Nir Tessler

Microelectronic & Nanoelectronic centersElectrical Enginnering Dept.

Technion, Israel Institute of TechnologyHaifa, Israel

www.ee.technion.ac.il/nir

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

450 500 550 600 650 700 750

PL (a

.u.)

Wavelength (nm)

n

0

0.5

1

1.5

2

2.5

200 250 300 350 400 450 500 550

Abs

orpt

ion

(OD

)

Wavelength (nm)

• Stoke Shift• 4 level system (not always true)

The ”original” motivation

Technological Advantages of “Plastic” Lasers

Gain and Glueproperties

Wavelength tuning through bending

Not sensitive toSurface recombination2D 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 1 Mirror 2

Input Power

OpticalAmplifier

OutputNoiseSource

+ X

OpticalFeedback

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

IElectrons

IHoles

Stimulated Emission

CaptureTransport

N

P

IElectrons

IHoles

Stimulated Emission

CaptureTransport

N

P

3D

2D

Quantum Well Lasers

Many issues had to be optimized

Most of them – material related!

InGaAsP

InGaAs

Ielectrons IlHoles

Optical M

ode

Gain and Absorption In PPV

Not 4 Level System

No net Gain (with Current Drive)

Abs

orpt

ion/

Gai

n (c

m-1)

Wavelength (nm)

10

100

1000

10 4

10 5

10 6

300 400 500 600 700 800 900 1000

Absorption

ExcitonicGain

Charge InducedAbsorption

Charge absorption is plotted forExcited State Density = 1018cm-3

Charge absorption is “band to band” High cross section

T

TT

S

SS

CC

NNBdtdN

NBdtdN

NBJdtdN

2C

2C

2

43

N 41

9

3 2 1 1

18 3

10

10 ;

10

S

C

Sec

cm V Sec

N cm

Charge

SingletExciton

TripletExciton

Rate Equations

1.0N 41

SC

C

S BNN

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 PolyarylaminesYamamoto Method

N

Cl Cl Cl

N

n

R1R2

R2

R1

R2+[Ni] catalyst

N

**n

R

Vary R group to optimise charge mobility

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

Ele

ctro

lum

ines

cenc

e (a

.u.)

Time (ns)

0

0.2

0.4

0.6

0.8

1

1.2

5 10 15 20 25

Ele

ctro

lum

ines

cenc

e (a

.u.)

Time (ns)

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. Osterbacka, et. al.SCIENCE VOL 287p.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-bandAbsorption

Conduction

Valence

Split-off

Introduce strainProblem

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 1m can we take the emission band beyond 1m?

OK – Lets mix

5 nm

PbSe

20nm

InAs/ZnSe

nMeO

O

nMeO

O

Conjugatedpolymers

ZnSe

0.99

1.26

CdSe

InAs

Shell

0.46

V

Eg0.92

0.8 1.2 1.6 2.0 2.4

Absorbance intensity (a.u.)

x4

Photon Energy (eV)

Q.Y

15

20

13

0.9

~0.7 ML~1.3 ML~2.2 ML

InAs/ZnSe

1st stage = Optimising the NC Emission yield

20% PL Yield in Solution(toluene)

Best PL for Shell Thickness

between 1 and 2 monolayer

U. Banin, Hebrew University,Jerusalem

1000 1200 1400 1600 1800Wavelength (nm)

1000 1200 1400 1600 1800

Lum

ines

cenc

e (a

.u.)

Wavelength (nm)

Size A Size B Size C

InAs PbSe

>10% PL Efficiency in Solid Films

Glass

Ca\Al (cathode)

PEDOT/ITO (Anode)

nMeO

O

nMeO

O

Polymer

nanocrystal

V-

+

Current/Energy is first injected into the polymer

Energy/ChargeTransfer to the nanocrystal

Light Emission

What do we hope to achieve by mixing

0

0.2

0.4

0.6

0.8

1

400 500 600 700 800 900

Abs

orpt

ion

(OD

)

Wavelength (nm)

50V%PPV 50V%NC

NC in Polystyrene (80V%)

Polymer absorbs 70% @ ~450nm Polymer contributes <20% Energy to NC

Energy Transfer Polymer NC is Negligible

400 450 500 550 600 650 700 750 800

Em

issi

on a

t 155

0nm

(a.u

.)

Excitation Wavelength (nm)

NC in Polystyrene

20v%NC 80v%PPV

50v%NC 50v%PPV

What is the transfer mechanism?

Energy Transfer

?

Charge Transfer(trapping)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25

Cur

rent

(mA

)

Applied Voltage (V)

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25Ele

ctro

lum

ines

cenc

e (a

.u.)

Applied Voltage (V)

Increase NC Content Increase NC Content

Experimental Efficiency-Optimization

Trapping in the governing excitation mechanism

10% 20% 33% 50%

Efficien

cy

Very High Loading Probably due to partial aggregation

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

Cha

rge

Den

sity

(1018

cm-3)

Distance From Contact (nm)

V

Non-CompleteTrapping5% Loading

NC near contactSuppressInjection

The effect of trapped charges

See also A. Shik et. al.Solid. State Elect.,

46, 61,2002

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2 4 6 8 10 12

Cur

rent

(mA

)

Voltage (V)

No NC 10% NC - HOMO offset=0.3eV

10% NC , offset+0.1eV Measurement

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2 4 6 8 10 12

Cur

rent

(mA

)

Applied Voltage (V)

No NC10% 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

Glass

n1=1.7

n2=2

n3=1.7

Metal2

Metal1

x3

OpticalMode (Core)

(Cladding)

x1 (Cladding)

x2

Glass

n1=1.7

n2=2

n3=1.7

Metal2

Metal1

x3

OpticalMode (Core)

(Cladding)

x1 (Cladding)

x2

The structure

1

10

100

1000

0 50 100 150 200 250 300 350 400Cladding Thickness (nm)

Prop

agat

ion

Loss

(cm

-1)

Al

Ag

Consider more sophisticated structures

• Light emitting FET? (there is a talk later)

0

0.2

0.4

0.6

0.8

1

1.2

500 520 540 560 580

Wavelength (nm)

Elec

trol

umin

esce

nce

(a.u

.)

0.5-10s50s

TPPV TCTCT THS

+-

RPPV

CPPV

RCTCT

CCTCT

P THS

RIFC

Current Heating Effects

Chemistry/Materials

Device Modeling

Device Design & measure

Analysis and extraction ofproperties

New Functionalities Novel Materials

Phil MackieCupertino Domenico

Aveciapolymers

Y. Talmon TEMChem. Eng. Technion

Uri BaninChem. Hebrew U.

NC

Israel Science Foundation

European Union FW-5$

Vlad MedvedevYevgeni PreezantYohai RoichmanNoam RapaportOlga SolomeshchAlexey RazinYair GanotSagi Shaked

EE Technion

Absorption spectrum of the blends

oc10

OMe

n

oc4

oc4

*

*o

n=o=0.5

0

0.2

0.4

0.6

0.8

1

400 600 800 1000 1200 1400 1600

Abs

orpt

ion

(OD

)

Wavelength (nm)

PPV Derivative

50V%PPV 50V%NC

NC in Polystyrene (80V%)

x10

0

0.2

0.4

0.6

0.8

1

400 600 800 1000 1200 1400 1600

Abs

orpt

ion

(OD

)

Wavelength (nm)

PPV Derivative

50V%PPV 50V%NC

NC in Polystyrene (80V%)

x10

1.85

1.852

1.854

1.856

1.858

1.86

1.862

1.864

1.866

0.096

0.098

0.1

0.102

0.104

0.106

0.108

0.11

10 20 30 40 50 60 70 80

Temperature (c)

Peak

Ene

rgy

(m

-1)

Peak

Wid

th (

m-1)

Electrical Pulse Set-Up

Pulse Generator

150-200ns

45HzV

AC Current Probe

Si Photo DiodeFast APD

TemperatureControl (-170oc,70oc)

Laser Diode

0

0.2

0.4

0.6

0.8

1

1.2

450 500 550 600 650

Wavelength (nm)

Elec

trol

umin

esce

nce

(a.u

.)

20oC70oC

10 30 50 70

Temperature (C)

Ener

gy/W

idth

Current Heating Effects