1998 MRS Spring Meeting & Exhibit

Chairs

Warren B. Jackson
Xerox PARC
Palo Alto, CA 94304
415-812-4196

Robert G. Leisure 
Dept of Physics 
Colorado State Univ 
Fort Collins, CO 80523-1875 
970-491-5370

Norbert N. Nickel
Applied Physics
Hahn-Meitner-Inst
Berlin, D-12489 GERMANY
49-30-67053-310

* Invited paper

SESSION H1: PLENARY SESSION 
Chair: Warren B. Jackson 
Monday Morning, April 13, 1998 
Golden Gate B3

8:30 AM *H1.1 
HYDROGEN IN METALS AND ON CONDENSED CARBON. Louis Schlapbach, Andreas Zuttel, Daniel Chartouni, Olivier Kuttel, Leon Diederich, Oliver Groning, Solid State Physics Research Group, University of Fribourg, Fribourg, SWITZERLAND, Nobuhiro Kuriyama, Tetsuo Sakai, Osaka National Research Institute, AIST, Osaka, JAPAN. 

A tutorial introduction to the reversible adsorption of hydrogen on metals and on diamond, to its solution in metals and to the reversible metal hydride formation by gas sorption and cathodic charging will be followed by illustrations of a few potential applications: reversible hydrogen, heat or electricity storage, reversible magnetic and optical properties, electron emitters, and diamond synthesis. Finally, new results on the nature of the reversible phase transitions elaborated by photoelectron spectroscopy and diffraction as well as by scanning probe microscopy will be presented. 

9:15 AM *H1.2 
THEORY OF HYDROGEN IN SEMICONDUCTORS. Chris G. Van de Walle Xerox Palo Alto Research Center, Palo Alto, CA. 

Hydrogen plays a crucial role in many technologically important processes in semiconductors. Passivation of defects on the surface and in the bulk has been studied and utilized for a long time. More recent examples of the richness of phenomena to which hydrogen gives rise are the exfoliation of Si layers driven by hydrogen implantation (used for fabrication of silicon-on-insulator structures), the increased stability of defect passivation at Si/SiO₂ interfaces through substitution of deuterium for hydrogen, and the improved p-type doping of GaN. In this talk I will present a comprehensive overview of the current theoretical understanding of hydrogen interactions with semiconductors. This level of understanding has in large part been achieved through detailed first-principles calculations of hydrogen in many different semiconductors, and in many different configurations (including single crystal, polycrystalline, and amorphous environments, and bulk, surfaces, and interfaces). I will highlight the unifying aspects and main driving forces for hydrogen's behavior, but also point out where deviations from the expected behavior occur (for instance, the structure of hydrogen-dopant complexes in GaN). I will also discuss areas where additional research could be highly fruitful. 

SESSION H2: HYDROGEN ON SURFACES 
Chair: Joseph W. Lyding 
Monday Morning, April 13, 1998 
Golden Gate B3

10:30 AM *H2.1 
HYDROGEN ON SEMICONDUCTOR SURFACES. J.A. Schaefer, T. Balster, V. Polyakov, U. Rossow, S. Sloboshanin, and S. Tautz, Inst. f. Physik, TU Ilmenau, GERMANY. 

The interaction of atomic hydrogen with semiconductor surfaces and its influence on electronic and structural properties of semiconductors is of great interest from a fundamental point of view and for a thorough understanding of technological processes such as growth and etching. In this paper we critically review physical and chemical processes when atomic hydrogen reacts with various semiconductor surfaces. Materials considered include Si, Ge, SiC, Si/Ge-alloys, III-V compounds, and diamond. Emphasis is also placed on the modification of bulk properties by incorporation of hydrogen, e.g. free carrier compensation. It is demonstrated that, in conjunction with other surface techniques like low energy electron diffraction (LEED), electron spectroscopy (UPS, XPS, AES), high-resolution electron energy-loss spectroscopy (HREELS) is a powerful tool to obtain fundamental understanding of the elementary steps involved in the hydrogenation of semiconductors. The frequencies of the observed vibrational modes and the scattering geometry give information about the chemical bonding of hydrogen. A deeper insight into the chemical environment is provided by state-of-the-art model calculations of adsorbate frequencies. Thus, atomic hydrogen and HREELS can be used as a local probe for chemical analysis (vibrational spectroscopy for chemical analysis, VISCA). On the other hand, electronic properties of the subsurface bulk region are determined by a simulation of the measured spectra on the basis of self-consistent electron density profiles. To illustrate the application of HREELS to these problems recent results at SiC surfaces and delta-doped GaAs are presented. 

11:00 AM H2.2 
HYDROGEN ADSORPTION ON INGAAS AND GAAS (001) SURFACES. L. Li, B. -K. Han, H. Qi, R.F. Hicks, Chemical Engineering Department, University of California, Los Angeles, CA. 

The adsorption of atomic hydrogen on InGaAs and GaAs (001) films grown by metalorganic vapor-phase epitaxy (MOVPE) has been studied by multiple internal reflection infrared spectroscopy, scanning tunneling microscopy and low-energy electron diffraction. The surfaces of these films are terminated with arsenic and gallium dimers. Hydrogen adsorbs on the dimers, producing a series of As-H infrared bands between 2200 and 1900 cm^-1, and Ga-H infrared bands between 1800 and 1200 cm^-1. When strained InGaAs films are grown on GaAs(001) substrates, many new surface reconstructions are observed, such as the (2x3), (3x2) and (5x2). Hydrogen adsorption experiments have revealed the distribution of As and Ga dimers on these surfaces. By combining these data with scanning tunneling microscopy, we have been able to elucidate the atomic structures of the new reconstructions formed on strained InGaAs films. These surfaces have unusual properties that are not found on unstrained GaAs (001) surfaces. The implications of these results for MOVPE growth of heterojunction devices will be discussed at the meeting. 

11:15 AM H2.3 
THE EFFECT OF FOREIGN ADATOMS ON HYDROGEN DIFFUSION AND DESORPTION FROM SILICON SURFACES. John E. Crowell, Gregory J. Batinica, Michael L. Jacobson, Michael C. Chiu, and Bob M.H. Ning, Dept. of Chemistry & Biochemistry, University of California, San Diego, La Jolla, CA. 

The surface chemistry of silicon can be significantly modified by foreign adatoms such as germanium, phosphorus, and boron. One of the most important reactions to occur on silicon surfaces involves the desorption of hydrogen. For instance, the rate limiting step in silicon deposition from hydride sources involves hydrogen desorption. We have used temperature programmed desorption (TPD) and Auger electron spectroscopy (AES) to study the effect of adatoms such as germanium and phosphorus on the surface chemistry of silicon. Germanium reduces the desorption energy of hydrogen from silicon surfaces, shifting the desorption maxima to lower temperatures. Phosphorus has the opposite effect, shifting the desorption maxima to higher temperatures. Phosphorus additionally changes the desorption order, and the relative stability of surface hydrides. We have also examined the combined effect when P and Ge are co-adsorbed on Si surfaces. The effect of surface structure and hydrogen diffusion in the presence of adatoms will additionally be discussed, as we have compared the effect of these adatoms on hydrogen desorption from both the Si(100) and Si(111) surfaces. The mechanism by which these adatoms modify the surface chemistry of silicon will furthermore be presented. 

11:30 AM *H2.4 
A NEW DEVELOPEMENT OF SCANNING HYDROGEN IMAGING SYSTEM AND IT'S APPLICATION. Kazuyuki Ueda, Ken'ichi Ishikawa and Masamichi Yoshimura,Toyota Technological Institute, Tempaku, Nagoya, JAPAN. 

When dangling bonds on the silicon surface are terminated by atomic hydrogen, this mediated surface is practically effective for hetero-epitaxial growth and keeping oxide-free surface. Only a few people have measured hydrogen on the solid surfaces. For example, possible methods are HREELS, elastic recoil detection analysis (ERDA), electron-stimulated desorption spectroscopy (ESD) and FTIR. Firstly, experimental results are reported on the adsorption of hydrogen on silicon surfaces using a time-of-flight type electron-stimulated desorption (TOF-ESD) spectroscopy. The TOF-ESD enables us to measure hydrogen adsorption states on solid surfaces with very high sensitivity. Nowadays it is popular to make a scanning image to view 2-dimensional distribution of elements on the surface such as a scanning Auger mircoscopy (SAM), scanning SIMS and scanning XPS. Secondarily, an imaging of 2-dimensional distribution of hydrogen is achieved using conbination of the TOF-ESD with SEM. We have used firstly LaB₆ filament in the electron gun in order to get high dense of primary current of which beam size of 3 m in diameter at 800 eV. In this case the ESD H⁺ image of Cu-mesh (600 mpi) was obtained about 6 m in lateral resolution. In contrast, a thermal field emitter(TFE) gives us a high quality electron beam, namely, the beam size of 300 nm at 800 eV. Finally we demonstrate ESD H⁺ imaging using a TFE-gun on the hydrogen covered silicon surface which is lithographed by continuous electron beam prior to imaging. 

SESSION H3: THEORY AND THERMODYNAMICS 
Chair: Peter Vajda 
Monday Afternoon, April 13, 1998 
Golden Gate B3

1:30 PM *H3.1 
FIRST-PRINCIPLES STUDIES OF NOVEL HYDROGEN-METAL SYSTEMS. M.Y. Chou, School of Physics, Georgia Institute of Technology, Atlanta, GA. 

In recent years, many interesting properties have been discovered in the hydrogen-metal systems. Examples include the switchable optical properties in rare-earth hydrides and the continuous and reversible change in the magnetic coupling in metal superlattices by hydrogen charging. In order to understand these novel properties, first-principles studies based on microscopic quantum theory have been carried out for these systems. In this talk, I will review current theoretical issues and present results on structural and electronic properties of several prototype hydrides. The calculations were carried out using the pseudopotential method with the local-density approximation in most cases. I will summarize our current understanding of the metal-insulator transition in rare-earth hydrides from the study of the interplay of electronic and structural properties. I will also report on our recent study of hydrogen in a thin layer of noibium. Topics of interest include lattice expansion in the strained layer and the change of band structure upon hydrogen charging which is responsible for the change of the magnetic coupling across the layer. The work is in collaboration with Y. Wang and C. Li. 

2:00 PM H3.2 
EFFECT OF HELIUM ON THE ELECTRONIC STRUCTURE OF PALLADIUM TRITIDE. R.P. Gupta, SRMP, CEA, Saclay, FRANCE; M. Gupta, Institut Sciences des Matéiaux, Université Paris-Sud, Orsay, FRANCE. 

Tritium is usually stored in the form of a metal tritide sinse 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 the reaction is very small, and not enough to create defects. Due to the production of He, the chemistry of the tritide changes since it is no longer a binary alloy but a ternary one. There is an increase in the lattice parameter as the tritide ages, and the pressure-composition isotherms are modified. Further, tritide becomes brittle. The question concerning the location of He in the matrix is crucial for understanding these transformations. We have performed ab-initio electronic structure calculations that show that in PdT, a considerable amount of He can be accommodated at the octahedral interstitial sites where it is produced. Our calculations show that the presence of He results in an overall enhancement in the strength of the metal-hydrogen 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 He. 

2:15 PM H3.3 
Abstract Withdrawn. 

2:30 PM H3.4 
ELECTRONIC STRUCTURE AND ENERGETICS OF LaNi₅, -La₂Ni₁₀H AND -La₂Ni₁₀H₁₄. H. Nakamura, D. Nguyen-Manh and D.G. Pettifor, Dept of Materials, Univ of Oxford, Oxford, UNITED KINGDOM. 

The relation between the stability, the hydrogen capacity and the local bonding of hydrogen-absorbing alloys has not been investigated in details, even for the most highly studied hydrogen-containing alloys of LaNi₅. In order to gain insights into this relation, we have calculated the electronic structure of LaNi₅ and its hydride, -La₂Ni₁₀H₁₄, with the linear muffin-tin orbital (LMTO) method within the atomic sphere approximation (ASA). We first discuss the nature of the bonding between hydrogen and the metal atoms in terms of the intersite bond orders and bond energies. We then demonstrate that the negative heat of formation is determined by a subtle balance between the chemical bonding and atomic size contributions. The preferable site occupation by hydrogen in LaNi₅ has been also investigated in terms of the total energy of La₂Ni₁₀H. It has been found that the 6m site is the most stable and the 12o site the least stable. We discuss the correlation between the site preferency and other factors such as interstitial charge density and interstitial hole size. 

2:45 PM H3.5 
EFFECT OF SUBSTITUTIONS AT THE NICKEL SITE ON THE ELECTRONIC STRUCTURE OF LaNi₅ AND ITS HYDRIDES. M. Gupta, Institut des Sciences des Matériaux, Université Paris-Sud, Orsay, FRANCE. 

Since the early 70's, a large number of experimental investigations on LaNi₅ and related compounds have been performed in relation with their exceptional hydriding properties. Substitutions at the La or Ni sites are used to modify the thermodynamic properties and play an important role in the selection of these alloys for specific technological applications, since they affect the stability and the hydrogen content of the hydride. Up to date, theoretical ab-initio studies of these systems are extremely limited. We have investigated the effect of Ni substitution by a 3d element Mn, Fe, Co, Cu and by s-p elements such as Al and Sn on the electronic structure of LaNi₅ and its hydrides using the L.M.T.O.-A.S.A. method. Our results are used to analyze: i) the modifications in the atomic interactions due to the effect of substitution at the Ni site and to hydrogen absorption ii) the changes in the properties at the Fermi level iii) the factors which control the stability. We discuss the relative importance of the lattice expansion and of the electronic effects on the observed decreased stability of the intermetallics. For the hydrides, the stability results from two competing effects: the formation at low energy of metal-hydrogen. bonding states and the shift of the Fermi energy towards higher energies. Our electronic structure results are compared with available experimental data such as photoemission, electronic specific heats... 

3:30 PM *H3.6 
HYDROGEN SOLUBILITIES IN INHOMOGENEOUS PD ALLOYS. Ted B. Flanagan, D. Wang, J.D. Clewley, Dept of Chemistry, Burlington, VT; J.G. Barker, National Institute of Standards and Technology, Center for Neutron Research, Gaithersburg, MD. 

It has been shown that  solubility provides a convenient means for the characterization of defects in Pd and its alloys because equilibrium can be readily established even at low temperatures where the effect of defects on the solubility is most pronounced. In view of this, it seems that  solubility may also be a convenient in situ technique for the characterization of the degree of inhomogeneity of Pd alloys. For alloys where compositional variations are introduced by phase separation,  isotherms may be useful in determining the degree of phase separation which occurs under various conditions. Disordered fcc Pd-Rh alloys are metastable with respect to phase separation into Pd-rich and Rh-rich phases below a critical temperature of  1150 K. Isotherms for the phase separated alloys will be measured and discussed in view of the known phase separation. Disordered Pd-Pt alloys form fcc solid solutions over the complete composition range. Appreciable concentrations of dissolved H at, e.g., 673 K, cause segregation of the metal atoms according to a ternary (Pd + Pt + H) phase equilibrium. Annealing in the absence of H at this same temperature causes the phase separated alloy to return towards its equilibrium, homogeneous state. Hydrogen solubility isotherms corresponding to these various conditions of homogeneity will be measured and discussed. In addition to measuring  solubilities in alloys of various degrees of homogeneity, small angle neutron scattering will be employed as a complement to the  solubility measurements. 

4:00 PM H3.7 
HYDROGEN ABSORPTION AND DESORPTION IN PD-RH-CO TERNARY ALLOYS. David F. Teter and Dan J. Thoma, Los Alamos National Laboratory, Los Alamos, NM. 

The effect of varying the Rh and Co compositions on the hydride plateau pressure and hydrogen capacity of several Pd-Rh-Co ternary alloys has been investigated using pressure-composition (PC) isotherms. Cycling the alloys through the absorption and desorption pressure-composition isotherms causes the extent of the hysteresis to decrease with increasing cycles, eventually converging to a stable PC isotherm. In binary Pd-X alloys (where X is Rh, Ni, Co and Cr), the plateau pressures follow the trend: Cr > Co > Ni > Rh; whereas the hydrogen capacities follow an opposite trend: Cr < Co < Ni < Rh. For this study, cobalt was chosen as a compromise in hydrogen absorption/desorption behavior between Cr and Ni, while retaining the capacity associated with the base Pd-Rh alloy. The trends in the hydrogen solubility and plateau pressure as a function of Rh and Co content will also be correlated to x-ray diffraction measurements of the lattice parameters as well as other physical properties of the alloys. Also, some of the pressure-composition isotherms of the alloys implicate the occurrence of hydrogen-induced phase separation. 

4:15 PM H3.8 
PROPERTIES OF Mg₂NiH₄ at 450-570K*. S.E. Guthrie, and G.J. Thomas, Sandia National Laboratories, Livermore, CA; D. Noreus and E. Ronnebro, Dept. of Structural Chemistry, Univ. of Stockholm, Stockholm, SWEDEN. 

It has been established that Mg₂NiH₄ undergoes a phase change at about 525ºK 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 are 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. Additionally, the effects of surface impurities on the two phases show very distinct optical and hydrogen kinetic properties. 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, hydrogen absorption/desorption kinetics, and surface properties of the two materials in the temperature range of 450-570ºK. 

SESSION H4: HYDROGEN TRANSPORT PHENOMENA 
Chairs: Robert G. Leisure and Stephen J. Pearton 
Tuesday Morning, April 14, 1998 
Golden Gate B3

8:30 AM *H4.1 
NUCLEAR MAGNETIC RESONANCE STUDIES OF HYDROGEN DIFFUSION IN LAVES-PHASE HYDRIDES. Günter Majer, Max-Planck-Institut für Metallforschung, Stuttgart, GERMANY. 

The diffusivity D of hydrogen in Laves-phase hydrides have been measured model-independently by means of pulsed-field-gradient NMR (PFG-NMR) over wide temperature ranges. Additional information on the hydrogen hopping frequency has been deduced from the spin-lattice relaxation rate . A review is given of the hydrogen diffusion in ZrV₂H_x, ZrCr₂H_x, , ZrMo₂H_x, , . 
Detailed studies of the variation of D and its temperature dependence with the hydrogen concentration x were performed. In the concentration range  2.5 hydrogen atoms occupy only so-called g-sites. Each occupied site blocks the neighbouring sites within a radius  2.1 Å and, thus, reduces the number of interstices available for hydrogen occupation. This explains the decrease of the diffusivity with increasing hydrogen concentration up to  2.5. For  2.5 hydrogen atoms start to occupy so-called e-sites, which at least partly compensates the blocking effect and results in an increase of the hydrogen mobility. The mechanisms of hydrogen diffusion are analysed by comparing the results of Monte Carlo simulations with the PFG and the  data. 
In ZrCr₂H_x hydrogen retains high long-range diffusivity down to low temperatures ( 10^-13m²s^-1 at 130 K). Over the entire range (480 K  130 K) the temperature dependence of D can be described quite naturally in terms of quantum mechanical mechanisms of diffusion. The diffusivities below about 200 K are ascribed to tunnelling between the ground states, whereas above 200 K the tunnelling occurs predominantly between excited states in neighbouring interstices. 
In the hydrogen-stabilized Laves-phase compound  the comparison of the D and  data allowed to distinguish between long-range diffusion and localized motion. 

9:00 AM H4.2 
HYDROGEN DYNAMICS IN LaH_3-x BY QUASIELASTIC NEUTRON SCATTERING. C. Karmonik, T.J. Udovic, J.J. Rush, NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg MD. 

Recently, rare-earth hydrides have attracted increased attention due to the discovery that these materials become optically transparent for H/metal stoichiometric ratios approaching three. The physics behind this optical transition has yet to be satisfactorily resolved. In the present study, we have used quasielastic neutron scattering (QENS) methods to characterize the hydrogen dynamics above room temperature in LaH_3-x in the H concentration range relevant for the optical transition. For all the QENS spectra, two quasielastic components, i.e. two Lorentzians with distinctively different halfwidths, are observed. The momentum-transfer dependence of the halfwidth of the narrow Lorentzian is in excellent agreement with an orientally-averaged Chudley-Elliott model, which is a good approximation for describing a long-range H diffusional motion. A preliminary analysis revealed a jump length on the order of the distance between the octahedral and tetrahedral interstitial sites. With decreasing x from 0.5 to 0.005, the jump rate of this process is found to increase considerably (about an order of magnitude). The behavior of the second broader quasielastic component suggests that the underlying dynamical process is a localized motion with a relatively small radius and is believed to be due to a rattling motion of the hydrogen in the relatively large octahedral interstices. This motion also exhibits a clear dependence on the hydrogen concentration. The activation energies of both processes are found to be in good agreement with values derived from NMR investigations. 

9:15 AM H4.3 
HYDROGEN DIFFUSION IN THE LAVES-PHASE ZrCr₂: A QUASIELASTIC NEUTRON SCATTERING STUDY. Alexander Skripov, Inst of Metal Physics, Urals Branch of the Academy of Sciences, Ekaterinburg, RUSSIA; Michael Pionke, Inst fuer Festkoerperforshung, Forschungszentrum Juelich, Juelich, GERMANY; Oliver Randl, Inst Laue-Langevin, Grenoble, FRANCE; Rolf Hempelmann, Inst fuer Physikalische Chemie, Univ des Saarlandes, Saarbruecken, GERMANY. 

The intermetallic compound ZrCr₂ may exist in the form of two structural modifications (the hexagonal C14 or the cubic C15) both of which absorb large amounts of hydrogen. In both C14- and C15-type ZrCr₂H_x with  0.5 the hydrogen mobility is found to be extremely high. In order to elucidate the microscopic picture of H motion in ZrCr₂, we have performed high-resolution quasielastic neutron scattering (QENS) measurements on C15-ZrCr₂H_0.45 and C14-ZrCr₂H_0.5 in the temperature range 10-340 K. For C15-ZrCr₂H_0.45 our measurements have revealed two types of diffusive H motion with different frequency scales. The faster process implies the localized H hopping within the hexagons formed by interstitial g (Zr₂Cr₂) sites. The slower process corresponds to the hopping of H atoms from one hexagon to the other. Similar microscopic picture of H motion has been earlier found for C15-type TaV₂H_x. Comparison of the QENS data for the C15-type ZrCr₂H_x and TaV₂H_x shows that there is a clear correlation between the exact g-g distances and the parameters of two types of H motion. The QENS data for C14-ZrCr₂H_0.5 appear to be very close to those for C15-ZrCr₂H_0.45. This reflects the fact that the sublattices of interstitial sites for the C14 and C15 structures are closely related. However, the hopping rates of hydrogen in the C14 system are found to be somewhat lower than the corresponding H hopping rates in the C15 counterpart. 

9:30 AM *H4.4 
QUANTUM DIFFUSION OF H(D) IN SEMICONDUCTORS AND METALS, AND THE ROLE OF THE INTERACTION WITH IMPURITIES. G. Cannelli,¹ R. Cantelli,² F. Cordero,³ E. Giovine,³ F. Trequattrini,² M. Capizzi ²and A. Frova,^2 1Dip. Fisica, Univ. della Calabria, Arcavacata di Rende (CS), ITALY, and INFM² Dip. Fisica, Univ. Roma ''La Sapienza'', Roma, ITALY and INFM³ CNR, Ist. di Acustica ''O.M. Corbino'', Roma, ITALY, and INFM.

Abstract not available.