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Yu-Ming Chang ( 張玉明 )
Center for Condensed Matter Sciences
National Taiwan University
April 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 ()
Pro
bed
SH
G 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/
R0(
SH
G)
(x10
-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 T
t
Coherent phonon spectroscopy
Coherent lattice oscillation :
)(22
2
)()2()(
2)( tQFtQf
t
tQ
t
tQ
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
VB
Depletion Region
< 0
EF
CB
VB
Edc>0
E=hv
Depletion Region
= 0Edc~ 0
EF
CB
VB
Depletion Region
> 0
Coherent phonon generation in the depletion region of GaAs(100)
m342mm
TIME
E field E~0Field
screeningby freecarrier
injectionEdc
EF
CB
VB
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/
R0(
SH
G)
(x10
-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 m
GaP 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 ban
d 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
-1
0
1
2
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.)
S
A1(LO)
L-
L+
Frequency (THz)
Fou
rier
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+
Ram
an S
igna
l (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)
E2
H
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
Pow
er S
pect
rum
Identification of surface optical phonon
Y.M. Chang and et. al., APL v90, 072110 (2007)
6 8 10 12 14 16 18 20 22 24
0.0
0.4
0.8
1.2
1.6
B
A
Fo
uri
er P
ow
er S
pec
tru
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
300 350 400 450 500 550 600 650
012345
(a)
(b)
(c)
(d)
Fou
rier
Pow
erS
pect
rum
(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)
Fou
rier
Pow
erS
pect
rum
(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
B
A
F
ourie
r T
rans
form
Mag
nitu
de (
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 e
m
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)()([2
1 22222222TOppLOpLO
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
B
A
F
ourie
r T
rans
form
Mag
nitu
de (
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
Fre
quen
cy (
TH
z)
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
Fre
quen
cy (
TH
z)
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 (
TH
z)
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 e
m
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.