Ordered Quantum Wire and Quantum Dot Heterostructures Grown

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) q Introduction q Self-ordering on nonplanar substartes q Neutral and charged low-D excitons q Contacting single QWRs and QDs q Summary and outlook ADMOL, Dresden, Germany, February 23 -27, 2004

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

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

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

Lateral Patterning during Epitaxial Growth: Controlling lateral fluxes with the surface chemical potential 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)

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

Excitons in Quantum Wires: Signatures of a 1 D system Experiment: PL-excitation spectra Theory: excitonic absorption Ø Excitonic transitions dominate (reduced Sommerfeld factor in 1 D) Ø Polarization anisotropy due to valence band mixing Ø Enhanced exciton binding energy (14. 5 me. V) deduced M. -A. Dupertuis et al. , to be published

Contacting a Single Quantum Wire: 1 D Electron Gas in V-Groove QWRs - + - QWs + - + - + + + + - + Etched Areas Current flow + + + - - wire - -- + + + - - 1 µm - + QWR Ø Moduation-doped V-groove QWR structure Ø Wire contacted via 2 D electron gas on sidewalls Ø Conductance quantized close to 2 e 2/h Ø Discrepancy due to quantum contact resistance D. Kaufmann et al. , Phys. Rev. B 59, R 10433. (1999) QWR

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

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

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

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

Single Quantum Dot Spectroscopy: Origin of optical transitions QWR ~ 3 -4 nm Back-Etched Pyramids 10 K, 1 W on single pyramid QD ~ 6 nm Micro-PL of Single Pyramids QW ~ 1 -1. 5 nm VQW Monochromatic CL Imaging 1. 60 e. V QD A. Hartmann et al. , J. Phys. : Condens. Matter 11 5901 (1999) 1. 94 e. V 1. 70 e. V QWR QW

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

Quantum Dots in an N-type Environment: Charged excitonic complexes A. Hartmann et al. , PRL 84, 5648 (2000) laser = 2. 42 e. V Experiment Theory Full CI model 3 e-2 h 4 X 3 X 2 X 2 e-h X 3 e-h 2. 5 n. W 3 e-h 4 e-h 5 e-h 6 e-h 4 e-h 30 p. W Single exciton regime Multi exciton regime 600 n. W 2 X 2 e-h 3 e-h 4 e-h 5 e-h 6 e-h X

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

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

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

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

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

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

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

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

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