Dihydride Groups Research Articles

FTIR Measurement of the Hydrogenated Si(100) Surface: The Structure-Vibrational Interpretation by Means of Periodic DFT Calculation

The periodic density functional theory (periodic DFT) method was employed for the interpretation of infrared radiation spectra (IR spectra) measured for the hydrogen-covered H/Si(100) surface after the standard step of native oxide removal by brief etching in 40% NH4F. The IR employed the attenuated total reflectance (ATR) method. The periodic DFT calculations of IR spectra focused on reconstructions of H/Si(100) that involved combinations of surface-terminating Si–H groups including the double-occupied dimer (DOD), dihydride (DH), and trihydride (TH). The IR spectra calculated with periodic DFT for H/Si(100) surfaces were compared with the IR spectra calculated by means of DFT in Si–H clusters. The periodic DFT provided considerably better and more reliable theoretical description of the IR spectra by keeping the periodicity of the silicon material that guaranteed proper spatial distribution of the Si–H species within H/Si(100). The calculated IR spectrum for H/Si(100) that involved a combination of two DOD and three DH groups (7 × 1 reconstruction) was in good agreement with the measured IR spectrum. Moreover, a combination of the calculated IR spectra for DH in 1 × 1 reconstruction and the spectra for adjacent DH and DOD in 3 × 1 reconstruction (no long-range ordering) also agreed with the experiment. The periodic DFT further indicated that the IR peaks measured above 2120 cm–1 can originate in mutual steric interactions of adjacent DH species within H/Si(100). Accordingly, we hypothesize that the H/Si(100) surface prepared by the standard treatment likely involves combinations of DOD and DH but not TH.

The modification of the properties of n-type conductivity porous silicon by argon ion irradiation

The results of the research into the influence of argon ion irradiation at 3 keV on the composition and structure of porous silicon are presented. At a certain angle of incidence of the particles relative to the surface of the monocrystallites, an undulating λ∼60 nm nanorelief is formed, while the crystallite sizes and structure remain unchanged. The IR-spectroscopy data show that SiH groups are mainly localized in a thin 120 nm near-surface layer. During exposure of the samples to the air in the dark, monohydride groups are removed from the surface within a month. Dihydride groups, located in deeper layers, are oxidized considerably more slowly than the monohydride ones. The experimental data show that the 0.1 μm-thick surface layer serves as a diffusion barrier preventing oxygen from penetrating deep into the porous silicon.

Hydrogen atom reactions with monolayer graphite edges on Pt(100) surfaces: hydrogenation and H abstraction

The interaction of thermal H (D) atoms with monolayers of graphite deposited on Pt(100) surfaces was studied with LEED, AES, TDS and HREELS. H (D) atoms react with C atoms at the zigzag and armchair edges of C islands via the formation of sp 3 CH (CD) and CH 2 (CD 2) edge groups. The initial transformation C→CH (CD) occurs with a constant probability via a precursor state which is restricted to the immediate vicinity of the edges. The hydrogenation of monohydride CH (CD) groups to dihydride CH 2 (CD 2) occurs with a cross-section of about 1.5 Å 2. The abstraction reactions of H (D) from monohydride and dihydride groups by H or D via H 2 (D 2) formation have cross-sections of 0.1 and 1 Å 2, respectively. The magnitude of these cross-sections are expected for direct Eley-Rideal-type reactions. Isotope exchange reactions were measured by subjecting dihydride CD 2 (CH 2) groups to fluences of H (D) atoms. The observed cross-sections suggest a direct exchange via H + CD 2→CDH + D.

Bonding and Thermal Reactivity in Thin a-SiC:H Films Grown by Methylsilane CVD

The bonding and thermal reactivity of thin a-SiC:H films have been studied and compared with that of methyl groups on single-crystalline Si and in thick polymer films. The films were deposited on silicon substrates at 200 K by hot-wire chemical vapor deposition (CVD) using methylsilane (CH3SiH3) as the precursor. The resulting films were probed by in situ multiple internal reflection−Fourier transform infrared (MIR−FTIR) spectroscopy, and the thermal decomposition products were measured by temperature-programmed reaction/desorption (TPR/D). According to MIR−FTIR measurements, hydrogen is present in the films in the form of mixed silicon hydride species (SiH, SiH2, and SiH3) and intact methyl groups. TPR/D and MIR−FTIR annealing studies following growth at 200 K indicate that the film is stable up to 550 K. Above 550 K, a number of thermal etching processes are observed, leading to silane and methylsilane evolution at 600 K, followed by the loss of methane and molecular hydrogen by 780 K. Annealing studies show that the SiH3 species are more reactive at thermal energies than are methyl groups. The stability of methyl groups and the silicon mono- and dihydride groups in the films largely parallels that observed on single-crystalline silicon, but more reaction pathways are available in the thin films. Mechanisms for the film growth and thermal decomposition are proposed.

Vibrational modes of hydrogen and deuterium at InP(110) surfaces

Vibrations at hydrogen- and deuterium-covered InP(110) surfaces were investigated by high-resolution electron energy-loss spectroscopy. With semi-insulating crystals, InH and PD bending modes were found at 438 and 465 cm −1 (54.3 and 57.7 meV), respectively. In the low-coverage regime two components of the PH stretching vibration at 2260 and 2300 cm −1 (280.2 and 285.4 meV) are resolved. They are explained by vibrational motions of mono- and dihydride groups. This indicates that InP(110) surfaces are disrupted even after small H exposures. Bond strengths are estimated from the isotope shifts of the stretching frequencies.

Analysis of Hydrogen in Aerosol Particles of a‐Si: H Forming during the Pyrolysis of Silane

AbstractThe hydrogen content in aerosol particles of a‐Si: H, forming during pyrolysis of silane, is studied by means of the methods of hydrogen evolution and IR spectroscopy. The conclusion is drawn that the evolution peaks at 650,730, and 750 to 850 K correspond to hydrogen present in the dihydride groups of polysilane chains, the monohydride groups of the clustered phase, and the monohydride groups of the dilute phase, respectively. In the IR spectra the monohydride groups of the clustered phase cause absorption near 2000 cm‐−1 and the monohydride groups of the dilute phase near 2100 cm‐−1. Under low silane conversion the main fraction of hydrogen is contained in (SiH2)n, under conversion more than 50% in monohydride groups.

Internal-state distributions of H2 desorbed from mono- and dihydride species on Si(100)

Following adsorption of atomic hydrogen on Si(100)–(2×1), the surface is heated and the desorbed H2 is detected via (2+1) resonance-enhanced multiphoton ionization (REMPI). H2 desorption correlated with the decomposition of dihydride groups on the surface (SiH2) is detected at a surface temperature Ts near 660 K, and with the monohydride species (SiH) near Ts=780 K. Although the H2 rotational distributions are nearly identical for the mono- and dihydride species, the vibrational distributions differ with roughly 0.2% and 1% of the population in H2(v=1) for the monohydride and dihydride, respectively. The enhancement in the [H2(v=1)]/[H2(v=0)] population ratio over that of a thermal distribution at Ts is, however, roughly 20 times for both mono- and dihydride species. The results are interpreted within a model that assumes desorption proceeds through a common intermediate, which is identified as the dihydride.

Dihydride 1 × 1 structures on group IV (100) surfaces

In the formation of dihydride structures on elemental group IV (100) surfaces the hydrogen atoms saturate dangling bonds and cause the 2 × 1 clean surface reconstructions to reduce to 1 × 1 topologies. It has also been suggested that repulsion between hydrogen atoms on adjacent dihydride groups may produce large deviations in the hydrogen bond angles from those of tetrahedral geometry. In this communication we first illustrate the features of these dihydride systems by calculating the structure of the Si(100)1 × 1:2H systems. The effect of hydrogen repulsion is then examined by comparing the dihydride surface structure corresponding to substrates of different lattice constant. For this purpose we have employed the C(100) surface, the carbon-terminated β-SiC(100) surface, and an adsorbed layer on Si(100). In each case the optimum surface topology has been determined by minimising the total energy of the system with respect to variations in the atomic co-ordinates of the hydrogen and the first four layers of the substrate. The results clearly show that the H-C-H bond angle decreases sharply with decreasing C-C internuclear distance.

Short-range order and microstructure in hydrogenated amorphous silicon

An experimental study has been made on the relationship between short-range order and microstructure in hydrogenated amorphous silicon films. The properties of the material have been varied by applying rf power of different magnitudes. The change in the short-range order has been characterized by Raman scattering measurements. Microstructure has been determined by means of field assisted ion exchange technique. The observed correlation between the two structural length scales suggests that the presence of dihydride groups in these materials is a key factor for the release of the silicon network strain.

Polyhydride Bonding Groups in PECVD Amorphous Si Thin Films.

AbstractIn glow discharge deposition (GD) of a-Si:H alloys, the concentration of electronically-active defects increases as the incorporation of bonded hydrogen in polymerized dihydride groups, (SiH2)n increases. Since the fraction of dihydride groups increases strongly with decreasing substrate temperature (Ts), the incorporation of these bonding groups can only effectively be reduced by restricting Ts to about 230°C or higher. We have found that a-Si:H films produced by remote Plasma Enhanced Chemical Vapor Deposition (remote PECVD) are qualitatively different than the GD films in the following ways: 1) the amount of bonded hydrogen for Ts > 100°C is less by about a factor of two; and 2) the fraction of polyhydride bonding is also significantly less than the monohydride fraction for Ts > 100°C. We develop a statistical model that provides a basis for translating any process-dependent representation of data for the polyhydride fraction into a universally-obeyed scaling relationship in which the independent variable is [H]. The agreement between the statistical model and the experimental data, replotted as a function of [H], supports our observation that the fraction of polyhydride bonding is not determined simply by Ts, but is a function of the particular reaction pathways in the deposition process chemistry.

Chemical bonding of hydrogen and oxygen in glow-discharge-deposited thin films of a-Ge:H and a-Ge:(H,O).

We have grown thin films of a-Ge:H and a-Ge:(H,O) by the glow-discharge process, and have used infrared (ir) absorption spectroscopy to study the local bonding of hydrogen and oxygen as a function of the substrate temperature ${T}_{s}$ and the amount of ${\mathrm{O}}_{2}$ and/or ${\mathrm{H}}_{2}$O added to the germane gas mixture in the plasma. The temperature dependence of the ir features associated with monohydride and dihydride groups in a-Ge:H alloys is different in these films, grown at relatively high plasma power, from what has been reported in films of a-Si:H grown from silane plasmas at lower power levels. In particular, dihydride incorporation in these a-Ge:H films displays a distinct temperature threshold, rather than a competition with the monohydride bonding as in the a-Si:H films. The bonding of H and O atoms in the ternary alloys a-Ge:(H,O) is also different from what has been reported for the corresponding Si ternary alloys, both in the nature of the local atomic arrangements of the O and H atoms, and in the relationship between these bonding environments and the source of the oxygen atoms (${\mathrm{O}}_{2}$ or ${\mathrm{H}}_{2}$O). These differences in bonding in a-Ge:H and a-Ge:(H,O) films with respect to the corresponding a-Si alloy films reflect differences in both the plasma-phase precursor chemistry and temperature-dependent reactions at the growth surface.

Hydrogenated amorphous silicon growth by CO2 laser photodissociation of silane

A new method of hydrogenated amorphous silicon deposition on large areas is reported in which silane is photodissociated by resonant absorption of cw CO2 laser radiation. The free radicals produced interact in a suitable geometrical configuration with the surface of a low-temperature substrate and grow a film having properties similar to the glow-discharge deposited material. The film amorphousness and the hydrogen presence either in monohydride and dihydride groups are evidenced by x-ray patterns and ir spectra.

Laser Induced Chemical Vapor Deposition of Hydrogenated Amorphous Silicon

ABSTRACTSome results on hydrogenated amorphous silicon growth by CO2 laser photodissociation of silane are reported. A 100 W CW CO2 laser was used as the excitation source. A horizontal configuration was adopted, where the laser beam is sent parallel to the substrate surface inside a flux reactor, and its energy is used to excite and dissociate the silane molecules flowing near the solid surface. The laser has no direct heating effect on the substrate, which is independently heated by an oven.The photoproduced radicals by interacting with the surface grow a film at a rate strongly depending on silane pressure, substrate temperature and laser intensity. This experimental configuration allows depositions on large areas, owing to the large number of reactant molecules involved in the photochemical process. Moreover, material can be produced with a continuously variable hydrogen content, since the substrate temperature required for obtaining depositions can be as low as room temperature and adjusted independently of the other process parameters.The film properties are similar to those of the glow discharge deposited material. The film amorphousness and the hydrogen presence either in monohydride and dihydride groups are evidenced by x-ray patterns and ir spectra. Results on the electrical and optical properties are also reported.

Studies of the mechanism of the decomposition of hydrogenated a-Si films

Abstract The dehydrogenation of glow-discharge deposited a-Si films has been studied by in situ infrared spectroscopy in order to resolve the apparent inconsistency between the interpretation of previous manometric data and the assignments of the infra-red spectral features in these films. Our studies show that, on heating at about 350°C, there is no preferential reduction in the integrated band intensities near 2090 cm−1, assigned to =SiH2 stretching modes, relative to the band near 2000 cm−1 assigned to the ≡SiH stretching mode. Accepting both infrared assignments our data conflicts with previous proposals that the low temperature dehydrogenation mechanism involves ≡SiH2 groups alone. It appears that the low temperature dehydrogenation proceeds via the molecular elimination of hydrogen from regions in the film where bound hydrogen atoms are in close proximity. These hydrogen atoms may be incorporated either as monohydride or dihydride groups. The model, therefore, allows a consistent interpretation of t...