Yu-Ming Chang ( 張玉明 )

Center for Condensed Matter Sciences

National Taiwan UniversityApril 12, 2007

Institute of Physics, NCTU

Carrier and Phonon Dynamics in InN and its Nanostructures

Outline Motivation Time-resolved second-harmonic generation (TRSHG) What is coherent phonon spectroscopy Coherent phonon spectroscopy of InN and its nanostructures

Identification of surface optical phonon

Direct observation of LO phonon and plasmon coupling Determination of the InN effective mass along the c-axis Determination of the InN plasma relaxation time Coherent phonon spectroscopy of InN ultrathin films

Conclusion

Band gap engineering of III-nitride semiconductors

Question: What can we play in this ball game ?

Our research strategy : Try to explore the transient carrier and phonon dynamics in InN and its nanostructures !

Time-resolved second-harmonic generation

Femtosecond laser pump and probe technique

lc

l

FemtosecondLaser pulse

Probe

Pump

BS

Mirror

ToSample

Time-resolved second-harmonic generation

Delay Time ()

Prob

ed S

HG

Femtosecond temporal resolution No-contact, no-damage, remote, and all optical configuration Better surface / interface sensitivity than other optical techniques

Probe

Pump

Sample

AC ProbedSHG

Signal AC

TRSHG can probe carrier and phonon dynamics in semiconductors

termsorderhigh

EEEEEP zkjijkzkjijkNL

i

__

)0()()(:)0()()(:)()2( )3()2(

)2(16)2(4)( 2

2

2

2

NLPc

Ec

Second Harmonic Generation:

Modulation of eff(2) due to the pump pulse:

)()()()2(

,,

)2()2(0,

)2( tQQ

tnn

t ii

effhe

he

effeffeff

)()(:)()2( )2(, kjijkeff

NLi EEP

Carrier Dynamics Phonon Dynamics

Coherent phonon spectroscopy: GaAs as example

0 500 1000 1500 2000 2500 3000

0.00

0.01

0.02

0.03

0.04

0.05

0.06

R/R

0(S

HG

)

Delay Time (fs)

0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

6

R

osc/R

0(S

HG

) (x1

0-3)

Delay Time (fs)

7 7.5 8 8.5 9 9.5 100

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10

-5

Frequency (THz)

o04180103

Time-Resolved Second-Harmonic Generation (TRSHG) measurement

Fourier Power Spectrum

Bulk LO phonon mode @ 8.8 THz

What is coherent phonon spectroscopy ?

)2(cos)(

tfeAtQ Tt

Coherent phonon spectroscopy

Coherent lattice oscillation :

)(22

2

)()2()(2)( tQFtQfttQ

ttQ

where A, T, f, and are the oscillation amplitude, dephasing time, frequency, and initial phase respectively.

Impulsively driving force can be …..(a) Raman scattering / electronic transition process(b) Transient depletion / piezoelectric field screening process(c) Transient local strain induced by thermal absorption

Driving force for launching coherent phonon

Impulsive stimulated Raman scattering Transient electric field screeningTransient electric field screening Displasive excitation due to electronic transition Impulsive thermal excitation

Laser pulse width < Phonon oscillation period Raman / IR active phonon mode Sample with built-in electric / piezoelectric fieldSample with built-in electric / piezoelectric field

Some Criterions :

Driving Mechanisms :

Femtosecond laser photoexcited carrier dynamics in the depletion region of GaAs

Edc>0

EF

CB

VBDepletion Region

< 0

EF

CB

VB

Edc>0

E=hv

Depletion Region

= 0Edc~ 0

EF

CB

VBDepletion Region

> 0

Coherent phonon generation in the depletion region of GaAs(100)

m342mm

TIME

E field E~0Field

screeningby freecarrier

injectionEdc

EF

CBVB

Depletion

Region

[100]

Coherent phonon spectroscopy: GaAs as example

0 500 1000 1500 2000 2500 3000

0.00

0.01

0.02

0.03

0.04

0.05

0.06

R/R

0(S

HG

)

Delay Time (fs)

0 500 1000 1500 2000 2500 3000-4

-2

0

2

4

6

R

osc/R

0(S

HG

) (x1

0-3)

Delay Time (fs)

7 7.5 8 8.5 9 9.5 100

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10

-5

Frequency (THz)

o04180103

Time-Resolved Second-Harmonic Generation (TRSHG) measurement

Fourier Power Spectrum

Semiconductor nanostructures

InGaP 200 nmGaAs 7 nmInGaP 500 nmGaAs buffer layer

Single Quantum WellSchottky InterfaceQuasi-2DEG

GaAs 9 nmAlGaAs 10 nmSi -doping layersAlGaAs 22 nmGaAs 1.5 m

Au 10 nmGaP n-type 30 mGaP substrateMetal electrode

Coherent phonon spectroscopy of InN

- Coherent LO phonon and plasmon coupling in the near surface region of InN

InN sample structure and its physical properties

This InN sample is n-type and its bulk carrier concentration is nd=3.7x1018 cm-3 determined by Hall measurement.

The electron mobility is measured as e=1150 cm2

/V·sec at room temperature.

The X-ray diffraction study shows that this sample is a high-quality wurtzite structured InN epitaxial layer formed with its c axis perpendicular to the substrate surface.

The photoluminescence spectrum indicates the band gap Eg~ 0.7 eV.

The absorption length at =800 nm is ~ 150 nm.

InN

AlNSi3N4

Si (111) substrate

Sample Structure

Provided by Prof. S. Gwo, NTHU

Coherent Phonon Generation in InN

Impulsive stimulated Raman scattering Transient electric field screeningTransient electric field screening Displasive excitation due to electronic transition Impulsive thermal excitation

Laser pulse width < Phonon oscillation period Raman / IR active phonon mode Sample with built-in electric / piezoelectric fieldSample with built-in electric / piezoelectric field

Some Criterions :

Driving Mechanisms :

Electron accumulation in the near surface region of InN

EF

Huge electric fieldE~4.7x106 V/cm

Electron accumulation in the near surface region of InN

Coherent Phonon Spectroscopy of InN Sample

0 200 400 600 800 1000 1200-2-1012

5 10 15 20 25 30 35 40 450

5

10

15

20

d(R

(2)

)/dt (

a.u.

)

Delay Time (fsec)

-4-20246

(c)

AC

(b)

(a)

R(2)

(a.u

.)

SA1(LO)

L-

L+

Frequency (THz)

Four

ier

Mag

nitu

de (a

.u.)

Spontaneous Raman spectroscopy of InN

10 12 14 16 18 20 22 24 26 28 30

0

100

200

300

B1(high)(?)

E2(high)

A1(LO)

LPP- LPP+

R

aman

Sig

nal (

a.u.

)

Frequency (THz)

InN Bulk

Coherent phonon spectrum vs. CW Raman spectrum

5 10 15 20 25 30 35 40 450

100

200

300

400

2A1(LO)

CW RamanS

A1(LO)

A1(LO)

E2H

L-

L- L+

L+

Ram

an S

igna

l

Frequency (THz)

-0.05

0.00

0.05

0.10

0.15

0.20

TRSHG

Fou

rier P

ower

Spe

ctru

m

Identification of surface optical phonon

Y.M. Chang and et. al., APL v90, 072110 (2007)

6 8 10 12 14 16 18 20 22 240.0

0.4

0.8

1.2

1.6

B

A

Four

ier P

ower

Spe

ctru

m (a

.u.)

Frequency (THz)

The phonon peak at 16.2 THz : a surface optical phonon ?!

InN

InN

Sample A

Sample B

2 nm LT-GaN

300 350 400 450 500 550 600 650012345

(a)

(b)

(c)

(d)

Four

ier P

ower

Spe

ctru

m (a

.u.)

Wave Number (cm-1)

0 100 200 300 400 500 600 700 800 900

-0.002

0.000

0.002

s-p (x50)

Ros

c (2)

/ R

o (2)

Delay Time (fsec)

-0.01

0.00

0.01

-0.06

-0.03

0.00

0.03

0.06

p-p

s-p

p-p

p-p

s-p

R (2)

/ R

o (2)

300 350 400 450 500 550 600 6500.0

0.4

0.8

1.2

1.6

(a)

(b)

(c)

(d)

Four

ier P

ower

Spe

ctru

m (a

.u.)

Wave Number (cm-1)

0 100 200 300 400 500 600 700 800 900

-0.01

0.00

0.01

Delay Time (fsec)

-0.01

0.00

0.01

Ros

c (2)

/ R

o (2)

-0.03

0.00

0.03

0.06

B

B

A

A

B

A

AC

R (2)

/ R

o (2)

Sample dependence SHG Polarization dependence

c-axis

(N)

(In)

AirInN

The vibration mode of InN surface optical phonon

A1(LO)-like (bulk-terminated) surface phonon mode

Direct observation of coherent A1(LO) phonon-plasmon coupling modes

Y.M. Chang and et. al., APL v90, 072111 (2007)

Y.M. Chang and et. al., APL v85, 5224 (2004)

0 5 10 15 20 25 30 35 40 45 500.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

D

C

BA

Fo

urie

r Tra

nsfo

rm M

agni

tude

(a.u

.)

Frequency (THz)

Coherent phonon spectroscopy : pump power dependence

nex~2x1018 /cm3

nex~1x1018 /cm3

nex~6x1017 /cm3

nex~2x1017 /cm3

Photo-injected carrier density

LO phonon-plasmon coupling in polar semiconductors

2 2 2

2 2 2( ) [1 ]p LO TO

p TO phi i

where ∞ is the optical dielectric constant, LO and TO are the bulk LO and TO

phonon frequencies, p p and ph are the damping constants of

plasmon and LO phonon respectively.

22

*

4 pp

e

N em

where Np is the plasma density and me* is the effective mass (i.e. ∞ = 6.7 and

me* = 0.033 me for InN, where me is free electron mass)

]4)()([21 22222222

TOppLOpLO

where p is plasma frequency and depends on the spatial carrier density in the

depletion region.

LO-plasmon coupling modes

Dielectric Function

Dielectric function: determine the LOPC frequencies

0 5 10 15 20 25 30 35 40 45 500.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

D

C

BA

Fo

urie

r Tra

nsfo

rm M

agni

tude

(a.u

.)

Frequency (THz)

Coherent phonon spectroscopy : pump power dependence

nex~2x1018 /cm3

nex~1x1018 /cm3

nex~6x1017 /cm3

nex~2x1017 /cm3

Photo-injected carrier density

1017 1018 1019 10200

5

10

15

20

25

30

35

40

L-

L+

Sample ASample BSample C

Freq

uenc

y (T

Hz)

Plasma Density (1/cm3)

Coherent LO phonon-plasmon coupling modes

A1(LO)A1(TO)

Plasmon

Large electron concentration in the surface region

1018 1019 1020200

300

400

500

600

700

800

BA

L+

L-

A1(TO) 447 cm-1

A1(LO) 587 cm-1

Wav

e nu

mbe

r (cm

-1)

Carrier Density (1/cm3)

Large electron concentration in the near surface region

InN

Y.M. Chang and et. al., APL v85, 5224 (2004)

Determination of the effective mass of electron along the c-axis of wurtzite InN

Y.M. Chang and et. al., APL v90, 072111 (2007)

1017 1018 1019 10200

5

10

15

20

25

30

35

40

L-

L+

Sample ASample BSample C

Freq

uenc

y (T

Hz)

Plasma Density (1/cm3)

Coherent LO phonon-plasmon coupling modes

A1(LO)A1(TO)

Plasmon

Determination of InN effective mass (m*//) along the c-axis

1x1018 2x1018 3x1018 4x101815

20

25

30

35

40 Sample A Sample B Sample C

LO

PC

Fre

quen

cy (T

Hz)

Plasma Density (cm-3)

me*=0.02 me me*=0.03 me me*=0.033 me me*=0.04 me me*=0.05 me

2

*

4 pp

e

N em

p d exN n n

Determination of the plasma relaxation time

(the following slides are deleted for confidential reason)

Y.M. Chang and et. al., in preparation (2007)

Coherent phonon spectroscopy of InN ultrathin films

(the following slides are deleted for confidential reason)

Y.M. Chang and et. al., in preparation (2007)

Coherent A1(LO) phonon-plasmon coupling modes of InN are observed for the first time. We obtain the following important physical properties :

(1) A1(LO) phonon dephasing time : 200~700 fsec

involving phonon-phonon, phonon-carrier, and phonon-defect scatterings (2) Plasma damping time constant : 50~150 fsec involving carrier-carrier and carrier-defect scatterings (3) Surface electron accumulation : > 1020 /cm3

(4) Bulk carrier concentration is overestimated by Hall measurement inhomogenous spatial distribution of carrier concentration

(5) InN effective mass (along c axis): ~ 0.033 me

nonparabolic conduction band

Summary

Conclusion Time-resolved second-harmonic generation (TRSHG) is capable of probing the carrier and phonon dynamics in InN and its heterostructures;

Surface optical phonon at 16.2 THz is observed and characterized for the first time.

We directly observe the coherent A1(LO) phonon and plasmon coupling in

the near surface region of InN.

The effective mass (m*//) of InN electron is determined to be ~ 0.033 me by

fitting the upper-branch of bulk A1(LO) phonon-plasmon coupling mode.

Coherent phonon spectroscopy of InN ultrathin film are carried out for comparison. The carrier and phonon dynamics are very different from those of the InN thick films. The analysis is in progress now.