Amorphous Hydrogenated Research Articles

Thermally Induced Changes in the Hydrogen Microstructure of Amorphous Hydrogenated Silicon Films, Analyzed Using In Situ Real Time Infrared Spectroscopy

Changes in the hydrogen content and bonding in amorphous hydrogenated silicon (a-Si:H) films during stepwise thermal annealing are measured using in situ real time infrared spectroscopy. The experimental spectra are fit using previously identified SiH stretching modes for hydrogen bonded at isolated network sites, hydrogen in platelet-like configurations and hydrogen at surfaces. Based on this mode separation, the release of hydrogen from surfaces and platelet configurations is found to occur at ∼320–370°C. By 470°C, these groups are completely released from the sample, whereas isolated SiH network sites are still present. This thermal annealing of the sample also irreversibly changes the microstructure and thereby the distribution of available hydrogen bonding sites in the amorphous network. Re-hydrogenation experiments show that isolated bonding sites are created and platelet and surface bonding sites are removed from the hydrogen density of states. This structural transformation during annealing is interpreted as the release of hydrogen from platelet like configurations and the reformation of Si–Si bonds in a-Si:H.

The electrochemical evaluation of ball milled MgNi-based hydrogen storage alloys

The electrochemical properties of amorphous MgNi-based hydrogen storage alloys synthesized by ball milling were evaluated. In order to improve the cycle life, the surface of the amorphous Mg50Ni50 alloy was coated with Ti, Al and Zr by ball milling. Among them Ti was very effective to improve cycle life. And two kinds of MgNi-based amorphous alloys were designed by the substitution of Ti and other elements for Mg of MgNi-based alloys, which were composed of four components. Thus, the cycle life of electrodes with these quaternary amorphous alloys was strongly improved.

Study of hydrogen polarization potential in MFM device using amorphous (Ba, Sr)TiO3 type thin films and hydrogen gas sensing preporties

Ferroelectric (Ba,Sr)TiO3 type thin films, with and without Ti and La doping, have been prepared by the sol-gel technology and characterized using TGA, DTA, X-ray diffraction, dielectric characterizations, and gas sensing measurements for H2 detection. The BST thin film capacitive devices are made on Si substrate to detect H2 gas and to study the hydrogen induced interfacial polarization potential. Experimental results show that the Schottky I-V behavior exhibits in these Pd/amorphous (Ba,Sr)TiO3 type thin film/metal capacitive devices, and that the enhanced interfacial polarization potential as large as 4.5 V at 1042 ppm H2 gas diluted in air has been observed, which is about 7 times larger than the best value reported in the literature. It has been clearly shown that the hydrogen induced interfacial polarization potential is closely correlated with the microstructure in ferroelectric thin films and the enhancement of this polarization potential is mainly attributed to the high dielectric constant of amorphous ferroelectric thin films. A hydrogen-blocking model is proposed to explain this interesting phenomenon. Other factors such as compositions, stoichiometry, processing conditions, are also discussed.

Deposition of Amorphous Hydrogenated Silicon (a-Si:H): in situ Gas Analysis by Time-of-Flight Mass Spectrometry

Deposition of Amorphous Hydrogenated Silicon (a-Si:H): in situ Gas Analysis by Time-of-Flight Mass Spectrometry

Observation of Blue Emission from ECR-CVD Deposited Amorphous Hydrogenated Silicon Carbide

ABSTRACTAmorphous hydrogenated silicon carbide (a-Sil-xCx:H) thin films have been deposited by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) technique at different microwave powers from 100W to 1000W. The films were characterized in terms of their optical absorption and photoluminescence (PL). Their optical band gap E04 ranged from 3.06eV to 3.54eV and when excited at 363.8nm from an Ar+ ion laser, the PL peak emission energy Epl, was found to range from 2.44eV to 2.79eV, corresponding to green to blue emission. The effects of excitation energy Eex, on Epl, and FWHM of the PL spectra were also investigated. A linear relation between the FWHM and Urbach tail width E0was noted, suggesting that the PL bandwidth is mainly contributed by static disorder. The blue emission observed in these a-S1-x-Cx:H films is promising for their applications in large area flat panel displays.

Fluorine Incorporation Into Hard Amorphous Hydrogenated Carbon Films Deposited by PECVD

ABSTRACTThe incorporation of fluorine into amorphous hydrogenated carbon films deposited by the plasma enhanced chemical vapor deposition technique (PECVD) was investigated. Different mixtures of CH4 and CF4 were employed as the plasma atmosphere, with the partial pressure of CF4 ranging from 0 to 100 %. For all depositions, the self-bias was kept at –350 V. In the case of atmospheres richer than ∼80 % of CF4, no film deposition but substrate erosion was observed. The composition of the films was determined by Ion Beam Analysis (IBA), revealing that fluorine is incorporated into the amorphous network by replacing hydrogen. Infrared (IR) transmission measurements confirmed these results. It was also found that the incorporation of fluorine has the effect of reducing both the internal stress and hardness due to atomic density reduction.

Application of a Semi‐insulating Amorphous Hydrogenated Silicon Nitride Film as a Resistive Field Shield and Its Reliability

In order to develop reliable high voltage integrated circuits (HVICs), the characteristics of semi‐insulating plasma‐deposited amorphous silicon nitride (a‐SiN:H) films as resistive field shields, are examined, the reliability of their application to HVICs are studied. The surfaces of these semi‐insulating films were unstable, and it was concluded that these films must be covered with a final passivation film such as an insulating plasma‐deposited a‐SiN:H film. The transverse electrical conduction mechanism of these films is briefly discussed.

Mechanism of the Initiation Step in Atomic Hydrogen-Induced CVD of Amorphous Hydrogenated Silicon–Carbon Films from Single-Source Precursors

A number of alkylsilanes and alkylcarbosilanes of widely different molecular structure are characterized in terms of their ability to form amorphous hydrogenated silicon–carbon (a-Si:C:H) films in atomic hydrogen-induced chemical vapor deposition (AHCVD). The compounds containing only Si–C and C–H bonds in the molecular skeleton appear to be inactive, while those with Si–Si or Si–H bonds are capable of a-Si:C:H film formation. The reactivity of the latter group of compounds is characterized by determining the AHCVD′s rate constants. For most of the investigated source compounds AHCVD was found to be a non-thermally activated process. Based upon the values of the rate constant and the identified low-molecular-weight and oligomeric products of AHCVD, a mechanism of the initiation step, as well as the nature of resulting film-forming precursor are proposed.

Mechanism of the Initiation Step in Atomic Hydrogen-Induced CVD of Amorphous Hydrogenated Silicon–Carbon Films from Single-Source Precursors

A number of alkylsilanes and alkylcarbosilanes of widely different molecular structure are characterized in terms of their ability to form amorphous hydrogenated silicon–carbon (a-Si:C:H) films in atomic hydrogen-induced chemical vapor deposition (AHCVD). The compounds containing only Si–C and C–H bonds in the molecular skeleton appear to be inactive, while those with Si–Si or Si–H bonds are capable of a-Si:C:H film formation. The reactivity of the latter group of compounds is characterized by determining the AHCVD′s rate constants. For most of the investigated source compounds AHCVD was found to be a non-thermally activated process. Based upon the values of the rate constant and the identified low-molecular-weight and oligomeric products of AHCVD, a mechanism of the initiation step, as well as the nature of resulting film-forming precursor are proposed.

Hydrogen energetics ina−Si:Has determined by a combination of mean-field modeling and experimental evolution data

The energetics of hydrogen configurations in amorphous silicon ($a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$) and hydrogen populations associated with these configurations remain an important unresolved issue. To date, hydrogen evolution data have been difficult to analyze due to lack of models accurately describing the coupling of the thermally activated reaction/diffusion/desorption processes, that control evolution. A rigorous mathematical model for hydrogen transport in, and evolution from $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ is described and validated here. The model improves on other evolution models by including three discrete hydrogen energy levels in the film, one mobile level and two independent trap levels, and by explicitly simulating adsorption/desorption processes at the surface. This multienergy level model with a floating boundary condition at the surface is used to simulate two types of evolution experiments: temperature programed evolution and isothermal evolution. Results from both types of experiments for glow discharge deposited films are modeled using the same energetics, with trap depths of 1.5 and 1.8--1.9 eV below the transport level, and 20--30 % of hydrogen residing in the deep traps. These results contrast past experimental analysis, which suggested that hydrogen in deep traps is more tightly bound, 2.1--3.4 eV below the transport level. The difference of 0.3--0.4 eV we observed between the two trap energy levels agrees well with published bonding energies estimated using ab initio quantum-mechanical calculations. The difference in trap energies is also similar to the measured defect formation activation energy of 0.2--0.5 eV, suggesting that the defect formation mechanism in $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ may involve hydrogen motion between the two energy levels. Combination of our results with quantum-mechanical calculations of various hydrogen energy levels in crystalline silicon suggests that the interstitial hopping may not be the primary hydrogen transport mechanism.

13C NMR Spectroscopy of Amorphous Hydrogenated Carbon and Amorphous Hydrogenated Boron Carbide

AbstractWe report the 13C NMR spectra of amorphous hydrogenated carbon and boron carbide. The amorphous hydrogenated carbon spectra consist primarily of a sp2 carbon peak at 40 ppm and a sp3 carbon peak at 140 ppm and are in reasonable agreement with recent theoretical calculations of Mauri, Pfrommer, and Louie, but there are some noteable discrepancies. The amorphous hydrogenated boron carbide spectra are very different from that of amorphous hydrogenated carbon, showing two sharp lines at 135 and 170 ppm at low boron concentrations and an intense, broader line at 15 ppm at high boron concentrations. We suggest the two sharp lines at 135 and 170 ppm could be due to carbon atoms in boron-containing aromatic rings, and the broader line at 15 ppm as due to carbon atoms in boron carbide icosahedra. These lines provide evidence of nanocrystalline structure imbedded in an amorphous hydrogenated boron carbide matrix.

The 1280 Cm−1 Absorption Line in Amorphous Hydrogenated Boron Carbide

AbstractWe report infrared absorption measurements that provide evidence for the presence of boron carbide icosahedra in amorphous hydrogenated boron carbide thin films. The infrared absorption spectra is dominated by an intense line at 1280 cm-1 with a FWHM of ≃320 cm-1. Similar lines have been previously reported in polycrystalline boron carbide, where boron carbide icosahedra make up the unit cell. In both systems, the linewidth narrows and the peak position shifts to higher energy with increasing carbon concentrations. From annealing studies of amorphous hydrogenated boron carbide, hydrogen plays a very small role in the 1280 cm-1 line. Finally, the integrated intensity of the 1280 cm-1 line is a sublinear function of the boron concentration, providing further evidence that the carbon concentration in these icosahedra increases as the carbon concentration of the film increases.

Photoluminescence and Electroluminescence of Wide-Gap Amorphous Hydrogenated Silicon Prepared under High Dilution with He

(a) Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10,16200 Praha 6, Czech Republic(b) IPCMS, Groupe d’Optique Nonlineaire et d’Optoelectronique, Unite Mixte 380064,CNRS-ULP-EHICS, F-67037 Strasbourg Cedex, France(c) Faculty of Chemistry, Technical University, Veslarska 230, 63700 Brno,Czech Republic(d) Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3,12116 Praha 2, Czech Republic(Received June 23, 1997)Absorption spectra in the visible and IR regions and photoluminescence (PL) spectra of wide-gaphydrogenated amorphous silicon (a-Si:H) are investigated. The possible origin of the room tempera-ture visible PL is discussed. First electroluminescence results on a p–i–n structure are reported.

Measurement of atomic momentum distributions by high-energy neutron scattering

Neutron Compton scattering (NCS) of high-energy neutrons from atomic nuclei, measures the momentum distribution of atoms in condensed matter. Typical energy transfers are in the 1–100 eV region with wave vector transfers between 30 and 200 Å −1. This is relatively new application of neutron scattering, made possible by the development of accelerator-based neutron sources, such as the ISIS source. Recent work includes measurements on quantum fluids and solids, metal hydrides, glasses and amorphous materials, hydrogen bonds, metals, catalysts and molecular hydrogen. The new applications and future prospects for this technique are discussed.

Interfacial interaction between Al-1%Si and phosphorus-doped hydrogenated amorphous Si alloy at low temperature

The interfacial interaction between phosphorus-doped amorphous silicon hydrogen alloy ((n+)a-Si:H) and thermal evaporated Al-1%Si layer after furnace annealing in the temperature range from 150 to 250 °C has been investigated in detail. The scanning electron microscope photographs show that many dendrites were formed on the original (n+)a-Si:H surface at annealing temperature higher than 170 °C. Raman spectroscopy and transmission electron diffraction show that the original (n+)a-Si:H film has been converted to polycrystalline Si with the crystalline Si dendrites on top. The drastic increase (∼4 orders of magnitude) of electrical conductivity of the 200 °C annealed (n+)a-Si:H films with the Al-1%Si removed is caused by the formation of polycrystalline silicon percolation channel in the background area between dendrites. Auger spectroscopy also provides evidence that no aluminum is incorporated into the converted film during silicon recrystallization and thus no SiAl alloy is formed.

Hydrogen-induced amorphization of YNi 2 enhanced by mechanical grinding

The C15 Laves phase YNi 2, which transforms into amorphous phase by hydrogenation, was mechanically ground under various hydrogen partial pressures up to 1.0 MPa, to investigate the effect of mechanical grinding on hydrogen-induced amorphization (HIA) processes. The thermal stability of the amorphous phase and hydrogen contents dissolved in the compound were also examined. Under an initial hydrogen pressure of 1.0 MPa, the HIA processes are remarkably enhanced by grinding. Under an initial partial hydrogen pressure of 0.2 MPa, on the other hand, the YNi 2-H system decomposes into two phases, the α- and α'-phase by grinding, because the amount of hydrogen in the grinding vial is limited. Further grinding leads not only to an amorphization of the α'-phase but also to a transfer of hydrogen from the α'-phase to an amorphous phase and to a phase transformation of YNi 2 into YNi 5. Excess Y left in the phase transformation is dissolved into the amorphous phase. The difference between the above two processes depends on the initial hydrogen pressure and is understood by considering the free energy variation of the YNi 2-H system during mechanical grinding.

High Rate Growth of Amorphous Hydrogenated Silicon Films and their Device Applications

Amorphous hydrogeneated silicon (a-Si:H) has already been accepted as a commercially viable solar photovoltaic material. Technique to grow a-Si:H films at high rates need to be accomplished to enlarge the application area to include other devices like photocopier drums, X-ray detectors, optically addressed spatial light modulators (OASLM) etc. The techniques like VHF glow discharge and pulse plasma deposition with dilution of feed gases with helium offer such opportunities. This article presents an account of the efforts being made in this direction at the National Physical Laboratory, both in growing a-Si:H films at high rates and incorporating them in a variety of devices inorder to cater to the needs of what has come to be known as “Large Area Electronics”.

ChemInform Abstract: Thermal Desorption Spectroscopy of Hydrogen from Amorphous Hydrogenated Silicon.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Optically Addressed Spatial Light Modulator with Highly Sensitive Layer of Amorphous Hydrogenated Silicon Carbide

Abstract The photoaddressed spatial light modulator (PLSM) with thin hydrogenated amorphous silicon carbide photoreceptor α-Si1−XCx:H and nematic liquid crystal has been fabricated. The films were prepared by rf plasma decomposition of SiH4 and CH4 gas mixture. They have a high dark resistivity and photosensitivity as a hydrogenated amorphous silicon, but a more transmittance in visible spectrum. The maximum of diffraction efficiency for PLSM was achieved for the write light intensity 40 μW/cm2, a resolution on the one half of maximum of diffraction efficiency was 54 mm−1. The optical response of PLSM was observed in the frequency region from 1 Hz to 100 Hz.

Study of a-Si:H / c-Si Heterojunctions for PV Applications

Abstracta-Si:H / c-Si heterojunction diodes were produced by PECVD with varying amorphous silicon layer thickness and hydrogen dilution of the gas phase. An accurate determination of the growth rate also in the initial stages of the deposition was made possible by an original chemical method based on the dissolution of the films followed by spectroscopical analysis of the obtained solution.The electrical characterization of the diodes confirms the generation - recombination - multitunneling nature of the transport. Although H2 dilution is important, however, beyond a certain level it is detrimental for the junction quality, probably due to the transition to a microcrystalline phase deposition. Solar cells were also produced, the best results being an open circuit voltage of 610 mV and an intrinsic efficiency of 14.2%.