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Session Chair: E.P. Kvam, School of Materials Engineering, Purdue University, West Lafayette, IN 47907-1289
2:00 pm INVITED
NEW INSIGHTS INTO THE STRESS DRIVEN 2D TO 3D TRANSITION: D.E. Jesson, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6030
An understanding of the stress driven 2D to 3D transition is central to many key issues in materials science including the growth of planar but strained semiconductor layers and the fabrication of quantum dots. We will overview our current understanding of the transition and identify the conditions under which stress driven instabilities occur during film growth. Nucleation mechanisms of roughening compete with instabilities and this will be illustrated by annealing experiments which demonstrate the formation of rippled surface morphologies by cooperative nucleation events.
2:40 pm INVITED
SPONTANEOUS COMPOSITION MODULATION IN SEMICONDUCTOR ALLOYS: Joanna Mirecki Millunchick, Sandia National Laboratory, Albuquerque, NM 87195-0601
Lateral composition modulation (CM) has been observed to occur spontaneously in III-V semiconductors, producing quantum wires. CM is presumed to originate from surface undulations arising from compositional inhomogeneity, as well as lattice mismatch between the epilayer and the substrate. We observe CM in homogeneous alloys or in short period superlattices. Samples were characterized using cross-sectional transmission electron microscopy (XTEM), polarized photoluminescence spectroscopy (PPL), reflection difference spectroscopy (RDS), and magneto-luminescence (ML). For example, XTEM micrographs of InAs/GaAs superlattices show contrast due to CM only in the projection, with a period 150Å. PL measurements show that emission is red-shifted and polarized with a ratio of up to 4. InAlAs buffer layers, on the other hand, have much higher polarization ratios (up to 10), indicating that these films may also be compositionally modulated. XTEM images of nominally lattice-matched InxGa1-xAs / InxAl1-xAs superlattice structures grown on InP (001) show that only the InAlAs layers exhibit contrast due to CM along the direction, in agreement with polarized PL results. The modulation period for such a structure deposited at Ts=475°C, for example, is 50Å. Preliminary temperature-dependent PL and ML results for InAlAs epilayers show that the emission is independent of T and B, suggesting carrier confinement due CM. These phenomena are under investigation in an effort to determine the dependence of the material system on CM. Finally, a model of the effect of CM on the band structure will be discussed. Supported by U.S. Department of Energy, OER/BES Division of Materials Science Grant No. DE-AC02-83-CH10093.
STRUCTURAL STABILITY OF LOW TEMPERATURE GROWN InGaAs/GaAs HETEROSTRUCTURES: Chanro Park, C.G. Park, Dept. of Materials Science and Engr., Pohang University of Science and Technology, Pohang 790-784, Korea; C.D. Lee, S.K. Noh, Materials Evaluation Center, Korea Research Institute of Standards and Science, Taejon, 305-340, Korea
Microstructural evolution of the InGaAs/GaAs heterostructures grown by MBE at low temperatures (200-250°C) has been studied using double crystal x-ray diffraction and transmission electron microscopy. Dislocation formation was suppressed at low growth temperatures, and the layers sustained metastable structures which would undergo microstructural change at high temperatures. Misfit dislocations formed at InGaAs/GaAs interfacial region during annealing relieved accumulated strain caused by lattice mismatch between the InGaAs and GaAs. Arsenic precipitates formed during the annealing process also played an important role for strain relaxation by forming misfit dislocations along matrix/As interface. Structural stability of the low temperature grown layers in discussed in terms of the microstructural evolution observed at various growth and annealing temperatures.
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