Program Organizer: Dr. David E. Jesson, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6030
Tuesday, AM Room: Orange County 4
February 6, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: D. E. Jesson, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6030
8:30 am Invited
STRESS- DRIVEN SELF- ORGANIZATION IN THREE- DIMENSIONAL "QUANTUM DOT" STRUCTURES: J. Tersoff, IBM T. J. Watson Center, PO Box 218, Yorktown Heights, NY 10598
In some systems, e.g. InAs on GaAs(001), pseudomorphic growth of strained material leads to formation of islands with a relatively narrow size distribution. Such islands are considered possible candidates for "quantum dot" devices. Moreover, several groups have observed that when such islands are grown on successive layers of a multi- layer structure, there may be strong vertical correlations between islands in different layers. I will discuss how the distribution of islands evolves with the growth of successive layers. Not only do the islands become vertically correlated, but in addition their lateral arrangement and size distribution is predicted to become more uniform. Comparison will be made with experimental data from C. Teichert et al.
9:00 am Invited
THE CHEMICAL AND MORPHOLOGICAL STABILITY OF LATTICE-MISMATCHED FILMS: Peter W. Vorhees, Dept of Matls Science, Northwestern University
The phenomenon of coherent (undislocated) island formation during heteroepitaxy is discussed using the results of recent Monte Carlo simulations as a framework. Through an approximate treatment of the elasticity problem, the growth model permits dynamic stress relaxation at the free surface of both 2D and 3D islands although misfit dislocations are disallowed by construction. It is assumed that misfit strain reduces the energy barrier to atom detachment from island edges. Coherent islands form above a critical value of misfit that depends on both growth conditions and the material parameters. Both the lateral size and width of the size distribution become smaller as misfit increases. The latter occurs despite the complete absence of substrate-mediated elastic repulsion between islands. Comparison is made with other theories of island formation.
9:30 am
COHERENT STRANSKI- KRASTANOV GROWTH: STUDY OF THE FORMATION OF 3D ISLANDS VIA THE POWER SPECTRAL DENSITY: G. C. Hsueh, Department of Chemical Engineering, C.M. Reaves, S. P. DenBaars, Materials Department, W. H. Weinberg, Center for Quantized Electronic Structures (QUEST), University of California, Santa Barbara, CA 93106
The evolution of the power spectral density (PSD) corresponding to the development of coherent 3D InP islands on GaInP/GaAs(100) is investigated. Analysis of the PSD of the pseudomorphic wetting layer follows the correspondence of wavelength scaling laws with associated atomic proccesses on unstrained and strained surfaces. Upon the formation of coherent 3D InP islands, the qualitative nature of the PSD changes profoundly, with the appearance of short- wavelength oscillations, indicative of a narrow distribution of island sizes. The PSD of the surface displaying 3D InP islands is interpreted in terms of particle scattering and interference. By assuming an analytic approximation to the surface morphology, the island size distribution and spatial correlation are extracted and deconvoluted from the PSD. Specifically, the island size distribution is modeled by an ensemble of cylindrical functions, while the spatial correlation is accounted through the use of a "hard- spheres" radial distribution function.
9:50 am
THE STRAINED STATE OF HETEROEPITAXIAL ISLANDS ON MISFITTING SUBSTRATES--FINITE ELEMENT CALCULATIONS AND EXPERIMENTS: S. Christiansen, M. Albrecht, H. Michler, H. P. Strunk, A. Borbely, Institut fur Werkstoffwissenschaften VII, Universitat Erlangen- Nurnberg, Cauerstr. 6, 91058 Erlangen, Germany; T. Ungar, Eotvos University Budapest, Institute for General Physics, POB 323, Budapest VIII, Hungary; B. Dietrich, Institut fur Halbleiterphysik, Walter Korsing Ring 2 15230, Frankfurt/Oder, Germany
Several attempts exist to calculate the strained state in nanoscaled laterally limited structures by analytical, semi- empirical, and numerical methods. However, the experimental confirmation of these data encounters several problems: high resolution methods, such as convergent beam electron diffraction and quantitative high resolution transmission electron microscopy have to deal with inhomogeneous thin foil relaxation of single strain components, while laterally averaging methods such as high resolution x- ray diffraction and Raman spectroscopy are limited in local resolution. In this paper, we compare both classes of methods for the case of facetted pyramidal islands obtained by solution growth of Ge on Si(001). The Raman measurements yield the average strain distribution within the layer, and the x- ray measurements yield the average strain distribution within the substrate. We compare these data to the strain distribution calculated with the three- dimensional finite element method for the substrate and for the islanded Ge layer. Steep strain gradients are present in certain regions of the structure and are corroborated by experiments using locally resolving convergent beam electron diffraction.
10:10 am BREAK
10:30 am Invited
NEW INSIGHTS INTO THE KINETICS OF THE STRESS- DRIVEN 2D- TO- 3D TRANSITION: K. M. Chen, D. E. Jesson, S. J. Pennycook, T. Thundat, R. J. Warmack, Oak Ridge National Laboratory, Oak Ridge, TN 37831- 6031
The stress- driven 2D- to- 3D transition associated with the Stranski- Krastanow growth mode has received considerable attention. In this work, the influence of temperature, film thickness, misfit strain, atomic steps, and supersaturation on the kinetics of the transition have been systematically investigated with RHEED and AFM. Our studies have revealed a number of surprising features, reflecting the interplay between kinetics and energetics during the initial and late stages of island growth. We establish that an energy barrier exists to the formation of 3D islands, and map the kinetic pathway to {501} faceted islands. It is demonstrated that the energy barrier exists before {501} faceting. We interpret the transition in terms of positive step free energy, which explains the observed sensitivity of the results to strain and temperature. In the late stages of growth, we observe quite different behavior including the fissioning of elongated islands (inverse Ostwald ripening), and we present a model for the remarkably narrow distribution of observed island sizes. This research was sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract No. DE- AC05- 840R21400 with Lockheed Martin Energy Systems.
11:00 am
IMPACT OF DEPOSITION SURFACE ON THE FORMATION OF SELF- ASSEMBLED InP EPITAXIAL ISLANDS: C. M. Reaves, G. C. Hsueh, W. H. Weinberg, S. P. DenBaars, Center for Quantized Electronic Structures, University of California, Santa Barbara, CA 93106
The formation of self- assembled, defect- free epitaxial islands in the Stranski-Krastanov growth mode has been observed for a wide range of semiconductor systems. Although various studies have focused on understanding the impact of growth conditions on these islands, one growth parameter that has not received much attention is the nature of the deposition surface. We have studied the formation of InP islands on a range of surfaces with the GaAs lattice constant (a lattice mismatch of 3.7 %). Comparisons between GaAs and GaInP surfaces, between rough and smooth GaInP surfaces, and between (100) and (311) surfaces will be presented. For example, InP islands grown on a GaInP (311)A surface exhibit a single distribution of islands of height 21 + 7 A and base diameter 850 + 120 A, whereas islands grown on a GaInP (100) surface exhibit a bimodal distribution, one mode of height 31 + 12 A and base diameter of 1320 + 200 A and another of height 280 + 15 A and base diameter of 1430 + 120 A. The impact of the surface on transport, >nucleation, and growth processes will be discussed.
11:20 am
ON THE INSTABILITY OF EPITAXIALLY- STRAINED ANISOTROPIC THIN FILMS: Jong K. Lee, Stephen A. Hackney, Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931
The morphological instability of an epitaxially- strained thin film is studied by means of a discrete atom method, which is predicated upon Hookean atomic interaction and Monte Carlo diffusion. Cubic systems of anisotropic ratios, A (= 2C44/(Cll - Cl2)) = 0.5, 1, and 2.33 are analyzed in dislocation- free, two dimensional crystals. If the surface normal is parallel to an elastically soft direction (> = 100 with A = 2.33), a planar thin film is found to be relatively stable at a low temperature, but prone to create islands at a high temperature apparently with the aid of thermal fluctuations. If the surface normal orientation is elastically hard or isotropic, however, a planar film tends to create a wavy surface with characteristic wavelengths. Stress analysis shows that within a hilltop, the strain energy density decreases as the free surface is approached from inside, but increases within a valley. The instability of the film- substrate interface and its elastic interaction with the free surface are also discussed. The research was supported by the U.S. Dept. of Energy under Grant DE- FG0287ER45315.
11:40 am
MOLECULAR DYNAMICS SIMULATION OF STRESS RELAXATION IN HETEROEPITAXIAL FILMS: Richard W. Smith, David J. Srolovitz, Department of Materials Science and Engineering, The University of Michigan, 2300 Hayward, Ann Arbor, MI 48109
Molecular dynamics simulations were performed to investigate the development
of voids and stress during the growth of heteroepitaxial films. During low
temperature deposition of atoms which possess a larger lattice constant than
the substrate material, a compressive stress develops in the first few layers
of the film. Void formation and dislocation formation were found to each play a
significant role in relaxing the misfit stress. Misfit stresses can be relieved
by the flexure of the free surfaces of the voids throughout the film. As the
lattice constant of the deposited atoms is increased, the critical thickness
for dislocation formation decreases and for sufficiently large mismatches,
dislocations form immediately at the interface between film and substrate.
While dislocations normally form at the film-
substrate
interface when no voids are present, when voids are present dislocations were
found to be preferentially located within the voids. However, if the misfit
strain were very large, dislocations were found to lie preferentially at the
interface. Dislocations at the interface move rapidly along the interface in
the absence of voids. A video tape showing stress and microstructure evolution
during film growth will be presented.
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