity 0.1(Ω-cm)⁻¹ with band gap 2.1 eV has been obtained.     

Microstructural evolution of NbF5-doped MgH2 exhibiting fast hydrogen sorption kinetics

The microstructure of MgH₂ with 1 mol% NbF₅, prepared by high-energy ball milling (HEBM), was studied using high resolution transmission electron microscopy (HR-TEM) with an X-ray energy dispersive spectrometer (EDS) before and after hydrogen sorption cycles. The TEM samples were prepared without any air exposure by a novel, focused ion beam (FIB) system specially designed for highly air sensitive materials. During HEBM, the doping agent, NbF₅, was distributed as an extremely thin, film-like, amorphous phase along the grain boundaries of the nanocrystalline MgH₂. After 10 sorption cycles, amorphous Nb-F phase was transformed into crystalline Nb hydrides. It is believed that the Nb hydride played a decisive role in improving the sorption kinetics of MgH₂.     

Catalytic effect of titanium nitride nanopowder on hydrogen desorption properties of NaAlH4 and its stability in NaAlH4

Single crystalline titanium nitride (TiN) nanopowder is synthesized by a mechano-chemical reaction between titanium chloride (TiCl₃) and lithium nitride (Li₃N) by means of high-energy ball milling. The TiN nanopowder has an average particle size of 6 nm and is introduced into sodium alanate (NaAlH₄) as a catalyst. During hydrogen sorption cycles, TiN-catalyzed NaAlH₄ exhibits a greater hydrogen desorption rate and higher hydrogen capacity than TiCl₃-catalyzed NaAlH₄. Contradicting thermodynamic predictions, in situ X-ray diffraction results reveal that TiN nanopowder remains stable and produces no by-products (e.g., Ti-Al compounds) in the reaction with NaAlH₄ during hydrogen desorption. In situ Raman spectroscopy also confirms the stability of TiN nanopowder in NaAlH₄. This implies that the sustained hydrogen sorption kinetics and hydrogen capacity of TiN-catalyzed NaAlH₄ originate from the structural and chemical stability of TiN nanopowder in NaAlH₄ for the given conditions of the hydrogen cycle test.     

Intermediate order in tetrahedrally coordinated silicon: evidence for chainlike objects

In this report, we describe the nature of intermediate order in silicon as determined by recent measurements on thin films using transmission electron microscopy (TEM) and Raman scattering. The TEM images show in addition to the expected continuous random network (CRN), the presence of highly ordered quasi-one-dimensional “chain-like objects” (CLO's) that are 1–2 nm wide and tens of nm long that meander and show some evidence of cross-linking with each other. The presence of these objects correlate to a Raman feature centered at 490 cm⁻¹ whose width is 35–40 cm⁻¹, and is used to quantify the heterogeneity in terms of the CLO and CRN (=475 cm⁻¹ scattering) concentrations. The 490 and 35 cm⁻¹ values are consistent with bond angle deviations approaching 0°, and thus reinforces an association with the CLOs. We find that in reference quality a-Si:H (made using pure SiH₄), the CLO concentration is about 5 vol%, while in state-of-the-art material using high H₂ levels of dilution during processing, it increases to about 15%. Increased stability of such material to light-soaking is thus not mediated by a direct volumetric replacement of poor with high-quality components. Rather, an important characteristic of intermediate order in silicon is the low-dimensional aspect of its order, which allows it to influence more total volume than which it is itself composed. Consistent with these and other recent findings, we propose a tensegrity model of amorphous silicon.     

XPS, TEM and NRA investigations of Zn(Se,OH)/Zn(OH)2 films on Cu(In,Ga)(S,Se)2 substrates for highly efficient solar cells

Structural and compositional properties of Zn(Se,OH)/Zn(OH)₂ buffer layers deposited by chemical bath deposition(CBD) on Cu(In,Ga)(S,Se)₂ (CIGSS) absorbers are investigated. Due to the aqueous nature of the CBD process, oxygen and hydrogen were incorporated into the ‘ZnSe’ buffer layer mainly in the form of Zn(OH)₂ as is shown by X-ray photoelectron spectroscopy and nuclear reaction analysis (NRA) measurements leading to the nomenclature ‘Zn(Se,OH)’. Prior to the deposition of Zn(Se,OH), a zinc treatment of the absorber was performed. During that treatment a layer mainly consisting of Zn(OH)₂ grew to a thickness of several nanometer. The whole buffer layer therefore consists of a Zn(Se,OH)/Zn(OH)₂ structure on CIGSS. Part of the Zn(OH)₂ in both layers (i.e. the Zn(Se,OH) and the Zn(OH)₂ layer) might be converted into ZnO during measurements or storage. Scanning electron microscopy pictures showed that a complete coverage of the absorber with the buffer layer was achieved. Transmission electron microscopy revealed the different regions of the buffer layer: An amorphous area (possibly Zn(OH)₂) and a partly nanocrystalline area, where lattice planes of ZnSe could be identified. Solar cell efficiencies of ZnO/Zn(Se,OH)/Zn(OH)₂/CIGSS devices exceed 14% (total area).     

CF4/O2 dry etching of textured crystalline silicon surface in a-Si:H/c-Si heterojunction for photovoltaic applications

We investigated a dry cleaning procedure of the crystalline substrate, both mono- and multi crystalline silicon, to leave an uncontaminated surface using an etching process involving CF₄/O₂ mixture. A detailed investigation was performed to find compatibility and optimisation of amorphous layer depositions both on flat and textured silicon by changing the plasma process parameters. We found evidence that plasma etching acts by removing the native oxide and the damages of textured silicon and by leaving an active layer on silicon surface suitable for the emitter deposition. SEM analysis confirmed that it is possible to find plasma process conditions where no appreciable damages and change in surface morphology are induced. By using this process we achieved on amorphous crystalline heterostructure a photovoltaic conversion efficiency of 13% on 51 cm² and 14.5% on 1.26 cm² active area. We also investigated compatibility of the process with industrial production of large area devices.     

基于TEM、FT-IR和XPS的PODE2-4/柴油混合燃料颗粒物特性

基于透射电镜（TEM）、傅里叶变换红外光谱（FT-IR）和X射线光电子能谱（XPS）试验对不同掺混比例与不同负荷下的聚甲氧基二甲醚(PODE_2-4)/柴油混合燃料排气颗粒物的理化特性变化规律进行研究.TEM与分形理论分析表明,当PODE_2-4掺混比例增加或负荷减少时,颗粒物的整体尺寸变小,排列更紧密,分形维数变大.FT-IR分析发现,当PODE_2-4掺混比例或负荷增加时,当量峰高比比值减少,颗粒物表面脂肪族碳氢官能团相对含量降低.XPS试验表明,随着PODE_2-4掺混比例的增加或负荷的减小,颗粒物表面的氧、碳元素物质的量比与碳原子杂化比比值增加,羟基、羰基以及总含氧官能团含量增大.结果表明:PODE_2-4掺混比例的增加和负荷的减小影响颗粒物的纳观结构和氧化活性,使其更易于被氧化,有利于颗粒物捕集器的再生.     

Synthesis and characterization of TiO2 doped polyaniline composites for hydrogen gas sensing

The Polyaniline (PANI) and Titanium dioxide (TiO₂)/PANI composite thin film based chemiresistor type gas sensors for hydrogen (H₂) gas sensing application are presented in this paper. Pure PANI and TiO₂/PANI composites with different wt% of TiO₂ were synthesized by chemical oxidative polymerization of aniline using ammonium persulfate in acidic medium at 0-5 °C. Thin films of PANI and TiO₂/PANI composites were deposited on copper (Cu) interdigited electrodes (IDE) by spin coating method to prepare the chemiresistor sensor. Finally, the response of these chemiresistor sensors for H₂ gas was evaluated by monitoring the change in electrical resistance at room temperature. It was observed that the TiO₂/PANI composite thin film based chemiresistor sensors show a higher response as compared to pure PANI sensor. The structural and optical properties of these composite films have been characterized by X-ray diffraction (XRD) and UV-Visible (UV-Vis) spectroscopy respectively. Morphological and structural properties of these composites have also been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively.     

Properties of amorphous silicon solar cells fabricated from SiH2Cl2

Chlorinated intrinsic amorphous silicon films [a-Si:H(Cl)] and solar cell i-layers were fabricated using electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) and SiH₂Cl₂ source gas. n–i–p solar cells deposited on ZnO–coated SnO₂ substrates had poor photovoltaic performances despite the good electronic properties measured on the a-Si:H(Cl) films. Improved open–circuit voltage (V_oc) of 0.84 V and fill factor (FF) of 54% were observed in n–i–p solar cells by providing an n/i buffer layer and by using Ga-doped ZnO coated glass substrates. However, the FF improvement was still rather poor, which is thought to originate from high interface recombination in the ECR deposited solar cells. The V_oc and the FF showed much stable feature against light soaking.