1. Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates Eli Kapon Laboratory of Physics of Nanostructures Swiss Federal Institute of Technology Lausanne (EPFL)  Introduction  Self-ordering on nonplanar substartes  Neutral and charged low-D excitons  Contacting single QWRs and QDs  Summary and outlook ADMOL, Dresden, Germany, February 23-27, 2004

2. Quantum Confinement: Compound semiconductor heterostructures Electron envelope functions : Schrödinger equation with heterostructure potential : AlGaAs GaAsAlGaAs Quantum Well Heterostructure AlGaAs Confined envelope functions AlGaAsGaAs Potential well Quantum Well Potential

3. Low-Dimensional Semiconductors: Quantum wells, wires and dots Density of states Quantum Well Quantum Dot Quantum Wire

4. Spontaneous Formation of Quantum Nanostructures: Self-formed quantum dots 400nmX400nm STM scan of MBE- grown GaAs (100) surface R. Grousson et al., Phys. Rev. B 55, 5253 (1997) « Natural » QDs Zhuang et al., J. Crystal Growth 201/202, 1161 (1999) TEM cross section of vertically-stacked SK-grown quantum dots Stranski-Krastanow QDs Surface fluxes of adatoms are not controlled: random nucleation and broad size distribution

5. Chemical potential: Surface flux: Lateral Patterning during Epitaxial Growth: Controlling lateral fluxes with the surface chemical potential StrainCapilarityEntropy of mixing G. Biasiol and E. Kapon, Phys. Rev. Lett. 81, 2962 (1998); G. Biasiol et al., Phys. Rev. B 65, 205306 (2002)

6. V-Groove Quantum Wires: Size and shape control by growth adjustments Surface Chemical Potential G. Biasiol et al., PRL 81, 2962 (1998); Phys. Rev. B 65, 205306 (2002) Size and Shape Control  Nano-template width adjusted by surface diffusion length  Wires/dots produced by switching surface diffusion length Self-limiting facet width

7. Excitons in Quantum Wires: Signatures of a 1D system Experiment: PL-excitation spectra  Excitonic transitions dominate (reduced Sommerfeld factor in 1D)  Polarization anisotropy due to valence band mixing  Enhanced exciton binding energy (14.5 meV) deduced Theory: excitonic absorption M.-A. Dupertuis et al., to be published

8. Contacting a Single Quantum Wire: 1D Electron Gas in V-Groove QWRs Etched Areas 1 µm Current flow QWR wire - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + - + + + + - - + - - QWs QWR D. Kaufmann et al., Phys. Rev. B 59, R10433.(1999)  Moduation-doped V-groove QWR structure  Wire contacted via 2D electron gas on sidewalls  Conductance quantized close to 2e 2 /h  Discrepancy due to quantum contact resistance

9. Groove axis (nm) Height profile (nm) Sidewalls Bottom (100) facet MLs steps Long range (~1µm) variations induced by lithography imperfection Short range (~100nm) variations induced by monolayer steps Structural Disorder Along a V-Groove QWR: Monolayer steps at the central (100) wire facet

10. Charged Excitons in V-Groove QWR: Binding energies and localization Micro-PL spectra through sub-  m apertures Modulation doped QWRs for charging control Sharp lines represent localized excitons Localization Effects

11. Self-Ordering of Pyramidal Quantum Dots: OMCVD growth on pyramidal patterns 1µm (111)B {111}A (111B) substrates patterning GaAs-support Substrate removal pump PL 1  m A l G a A s GaAs QD Self-limited OMCVD growth QDs self-formed at a dip in the surface chemical potential

12.  >99% of QDs emit light  Highly uniform dot arrays Ground state CL image (7 meV window) 1  m 950 QDs 7 meV CL Intensity (arb. units) Photon Energy (eV) T = 7K CL spectrum Dense Site-Controlled Pyramidal QD Arrays: Cathodoluminescene spectroscopy

13. Single Quantum Dot Spectroscopy: Origin of optical transitions Back-Etched Pyramids Micro-PL of Single Pyramids Monochromatic CL Imaging QD 1.60 eV QWR 1.70eV QW 1.94eV 10 K, 1  W on single pyramid QD ~ 6 nm QWR ~ 3-4 nm QW ~ 1-1.5 nm VQW A. Hartmann et al., J. Phys.: Condens. Matter 11 5901 (1999)

14. Multi-Particle States in Quantum Dots: Excitonic states and charging mechanism l = -1 0 +1 s p s p Energy Emission X X-X- X - - 2X 2D harmonic oscillator model QD AlGaAs n ~ 10 17 cm -3 background doping Chrage control by photoexcitation

15. Quantum Dots in an N-type Environment: Charged excitonic complexes Single exciton regime Multi exciton regime X 2X 3e-2h 2e-h 3e-h 4e-h 5e-h 6e-h 5e-h 4e-h 3e-h Theory Full CI model X 4e-h 5e-h 3e-h 4e-h 6e-h 3e-h 2e-h 2X 3X 4X laser = 2.42 eV Experiment 30 pW 2.5 nW 600 nW A. Hartmann et al., PRL 84, 5648 (2000)

16. TiSa Laser Diode Laser c u n t e r Pulse. Analyz. i l QD sample monochromator A monochromator B time delay photon counter  Single QDs are readily observed and probed  Photon antibunching observed at X line M. Baier et al., Appl. Phys. Lett. 84, 648-650 (2004) Pyramidal QDs as Single-Photon Emitters: Hanbury Brown and Twiss correlation measurements

17. Controlled Photon Emission from 0D Excitons: Exciton dynamics probed by photon correlations QD PL spectra X-X correl. X - -X - X - -X 2X-X 2X-X -

18. Carrier Transport into Quantum Wires: Preferential Injection via connected quantum wells  Low-energy QWs form next to wires  Carriers injected via QWs into quantum wires H. Weman et al., Appl. Phys. Lett. 73, 2959 (1998);79, 1402 (2001)

19. Electronic States in Pyramidal QDs: Finite element k.p modeling ground state first excited state  t t qw h w quantum dot lateral quantum wells Z Y [112] [111 ] [110] X F. Michelini et al.

20. Electronic States in Pyramidal QDs: Impact of vertical quantum wire ground state second excited state Without WireWith Wire F. Michelini et al.

21. Single Quantum Dot Light Emitting Diode: Preferential carrier injection into a single dot quantum dot Vertical Quantum wire + - QWR s VQWR GaAs VQWQWs QD PL EL VQWR  Quantum dot light emitting diode structure  Emission from vertical QWR and QD only (at low current) QD VQWR M. Baier et al., APL, 2004 (in print)

22. QDs Embedded in Photonic Crystals: Energy tuning of ground and excited state transitions QD in Hexagonal PhC « Defect » S. Watanabe et al. Wavelength-Dispersive CL images  QD positioned in a photonic crystal microcavity  Emission energy tuned by epitaxial growth effect

23. Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates Summary: - Self-ordering during epitaxial growth on non-planar substrates is useful for producing high quality QWRs and QDs -New excitonic states are made stable by lateral quantum confinement in QWRs and QDs -Low-dimensional quantum nanostructures should be useful in novel optoelectronic devices such as single photon emitters and optically active photonic crystals

24. Collaborators: Crystal growth: A. Rudra, E. Pelucchi Nanofabrication and nanocharacterization: B. Dwir, K. Leifer, S. Watanabe, C. Constantin Optical spectroscopy: D. Oberli, H. Weman, A. Malko, T. Otterburg, M. Baier Theory: M.-A. Dupertuis, F. Michelini Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates