High Hydrogen Exposures Research Articles

Hydrogen adsorption and desorption on Si(100) and Si(111) surfaces investigated by in situ surface infrared spectroscopy

The hydrogen adsorption and desorption on Si(100)(2×1) and Si(111)(7×7) were investigated by using in situ surface infrared spectroscopy in the multiple internal reflection geometry. At initial stages of room-temperature adsorption of atomic hydrogen, dangling bonds of surface silicon atoms are terminated by hydrogen with the surface reconstruction structure retained, producing the so-called ‘double-occupied dimer’ (HSi–SiH) on Si(100)(2×1) and the monohydride Si (Si–H) perpendicular to the surface on Si(111)(7×7). On Si(111)(7×7), hydrogen adsorption onto the adatom and the rest atom have been distinguished, which have two different Si–H vibration frequencies. For high hydrogen exposure, atomic hydrogen breaks the surface Si–Si bonds (dimer bonds and backbonds) to produce higher hydride species: dihydride (SiH 2) and trihydride (SiH 3). Upon annealing of the hydrogen-exposed surface at moderate temperatures, trihydride species are thoroughly etched away with monohydride and dihydride remaining. We find that the conversion from the monohydride to the dihydride phase occurs during thermal annealing of the hydrogen-saturated Si(100)surface. Thermal annealing of the hydrogen-exposed Si(111) surface produces hydrogen-terminated (1×1) domains.

Ab initio cluster calculations of the chemisorption of hydrogen on the Si(111)√3 × √3R30°-B surface

First principles all-electron Hartree-Fock and density functional theory cluster calculations have been performed to investigate the chemisorption of atomic hydrogen on the Si(111)√3 × √3R30°-B surface. Both the equilibrium geometries corresponding to n hydrogen atoms (1 ≤ n ≤ 5) chemisorbing on the B-S 5 and B-T 4 reconstructed structures, and the desorption energies for a silicon or boron adatom bonded to x hydrogen atoms (0 ≤ x ≤ 3), have been obtained. As successively more hydrogen is chemisorbed, a silicon or boron adatom is found to move from its original threefold site to an adjacent bridge site, and then to a neighbouring on-top site. It is also found that boron will most likely occupy a subsurface substitutional S 5 site at low hydrogen coverages (≤ 0.67ML), but appear as an adatom at an on-top site directly above one of the first-layer silicon atoms for hydrogen exposures higher than 0.67 ML. This boron segregation at high hydrogen exposures prevents the formation of SiH 2 and SiH 3 complexes and leads to the prediction that only SiH and BH 2 will be observed on an Si(111):B hydrogenated surface. It also provides an explanation for the lack of Si surface etching at high hydrogen exposure. All of these results are in good agreement with the available experimental data on hydrogenated B/Si(111) surfaces.

Si(100)4 × 3-In surface phase: identification of silicon substrate atom reconstruction

The arrangement of Si substrate atoms in Si(100)4 × 3-In surface phase was studied using removal of In atoms at exposure of the surfaces to atomic hydrogen and monitoring the surface transformations by low energy electron diffraction and Auger electron spectroscopy. The Si(100)4 × 3-In surface was found to transform at H exposure to Si(100)4 × 1-H(In) surface showing definitely that Si substrate atoms are reconstructed. The persistence of 4 × 1 structure at high hydrogen exposures (when no Si dimerization can be preserved) unambiguously reduces the choice of possible structural models of Si substrate reconstruction to only three candidates. In turn, STM data [A.A. Baski et al., Phys. Rev. B 43 (1991) 9316] of ∼0.5 ML Si atom density in Si(100)4 × 3-In phase leaves a single realistic structure, namely Si(100) surface having every second Si atom double row missing.

Influence of Hydrogen on Si-Doped GaAs(100) in the Space Charge Regime

The influence of hydrogen on collective excitations of free electrons in the conduction band (surface/interface plasmons) for homogeneously doped (Si) (≈7.5×1018 cm−3) GaAs(100)-c(4×4) surfaces was studied by high-resolution electron-energy-loss spectroscopy (HREELS). A simple two-layer model based on the semiclassical dipole approach was applied to analyse the experimental energy-loss spectra. A spatial dispersion of the surface plasmons in the form of the long-wavelength limit of random-phase approximation (Thomas-Fermi model) and the nonparabolicity of the conduction band were incorporated to the model for fitting the measured HREELS spectra. It was shown that the two competing effects of free-electron compensation and band bending, which modify the spatial dispersion of the surface plasmon, are responsible for the red shift of the surface plasmon energy observed at high hydrogen exposures. Furthermore, results about the influence of hydrogen exposure upon annealed GaAs(100)-c(8×2) surfaces are presented. The findings of the present study prove that the effective in-diffusion of hydrogen atoms occurs and that the creation of hydrogen-induced acceptor levels in the bandgap of heavily-doped n-type GaAs is accompanied by an appreciable reduction of free-electron density in the conduction band. Simultaneously, the high hydrogen exposures cause the large band bending of GaAs(100).

Evolution of Auger peaks in GaP(110) with hydrogen chemisorption

Auger spectroscopy flanked by energy loss spectroscopy was used to investigate the evolution of GaP(110) surface electronic structure upon hydrogen exposure. The paper is focussed on the modification of PL 2,3 VV Auger line shape as a function of hydrogen exposure in the exposure range between zero and Ga3d surface exciton quenching. The discussion is concentrated on the Auger emission region about 9 eV below the emission from valence band top. The line shape change can be explained on the basis of present knowledge of electronic structure of H:GaP(110) surface and ascribed to the birth of a hydrogen-induced state ( H 4) at ∼ 4.4 eV below valence band top. Moreover the surface stoichiometry was monitored by Auger spectroscopy through the PL 2,3 VV to GaM 2,3M 4,5M 4,5 peak-to-peak ratio which was found to be extremely affected at high hydrogen exposures.

Structure and dynamics of hydrogenated GaAs(110) and InP(110) surfaces

We have investigated the structural and vibrational properties of hydrogen-covered GaAs(110) and InP(110) surfaces in the framework of the density-functional theory. The plane-wave pseudopotential method combined with the slab supercell description for the surfaces has been employed to determine the relaxation geometries of H:GaAs(110) and H:InP(110) for the half- and one-monolayer coverages. Our results are in good agreement with all available experimental data. For both surfaces and for all coverages we have determined the surface vibrations by means of an ab initio linear-response formalism. The calculated bond-stretching frequencies compare very well with the results from high-resolution electron-energy-loss spectroscopy. The half-monolayer coverages on InP(110) exhibit vibrational states which are related to the mechanism which leads to the structural decomposition of the surface experimentally observed at high hydrogen exposures.

Hydrogen-induced modification of the optical properties of the GaAs(100) surface

The influence of hydrogen adsorption on the surface order, dielectric function, and dielectric anisotropy was investigated for the three main reconstructions of the GaAs(100) surface [c(4×4), (2×4), (4×2)] by low-energy electron diffraction, spectroscopic ellipsometry, and reflectance difference spectroscopy. For all reconstructions, low hydrogen exposure removes the surface dimers, whereas surface roughening is observed at high hydrogen exposures. However, detailed differences can be resolved depending on the surface reconstruction. On the As-rich c(4×4) surface, the outermost As-dimers are removed at low H-exposures leaving behind a still As-rich surface of (1×1) symmetry. Thereafter, more As is removed and a new surface anisotropy develops due to the exposed Ga-dimers arranged in a mixture of (1×2) and (√2×√2)-reconstructions. For H-adsorption on the (2×4)- and (4×2)-surfaces, in contrast, only the subsequent reduction of the surface reconstruction is observed with increasing exposure, but no structural rearrangement. The removal of the As- [Ga-] dimers modifies the surface symmetry from (2×4) [(4×2)] through (1×4) [(4×1)] to (1×1). Finally, for large exposures, inhomogeneous surface etching by hydrogenation leads to rough, disordered surfaces for all three reconstructions.

Existence and thermal stability of mono and dihydride phases on Si(1 1 1) and Ge(1 1 1) surfaces: A photoemission study

Based on UPS and XPS investigations, it is concluded that a monohydride phase forms at first on the Si(1 1 1)7 × 7 surface. Upon further hydrogen dosing at room temperature, a dihydride phase develops and superposes to the previously formed monohydride phase. The dihydride phase desorbs completely around 250°C and the monohydride phase at about 550°C. A pure dihydride phase obtained by H adsorption cannot be observed on a silicon surface. Silane or disilane adsorption at room temperature exhibits the characteristic features of the dihydride phase without the associated monohydride phase. The obtained phase desorbs at the same temperature as the H induced dihydride phase. That is to our mind the only possibility to obtain a pure dihydride phase.For germanium in careful conditions we observe only a monohydride phase which desorbs at 150°C. For high hydrogen exposures, we obtain a new phase but XPS measurements indicate oxygen contamination. This place desorbs at 225°C and allows clear distinction between H adsorption and contamination. It is concluded that Ge and Si surfaces have different reactivities for hydrogen adsorption. These conclusions are extended to all Ge and Si surfaces either crystallized or amorphous.

Monohydride phase on Ge(111) and amorphous germanium surfaces: a photoemission study

This is a study of the adsorption of hydrogen at various temperatures on Ge(111)c(2 × 8) and amorphous Ge surfaces. We observe that the only phase forming on both surfaces for temperatures above room temperature is a monohydride phase. This phase desorbs completely at 150°C. For high hydrogen exposures, coadsorbed oxygen impurities build up for substrate temperatures below 150°C. The oxygen contamination desorbs completely at 225°C. This thermal evolution allows clear distinction between the monohydride phase and oxygen contamination.

Active sites of adsorption on cleaved and sputtered indium phosphide surfaces

From the initial uptake of atomic hydrogen at room temperature the active sites on cleaved and sputtered InP(110)-surfaces were determined using high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS, XAES). It is found that the In-H stretching mode intensity is dominant for the cleavage surface, which is known to be P-rich due to surface relaxation. For the Ar+-ion bombarded surface (In-rich) the P−H stretching mode is very strong if compared to the corresponding In-H mode. From these observations it may be deduced that both, the chemical bonding situation and the geometric configuration, are reflected in our HREELS-results. For the cleavage surface the initial adsorption for atomic hydrogen is interpreted in the framework of existing models for the (1×1) relaxation in terms of its chemical bonding configuration. The sputtered surface is assumed to be amorphous consisting of In-clusters and predominantly broken P-bonds, which are passivated after hydrogen exposure. For high hydrogen exposures, a strong decrease of the phosphor hydride lines may be interpreted as phosphine desorption.

Formation and atomic configuration of Si(100)c(4 × 4) structure

It was reported that the Si(100)c(4 × 4) structure appears rarely during heat treatments of surface cleaning in ultra-high vacuum. In the present experiment, the c(4 × 4) structure is formed reproducibly by special surface processing at a hydrogen exposure higher than 10 −2 L (10 −6 Torr·.s) and afterwards annealing at 570–690 °C. The high hydrogen exposure produces missing-dimer defects by vaporization of volatile silane and subsequent annealing causes order of missing-dimer defects. By intensity analysis of low energy electron diffraction patterns of c(4 × 4) and the hydrogenated surface c(4 × 4)-H. it is concluded that c(4 × 4) is one of the ordered structures with missing-dimer defects formed on the basic 2 × 1 structure.

Thin-film palladium and silver alloys and layers for metal-insulator-semiconductor sensors

The addition of Ag to Pd in the gate metal of a metal-insulator-semiconductor gas sensing diode can improve the performance and change the selectivity of the sensors for a variety of reactions. Data on the response of diodes with 12 different ratios of Ag to Pd in alloys and layers of Pd and Ag to hydrogen and other gases are reported. Diodes with as much as 32% Ag respond very well to H2 gas and the films are much more durable to high hydrogen exposure than pure Pd films. Improvements in the rate of response and aging behavior are found for certain Ag combinations; others give poorer performance. The presence of Ag on the surface changes the catalytic activity in some cases and examples of H2 mixed with O2 and/or NO2, propylene oxide, ethylene, and formic acid are given. Such selectivity forms the basis for miniature chemical sensor arrays which could analyze complex gas mixtures.