AVS1996 Session NS+SS-ThA: Properties of Nanostructures II

Thursday, October 17, 1996 1:30 PM in Room 202 A/B

Thursday Afternoon

Start	Invited?	Item
1:30 PM	Invited	NS+SS-ThA-1 From Clusters and Nanophase Assemblies to Nanowires U. Landman (Georgia Institute of Technology) Materials systems exhibit dependencies on the degree, mode, form, and state of aggregation. Elucidation of size-evolutionary patterns in materials and manipulations of materials properties on the atomic scale are among the main challenges of modern materials science. In this lecture, theoretical investigations of these issues, combining large-scale classical and quantum computer simulations and analytical methods, will be presented. Topics that will be discussed include: geometrical structure, electronic spectra, chemical reactivity, and stability of atomic and molecular clusters in various environments; structure, dynamics, and thermodynamics of passivated metal nanocrystallites as gas phase clusters, when adsorbed on a surface, and in superlattice assemblies; formation mechanisms, mechanical properties, conductance quantization, and magnetoresistance of metallic nanowires.Work supported by DOE and AFOSR.
2:10 PM		NS+SS-ThA-3 Transport Properties Along Finite-Length Atomic Chain T. Yamada, Y. Yamamoto (Stanford University) Electron transport properties along a metallic atomic chain [1] on the flat insulating substrate artificially created using an STM tip are studied theoretically. If the chain length L is infinite, the electron momentum is continuous, and the usual conductance quantization is expected with ideal ohmic contact as in the case of mesoscopic systems. However, the momentum is discretized for finite L, and quantized energy levels form. Details of this quantization depends on the boundary condition or the actual physical processes at the ohmic contacts. We assume that if the highest occupied level is partially filled, the Fermi levels of the reservoirs align to this level in thermal equilibrium, while if fully filled, the Fermi levels align to the middle of the highest occupied and the lowest unoccupied levels. This energy alignment assures the same ionization energy for both addition or removal of an electron. The low-field transport properties will be significantly different depending on whether the highest occupied level is partially or fully filled. In the partially occupied case, a current is expected for an infinitesimally small voltage. The involved energy level has an intrinsic transmission coefficient, which determines the current magnitude that is insensitive to the applied voltage until the next energy level is involved in the transport. In the fully occupied case, a current cannot flow until the applied voltage overcomes the energy difference between the highest occupied and the lowest unoccupied levels. [1] T. Yamada, Y. Yamamoto, and W. A. Harrison, J. Vac. Sci. Technol. B 14, 1243 (1996)
2:30 PM		NS+SS-ThA-4 Nanostructural and Local Electronic Properties of Fe/W(110) Correlated by Scanning Tunneling Spectroscopy M. Bode, R. Pascal, R. Wiesendanger (Institute of Applied Physics and Microstructure Research Center, Germany) Although there is a large lattice mismatch of 10% between the two bcc-metals Fe and W, Fe grows epitaxially on W(110) substrates up to a coverage of 1.3 ML. Above the stressed Fe-film a peak in the dI/dU spectra is found which is centered at +0.2 eV as measured by scanning tunneling spectroscopy (STS). This peak does not appear above relaxed, quasi three-dimensional Fe-islands. Instead, a peak at -0.5 eV was found. It is shown directly in real space by STS on nanometer-scale wedges of Fe/W(110) that the density of empty-states is diminished on a lateral scale of a few \Ao\, wherever the stressed Fe-film begins to relax. The change in local differential conductivity can therefore be explained by a stress-induced change of electronic structure for the first two monolayers due to the large misfit between film and substrate. STS-measurements on the system Fe/W(110) were also performed with in-situ iron-coated tungsten tips. In contrast to the results obtained with the non-coated tungsten tips an additional peak in the dI/dU-spectra above iron islands of the second monolayer was observed.
2:50 PM		NS+SS-ThA-5 Photoluminescence Spectra of Self-organized SiGe Quantum Dot Lattices J. Lin, C. Teichert, L. McCaughan, M. Lagally (University of Wisconsin, Madison); J. Bean (Bell Laboratories) The recent observation of stress-driven self-organization of 3D nanoscale structures in SiGe/Si multilayer films provides technical possibilities of applications [1]. A progressively uniform size and spacing of the 3D islands that already form in a thin SiGe film is achieved by using a multilayer stack. The reduced dimensionality and size of these quantum dot structures suggest an enhanced oscillator strength of photoexcited carriers, a factor particularly important to quantum structures in which the constituents are indirect elemental semiconductors, such as Si and Ge. We have investigated the characteristic photoluminescence (PL) of SiGe quantum dots (QD) to determine the optical response in this system. A broad PL band of the excitons confined in the clusters is observed, suggesting an effective capture of photoexcited carriers induced by the strain field that drives the carriers towards the islands. As we change the SiGe alloy concentration and the bilayer number, the evolution of this emission band can be directly related to the improved self-organization of this QD lattice, as observed by the atomic force microscopy. A distinct spectral component develops at the high-energy tail of this broad band as the 3D nanoscale structures reach a high degree of uniformity. The enhanced oscillator strength and relatively narrow spectral width of this exciton line are interpreted as the consequences of quantum confinement in 3D. The persistent excitonic resonance in the PL spectrum from low temperature to T = 190 K adds support to our identification. This work is supported by NSF Grant No. DMR91-21704 and AFOSR. [1] J. Tersoff et al. Phys. Rev. Lett. 76, 1675 (1996); C. Teichert et al. to be published in Phys. Rev. B, 1996.
3:10 PM		NS+SS-ThA-6 Tunneling Spectroscopy in High Mobility GaAs/AlGaAs Heterostructure Subject to Surface Field Effect Induced Periodic Potentials M. Hannan, R. Giannetta, R. Grundbacher, I. Adesida (University of Illinois, Urbana) Transport measurements are reported for a high mobility GaAs/AlGaAs heterostructure subject to lateral surface induced periodic potentials. The potentials are electrostatically induced by interdigitated Ti/Au Schottky gates located 55 nm above the 2DEG. Measurements performed at 0.3 K and 1.5 K using low frequency techniques suggest sequential resonant tunneling between spatially separated 1D regions which exist between the gates. Conductance oscillations with and without magnetic fields provide evidence of 1D subbands formation in the confined islands. The dI/dV\sub ds\ data suggests tunneling through the surface-gate induced barriers which is modified by the perpendicular magnetic field. insert figure
3:30 PM	Invited	NS+SS-ThA-7 In Situ, Growth-Controlled Fabrication of Semiconductor Nanostructures A. Madhukar (University of Southern California) Semiconductor structures with electronic states confined in two or three dimensions (i. e. quantum wires and boxes, respectively) offer a potential basis for advanced electronics and optoelectronics in the 21st century. The first step in this direction is exploring materials and methodologies for creation of such structures. This talk will focus on in-situ realization of semiconductor nanostructures via, (1) growth on non-planar patterned substrates and (2) formation of 3-dimensional islands in highly strained epitaxy such as InAs/GaAs, InAs/InP, Ge/Si, etc. For the former, emphasis will be on GaAs/AlGaAs and InAs/GaAs quantum wires and boxes realized via substrate-encoded size-reducing epitaxy on appropriately oriented GaAs mesa tops. For the latter, singly and multiply stacked (i. e. vertically self-organized) quantum box structures, and lasers based upon them, will be emphasized. In both cases, the evolving nature of the stress fields, and hence the mechano-chemical nature of the surface kinetic processes such as adatom migration, incorporation, detachment, etc. during growth will be shown to be the common underlying feature responsible for nanostructure formation via growth control.
4:10 PM		NS+SS-ThA-9 Microscopic Processes of Crystal Growth: How can we Control Surface Roughness and Self-organization of Nano-scale Islands P. Ruggerone, B. Yu, C. Ratsch, M. Scheffler (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) Density functional theory calculations are discussed for atoms ``raining down'' and diffusing at surfaces. In order to determine the dependence of growth phenomena on temperature and deposition rate we have complemented the ab initio total-energy results by a kinetic Monte-Carlo simulation. --It will be shown how different microscopic aspects of surface diffusion determine the quality of the growing surface. For example, we explain why silver grows very rough at the (111) surface, but nice and flat at (100). Furthermore, it is shown how the kinetics of growth can be utilized by properly choosing the temperature and deposition rate to set conditions for a self-assembling of islands with nano-scale dimensions and well defined shape, size, and density.
4:30 PM		NS+SS-ThA-10 Growth and Properties of Self-Organized Si/Si\sub 1-x\Ge\sub x\ Clusters in Multilayer and Single Films J. Sullivan, C. Teichert (University of Wisconsin, Madison); J. Bean (Bell Laboratories); D. Savage, M. Lagally (University of Wisconsin, Madison) Under specific conditions of alloy composition, temperature, growth rate, and layer thickness, growth of Si\sub 1-x\Ge\sub x\ films on Si(001) leads to the formation of very small three-dimensional crystalline islands that may be suitable as "quantum dot" structures. In a Si/Si\sub 1-x\Ge\sub x\ multilayer, Si\sub 1-x\Ge\sub x\ islands in neighboring alloy layers communicate via strain fields through the Si layer separating them. The strain fields provide a mechanism for vertical correlation and self-organization (Tersoff et al, Phys. Rev. Lett. 76, 1675 (1996)). We use atomic force microscopy to determine the effect of various process conditions on self-organization of the three-dimensional islands (size, shape, and separation) and near-field scanning optical microscopy, scanning tunneling luminescence, and photoluminescence to probe their optoelectronic properties. We compare growth, vertical correlation, and self-organization of three-dimensional islands grown by molecular beam epitaxy and by rapid thermal chemical vapor deposition using SiH\sub 4\ and GeH\sub 4\. We discuss variations in terms of possible differences in the atomistic kinetic mechanisms in MBE and CVD.