AVS1997 Session SS2-MoM: Atom and Vacancy Diffusion on Surfaces

Monday, October 20, 1997 8:20 AM in Room A1/2-A

Monday Morning

Start	Invited?	Item
8:20 AM		SS2-MoM-1 Diffusion of Hydrogen on Si(001) by Hot STM and Atomistic Modelling J.H.G. Owen (University of California, Santa Barbara); D.R. Bowler (University of Oxford, United Kingdom); K. Miki (Electrotechnical Laboratory, Japan); G.A.D. Briggs (University of Oxford, United Kingdom) An understanding of the diffusion of hydrogen on Si(001) is important in the gas-source growth of silicon and silicon-germanium alloys, because the surface hydrogen can block diffusion and, acting as a surfactant, prevents germanium segregation. Elevated-temperature STM experiments of the Si(001):H surface¹ has found that there are two different diffusion mechanisms. At low coverages, single hydrogen atoms adsorb onto one end of a surface dimer, and diffuse independently above 550 K. The experimentally-measured barrier for this process is 1.68 eV with a prefactor of 10¹³ s^-1. At higher coverages, closer to typical growth conditions, the hydrogen is paired up on the surface dimers. This paired configuration is energetically favourable. At around 650 K, the hydrogen atoms begin to diffuse in a concerted fashion in order to maintain the paired configuration. The barrier for this motion is 2.0 eV. In conjunction with the STM investigations, ab initio and semi-empirical modelling have been used to calculate the barriers for single-atom and concerted-atom diffusion, which agree well with the experimental barriers. Two possible routes for the concerted diffusion have been modelled. Only one of these routes has a barrier which is consistent with the experiment, leading to a conclusive identification of the diffusion mechanism. ¹Examples from our movies of hydrogen diffusion may be found at: http//squid.ucsb.edu/~jhgowen/Hpaper.html.
8:40 AM		SS2-MoM-2 Adsorbate Influence on the Diffusion of Individual Pt Atoms on Pt(110) S. Horch, T.R. Linderoth, E. Laegsgaard, F. Besenbacher (University of Aarhus, Denmark) The surface self-diffusion of Pt adatoms on the Pt(110)-(1x2) surface is studied using variable-temperature Scanning Tunneling Microscopy. In the temperature range investigated here (270 K up to 380 K), the adatoms are confined to the troughs of the missing-row reconstruction, thus realizing a 1D model system. Using atomically resolved STM movies, we are able to observe and measure the rate of diffusion of individual adatoms. If the surface is exposed to different gases while taking such a film, the response of the individual Pt adatoms to the presence of the gas can be evaluated from such films. It is found, that hydrogen exposure leads to an increased root-mean square displacements of the adatoms. A more detailed analysis of the films reveals the existance of two different types of diffusion promotion: (I) The diffusion rate for the entire ensemble of adatoms increases with the hydrogen exposure. (II) For limited time intervals, single adatoms spontaneously exhibit a much higher diffusivity, thereafter behaving as before. Currently, we are also investigating the influence of oxygen and CO exposure on the self-diffusion of Pt on Pt(110).
9:00 AM	Invited	SS2-MoM-3 Diffusion of Hydrogen and Deuterium on Ni(111) over a Wide Temperature Range: Exploring Quantum Diffusion on Metal Surfaces X.D. Zhu (University of California, Davis) Using a linear optical diffraction technique and varying the periodicity of the adsorbate density modulation from 0.4 to 61 microns, we were able to measure the diffusion rates of hydrogen (¹H) and deuterium (²H) on Ni(111) from 65 to 240 K. The diffusion rates in this temperature range vary by 8 orders of magnitude. We have also measured the azimuthal dependence of the diffusion rates within the surface plane. The temperature dependence of the diffusion for both isotopes are characterized by a cross-over from one thermal activated behavior to another activated behavior with a smaller activation energy at low temperatures. At high temperatures (above110 K), the activation energy and diffusivity for hydrogen are E₁(¹H) = 4.5 kcal/mol and D₁₀(¹H) = 2.8 x 10^-3 cm²/sec, and for deuterium, E₁(²H) = 5.0 kcal/mol and D₁₀(²H) = 3.4 x 10^-3 cm²/sec, respectively. At low temperatures (below 110 K), the activation energy and diffusivity for hydrogen are E₂(¹H) = 2.4 kcal/mol and D₂₀(¹H) = 2.4 x 10^-7 cm²/sec, and for deuterium, E₂(²H) = 2.4 kcal/mol and D₂₀(²H) = 1.6 x 10^-8 cm²/sec. From both the temperature dependence and azimuthal dependence of the diffusion rates, we can reasonably conclude that the cross-over is most likely from a classical over-barrier hopping to a quantum mechanical under-barrier tunneling diffusion.
9:40 AM		SS2-MoM-5 Hydrogen Inhibition of Exchange Diffusion on Pt(100)¹ G.L. Kellogg (Sandia National Laboratories) Field ion microscope observations show that the rate of exchange displacements for individual Pt atoms on Pt(100) is significantly reduced when the surface is exposed to hydrogen. A hydrogen background pressure of 1x10^-10 Torr causes the self-diffusion rate to decrease by ~3 orders of magnitude over 2-3 hours. This effect is opposite to that for self diffusion on Rh(100)¹, indicating that hydrogen promotes hopping displacements, but inhibits exchange displacements. Higher partial pressures of hydrogen suppress exchange displacements on Pt(100) to the point where hopping displacements become energetically accessible. The temperature at which hopping displacements are observed is consistent with a previous study in which exchange displacements were inhibited by an external electric field². Observations of small clusters indicate that the presence of hydrogen inhibits their diffusion as well. The ability to influence both the displacement rate and the transport mechanism of atoms on surfaces suggests that hydrogen may be used to control thin-film growth at the atomic level. Work supported by the U. S. DOE under Contract DE-AC04-94AL85000. Sandia is operated by Sandia Corporation, a Lockheed Martin Company, for the U. S. DOE. ¹G. L. Kellogg, Phys. Rev. B 55, 7206 (1996). ²G. L. Kellogg, Phys. Rev. Lett. 70, 1631 (1993)
10:00 AM		SS2-MoM-6 Diffusion of Pt Adatoms and Dimers on Pt(111) K. Kyuno, A. Gölzhäuser, G. Ehrlich (University of Illinois, Urbana-Champaign) Although the mobility of adatoms and clusters plays a crucial role in understanding the morphology of growing crystals, only little direct information is available about mobilities important in the growth of Pt(111), one of the most carefully studied examples of homoepitaxy. In this context, diffusivities of individual Pt atoms and dimers have been examined on Pt(111) using a low-temperature field ion microscope. Pt atoms are deposited on top of Pt(111) islands formed by field evaporation with a radius of ~16 nearest-neighbor spacings, and their movement and association are observed after warming to temperatures between 40 and 160K. The distribution of Pt dimers on Pt(111) islands is not uniform: an empty zone separates the central region from the cluster edges. Dimers nucleated in the central region have to overcome an energy barrier to reach the island edges. Diffusion measurements have been performed in the central region, and diffusivities have been deduced from mean-square displacements at temperatures between 85 and 100K for single atoms and between 140 and 160K for dimers. The activation energy for monomer migration is found as 0.26 eV/atom, in accord with previous estimates; for dimers, the activation energy is 40% larger than that of the monomer. Comparison with diffusion on other platinum family metals reveals quite unusual trends. Work supported by the Department of Energy under Grant DEFG02-91ER-45439. K.K. is on leave from the University of Tokyo, Japan. A.G. is currently at the Institute for Applied Physical Chemistry, Heidelberg, Germany.
10:20 AM		SS2-MoM-7 In-Situ STM Experiments and Predictive Modeling of Grain Growth Kinetics in Two Dimensions. A.K. Schmid, N.C. Bartelt, R.Q. Hwang (Sandia National Laboratories) The kinetics of grain growth is a subject of great technological relevance, particularly regarding two-dimensional polycrystals because of the importance of thin film applications such as electronic interconnects. In this talk we present our study of the coarsening of a prototypical 2D polycrystal which can be directly compared to the classical coarsening models. We have prepared oxygen adlayers on Ru(0001). At a coverage corresponding to 0.25 monolayer, the oxygen overlayers order into a (2x2) superstructure. The registry between the film and the substrate leads to the existence of four types of antiphase domains (grains). During the oxygen adsorption, the system rapidly passes through the (2x2) ordering transition. This gives rise to starting configurations containing a dense domain structure. Within a few hours at room temperature, the grain size increases by more than an order of magnitude. The grain growth involves both wall motion favoring larger grains as well as the coalescence of grains with matched registry. Using time resolved scanning tunneling microscopy, we have characterized the dynamics of antiphase walls between the domains both on the scale of individual atomic events as well as on the scale of overall grain structure. The fourfold degeneracy characterizing this physical system prompts the comparison with the two-dimensional four-states Potts model. We have used this model to perform Monte Carlo simulations of experimentally observed configurations. Quantitative comparison with experiments demonstrates the excellent predictive capability of the model: By measuring the hops of individual atoms on a timescale of seconds, we are able to fairly precisely predict for several hours the evolution of a grain structure involving several hundred thousand atoms. This work was supported by DOE under Contract No. DE-AC04-94AL85000
10:40 AM		SS2-MoM-8 Separating Out the Prefactor and the Step Edge Barrier of Interlayer Diffusion for Ag/Ag(111) Homoepitaxy K.R. Roos (Bradley University); M.C. Tringides (Ames Lab-USDOE & Iowa State Univ.) An important process in epitaxial growth which determines whether the growth mode is 2d or 3d is interlayer diffusion (i.e. the transfer of mass from higher to lower levels). Interlayer diffusion is determined by the probability p=(ν_s/ν_d)exp(-E_s/kT), where ν_s/ν_d is the ratio of the prefactor for hopping between terraces to hopping on a terrace, and E_s is the extra barrier to diffusion at a terrace edge. It is important that methods be developed to separate out the contribution of the step edge barrier from the ratios of the prefactors. We re-examine a recently developed STM-based method¹ and show how the prefactor and the barrier can be uniquely separated out. In addition, we have developed a diffraction based method² that uses the step density of nucleated islands at large D/F ratios (where D is the terrace diffusion and F the flux rate). At fixed total deposited amount, the step density decreases as the probability, p, to hop from higher to lower adjacent terraces increases. We use the RHEED peak intensity decay as a function of coverage during submonolayer growth of Ag/Ag(111) for T=150-315 K to measure the step edge density under high D/F conditions. By modeling the growth process we determine how the step density can be used to determine ν_s/ν_d and E_s. The values, E_s=0.13 eV and ν_s/ν_d=3x10³, found are in good agreement with those obtained with STM based methods. ¹K. Bromann, H. Brune, H. Roder and K. Kern, Phys. Rev. Lett. 75 (1995) 677. ²K.R. Roos, R. Bhutani, and M.C. Tringides, Surf. Sci. (in press).
11:00 AM		SS2-MoM-9 Surface Segregation of Low-Energy Ion-Induced Defects in Silicon P. Bedrossian, T. Diaz de la Rubia (Lawrence Livermore National Laboratory) We demonstrate a novel technique for studying the surface segregation of point defects induced by low-energy ions in Silicon. Starting with atomically-clean surfaces of Si(100) and Si(111) held at 100K, irradiation by He ions creates Frenkel pairs in the bulk silicon. The silicon is then annealed isochronally while monitoring the specular RHEED (Reflection High-Energy Electron Diffraction) intensity. During annealing, point defects initially created in the bulk during irradiation migrate to the surface, resulting in changes in the RHEED intensity relative to that of a non-irradiated specimen. Defects which arrive at the surface initially roughen it, leading to a diminution of the antiphase RHEED intensity. As these defects are annihilated, the RHEED intensity increases. We detect surface segregation of neutral point defects as low as 130K. We compare results with various ion kinetic energies (3 and 5 keV) and doses (<5 x 10¹³ ions/cm²). Our RHEED results will be compared with atomistic hybrid molecular dynamics/kinetic Monte Carlo simulations, as well as our previous STM results showing depopulation of adatoms with the arrival of vacancies at the surface, and repopulation of the surface with the arrival of interstitials from the bulk. Work was performed at Lawrence Livermore National Laboratory under the auspices of the US Dept. of Energy under Contract W-7405-Eng-48.
11:20 AM		SS2-MoM-10 Quantum Coherence in Tip-Surface Transfer of Adatoms in AFM/STM I.S. Tilinin, M.A. Van Hove, M. Salmeron (Lawrence Berkeley National Laboratory) The problem of the spontaneous adsorbate transfer in AFM/STM from the surface to the tip and vice versa is closely related to heavy interstitial diffusion in solids. Owing to strong coupling with phonon and electron-hole excitations, which play the role of dissipation, the adsorbate transfer occurs incoherently at temperatures comparable with or larger than the Debye temperature. As the temperature decreases the phonon and electron-hole relaxation times may become large compared to the inverse coherent band width so that synchronized (coherent) transitions between two or several levels of a system are possible. AFM/STM offers a unique opportunity to vary the coherent band width Δ in a controlled way by approaching the tip to an adsorption site. As the transition matrix element value is very sensitive to the distance between the adsorbate equilibrium positions, the quantity Δ for a transition from the surface to the tip may be made several orders of magnitude larger than in the bulk. In this report, we show that at temperatures below 1 K the phonon and electron-hole excitations may become negligible and the transfer probability of an adatom exhibits quantum oscillations as a function of time, with the frequency proportional to the average value of the energy level splitting of the ground state of the adsorbate. At sufficiently low temperature and for typical STM tip velocities and separation distances, coherent transition rates may not only be comparable to but also exceed significantly those observed at room temperature for thermally activated tip-surface transfer of adatoms ¹. Work supported by U.S. DOE. ¹B. J. McIntyre, M. Salmeron and G.A. Somorjai, Catal. Lett. 39, 5 (1996).
11:40 AM		SS2-MoM-11 Elementary Steps of Lateral Translation of Cu, Pb, Pb2 and CO Studied by Low Temperature STM L. Bartels, G. Meyer, K.H. Rieder (Freie Universität, Germany) We have studied the elementary steps involved in lateral manipulation of Cu and Pb atoms, Pb dimers and CO molecules on Cu(211) at 30 K. The interaction between the metallic species and the tip appears to be attractive, while CO molecules could be repulsively pushed across the surface. Recording the tip height during lateral manipulation of the adspecies at constant current yields tip height curves containing valuable information on the adparticle movement across the surface: i) the metallic species can be pulled across the surface with the tip apex moving in front and the adspecies taking hops from one adsite to the next following the tip whenever the lateral distance between the tip apex and the center of the adspecies becomes greater than a threshold value, which depends on the chosen bias and current. ii) while single atoms regularly perform single adsite hops while being pulled across the surface, double and triple adsite hops can be induced at Pb dimers. iii) Pb atoms can be slid across the surface at approx. constant tip apex-Pb atom distance. Hence, the tip height curve monitors the substrate corrugation approx. as it appears to a Pb atom. iv) individual and rows of CO molecules can be pushed across the surface performing single hops whenever the oncoming tip approaches nearer than a threshold value depending on the chosen bias and current. Thus, our measurements allowed us to precisely determine the lateral position at which the interaction between the tip and the adspecies is sufficient to overcome the energetic barrier between different adsites. This renders not only the control of final sites in lateral manipulation to single adsite precision easily possible but also sheds some light on the way the tip modified diffusion barrier is overcome.