PhD Final Exam

Lifang XuApril , 2007

Electrical and Computer Engineering DepartmentColorado State University

Study of Carrier and Gain Dynamics in InGaAsN Quantum Well

4/6/2007 L. Xu 2

Motivation

Thesis Work

Optical properties studies• Localization

• Measurement of conduction band effective mass me*

• Carrier recombination and dynamics

Nonlinear gain dynamics

Carrier capture/escape processes

To unveil the role of nitrogen incorporation on the performance of a high-speed laser device in strained compensated In0.4Ga0.6As0.995N0.005/GaAs QW laser diode.

4/6/2007 L. Xu 3

Outline of presentation

Experiments description

Results Analysis&

Discussion

Introduction &

Background

Analytical Model

Optical Propertystudies

Nonlinear GainDynamics

Carrier Capture & Escape

Summary&

Conclusion

4/6/2007 L. Xu 4

Why InGaAsN

Thermal Problem (High temperature sensitivity of Jth due to small conduction band offset and associated carrier leakage).

No efficient DBR can be found with lattice matched to InP substrate. While GaAs can lattice match with the high efficient AlGaAs/GaAs DBR.

Problems with current InP-based InGaAsP long wavelength laser diode

⎯ Need a suitable active region emitting at 1310 nm lattice matched to GaAs⎯ Introducing N into InGaAs can obtain this wavelength

4/6/2007 L. Xu 5

InGaAsN/GaAs QWsInGaAsN/GaAs QWs

Big bowing effect due to the large electronegativity reduces the band-gap energy1310 nm emission can be easily obtainedBetter electron confinement, improves temperature performanceLattice matches to GaAs VCSEL

∆Ec

CB

VB

Eg

∆Ec

me*

4/6/2007 L. Xu 6

Strain compensated InGaAsN/GaAsStrain compensated InGaAsN/GaAsQWsQWs

Barrier Region

100-A GaAs

60-A In0.4Ga0.6As0.995N0.005 QW

75-A GaAs0.85P0.15

High compressive strain ~2.7%

Lowest Jth~200−210 A/cm2

High strain limits the growth of thick freestanding epilayers (typically ~1-3µm), thus no direct determination of the me* has been reported for this type of InGaAsN.

The effect of an enhanced me* on the performance of laser diodes is still in debate.

Still shows deteriorated optical quality.

Gain nonlinearity and carrier capture & escape ratio are barely investigated

Unversity of wisconsin

4/6/2007 L. Xu 7

Analytical gain modelAnalytical gain model

Parabolic E-k dispersion assumption is tested against published k*p simulation.

Band anti-crossing (BAC) approach is applied in interband transition energies calculation.

Predict general laser behavior due to nitrogen incorporation.

Explore the effect of an increase in me* on differential gain.

Help to understand the discrepancy between experiments and theory.

4/6/2007 L. Xu 8

General results of gain modelGeneral results of gain model

0.92 0.94 0.96 0.98 1.00-1500

-1000

-500

0

500

1000

N=1.2*1018 cm-3

T=10 C

T=40 C

Gai

n (c

m-1)

Energy (eV)0 20 40 60 80 100

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

InGaAsN InGaAs

Ntr(1

018cm

-3)

Temperature(oC)

Effect of carrier heating on gain compression

The introduction of nitrogen into InGaAs leads to an increase of Ntr.

4/6/2007 L. Xu 9

Effect of enhanced me* on differential gain from gain model

me* affects dg/dn through the changes of both M11 and Quasi-Fermi level , thus fc-fv. The former is, not only determined by the band-edge electron effective mass, but also determined by the chosen orbital function.

momentum matrix element

g T n, E,( )

c v,( )

C0 E( )2Lz

⋅

0

∞

kM11T k,( )

γk

π

Ecv T n, k,( ) E−( )2 γk2

+

⋅ fc T n, k,( ) fv T n, k,( )−( )⋅kπ 2⋅

⋅

⌠⎮⎮⎮⎮⌡

d⋅∑:=

Fermi-Diracinversion factor

Line width

Gain spectra model•• Increase of me* cause a decrease in dG/dN• k*p model using different band structure

actually yields different change in dG/dN?

4/6/2007 L. Xu 10

• Effect of me* only on Quasi- Fermi level leads to 30% of the enhancement in dg/dn.

• Momentum matrix element is determined by the state function chosen in the model as well as by me*, which brings uncertainty in the results.

Effect of enhanced me* on differential gain from gain model

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4-500

0

500

1000

1500

2000

2500

3000

3500

dg/dn=3.812*10-15cm2

dg/dn=2.015*10-15cm2

dg/dn=2.869*10-15cm2

Peak

Gai

n (c

m-1)

Carrier density (cm-3)

InGaAs InGaAsN, effect of me* on fc-fv InGaAsN, effect of me* on both M11 and fc-fv

4/6/2007 L. Xu 11

Optical Property studies

Motivation- Deteriorated optical quality- No direct determination of the me* has been reported for this type

of InGaAsN

Experimental design− Time integrated photoluminescence with various temperature− Time resolved photoluminescence measurements− Polarization dependent photocurrent measurement

Results, analysis and discussion– Carrier localization effect– Derive electron effective mass me*– Carrier recombination feature analysis – Carrier recombination dynamics– Carrier recombination lifetime measurement

4/6/2007 L. Xu 12

Steady state PL spectraSteady state PL spectra

By selecting the excitation wavelength, both in and outside well excitations have been employed in low and high excitation regime (2×1010~1012cm-2).

Sample is mounted on a closed-cycle Helium cryostat that allows varying the temperature from 10K to 300K.

Ti-Sapphire: 100 fs pulse centered at 800 nm

OPO: ~150fs pulse, wavelength 1-1.35µm

Cw 980nm laser diode

spectrometer Nitrogen cooled InGaAs

Photoluminescence

Sample on cryostat

Ti-Sapphire laser or OPO

Lock-in amplifierChopper controller

chopper

Experiment

4/6/2007 L. Xu 13

Polarization dependent Photocurrent measurement

Spectrometer

White light source

Polarizer

TM

TE

A

Chopper Controller

Lock-in amplifier Computer

to help characterizing different features observed in TIPL

R

4/6/2007 L. Xu 14

Time-resolved Photoluminescence Spectra measurement

Schematical drawing of the luminescence up-conversion

Beta-barium-borate (BBO) crystal, thickness: 250B

Time and spectral resolution of the up-conversion process are mainly limited by the pulsed duration (~100fs) and spectral width (9meV) of the 800nm gating laser, respectively.

CCD

Spectrometer

Cryostat Computer

Stepper controller Gating pulse

Excitation pulse

BBO

Stepper

4/6/2007 L. Xu 15

Carrier recombination lifetime measurement

Trigger from Ti-sapphire

Sampling Oscilloscope

25GHzInGaAs detector

Cryostat

The decay curve used for fitting is the curve deconvolved with a system response.

The system response function is obtained by replacing the PL sample with a piece of brass under the same setup.

4/6/2007 L. Xu 16

Temperature dependent steady state PL spectra

Temperature dependence of time integrated PL spectrum with excitation intensity 1×1012cm-2 in

logarithmic scale.

0.8 0.9 1.0 1.1 1.2 1.3

InGaAsN

Energy (eV)0.8 0.9 1.0 1.1 1.2 1.3

InGaAs

Energy (eV)

300K 200K 150K 100K 80K 60K 50K 40K 30K 20K 10K

0 50 100 150 200 250 300

0.98

0.99

1.00

1.01

1.02

1.03

1.04

1.05

1.06

InGaAsN

PL p

eak

post

ion

(ev)

PL p

eak

Posi

tion

(ev)

Temperature (K)

1.02

1.03

1.04

1.05

1.06

1.07

1.08

1.09

1.10

1.11

InGaAs

Both samples follow similar trace described as Vashni equation

The well known ‘S’ shape variation for InGaAsN QW, suggests the existence carrier localization

An additional PL emission presented at the higher energy side of the spectra in both samples.

,)0()(2

βα+

−=T

TETE

4/6/2007 L. Xu 17

Carrier localizationCarrier localization

1.00 1.05 1.10 1.15 1.20

As grown InGaAs

PL (a

.u)

Energy (ev)

PL Spectra with InPL Spectra with In--Well CW Well CW Low Excitation.Low Excitation.

Low energy tail is fitted byLow energy tail is fitted by

0.85 0.90 0.95 1.00 1.05 1.10 1.15

∆=38mev

As grown InGaAsN

PL (a

.u)

Energy (ev)

0.85 0.90 0.95 1.00 1.05 1.10 1.15

0.0002

0.0004

annealed InGaAsN

?=20mev

PL

(a.u

)

energy (ev)

∆−∝ /)( EeEρ

At T=50K, ∆~38 meV for as grown InGaAsN and dropping to 20 meV after annealing, indicates that annealing can smear some shallow localized state, resulting in a less localization potential .

No obvious exponential lower energy tail was observed in InGaAs QW,

4/6/2007 L. Xu 18

Carrier recombination channelsCarrier recombination channels

0.9 1.0 1.1 1.2 1.3

PL

TMTE

NP e1-hh1

e1-lh1

InGaAsN Photocurrent at RT

Pho

tocu

rren

t (a.

u)

Energy (eV)0.9 1.0 1.1 1.2 1.3

Phot

ocur

rent

(a.u

)

PL

TMTE

NPe1-lh1e1-hh1

InGaAs Photocurrent at RT

Energy (eV)

The higher energy transition shows TM polarization, thus we assign it to e1-lh1 QW transition

0.9 1.0 1.1 1.2 1.3 1.4 1.5

0.01

0.1

1

InGaAsN at T=50K

ND0.8 ND1 ND2 ND3

PL (a

.u)

Energy (eV)0.9 1.0 1.1 1.2 1.3 1.4 1.5

InGaAs T=50K

energy (ev)

ND0.8 ND1 ND2 ND3

4/6/2007 L. Xu 19

Band diagram calculation

Conduction Band offset ratio (∆Ec/∆Eg) is 82% for InGaAsN/GaAs and 65% for InGaAs/GaAs .

Compressive strain: 2.7% for InGaAs and 2.5% for InGaAsN .

Electrons are more confined while holes are less confined in InGaAsN

This is contrary to our photocurrent measurement result, which show that the light hole is a confined state in the well.

hhhh 99..44 mmeeVVhhhh 5522 mmeeVV

∆Ehh=105 meV

∆EC=431 meV

e1 71meV

e2 265meV

In0.4Ga0.6As0.995N0.005/GaAs

0.907 eV

∆EC=251 meV

e1 74meV

e2 249meV

∆Ehh=177 meV

hhhh 1111..77 mmeeVV hh 59 meVhh 125meV

0.996 eV

In0.4Ga0.6As/GaAs

hhhh 110022 mmeeVV

(e1 78 meV)

(e2 284meV)

4/6/2007 L. Xu 20

Optical method to determine me*

detailed line shape analysis

Temperature dependent PL spectra

the ratio between free carriers and exciton

Exciton binding energy from 2-D mass law

Electron effective mass

Selected exciton binding energy

4/6/2007 L. Xu 21

Optical method to determine me*

0.90 0.95 1.00 1.05 1.10 1.15 1.20

300K 230K190K

160K120K

80K40K

20K

InGaAsN

PL in

tens

ity (a

.u)

Energy (eV)

PL spectra at low excitation

0.95 1.00 1.05 1.10 1.15 1.20 1.25

300K230K190K160K

120K80K

40K20K

InGaAs

PL in

tens

ity (a

.u)

Energy (eV)

4/6/2007 L. Xu 22

Optical method to determine me*>>> Line shape analysis on PL spectra

,)

)(exp(

))())((

exp()(

TkTEET

TEEAEI

B

HH

Hxx −

+σ−

−=1

12 2

2

,)

)(exp(

|)(|exp()

)()(

exp()(

TkTEE

TEER

TTEE

AEI

B

C

CC

CCC −

+−

π−+σ−

−+=

1

1

21

2

1

1

0 .9 5 1 .0 0 1 .0 5 1 .1 0 1 .1 5 1 .2 0

In G a A s

3 0 0 K

1 9 0 K

8 0 K

E n e r g y (e V )0 .9 0 0 .9 5 1 .0 0 1 .0 5 1 .1 0 1 .1 5

In G a A s N

3 0 0 K

1 9 0 K

8 0 K

PL in

tens

ity (a

.u)

E n e r g y ( e V )

PL spectrum is assumed to be a superposition of both excitonic and free carrier recombination.

Exciton

Free carrier

xNhNeNRatio of exciton

and free carrier emissions

∝

Fit of the PL spectra with sum of Eq.(1) and Eq.(2)

1)

2)

4/6/2007 L. Xu 23

Optical method to determine me*

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

300K 80K

InGaAs QW (9.72 meV)

InGaAsN QW (17.5 meV)

ln(N

eNh/(

NxT

)

1/kBT (meV-1)

Exciton binding energy R

9.72 meV InGaAs

17.5 meV InGaAsN

me*_InGaAs=0.049±0.007m0

me*_InGaAsN=0.11±0.015 m0

Well established me*_InGaAs=0.047m0

TkR

B

he

he

x

he BeTk

mmmm

NNN −

π+∝=ρ 2h

yR)(R 2

12−α

=

Hy Rm

R0

20 )( µεε

=*

1*

11

he mm+=

µ

α = 2.248 InGaAsα = 2.199 InGaAsN

, where

4/6/2007 L. Xu 24

Carrier recombination dynamics>>> Time evolution of PL spectra

0.8 0.9 1.0 1.1 1.2 1.3 1.4Energy (eV)

InGaAs T=25K

1ps10ps24ps47ps

235ps470ps

940ps

1410ps

1880ps

0.8 0.9 1.0 1.1 1.2 1.3 1.4

Energy (eV)

10ps2ps

47ps

235ps

470ps

940ps

1410ps

1880ps

InGaAsN (T=25K)

T=25K

QW transition e1-hh1and shoulder e1-lh1 are populated simultaneously for both InGaAsN and InGaAs QW.

The lower energy peak,shows a slower decay time constant and a monotonous redshift, which is a measure of the population to the localized states

4/6/2007 L. Xu 25

Carrier recombination dynamics>>> Time evolution of PL spectra

T=300K

0.8 0.9 1.0 1.1 1.2 1.3 1.4

30mW InGaAs T=300K

235ps

470ps

940ps

1410ps

10ps

1880ps

24ps47ps

2350ps

Energy (eV)0.8 0.9 1.0 1.1 1.2 1.3 1.4

30mW InGaAsN T=300K

47ps

5ps10ps

235ps

376ps

470ps705ps940ps

Energy (eV)

The fast population at light hole state will affect the carrier distribution at high injection, therefore likely impact the device performance.

Localization is less pronounced as temperature increases in InGaAsN, it competes with nonradiative recombination.

4/6/2007 L. Xu 26

Carrier recombination dynamics>>> Carrier localization

0 500 1000 1500 2000

1.04

1.06

1.08

1.10

1.12

1.14

1.16

1.04

1.06

1.08

1.10

1.12

1.14

1.16

T=150K

T=50K

T=150K

T=50K

InGaAsN

InGaAs

PL p

eak

posi

tion

(ev)

Time delay (ps)

Red shift of the peak indicates the population time to localized states.

Localized states decrease as temperature increases.

4/6/2007 L. Xu 27

Carrier lifetime as function of temperature

Fast detector measurement

0 5 10

(c)

(b)

(a)

PL (a

.u)

Time Delay (ns)

InGaAs

InGaAsN

System response

The decay curve used for fitting is the curve deconvolved with system response (red line).

4/6/2007 L. Xu 28

Carrier lifetime as function of temperature

0 50 100 150 200 250 3000.4

0.5

0.6

0.7

0.8

0.9

1.0

InGaAsN

Life

time

(ns)

Temperature (K)0 50 100 150 200 250 300

0.5

1.0

1.5

2.0

2.5

3.0

InGaAs

Life

time

(ns)

Temperature (K)

Localization dominant

Nonradiative recombination dominant

4/6/2007 L. Xu 29

Nonlinear Gain Dynamics

Motivation

001124

2PN

)PN()esc/cap(p

wdN/dggv

Rf ⋅ε+⋅ττ+τπ

⋅=

The high speed modulation is limited by

• The effect of gain compression ε

• The capture/escape time ratio τcap/τesc

0.92 0.94 0.96 0.98 1.00-1500

-1000

-500

0

500

1000

N=1.2*1018 cm-3

T=10 C

T=40 C

Gai

n (c

m-1)

Energy (eV)

Effect of carrier heating on gain compression

4/6/2007 L. Xu 30

Nonlinear Gain Dynamics

Biased Laser

Efc

Ec

Probe Pump

τ

INITIAL CONDITION

NON-FERMI DISTRIBUTION

∆N + UNDEFINED TEMPERATURE

FERMI DISTRIBUTION

∆N + ∆T

T = T0 ∆N

EQUILIBRIUM

Two photon absorption Free carrier absorption Interband transition

Carrier-carrier interaction (thermalization)

Carrier phonon interaction

Carrier recombination Carrier escape

Le)(prI)Lz(prI α−== 0

δα∝=

=⋅⋅δα−=

=

=δ=

∆)Lz(prI

)Lz(prIL

)Lz(prI

)Lz(prI

TT

4/6/2007 L. Xu 31

Cross polarization Pump-Probe transmission setup

Nonlinear gain dynamics

Generally the pump beam is set to TE-polarized and the probe beam TM-polarized. The polarization of the pump beam is selected to be that of laser’s that stimulated emission. λpump=λprobe.

4/6/2007 L. Xu 32

Nonlinear gain dynamics study

-2 0 2 4 6 8 10 12 14

4

3

2

1

h(t)

Delay (ps)

[ ] )t(a)/texp(a)/texp(a)/texp(a)t(u)t(h δ+τ−⋅+τ−⋅+τ−⋅= 4332211

1 2 3 4

1 Interband relaxation

2 Carrier heating relaxation

3 Absorption bleaching

4 Two photon absorption

4/6/2007 L. Xu 33

Nonlinear gain dynamics>>> selection of regime

Selection of operation region : Gain, Transparency, Absorption

1200 1240 1280 1320

-2000

-1000

0

1000

2000

3000

InGaAsN

12 mA 10 mA 8 mA 6 mA 4 mA 2 mA

Wavelength (nm)

Absorption

Gain

λpump=λprobe

4/6/2007 L. Xu 34

Nonlinear gain dynamics study>>> Results

-2 0 2 4 6 8 10 12 14

(c)

Delay (ps)

-2 0 2 4 6 8 10 12 14

(b)

∆T

-2 0 2 4 6 8 10 12 14

InGaAs

(a)

-2 0 2 4 6 8 10 12 14

(c)

Delay (ps)

-2 0 2 4 6 8 10 12 14

(b)

-2 0 2 4 6 8 10 12 14

InGaAsN

(a)

Pump-probe measurements and fitting: (a) absorption (b) transparency (c) gain

4/6/2007 L. Xu 35

Nonlinear gain dynamics study>>> Fitting results

-0.250.16 ps42.3 ps-0.40.86(absorption)

-0.850.21 ps52.5 ps-0.50.05(transparency)

-1.60.217.82.4 ps-0.77-0.18InGaAsN

(gain)

00.16 ps3.82.9 ps-0.911.05(absorption)

-10.16 ps02.7 ps-0.170.05(transparency)

-0.70.16 ps02.1 ps-0.13-0.17InGaAs (gain)

a4τ3a3τ2a2a1Diode

TPAAbsorption bleachingCarrier heatingStep

[ ] )t(a/texp(a)/texp(a)/texp(a)t(u)t(h 4332211 δ+τ−⋅+τ−⋅+τ−⋅=

In InGaAsN laser carrier heating has a more significant effect on gain compression.

The relaxation time constant of carrier heating has similar values in both samples.

4/6/2007 L. Xu 36

Carrier capture & escape time measurement

τThermal

τtunnel

τcap-_QW

τtrans

001124

2PN

)PN()esc/cap(p

wdN/dggv

Rf ⋅ε+⋅ττ+τπ

⋅=

Motivation

τr

τcap= τtrans+ τcap-_QW

tunnthermesc τ+

τ=

τ111

4/6/2007 L. Xu 37

Two color pump-probe transmission measurement

0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

OPO - probe

Dilute Nitride laser

Ti-sapphire Pulse - pump

Inte

nsity

(a.u

)

wavelength (µm)

τ

Pump Probe

Color Filter

Pump Probe Probe

(a)

(b)

Avoid high power requirement for probe in previous broadband pump-probe experiment; Easy to distinguish pump and probe pulses.

4/6/2007 L. Xu 38

Two color pump-probe transmission setup

Mode-locked Ti-sapphire

OPO

Stepper

Pump

Probe

Lock-in Amplifier

Chopper Controller

Stepper Controller

Computer

Detector LPF Sample

A

BC

Carrier capture and escape time in the QW laser active region

4/6/2007 L. Xu 39

Carrier escape time measurement

0 20 40 60 80 100 120 140

I=8 mA

Time delay (ps)

0 20 40 60 80 100 120 140

I=6 mA

0 20 40 60 80 100 120 140

I=4 mA

∆T

0 20 40 60 80 100 120 140

I=2 mA

0 20 40 60 80 100 120 140

InGaAsNo bias

0 20 40 60 80 100 120 140

InGaAsN

I=8 mA

0 20 40 60 80 100 120 140

I=6 mA

0 20 40 60 80 100 120 140

I=4 mA

0 20 40 60 80 100 120 140

I=2 mA

0 20 40 60 80 100 120 140

No bias

Time delay (ps)

4/6/2007 L. Xu 40

Carrier capture and escape processes>>> Origin of carrier escape

)TkE)N(E

exp(

)Tk

qVexp(

)Tkm

m(

mL

)N(

B

_efc

B

e

Bb_e

w_e

b_e

qwe_therm

1

212

1−

+

πω=τ

h

)))E)N(E(V()N(Fe*m

exp(~)N( /_efce

ee_tun

231

234

−−τh

Thermionic escape model (solid line)

Tunneling effect (dotted line)

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

10

100

InGaAsN

InGaAs

Carrier Density (1018 cm-3)

τ esc

(ps)

4/6/2007 L. Xu 41

Carrier capture/escape ratio

001124

2PN

)PN()esc/cap(p

wdN/dggvRf ⋅

ε+⋅ττ+τπ

⋅=

τcap=5.3 ps for both InGaAs and InGaAsN QW

0.0 0.5 1.0 1.5 2.0 2.5

0.0

0.5

1.0 InGaAsN InGaAs

R=τ

cap/τ

esc

Carrier Density N (1018 cm-3)

0.06

1.2

9401

1 .InGaAs

esc

cap=

τ

τ+

4501

1 .InGaAsN

esc

cap=

τ

τ+

fR decreases by factor of 1.4 in InGaAsN lasers

4/6/2007 L. Xu 42

Summary

The incorporation of Nitrogen creates carrier localization and nonradiative recombination centers. The former only exists at low temperature (T<100K) while the latter is dominant after T>100K and deteriorates the luminescence efficiency of InGaAsN sample.

The optical method allows to extract the electron effective mass by using a fractional parameter model. This approach gives me*=(0.049±0.007)m0 and me*=(0.11±0.015)m0 for InGaAs and InGaAsN.

The fast population at light hole state will affect the carrier distribution at high injection, therefore likely impact the device performance.

In InGaAsN laser carrier heating has more significant effect on gain compression in the gain regime. The relaxation time constant associated with carrier heating in both InGaAs and InGaAsN lasers has similar values.

Independably measured carrier escape times τesc. The decrease of τesc in dilute nitride limits the bandwidth by factor of 1.4. Both tunneling effect and thermal emission are needed to explain the decrease of τesc

4/6/2007 L. Xu 43

Conclusion and Future work

Material related

Conclusion:The incorporation of nitrogen in the growth introduces carrier localization and

nonradiative recombination centers.

The increase of me* in InGaAsN is intrinsic and probably has a positive influence on high speed device performance. But increase of me* leads to higher transparency carrier density, therefore, threshold.

Gain compression ε is more pronounced in dilute nitride compared to nitrogen free device. Incorporation of nitrogen does not affect the relaxation rate of ε.

Future work:Improvement of the material growth procedure and post treatments

Further efforts in the k*p theoretical modeling are needed

4/6/2007 L. Xu 44

Conclusion and Future work

QW structure related

Conclusion:An increase of hole escape rate through thermal emission and carrier

tunneling was discovered in dilute nitride lasers, which leads to reduction of the bandwidth and cause a rapid increase of the threshold current.

Future work:High indium content strain compensated dilute nitride lasers requires

careful adjustment of the band structure, such as super-lattice barriers or managing the strain to realize a better hole confinement.

4/6/2007 L. Xu 45

• Dr. J.R. Sites, Dr. M.C. Marconi, Dr R.Bartels and Dr. J. Pikal, Committee members

• Dr. C.S Menoni, advisor

• Dr. D. Patel and Dr. O. Anton, group members

• Dr. L. Mawst, Dr. J.Y.Yeh and Dr. N.Tansu, collaborators

Acknowledgments

4/6/2007 L. Xu 46

0.94 0.96 0.98 1.00 1.02 1.04 1.06-2000

-1000

0

1000

2000

3000In0.4Ga0.6As/GaAs

n∗1018cm-3

1.8 1.6 1.4 1.2 1

Gai

n (c

m-1)

Energy (eV)0.94 0.96 0.98 1.00 1.02 1.04 1.06

-2000

-1000

0

1000

2000

3000

n*1018 cm-3

In0.4Ga0.6As0.995N0.005/GaAs

Gai

n (c

m-1)

Energy (eV)

2 1.8 1.6 1.4 1.2

4/6/2007 L. Xu 47

0 2 4 6 8 10

0.0

0.4

0.8

1.2

1.6

2.0

2.4

InGaAs

InGaAsN

Car

rier D

ensi

ty (1

018 c

m-3)

Current (mA)

4/6/2007 L. Xu 48

Nitrogen composition (%)

dG/d

N (1

0-15 cm

2 )

4/6/2007 L. Xu 49

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

FWHM=203 fsOPO

Aut

ocor

rela

tion

(a.u

.)

Delay (ps)

(a)

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Ti Sapphire

FWHM=95.8 fs

Aut

ocor

rela

tion

(a.u

.)

Delay (ps)

(b)