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Optical properties of excitons in ZnO-based quantum wells

Optical properties of excitons in ZnO-based quantum wells. RIKEN (The institute of Physics and Chemistry) Tohoku University Department of Physics Prof. Segawa In collaboration with Institute for Material Research Tohoku University Prof. Kawasaki. CHIA CHIN HAU 謝振豪 國立交通大學 電子物理所 褚德三教授.

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Optical properties of excitons in ZnO-based quantum wells

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  1. Optical properties of excitons in ZnO-based quantum wells RIKEN (The institute of Physics and Chemistry) Tohoku University Department of Physics Prof. Segawa In collaboration with Institute for Material Research Tohoku University Prof. Kawasaki CHIA CHIN HAU謝振豪 國立交通大學 電子物理所 褚德三教授

  2. Background and Research motivation ZnOLarge exciton binding energy（60meV) Exciton-related lasing action ZnO thin filmsObservation of RT lasing [1] promised material for UV-based optoelectronics Ohtomo et. al. [2] lattice-matched substrategood quality thin films and superlattices ZnO-based QWInvestigation of excitonic properties Realization of optical phenomena and application [1] P. Yu et. al., Solid State Commun. 103, 459 (1997) [2] A. Ohtomo, Ph. D. Thesis (Tokyo Institute of Technology, 2000)

  3. Exciton E electron Conduction band n=∞ n=2 Exb n=1 Eg Exciton Ex hole Exciton energy Valence band （Exciton binding energy）

  4. Symmetry of ZnO exciton ConductionZn-4s state Valence O-2p state G7 G5 - A exciton G5 – B exciton G1 – C exciton G7 G9 G7 Crystal field splitting Spin-orbit interaction

  5. Sample structure Made in Kawasaki lab by Dr. Ohtomo Zn1-xMgxO barrier (5nm) x=0.12 and 0.27 10-periods ZnO well Lw= 4.7, 4.2, 3.7, 2.8, 2.4, 1.8, 1.2, 0.9, 0.7 nm Combinatorial laser ablation MBE Heater Sample ScAlMgO4 substrate KrF excimer laser Characteristic During the growth, the parameter is constant Statistical error is small Target Mask Pattern Mask Rotation

  6. Substrate and Bandgap of barrier ZnMgO RT a=3.249 Å a=3.246 Å Lattice-mismatch is small (0.09%) Increase of Mg bandgap increase

  7. Lattice constant of ZnMgO Increase of a-axis length is only about 1% even for Mg content of 0.33 Cited from Ref : A. Ohtomo et. al., Appl. Phys. Lett 72, 2466 (1998)

  8. Well-width dependence of PL and absorption Mg 0.12 Mg 0.27

  9. ZnMgO ZnO ZnMgO subband Exciton binding energy n =1 Exb DEc Eg Well bandgap Ex Exciton energy DEv n =1 Confinement energy Ex = Eg (ZnO)+ DEc + DEv - Exb

  10. Determination of exciton binding energy Inelastic exciton-exciton scattering Energy Continuum n=∞ 1 Eg P∞ Ex P-band [1] H. D. Sun et. al., Appl. Phys. Lett 77, 4250 (2000)

  11. Quantum size effect of exciton binding energy Mg 0.27 High Mg Low Mg Mg 0.12 Theoretical Calculation G. Coli, K. K. Bajaj, Appl. Phys. Lett. 78, 2861 (2001)

  12. Absorption peak energy Calculation Square potential well model Finite barrier Mg 0.27 Mg 0.12

  13. Quantum size effect Large Mg concentration larger blueshift Small well-width exciton energy increases Combinatorial samples Peak energy increases smoothly data fluctuation is small Stoke’s shift is well-width dependence Localized exciton

  14. Time-resolved PL PL decay time is energy-dependent relaxation to localized states [1] tPL : Localized exciton lifetime Eme : Mobility edge Eo : localized potential depth [1] C. Gourdon et. al., Phys. Sol. Stat. (b) 153, 641 (1989)

  15. Mg 0.27 InternalE |C> LE QCSE |V>

  16. Spectral distribution of PL decay time for ZnO QW (0.9 nm) THG of Ti:S laser 82 MHz、2 ps Streak camera

  17. Spectral distribution of PL decay time for ZnO QW (4.2 nm) Overlap of electron-hole pairs wavefunction decreases by the internal electric field, thus, transition probability decreases lifetime increases

  18. LO-sidebands of PL Large Huang-Rhys factor S Exciton-LO phonon interaction depends on mutual distance of electrons and holes [Hopfield, J.Phys.Chem.Sol.,10,p.110 (1959)] Separation of electron and hole increased by internal electric field

  19. Excitation intensity dependence of PL Screening of internal electric field、PL blueshifts |C> |V> Increasing power

  20. Summary for linear properties of exciton • Quantum confinement effect in ZnO/ZnMgO MQWs is identified. As well-width decreases, the excitonic absorption energy blueshifts. • Localization effect due to well-depth fluctuation and barrier alloy composition fluctuation, is found in the QW samples adopted in this study. The origin of PL is radiative recombination of localized exciton. • Spectra for High Mg, Large well-width sample characterized by strain-induced internal electric field.

  21. Biexciton of ZnO bulk [1] ZnO bulk ---- (AA)G1 Biexciton 14.7 meV (AB)G5,6 Biexciton 9.5 meV (BB)G1 Biexciton 3.3 meV [2] ZnO thin film ----- 15meV Two-photon reabsorption spectroscopy PL lineshape analysis [1] J. M. Hvam, Phys. Status. Solidi B 118, 179 (1983) [2] A. Yamamoto, K. Miyajima, T. Goto, H. K. Ko, and T. Yao, J. Appl. Phys. Lett. 90, 4973 (2001)

  22. Formation of biexciton Energy 2 electrons 2Ex Exxb Exx Induced absorption Ex Two photon absorption 2 holes Biexciton energy Wavevector k （biexciton binding energy）

  23. Biexciton emission from ZnO QW Low threshold power for stimulated emission using biexciton emission High excitation Dye laser LX : Localized exciton LXX: Localized biexciton P: exciton-exciton scattering [2] H. D. Sun et. al., Appl. Phys. Lett 78, 3385 (2001)

  24. Excitation density dependence of PL LXX LX Support assignment of biexciton emission

  25. LX LXX？P？ Overlap of LXX and P emission band at RT Can the biexciton in ZnO QW be stable in RT ? Biexciton binding energy ? LXX LX

  26. Formation of biexciton by pump-probe 2|x> Ex Induced absorption Exxb |xx> Localization effect Biexciton binding energy unable to determine from PL loc|xx> emission Exx |x> loc|x＞ Ex |g>

  27. Set-up for pump-probe Monochromator f sample M f Pump Beam splitter shutter Excimer laser XeCl(308nm) f M M Dye cell (EXA351) M f AP Probe

  28. Differential absorption spectra I Differential absorption spectra ＝ After pump spectra – before pump spectra positive：induced absorption negative：absorption bleaching EIA: induced absorption energy Ex: exciton energy ABS Induced absorption Exciton to biexciton states Absorption bleaching Saturation of exciton states

  29. Differential absorption spectra II 0.00 0.00 0.00

  30. Band diagram hpump=4.0255eV 2|x> Exxb free|xx> loc|xx> EIA Induced absorption Ex free|x> loc|x> Ex |g> Free exciton to free biexciton states

  31. Fitting of absorption spectra Voigt function inhomogeneous homogeneous C: absorption strength;G: Gaussian width; W: Lorentzian width; P: peak energy

  32. Fitting of differential absorption spectra After pump Before pump Before pump C, G, W, P :taken from fitting results of absorption spectra After pump Gaussian G: suppose unchanged by extrinsic broadening Peak energy P: experimental results W’: fitting parameters C’ = C/(1+N/NS):reduction of exciton oscillator strengths

  33. Excitation density dependence of absorption spectra Peak energy is constant

  34. Fitting results I Induced absorption Induced absorption Lw= 2.8 nm Lw= 3.7 nm Saturation of exciton states Saturation of exciton states Absorption bleaching is due to reduction of exciton oscillator strength

  35. Fitting results II Induced absorption Induced absorption Lw= 2.4 nm Saturation of exciton states ABS

  36. Induced absorption band Biexciton binding energyExxb = Ex - EIA

  37. Well-width dependence of biexciton binding energy Largest well-width larger than bulk Smallest well-width 26 meV c.f. [1] (AA)G1 biexciton (15 meV) c.f. [2] (AB)G5,6biexciton (9 meV) [1] H. D. Sun et. al., J. Appl. Phys. 91, 1993 (2002) [2] J. M. Hvam, Phys. Status. Solidi B 118, 179 (1983)

  38. GaAs QW - Comparison c.f. [4] c.f. [1,2,3] [1] S. Adachi et. al., Phys. Rev. B.55, 1654 (1997) [2] D. Birkedal et. al, Phy. Rev. Lett.76, 672 (1995) [3] G. O. Smith et. al., Solid State Commun.92, 325 (1994 [4] J. J. Liu et. al., J. Appl. Phys.84, 2638 (1998) [5] D. A. Kleinman, Phys. Rev. B 28, 871 (1983) [6] I.-K. Oh et. al, Phys. Rev. B 60, 2528 (1999)

  39. Conclusion about biexciton in ZnO QW • Induced absorption of free excitons to free biexcitons was observed in nanosecond pump-probe measurement. • Biexciton binding energies were determined by extracting induced absorption bands from differential absorption bands. • Biexciton binding energy enhances as well-width decreases, from 16 meV in ZnO QW of 3.7 nm to 26 meV in QW of 1.8 nm。 • In the thinnest QW sample, biexciton binding energy is comparable to RT energy.

  40. ZnO/ZnOバッファ薄膜の反射スペクトル SCAM基板上直接成膜したZnOに比べて、バッファ上に成膜したZnOの反射スペクトルには高次励起子の構造がはっきりする。

  41. ZnO/ZnMgOバッファ薄膜の透過スペクトル 吸収スペクトルのA（ｎ=2）励起子共鳴エネルギーが発光スペクトルのそれとほぼ一致した。

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