Program Organizer: Dr. David E. Jesson, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6030
Monday, PM Room: Orange County 4
February 5, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: C. J. Palmstrom, Department of Chemical Engineering and Materials Science, University of Minnesota, MS
2:00 pm Invited
MANIPULATING EPITAXIAL GROWTH AT THE ATOMIC SCALE: FROM SURFACTANTS TO QUANTUM WIRES: Juan José de Miguel, Juan de la Figuera, and Rodolfo Miranda, Laboratorio de Fisica de Superficies, Departamento de Física de la Materia Condensada, Instituto de Ciencia de Materiales "N. Cabrera", Universidad Autónoma de Madrid, Spain
Heteroepitaxial growth is usually controlled by kinetic barriers which determine the island shape and density, the film morphology or the stacking of layers. In order to achieve thin films or superlattices with the desired properties it is often necessary to find ways to overcome these kinetic limitations. The purposeful use of surface active agents has been employed to modify crystal growth. We will review the use of surfactants to control the mode of growth of metallic films, the defect structure of vertical superlattices and the fabrication of lateral superlattices of magnetic wires by a multitechnique approach including TEAS, STM, LEED, surface- sensitive XRD and Molecular Dynamics.
2:30 pm
INFLUENCE OF SURFACTANTS ON SURFACE MORPHOLOGY OF Ge/Si HETERO EPITAXIAL FILMS STUDIED BY HIGH RESOLUTION ELECTRON DIFFRACTION: M. Horn von Hoegen, Universitat Hannover, Institut fur Festkorperphysik, Appelstr. 2, D- 30167 Hannover, Germany
Surfactants have been very effectively used for growth mode engineering of hetero epitaxial systems. For the Ge/Si system Sb, As, Bi or H has been tested to act as surfactant by forcing layer growth instead of island formation as observed without surfactant. Two growth regimes have to be considered: Elastic relaxation. Prior to the generation of dislocations or defects Ge growth pseudomorphic with the Si lattice constant and is strongly strained. The continuous Ge film can relieve strain by forming a rough and open surface (microroughness on a nm scale) which allows a partial elastic relaxation towards the bulk lattice constants. The variations of the parallel and vertical lattice constant could be observed with LEED. At very high temperatures on Si(001) the Ge forms regular arranged cones with a uniform size of 30 nm covering the whole surface. Reversible changes of the shape of the cones could be induced by variations of the surfactant coverage (i.e., by changes of the surface free energy). Plastic relaxation. After a particular "critical" thickness finally dislocations are generated. On Si(111) those dislocations are arranged in a threefold regular array at the interface and matches exactly the lattice constants. For the first time the formation and evolution of such a network was observed in situ during deposition with high resolution LEED.
2:50 pm
SILICIDE FORMATION BY Mo INCORPORATION IN Si(100): Peter J. Bedrossian, L- 350, Lawrence Livermore National Laboratory, Livermore CA 94551
The binary Mo/Si system has a particularly rich bulk phase diagram with a variety of stoichiometries. The structure of Mo- silicide thin films is complicated even further by the influence of interfacial energies, the importance of which is underscored by the ready observation of thin- film phases which do not even appear in the bulk binary phase diagram. Previous investigations of Mo adsorption on clean Si(100) have reported the formation of an amorphous interface and transitions among various disilicide phases, but the transition temperatures were found to be highly process- dependent. Using STM and glancing- incidence x- ray diffraction, we have identified the temperature- and coveragedependence of the formation and interconversion of a variety of surface and thin- film phases resulting from adsorption of up to five monolayers of Mo on clean Si(100). Submonolayer Mo adsorption below 650deg.C leads to metastable, globally- homogeneous surface phases which exhibit short- range order in one- dimension, but were previously thought to be amorphous. Above 650deg.C, both the disilicide precipitates and precursors to grain boundaries can be identified even after initial formation of disilicide microcrystals. Microcrystals of two different phases of MoSi2, which form at different temperatures, can be identified with STM and x- ray diffraction. Recent modeling of the electronic structure of Mo- silicides offers insights which account qualitatively for many of these experimental observations. Despite the tremendous complexity of this prototypical transition metal- silicide system, it is now possible to identify in detail the microscopic consequences of various processing parameters on evolving thin- film morphology.
3:10 pm
CoSi2 FORMATION IN Ge/Si HETEROSTRUCTURES: G. R. Carlow, M. ZinkeAllmang, Department of Physics, University of Western Ontario, London, Ontatrio, Canada
The evolution of Co/Ge films on Si substrates has been investigated. Room temperature deposition of Ge films followed by Co films was done by Molecular Beam Epitaxy (MBE) and post- deposit annealing was done at 700deg.C. While planar CoSi2 films are formed at this temperature when no intermediate Ge layer is present, the effect of the Ge creates a clustered morphology and results in the formation of non- planar CoSi2 regions. Using Rutherford backscattering spectroscopy and transmission electron miscroscopy in combination with electron energy loss spectroscopy, we detail the evolution of the cluster morphology and the spatial distributions of the Co and Ge.
3:30 pm BREAK
COMPOSITIONAL ORDERING
Session Chairperson: L. J. Schowalter, Dept. Physics and Ctr. Integrated Electronics and Electronic Manufacturing, Rensselaer Polytechnic Institute, Troy, NY 12180
3:50 pm Invited
SURFACE STRUCTURE AND ORDERING IN GaInP: G.B. Stringfellow, L. C. Su, Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112
The surface structure of GaInP layers grown by organometallic vapor phase epitaxy lattice- matched to GaAs,substrates has been studied using atomic force microscopy. For exactly (001) oriented substrates, islands 30- 60 Å in height are observed that are formed of bilayer (5.9 Å) steps. The results allow an understanding of the microstructure of the Cu- Pt type ordering observed in these layers, particularly the presence of order twin boundaries. For layers grown on substrates intentionally misoriented by angles of from 3 to 9deg. to produce [110] steps on the surface, the steps are found to bunch, forming (001) facets, (lln) facets, and vicinal regions. Both the step height and the value of n are found to depend on growth conditions. The lateral dimensions of the structures formed are found to scale as approximately (growth rate)- _, indicating a diffusion limited size. The supersteps are found to correlate with the anti- phase boundaries observed in the ordered structure.
4:20 pm Invited
SURFACE RECONSTRUCTION AS A DRIVING FORCE IN ATOMIC LONG RANGE ORDERING: Alex Zunger, NREL, Golden, CO 80401
Vapor- Phase (MOCVD, MBE, ALE) growth of many Al- xBxC semiconductor alloys results in spontaneous long- range order, most often in the form of monolayer- alternation (AC)1(BC)1 superlattices along the (111) direction (the "CuPt" Structure). At the same time, it is known theoretically that: (i) the lowest energy state of bulk alloys is phase- separation into AC + BC, and that (ii) the lowest energy state of epitaxial alloys is the ABC2 chalcopyrite structure. A combination of first- principles total energy calculations and lattice- gas thermodynamic models clarifies that: (i) phase- separation is inhibited by the epitaxial coherence with the substrate, (ii) the chalcopyrite structure is surface-unstable relative to the CuPt structure, and (iii) dimerization, buckling and titling of surface actions stabilizes even at T~900K a special variant ("CuPt- B") of the CuPt structure. Detailed calculations on surface reconstruction in the GaxInl-xP/GaAs (001) epitaxial films will be presented. They show that (1) the (2X2) cation terminated reconstruction leads to CuPtg ordering and In surface segregation, (2) the 2(4X2) leads to CuPtA ordering and Ga segregation, (3) the 2(2X4) leads to CuPtB ordering and In segregation, and (4) the C(4X4) leads to CuPtA ordering and In segregation. Spontaneous ordering changes profoundly the band structure of the alloy, leading to (a) bandgap reduction, (b) splitting of the degeneracy of the valence band maximum, and (c) new polarization selection rules. In collaboration with S. B. Zhang and S. Froyen.
4:50 pm
X- RAY DIFFRACTION STUDIES OF NONEQUILIBRIUM ORDER AND DOMAINS IN SixGel- x FILMS ON MISCUT Si(001): J. D. Budai, J. Z. Tischler, D. E. Jesson, P. Zschack, Oak Ridge National Laboratory, Oak Ridge TN 37831- 6030; J.- M. Baribeau, D. C. Houghton, National Research Council of Canada, Ottawa, Canada
SixGe1-xfilms grown on Si(00l) continue to provide ideal model systems for investigating the rich interplay between surface morphology during growth and the resulting nonequilibrium compositional order. We have utilized synchrotron x- ray scattering measurements to characterize the atomic structure, the order parameter, the domain populations, and the domain sizes for Si0.5Ge0.5 films. The films were grown by MBE on substrates with a large range of miscut angles away from Si(001), including the Si(105) surface. Measurements of integral and half- integral reflections reveal that compositional modulations along both (111) and (100) type directions are present in all samples. The substrate miscut controls the domain orientation, consistent with a growth mode governed by ordering at surface ledges. The coherence lengths for the composition modulations in the two different directions are approximately equivalent over a widely varying range of values, suggesting that the structures coexist rather than compete. Research sponsored by Division of Materials Sclences, U.S. DOE. under contract DE- AC05- 840R21400 with Lockheed Martin Energy Systems, Inc.
5:10 pm Invited
CONTROL OF THE GROWTH AXIS AND CHEMICAL ORDERING IN INTERMETALLIC MAGNETIC ALLOY FILMS: R. F. C. Farrow, D. Weller, R. F. Marks, M. F. Toney, IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120- 6099
The magnetic and magneto-
optical
properties of intermetallic Co-
Pt
and Fe-
Pt
alloy films are of considerable current interest. For example, the magnetic
anisotropy of equiatomic FePt correlates with the degree of chemical ordering
and is predicted to reach the highest value of all transition metal alloys for
the fully-
ordered
L1o phase. Films containing this phase of FePt or CoPt are being studied as high-
anisotropy
longitudinal magnetic recording media. The challenge in growth of intermetallic
alloy films is in controlling both the extent of chemical ordering and its
orientation. We show how this control can be achieved in the case of FePt by
using seeding techniques to select the orientation of the tetragonal c-
axis.
Films were grown by co-
evaporation
(MBE) of Fe and Pt onto a seed film of Pt grown on MgO substrates.
Perpendicular or in-
plane
ordering was controlled by selecting the (001) or (110) plane, respectively, of
substrate and seed film. The long-
range
order parameter (S) increased smoothly with substrate temperature and near-
complete
chemical ordering (S=0.950.05) was achieved for growth at 500deg.C.This work
raises several interesting questions. Why does one unique orientation of the c-
axis
(c-
axis
in plane and along Pt[001]) dominate for growth of FePt on Pt(110)? Why do
minority orientations occur more readily for compositions which are Pt-
rich?
These issues will be discussed and examples given of the growth of other
intermetallic compounds including the newly-
discovered
hcp ordered phase of Co3 Pt.
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