Program Organizer: Dr. David E. Jesson, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6030
Wednesday, AM Room: Orange County 4
February 7, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: J. Tersoff, IBM T. J. Watson Center, PO Box 218, Yorktown Heights, NY 10598
8:30 am Invited
ATOMIC EVENTS IN EPITAXIAL GROWTH AT LOW TEMPERATURES: Gert Ehrlich, Materials Research Laboratory and Department of Materials Science and Engineering, University of Illinois at Urbana- Champaign, Urbana, IL 61801
Using the field ion microscope, it is now routinely possible to examine individual atomic events that dictate the morphology of epitaxial films. At low temperatures, transport of atoms over crystal facets, diffusion on top of surface clusters, and transport between adjacent crystal layers are decisive in determining whether growth is three- dimensional or layer- by- layer. Direct observations of these atomic processes on closepacked metal surfaces will be reviewed, with special emphasis on the role of transient events. Supported by the Department of Energy under Grant DEFG02- 91ER- 4539.
9:00 am Invited
DIRECT MEASUREMENTS OF THE KINETICS OF Si ADDIMERS ON Si(001) USING ATOM- TRACKING SCANNING TUNNELING MICROSCOPY: B. S. Swartzentruber, Sandia National Laboratories, Albuquerque, NM 87185- 1413
The rotation and diffusion of Si addimers on the Si(001) surface at temperatures between room temperature and 128deg.C is measured using a novel atom tracking technique. The atom tracker employs lateral- positioning feedback to lock the scanning tunneling microscope (STM) probe tip into position above selected atoms with sub- Angstrom precision. Once locked the STM tracks subtle changes in the addimer configuration as well as their position as they migrate over the crystal surface. The addimers perform random walks on top of the substrate dimer rows and are reflected by defects. The average hopping rate increases from 0.001 sec-1 at 25deg.C to 10 sec- 1 at 128deg.C yielding an activation barrier for diffusion of 0.94 eV. This work performed at Sandia National Laboratories is supported by the U. S. Department of Energy under contract DE- AC0494AL85000.
9:30 am Invited
ADATOM CONCENTRATION DURING GROWTH OF GaAs: Brad Orr, Mark Johnson, Dept. of Physics, The University of Michigan, Ann Arbor, MI 48109- 1120
We present a study of the concentration of adatoms on GaAs(001) during MBE annealing. By rapidly cooling the sample from typical growth temperatures and typical As overpressures, the thermal concentration of Ge adatoms can be frozen into small islands on the terraces. The area of the resulting islands which are far from terrace steps is measured with STM giving an estimate of the concentration of adatoms during equilibrium. We find a large concentration of adatoms (0.18 monolayer coverage when cooled from 600deg.C) is present for typical growth temperatures. Interestingly, Monte- Carlo simulations which are successful at predicting growth dynamics show less than 1/20 of the adatom concentration as measured with STM.
10:00 am BREAK
10:20 am Invited
EPITAXY AND STRESS AT SURFACE DEFECTS: J. M. Gibson, R. D. Twesten, O, Pohland, University of Illinois, Urbana, IL; D. Loretto, Lawrence Berkeley National Laboratory, Berkeley, CA
We use an ultra- high vacuum transmission electron microscope to study in- situ deposited thin films on clean surfaces using transmission electron microscopy. Samples are observed in the plan- view geometry. Of particular interest is the observation of long- range stress fields originating from surface defects such as steps and domain boundaries in reconstructed surfaces. These originate from fundamental surface stresses, which have been measured by image matching. When layers of Ge are deposited on such surfaces, using molecular beam epitaxy, the step stress field is radically altered. This alteration can be useful in identifying the nature of the interfacial and surface steps for epitaxial films. In addition, we consider the significant impact of these long- range stress fields on the epitaxial growth process.
10:50 am Invited
DYNAMICS OF GROWTH AND ETCH STRUCTURES ON CRYSTAL SURFACES: Georg Rosenfeld, Karina Morgenstern, George Comsa, Institut fur Grenzflachenforschung und Vakuumphysik, KFA/Forschungszentrum Julich GmbH, D- 52425 Julich, Germany
The dynamics of growth or etch morphologies created by deposition on or removal from crystal surfaces are discussed and illustrated by experimental examples including motion and decay of islands and pits on Ag(111) studied by high- speed STM. In particular, it will be shown that at room temperature, vacancy islands on Ag(111) perform a random walk according to the laws of Brownian motion of free particles. The diffusion coefficient of the vacancy islands scales with the inverse of the island area, indicating that the microscopic mechanism behind the motion is adatom diffusion across the vacancy island rather than along the island boundary. This interpretation is confirmed by the fact, that at the same temperature, adatom islands decay via evaporation of adatoms. The decay rate of adatom islands is studied in detail and is found to increase with decreasing island size. However, the time dependence of the island decay does not follow a simple power law. The possibility of estimating characteristic parameters like diffusion coefficients, step edge barriers, and step free energies from these experiments is examined.
11:20 am Invited
STEP DYNAMICS OF THE RELAXATION OF FABRICATED STRUCTURES ON Si(111): M. D. Johnson, Department of Physics, University of Maryland, College Park, Maryland 20742
The relaxation of step bunched Si(111) surfaces has been investigated using scanning tunneling microscopy (STM). Initially, meta- stable step bunch regions containing approximately 10 steps separated by 2 m terraces are prepared by direct current heating. Subsequent heating by alternating current, direct current in the step up direction, or electron emission heating at 900deg.C allowed the surface to relax slowly to evenly spaced steps. The time evolution of this process was characterized by quenching the surface at selected points during the relaxation and imaging with STM. These results are compared with several models based on previous results of step- step interactions on Si(111) in order to better understand the microscopic mechanism primarily responsible for the relaxation. The models, which should be generally applicable to a variety of materials, can be used to predict the relaxation rate of artificially created structures such as quantum dots and devices created with selected area epitaxy.
MEASUREMENT OF INTRA- AND INTERLAYER DIFFUSION BARRIERS IN METAL EPITAXY AND THEIR SENSITIVITY TO STRAIN: H. Brune, K. Bromann, K. Kern, Institut de Physique Experimentale, EPF Lausanne, CH- 1015 Lausanne, Switzerland
A general concept is shown how to measure both, the barriers for terrace and step- down diffusion for an epitaxial system with high accuracy by application of mean- field nucleation theory to variable temperature STM data. Terrace diffusion is studied via the variation of saturation island density with temperature at a critical nucleus size of one. The additional barrier for an adatom to descend the step edge (first discovered by Ehrlich and Hudda, as well as by Schwoebel in 1966) can be determined through measurement of the nucleation rate on top of islands as a function of island size and temperature. This approach is applied to the growth of Ag on the (111) surfaces of Ag and Pt. The first Ag layer on Pt(111) grows pseudomorphic and is thus under a considerable compressive strain of 4.2%. This compression drastically lowers, first, the barrier for terrace diffusion (60 meV, compared to 97 meV for Ag/Ag(111) and 157 meV for Ag/Pt(111)), and second, the additional step- edge barrier for interlayer diffusion from Es=(12015) meV for Ag(111) homoepitaxy to Es=(305) meV for jump down from the first Ag layer on Pt(111). These examples illustrate the effect of the layer dependent strain on the intra and interlayer masstransport and thus on the film morphology in the kinetic growth regime. Potential origins of this pronounced effect will be discussed.
RATE AND DIFFUSION EQUATION ANALYSES OF MASK DEPOSITION PROCESSES: J. A. Venables, School of MaPS, University of Sussex, Brighton BN1 9QH, UK; A. Sugawara, Box 871504, Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287- 1504
In many low temperature deposition processes, the diffusion length of adatoms
on the surface before evaporation can approach macroscopic dimensions. In this
case the actual adatom diffusion length is determined by competing surface
processes, such as nucleation of islands, interaction with steps, or surface
reactions such as segregation or interdiffusion. Experiments can be performed
which use a deposition mask to define the area where atoms are deposited; the
mask width is a constant length, which can be compared with the other
lengths. Examples will be given of recent UHV-
experiments involving deposition through 20-
micron masks, where temperature dependent diffusion lengths are observed. The
present state of the analysis will be described, and activation energies for
specific surface processes extracted. The scope for mask deposition experiments
at higher spatial resolution will be discussed.
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