Nir Tessler Microelectronic & Nanoelectronic centers Electrical Enginnering Dept. Technion, Israel Institute of Tech

1. The Quest for Electrically Pumped Lasers Nir Tessler Microelectronic & Nanoelectronic centers Electrical Enginnering Dept. Technion, Israel Institute of Technology Haifa, Israel www.ee.technion.ac.il/nir

2. Outline Introduction Some of the problems One of the ways to approach the problems

3. These materials can now be taken seriously for demanding applications The issue of electrically pumped organic laser is now relevant Historical Perspective Lasers - Schawllow&Towns 1958 Organic Molecules Lasers - In Solution (Lempicki,1962) Fibre Laser (RCA, 1963) In a Matrix Energy Transfer (Morantz,1962) Triplet Laser (reported but….) Photonic Structures DBR + DFB (Kogelnik,1971) Whispering Gallery (Kuwatagonokami,1992) Conjugated Polymer Lasers & Small molecule based lasers

4. n PL (a.u.) Wavelength (nm) 450 500 550 600 650 700 750 2.5 2 1.5 1 Absorption (OD) 0.5 0 200 250 300 350 400 450 500 550 Wavelength (nm) The ”original” motivation PPV • Stoke Shift • 4 level system (not always true)

5. Technological Advantages of “Plastic” Lasers Wavelength tuning through bending Stamp Gain and Glue properties Not sensitive to Surface recombination 2DBandgap

6. There is a great potential So how come we can’t make it happen Or at least prove that it did happen

7. Optical Feedback Optical Amplifier Noise + X Output Source Input Power Material Mirror 1 Mirror 2 Device structure Light - Amplifier The most Common Laser

8. MQW Laser Structure E P+InGaAS P-InP InGaAsP QW InGaAs InGaAsP N-InP N-InP, Substrate We are interested in molecular materials Similar to quantum confinement based lasers

9. Quantum Well Lasers InGaAsP InGaAs Many issues had to be optimized Most of them – material related! N. Tessler et. al. JQE, 1993

10. Optical Mode Ielectrons IlHoles

11. Gain and Absorption In PPV 6 10 5 10 4 10 1000 100 10 300 400 500 600 700 800 900 1000 Not 4 Level System No net Gain (with Current Drive) Absorption Charge absorption is plotted for Excited State Density = 1018cm-3 Charge Induced Absorption Absorption/Gain (cm-1) Excitonic Gain Wavelength (nm)

12. Charge absorption is “band to band”  High cross section

13. Rate Equations Charge Exciton Generation Singlet Exciton Triplet Exciton Exciton Generation = Bottleneck

14. How to Enhance the Probability 1. Material with high mobility (crystals looked promising) 2. Material with low charge induced absorption

15. Synthesis of Polyarylamines Yamamoto Method Vary R group to optimise charge mobility

16. Even if we won’t make electrically pumped laser we have made the basic unit for 100MHz (500MHz) data link. Fast Switching This initial set of devices & materials requires above 20V to achieve rise time of less then 10ns. (new materials have much better mobility)

17. How to Enhance the Probability 1. Material with high mobility (crystals looked promising) 2. Material with low charge induced absorption

18. Are there other structural effects that can move the charge absorption oscillator strength away from the emission band? Two-Dimensional Electronic Excitations R. Osterbacka, et. al. SCIENCE VOL 287 p.839 Charge induced absorption band at the visible is reduced when chains are coupled

19. Introduce strain Conduction Valence Split-off Anything to learn from inorganic lasers? Low bandgap Inorganics Problem Conduction Valence Split-off Inter Valence-band Absorption

20. The Organic equivalent Hole - Polaron Exciton - Polaron HOMO HOMO LUMO LUMO

21. Is there an alternative solution? Charge absorption covers visible range and up to 1mm  can we take the emission band beyond 1mm? OK – Lets mix O O n n MeO MeO 5 nm PbSe InAs/ZnSe Conjugated polymers 20nm

23. InAs PbSe >10% PL Efficiency in Solid Films

24. Current/Energy is first injected into the polymer Energy/Charge Transfer to the nanocrystal Light Emission O O n n MeO MeO What do we hope to achieve by mixing Ca\Al (cathode) Polymer - V + nanocrystal PEDOT/ITO (Anode) Glass

25. What is the transfer mechanism?

26. Charge Transfer (trapping) ? Energy Transfer

27. ~1% EL External-Efficiency Tessler et. al., Science, 2002

28. 20nm Experimental TEM Top View of =1500nm NC in PPV (30v% NC) Partial segregation PPV “pin-hole” “Good” Surface Coverage Y. Talmon

29. Optimization Requires Dedicated Modeling V V 5V% NC 2D Mesh with Traps (NCs) Randomly Positioned at a given density (trap depth = 0.4eV)

30. V Charge Density (1018cm-3) Non-Complete Trapping Distance From Contact (nm) 5% Loading Suppress Injection NC near contact The effect of trapped charges See also A. Shik et. al. Solid. State Elect., 46, 61,2002

31. Measurement No NC 10% NC 20% NC 30% NC 10% NC , offset+0.2eV HOMO offest ~0.3eV Simulation No NC 10% NC - HOMO offset=0.3eV 10% NC , offset+0.1eV

32. Let Us Assume someone will solve all material issues Related to Lasers

33. The structure

34. 1000 Al 100 Propagation Loss (cm-1) Ag 10 1 0 50 100 150 200 250 300 350 400 Cladding Thickness (nm)

35. Consider more sophisticated structures • Light emitting FET? (there is a talk later)

36. 1.2 50ms 0.5-10ms 1 0.8 Electroluminescence (a.u.) 0.6 0.4 TPPV TCTCT THS RCTCT RIFC RPPV + - 0.2 THS P CPPV CCTCT 0 500 520 540 560 580 Wavelength (nm) Current Heating Effects

37. New Functionalities Novel Materials Chemistry/Materials Analysis and extraction of properties Device Modeling Device Design & measure

38. EE Technion Vlad Medvedev Yevgeni Preezant Yohai Roichman Noam Rapaport Olga Solomeshch Alexey Razin Yair Ganot Sagi Shaked Avecia Phil Mackie Cupertino Domenico polymers Chem. Eng. Technion TEM Y. Talmon Chem. Hebrew U. Uri Banin NC Israel Science Foundation European Union FW-5 $

39. Absorption spectrum of the blends n=o=0.5

40. 1.866 0.11 1.864 0.108 1.862 0.106 1.86 Peak Energy (mm-1) 0.104 1.858 Peak Width (mm-1) 0.102 1.856 0.1 1.854 0.098 1.852 0.096 1.85 10 20 30 40 50 60 70 80 Temperature (c)

41. Electrical Pulse Set-Up 150-200ns Pulse Generator V 45Hz AC Current Probe Si Photo Diode Fast APD Laser Diode Temperature Control (-170oc,70oc)

42. 1.2 1 70oC 20oC 0.8 0.6 Electroluminescence (a.u.) 0.4 0.2 0 450 500 550 600 650 Wavelength (nm) Current Heating Effects Energy/Width 10 30 50 70 Temperature (C)