Charged exciton complexes (trions) in low dimensional structures

1. Charged exciton complexes (trions) in low dimensional structures by Dmitri Andronikov at JASS 2005

2. Outline • Introduction • Experiment • Results and discussion • Conclusions

3. Outline • Introduction • Experiment • Results and discussion • Conclusions

4. What are Trions? X- Trion X+ Trion + + - electrons hole holes electron Similar to ionized hydrogen molecule H2+ if me<<mh Similar to hydrogen ion H-if me<<mh

5. Short history of trions • 1958 Theoretical prediction of trion existence by Lampert • 1977-78 Discovery of charged excitons in bulk Ge, Si and CuCl • 1993 First report on X- trion in a CdTe/CdZnTe semiconductor QW structure • 1995 First report on X+ trion in a GaAs QW

6. Singletandtriplettrion states Wavefunction of an ion with two electrons Sz = +1 Sz = 0 triplet Sz = -1 Spatial part of the wavefunction Sz = 0 singlet + singlet state - triplet stateUnlm¹ 0 , only ifL ¹ 0 (1S state and 2P state) e or if one electron is in 1S state, and the other is in 2S state Spin part of the wavefunction Sz = 0 Sz = ±1,0 singlet state triplet state

7. Singletandtriplettrion states in the magnetic field In the magnetic field each trion level splits into two due to Zeeman effect thus giving 6 triplet and 2 singlet levels The allowed optical transitions must satisfy the optical selection rule Sfinal+Sinit = ±1. This gives 6 possible transitions to/from trion singlet and triplet states

8. Exciton and trion binding energies in 3D and 2D structures GaAs CdTe ZnSe 4.2 meV 10 meV 20 meV 9 meV 20 meV 30 meV 1 meV 2.5 meV 5 meV 13.6 eV 3D EXbulk EXQW 2D 100A QW ETQW

9. Structures with modulation doping 2DEG 2DEG concentration varies fromne=5*109cm-2to9*1011cm-2

10.  electron trion photon exciton Trion PL and reflectivity spectra The formation time of a trion inZnSe based structures is of the order of 2 –4 ps compared to radiative lifetime of the exciton 40-50ps

11. Singlet trion in the magnetic field

12. Landau levels The polarization of the trion reflectivity line can be used to determine the electron concentration by a pure optical method For the filling factors less than 1 only the lowest electron Landau level will be populated leading to full spin polarization of the 2D electron gas e (-1/2) X(+1/2, -3/2) s- T(-1/2, +1/2, -3/2)

13. Outline • Introduction • Experiment • Results and discussion • Conclusions

14. Experimental structures 2DEG Two samples with different 2DEG concentration were analyzedne=3*1010cm-2and3.7*1011cm-2

15. Experimental setup Magnetic field pulse

16. Pulse magnet power source Supplying power to the world's strongest long-pulse magnet at Los Alamos' National High Magnetic Field Laboratory is a 1.4 billion-watt generator, itself the largest among magnetic power sources. It can produce enough energy to power the entire state of New Mexico.

17. Outline • Introduction • Experiment • Results and discussion • Conclusions

18. Polarized magnetoluminescence of the sample with ne=3*1010cm-2 triplettrion line has been found

19. Luminescence spectra analysis I PL maxima 3 meV The dark triplet trion line appears at 20T magnetic field and splits from the exciton line with the maximum splitting of 3meV at 45T

20. Luminescence spectra analysis II PL intensity The Gauss fit of the exciton-dark triplet trion band allowed us to distinguish the triplet trion line from the exciton line down to magnetic field lower than 15T

21. Luminescence spectra analysis III Field dependence of the PL lines for the sample withne= 3.7*1011cm-2 For the sample with higher electron concentration the triplet line can be followed down to the fields as low as about 13T

22. Reflectivity At 6T magnetic field the reflectivity line of the singlet trion is fully polarized Magnetic field In reflectivity no line is found that would energetically coincide with the position of the dark triplet trion line found in the PL

23. Singlet and triplet trion states formation mechanism The triplet trion level is formed from the lowest Zeeman sublevels of both electrons and excitons and thus will be the most populated

24. Calculation of the exciton-electron energetic system tform = 10 ps, tX = tX- = 30 ps, tK= 12 ps, trecX= 40 ps, trecTs = 60 pstrecTt= 50trecTs.

25. The results of the PL calculation calculation experiment The intensity of the triplet PL is already noticeable at sufficiently low magnetic field

26. Outline • Introduction • Experiment • Results and discussion • Conclusions

27. Conclusions • Optically active and forbidden singlet and triplet trion states have been observed in reflectivity and PL spectra. • A model explaining the appearance of the dark triplet trion state in the PL spectra from the studied structures state has been suggested.It is based on the fact that the singlet trion state formation mechanism is suppressed by the magnetic field, meanwhile the formation of the optically inactive triplet trion state, observed in the experiment, is stimulated in the magnetic field at low temperatures. • A model calculation based on the system of kinetic equations of the PL of the trion exciton energetic system has been carried out. The model took into account the singlet and triplet trion formation mechanisms in the magnetic field. The results of the calculation are in good agreement with the experiment.

28. THANK YOU!!!!