Nir Tessler Microelectronic & Nanoelectronic centers Electrical Enginnering Dept. Technion, Israel Institute of Technology Haifa, Israel The Quest for Electrically Pumped Lasers

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

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

PPV PL (a.u.) Wavelength (nm) n Absorption (OD) Wavelength (nm) Stoke Shift 4 level system (not always true) The ”original” motivation

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

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

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

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

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

I electrons I lHoles Optical Mode

Gain and Absorption In PPV Not 4 Level System No net Gain (with Current Drive) Absorption/Gain (cm -1 ) Wavelength (nm) Absorption Excitonic Gain Charge Induced Absorption Charge absorption is plotted for Excited State Density = 1018cm -3

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

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

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

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. O sterbacka, 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?

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

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

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

InAs PbSe >10% PL Efficiency in Solid Films

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

What is the transfer mechanism?

Energy Transfer ? Charge Transfer (trapping)

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

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

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

Charge Density (10 18 cm -3 ) Distance From Contact (nm) V Non-Complete Trapping 5% Loading NC near contact Suppress Injection The effect of trapped charges See also A. Shik et. al. Solid. State Elect., 46, 61,2002

No NC10% NC - HOMO offset=0.3eV 10% NC, offset+0.1eV Measurement No NC 10% NC 30% NC 20% NC 10% NC, offset+0.2eV Simulation HOMO offest ~0.3eV

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

The structure

Cladding Thickness (nm) Propagation Loss (cm -1 ) Al Ag

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

Current Heating Effects

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

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

Absorption spectrum of the blends n=o=0.5

Electrical Pulse Set-Up Pulse Generator ns 45Hz V AC Current Probe Si Photo Diode Fast APD Temperature Control (-170 o c,70 o c) Laser Diode

Temperature (C) Energy/Width Current Heating Effects