We review structural and electronic aspects of the reaction of hydrogen with semiconductor surfaces. Among others, we address the Si(100), GexSi1-x(100), GaAs(100), InP(100), SiC(100), SiC(0001) and SiC(0001) surfaces. It is demonstrated that high resolution electron energy loss spectroscopy (HREELS) in conjunction with a number of other surface sensitive techniques like low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS/UPS) can yield important information about the surface atomic structure, the effects of hydrogen passivation and etching and on electronic properties of the surfaces.

Scanning type electron-stimulated desorption (ESD) spectroscopy to detect hydrogen on solid surfaces has been described. To analyze a two-dimensional distribution of hydrogen on solid surface, a pencil-type fine-focused electron gun, of which spot size is less than 300 nm at 800 eV, has been developed using a thermal field emitter. The lateral resolution of analysis is achieved less than 1 μm.

Letters were drawn on a hydrogen-terminated Si(100) surface by irradiation of continuous electron beam to remove hydrogen from the surface, and by the following scanning ESD measurement, a clear letters were confirmed on this surface. Direct lithography by an electron beam on silicon surfaces has been realized.

Properties and characteristics of hydriding alloys are strongly dependent on surface compositions and morphologies. For instance, oxides such as La203 on AB5 alloys and ZrO2 on AB2, AB, and body-centered-cubic (BCC) alloys act as the barriers for the conversion of molecular and ionic hydrogen to atomic hydrogen at the surface, thus reducing the kinetics in both the gas-solid and electrochemical reactions.

Alloy surfaces chemically treated by an aqueous F-ion containing solution have been developed to solve such problems. F-treated surfaces exhibit significantly improved characteristics in regard to the hydrogen uptakes and the protection against impurities and electrolyte solution. In addition, highly conductive metallic Ni layers can be formed on the surface of the alloy particles by the fluorination.

The authors report the properties and characteristics of fluorinated hydriding alloys, mainly of a typical AB2 Laves phase material which represents the difficult activation characteristics and poor long-term durability during electrochemical charge/discharge cycles.

The oxidation of H/Si(100) and H/Si(111) with high concentration ozone gas was investigated with X-ray photoelectron spectroscopy(XPS). The ozone oxidation of partially hydride-covered surface was observed. The hydrogen termination reduced the rate of oxygen insertion into silicon backbond. The reduction of oxygen insertion rate by the H-termination for H/Si(100) was larger than that for HSi(111). The dissociation rate of ozone molecule on H/Si was estimated to be ≃0.2 with a directional mass analyzer.

While the bulk kinetics of the uranium-hydrogen reaction are well understood, the mechanisms underlying the initial nucleation of uranium hydride on uranium remain controversial. In this study we have employed environmental cell optical microscopy, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy, (AFM) in an attempt to relate the structure of the surface and the microstructure of the substrate with the susceptibility and site of hydride nucleation. Samples have been investigated with varying grain size, inclusion (carbide) concentration, and thermal history. There is a clear correlation to heat treatment immediately prior to hydrogen exposure. Susceptibility to hydride formation also appears to be related to impurities in the uranium. The oxidized surface is very complex, exhibiting wide variations in thickness and topography between samples, between grains in the same sample, and within individual grains. It is, however, very difficult to relate this fine scale variability to the relatively sparse hydride initiation sites. Therefore, the surface oxide layer itself does not appear to control the sites where hydride attack is initiated, although it must play a role in the induction period prior to hydride initiation.

Niobium single crystals are used as substrate material for constructing superconducting quantum interference device or SQUID. The use of niobium is prompted by the fact that it is a low Tc metallic superconductor. In order to fabricate the device the surface of the crystal has to be polished flat. This is achieved by combination of mechanical polishing and electrochemical polishing. It has been reported that during electrochemical polishing hydrogen can enter the material forming Niobium hydrides. These can result in surface roughening of a magnitude greater than the thickness of the films subsequently deposited. This leads to failure of the films. The problem can be solved by annealing the material at 300° C to remove any hydrogen that might be present. However it is phenomenologically interesting to study the effect of electrochemical hydrogen charging on the surface of Niobium.

This paper treats the subject of hydrogen in semiconductors from various perspectives. First, a brief historical overview is given. Then, some basic principles governing the interaction between hydrogen and semiconductors are outlined. Finally, specific examples will emphasize the impact of hydrogen on technological applications. While the general treatment applies to interactions of hydrogen with any semiconductor, the applications will focus mainly on hydrogen interacting with silicon.

Tritium is usually stored in the form of a metal tritide since it is safe to handle in this form, easily recoverable, and further large quantities of tritium can be stored. However, since tritium is radioactive it decays into 3He and an electron. Helium recoil energy in this reaction is very small, and not enough to create defects. We have performed ab-initio electronic structure calculations that show that in PdT, a considerable amount of 3He can be accommodated at the 3He results in an overall enhancement in the strength of the metal- tritium bonding that leads to the lowering of the plateau pressure. We also find that there is a weakening of the metal-metal bonds due to the repulsive interaction with 3He.

The electronic structure and energetics of LaNi5, its hydrogen solution (α-La2Ni10H) and its hydride (β-La2Ni10H14) were investigated by means of the tight-binding linear muffin-tin orbitals method within the atomic sphere approximation (TB-LMTO-ASA). Preferred site occupancy by the absorbed hydrogen atoms was investigated in terms of the charge density of the interstitial sites and the total energy, both of which indicate that the 6m site in the P6/mmm symmetry is the most preferred. A negative heat of formation of La2Ni10H14 was obtained from the total energy calculations.

The effect of Ni substitution in LaNi5 by 3d and s-p elements on the electronic structure of the intermetallic and its hydrides has been investigated using the self consistent linear muffin tin orbital (LMTO) method in the atomic sphere approximation (ASA). The Fermi level, EF, of LaNi4M (M = Fe,Co,Mn) is found to lie in the narrow additional M 3d subband above the Ni d states, leading to an increase in the density of states (DOS) at EF. In contrast, the substitution of Ni by an s element of the 3d series, Cu, or by an s-p element: Al or Sn results in a progressive filling of the Ni-d bands and in a decrease of the DOS at EF. In all the substituted intermetallic compounds, we find that the lattice expansion accounts for less than 50% of the observed decreased stability, this shows the importance of the effect of chemical substitution. We also discuss the factors which affect the electronic structure and the stability of the hydrides and compare our results with available experimental data.

As-cast, arc-melted Pd-Ni alloys are inhomogeneous and the H2 isotherms for these differ from their homogeneous counterparts in the two phase, (dilute + hydride), regions but not in the dilute phase regions. Pd-Ni alloys, which become inhomogeneous via a ternary (Pd + Ni + H) equilibrium phase change, have H2 isotherms which differ from those of the homogeneous alloy in both the two-phase and the dilute phase regions. These results are discussed with respect to the expected type of inhomogeneities.

It has been established that Mg2NiH4 undergoes a phase change around 500°K in which the orientation of the NiH4 complex is quenched in a monoclinic distortion of the cubic high temperature phase. This results in the formation of domains in which the lattice distortion is accommodated by microtwinning. These effects can be absent when the hydride phase is formed below the transition temperature. Microscopic analysis verifies a similar basal cubic structure in the low temperature phase; however, the domains and microtwins are absent in this material and it can readily be destabilized by thermal stresses induced by the electron beam. It is of interest to measure and compare the effect of the lattice differences on the thermodynamic properties of the low temperature versus the high temperature hydride phases. We report the equilibrium PCT data and hydrogen desorption kinetics of the two hydrides in the temperature range of 450–570 K.

The theoretical investigation of solubility isosteres of adsorbed hydrogen has been performed for free face (0001) of crystals with hexagonal close-packed lattice A3 of Mg type. The face free energy has been calculated and its dependence on temperature, pressure, hydrogen concentration and character of hydrogen atoms distribution over surface interstitial sites of different type has been defined. The equations of thermodynamic equilibrium and solubility of adsorbed hydrogen have been defined. The plots of isosteres in the region of phase transition from isotropic to anisotropic state have been constructed and it has been established that in anisotropic state the order in distribution of hydrogen atoms over interstitial sites of different type must become apparent. Comparison of the theoretical isosteres with experimental for ruthenium has been carried out, the isotropic-anisotropic state transition can stipulate a stepwise and break-like change in isosteres.

The diffusivities D of hydrogen in cubic Laves-phase hydrides have been measured by means of pulsed-field-gradient nuclear magnetic resonance over wide temperature ranges. A review is given of the diffusion coefficients of hydrogen in ZrCr2Hx, rTi2Hx, ZrV2Hx, ZrMo2Hx. and HfV2Hx. with special emphasis on the variation of D with the hydrogen concentration x. The formation of ordered low-temperature phases in ZrV2Hx results in a substantial reduction of the hydrogen diffusivity. The dependence of D on the lattice parameter of the host compound is considered.

The mobility of hydrogen and its isotopes in metals has been the object of investigation for several years, whereas the diffusion studies of H in doped semiconductors started more recently. Although the H diffusion coefficient in metals may be several orders of magnitudes higher than in semiconductors, the dynamics of H in metals and semiconductors presents many common features, like precipitation, trapping by heavier impurities and, as indicated by recent results, quantum tunneling at low temperature.

In boron doped silicon, the relaxation rates τ−1(T) of H around B obtained from anelastic relaxation were joined with those from infrared absorption: the remarkably wide range obtained (11 decades) clearly shows a deviation of τ−1(T) from the classical dependence at low temperature. However, the results obtained and their analysis do not allow yet to draw conclusions on the mechanism governing the H(D) dynamics.

Recently, the investigation of the dynamics of H(D) in GaAs doped with Zn revealed a dissipation peak at 20 K in the kHz range. This relaxation has the highest rate found for H in a semiconductor: more than 15 orders of magnitude higher than in all the other semiconductors measured so far. The analysis of the dissipation curves clearly indicates that the nature of the H reorientation is quantistic.

In metals two regimes of the H mobility are observed: hopping with deviations from a classical Arrhenius motion, and a much faster tunneling within few close sites. In the latter regime the H dynamics does not consist of jumps but of transitions between the quantized energy levels of the tunnel systems. The types of interactions assisting the H transitions and the geometry of the tunnel systems are an open problem: although the two-level tunnel system (TLS) has been widely used to explain neutron diffusion, specific heat, and acoustic spectroscopy results in interstitial solutions (NbOxHy), recently this model has appeared not to be valid in substitutional solutions (NbZrxHy, NbTixHy) where the tunnel systems have a higher symmetry. The four-level systems seem to be more appropriate, although the corresponding model has not been developed as much as the TLS yet.

A new model on hydrogen permeation is proposed, considering absorption and desorption processes on the sample surfaces. Analytical solution, satisfying the flux continuity rather than the concentration boundary conditions, is derived from the model. Drift velocity through surface and drift velocity in bulk are introduced and their ratio determines the validity of the time-lag model. When the ratio of drift velocity through surface over that in bulk approaches infinity, the proposed model is reduced to the time-lag one. The diffusivity and the drift velocity through surface can be evaluated by fitting the entire normalized permeation curve. The obtained results can predict the effects of temperature, sample thickness and energy barriers of absorption and desorption on the permeation process. The thickness effect occurred in using the time-lag model is well explained by the effects of absorption and desorption on the permeation process.

In order to verify the results predicted by the model in Part I of this work [1], permeation experiments were conducted at room and high temperatures on fully-annealed-commerciallypure iron with two kinds of surface treatment, one group with plasma cleaning and presputtering and the other without it. The experimental results show that the diffusivity evaluated by the new model is independent of sample thickness and surface treatment, while the diffusivity evaluated by the time-lag model varies two orders of magnitude. The experimental results confirm that a fine surface treatment yields a low energy barrier for desorption. The energy barrier for either group is higher than the activation energy of diffusion. Consequently, the ratio of drift velocity through surface to that in bulk increases with increasing temperature and makes the time-lag method appropriate at elevated temperatures.

The hydrogen dynamics in LaHx was studied for different concentrations x ≥ 2.5 at different temperatures by quasielastic neutron scattering. The neutron spectra show the existence of both long-range translational diffusion and localized motion. The prefactor of the jump rate for the first process exhibits a maximum around x= 2.75. The radius for the second motion is in agreement with neutron powder diffraction measurements which support a localized motion of H confined in an octahedral site.

The authors of present research have used in experiments the atomic hydrogen and metallic foil 25–30 jim thick.

It have been supposed that these technical operations will permit to exclude the influence of surface and diffusional processes on the rate of Me-H interaction.

The series of experiments have been carried out and they confirm this assumption. It have been shown that hydrogenation reaction of zirconium foil in atomic hydrogen conform to the topochemical model of volume segregation of interaction product and the rate of its flow is independent of the surface processes and hydrogen diffusion in volume.

It is shown that a reduction of the three-level energy diagram proposed by Van de Walle (Phys. Rev. B 53, 11292, 1996) to describe the relaxation of non-equilibrium hydrogen distributions, to just the interstitial transport level and a distribution of traps, allows an essentially equivalent formulation of the hydrogen relaxation kinetics. The modified formulation offers the possibility of accounting for dispersive diffusion while preserving the essential multiple retrapping aspect of the original proposal.