Meetings & Events

Spring 1999 logo1999 MRS Spring Meeting & Exhibit

April 5-9, 1999 | San Francisco
Meeting Chairs: Katayun Barmak, James S. Speck, Raymond T. Tung, Paul D. Calvert

Symposium W—Semiconductor Quantum Dots


Paul Alivisatos 
Dept of Chemistry 
Univ of California-Berkeley 
B62 Hildebrand Hall #1460 
Berkeley, CA 94720-1460 

Anupam Madhukar
Dept of Matls Science & Physics
Univ of Southern California
Photonic Matls & Dev Lab VHE506
Los Angeles, CA 90089-0241

Alex Zunger
National Renewable Energy Lab
Golden, CO 80401-3393

Proceedings published as Volume 571 
of the Materials Research Society 
Symposium Proceedings Series.
* Invited paper
Chairs: Louis E. Brus and Michel Lannoo 
Monday Morning, April 5, 1999 
Golden Gate C2 (M)
8:30 AM W1.1 
ULTRAFAST ELECTRON DYNAMICS IN Ge QUANTUM DOTS. A. Stella , P. Tognini, Dipartimento di Fisica, A. Volta, Universite di Pavia, Pavia, ITALY; S. De Silvestri, M. Nisoli, Dipartimento di Fisica, Politecnico, Milano, ITALY; P. Cheyssac, R. Kofman, Laboratoire de Physique de la Matiëre Condensée, Université de Nice-Sophia Antipolis, Nice Cedex, FRANCE.

Ultrafast electron dynamics in Ge quantum dots, grown by means of an evaporation condensation technique described in details elsewhere [1], has been analyzed using a pump and probe technique. The systems investigated are constituted by Ge nanoparticles embedded in a transparent amorphous Al2O3 matrix on sapphire substrates. Preliminary TEM and optical characterization allowed to obtain the size distribution of the nanocrystals and the static optical functions of the composite films. The pumping Ti:sapphire laser provided pulses of (150 fs duration at 780 nm (fundamental frequency), 390 nm (2nd harmonic) and 260 nm (3th harmonic), with energies up to 750  J and 1 KHz repetition rate. The samples were probed in the wavelength range 450 - 750 nm, at variables delays with respect to the pump (from 0.4 up to 40 ps). In such a configuration, it has been possible to study the electron and hole dynamics, involving the electronic density of states in a wide energy range, i.e. in a wide portion of the Brillouin Zone, during the relaxation process which leads to carrier thermalization. The main results can be summarized as follows: - fundamental role of the renormalization process also above the band gap in explaining the response (which is basically different from what observed in metallic nanoparticles [2]) after the initial subpicosecond regime; - transient transmissivity data amenable to a bulk-like crystalline band structure down to 5 nm (less than Bohr radius of fundamental excitons); - evidence of E1 - E1+(1 structures, whose partially excitonic structure can be analyzed as a function of energy and time; - initial signatures of quantum confinement both in subpicosecond and picosecond regimes. 

8:45 AM W1.2 

BREAKDOWN OF  k-CONSERVATION RULE IN Si NANOCRYSTALS. D. Kovalev , H. Heckler, G. Polisski, F. Koch, Technische Universitaet Munchen, Physik-Department, Garching, GERMANY; M. Ben-Chorin, Weizmann Institute of Science, Department of Chemical Physics, Rehovot, ISRAEL.

We show that light emission from different systems of silicon nanocrystals does behave as expected for indirect bandgap quantum dots. Photoluminescence excited on the low energy part of the distribution of Si nanocrystals exhibits a set of narrow peaks associated with Si TA- and TO- momentum conserving phonon-assisted optical transitions. These spectra allow us to determine the ratio of no-phonon transitions to TA- and TO-phonon assisted processes over a wide range of confinement energies. The ratio between these recombination channels changes by two orders of magnitude with increasing confinement energy. For confinement energies above 0.7 eV the radiative transitions are governed by no-phonon quasi-direct processes.

9:00 AM W1.3 
FEMTOSECOND SPECTROSCOPY OF SI AND GE QUANTUM DOTS. Howard W.H. Lee , Lawrence Livermore National Laboratory, Livermore, CA; Peter A. Thielen, Dept of Applied Science, Univ of California at Davis, Livermore, CA; B.R. Taylor, Chung-Sung Yang and Susan M. Kauzlarich, Dept of Chemistry, Univ of California at Davis, CA.

The electronic structure and dynamical processes of indirect bandgap semiconductor quantum dots (QDs) are interesting due to the complexity of the physics, the difficulty in obtaining useful experimental results, and the increasing interest in technological applications. Unlike direct bandgap QDs, steady-state linear optical spectroscopy reveals little useful information due in part to the indirect nature of the lowest optical transition of the bulk material. We present results of femtosecond pump-probe spectroscopies on Si and Ge QDs including femtosecond hole burning, differential transmission, and photoluminescence decays. The QDs were fabricated via a chemical synthetic route that provides excellent surface passivation with various organic alkyl groups. These QDs were characterized by electron microscopies and steady-state linear optical spectroscopies which reveal moderate size distributions and quantum confinement. The femtosecond hole burning and differential transmission experiments indicate an electron-phonon interaction, as manifested by the temperature dependent linewidth and Stokes shifts, that is smaller than indicated by steady-state optical spectroscopy. This result agrees with tight binding models that predict Stokes shifts of a few meVís. Features in the femtosecond hole-burning are interpreted as excited state electronic structure that shift with particle size. Our results also address the controversial issue of the origin of light emission reported in Si and Ge QDs. We observed a rapid localization (100s fs) of the initial electron-hole excitation into several trap states and their subsequent sequential relaxation. Our results support a model in which low energy emissions (red, blue, green) result not from direct recombination processes but rather from trap emission. A specific UV-blue emission exhibit dynamics more representative of direct recombination processes from the QD bandedge. Work at LLNL was performed under the auspices of the U.S. DOE by LLNL under contract No. W-7405-ENG-48.

9:15 AM W1.4 
TWO-PHOTON-EXCITED PHOTOLUMINESCENCE FROM SILICON NANOCRYSTALS. J. Diener , Y.R. Shen1, D.I. Kovalev, G. Polisski and F. Koch, Technische Universitát München, Physik-Department, Garching, GERMANY; 1Department of Physics, University of California Berkeley, CA.

The optical properties of semiconductor nanocrystals are currently intensively investigated in order to understand the role of quantum confinement effects in these systems. A wide variety of nanostructured semiconductors emitting light from the blue down to the near infrared spectral region are nowadays available and they are promising candidates for novel electro-optical or even biological applications. In many of the nanoparticle systems the photoluminescence (PL) polarization memory effect is observed. The essence of the effect is the linear polarization of the PL parallel to the polarization vector of the exciting linearly polarized light. Analysis of polarization properties of the PL provides certain information about the structure of the electronic states in the nanoparticles as well as about the shape of the crystallites. We have studied generation of visible PL by two-photon excitation in porous Silicon (PSi) using a tunable pulsed infrared source. While the main properties of the photoluminescence excited by one- and two-photon excitations are identical, the degree of polarization of the photoluminescence in the two-photon case is significantly higher and depends on the orientation of the polarization vector of the incident light relative to the fundamental crystallographic axes of Silicon in the (100) plane of the sample. The enhancement has a maximum if the vector polarization of the exciting light is along the [110] direction in the (100) plane for specimen prepared from (100) silicon substrate. This is caused by an angular dependence of the two-photon absorption by aspherical nanocrystallites in PSi resulting from an anisotropic third order nonlinear susceptibility .

9:30 AM W1.5 

The accurate prediction of the optical bandgap of semiconductor nanocrystals is an important challenge for material physicists. Classically, it requires the evaluation of two quantities: the quasiparticle gap and the excitonic correction. The quasiparticle gap is usually approximated by the one-electron gap obtained from ab initio local density or semi-empirical techniques. To check the accuracy of such predictions, one needs to go beyond these approximations using for example the so-called GW method which allows to calculate the self-energy operator. The application of GW using ab initio wave functions defined in plane wave basis becomes extremely heavy for clusters. We thus propose a model GW formalism based on the tight binding framework. We verify that it gives correct results for the self-energy corrections of bulk Si, Ge and diamond. Applying the method to clusters containing up to 300 atoms, we show that the self-energy correction increases with the confinement. We identify two contributions in the correction. The first one comes from the well-known electronic relaxation in finite systems, euivalent to the electrostatic image charge effect. The second one, which is smaller, varies linearly with the inverse dielectric constant of the crystallite. We show that the excitonic interaction also contains an image charge contribution which almost compensates the equivalent term in the self-energy correction. We conclude on the accuracy of semi-empirical and ab initio calculations of the bandgap.

10:15 AM W1.6 
NEW LIGHT ON THE OPTICAL PROPERTIES OF Si NANOCRYSTALS EMBEDDED IN SiO2. M.L. Brongersma , P.G. Kik and A. Polman, FOM Institute for Atomic and Molecular Physics, Amsterdam, THE NETHERLANDS; K.S. Min, E. Boer, T. Tambo, H.A. Atwater, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology.

Ion implantation is a well-established technique to form Si nanocrystals in a SiO2 matrix in a controlled way. However, many issues concerning the optical properties of these nanocrystals have remained unsolved. In this work, we have synthesized 1-5 nm diameter Si nanocrystals in thermally grown SiO2 films by 35 keV Si ion implantation followed by thermal annealing at 1100C. Using high-resolution TEM, RBS, XPS, PL spectroscopy, as well as selective passivation, etching and oxidation experiments we determined: 1) the depth distribution of luminescent nanocrystals, and find a large concentration of small nanocrystals near the two interfaces of the oxide film. 2) that a significant contribution of the luminescence originates from irradiation-induced defects, that can be passivated by introduction of H or D. 3) that the nanocrystal size can be decreased by oxidation, and the PL emission wavelength tuned all over the spectrum from 500 to 1100 nm. 4) that a large exchange splitting (about 15 meV) of the energy levels of the quantum confined excitons must be taken into account to describe the dependence on temperature of the PL intensity and lifetime. 5) that interaction and excitation migration between nanocrystals may affect the luminescence lifetime and spectral shape. The results may be used to engineer the size distributions and optical properties of Si nanocrystals for a variety of applications, like nanocrystal based waveguide lasers, single electron memories, and LED's, as will be discussed in this presentation.

10:30 AM W1.7 
LUMINESCENCE OF Si QUANTUM DOTS EMBEDDED IN SiO2: CORRELATION WITH DOT SIZE DISTRIBUTION. S. Coffa , S. Lombardo, C. Spinella, CNR-IMETEM, Catania, ITALY; C. Gerardi, F. Ferrari, STMicroelectronics, Central R & D, Catania, ITALY.

We have studied the photoluminescence of Si quantum dots embedded into an SiO2 matrix. The dots have been obtained by high temperature annealing of silicon rich oxides (SRO) prepared either by Si ion implantation in thermally oxidized Si substrates or by plasma-enhanced chemical vapor deposition (PECVD) of SiH4 and O2. Transmission electron microscopy (TEM) analysis in high resolution, bright, and dark field allows a clear determination of dot size distribution under different preparation conditions. Photoluminescence (PL) measurements from 300 to 1700 nm were performed using an Argon ion laser as the excitation source and varying the sample temperature in the range 10 K-300 K. Analysis of temperature dependence of the luminescence yield and time decay measurements have been used to determine the strenght of radiative and non radiative recombination processes. Luminescence and dot size distribution are expected to be strongly correlated. In fact, it has been theoretically shown that band-gap width and radiative oscillator strenght are determined by the dot size. Therefore, by assuming that a dot of a given size is characterized by a well-defined and sharp luminescence spectrum, we expect that the broad experimental luminescence is the sum of the contribution of dots of different sizes weighted by the dot size distribution. In this work, based on the experimental dot size distribution, we calculate the theoretically predicted luminescence lineshape. The comparisons with the experimental luminescence spectra in a wide dot size range (1-10 nm) are shown and discussed.

10:45 AM W1.8 
INTRABAND ABSORPTION IN Ge-Si SELF-ASSEMBLED QUANTUM DOTS. P. Boucaud , V. Le Thanh, S. Sauvage, D. Debarre, D. Bouchier, IEF, Universite Paris XI, Orsay, FRANCE; F. Fortuna, CSNSM, Universite Paris XI, Orsay, FRANCE.

We have observed intraband absorption in Ge-Si self-assembled quantum dots. The quantum dots are grown by ultra-high-vacuum chemical vapor deposition on Si(001) using silane and germane as gas precursors. The structural parameters of the quantum dots are investigated by cross section transmission electron microscopy and atomic force microscopy. It is found that the quantum dots have a square-based pyramidal shape. Intraband absorption in the valence band is observed around 300 meV using a photoinduced absorption spectroscopy technique. The intraband absorption which is in-plane polarized is attributed to bound-to-continuum transitions since the intraband energy corresponds to the energy difference between the Si band gap and the photoluminescence energy of the quantum dots. The magnitude of the photoinduced intraband absorption saturates when the ground level of the quantum dots is filled. A very large value (2 x 10-13cm2) of the in-plane absorption cross section of the intraband transition is deduced from the experiment. Potential applications of this intraband absorption for quantum dot infrared photodetection at normal incidence will be discussed.

11:00 AM W1.9 
INTER-SUB-LEVEL SPECTROSCOPY OF P-TYPE MODULATION-DOPED GE QUANTUM DOTS. J.L. Liu , W.G. Wu, G. Jin and K.L. Wang, Device Research Laboratory, Department of Electrical Engineering, University of California at Los Angeles, CA.

Recently, the growth of self-organized Ge quantum dots by the Stranski-Krastanow process and their optical properties have attracted much attention. Besides the interband optical properties of the quantum dots that have been widely reported in the literatures, the inter-sub-level absorption in these quantum dots is of interest since most of the band gap discontinuity (between Si and Ge) goes to the valence band as well as the small hole effective mass favors hole inter-sub-level transitions for mid-infrared applications. In this presentation, we will present inter-sub-level spectroscopy of p-type modulation-doped Ge quantum dots. A series of the samples, which consist of 30 periods of Ge quantum dots with various size stacked with 6 nm boron-doped Si layers, are grown on Si (100) substrates by a solid source molecular beam epitaxy. Cross-sectional transmission electron microscopy and atomic force microscopy are used to characterize the structural properties of these Ge dots. The inter-sub-level transitions in these Ge dots are then studied by using Raman spectroscopy and Fourier transform infrared spectroscopy. The observed absorption peak position in the mid-infrared range dependence of dot size reveals the nature of the transitions, which is strongly related to the first two quantized heavy hole states of the Ge quantum dots. The polarization dependence measurement is also used to study the polarization selection rules of inter-sub-level transitions, indicating that the selection rules strongly depend on the Ge dot spatial distribution and the symmetry of the levels involved in the transitions. This study provides an impetus for developing improved Si-based mid-infrared detectors.

11:15 AM W1.10 
DIRECT EVIDENCE OF QUANTUM CONFINEMENT EVALUATED FROM SIZE-DEPENDENCE OF THE PHOTOLUMINESCENCE OF SILICON QUANTUM WIRES. D.P. Yu , Z.G. Bai, J.J. Wang, Y.H. Zou, W. Qian, H.Z. Zhang, Y. Ding and S.Q. Feng, Department of Physics, National Key Laboratory of Mesoscopic Physics, Peking University, Beijing, CHINA; L.P. You, J. Xu, Electron Microscopy Laboratory, Peking University, Beijing, CHINA.

The recent success of bulk synthesis of pure nano-scale silicon quantum wires (SiQW's) enables us to evaluate their photoluminescence (PL) characteristics under ultra-violet photo-excitation. The as-grown deposit was found grown into bundles of millimeter-size. Each bundle consists of pure silicon quantum wires at an average diameter around 13 nm and length up to hundreds of micrometers. To evaluate the size-dependence of the quantum confinement of the SiQW's, the samples were oxidized at 700C for different time in air to refine the diameter of the SiQW's, and the corresponding PL were measured. The PL spectra correspond to samples oxidized for 5 min, 10 min, and 15 min, with an average diameter of the crystalline silicon core around 7 nm, 5.5 nm, and 4.5 nm, respectively. Intensive multiple light emissions ranging from dark red, green, to blue regions were evidenced for the partially oxidized SiQW's samples. The red band has a blueshift of about 30 nm upon oxidation. Paramagnetic defects were revealed by electron paramagnetic resonance (EPR) spectrum for as-grown SiQW's, which were believed to quench the PL. Oxidation procedure in one hand annealed these defects, and on the other hand diminished the mean diameter of the quantum wires. As a result, the red emission band increases in intensity with increase of oxidation time. The red light emission was ascribed to quantum confinement effect originating from the crystalline core part of the SiQW's which is seemingly mediated by interface states. However, the PL emission from green to blue is found to be definitely unrelated to quantum confinement, instead they are attributed to the radiative recombination from defect centers in the over-coating layer of the amorphous silicon oxide outside the SiQW's, because green-blue emissions are the characteristic of the completely oxidized sample in which the red emission disappeared completely.

11:30 AM W1.11 
ENCAPSULATING SiGe 3-D ISLANDS. D.E. Savage , J.S. Sullivan, K. Moloni, F. Flack, J.R. Goomey, E.M. Rehder, T. Larsen, T.F. Kuech, L. McCaughan and M.G. Lagally, Univ. of Wisconsin, Madison, WI.

Self-assembled dislocation free {105} faceted 3-D islands of SiGe are readily formed on Si(001) through the modified Stranski-Krastanov growth mode. While narrowing of the island size distribution and lateral ordering of islands have been demonstrated in superlattices, island shape is not always maintained during overgrowth by Si. For example, recent LEEM results have shown that the combination of elevated temperature and Si deposition destabilizes the islands by incorporating Si into (105) facets and, therefore, changing island composition, size, and shape[1]. In this paper we investigate the morphology of SiGe islands formed in superlattices using UHV chemical vapor deposition as a function of the temperature at which the islands are initially encapsulated. Buried-island morphology is obtained using cross-sectional TEM. The shape is well maintained for encapsulation at or below 525C, while at higher temperatures islands expand laterally and are less distinct. Destabilization is associated with the disordering of the reconstruction of the (105) facet. An atomistic model for destabilization will be presented. The importance of controlling buried island morphology is demonstrated by photoluminescence and optical absorption measurements that show variation with buried island size, shape, and composition. * Research supported by NSF-MRSEC. [1] P. Sutter and M. G. Lagally, Phys. Rev. Lett. 81, 3471 (1998).

11:45 AM W1.12 
LOW TEMPERATURE Si DOT THIN-FILM-TRANSISTOR MEMORY. Kazumasa Nomoto , Dharam Pal Gosain, Takashi Noguchi, Setuso Usui, Yoshifumi Mori, Sony Corporation, Research Center, Yokohama, JAPAN.

We present a novel poly-Si Thin-Film-Transistor (TFT) based memory with Si-nano-crystal (Si dot) floating gate (FG) fabricated on a quartz subtrate at temperatures 300-400C compatible with the use of a low-cost glass substrate. This fact opens up possibility of consisting of a display and a memory in a single panel. In a conventional EEPROM or Flash memory, a FG consists of a poly-Si plate. Since a charge extends in such a continuous FG, charges stored in the FG can flow through defects at any positions. Therefore we cannot apply a continuous FG to a memory with a leaky tunnel barrier fabricated at low temperature. If discrete Si dots are used as an FG, we can hold charges in Si dots surrounded by a defect-free part of an oxide. Therefore we consider that a memory device with Si-dot-based FG is promising for a low temperature NVM (LTNVM). For fabrication of an LTNVM, we developed a novel technique to fabricate Si dots, in which XeCl excimer laser annealing of Si-rich oxide film induces a separation of excess Si as Si dots in SiO2 at room temperature. The densiy of Si dots is controlled by Si/O atomic ratio in a Si-rich oxide film. Their size distribution is controlled by the laser energy density. A preliminary device shows threshold voltage shift of > 1 V with a gate voltage pulse of 20 V, 10 ms. Data retention time is 103 s at room temperature. The device works in 104 write/erase cycles without considerable degradation. 

Chairs: Moungi G. Bawendi and Alex Zunger 
Monday Afternoon, April 5, 1999 
Golden Gate C2 (M)
1:30 PM *W2.1 
SPECTROSCOPY OF COLLOIDAL InP AND STRAIN-INDUCED S-K GaAs QUANTUM DOTS. Arthur J. Nozik , Olga I. Micic, Dietrich Bertram, Mark Hanna.

Quantum dots of colloidal InP in the size range of 25 to 75 , prepared by colloidal chemistry , can be studied as isolated nanocrystals in colloidal solution, as closed-packed quantum dot arrays in films, as quantum dots buried in a larger bandgap semiconductor matrix, or as isolated single quantum dots. GaAs quantum dots can be produced by strain-induced 3-D confinement in GaAs/AlGaAs quantum wells created by S-K growth of InP stressor islands on top of the quantum well barrier layer. Various cw and time-resolved spectroscopic measurements have been made on these various III-V quantum dot configurations to obtain information on their electronic structure, carrier relaxation dynamics, inter-dot electronic coupling and transport, energy transfer, and optical properties. These results will be presented and discussed.

2:00 PM W2.2 
SPECTROSCOPY OF SINGLE CdSe NANOCRYSTALS. Stephen Empedocles , Robert Neuhauser, Kentaro Shimizu and Moungi Bawendi, M.I.T., Dept of Chemistry, Cambridge, MA.

We use the techniques of single molecule spectroscopy to study the emission from single CdSe nanocrystallite quantum dots. Inhomogeneous spectral broadening which results primarily from the distribution of nanocrystal sizes is eliminated, leading to the discovery of new and previously unexpected spectral phenomena. Surprisingly, single nanocrystal spectra are seen to shift in energy as much as 80meV over short periods of time. Single nanocrystal Stark measurements indicate that spectral shifting is the result of large local electric fields which are thought to result from ionization of the nanocrystals over time. Polarization spectroscopy of single nanocrystals allows the direct measurement of individual transition dipoles. Data suggest that emission proceeds through a degenerate transition dipole oriented in the x-y plane of the nanocrystal. The 2-dimensional nature of the transition dipole allows the direct determination of the 3-dimensional orientation of individual nanocrystals.

2:15 PM W2.3 
PSEUDOPOTENTIAL THEORY OF EXCITED STATES IN SEMICONDUCTOR QUANTUM DOTS. Alberto Franceschetti , L.W. Wang, H. Fu and A. Zunger, National Renewable Energy Laboratory, Golden, CO.

We present a unified theoretical approach to the calculation of excited states in semiconductor quantum dots - including single excitons, multi excitons and charged excitons. Our method is based on a many-body expansion of the excited-state wavefunctions in terms of Slater determinants obtained from single-particle pseudopotential wavefunctions. This method permits to fully include intra-configuration electron-hole, electron-electron, and hole-hole Coulomb and exchange interactions. In addition, correlation effects can be included in a controlled way by allowing configuration interaction. We apply our method to the calculation of excited states of CdSe and InP nanocrystals in the strong-confinement regime. We find that the electron-hole Coulomb energy does not scale as 1/R with the nanocrystal radius R, as previously thought, and that the electron-hole exchange energy does not scale as 1/R3. Instead, the exchange interaction is characterized by a large, previously neglected long range component, which is responsible for the experimentally observed  scaling of the red-shift between the emission and the absorption peaks in zinc-blende nanocrystals. We discuss in detail the fine structure of the lowest-energy exciton states, and compare our results with recent experimental measurements obtained by size-selective optical spectroscopies. Our calculations illustrate the interplay between spin-orbit coupling, electron-hole exchange interaction, and configuration interaction in determining the splitting of the lowest excitonic multiplets. By analyzing the calculated excitonic energy levels with a simple model, we provide a practical expression to predict the dependence of the exciton splitting on the nanocrystal size. This work was supported by OER contract DE-AC36-83CH10093.

2:30 PM *W2.4 
OPTICAL PROPERTIES OF CdS QUANTUM DOTS:THE KEY ROLE OF THE SPIN-ORBIT AND THE COULOMB INTERACTIONS. Maria Chamarro , Valia Voliotis, GPS, Univ. Paris 6 and 7, Paris, FRANCE, Univ. d'Evry Val d'Essonne, Evry, FRANCE; Mohamed DIB, GPS, Univ. Paris 6 and 7, Paris, FRANCE; Thierry Gacoin, LPMC, Ecole Polytechnique, Palaiseau, FRANCE; Christophe Delerue, Guy Allan, Michel Lannoo, Dept ISEN, IEMN, Villeneuve d'Ascq, FRANCE.

We study theoretically and experimentally the effects of the Coulomb interaction and the spin-orbit (SO) coupling on the electronic structure of small quantum dots. A tight-binding calculation with restricted configuration interaction is developped in a typical case: very small cubic quantum dots for which the electron-hole (e-h) exchange interaction is of the order of magnitude of the SO interaction. Experimentally, resonant photoluminescence (PL) and photoluminescence excitation are used to obtain information about a single size of CdS quantum dots obtained by a chemical growth method. The good agreement between theoretical and experimental results allows us to attribute the red-shift of the resonant PL to the splitting between an optically allowed exciton state and the lowest state, a spin-forbidden state. We show that the red-shift is a function of both the e-h exchange energy and the SO splitting. We also show that the e-h exchange energy for small sizes (radius < 17 ) varies linearly with the gap which is in discrepancy with the predictions of the effective mass approximation.

3:30 PM W2.5 
OPTICAL PROPERTIES OF ZERO-DIMENSIONAL EXCITONS IN CdSe/ZnSe QUANTUM DOT STRUCTURES. U. Woggon , F. Gindele, W. Langbein, University of Dortmund, Institute of Physics, GERMANY; K. Leonardi, D. Hommel, H. Selke, University of Bremen, Dep. of Solid State Physics, GERMANY.

The optical properties of CdSe nanostructures grown by migration enhanced epitaxy (MEE) of CdSe on ZnSe are studied by time-, energy-, and temperature-dependent photoluminescence and excitation spectroscopy, as well as by polarization-dependent four-wave mixing and two-photon absorption experiments.