Positioning of single nanostructures - Single quantum devices · Electron escape from InAs/GaAs...

Positioning of single nanostructures - Single quantum devices






Second international workshop on

Positioning of single nanostructures -

Single quantum devices

November 15 - 16, 2007

Freudenstadt-Lauterbad

Funded by the Deutsche Forschungsgemeinschaft (DFG)

THURSDAY, November 15, 2007

10:35 – 10:45 Workshop opening P. Michler

10:45 – 12:30 Nanostructures for electronics

10:45 – 11:30 Electron escape from InAs/GaAs quantum dots

O. Engström

11:30 – 12:00 Lateral positioning of SiGe/Si(001) islands

M. Stoffel

12:00 – 12:30 Ge-dot based diodes A. Karmous

12:30 – 14:00 Lunch break

14:00 – 15:45 Light matter interactions

14:00 – 14:45 Gold nanoparticles and light beams: a close look in the far field

A. Tchebotareva

14:45 – 15:15 Coupling of excitons to plasmonic structures

H. Giessen

15:15 – 15:45 Perspectives for single atom deposition using cold atoms

T. Pfau

15:45 – 16:15 Coffee break

Program

16:15 – 17:30 Coherence and entanglement

16:15 – 17:00 Biphoton interference and coherence of a quantum dot source of entangled photons

R. J. Young

17:00 – 17:30 Triggered polarization-entangled photon pairs from a single quantum dot up to 30 K

R. Hafenbrak

17:35 – 18:30 Meeting of the FG project leaders

19:00 Dinner

FRIDAY, November 16, 2007

08:30 – 10:15 Single molecules, quantum dots and wires

08:30 – 09:15 Core-shell nanowires N. Sköld

09:15 – 09:45 Recombination electroluminescence in polyaromatic host-guest systems

M. Nothaft

09:45 – 10:15 Electroluminescence from the contact point of the scanning tunneling microscope

A. Kabakchiev

10:15 – 10:45 Coffee break

10:45 – 12:00 Quantum dot molecules

10:45 – 11:30 Self-assembled quantum dots and quantum dot molecules: Their basic properties and potential application

S. Panyakewo

11:30 – 12:00 Lateral GaAs/AlGaAs and InAs quantum dot molecules

L. Wang

12:00 – 13:30 Lunch break

13:30 – 15:15 Single quantum devices

13:30 – 14:15 Patterned InAs quantum dot and nanopillars: Formation and characterization

D. Huffaker

14:15 – 14:45 Wavelength tuning of emission from semiconductor quantum dots in optical resonators

S. Kiravittaya

14:45 – 15:15 Post-growth control of QD emission characteristics by lateral electric fields and local annealing

S. Ulrich

15:15 Conclusive remarks

Electron escape from InAs/GaAs quantum dots O. Engström1 and M. Kaniewska2

1Chalmers University of Technology, Microtechnology and Nanoscience, SE-412 96 Göteborg, Sweden,Email: [email protected] 2Institute of Electron Technology, Analysis of Semiconductor Nanostructures, Al. Lotników 32/46, 02-668 Warsaw, Poland For prospective use of quantum dots (QDs) as elements in semiconductor memories, the retention time of captured charge carriers representing the stored information is of primary importance. In the light of these applications, this presentation will give an overview of results for characterizing the escape of electrons from InAs QDs. A practical material structure for investigating the charge carrier emission properties is obtained by positioning the QDs in the depletion region of a p-n junction or a Schottky diode and measure charge carrier escape by deep level transient spectroscopy (DLTS). For InAs/GaAs QDs with height/base dimensions of about 10/20 nm and prepared by Stranski-Krastanov technique, a number of problems occur in the interpretation of data, which are seldom met in traditional DLTS procedures. First, as a result of the preparation technique, there will be a certain distribution in size between individual QDs. This gives rise to a corresponding distribution of energy eigenvalues for captured carriers. Second, due to multiple energy levels and the presence of an electric field, a number of different emission paths related to thermal and tunneling processes are possible. Third, for the dimensions mentioned above, six electrons may be captured in each QD, which means that multiple charge carrier statistics needs to be developed in order to interpret experimental results. In recent work, the statistics was expounded for this type of QDs, taking into account the possible escape possibilities for thermal and tunneling processes [1,2]. This theory has been applied to measured results [3], where DLTS spectra were presented as a three-dimensional mountainous data terrain on a coordinate plane spanned by tempera-ture and Schottky barrier reverse voltage (TV-DLTS) as shown in Fig. 1. From such a scheme, different emission processes can be identified and retention time maps can be defined for specified electronic conditions. The “DLTS-landscape” reveals regions where tunneling dominates and regions where thermal processes occuring as two-step excitations or combined thermal – tunneling steps can be recognized [3, 4]. [1] O. Engström and P.T. Landsberg, Phys. Rev. B 72, 075360 (2005) [2] O. Engström, P. T. Landsberg and Y. Fu, Mat. Sci. Eng. C 26, 739 (2006) [3] O. Engström, M. Kaniewska, W. Jung and M. Kaczmarczyk, Appl. Phys. Lett. 91, 033110 (2007) [4] O. Engström, M. Kaniewska, M. Kaczmarczyk and W. Jung, Appl. Phys. Lett. 91, 133117 (2007)

Fig. 1 TV-DLTS spectrum for InAs/GaAs QDs

Fig. 1 TV-DLTS spectrum for InAs/GaAs QDs

Second international workshop on "Positioning of single nanostructures"Hotel Zollernblick, Freudenstadt-Lauterbad (Germany), November 15 - 16, 2007

Lateral positioning of SiGe/Si(001) islands

M. Stoffel1, J. J. Zhang1,2, A. Rastelli1,3, O. G. Schmidt1,3 1 Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1,

D-70569 Stuttgart-Germany 2 Institut für Halbleiter und Festkörperphysik, Johannes Kepler Universität,

A-4040 Linz-Austria 3 Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstrasse 20,

D-01069 Dresden-Germany

Strain-driven self-assembly of semiconductor islands or “quantum dots” has been a very active research field during the last fifteen years. Among the different strained material combinations investigated so far, the Ge/Si(001) heteroepitaxial system is the simplest and has thus been the subject of numerous fundamental studies. In most cases, the growth is performed on planar substrates leading thus to random island formation. In view of the integration of self-assembled SiGe islands in novel nanoscale device architectures [1], a precise control of their nucleation sites is required. In order to achieve this goal, a combination of substrate patterning and self-assembled growth was used and impressive results were already reported. In this contribution, we report on the morphological evolution of both pits and SiGe islands on Si(001) substrates patterned by optical lithography and reactive ion etching. When the Si buffer layer thickness increases, the patterned holes transform into multifaceted pits with {113}, {103} and {11n} facets before evolving into inverted truncated pyramids with {11n} facets. By subsequently depositing Ge, we were able to fabricate perfectly ordered arrays of SiGe islands with a pit spacing varying between 600 nm and 1000 nm. These results are of paramount importance both for fundamental investigations of single SiGe/Si(001) islands as well as for their integration in single dot based devices. A systematic variation of the Ge coverage and pit spacing allows us to shed new light into the evolution of SiGe islands on patterned Si(001) substrates [2]. In particular, the islands forming at the pit bottom are generally more symmetric than their counterparts on flat Si(001) surfaces, especially in the case of islands with intermediate shape between pyramids and domes. In addition, a noticeable difference appears in the island shape distribution. While on planar substrates, monomodal shape distributions can only be obtained in narrow growth parameter windows due to coarsening, the presence of pits allows the fabrication of uniform island arrays with any of their equilibrium shapes. This indicates that growth on pit-patterned substrates is a viable path to achieve not only spatially, but also morphologically ordered island ensembles. Finally, quantitative data on the influence of the pattern periodicity on the SiGe island volume are discussed. References: [1] O. G. Schmidt and K. Eberl, IEEE Trans. Electron. Devices 48, 1175 (2001). [2] J. J. Zhang, M. Stoffel, A. Rastelli, O.G. Schmidt, V. Jovanovic, L. K. Nanver and G. Bauer, Appl. Phys. Lett. 91, 173115 (2007).


Ge-dot based diodes

A.Karmous, O.Kirfel, E.Kasper

Institut für Halbleitertechnik The aim of this study is to introduce Ge dots in diode structures starting from Schottky type diodes (Fig. 1) and reducing the barrier width until tunnelling contributes significantly to the current. DC and RF electrical characterisation of diodes having a layer of Ge dots in their structures will be presented. In order to form electrical device structures with single dots, Ge growth on oxide windows is investigated. Windows in oxide layer are obtained by either Optical Lithography (OL) or Electron Beam Lithography (EBL). Chemical Mechanical Polishing (CMP) is used to remove the materials deposited on the oxide. The electrical characterizations of Schottky diodes grown by Differential Epitaxy (DE) (Fig. 2: growth on window + CMP) show that the DE is compatible with device fabrication. Depending on the surface preparation before the growth, either single dot (Fig. 3) or ring-like shaped structure (Fig. 4) are obtained in the windows.

Fig. 1 Schottky Diode structure with a dot layer.

Fig. 2 Differential Epitaxy

Fig. 3 Ge dot on EBL patterned oxide windows

Fig. 4 Ring-like shaped Ge nanostructure at the edge of EBL patterned oxide windows


Gold Nanoparticles and Light Beams: a Close Look in the Far Field

A. L. Tchebotareva a, M. van Dijk a, P. Ruijgrok a, M. Lippitz b, and M. Orrit a a MoNOS, Huygens Laboratory, University of Leiden, Niels Bohrweg 2, 2333CA Leiden, The Netherlands

b Max Planck Institute for Solid State Research Heisenbergstrasse 1, D-70569 Stuttgart, Germany

Single nanoparticle studies are of crucial importance for understanding how shape, size and local environment influence the properties of solids on the nanoscale. We will show how the combination of far-field optical studies with the scanning electron microscopy (SEM) on the very same nanoparticle provides a deeper insight into the dependence of the nanoparticle’s properties on its exact size and shape. We have combined optical reflectivity, time-resolved spectroscopy, and scanning electron microscopy on individual gold nanoparticles. We find that the optical response of the particle is strongly dependent on its morphology. Most notably, dumbbells differ strongly from single colloidal nanospheres. The optical response of dumbbells shows two distinct peaks, related to the plasmon modes along the two principal axes. Time-resolved pump-probe measurements allowed us to detect the elastic vibrations launched in the very same particles upon excitation by a short laser pulse. We have recorded the signal change at zero delay between the pump and probe pulses, for different probe wavelengths. In this way, we are able to measure the change of the optical response due to the heating of the electron gas of the metal particle. On longer time scales, the energy is transferred from electrons to the lattice by electron-phonon scattering. This leads to a sudden thermal expansion of the lattice and launches elastic vibrations of the particle, which results in a periodic change of its optical response. By detecting this change at the probe wavelength, we follow the vibration of individual particles. The elastic response of a single gold nanosphere of 80 nm diameter is dominated by a vibration at a frequency of ~40 GHz, which is associated with the fundamental breathing mode. The elastic response of a dumbbell displays an additional frequency peak around 17 GHz. This frequency corresponds to a higher-order ellipsoidal mode. The excitation of this mode in a dumbbell is enhanced by the mechanical interaction between the particles in the contact area.


Coupling of excitons to plasmonic structures

H. Giessen

4. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70550 Sttuttgart [email protected]

Abstract not available


Perspectives for single atom deposition using ultracold atoms Tilman Pfau 5. Physikalisches Institut Universität Stuttgart Over the last decade techniques have been developed to structure atomic beam deposition by the interaction of quasi resonant light masks with the electronic structure of the atoms. Feature sizes on the order of 10 nm and periodicities down to 60 nm have been demonstrated [1]. At the same time atomic beam sources have advanced from thermal to laser-like, by the advent of Bose Einstein condensates. Recently such atom laser sources became available for technologically relevant materials like Cr [2], and Yb [3]. Of course the flux of those sources is still very limited (<10^4 atoms/sec), but an effort has been made to improve this flux of ultracold atoms. In case of chromium a magnetically guided flux (of still thermal but ultracold atoms) of 6 10^9 atoms/sec has been achieved in a flux oriented setup [4]. We plan to use these sources to study single atom deposition. In the ultracold regime not only the methods of coherent atom optics (like matter wave holography) can be applied. More importantly the interaction among the atoms which can de tuned over a wide range [5] can be used to influence the statistics of the source. In contrast to Poissonian number fluctuations of a thermal beam an interacting quantum degenerate source e.g. in an optical light mask potential (optical lattice) can suppress number fluctuation substantially and by that providing single atom control. In order to detect single deposited chromium atoms we intend to use well known methods of photoluminescence of single fluorescence centres. First studies indicate, that single deposited chromium atoms can be converted in to single fluorescence/luminescence centre which can be detected by confocal microscopy. [1] For an overview see: M. Oberthaler and T. Pfau "One-, two- and three-dimensional nanostructures with atom lithography" J. Phys.: Condens. Matter 15, R233(2003) [2] A. Griesmaier, J. Werner, S. Hensler, J. Stuhler and T. Pfau "Bose-Einstein condensation of chromium" Phys. Rev. Lett. 94, 160401 (2005) [3] Y. Takasu, K. Maki, K. Komori, T. Takano, K. Honda, M. Kumakura, T. Yabuzaki, and Y. Takahashi “Spin-Singlet Bose-Einstein Condensation of Two-Electron Atoms” Phys. Rev. Lett. 91, 040404 (2003) [4] A. Greiner, J. Sebastian, P. Rehme, A. Aghajani-Talesh, A. Griesmaier, T. Pfau "Loading chromium atoms in a magnetic guide" J. Phys. B, 40 F77 (2007) [5] J. Werner, A. Griesmaier, S. Hensler, J. Stuhler, T. Pfau, A. Simoni and E. Tiesinga "Observation of Feshbach Resonances in an Ultracold Gas of 52Cr" Phys. Rev. Lett. 94, 183201 (2005) Th. Lahaye, T. Koch, B. Fröhlich, M. Fattori, J. Metz, A. Griesmaier, S. Giovanazzi, T. Pfau "Strong dipolar effects in a quantum ferrofluid" Nature 448, 672 (2007)


Biphoton interference and coherence of a quantum dot source of entangled

photons R. J. Young

1, R. M. Stevenson

1, A. J. Hudson

1,2, P. Atkinson

2, K. Cooper

2, D. A. Ritchie

2, and A. J. Shields

1

1 Toshiba Research Europe Ltd., 208 Cambridge Science Park, CB4 0GZ, UK. 2 Cavendish Laboratory, Cambridge University, CB3 0HE, UK.

A well controlled, triggered source of entangled photons is desirable for many applications

in quantum information processing. The two-photon cascade from a biexciton state, two

electrons and holes, confined in a quantum dot can be such a source of polarisation-

entangled photons provided the two decay paths from the biexciton carry no “which-path”

information. In recent years many techniques have been employed to make the two optical

decay paths from the biexciton state indistinguishable. A number of these techniques will be

discussed and entangled-photon emission from a single quantum dot with a high fidelity in

the expected Bell-state from the cascade will be demonstrated.

This source of entangled photons allows optical interferometry beyond the limits imposed

by the photon wavelength. Interference fringes of the entangled biphoton state reveals a

periodicity half of that obtained with the single photon, and much less than that of the pump

laser. High fringe visibility indicates that biphoton interference is less sensitive to

decoherence than interference of two sequential single photons.

The effect of the exciton fine-structure splitting on our entangled photon source will be

shown. Surprisingly the entanglement is found to persist despite relatively large separations

between the intermediate energy levels of up to 4µeV. Measurements demonstrate that

entanglement of the photon pair is robust to the dephasing of the intermediate exciton state

responsible for the first order coherence time of either single photon. We distinguish

between the first-order coherence time, and a parameter defined as the cross-coherence time,

this is illustrated in the figure.

XH

time

XV

~τ2*

~ττττHV

exciton

-ph

oto

n

wa

vefu

nction

(c)

~τ2*~τ2

*

XH

time

XV

~τ2*

~ττττHV

exciton

-ph

oto

n

wa

vefu

nction

(c)

~τ2*~τ2

*

The exciton-photon wavefunction evolution of the superimposed intermediate states showing three dephasing

events. The first two are single-photon decoherence events and do not affect the phase relationship of one field

relative to the other. The third event is a cross-coherence dephasing event and randomises the relative phase.

time


Triggered polarization-entangled photon pairs

from a single quantum dot up to 30 K

R. Hafenbrak, S. M. Ulrich, P. Michler, L. Wang, A. Rastelli and O. G.Schmidt

The radiative biexciton-exciton decay in (In,Ga)As semiconductor quan-tum dots has the potential of being a source of triggered polarization-entangledphoton pairs. However, this entanglement is in general reduced by theanisotropy-induced exciton fine structure splitting.

Here we present measurements on improved quantum dot structures, pro-viding both significantly reduced inhomogeneous emission linewidths andnear-zero fine structure splittings.

A high-resolution detection technique is introduced which allows us to ac-curately determine the fine structure in the photoluminescence emission andtherefore select appropriate quantum dots for quantum state tomography.

We were able to verify the conditions of entangled or classically corre-lated photon pairs in full consistence with observed fine structure properties.Furthermore, we demonstrate reliable polarisation-entanglement for elevatedtemperatures up to 30 K. The fidelity of the maximally entangled state de-creases only little from 72 % at 4 K to 68 % at 30 K.


Core-Shell Nanowires Niklas Sköld1, Jakob B. Wagner2, Johanna Trägårdh1, Werner Seifert1, Lars Samuelson1,

Mats-Erik Pistol1 1Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden, and

2Materials Chemistry/The Nanometer Structure Consortium, Box 124, SE-221 00 Lund, Sweden Au assisted growth of nanowires through the vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) mechanisms have enabled production of freestanding, high aspect ratio, 1D-structures. The high degree of control of the size and position of the nanowires as well as the size and position of heterostructures within the nanowires make this a promising route for manufacturing single quantum devices. As these nanowires have a large surface to volume ratio, surface oxide and surface trapped impurities can have a severe impact on the energy structure by forming deep levels resulting in band bending and non-radiative recombination. Surface passivation can be done chemically, post growth, by e.g. H- or S-passivation1, 2. This, however, only offers a short-term surface stability. A more viable scheme is to grow a large band gap shell around the nanowire and thus move the surface states away from the charge carriers confined in the core. Core-shell nanowires have shown an increase in photoluminescence (PL) intensity of several orders of magnitude3, 4 as carriers in the core cannot couple to the surface states. We have studied the effect of different shells, AlGaAs, GaInP and AlInP on GaAs nanowires. The wires were grown by metal-organic vapor phase epitaxy using two different temperature regimes for the core and the shell respectively. PL measurements showed that an increase in emission efficiency of 2-3 orders of magnitude can be achieved. Strain effects of lattice mismatched shells on the energy spectrum were studied and modeled by strain-dependent k·p calculations. By varying the mismatch from -1.3% to 1.3% the band gap of the GaAs core was tuned over a range of 240 meV. The PL spectra showed multiple peaks from the shell. This indicates an inhomogeneous composition and the existence of local energy minima. Detailed studies of the nanowire interior were therefore performed by transmission electron microscopy (TEM), cross-sectional TEM and cross-sectional scanning tunneling microscopy (STM). The nanowires were sliced both lengthwise as well as across. Defects, impurities and phase segregations were observed and investigated. The studies confirmed that phase segregation occurs in ternary shells due to variations in the chemical potential around the circumference of the wire in combination with different diffusion lengths for the growth species. 1 Mattila, M.; Hakkarainen, T.; Lipsanen, H.; Jiang, H.; Kauppinen, E. I. Appl. Phys. Lett. 2007, 90, 033101. 2 Suyatin, D. B.; Thelander, C.; Björk, M. T.; Maximov, I.; Samuelson, L. Nanotechnology, 2007, 18, 105307. 3 Noborisaka, J.; Motohisa, J.; Hara, S.; Fukui, T.; Appl. Phys. Lett. 2005, 87, 093109 4 Tchernycheva, M.; Cirlin, G. E.; Patriarche, G.; Travers, L.; Zwiller, V.; Perinetti, U.; Harmand, J. C.; Nano Lett. 2007, 7, 1500.


580 600 620 640 660 680 700 720 7400

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710nm

650nm

593nm

optical excitation (532nm) electrical excitation (15kV)

Inte

nsity

[a. u

.]

Wavelength [nm]

Fig.1: Comparison between the optical and electrical excitation of a p-terphenyl crystal doped with pentacene.

Recombination electroluminescence in polyaromatic host-guest systems

M. Nothaft, F. Jelezko, J. Wrachtrup, J. Pflaum

3. Physikalisches Institut, Universität Stuttgart, 70569 Stuttgart

The goal of our project is to demonstrate electroluminescence of single molecules inside an organic host crystal as a proof of concept for its use as an electrically driven single photon source on demand operating at room temperature. As a starting point we chose polyaromatic host-guest crystals, namely terrylene and pentacene in p-terphenyl as well as dibenzoterrylene in anthracene, both of which exhibiting single photon emission upon optical excitation. To provide electroluminescence it is a necessity to inject electron and holes into the host crystal where the dopant molecules act as individual recombination centers.

As a major challenge the effective injection of electrons and holes into the host has to be achieved despite the energetically high position of the HOMO (5.5 – 6.5eV) and low position of the LUMO (1.5 - 2.2eV). We therefore demonstrate two possible approaches to circumvent this problem by a) direct injection of free electrons and b) survey of suited contact materials.

a) Injection of free electrons was performed by using an electron microscope operating at high voltages up to 30keV. Correlation between the excitation via cathodoluminescence and optical excitation was performed on a p-terphenyl crystal doped with pentacene at a concentration of 10-6 mol/mol (Fig.1). As will be shown in the talk it is, in general, possible to identify the four different lattice sites where the pentacene is inserted in the host crystal. However, it turned out that sample degradation as well as low electroluminescence yield avoid an efficient application of this injection method for further time-resolved (anti-bunching) studies.

b) In the second approach we characterized various materials with respect to their injecting abilities by means of space-charge-limited current measurements. For the host-guest system anthracene doped by dibenzoterrylene (DBT) we will discuss the I(V)-characteristics and show that a double layer of Cs2CO3 and Al enables sufficiently good electron injection and that the observation of electroluminescence is possible. From the spectrally resolved electroluminescence signal of the anthracene host doped with DBT we deduced the relation between the injected current density and the resulting intensity of electroluminescent peaks.

Finally, strategies to transfer the gained knowledge on the injection properties to samples with nanostructured volume and nanostructured electrodes will be highlighted, respectively.


Electroluminescence from the contact point of the scanning tunneling microscope

Alexander Kabakchiev, Klaus Kuhnke, Theresa Lutz, Giovanni Costantini, and Klaus Kern Max-Planck Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart The goal of the project is the fluorescence excitation of single organic molecules and quantum dots by the tunneling current in an STM. For this we designed a novel setup for the detection of luminescence from the STM tunnel junction. Three optical paths are introduced into an existing low temperature STM. We employ free optical light propagation using lenses and mirrors which fully preserve essential information carried by the light, such as angular distribution, polarization, and emission time. As a first step, light emission measurements on a Cu(110) clean surface and W tip were performed. This study is a prerequisite for later measurements of molecular luminescence, since plasmons have a distinct influence on the light emission near a metal surface. A two-dimensional intensity map of luminescence spectra as a function of bias voltage covering the range from -10 to +10 V reveals intensity peaks attributed to localized surface plasmons. The spectral properties measured at a constant tip-sample distance as well as in the constant current mode will be discussed and interpreted. Earlier experiments demonstrated that molecular fluorescence on a bare metal surface is quenched due to non-radiative energy dissipation. Thus the molecule has to be decoupled from the metal substrate used in the STM. As an insulating layer between the molecule and the metal surface we utilize KCl. It can be evaporated easily and covers partially the underlying substrate building 1 to 3 monolayers. STM-topography measurements of pentacene on Cu(110) and on KCl/Cu(110) will be presented. Preferential accumulation of pentacene on the bare metal regions is observed, most probably due to a high mobility of the molecules on the insulating layer. Several approaches in the sample preparation procedure are presently employed in order to increase the amount of pentacene on the salt layer. Furthermore, we plan to investigate different self-organized semiconductor nanostructures (e.g. uncapped and capped quantum dots, quantum dot molecules, quantum rings) in close collaboration with the group of O. G. Schmidt. The scanning probe and photon spectroscopy will provide information about the correlation between the local electronic and optical properties and the shape of the nanostructures.


Self-Assembled Quantum Dots and Quantum Dot Molecules :

Their Basic Properties and Potential Applications

S. Panyakeow

The Semiconductor Device Research Laboratory (SDRL), CoE Nanotechnology Center of Thailand, Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University,

Phyathai Road, Bangkok 10330, Thailand. Phone +662 218 6524, Fax. +662 218 6523, E-mail: [email protected]

Extended Abstract Self-assembled quantum dots (QDs) and quantum dot molecules (QDMs) are grown by

Molecular Beam Epitaxy (MBE). With an original MBE growth techniques called multiple thin-capping-and-regrowth process at particular low capping temperature, as-grown QDs are transformed to camel-like nanostructures with nanoholes due to anisotropic strain along ]011[ crystallographic direction. These nano-templates become the origins of different types of quantum dot structures after appropriate conditions of regrowth process, namely Quantum Dot Molecules (QDMs), Quantum Rings (QRs), Quantum Dot Pairs (QDPs), Quantum Dot Chains (QDCs) and High Density Quantum Dot Molecules (HD-QDMs).

Most of the quantum dot nanostructures are based on InAs/GaAs material system. InP/GaAs quantum nanostructures are also grown by droplet epitaxy providing QRs and QD-Rings.

The basic properties of all quantum dot nanostructures are characterized by photoluminescence (PL) measurement and Atomic Force Microscopy (AFM). Some samples are mesa-etched for micro-PL analysis.

We demonstrate the potential application of HD-QDMs for high efficiency solar cells due to wider spectrum response giving high short circuit current density. Quantum Dot Solar Cells show stable performance at high concentrated sunlight. It would be an alternative for cost-down of terrestrial photovoltaic power generation in the near future.


Lateral GaAs/AlGaAs and InAs quantum dot molecules

Lijuan Wang1, Armando Rastelli2,Suwit Kiravittaya1, Mohamed Benyoucef2, Oliver. G. Schmidt2

1Max-Planck-Institute für Festkörperforschung, Stuttgart, Germany 2Institute for Integrative Nanosciences, Leibniz Institute for Solid State and Materials Research, Dresden, Germany Email address: [email protected] Coupled semiconductor quantum dots (QDs) are attracting growing interest due to their potential application as solid-state quantum gates. Substantial progress towards the experimental implementation of such quantum dot molecules (QDMs) has been achieved both for electrically defined QDs and for self-assembled, vertically-stacked QDs. To explore the possibility of coupling a larger number of self-assembled QDs, investigations on lateral coupling are required. In this work, we demonstrate the fabrication of lateral GaAs QDMs and provide evidence of lateral coupling between two nearby QDs. This coupled quantum system is created by a method based on selective etching of buried self-assembled InAs QDs and subsequent overgrowth with AlGaAs. With a proper choice of etching and overgrowth conditions, the original etching-resultant single holes are found to split into two closely-spaced holes aligned in the [110] direction, revealed by atomic force microscopy. GaAs filling of the resultant biholes is applied subsequently to convert the hole structures into the GaAs QDMs below a thin quantum well embedded in AlGaAs barriers. Photoluminescence spectra (PL) of different QDMs show common spectral features. Because of fluctuations inherent in the self-assembled growth, two QDs composing a molecule are generally not identical and thus their mutual coupling is tuned by an external electric field parallel to the [110] direction. An intricate behavior, consisting of spectral line crossings and avoided crossings is observed for different molecules. Anticrossing patterns in the photoluminescence spectra provide direct evidence of the lateral coupling between two nearby quantum dots. A simple calculation suggests that the coupling is mediated by electron tunneling, through which the states of direct and indirect exciton are brought into resonance. The selective etching of buried self-assembled InAs QDs combined with overgrowth process could also be used to fabricate lateral InAs QDMs. Here we use wet chemical etching to remove the GaAs cap and to obtain useful structural information, which is normally difficult to determine since the QDMs are encapsulated in GaAs matrix. Furthermore, a finite element calculation using structural parameters obtained suggests a lateral coupling between the two dots.


Patterned InAs Quantum Dot and Nanopillars

Formation and Characterization

Diana L. Huffaker Department of Electrical and Computer Engineering, Center for Nanosystems Integration,

UCLA, Los Angeles, CA 90095 We overview our work in controlled patterned nanostructure formation and dependence MOCVD growth parameters. Our patterned quantum dot (PQDs) are formed atop the (001) apex of a GaAs pyramidal buffer to achieve sufficiently small growth platform for quantized carrier confinement and to separate the recombination region from the processed interface. The GaAs pyramids are characterized by well-defined equilibrium crystal shapes (ECS) defined by three crystal plane families including {11n}, {10n} and (001). Subsequent patterned QD (PQD) nucleation on the GaAs pyramidal facets is highly preferential towards the (11n) planes due to superior energy minimization and the shape of the QDs on the (11n) planes is also highly predictable and uniform. The GaAs pyramid formation strongly correlates to the pyramidal shape and to the subsequent PQD PL characteristics. The wavelength of the patterned In(Ga)As QDs can be controlled and ranges from 950 nm to as long as 1.6 µm. Several aspects to be discussed are the effects of crystallographic structure measured using photoluminescence and SEM. By controlling crystal faceting, we are able to form coupled quantum clusters along with truly isolated QDs. This initial work correlates the basic PQD characteristics to the GaAs pyramidal buffer formation. Our ongoing studies include time resolved photoluminescence and photo-excitation luminescence studies to further elucidate band-structure. Planarization and overgrowth for room temperature light emitting diodes will also be described. Professor Diana Huffaker received her Ph.D. in Electrical Engineering from the University of Texas at Austin with dissertation studies focused on vertical cavity microlasers and other quantum dot devices. Prior to joining the University of California at Los Angeles, she was Associate Professor of Electrical Engineering at the Univeresity of New Mexico at the Center for High Technology Materials. She has also served as Senior Research Scientist at Picolight Incorporated in Boulder, CO. Her research interests include directed and self-assembled nanostructure solid-state epitaxy, optoelectronic devices for energy and biosensing applications with special emphasis in III-V/Si photonics. Professor Huffaker has co-authored over 120 refereed journal publications, 2 awarded patents with 8 disclosures pending, 2 book chapters and has reported her work through many invited presentations. She has been awarded the 2002 Compound Semiconductor International Symposium Young Scientist Award for developments in novel quantum dot and selectively oxidized optoelectronic materials and devices including the first oxide-confined VCSEL and the first 1.3 µm self-organized quantum dot laser. She recently received the 2004 Alexander von Humboldt research fellowship to study (In)GaN quantum dot light emitters at Technical University Berlin. She is an active participant in the technical community with appointments in IEEE/LEOS, SPIE, WISE, MRS, OSA and TMS. She is an elected member of the IEEE/LEOS Board of Governors and IEEE WIE Region 6 chairman.


Wavelength tuning of emission from semiconductor quantum

dots in optical resonators S. Kiravittaya1,*, S. Mendach1, M. Benyoucef1,2, A. Rastelli1,2, and O. G. Schmidt1,2 1 Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany 2 Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstr. 20, D-01069 Dresden, Germany A bi-directional wavelength tuning of photon energy emitted from single quantum dots (QDs) in microtube resonators is theoretically and experimentally demonstrated. Within the framework of linear elasticity, we predict the possible range of experimental parameters to obtain a state where the emission of two QDs is tuned into resonance with an optical mode. By mean of in-situ deformation using a glass needle, we experimentally show that both redshift and blueshift of the QD emission from the same QD can be obtained by simply changing the pressing positions. Furthermore, the emission from a QD can also be brought into resonance with either another QD or an optical mode. Corresponding author: Dr. Suwit Kiravittaya MBE Group, Max-Planck-Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany Room: 3B18 Tel. (office): ++49-(0)711-689-1313 Fax: ++49-(0)711-689-1010


Post-Growth Control of Single Quantum Dot Emission Characteristics by Lateral Electric Fields and Local Thermal

Annealing

S. M. Ulrich(1), M. M. Vogel(1,2), R. Hafenbrak(1), P. Michler(1),L. Wang(3), A. Rastelli(4), and O. G. Schmidt(4)

(1) Institut für Halbleiteroptik und Funktionelle Grenzflächen, Universität Stuttgart, Germany, (2) Institut für Strahlwerkzeuge, Universität Stuttgart, Germany,

(3) Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany, (4) Institute for Integrative Nanosciences, IFW Dresden, Germany.

The quantum-confined Stark effect (QCSE) was studied on different excitonic carriercomplexes of single self-assembled (In,Ga)As/GaAs quantum dots (QDs) in terms of low-temperature micro-photoluminescence (µ-PL) spectroscopy under lateral electric fields. For neutral excitons and biexcitons as well as a charged QD state, similar Stark shifts could be observed in parallel. Our investigations suggest the absence of a permanent dipole moment in the lateral quantum dot plane whereas comparable values for the polarizability have beenderived from all investigated carrier complexes. In addition, field-dependent investigations on the relative µ-PL intensities of different radiative decay channels revealed the possibility to reversibly control and promote charged versus neutral dot configurations. Furthermore, high-resolution Fabry-Pérot interferometry was applied to resolve the excitonic emission fine structure splitting and to trace the influence of a lateral electric field. For a single dot, thesplitting could be tuned to zero, thus affording the possibility to create electrically controlled entangled photon pairs.

The second part of the talk addresses recent results of laser-induced post-growth localthermal annealing experiments on individual (In,Ga)As/GaAs QDs in microdisk cavities. Withthis technique, we investigate the possibility to thermally tune individual QD excitonic resonances with respect to the high quality whispering gallery-type eigenmodes of a surrounding microresonator. By utilization of the different shift behaviour observed from cavity modes and single QD emission spectra, resonance tuning of individual excitonic PL channels and resonator modes has been achieved.

Fig. (a): Results of polarization-dependent high-resolution µ-PL measurements on the excitonicemission line of a single QD. By application of a variable in-plane electric field, the QD emission fine structure can be tuned to zero. Inset: Top view of the metal finger contactstructure.

Fig. (b): Results of local thermal annealing series, demonstrating stepwise tunability between asingle microdisk mode (top spectra) and the emission channel of an individual QD

(bottom spectra). (SEM picture by courtesy of IFW Dresden).


Second Workshop on “Positioning of single nanostruc-tures – Single quantum devices”

Hotel Zollernblick, Freudenstadt-Lauterbad

November 15 – 16, 2007 List of participants Paola Atkinson Max-Planck-Institut für Festkör-

perforschung, Stuttgart [email protected]

Mohamed Benyoucef Leibniz-Institut für Festkörper- und Werkstoffforschung, Dresden

[email protected]

Michiel de Dood Molecular Nano-Optics and Spins, Leiden University

[email protected]

Olof Engström Department of Microtechnology and Nanoscience, Chalmers Uni-versity of Technology

[email protected]

Harald Giessen 4. Physikalisches Institut, Univer-sität Stuttgart

[email protected]

Hongcang Guo 4. Physikalisches Institut, Univer-sität Stuttgart

[email protected]

Robert Hafenbrak Institut für Halbleiteroptik und Funktionelle Grenzflächen, Uni-versität Stuttgart

[email protected]

Claus Hermannstäd-ter

Institut für Halbleiteroptik und Funktionelle Grenzflächen, Uni-versität Stuttgart

[email protected]

Diana Huffaker Center for High Technology Mate-rials, University of New Mexico

[email protected]

Fedor Jelezko 3. Physikalisches Institut, Univer-sität Stuttgart

[email protected]

Michael Jetter Institut für Halbleiteroptik und Funktionelle Grenzflächen, Uni-versität Stuttgart

[email protected]

Alexander Kabak-chiev

Max-Planck-Institut für Festkör-perforschung, Stuttgart

[email protected]

Alim Karmous Institut für Halbleitertechnik, Uni-versität Stuttgart

[email protected]

Suwit Kiravittaya Max-Planck-Institut für Festkör-perforschung, Stuttgart

[email protected]

Olaf Kirfel Institut für Halbleitertechnik, Uni-versität Stuttgart

[email protected]

Klaus Kuhnke Max-Planck-Institut für Festkör-perforschung, Stuttgart

[email protected]

Markus Lippitz Max-Planck-Institut für Festkör-perforschung, Stuttgart

[email protected]

Theresa Lutz Max-Planck-Institut für Festkör-perforschung, Stuttgart

[email protected]

Peter Michler Institut für Halbleiteroptik und Funktionelle Grenzflächen, Uni-versität Stuttgart

[email protected]

Maximilian Nothaft 3. Physikalisches Institut, Univer-sität Stuttgart

[email protected]

Somsak Panyakeow Semiconductor Device Research Laboratory, Chulalongkorn Uni-versity

[email protected]

Tilman Pfau 5. Physikalisches Institut, Univer-sität Stuttgart

[email protected]

Markus Pfeiffer 4. Physikalisches Institut, Univer-sität Stuttgart

[email protected]

Jens Pflaum 3. Physikalisches Institut, Univer-sität Stuttgart

[email protected]

Niklas Sköld Division of Solid State Physics, Lund University

[email protected]

Armando Rastelli Leibniz-Institut für Festkörper- und Werkstoffforschung, Dresden

[email protected]

Matthias Reischle Institut für Halbleiteroptik und Funktionelle Grenzflächen, Uni-versität Stuttgart

[email protected]

Robert Roßbach Institut für Halbleiteroptik und Funktionelle Grenzflächen, Uni-versität Stuttgart

[email protected]

Oliver Schmidt Leibniz-Institut für Festkörper- und Werkstoffforschung, Dresden

[email protected]

Heinz Schweizer 4. Physikalisches Institut, Univer-sität Stuttgart

[email protected]

Mathieu Stoffel Max-Planck-Institut für Festkör-perforschung, Stuttgart

[email protected]

Takayuki Suzuki Max-Planck-Institut für Festkör-perforschung, Stuttgart

[email protected]

Anna Tchebotareva Molecular Nano-Optics and Spins, Leiden University

[email protected]

Sven Ulrich Institut für Halbleiteroptik und Funktionelle Grenzflächen, Uni-versität Stuttgart

[email protected]

Ralf Vogelgesang Max-Planck-Institut für Festkör-perforschung, Stuttgart

[email protected]

Lijuan Wang Max-Planck-Institut für Festkör-perforschung, Stuttgart

[email protected]

Christian Wolpert 4. Physikalisches Institut, Univer-sität Stuttgart

[email protected]

Robert J. Young Toshiba Research Europe Limited, Cambridge Research Laboratory

[email protected]

Positioning of single nanostructures - Single quantum devices · Electron escape from InAs/GaAs...

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