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Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates

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

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Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates

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

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

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

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

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

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

  8. 1 µm + - - QWs + + - - - + + - - - Etched Areas - + + - + + - - + Current flow + - - wire + + + + + - - - - - - + + + + QWR + + + + + QWR • Moduation-doped V-groove QWR structure • Wire contacted via 2D electron gas on sidewalls • Conductance quantized close to 2e2/h • Discrepancy due to quantum contact resistance D. Kaufmann et al., Phys. Rev. B 59, R10433.(1999) Contacting a Single Quantum Wire: 1D Electron Gas in V-Groove QWRs

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

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

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

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

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

  14. l = -1 0 +1 p QD s s AlGaAs p l = -1 0 +1 X X- - X- 2X Emission Chrage control by photoexcitation Energy 2D harmonic oscillator model n ~ 1017 cm-3 background doping Multi-Particle States in Quantum Dots:Excitonic states and charging mechanism

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

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

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

  18. 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) Carrier Transport into Quantum Wires:Preferential Injection via connected quantum wells

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

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

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

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

  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. Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates 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

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