Vladan Mlinar 2009 American Physical Society March Meeting

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Contributed talk at the 2009 American Physical Society March Meeting:

http://meetings.aps.org/Meeting/MAR09/SessionIndex2/?SessionEventID=104219

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Vladan Mlinar 2009 American Physical Society March Meeting

  1. 1. Effect of Atomic-Scale Randomness on the Optical Polarization of Semiconductor Quantum Dots Vladan Mlinar and Alex Zunger National Renewable Energy Laboratory, Golden, Colorado USA Vladan.Mlinar@nrel.gov
  2. 2. Structure – Spectra relationship: QD morphology: Single-dot spectroscopy: Ga1-xInxAs QDs (P. A. Dalgarno & (~105 atoms) R. J. Warburton)
  3. 3. Structure – Spectra relationship: QD morphology: Theory: Single-dot spectroscopy: Input structure Ga1-xInxAs QDs Strain minimization (P. A. Dalgarno & (~105 atoms) R. J. Warburton) Solve the single-particle problem Solve many-particle Problem (CI calc.) Emission Spectra
  4. 4. Structure – Spectra relationship: QD morphology: Theory: Single-dot spectroscopy: Input structure Ga1-xInxAs QDs Strain minimization (P. A. Dalgarno & (~105 atoms) R. J. Warburton) Solve the single-particle problem Solve many-particle Problem (CI calc.) Emission Spectra
  5. 5. Self-assembled QDs are usually alloyed: Ga1-xInxAs: As In Ga In Random realization (RR) - particular random assignment of the In and Ga atoms onto the cation sublattice.
  6. 6. Self-assembled QDs are usually alloyed: Ga1-xInxAs: As In Ga In Random realization (RR) - particular random assignment of the In and Ga atoms onto Different local environments: the cation sublattice.
  7. 7. Optical Properties vs Atomic-Scale Randomness Ga1-xInxAs: Large bulk solids: different random realizations (RRs) get self- averaged, so the measured physical property does not resolve features of individual RRs. Finite nanosystems (≤105 atoms): self-averaging of RRs may not be complete, so we can observe the effect of individual RRs (atomic- scale alloy randomness effect). What is the effect of atomic-scale randomness on the optical properties of Gax-1InxAs QDs?
  8. 8. X0: Fine structure splitting (FSS)
  9. 9. X0: FSS & Polatization directions vs RRs? S. Seidl et al., Physica E 40, 2153 (2008) –Conference proceedings.
  10. 10. X0: FSS & Polatization directions vs RRs? ? S. Seidl et al., Physica E 40, 2153 (2008) –Conference proceedings.
  11. 11. X0: Linear Polarization Ratio (P) vs RRs? • P is a measure of the in-plane polarization anisotropy P = (Ix - Iy)/(Ix + Iy) Where Ix and Iy are intensities defined along [110] and [1-10] direction • Atomic-scale randomness vs geometrical anisotropy? I. Favero et al., APL 86, 041904 (2005).
  12. 12. X0: Linear Polarization Ratio (P) vs RRs? ? • P is a measure of the in-plane polarization anisotropy P = (Ix - Iy)/(Ix + Iy) Where Ix and Iy are intensities defined along [110] and [1-10] direction • Atomic-scale randomness vs geometrical anisotropy? I. Favero et al., APL 86, 041904 (2005).
  13. 13. X0: FSS changes by more than a factor 7 with RRs
  14. 14. X0: FSS changes by more than a factor 7 with RRs FSS exhibits significant dependence on the RRs (from 1.1 - 8.5 μeV)
  15. 15. X0: FSS changes by more than a factor 7 with RRs FSS exhibits significant dependence on the RRs (from 1.1 - 8.5 μeV) FSS shows almost no sensitivity to piezoelectric field, irrespective of piezoelectricity was included via linear term only, or both linear and nonlinear terms.
  16. 16. X0: Polarization Directions vs RRs Vladan Mlinar and Alex Zunger, Phys. Rev. B 79, 115416 (2009)
  17. 17. X0: Polarization Directions vs RRs Vladan Mlinar and Alex Zunger, Phys. Rev. B 79, 115416 (2009)
  18. 18. X0: Polarization Directions vs RRs Vladan Mlinar and Alex Zunger, Phys. Rev. B 79, 115416 (2009)
  19. 19. X0: P is not affected only by geometrical elongation • Even 50% elongation in [100] direction gives the same range of P, as in geometrically symmetric QD! • Measuring P cannot tell: (1) the geometrical anisotropy (2) composition
  20. 20. FSS & Polarization Directions of Multiexcitons • QD can be charged by controllable number of electrons and holes: X-2, XX0, XX-1, XX+1 • FSS of multiexcitons is sensitive to RRs. • Optically active transitions of different multiexcitons do not have fixed polarization directions Poem et al., PRB 76, 235304 (2007): Different multiexcitonic transitions have well defined polarization directions.
  21. 21. X-2: Conflicting experimental results Poem et al., PRB 76, 235304 (2007): Ediger et al., PRL 98, 036808 (2007): • Polarization directions of the optically Polarization directions of the optically active transitions oriented along active transitions oriented along [120] and [2-10] [110] and [1-10] • Optically active transitions of X-2 have well define polarization directions
  22. 22. X-2: Conflicting experimental results Poem et al., PRB 76, 235304 (2007): Ediger et al., PRL 98, 036808 (2007): • Polarization directions of the optically Polarization directions of the optically active transitions oriented along active transitions oriented along [120] and [2-10] [110] and [1-10] • Optically active transitions of X-2 have well define polarization directions Different X-2 emission lines DO NOT have fixed polarization directions!
  23. 23. Conclusions: We provided a clear evidence for the effect of atomic-scale randomness on the optical properties of alloyed Ga1-xInxAs QDs. • Fine structure splitting of the monoexciton changes by more than a factor of 7 with RRs. • For multiexcitons, the polarization directions strongly depend on the atomic scale randomness, so different multiexciton emission lines do not have fixed polarization directions. • Optical polarization is affected both by atomic-scale randomness and by possible geometric elongation of the QD in one direction. Vladan Mlinar and Alex Zunger, Phys. Rev. B 79, 115416 (2009) Thank you for your attention!
  24. 24. Comparison with other calculations: PRB 77, 113305 (2008): This work: • Investigated effect of intermixing profiles • Investigated effect of atom-by-atom random substitution • EBOM does not have atomic resolution • LCBB full atomic resolution •Uniform composition profile vs. •Uniform composition profile, but nonuniform profile different random realizations • Linear polarization ratio as a tool for • Linear polarization ratio cannot be used structural characterization as a tool for structural characterization
  25. 25. FSS & Polarization Directions of Multiexcitons
  26. 26. FSS vs RRs of Multiexcitons:
  27. 27. Polarization Directions of Multiexcitons:

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