Chu-Shou Yang

1. Growth and optical properties of II-VI self-assembled quantum dots Chu-Shou Yang Prof. W.C. Chou (NCTU) Prof. J.S. Wang (CYCU) Prof. J.L. Shen (CYCU) Prof. J. Lee (CYCU) Prof. D.J. Jang (NSYU) Dr. Y.C. Cheng (CSIST) 2005/8/4

2. Outline • Introduction of II-VI self-assembed quantum dots (SAQDs) • Growth mechanisms and optical properties of CdSe SAQDs • Growth and optical properties of type-II ZnTe SAQDs • The fabrication of the ZnTe SAQDs light emitting diode (LED) • Conclusion

3. Progression of Jth on GaAs based lasers DOS DOS DOS DOS E E E E GaAs p-n • Low threshold current • High thermal stability • High efficiency N. N. Ledentsov, et al., IEEE J. Select. Topics Quantum Electron. 3, 196 (2000).

4. ( Interface energy + epilayer energy ) compare with ( substrate surface energy ) coherentincoherent interfacelarge Ge QDs/Si layer by layer3MLSK small Ge QDs/Si critical island critical thicknessdislocation-free size for dislocation Key: reducing island size will lead to dislocation D.J. Eaglesham and M. Cerullo, PRL 64, 1943 (1990)

5. The different of growth dynamics of III-V and II-VI SAQDs S. Mackowski et al. Phys. Rev. B69, 205325 (2004)

6. Energy Gap vs Lattice Constant

7. Vecco Applied EPI molecular beam epitaxy (MBE) (CYCU) Sources: Zn, Mn, Se, Te, Cd, , ZnCl2 and O2 → ZnSe, ZnTe, CdSe, CdTe, ZnSeTe, ZnCdSe, ZnMnSe, CdSeTe etc., and n-type compound

8. MBE (CSIST) Riber 32P Two chamber: II-VI and III-V Sources(II-VI): Zn, Mg, Se, Te, Cd, ZnS, ZnCl2 and N2

9. Growth mechanisms and optical properties of CdSe SAQDs • Grow CdSe with 2.0, 2.5, 2.7, 3.0, and 3.3 mono-layers(MLs) by MBE • Growth meachnisms • 2D platelets → 2D platelets + QDs →fully developed QDs • Temperature dependent PL more than 3.0 MLs Below 2.5 MLs

10. (a) ZnSe (b) 2.0 MLs (c) 2.5 MLs (d) 2.7 MLs 1μm AFM images of different average CdSe coverage (e) 3.0 MLs (f) 3.3 MLs (g) 3.5 MLs (h) 3.7 MLs (TG= 260 ℃)

11. (a) ZnSe only (b) CdSe < 2.5 MLs (c) CdSe ~2.7 MLs (d) CdSe ~3.0 MLs ZnSe buffer wetting layer CdSe QDs The schematic diagram of growth dynamics for self-assembled CdSe QDs. 2.3 MLs HRTEM M. Strassburg et al. APL 76, 685 (2000) 1.5 ML STEM C.S. Kim et al. PRL 85 1124 (2000)

12. PL of different CdSe coverage CdSe QD state 3D-QDs 3D-QDs 2D-QW 2D-QW (i) (ii) ZnSe ZnSe ZnCdSe QW

14. CdSe QD state

15. Optical properties ZnTe quantum dots • The coverage of ZnTe were change from 2.0 to 4.0 MLs • Temperature dependent PL • PL peak energy • Full width at helf maximum

16. buffer 1.8 MLs 2.8 MLs 2.5 MLs RHEED patterns and AFM of ZnTe QDs Buffer, roughness~0.56 1.8MLs, roughness~0.5 2.8 MLs roughness~1.039 density~9x108 cm-2 2.5 MLs, roughness~1.03 2.0 MLs < ZnTe coverage < 4.0 MLs No dot

17. ZnTe QDs ZnSe GaAs The AFM image of ZnTe QDs Lattice mismatch～ Dot density ~ 1.5x109 cm-2 hight ~ 7.86 nm diameter ~ 40.5 nm

18. The PL spectra of ZnTe QDs with different coverage

19. H △EC ~ 0.6 eV 1.8 eV △EV ~ 1.0 eV ZnSe ZnTe ZnSe The schematic type II ZnTe/ZnSe band structure D ZnSe H ~ 8 nm ZnTe Wetting layer ZnSe

20. ZnSe ZnTe ZnSe

21. ZnSe ZnTe ZnSe Power dependent PL

23. I and I’ : ZnTe QDs III : ZnTe WL or ZnSe matrix * M. Funato et al., Appl. Phys. Lett., 80, 443 (2002).

24. I (I’) and II (II’) : ZnTe QDs III : ZnTe WL or ZnSe matrix * M. Funato et al., Appl. Phys. Lett., 80, 443 (2002).

25. Γ(T) = Γ0 + Γa T+ + Γi exp (- <Eb> / kT) Where,Γ(T) and Γ0 : the line-width at temperatures T and 10 K Γa : a coefficient of the exciton-acoustic phonon interaction ΓLO1 : the strength of the exciton-longitudinal optical phonon (LO1) interaction ħω : the LO phonon energies for the ZnTe epilayers Γi : accounts for the density of the impurity center <Eb> : the binding energy averaged over all possible impurities

26. I (I’) and II (II’) : ZnTe QDs III : ZnTe WL or ZnSe matrix where I0 (T) : the integrated PL intensity at temperatures T I0 (0) : fitting parameter k : Boltzmann constant D : a fitting constant Ea : the activation energy

27. p+ ZnTe p-ZnTe/ZnSe SL p-ZnSe p-ZnSSe ZnSS ZnSe Zn0.88Cd0.12Se (10 nm) ZnTe QDs Zn0.88Cd0.12Se (10 nm) ZnSe n-ZnSe p+ ZnTe (50 nm) n-GaAs p-ZnTe/ZnSe SL (b) LED No2 p-ZnSe (~700 nm) p-ZnSSe (50 nm) ZnS0.13Se0.87 (5 nm) ZnTe QDs 3.0 MLs ZnSe (10 nm) n-ZnSe (~700 nm) n-GaAs (a) LED No1 The fabrication of the ZnTe SQDs light emitting diode (LED)

28. ZnTe p+ ZnTe ZnCdSe ZnCdSe p-ZnTe/ZnSe SL p-ZnSe p-ZnSSe CB ZnSS ZnSe 2.4 eV Zn0.88Cd0.12Se (10 nm) ZnTe QDs 2.8 eV 2.5 eV Zn0.88Cd0.12Se (10 nm) ZnSe n-ZnSe VB n-GaAs LED No2 ZnSe (1 nm) The schematic band diagrams

29. P contact by ITO N contact by In metal

30. p+ ZnTe p-ZnTe/ZnSe SL p-ZnSe p-ZnSSe ZnSS ZnSe Zn0.88Cd0.12Se (10 nm) ZnTe QDs Zn0.88Cd0.12Se (10 nm) ZnSe n-ZnSe n-GaAs LED No2 The thickness of un-doped ZnSe of LED No2 and No3 are 100 and 50 nm, respectively.

31. The electroluminescence of ZnTe QDs LED Bias ~ 20 V Current ~ 30 mA E1 ～ 582.1 nm(2.13 eV) E2 ～ 629.3 nm (1.97 eV)

32. ZnTe ZnCdSe ZnCdSe CB 2.4 eV 2.8 eV 2.5 eV VB Temperature dependent PL of LED No.2 Pump laser: 457 nm 50 mW 457 nm (2.71 eV) can pump all region

33. ZnTe ZnCdSe ZnCdSe CB 2.4 eV 2.8 eV 2.5 eV VB Temperature dependent PL of LED No.2 Pump laser: 514 nm 50 mW 514 nm (2.41 eV) only can pump this region

34. Conclusions • The growth mechanism of CdSe SAQDs start from 2D platelets. When coverage more than 2.5 MLs, the small dot form in the 2D platets. After 3.0 MLs coverage was deposited, the fully developed QD was taken shape. • In the work of ZnTe QDs, the PL peak energy is found to decrease with the dot size. • For the 2.5 and 3.5 MLs samples, the PL peak energy decreases monotonically with the increasing temperature. However, for the 3.5 MLs sample, as the temperature is increased, the PL peak energy exhibits blue shifted first then red shifted as the temperature is increased above 40K. • Temperature dependent line width broadening model is employed to fit the high temperature data. • Finally, the orange light emitting diodes (LEDs) have been successfully fabricated by using the type II ZnTe QDs.