The Quest for Electrically Pumped Lasers Nir Tessler

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

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

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 These materials can now be taken seriously for demanding applications The issue of electrically pumped organic laser is now relevant

The ”original” motivation PL (a. u. ) PPV 2. 5 450 2 Absorption (OD) n 500 550 600 650 700 750 Wavelength (nm) 1. 5 1 0. 5 0 200 250 300 350 400 Wavelength (nm) 450 500 550 • Stoke Shift • 4 level system (not always true)

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

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

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

We are interested in molecular materials Similar to quantum confinement based lasers MQW Laser Structure P+In. Ga. AS E P-In. P In. Ga. As. P QW In. Ga. As. P N-In. P, Substrate

Quantum Well Lasers In. Ga. As. P In. Ga. As Many issues had to be optimized Most of them – material related! N. Tessler et. al. JQE, 1993

l ca e d o M ti p O Ielectrons Il. Holes

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

Charge absorption is “band to band” High cross section

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

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

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

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

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

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 Are there other structural effects that can move the charge absorption oscillator strength away from the emission band?

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

The Organic equivalent Hole - Polaron Exciton - Polaron HOMO LUMO

Is there an alternative solution? Charge absorption covers visible range and up to 1 mm OK – Lets mix can we take the emission band beyond 1 mm? Pb. Se 5 nm 20 nm In. As/Zn. Se O Conjugated polymers Me. O n

In. As Pb. Se >10% PL Efficiency in Solid Films

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

What is the transfer mechanism?

Energy Transfer Charge Transfer (trapping) ?

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

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

Optimization Requires Dedicated Modeling V 5 V% NC V 2 D Mesh with Traps (NCs) Randomly Positioned at a given density (trap depth = 0. 4 e. V)

Charge Density (1018 cm-3) The effect of trapped charges Di V sta nc e Fr om Co nt ac See also A. Shik et. al. Solid. State Elect. , 46, 61, 2002 t( 5% Loading nm ) NC near contact Non-Complete Trapping Suppress Injection

Simulation No NC 10% NC - HOMO offset=0. 3 e. V 10% NC , offset+0. 1 e. V Measurement No NC 10% NC 20% 10% NC , offset+0. 2 e. V 30% HOMO offest ~0. 3 e. V

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

The structure

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

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

Electroluminescence (a. u. ) Current Heating Effects 1. 2 50 ms 1 0. 5 -10 ms 0. 8 0. 6 0. 4 0. 2 0 TPPV P 500 TCTCT RPPV CPPV 520 THS RCTCT RIFC + - CCTCT 540 Wavelength (nm) 560 THS 580

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

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

Absorption spectrum of the blends n=o=0. 5

0. 11 1. 864 0. 108 1. 862 0. 106 1. 86 0. 104 1. 858 0. 102 1. 856 0. 1 1. 854 0. 098 1. 852 0. 096 1. 85 10 20 30 40 50 60 Temperature (c) 70 80 Peak Width (mm-1) Peak Energy (mm-1) 1. 866

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

1. 2 1 0. 8 Energy/Width Electroluminescence (a. u. ) Current Heating Effects 70 o. C 20 o. C 0. 6 10 30 50 70 Temperature (C) 0. 4 0. 2 0 450 500 550 600 Wavelength (nm) 650