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Mechanism for suppression of free exciton no-phonon emission in ZnO tetrapod nanostructures

Mechanism for suppression of free exciton no-phonon emission in ZnO tetrapod nanostructures. S. L. Chen 1) , S.-K. Lee 1) , D. Hongxing 2) , Z. Chen 2) , W. M. Chen 1) , and I. A. Buyanova 1) 1) Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden

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Mechanism for suppression of free exciton no-phonon emission in ZnO tetrapod nanostructures

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  1. Mechanism for suppression of free exciton no-phonon emission in ZnO tetrapod nanostructures • S. L. Chen1), S.-K. Lee1), D. Hongxing2), Z. Chen2), W. M. Chen1), and I. A. Buyanova1) • 1)Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden • 2) Surface Physics Laboratory, Department of Physics, Fudan University, Shanghai, China

  2. Outline • Introduction and motivation • Experimental • Samples • Methods • Results • Temperature dependent PL of tetrapods ensemble and bulk ZnO • Spatially resolved CL of individual tetrapods • Mechanism for suppression of no-phonon free exciton emission • Summary

  3. Motivation and Objectives • Why ZnO? • Attractive fundamental properties: • Wide and direct band-gap: ~ 3.37 eV • Large exciton binding energy: ~60 meV • A large variety of interesting morphologies, e.g. tetrapod nanostructures • Promising for various applications: • Novel nano UV light emitters • Gas sensors, etc. • Requires understanding of optical properties 3

  4. Motivation and Objectives ZnO nanostructures: weak no-phonon (NP) free exciton (FX) emission, strong longitudinal optical (LO) phonon-assisted FX transitions ? Enhanced exciton-phonon coupling? Objectives:To clarify this issue by employing PL and cathodoluminescence (CL) spectroscopies

  5. Samples and Methods • Samples: • Tetrapod ZnO nanostructures: • Growth method: thermal evaporation • Growth temperature : ~900 ºC • Reference, commercially available bulk ZnO • Methods: • Scanning electron microsocpy (SEM) • Cathodoluminescence • Measurement temperature: 300 K • Detection: a grating monochromator with a photomultiplier tube • Cw- photoluminescence (PL) • Excitation: a Verdi/MBD-266 laser system (λ=266 nm) and Ar+ laser (λ=351 nm) • Measurement temperature: 5-300 K • Detection: a grating monochromator with a charge-coupled device (CCD)

  6. PL : bulk & tetrapod ZnO [5K] Bulk ZnO : Neutral donor-bound exciton (BX), 3.361 eV Tetrapod ZnO : Surface exciton (SX), 3.367 eV  Higher contribution of the SX in the tetrapod structure due to increased surface to volume ratio

  7. PL : bulk & tetrapod ZnO [Temperature dependence] • T↑  FE emission • Intense FX-nLO transitions Expected for highly polar ZnO, promoted by a strong Fröhlich interaction • No-phonon FX line is more pronounced in bulk ZnO • Origin? PL Intensity (a.u)

  8. Suppression of the NP FE emission : Role of surface band bending Photon Energy (eV) • Exciton-phonon coupling: • Depends on spatial distributions of electron and hole charge densities • Enhanced in proximity to the surface and, therefore, in nanostructures? • PL under different Wex: • Constant relative intensities of the NP and LO assisted transitions • The surface band bending is NOT the origin of the strong FX-1LO transitions in the tetrapods Wavelength (nm)

  9. Suppression of the NP FE emission : Role of surface band bending Photon Energy (eV) • Depth profiling using CL: • Excitation depth L= 0.2 mm (Va= 5 keV)  Strong FX line • Excitation depth L= 2 mm (Va= 20 keV)  Weak FX line • The surface band bending is NOT the origin of the strong FX-1LO transitions in the tetrapods Wavelength (nm)

  10. CL : individual tetrapod [RT] • SEM: Changes in morphology and shape within the tetrapod • multi-faceted shape close to the core • round shape in the main part of the leg • CL: Weaker NP emission from the core regions • Origin • Strong absorption in tetrapods

  11. Mechanisms : Re-absorption of the NP FX • Ray optics: • More probable internal reflections for multi-faceted structures  Suppression of the NP FX line

  12. Mechanisms : Re-absorption of the NP FX • Refractive index: • Strong spectral dependence, ↓ close to the FX resonance • Strong increase of the critical angle in the vicinity of the FX resonance • Strong internal reflections and re-absorption of the NP FX transitions

  13. Summary The NP FX transition is suppressed in ZnO tetrapod structures The suppression is not affected by changes in surface band bending The suppression depends on the structural morphology of the tetrapods and is the most pronounced in the faceted regions The effect is attributed to enhanced re-absorption due to multiple internal reflections, which become especially pronounced in the vicinity of the FX resonance. Appl. Phys. Lett. 96, 033108 (2010) 13

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