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Thang Ba Hoang

Investigation of Single Semiconductor Nanowire Heterostructures Using Polarized Imaging Spectroscopy (CdS, GaAs/AlGaAs). Thang Ba Hoang. Department of Physics, University of Cincinnati, Cincinnati, OH. Nanowires for opto-electronic nano-devices. Nanowire high-performance FET.

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Thang Ba Hoang

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  1. Investigation of Single Semiconductor Nanowire Heterostructures Using Polarized Imaging Spectroscopy (CdS, GaAs/AlGaAs) Thang Ba Hoang Department of Physics, University of Cincinnati, Cincinnati, OH

  2. Nanowires for opto-electronic nano-devices Nanowire high-performance FET Hybrid single-nanowire photonic crystal Nanowire biosensor Single Photon Emitter Nanowire avalanche photodiode

  3. Nanowire related publications (Source: ISI Web of Knowledge - http://isiknowledge.com/)

  4. Content • Introduction • Nanowires and nanowire growth • Micro-photoluminescence techniques for studying single nanowires • Optical properties of single CdS nanowires • Defect and surface related states • Temperature dependent photoluminescence • Photoluminescence imaging of defect states • Optical properties of single core-shell GaAs/AlGaAs nanowires • General photoluminescence properties • Polarization of photoluminescence • Resonant excitation photoluminescence • Polarized resonant excitation • Model for spin relaxation in single nanowires

  5. Nanowires • Nanowires are nanostructures which have diameters ranging from 30 nm to 150 nm, length from 5-20 mm • No significant quantum confinement because wire’s diameter > Bohr exciton diameter • For example: CdS Bohr exciton radius ~2.9 nm GaAs Bohr exciton radius ~10 nm • What make a nanowire different from bulk material? • Huge surface-to-volume ratio: (~108 m-1 for nanowires compared to ~102 m-1 for bulk materials) • strong sensitivity of the excitons to surface states and defects and structural inhomogeneities • Single nanowire measurements are required

  6. Nanowire growth Vapor-Liquid-Solid (VLS) growth GaAs Nanowires Pre-growth AsH3 Ga AsH3 Au 450oC, 30min reactants GaAs 600oC, 10 min desorb surface contaminants and form eutectic alloy. wire diameter: controllable by Au catalyst

  7. core GaAs (650oC, 15 min) shell AlGaAs Core-Shell GaAs/AlGaAs Shell AlGaAs: to increase the quantum efficiency by reducing non-radiative surface recombination Field-Emission Scanning Electron Microscope (FESEM) image: nanowires have tapered shape.

  8. Single nanowire studies GaAs AlGaAs AFM ~40 nm ~80 nm Core-shell GaAs/AlGaAs Nanowires were removed from the growth substrate into solution and deposited onto a silicon substrate

  9. Conduction band (CB) k E ħ~Eg Photoluminescence Valence band (VB) Photoluminescence (PL) A light source (laser) excites electrons from the valence band to the conduction band, leaving holes in valence band. Electrons and holes recombine to emit photons.

  10. e- h+ E Ebin Eg K Excitons • Electron-hole correlation: Exciton • Hydrogen-like bound state of an electron-hole pair • Smaller binding energy (1/1000) • Larger Bohr radius(100 times) • Energy spectrum of an exciton vacuum

  11. Spectrometer L - Lens BS - Beam Splitter DP - Dove Prism 1.5 mm spatial resolution 2D CCD image L CCD Defocusing lens DP Y spatial laser BS Tunable E sample emission energy T=10 K X-Y-Z translation stage Slit-confocal microscopy Experimental setup

  12. PL image rotated PL image from sample Dove prism y Image on CCD camera slit Dove prism to rotate PL image Dove Prism (DP) can be used to rotate image of a nanowire (rotate an image twice the angle that it rotates through ) (Edmund Optics Co.http://www.edmundoptics.com/ )

  13. Single CdS nanowires

  14. CdS nanowires • Majority of nanowires are straight and uniform • Few have significant irregularities SEM • Individual nanowire: ~ 50 – 200 nm in diameter ~ 10 – 15 μm long • Nanowires were removed from the growth substrate into solution and deposited onto a silicon substrate (for single wire study)

  15. Nanowire morphology and optical properties Room temperature Low temperature Irregular-shaped wires Uniform wires Room temperature emission is similar regardless of wire morphology: Near Band Edge emission Low temperature PL differs significantly

  16. A Uniform Wire ~45 – 50 nm Irregular Wire ~100 – 200 nm B B’ A’ 45 nm 150 nm Single nanowire studies We show two representative nanowires: AFM images of the two nanowires: with morphological irregularities straight and uniform

  17. Room-Temp. PL vs Low-Temp. PL Room temperature Low temperature • Low Temperature: the PL properties of the 2 wires differ significantly • Sharp lines are attributed to defect or surface state related emission • Room Temperature: the PL spectra of the 2 wires are alike • Single NBE (Near Band Edge) line emission

  18. Low-Temp. PL imaging Narrow lines occur at specific localized positions along the wires Only NBE emission with occasional small energy variation Excitons localized to particular positions along the wire Compositional/strain fluctuation

  19. NBE Defect-related emission Time-resolved photoluminescence

  20. Time-resolved photoluminescence

  21. Temperature dependence b) - Narrow lines start decreasing in intensity at 30 K and disappear by 90 K - NBE emission becomes the only peak as the temperature increases - Energies of the NBE emission and the localized states follow the band edge as temperature increases - Indicates that localized states are not deep levels but are excitonic This is consistent with time-resolved PL measuring of lifetime!

  22. Single Core/shell nanowires core GaAs (40nm) shell AlGaAs (~20nm)

  23. Core-shell GaAs/AlGaAs Bare (uncoated) GaAs nanowires: low quantum efficiency due to nonradiative surface recombination Undoped MBE-grown GaAs epilayer (PL at 5K) (Heiblum et al. J. Vac. Sci. Tech. B2 233 (1984))

  24. Single nanowire imaging • 2D image and extracted PLs: • Broad spectrum (FWHM ~25 meV) • Emission energy and line-shape are uniform • Lower energy shoulder: defect-related • No evidence of quantum confinement

  25. Temperature dependent PL • PL quenches at T > 140K (activation energy ~17 meV) • Presence of non-radiative centers

  26. Dielectric mismatch “Perfect, infinitely long cylinder” Dielectric mismatch: Excitation: Laser into page (Ruda et al. PRB 72 115308) Emission: PL out of page For GaAs:

  27. photoluminescence laser GaAs/AlGaAs NW: PL Polarization

  28. ┴ NW ║ Exciton Non equilibrium spin dynamics Our motivation: dielectric “confinement” of exciton dipole field (D<<l): Exciton densities Photoluminescence intensities N║= N┴ I║ >> I┴ Spin relaxation time ts ? t║ << t┴ We are interested in exciton spin dynamics in single nanowires

  29. core GaAs Tune excitation energy, E Laser , record PL intensity (PLE) Elaser shell AlGaAs AlGaAs AlGaAs GaAs E 2-LO resonances 1-LO GaAs PL hemission hexcitation real space k-space Resonant excitation r

  30. Resonant excitation Clear resonances at 36, 73 and ~133 meV above free exciton energy.

  31. Resonant excitation 1-LO and 2-LO GaAs phonons Resonance at ~133 meV: • Defect-AlGaAs related. or • Bottom of AlGaAs band -Al concentration ~10%, instead of nominal growth concentration 26% How does the polarization depend on excitation energy?

  32. Excitation energy dependent polarization Polarization changes with excitation energy!

  33. Resonant excitation creates non-equilibrium exciton spin distributions • As excitation comes closer to free exciton energy: • Excite parallel: polarization increases • Excite perpendicular: polarization decreases • Wire 2: thermal equilibrium N║ = N┴

  34. ts ts Modeling exciton dynamics Fermi’s golden rule At thermal equilibrium (highest energies) assume:

  35. Spin relaxation time Steady state: Spin relaxation time depends on excitation energy ts ~ 1 - 50 ps tnr ~ 50 ps “Non-Equilibrium Exciton Spin Dynamics in Resonantly Pumped Single Core-Shell GaAs-AlGaAs Nanowires” Thang. B. Hoang, L.V. Titova, J. M. Yarrison-Rice , H. E. Jackson, , A. O. Govorov, Y. Kim, H. J. Joyce, H. H. Tan, C. Jagadish, L. M. Smith Nano Letters 7 588 (2007)

  36. Summary We study optical properties of single CdS and GaAs/AlGaAs Nws: • CdS nanowires: • Room temperature nanowire PL not sensitive to morphological irregularities or defects. • Low temperature (< 20 K) extremely sensitive to such defects. • Spatially resolved PL imaging of defect states • Time-resolved PL shows quantum efficiency can be increased by removing such defects or surface states • Low temperature PL provides quick and non-destructive method for rapidly characterizing NW growth • Exciton spin dynamics from resonantly excited single GaAs-AlGaAs nanowires: • Resonances observed at 1-LO and 2-LO and ~133meV (AlGaAs related) above the PL emission line • Dependence of PL polarization on excitation energies • Spin relaxation time varies from ~1 ps at high energies to ~50 ps near resonance

  37. Rate equations

  38. E Conduction Band Eg k hh Valance Band ESO lh lh (SO) Optical “bright” states - “Bright” states: Δm = ±1 - Ignore the split-off bands

  39. Acknowledgements

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