Program Organizer: Dr. David E. Jesson, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6030
Tuesday, PM Room: Orange County 4
February 6, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: R. C. Pond, Department of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX
2:00 pm Invited
MISFIT DISLOCATION FORMATION INDUCED SURFACE MODIFICATIONS STUDIED BY UHV- SCANNING TUNNELING MICROSCOPY: G. Springholz, N. Frank, Y. Ueta, V. Holy, G. Bauer, Institut fur Halbleiterphysik, Universitaet Linz, A- 4040 Austria
Surface modifications caused by misfit dislocation formation in MBE- grown EuTe on (111) PbTe buffer layers (lattice mismatch: 2.1%) were studied using ultra- high- vacuum scanning tunneling micrsocopy. In agreement with the Matthews Blakeslee mechanism for strain relaxation, at the critical layer thickness straight monolayer step lines appear due to the glide of preexisting threading dislocations, grown- in from the PbTe buffer. Apart from these steplike features at the surface local wave- like deformations with an amplitude equal to the normal component of the Burgers vector are induced above the misfit dislocations at the epilayer/substrate interface, caused by the local lattice deformations around the misfit dislocation segments. The exact form of these surface deformations depends on the orientation and magnitude of the Burgers vector and furthermore dislocation reactions have their direct signature in the accompanying surface deformations.
2:30 pm Invited
A THERMALLY- INDUCED STRESS- ASSISTED COOPERATIVE DISLOCATION GENERATION MECHANISM: APPLICATION TO STRAINED EPITAXIAL FILMS: M. Khantha, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104- 6272
There are many phenomena where dislocation generation occurs suddenly over large length scales. Some well- known examples include the massive dislocation activity which occurs near a crack tip at the brittle- to- ductile transition, the yielding of whiskers and the generation of misfit dislocation network in epitaxial films. The mechanism by which dislocations are generated when a suitable control parameter such as temperature, stress, or film thickness is varied is not well understood. We have recently proposed a new thermally- induced stress-assisted cooperative dislocation generation mechanism, based on the Kosterlitz-Thouless concept of dislocation screening, and used it to explain the general characteristics of the brittle- ductile transition. We discuss how this mechanism can be adapted to explain the generation of misfit dislocations in epitaxial films above a critical film thickness for a given temperature.
3:00 pm Invited
DISLOCATION MECHANICS IN ATOMISTIC DETAIL: THE CASE OF SILICON: Vasily V. Bulatov, MIT, Dept. of Mechanical Engineering, 77 Massachusetts Ave., Cambridge, MA 02139
Results are presented of extensive computer simulations of full and partial dislocations of the 111-guide subsystem in silicon. New principal findings include a prominent role in dislocation motion of the (2x1) core reconstruction and the reconstruction defects, and a strong left-right asymmetry of dislocation kink propagation (in 30-partial dislocation). Computer simulation techniques are developed to examine atomistic structure effects on short-range interactions between two dislocations. The results show that certain pre-assumptions common in the classical (continuum) dislocation mechanics have to be re-examined and that details of the dislocation core structure have implications for dislocation mobility and interactions, including processes of cross-slip and cutting of the forest dislocations. Such core aspects can be explicitly addressed only within a fully atomistic approach. This work clearly shows both necessity and feasibility of atomistic simulations for the development of a quantitative theory of dislocations in silicon.
3:30 pm BREAK
Session Chairperson: C. B. Carter, Department of Chemical Engineering and Materials Science, Amundson Hall, University of Minnesota, Minneapolis, MN 55455
3:50 pm Invited
INTERFACIAL DEFECT ARRAYS AND MISORIENTED EPITAXIAL LAYERS: M. Aindow, School of Metallurgy and Materials, The University of Birmingham, Elms Road, Edgbaston, Birmingham, B15 2TT; R. Beanland, 2GMMT Caswell, Towchester, Northants, NN12 8EQ; R. C. Pond, Department of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX
The evolution of the microstructure and surface morphology of epitaxial deposits is strongly influenced by the degree of misfit and the condition of the substrate surface. In particular, epitaxial films may be misoriented by up to a few degrees away from a state where low-index directions in the substrate. Such misorientationed layers are referred to as being tilted, and this paper is a review of experimental observations reported over the last twenty years or so. It will be shown that the tilting layers can be explained comprehensively by topological considerations of defects at the film/substrate interface. Such defects arise at interfaces either to accommodate mismatch at singular or vicinal substrate surfaces, or as a consequence of deformation processes initiated in epitaxial layers to relieve long-range strain. When the total interfacial dislocation content is assessed, a component causing tilting, and not contributing to misfit relief may accrue. The topological theory of interfacial defects and arrays of defects will be reviewed briefly and subsequently used to understand the occurrence of tilted layers.
4:20 pm Invited
INDENTATION INDUCED DISLOCATION NUCLEATION AT EPITAXIAL INTERFACES: W. W. Gerberich, C. J. Palmstrøm, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455; P. O. Cohen, Department of Electrical Engineering, University of Minnesota, Minneapolis, MN 55455; P. M. Anderson, Materials Science and Engineering, Ohio State University, Columbus, OH 43210
Recently, Page at Newcastle, Michalske at Sandia National Laboratories, and ourselves have detected the onset of plasticity in bulk ceramic and metallic systems using nanoindentation. Our measurements have principally dealt with Fe- 3wt%Si single crystals and in those, it has been shown that dislocation loops initiate at tip forces in the range of 30 to 300 N. The measurements are sufficiently accurate to isolate the exact tip forces at which the first dislocation loop nucleates. We have analyzed the conditions and found that tip forces, as balanced by image forces and the Peierl's barrier are greatly affected by tip radius and oxide film thickness. Epitaxial films become doubly interesting because of the two interfaces at which the dislocation may nucleate and the epitaxial stress as a potential driving force for nucleation. A series of intermetallic and rare- earth group V semi- metallic films have been grown on InGaAs and GaAs with various misfit strains. The films have B2 and rock salt crystal structures, respectively. First order theoretical analysis of the various contributions to film stability have been made, and it is demonstrated, experimentally, that the stored elastic energy in the thin film lowers the onset of dislocation nucleation as expected.
EPITAXIAL GROWTHS OF HIGHLY CONDUCTIVE RuO2 THIN FILMS ON YSZ AND LaAlO3: Shihong G. Song, Quan Xi Jia, Xin Di Wu, Los Alamos National Laboratory, Los Alamos, NM 87545
Epitaxial growths of highly conductive RuO2 thin films on YSZ and LaAlO3 substrates have been observed using cross- sectional HRTEM techniques. The matrix/film interface in either case is seen to be atomically sharp. The epitaxial lattice relationships were determined using X- ray and electron diffractions. Although both films exhibit diagonal epitaxy, behavior details of the lattice relationship are distinctively different. Crystallographic variants are seen in the thin films as a result of the diagonal lattice relationship. Small lattice misfit is present in both cases, and a dislocation mechanism is provided to interpret the interface structures.
CHARACTERIZATION OF DEFECTS IN GaP/Si HETEROEPITAXY: Srikanth B. Samavedam, E. P. Kvam, School of Materials Engineering, Purdue University; T. P. Chin, J. M. Woodall, School of Electrical Engineering, Purdue University, W. Lafayette, IN 47907
Integration of III-
based devices with silicon-
technology may be possible through the growth of good quality gallium phosphide
layers on silicon (GaP/Si). GaP/Si films were grown using molecular beam
epitaxy (MBE) under various processing conditions and substrate orientations,
and were characterized primarily using transmission electron microscopy (TEM).
The observations seemed to indicate that islanding during initial periods of
GaP film growth might be responsible for the high density of stacking defects
observed. The problem was magnified by low GaP fault energy and surface energy
anisotropy. There were strong indications that anti-
domain boundaries (APBs) could be avoided by resorting to miscut Si(100)
substrates for growth.
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