-MRS-
Chairs
Max Lagally, Univ of Wisconsin-Madison
Pierre Petroff, Univ of California-S Barbara
R. Stanley Williams, Hewlett Packard Co
Symposium Support
* Invited paper
SESSION C1: SELF-ORGANIZED NANOSTRUCTURES I
Chair: R. Stanley Williams
Wednesday Morning, December 3, 1997
Essex West (W)
8:30 AM *C1.1
TUNING QUANTUM INTERACTIONS IN METAL NANOCRYSTAL SUPERLATTICES. James R. Heath, UCLA, Dept of Chemistry and Biochemistry, Los Angeles, CA.
Ordered superlattices of nanocrystals are exciting new materials in which the optical and electrical properties of the bulk material can potentially be tuned by controlling quantum mechanical wavefunction overlap between particles. In this talk, recent experiments will be presented in which pressure is used to tune interparticle quantum interactions within an ordered, 2D metal nanocrystal superlattice. The nanocrystals can be reversibly compressed through a metal insulator-transition. Linear and nonlinear optical measurements, and electrical transport measurements, are used to characterize the system.
9:00 AM *C1.2
GIGANTIC QUANTIZATION EFFECTS IN THIO-NANAURITE MOLECULES, Aun(SR)m. Robert L. Whetten, School of Physics & Chemistry, Georgia Institute of Technology, Atlanta, GA.
Small metal particles are known to show enormous enhancements in optical and electron-transfer properties arising from the quantization of energy- and charge-states, but these have been difficult to realize in practical materials because of dispersity effects, sensitivity to degradation, or oxidative surface-chemical bonding A recently identified series of five thio-nanaurite molecules, AuN(SR)M, are comprised of crystalline neutral-gold (aurite) cores with N = 38 to l90 Au atoms (1.1 to 1.8 nm equivalent diameter) encapsulated by a compact monolayer of various interchangeable thio groups (SR). These environmentally and thermally robust molecules are separable from each other as distinct compounds, form crystalline molecular solids, and undergo reversible charging comparable to that of metallofullerenes, e.g. La2C80. They exhibit distinct Coulomb-staircase type conductance curves at room temperature either in single-molecule (double tunnnel-junction) or ensemble (electrochemical) configurations. Extraordinary enhancements us near-infrared optical response, above the Kubo gaps, are also observed along why discrete level structure of the conduction-band orbitals delocalized over volumes on the order of 103 . These massive molecules can be integrated as active elements into various nanoelectrode and thin-film structures, and their electronic structure may be open to systematic modification by suitable doping or substitution.
9:30 AM *C1.3
SEMICONDUCTOR NANOCRYSTALS: NEW MATERIALS THROUGH CONTROL OF SIZE. Paul Alivisatos, Univ of California, Berkeley, Dept of Chemistry, Berkeley, CA.
In recent years there have been significant advances in the preparation of semiconductor quantum dots by colloidal chemistry routes. CdSe and InAs are examples of materials which can be made as nanocrystals of high quality. This talk will focus on recent efforts to integrate colloidal nanocrystals into more complex structures. One example involves blends of nanocrystals with the semiconductor polymer MEH-PPV. Depending on the nanocrystal surface chemistry, the local morphology of the blends can be adjusted, providing control over the electrical characteristics. A second example involves the integration of single nanocrystals into lithographically prepared transistors. Finally, efforts to use DNA to create complex spatial arrangements of nanocrystals will be described.
10:30 AM *C1.4
SPECTROSCOPY OF RARE-EARTH DOPED METAL-DICHALCOGENIDE NANOCRYSTALS. David F. Kelley, Department of Chemistry, Colorado State University, Fort Collins, CO.
The synthesis, doping and characterization of metal dichalcogenide nanocrystals are reported. For example, undoped PtS2 nanocrystals have an indirect band gap of ca. 1.4 eV compared to 0.87 eV for bulk PtS2. For Eu-doped PtS2 nanocrystals, emission and fluorescence excitation spectra show that 3.4 eV optical excitation of the nanocrystal results in energy transfer and subsequent luminescence of the Eu dopant. From the relative intensities of the Eu emission lines, it is concluded that Eu3+ ions are situated in the near-octahedral holes of the PtS2 nanocrystals. Chemical passivation of the nanocrystal surface trap states increases the intensity of the Eu luminescence, which demonstrates that trapping competes with electron/hole recombination on the dopant ion.
11:00 AM *C1.5
PSEUDOPOTENTIAL ELECTRONIC STRUCTURE THEORY OF SEMI-CONDUCTOR QUANTUM-DOTS. Alex Zunger, National Renewable Energy Laboratory, Golden, CO.
The energy levels, wave functions, strain-profiles, and electron-hole Coulomb and exchange interactions of (a) pyramidal InAs/GaAs dots and (b) free-standing InP and CdSe spherical dots are calculated within a novel approach that does not require effective-mass like approximation.
11:30 AM *C1.6
QUANTUM SIZE LEVELS AND ABSORPTION SPECTRA OF NARROW GAP SEMICONDUCTOR NANOCRYSTALS. Al. L. Efros, M. Rosen, Naval Research Laboratory, Nanostructure Optics Section, Washington, DC; W. Jaskolski, Instytut Fiziki, Uniwersytet Mikolaja Kopernika, Torun, POLAND; G.W. Bryant, NIST, Gaithersburg, MD; U. Banin, A.P. Alivisatos, Department of Chemistry, UCB, Berkeley, CA.
A theory of the Quantum Size Levels of electrons and holes in nanosize crystals of direct gap semconductors has been developed. The calculations are done within the 8 band model proposed by Pigeon and Brown and simultaneously take into account nonparabolicity of the electron and hole dispersion and the complicated structure of the valence band. The nonparabolicity of the electron and hole dispersion has not previously been taken into account and considerably affects the positions of the QSLs in narrow gap semiconductors. Calculations for InAs nanocrystals using bulk InAs energy band parameters give an excellent description of the absorption spectra.
SESSION C2: SELF-ORGANIZED NANOSTRUCTURES II
Chair: Pierre M. Petroff
Wednesday Afternoon, December 3, 1997
Essex West (W)
1:30 PM *C2.1
EVOLUTION OF SELF-ORGANIZED SixGe1-x ISLAND ARRAYS. Eric Chason, Jerry A. Floro, Michael B. Sinclair, John A. Hunter, Ray D. Twesten, Sandia National Laboratories, Albuquerque, NM; Robert Q. Hwang, Sandia National Laboratories, Livermore, CA; Gene Lucadamo, Lehigh University, Bethlehem, PA.
Heteroepitaxial layers can accommodate misfit strain by forming 3-dimensional islands. The size and shape of the islands is determined by a balance between the reduction in strain energy of high aspect ratio features and the increase in surface energy from their lower coordinated faces. Because the strain fields are long range, the islands can form well-organized periodic arrays with a narrow distribution of island size. We present measurements of the evolution of regular arrays of islands during the heteroepitaxial growth of SixGe1-x layers on Si(100) surfaces. Island size distributions, facet angles and orientation are determined using atomic force microscopy and scanning electron microscopy. A novel approach using spectroscopic light scattering has also been developed to measure the mean island spacing and the coherence length of the island ordering. Application of the light scattering technique for in situ measurements allows us to study the kinetics of the island self-organization process in real time. As the thickness of the layer increases, we observe transitions from continuous layer growth to islands with (501) surfaces (hut clusters) followed by higher-aspect-ratio coherent islands (dome clusters). Correlations with real-time stress relaxation measurements using in situ wafer curvature enable us to determine the relaxation in coherency stress associated with the different island morphologies. At elevated temperatures where surface mobility is high, the hut-cluster islands align themselves periodically along preferred 4-fold crystallographic orientations while the dome clusters develop a 6-fold symmetry and close-packed structure. Annealing of the islands indicates that the hut cluster can undergo significant coarsening without changing their shape.
2:00 PM *C2.2
REAL-TIME OBSERVATIONS OF THE NUCLEATION AND GROWTH OF NANOSIZE Ge ISLANDS ON Si(100). F.M. Ross, J. Tersoff and R.M. Tromp, IBM T.J. Watson Research Center, Yorktown Heights, NY.
We have examined the growth and evolution of Ge islands on the Si (100) surface using a UHV transmission electron microscope with in situ growth capabilities. Islands were grown on a thin Si substrate by CVD using Ge2H6 gas introduced to the specimen area through a capillary tube. At substrate temperatures around 650C and pressures of 1x10-6 Torr, initial growth (after the wetting layer) is in the form of small strained islands. Video recordings made during growth enable us to track the development of individual islands from nucleation onwards, using dark-field strain contrast to measure the island size. We find that all island nucleation events take place within a few seconds of each other. After nucleation a coarsening process is seen, in which about one third of the islands continue to grow while the others disappear. This coarsening occurs even while the flux is still on, and a bimodal distribution of island sizes develops. The first dislocations appear as the largest islands grew above 75nm diameter and the dislocated islands then grow rapidly compared to nearby stained islands. We explain the bimodal size distribution by a simple modified coarsening process in which the equilibrium island shape changes with size from a shallow pyramid to a complex form with more facets. Such a size-dependent island shape has already been observed, but the essential feature of this model is that the formation of additional facets reduces the chemical potential locally and allows an island to grow at the expense of its more slowly developing neighbours. In this presentation we will show videos of island growth and describe how the model can explain both the evolution of individual islands and the bimodal size distribution of the ensemble of islands. The insight which the model provides into the understanding of island growth can be used to develop arrays of uniformly or bimodally sized quantum dots for a variety of potential electronic applications.
2:30 PM *C2.3
SHAPE TRANSITION OF GE NANOCRYSTALS ON Si(001):FROM PYRAMIDS TO DOMES. Gilberto Medeiros-Ribeiro, Alexander M. Bratkovsky, Theodore I. Kamins, and Douglas A.A. Ohlbert, Hewlett-Packard Laboratories, Palo Alto, CA.
Understanding the size and shape of crystallites during their initial stages of growth is an important step in the rational synthesis of nanomaterials, since the chemical and physical characteristics of molecular-scale solids depend as much on their geometry as on their composition. We have measured, with an in situ Scanning Tunneling Microscope, the volume and surface area of thousands of nanocrystals growth by physical vapor deposition of Ge onto a Si(001) substrate at 600C. The topographs reveal two distinct families: (l) square based pyramids, which nucleate, grow to a critical size, and then transform abruptly into (2) multifaceted domes. Both nanocrystal shapes have size-dependent energy minima that result from strain relaxation at the facets and stress concentration at the edges. The size distributions of the pyramids and domes have been used to construct an approximate free energy model for the shape transition that includes the rearrangement of over 100,000 Ge atoms.
3:30 PM *C2.4
HYBRID ELECTROCHEMICAL/CHEMICAL SYNTHESIS-A NEW METHOD FOR THE SIZE-SELECTIVE SYNTHESIS OF SUPPORTED, LUMINESCENT SEMICONDUCTOR NANOCRYSTALS. S. Gorer, M.A. Anderson, G. Hsial, andR.M. Penner, Institute for Surface and Interface Science, Department of Chemistry, UC Irvine, CA.
A fundamentally new approach for synthesizing semiconductor nanocrystals - size-selectivity - is described in this talk. -CuI and CdS nanocrystallites (NCs) have been synthesized on the atomically smooth graphite basal plane surface using a hybrid electrochemical/chemical (henceforth E/C) method. This method involves the following steps: 1) Electrochemical deposition of metal (e.g. Cu, Cd) NCs onto an electrode surface, 2) Electrochemical oxidation of these metal NCs to yield either an oxide or a hydroxide intermediate (e.g. Cu2O3 Cd(OH)2) and, 3) Displacement of oxygen (or hydroxide) by iodide (or sulfide) in an aqueous KI (Na2S) solution. The dispersions of nanocrystals generated by the E/C method posses the following characteristics: Single crystallinity, good-to-excellent size monodispersity, epitaxial alignment (with the hexagonal periodicity of the graphite (0001) surface). In addition, E/C deposited particles on graphite exhibit strong room-temperature luminescence.
4:00 PM *C2.5
SELF ASSEMBLY OF A TWO-DIMENSIONAL SUPERLATTICE OF ELECTRONICALLY LINKED, NANOMETER-DIAMETER, METAL CLUSTERS. Jia Liu, Michael Buss, and Ronald Andres, School of Chemical Engr, Purdue University, West Lafayette, IN.
We describe the self assembly of lateral nanostructures consisting of nanometer diameter metal clusters (metal quantum dots) covalently linked to each other by rigid organic molecules (organic molecular wires) to form a uniform two-dimensional superlattice which we have termed a Linked Cluster Network (LCN). There are four steps involved in the synthesis of a LCN: (1) Synthesis of ultrafine metal crystals with uniform diameters, (2) Adsorption of a self-assembled monolayer of organic molecules on the surface of these particles to produce stable macromolecular entities that can be manipulated, (3) Formation of a monolayer film of these encapsulated particles on a solid substrate, and (4) Displacement of the organic surfactant coating the particles with a molecular interconnect that covalently bonds adjacent particles to produce a linked electronic network. The flexibility inherent in the ability to vary cluster material and size, linking molecule, and supporting substrate makes the LCN an exciting nanoengineered material. Various methods for patterning LCN's and measurements of the electronic transport in LCN's consisting of gold clusters interconnected by aryl dithiol molecules will be presented.
4:30 PM *C2.6
STUDIES OF PT AND AU NANOCLUSTER SUPERLATTICES. P.P. Newcomer, J.P. Wilcoxon, J.E. Martin, J.G. Odinek, Sandia National Laboratories, Albuquerque, NM.
We have recently synthesized superlattices of unfractionated Pt and Au nanoclusters grown by inverse micelle techniques. Two-dimensional hexagonal arrays readily form on surfaces, and large twinned three-dimensional crystals form from solution. Both the cluster size and the size of the capping ligand can be controlled to form superlattices having a range of lattice parameters. We have characterized the long-range order in these superlattices by TEM and X-ray diffraction, and have found that the capping agent exerts a marked influence on the tendency of these clusters to order. The effect of nanoparticle orientation on long-range order of the superlattice is discussed.
SESSION C3: SELF-ORGANIZED NANOSTRUCTURES III
Chair: R. Stanley Williams
Thursday Morning, December 4, 1997
Essex West (W)
8:30 AM *C3.1
NUCLEATION AND GROWTH OF METAL ATOMS ON IONIC SUBSTRATES. J.A. Venables, and J. H. Harding *, Depts of Physics and Astronomy, Arizona State University, Tempe AZ, and CPES, University of Sussex, Brighton, UNITED KINGDOM; *University College London, London, UNITED KINGDOM.
Nucleation and growth models used for metal films on ionic substrates are briefly reviewed. Classical simulation methods have been used to predict adsorption, diffusion and binding energies of Ag and Au monomers and dimers on several alkali halide (100), CaF2(111), and MgO(100) surfaces[1]. These results are compared with experimental values derived from rate equation analyses, where the need to include small cluster mobilty is often important. Selected calculations have been performed of trap energies at surface vacancies and at steps[1]. These results are compared with available experiments, using analyses from rate-diffusion equations containing traps[2].
9:00 AM *C3.2
AN ATOMISTIC KINETIC VIEW OF 3D ISLAND FORMATION ON SURFACES. Anupam Madhukar, Deptartments of Materials Science and Physics, University of Southern California, Los Angeles, CA.
(Abstract not available)
9:30 AM *C3.3
SELF-ASSEMBLED DISLOCATION FREE ISLAND FORMATION: AN EQUILIBRIUM THEORY. Albert-Laszlo Barabasi and István Daruka, Univ of Notre Dame, Dept of Physics, Notre Dame, IN.
Heteroepitaxial growth of highly strained structures offers the possibility to fabricate islands with very narrow size distribution, coined self-assembling quantum dots (SAQD). In spite of the high experimental interest, the mechanism of the SAQD formation is not well understood. The nonlocal strain existing in heteroepitaxial systems changes many aspects of the island formation process. Thermodynamic approaches will be presented that predict the equilibrium island sizes and densities, as well as the nature and the magnitude of the critical thickness needed to be deposited for SAQD formation. The obtained equilibrium phase diagram will be compared with experimental results on SAQDs observed in various systems, including InAs and InP grown on GaAs, CdSe grown on ZnSe and Ge grown on Si.
10:30 AM *C3.4
SELF-ORGANIZATION OF COHERENTLY STRAINED ISLANDS BY COOPERATIVE NUCLEATION. D.E. Jesson, G. Chen, K.M. Chen, S.J. Pennycook, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN.
The self-assembly of coherently strained islands by direct deposition or annealing encompasses a rich variety of interesting phenomena involving the interplay between surface and elastic energy. We will discuss the physics of cooperative nucleation1 and island fissioning phenomena which offer new approaches to self-organization in the SixGel-x system.
Understanding the growth kinetics of individual islands is of particular importance for obtaining narrow island size distributions. A detailed study of island nucleation and growth has been undertaken using atomic force microscopy and the emphasis has been placed on identifying the role of strained facets. We will demonstrate how the presence of faceting can be used to narrow island size distributions in low misfit systems. Surprisingly, shape instabilities are also found to arise quite naturally as a consequence of facet growth kinetics. Finally, we will discuss the self-organization of islands via the introduction and propagation of misfit dislocations .
11:00 AM *C3.5
NUCLEATION OF SEMICONDUCTOR NANOSTRUCTURES. Bruce W. Wessels, Northwestern University, Dept. of Materials Science and Engineering, Evanston, IL.
The nucleation of quantum dots formed from lattice mismatched semiconductors is analyzed for the case where the dots are isolated, coherent islands. In the model, partial strain relaxation in the underlying strained-layer is taken into account. Using a mean feld approximation the interaction energy is taken to be linearly proportional to the strain energy in the film. The model predicts that the quantum dot radius is inversely proportional to the square of the misfit. The predicted dependence is in quantitative agreement with recently reported experiments on the formation of quantum dots of InGaAs.
11:30 AM *C3.6
ADIABATIC PHASE TRANSFORMATIONS IN CONFINEMENT. Alexander Umantsev, Dept of Materials Science and Engineering, Northwestern University, Evanston, IL.
The phase diagram of small one-component particles will be analyzed under conditions of thermal insulation, i.e. conservation of energy. In large isolated systems the absolute stability belongs to heterogeneous states with phase separation. However, for small particles the global stability analysis shows a considerable extension of the single-phase regions into a two-phase zone of the phase diagram. Moreover, for very fine particles with sizes only 5-20 times exceeding interfacial thickness phase separation does not occur at all and the equilibrium is achieved on homogeneous transition states that can never be obtained in bulk samples because of their absolute instability. The thermodynamic and dynamical explanations are presented. This type of a small-particle phase diagram may be relevant to the theory of amorphization, magnetocaloric effect, and nanophase composite materials where small particles or thin whiskers capable of undergoing a transition are immersed into a poorly conducting matrix. In case of small particles of solid solution, where mass conservation replaces the conservation of energy, present results predict the appearance of new stable phases with compositions deeply inside the miscibility gap.
SESSION C4: SELF-ORGANIZED NANOSTRUCTURES IV
Chair: Pierre M. Petroff
Thursday Afternoon, December 4, 1997
Essex West (W)
1:30 PM *C4.1
INFRARED SPECTROSCOPY OF INTRABAND TRANSITIONS IN SELF-ORGANIZED QUANTUM DOTS. Sebastien Sauvage, Philippe Boucaud, Francois H. Julien, Institut d'Electronique Fondamentale, URA CNRS 22, University Paris XI, Orsay, FRANCE; Jean-Michel Gerard, France Telecom, CNET Bagneux, Bagneux, FRANCE; Jean-Yves Marzin, L2M-CNRS Bagneux, Bagneux, FRANCE.
Infrared absorption spectroscopy is an appropriate tool to investigate intraband properties of quantum dots since it provides a direct measurement of confinement energies of both electrons and holes. As a representative system, we have studied undoped InAs/GaAs quantum dot superlattices obtained by self-organized growth. The infrared absorption is measured by a sensitive photo-induced infrared spectroscopic technique. Quantum dots with different sizes have been investigated as a function of temperature, interband pump photon energy, intensity and infrared polarization. We show that in the 90-250 meV energy range, the quantum dots exhibit infrared absorptions polarized along the growth axis as for usual conduction intersubband transitions in quantum wells. Based on their energy position and their temperature dependence, the infrared resonances are respectively attributed to intraband transitions between confined holes and to bound-to-continuum transitions of electrons. The hole and electron intraband resonances respectively shift to high energy and low energy as the dot size is decreased. We then discuss excitation spectroscopy measurements of the infrared absorption and we will show that the intraband dipole moments can be deduced from saturation experiments. The assignment of the intraband transitions is further confirmed by separate infrared spectroscopic measurements performed on n-doped InAs/GaAs quantum dots. Finally, we will discuss potential applications to infrared photodetectors and lasers.
2:00 PM *C4.2
ZERO DIMENSIONAL EXCITION UNDER VARIOUS FIELD AND CHARGING CONDITIONS. K.H. Schmidt, U. Kunze, Werkstoffe der Elektrotechnik, Ruhr-Universität Bochum, Bochum, GERMANY; G. Medeiros-Ribeiro, Hewlett Packard Co., Palo Alto, CA; J.M. Garcia, P.M. Petroff, Materials Department, University of California, Santa Barbara, CA.
We have used photocapacitance, photocurrent and photoluminescence spectroscopy to study the dynamics and radiative recombination of optically excited electron hole pairs in InAs self-assembled quantum dots (QDs) under various field and charging conditions. For our investigations the InAs QDs are embedded in the intrinsic GaAs region of a Schottky diode with a n-doped GaAs back contact. By changing the bias it is possible to control the electric field and the number of charges in the QD. At high reverse bias the internal electric field is strong enough to overcome the excitonic binding energy and to separate the optically excited electron hole pairs. The electrons as well as the holes tunnel out the QDs. Due to the rapid loss of optically generated carriers the photocurrent signal is high while the PL signal almost vanishes in this field region. At low electric fields the holes as well as the electrons stay in the QD and recombine. Since the optically excited carriers remain in the QD, the photocurrent signal disappears. Instead, a good photoluminescence signal is detected. In this field regime tunneling of electrons from the back contact into the ground and first excited QD levels occurs which is indicated by peaks in the capacitance spectra. Thus, it is possible to study zero dimensional excitons in charged QDs. The number of charges in the dots can be controlled by capacitance spectroscopy while photoluminescence is an excellent technique to study the excitonic recombination. When the QDs are loaded with electrons from the back contact, the ground state transition in the PL spectra shows a red shift of about 15 meV. The energy shift is accompanied by a decrease in PL intensity and a broadening of the ground state transition. Field effects (Quantum Confined Stark effect) being responsible for the observed change of photoluminescence signal can be excluded since the energy shift does not appear at high fields. In addition, field effects would cause a blue shift of the ground state feature with decreasing field while the opposite behavior is observed in the experiment.
2:30 PM *C4.3
ELECTRONIC INTERACTIONS IN QUANTUM DOTS. Joerg P. Kotthaus, Ludwig-Maximilians-Universitaet, Munich, GERMANY.
Nanofabrication technologies make it possible to create cage-like quantum dots in semiconductor heterostructures in which the number of confined electrons can be voltage-tuned. Embedding InAs quantum dots created by self-assembled growth into suitable field-effect devices allows the injection of a few electrons into such quantum dots one by one.1 The few-electron ground states in their dependence on electron number and magnetic fields are studied with high resolution capacitance spectroscopy, which allows us to sample small ensembles of quantum dots2. Single electron charging of s-, p- and d-like energy shells is observed and investigated in its dependence on magnetic fields. The evolution of Coulomb charging energies with electron number and magnetic field is found to generally agree with a recent theoretical model3. On larger ensembles of about 108 quantum dots we can also study the electronic excitations with far-infrared spectroscopy.4 In particular, we observe a qualitative change in the electronic excitations when the p-like shell is filled with electrons, in agreement with theoretical expectations. Comparison of the infrared and capacitance spectra enables us to gain further insight into the effects of electronic interactions of such few-electron systems. Attempts to study the electronic interactions between neighboring quantum dots will be discussed.
3:30 PM *C4.4
PHOTOLUMINESCENCE SPECTROSCOPY OF SINGLE CDSE NANOCRYSTALLITE QUANUTM DOTS. Stephen Empedocles, David Norris, Moungi Bawendi, Massachusetts Institute of Technology, Dept of Chemistry, Cambridge, MA.
We use the techniques of single molecule spectroscopy to study the emitting state of single CdSe nanocrystallite quantum dots. Using far-field epifluorescence microscopy, we are able to image single quantum dots and obtain fluorescence spectra that are more than 50x narrower than what can be achieved using ensemble techniques. Resolution limited linewidths as narrow as 120eV demonstrate the true zero-dimensional nature of this unique physical system, reinforcing their description as ''artificial atoms''. The elimination of inhomogeneous spectral broadening reveals, for the first time, light driven spectral diffusion with shifts as large as 80meV observed over several minutes. Electric field studies on these single dots suggest a ''Stark'' mechanism behind these large shifts. In addition, single dot Stark studies reveal that the lowest excited state in these dots possesses both polar and polarizable character. This fact provides additional insight into the nature of this material and may require a reevaluation of many of the current theories on the excited states of these quantum dots. Individual dots show stark shifts as large as 60 meV under moderate fields strengths. These shifts are several orders of magnitude larger than the inherent linewidth, suggesting the potential use of these dots in optical switching devices.
4:00 PM *C4.5
QUANTUM CONFINEMENT AND TRAPPING IN GROUP IV SI AND GE NANOCRYSTALS. Howard W.H. Lee, Lawrence Livermore National Laboratory, Livermore, CA; Gildardo R. Delgado, Dept of Applied Science, University of California at Davis, Davis, CA; Susan M. Kauzlarich, Richard A. Bley, Boyd R. Taylor, Daniel Mayeri, Chung-Sung Yang, Dept of Chemistry, University of California at Davis, Davis, CA.
We report on and compare the optical properties of colloidal suspensions of Si and Ge nanocrystals that were made with different techniques and were surface-terminated with oxides or a variety of organic groups. Evidence is presented for the control of the particle size distribution and the complete passivation of the surface. Our results show a remarkable parallelism in the optical properties of Si and Ge nanocrystals. Both systems have blue and red photoluminescence (PL) peaks that show signs of quantum confinement as manifested, for example, by spectral shifts in size selective spectroscopy. For the appropriate particle sizes, tunable and efficient PL ranging from 320 to 460 nm and from 364 to 600 nm for Si and Ge nanocrystals, respectively, was observed. Comparison of our detailed optical results with recent pseudopotential and tight binding calculations indicates that the observed blue PL originates from quantum confinement effects on the bandedge emission from the nanocrystalline core, while the red PL likely results from traps. These conclusions are further supported by the observation of nearly identical optical behavior regardless of the nature of the surface passivation. The similarities between Si and Ge nanocrystals perhaps point to a consistency or general trend within Group IV indirect bandgap materials under quantum confinement. For optoelectronic devices, spectral shifts are important for the control of the emission wavelength by controlling the size and shape of the nanocrystals. Work at LLNL was perfomred under the auspices of the U.S. DOE by LLNL under contract No. W-7405-ENG-48.
4:30 PM *C4.6
NANOCRYSTALLINE SILICON SUPERLATTICES: FABRICATION, LUMINESCENCE AND CARRIER TRANSPORT. L. Tsybeskov, K.D. Hirschman, S.P. Duttagupta, M. Zacharias and P.M. Fauchet, Department of Electrical Engineering, University of Rochester, Rochester, NY; J.P. McCaffrey and D.J. Lockwood, Institute for Microstructural Sciences, National Research Council, Ottawa, CANADA.
Nanocrystalline silicon (nc-Si) superlattices have been produced by controlled recrystallization of amorphous Si/SiO2 multilayers. The recrystallization is performed by a two-step procedure: rapid thermal annealing (RTA) at 600C-1000C, and furnace annealing at 1050C. The samples are characterized by Raman scattering, electron diffraction, X-ray diffraction and X ray reflection. The TEM image of the recrystallized superlattice depicts an ordered structure, with Si nanocrystals confined between SiO2 layers. The size of the Si nanocrystals is determined by the thickness of the a-Si layer which we varied from 3 nm to > 10 nm, and their shape is nearly spherical. The thermal oxidation and the doping of the nc-Si superlattices are studied. The efficient room temperature photoluminescence (PL) of the nc-Si superlattices is shown to bc due to the spatial confinement and the increase of the binding energy of the free exciton. The increase in the PL efficiency in doped samples correlates with the increase in the intensity of no-phonon (NP) PL line, The carrier transport in the nc-Si/SiO2 multilayers is explained by field-assisted tunneling which is controlled by the thickness and quality of the SiO2 layers. The use of the nc-Si/SiO2 superlattices in quantum devices and practical light emitters will be discussed.
SESSION C5: POSTER SESSION:
SELF-ORGANIZED NANOSTRUCTURES
Chairs: Pierre M. Petroff and R. Stanley Williams
Thursday Evening, December 4, 1997
8:00 P.M.
America Ballroom (W)
C5.1
MECHANISM OF SELF-ORGANIZATION OF 3D CLUSTERS IN SiGe/Si MULTILAYERS, E. Mateeva, P. Sutter, M.G. Lagally, University of Wisconsin, Madison, WI; J.C. Bean, University of Virginia, Charlottesville, VA.
The spontaneous formation of small coherent 3-dimensional (3D) islands during the growth of SiGe on Si(100) is of particular interest as they are expected to exhibit the charge confinement properties of quantum dots. These ``hut'' islands were first observed by scanning tunneling microscopy [1] and studied further using atomic force microscopy (AFM)[2,3]. The AFM investigation showed that the ``hut'' islands undergo a transition from a broad distribution in size for a single layer of SiGe alloy on Si(100) to intraplanar size equalization in SiGe/Si multilayers. Based on the surface morphology studies a model was proposed to account for the self-assembled growth of the islands[3]. In the present work we provide the first direct observation of the vertically correlated arrangement of coherent clusters in SiGe/SI multilayers grown by molecular beam epitaxy (MBE). We study by cross-sectional transmission electron microscopy the evolution of the clusters to a uniform size distribution, which is found to involve two types of interaction between islands of subcritical size, as well as a pronounced stabilization of the island size when it reaches a critical value. Apart from the microscopic evolution of the arrangement of 3D clusters in a multilayer, we observe a change of shape of the islands from a {105}-faceted ``hut'' cluster to a rounded cluster upon overgrowth with Si. These findings are important in that they provide a pathway towards tailoring both the size and shape of the islands.
C5.2
STRUCTURE PARAMETERS FOR ELECTRON SPIN ORDERING IN InGaAs/GaAs COUPLED QUANTUM DOT SYSTEMS. Akira Sugimura, Ikurou Umezu and Itsushi Tadamasa, Dept of Applied Physics, Konan Univ. Kobe, JAPAN.
Quantum dot structure has attracted much interest because it has a possibility of showing new quantum phenomena. Till now, most of the research works have been restricted to the quantum effects inside indivisual dot. We previously showed that GaAs/AlGaAs quantum dot array has a possibility of exhibiting magnetic orders when the inter-dot spacing is short enough (Jpn. J. Appl. Phys. vol.29 L2463 '90). Dense dot distribution will be achieved more easily in self-organized quantum dot structures. We here discuss the structure parameters for the InGaAs/GaAs self-organized quantum dot system which shows the electron spin orderings. Energy levels were estimated using envelope function approximation for the cubic-shape dot as a function of the dot size. Electron transfer rate from a dot to its nearest neighbor was calculated by the overlap integral of the electron wavefunctions located on the neighboring dots. When two electrons are located on the same dot, Coulomb interaction occurs to cause the electron repulsion, whose energy was estimated by the Coulomb integral for the electron wavefunction confined in a dot. The calculated result indicated that there is a parameter region where the electron system can be described by the one band Hubbard model. We studied the magnetization of this electron system using the mean field approximation and obtained the phase diagram of the magnetic states. The result calculated for the dot system arranged in two dimensional square lattice indicated ferro-magnetic and antiferro-magnetic orderings when the dot widths and dot spacings are of the order of 10 nm. Phase transition temperature for these ordered states were also studied. The transition temperature increases as the dot spacing decreases. Finally, we will discuss the structure parameters and experimental condition for observing the spin-ordered state in this quantum dot system.
C5.3
ZnO NANOCRYSTALLINE THIN FILMS AS AN ULTRAVIOLET EMITTING MATERIAL. Akira Ohtomo, Masashi Kawasaki, Dept. of Innovative and Engineering Materials, Tokyo Inst. of Tech., Yokohama, JAPAN; Hideomi Koinuma, Materials and Structures Lab., Tokyo Inst. of Tech., Yokohama, JAPAN; P. Yu, Z. K. Tang, George K.L. Wong, Physics Dept., Hong Kong Univ. of Sci. and Tech., Kowloon, HONG KONG; Takashi Yasuda, Yuzaburou Segawa, RIKEN, Sendai, JAPAN.
C5.4
FABRICATION AND CHARACTERIZATION OF MOS CAPACITORS WITH SELF-ASSEMBLED SILICON QUANTUM DOTS AS A FLOATING GATE. Atsushi Kohno, Mitsuhisa Ikeda, Hideki Murakami, Seiichi Miyazaki and Masataka Hirose, Dept. of Electrical Engineering, Hiroshima University, Higashi-Hiroshima, JAPAN.
Charging and discharging characteristics of silicon quantum dots embedded in gate oxides of MOS capacitors have been studied to develop floating gate memory devices. Single-crystalline Si quantum dots with an areal density of 4x1011 cm-2 were self-assembled by LPCVD of pure SiH4 at 560C on 3.5nm-thick SiO2 thermally grown on p-Si(100) [1]. The average height and diameter of the as-grown Si-dots were evaluated to be 5 and 10nm, respectively, by AFM. The dot surface was covered by 1 nm-thick native oxide. Subsequently, a 3nm-thick amorphous Si layer was grown on the Si quantum dots by LPCVD at 440C and fully oxidized in dry O2 at 1000C to cover the Si-dots with a 7nm-thick oxide layer. Finally 300nm-thick n+ poly-Si gates were fabricated to complete MOS structures. For the gate bias from -3 to 6V with a sweep rate of 90mV/s, the C-V curve agrees with the theoretical one in which the Si-dots act as a 5nm-thick dielectric layer. On the other hand, as the gate bias is decreased from 6 to about 3V, the C-V curve shows a positive flat-band shift by 3.5V. In the bias range from 3 to 0.5V, the capacitance remains almost constant. Further decrease in the gate bias results in a positive flat-band voltage shift of 0.7V which indicates an electron density of 1x1012 cm-2 is stored in the dots and indicates a clear peak at -0.8V, beyond which the capacitance drops to coincide with the initial curve. No significant change in the hysteresis of C-V curves observed in this frequency range of 1-100kHz. This is interpreted in terms of electron charging and discharging of the Si-dots through the 3.5nm-thick bottom oxide. From the C-V curve measured in a steady state, an electron density of 4x1011 cm-2 is stably stored in the quantum dots. This indicates that in average one electron is charged on each of quantum dots.
C5.5
InAs SELF-ORGANIZED QUANTUM DOTS EMITTING IN THE 2 m RANGE. V.M. Ustinov, A.E. Zhukov, A.Yu. Egorov, A.R. Kovah, A.F. Tsatsul'nikov, and P.S. Kop'ev, A.F. Ioffe Physico-Technical Institute, St. Petersburg, RUSSIA; S. Ruvimov, Z. Liliental-Weber, and E.R. Weber, Lawrence Berkeley National Laboratory, Berkeley, CA.
When an InGaAs/InAlAs heterostructure on InP substrate is used as matrix for InAs quantum dots, it allows to achieve the practically important 2m optical emission range. The properties of quantum dots can be controlled through changing strain via controllable lattice mismatch and band-gap engineering via various combinations of heterobarriers and quantum wells. In this work the effect of barrier layers and effective thickness of deposited InAs on the structural and optical characteristics of InAs quantum dots on InP substrate has been studied. Coherently strained quantum dots are formed in this system within the 3-9 ML InAs coverage. The dot size in InGaAs has been found to be 3-4 times larger but the density about one order of magnitude smaller than that in InAlAs, which is attributed to the regimes of the InAIAs growth used here. PL emission wavelength as high as 2m at 77K has been demonstrated for the InAs/InGaAs quantum dots. The barrier band-gap strongly affects the optical transition energy of quantun dots, and the carrier localization energy is increased in a similar manner both for the InAs/InGaAs and InAs/InAIAs structures upon the quantum dot formation. Vertical coupling of quantum dots increases carrier localization energy as compared to individual dots similar to the InAs/GaAs system. Intentional barrier mismatch causes shifts in the PL line position due to changing the effect of strain on quantum dots. The lasing characteristics in this system are also discussed.
C5.6
PHONON BOTTLENECK IN ANNEALED SELF-FORMED InGaAs/GaAs QUANTUM DOTS. Kohki Mukai, Nobuyuki Ohtsuka, Hajime Shoji, and Mitsuru Sugawara, Fujitsu Laboratories Ltd, Kanagawa, JAPAN.
We proved clearly the existence of the phonon bottleneck, measuring carrier lifetimes in sublevels of self-formed InGaAs/GaAs quantum dots before and after annealing. The bottleneck was the phenomena in zero dimension where the carrier relaxation between discrete sublevels slowed down due to the requirement of phonon, energy of which agrees with the level spacing. The relaxation lifetime calculated varies from less than picosecond to more than nanosecond among theories, and experiments have not yet finished the arguments. Measuring time-resolved photoluminescence, we determined the carrier relaxation and the recombination lifetimes at the ground and the excited sublevels of the dots. In as-grown samples, we found that the carrier relaxation lifetime varied between 1E-9 and 1E-11 s depending on temperature and level order, that is quite longer than that in systems of higher dimensionality (e.g., 1E-14 s in GaAs bulk and 1E-13 s in GaAs quantum well). Annealing at over 600°C caused intermixing and drastically reduced the emission intensity of the ground level at 300 K while the intensity of the second level was not strongly influenced. The change of relative emission intensity suggests that the proportion of the radiative recombination lifetimes and the relaxation lifetimes changed. We confirmed that though the recombination lifetimes of as-grown dot were independent of temperature (1E-9 s), the recombination lifetime at the ground level became temperature-dependent and almost the same as the relaxation lifetime at the second level after annealing while the recombination lifetime at the second level was not affected much. The relaxation lifetimes at the second level decreased slightly after annealing. We simulated the luminescence spectra with the measured lifetimes, and found that the slow carrier relaxation made the relative emission intensity of sublevels quite sensitive to the radiative recombination lifetime.
C5.7
NANOCRYSTALS OF RARE EARTH CHALCOGENIDES: PREPARATION, PHOTOPYSICAL CHARACTERIZATION AND ELECTROOPTICAL PROPERTIES. Yongchi Tian, Frederic Guerin and Janos H. Fendler, Center for Advanced Materials Processing, Clarkson University, Potsdam, NY
We report here the study of a new family of nanoparticles, rare earth chalcogenides (Re2E3: Re=Nd, Pr, Eu, Tb, Dy; E=S, Se). The insertion of highly localized f-electrons in between the valence band and conduction band, and the interplay between the f-electrons and the free electrons have been shown to bring about unique and fascinating properties. The transparent sol of Re2E3 was synthesized in water, ethanol and 2-propanol media. Quantum size effects were observed during particle growth and at the final stage in terms of blue shift of band-to-band transition edge and f-to-band transition as well. A simplified molecular orbital scheme is proposed to interpreted the effects. Photoexcited luminescence of the particles took place via two photophysical pathes: free charge carriers recombination and f-to-f localized transition. The two kinds of transition events are seen to interact each other, which leads to size-dependent fluorescence emissions. TEM, XRD, NSOM and TDA were used to characterize the nanocrystals.
C5.8
EFFECTS OF As/P EXCHANGE REACTION ON THE FORMATION OF InAs SELF-ASSEMBLED QUANTUM DOTS GROWN ON (001) InP BY LP-MOCVD. Sukho Yoon, Young-Boo Moon, Tae-Wan Lee, and Euijoon Yoon, School of Materials Science & Engineering and Inter-university Semiconductor Research Center, Seoul National University, Seoul, SOUTH KOREA.
Growth of InAs self-assembled quantum dots (SAQDs) on InP is very promising for long-wavelength optoelectronic device applications. Strained InAs quantum structures on InP have advantages due to its smaller effective masses of the carriers than those grown on GaAs substrates. Nevertheless, there has been few results reported in the growth of InAs SAQDs on (001) InP since it is expected that the As/P exchange reaction may degrade the InAs/InP interface quality. The As/P exchange reaction is responsible for the degradation of the interface abruptness in many heterostructures. It is not clear, however, how and to what extent the As/P exchange reaction affects the formation of SAQDs.
In this study, InAs SAQDs were grown on (001) InP at various V/III ratios and growth temperatures by low pressure metalorganic chemical vapor deposition (LP-MOCVD). Changes in size distribution and density with various degrees of As/P exchange reaction were analyzed by atomic force microscopy (AFM) and photoluminescence (PL). InAs SAQDs with a high density (4.5x1010/cm2) and a uniform size (45nm10) were obtained as In surface migration and As/P exchange reaction were suppressed at a condition of low temperature (550C) and low V/III ratio (V/III=30). Low-temperature PL measurement for the InP-capped SAQDs confirmed the presence of SAQDs. On the other hand, the island density decreased and the island size increased drastically at a higher V/III ratio (V/III=300) or at a higher temperature (600C). It is presumed that the strain fields developed at the island edges and between islands enhance the As/P exchange reaction to reduce the total strain energy. Additional supply of surface In atoms from the As/P exchange reaction at high V/III ratios is responsible for the enhanced three-dimensional growth. Effects of other process parameters on the formation of InAs SAQDs will be reported.
C5.9
GROWTH AND CHARACTERIZATION OF SELF-ASSEMBLED InAs AND InAsxP1-x DOT STRUCTURES ON InP GROWN BY METALORGANIC VAPOUR PHASE EPITAXY. Niclas Carlsson, Tobias Junno, Lars Samuelson and Werner Seifert, Solid State Physics, Lund University, SWEDEN.
The formation of self-assembled InAs and InAsxP1-x dots on InP have been studied, especially for deposition conditions where mainly coherent dots are developed. The samples were grown by metalorganic vapour phase epitaxy. Morphological investigations were performed by atomic force microscopy (AFM), with the instrument working in contact mode as well as in non-contact mode. Surface densities and height distributions were extracted, as function of growth conditions. It is found that the average dot height decreases with increasing InAs deposition in the thickness range where coherent dots are formed. For a nominal deposition of 1.5 ML InAs, an average dot height of 7ñ8 nm was found, together with a surface density of 3 x 109 dots/cm2. When the InAs deposition was increased to 3.0 ML the average height was reduced to 5ñ6 nm while the surface density increased to 3 x 1010 dots/cm2. By AFM measurements with an extremely sharp tip under non-contact mode operation the lateral dot size could be estimated. The dots are elongated in the [-110] direction, having a typical width of 45ñ50 nm in this direction. In the [110] direction the width is smaller, typically 35ñ40 nm. Photoluminescence (PL) measurements were used in addition, for investigations of the optical properties of capped InAs dots, formed under equivalent conditions. Comparisons between the two characterization techniques, show a qualitative agreement with respect to the planar dot density as well as the size homogeneity. It is also indicated that dots of binary InAs can essentially be formed at deposition temperatures not higher than about 500C. Elevated temperatures in this process result in an unintentional alloying mechanism due to exchange reactions at the interface, leading to the formation of ternary InAsxP1-x dots, which can bee seen as a simultaneous increase in the light emission energy and the average dot size, indicating the widening of the energy gap in the quantum dots, which counteracts the decreased energy quantization in the larger dots formed at higher temperatures.
C5.10
RECOMBINATION PROCESSES IN InAs/GaAs SELF-ASSEMBLED SINGLE QUANTUM DOTS. Hong-Wen Ren, Mitsuru Sugisaki, Shigeo Sugou, Yasuaki Masumoto, Single Quantum Dot Project, ERATO, JST, JAPAN; Kenichi Nishi, Opto-electronics Research Labs, NEC Corp, Tsukuba, JAPAN.
Self-assembled semiconductor quantum dots (SADs) exhibit distinct properties due to three dimensional confinement. They are expected to be suitable for fabricating novel optical and electronic devices. In order to study the energy quantization and recombination processes in SADs in relation to their geometric structure, composition and strain distribution, we prepared InAs/GaAs SADs on GaAs (100) vicinal surfaces by gas-source molecular beam epitaxy. The amount of InAs deposition was fixed to be 1.7 monolayers which was confirmed by RHEED transition from streaky pattern to spotty, while the InAs island size was controlled by growth temperature. Moreover, the temperature for growing the GaAs cap-layer was found to greatly modify the SAD structure. By suppressing indium segregation, we obtained samples containing SADs with average diameters 35, 28, 22, and 15nm, respectively. Photo-luminescence (PL) studies revealed that the number of peaks from the dots was five, three, two and one, respectively. From excitation power dependence of PL spectra, these peaks are considered to be ground and excited states of SADs rather than multiple ground states from dots of different size distributions. The ground state energy increased but the energy separation between neighboring states did not show obvious increase with decreasing dot size probably due to reduced confining barrier and leakage of wavefunctions. By lithographic wet etching of the samples with low surface SAD densities, square mesas 1 and 3m in sizes were obtained. Micro-PL of the mesa showed a few clusters consisting of well-resolved emission lines. The line widths were below the spectral resolution of 0.3 meV. Excited states were found to be degenerated. The intensities of the emission lines were saturated gradually with increasing excitation intensity. Origins on the recombination processes in single qua ntum dots were analyzed.
C5.11
SELF-ORGANIZED Ge NANOCRYSTALS PRODUCED IN SiO2 LAYERS BY ION BEAM SYNTHESIS. K.H. Heinig, B. Schmidt, R. Groetzschel, A. Markwitz, J. von Borany and M. Strobel, Forschungszentrum Rossendorf, Dresden, GERMANY.
It is an attractive goal to combine, on the same substrate, the excellent data processing performance of Si-based electronic devices with the unrivaled capability of light in the transmission of information. The photo- and electro-luminescence from Ge+ implanted SiO2 layers on Si wafers offers such a possibility. The understanding of the formation and self-organization of Ge nanocrystals during Ge+ implantation and subsequent thermal treatment is a prerequisit for a physically guided optimization of the luminescence.
Here we present systematic experimental studies and comprehensive computer simulations of the ion beam synthesis of Ge nanocrystals. 100 and 500 nm thick thermally grown SiO2 layers have been implanted with Ge+ cm-2 and annealed at T=600C for t= min in different ambients. The Ge redistribution and nanocrystal formation have been studied by RBS and cross-section TEM. For 500 nm thick SiO2 layers the annealing in N2 and Argon leads to a bimodal Ge depth distribution and a decoration of the Si/SiO2 interface by Ge. Samples annealed in Ar+7%H2 behave different: Due to strongly enhanced Ge diffusion we observed only one narrow band of huge Ge nanocrystals at lower T. For 100 nm thick SiO2 layers we found additionally a narrow band of Ge nanoclusters close ( 10 nm) to the Si/SiO2 interface. This band forms when at the Si/SiO2 interface the as-implanted Ge concentration is sufficiently high. By means of kinetic 3D lattice Monte-Carlo simulations and a direct integration of rate equations of growing nanoclusters it will be shown that some aspects of the Ge redistribution and Ge nanocrystal ordering can be explained by self-organization of nanoclusters duri ng nucleation and Ostwald ripening.
C5.12
RESONANT TUNNELING SPECROSCOPY OF INHOMOGENEOUS STRAIN RELAXATION IN INDIVIDUAL QUANTUM DOTS. C. D. Akyüz, Department of Physics, Brown University, Providence, RI; A. Zaslavsky, Division of Engineering, Brown University, Providence, RI ; L. B. Freund, Division of Engineering, Brown University, Providence, RI; D. A. Syphers, Physics Department, Bowdoin College, Brunswick, ME; T. O Sedgwick, SiBond L. L. C., Hopewell Junction, NY.
We have for the first time probed the inhomogeneous strain relaxation in individual quantum dots with transport measurements. Since in a bulk lattice-mismatched heterostructure, the strain is biaxial and homogeneous, its effects in large devices can be calculated explicitly. On the other hand, in nanostructures, the free surface allows the relaxation of the built-in strain removing the translational invariance and leading to new and interesting electronic properties.
Our low temperature (T = 1.7 K) resonant tunneling I(V) and dI/dV data of strained p-Si/Si0.75Ge0.25 devices with nominal diameters, D, in the range of: , shows shifts in the heavy- and light-hole resonant peaks corresponding to an average strain relaxation. In devices with , the heavy-hole peak develops additional fine structure. The light-hole peak develops a similar fine structure at ; at which point the heavy-hole peak breaks up into two major steps. We attribute the development of fine structure in the resonant current with decreasing D to lateral quantization of holes in the inhomogeneous-strain-induced confinement potential. The magnetotunneling measurements in parallel field () reveal that the fine structure in the I(V) is quenched at T.
We calculate the strain distribution by a finite element simulation based on a linear elastic model and estimate the confining potentials for various D with the usual six-band Luttinger-Kohn Hamiltonian. In structures with , the confining potential for the heavy-hole subband has the form of a 2D parabola with a sharp ring-like perturbation along the perimeter, while for , the confinement becomes entirely ring-like.