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Terahertz Radiation from InAlAs and GaAs Surface Intrinsic-N + Structures and the Critical Electric Fields of Semiconductors J. S. Hwang, H. C. Lin, K. I. Lin and Y. T. Lu Department of Physics, National Cheng Kung University, Tainan, Taiwan. Outline. Introduction to Terahertz (THz) Radiation - PowerPoint PPT PresentationTRANSCRIPT
Terahertz Radiation from InAlAs and GaAs Surface Intrinsic-N+ Structures and the Critical E
lectric Fields of Semiconductors
J. S. Hwang, H. C. Lin, K. I. Lin and Y. T. LuJ. S. Hwang, H. C. Lin, K. I. Lin and Y. T. LuDepartment of Physics,Department of Physics,
National Cheng Kung University, Tainan, TaiwanNational Cheng Kung University, Tainan, Taiwan
OutlineOutline
Introduction to Terahertz (THz) Radiation
Motivation
System for generation and detection of THz radiation
Experimental Results and Discussions
Summary
What is Terahertz Radiation (THz or T-ray) ?What is Terahertz Radiation (THz or T-ray) ?
Terahertz region : 0.1 ~ 30 THz1 THz = 1012 Hz ~ 300 µm ~ 4.1 meV ~ 47.6 K
THz Gap
Application of Terahertz RadiationApplication of Terahertz Radiation
• Material characterization ex: carriers dynamics (concentration, mobility..), refraction index, superconductor characterizations…
• THz Imaging ex: security screening, distinguish cancerous tissue …
• Biomedicine application
ex: molecule (or protein) vibration modes in THz range,
cancer detection, genetic analysis…
• THz Laser
medical imaging and diagnosis :
cancer (oncology) , cosmetics , oral healthcare
pharmaceutical applications :
drug discovery & formulation , proteomics
security
non-destructive testing
TeraView.Ltd TeraView.Ltd (2001 UK)(2001 UK) => http://www.teraview.com
THz imaging
Science, vol. 297, 763 (2002)
Powder distribution in an envelope
Motivation
During the past ten years, the research activities in our lab are mainly concentrated in the field of modulation spectroscopy of photoreflectance. Three years ago, we started to set up the system for the generation and detection of THz radiation. We did not have any fund to buy the equipments for THz image or THz spectroscopy. In addition, we are unable to grow any semiconductor microstructures or devices. Therefore, we put all the semiconductor samples we have studied in the modulation spectroscopy to the THz system as the emitter.
We tried to find the most effective THz emitter or to find any new physicWe tried to find the most effective THz emitter or to find any new physical mechanism involved in the THz radiational mechanism involved in the THz radiation..
Thank to
Prof. Hao-Hsiong Lin, Dept. of Electric Engineering, National Taiwan University.
Prof. Jen-Yin Chyi, Dept. of Electric Engineering, National Central University.
System for generation and detection of THz radiation
Ti:Sapphire pulse laser (Tsunami, Spectro-Physics)Power : 700 mw (max); Wavelength : 790 nm ; Pulse width : 80 fs;Repetition rate : 82 MHz; Pulse power ~ 8.0 nJ
Voltagesource
Semiconductor crystal
THz pulse
optical beam
reflected optical beam& THz pulse
E1
E t E2
Laser pulse
THz pulse ETHz(t, )
t
)t(J)t(ETHz
(1) laser pulse + semiconductor
(2) create transient photocurrent
(3) far field THz radiation
gE
bEe)t(n)t(J
t
)t(J)t(ETHz
THz413 E
Lrn
II
ZnTe
Wollastonpolarizer
[1,-1,0]
[1,1
,0]
/4 plate
pellicleprobe beam
THz
beam
s p
polarizerE
E
detector
I
System for generation and detection of THz radiation
Ti:Sapphire pulse laser (Tsunami, Spectro-Physics)Power : 700 mw (max); Wavelength : 790 nm ; Pulse width : 80 fs;Repetition rate : 82 MHz; Pulse power ~ 8.0 nJ
t=t1t=t2t
signal
t
Porbe beam pulse
THz pulse
t=t0t
0 2 4 6
Inte
nsity (
a.u
)
Time delay (ps)
t=t1
t=t2
t=t0
0 2 4 6 8 10 12
GaAs
Inte
nsity
(a.
u)
Time delay (ps)
0 1 2 3
GaAs
Am
plit
ud
eFrequency (THz)
Time-domain THz spectroscopyTime-domain THz spectroscopy FFT of THz spectroscopyFFT of THz spectroscopy
System for generation and detection of THz radiation
Ti:Sapphire pulse laser (Tsunami, Spectro-Physics)Power : 700 mw (max); Wavelength : 790 nm ; Pulse width : 80 fs;Repetition rate : 82 MHz; Pulse power ~ 8.0 nJ
Generation : Photoconductive: 1. Ultra-fast laser pulse with photo energy greater than semiconductor band gap. Electron-hole pairs created.2. Static electric field at surface or interface.3. Carriers driven by field form a transient photocurrent.4. The accelerated charged carrier or fast time-varying current radiates electromagnetic waves.
locph
THz Eet
tn
t
JtE
)(
)(
where J : transient current e : the electron charge nph(t) : the number of photo-excited carriers μ : carrier mobility Eloc : the built-in electric field or external bias over the sample surface illuminated by the pump beam
• Detection : Electro-Optical Sampling1. Stop THz pulse => rotate λ/4 wave-plate => balance s- , p-polarized intensity .2. While THz pulse and Probe pulse arrived ZnTe at the same time => optical axis of ZnTe will be rotated => balance detector measures a difference signal ΔI .3. ΔI is proportional to THz Field .
Sample StructuresSample Structures
InIn0.520.52AlAl0.480.48As SINAs SIN++
InP (100)InP (100)Semi-insulatedSemi-insulated
InIn0.520.52AlAl0.480.48As (100)As (100)
11μμmmSi-doped 1*10Si-doped 1*101818cmcm-3-3
InIn0.520.52AlAl0.480.48As (100)As (100)
Thickness Thickness dd
d = 200, 120, 50, 20 nmd = 200, 120, 50, 20 nm
GaAs SINGaAs SIN++
GaAs (100)GaAs (100)Semi-insulatedSemi-insulated
GaAs (100)GaAs (100)11μμmm
n-doped 1*10n-doped 1*101818cmcm-3-3
GaAs (100)GaAs (100)Thickness Thickness dd
d = 100 nmd = 100 nm
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
10
20
30
40
50
GaAs waferGaAs SIN+ d=100nmInAlAs SIN
+d=104nm
InAlAs SIN+ d=200nm
Pha
se (
rad)
Frequency (THz)
Am
pli
tud
e (a
rb. u
nit
)
Frequency (THz)
0 2 4 6 8 10-1.0
-0.5
0.0
0.5
1.0
1.5
Am
pli
tud
e (a
rb.u
nit
s)
time delay (ps)
GaAs wafer
GaAs SIN+ d=100nm
InAlAs SIN+ d=104nm
InAlAs SIN+ d=200nm
(a)
Time domain THz radiation spectrum:
Frequency domain THz radiation (FFT) spectrum:
-200 -100 0 100 2000.0
0.5
1.0
1.5
2.0
2.5
20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
Top layer thickness (nm) TH
z am
plit
ude
(nA
)
TH
z am
plit
ude
(nA
)
Top layer thickness (nm)
InAlAs SIN+
etching from 200 nm
as grown
GaAs SIN+
etching from 100 nm
-200 -100 0 100 200
50
100
150
200
250
300
350
Intrinsic Layer thickness d (nm)
Surface field of different SIN + StructureGaAs (Etched from 100 nm) InAlAs ( As Grown )InAlAs ( Etched from 200 nm )
Bui
lt-i
n el
ectr
ic f
ield
(k
V/c
m)
Intensities of THz radiation from InAlAs SIN+ structures with variousintrinsic layer thicknesses d :
It is widely believed that the amplitude of THz is proportional to the surface electric field. However, compared with the electric fields measured from PR spectroscopy,
the amplitude is not proportional to the surface field !
eNS r /2 0
sd
0 0ph dx)xexp(Icos
)R1()d(n
r
On the other hand, the number of photo-excited free charged carriers can be estimated as function of the intrinsic layer thickness d by
where R : the reflectivity of the emitter;α : the absorption coefficient; η : the quantum efficiency; d : the thickness of the intrinsic layer in the SIN+ structure used as an emitter, : the photon energy of the pump beam;Θ : the incident angle of the pump beam;γ : the repetition rate of the pump beam; Io : the pump beam power;
S : the width of the charge depletion layer defined by
where is the dielectric constant of the semiconductor and is the potential barrier height across the interface or the charge depletion layer on surface.I0 : maintained at 200mW over an area with radius of 500μm.
Surprisingly the dependence of the number of the photo-excited carriers is the same as the dependence of the THz amplitude on the intrinsic layer thickness.
We have :
-200 -100 0 100 2000.0
0.5
1.0
1.5
2.0
2.5
20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
Top layer thickness (nm) TH
z am
plit
ud
e (n
A)
TH
z am
plit
ude
(nA
)
Top layer thickness (nm)
InAlAs SIN+
etching from 200 nm
as grown
GaAs SIN+
etching from 100 nm
locph
THz Eet
tn
t
JtE
)(
)(
-200 -100 0 100 2000.0
0.5
1.0
1.5
2.0
2.5
20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
Top layer thickness (nm) TH
z am
plit
ude
(nA
)
TH
z am
plit
ude
(nA
)
Top layer thickness (nm)
InAlAs SIN+
etching from 200 nm
as grown
GaAs SIN+
etching from 100 nm
Let’s come back to the equation:
In the instantaneous photo-excited case:
Carrier life time c (~1ps) >> laser pulse duration (~80fs)
ctphph entn /)(
locphc
THz EetntE
)()1
()(
The THz amplitude: phTHz nE )0(
The critical electric field : depends on the energy difference between the Γ to L valley (intervalley threshold, L valley offset ) in the semiconductor.
Why is ETHz independent of Eloc ?The critical electric field introduced by Leitenstorfer et al. in Appl. Phys. Lett. 74 (1999) 1516. Phys. Rev. Lett. 82 (1999) 5140.
In low field limit : the maximum drift velocity is proportional to the electric fieldIn high-field limit (as the field rises above the critical electric field) : the maximum drift velocity declines slightly as the field increases. The drift velocity of free carrier reaches its maximum at the critical electric field
The critical electric field:Appl. Phys. Lett. 74 (1999) 1516 :
GaAs : ΔE = 330meV, Ec = 40 kV/cm
Phys. Rev. Lett. 82 (1999) 5140 :InP : ΔE = 600meV, Ec = 60 kV/cmSolid State Electron. 43 (1999) 403 :
InAlAs : ΔE = 430meV, Ec ~ 47 kV/cm (estim
ated)
The surface fields in our samples exceed their corresponding critical electric fields
-200 -100 0 100 200
50
100
150
200
250
300
350
Intrinsic Layer thickness d (nm)
Surface field of different SIN + StructureGaAs (Etched from 100 nm) InAlAs ( As Grown )InAlAs ( Etched from 200 nm )
Bui
lt-i
n el
ectr
ic f
ield
(k
V/c
m)
All the surface fields are larger than their corresponding critical fields, therefore; the amplitudes of THz are independent of the surface field.
InIn0.520.52AlAl0.480.48As SINAs SIN++
d (nm)d (nm) Field (kV/cm)Field (kV/cm)
200200
120120
5050
2020
47.2547.25
53.3353.33
122.90122.90
255.30255.30
GaAs SINGaAs SIN++
d (nm)d (nm) Field (kV/cm)Field (kV/cm)
100100 61.1561.15
These results have been published in APL 87,121107 (2005).
-4000 -2000 0 2000 4000 6000 8000
0
2
4
6
8
10
12
14
16
TH
z A
mp
litu
de
(n
A)
Thickness(Å)
GaAs SINGaAs SIN++
GaAs (100)GaAs (100)Semi-insulatedSemi-insulated
GaAs (100)GaAs (100)11μμmm
n-doped 1*10n-doped 1*101818cmcm-3-3
GaAs (100)GaAs (100)Thickness Thickness dd
d = 800 nmd = 800 nm
THz Amplitude v.s. Thickness
-4000 -2000 0 2000 4000 6000 8000
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14
16
18
THz T
Hz
Am
plit
ud
e (
nA
)
Thickness (Å)
Ca
rrie
r N
um
be
r (1
08 )
Carrier
-4000 -2000 0 2000 4000 6000 8000
0
2
4
6
8
10
12
14
16
0
50
100
150
200
250
300
THz
TH
z A
mp
litu
de
(n
A)
Thickness(Å)
Fie
ld (
kV/c
m)
Field
THz Amplitude and Carriersv.s.
Thickness
THz Amplitude and Fieldv.s.
Thickness
-4000 -2000 0 2000 4000 6000 8000
0
2
4
6
8
10
12
14
0
5
10
15
20
25
THz
TH
z A
mp
litu
de
(n
A)
Thickness(Å)
Carrier Field
Ca
rrie
rFie
ld (
109
kV/c
m)
THz Amplitude and v.s.
Thickness
En
effectiveEnTHz Amplitude and v.s.
Thickness
Summary• THz radiation from series of GaAs and InAlAs SIN+ structures without externa
l bias was studied.
• The amplitude of THz waves radiated is independent of the built-in electric field when the built-in electric field exceeds the critical electric field.
• The THz amplitude is proportional to the number of photo-excited free charged carriers. (while bias field exceeds the critical electric field).
• If the critical electric field determined from the THz amplitude as a function of the electric field
=> It would be to determine the Γ to L valley splitting in semiconductors.
• The most efficient SIN+ structure THz emitter would be the built-in electric field equal to the critical field while the thickness of the intrinsic layer equal to the penetration depth of pump laser.
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5. J. S. Hwang, S. L. Tyan, W. Y. Chou, M. L. Lee, D. Weyburne and Z. Hang: Appl. Phys. Lett. 64 (1994) 3314.
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The End.The End.
Thanks for your attention !Thanks for your attention !
)2cossin22sin(cos2
),( 413
c
LrEnII THz
p
ZnTe CrystalZnTe Crystal
Z(001)
X(100) Y(010)Kp , KTHz
(110)
ETHz
Ep
pI
L
Probe beam intensityRefraction index of ZnTeElectro-optical coefficient of ZnTeThickness of ZnTe
41rn
0 90 180 270 360-8
-6
-4
-2
0
2
4
6
8
Inte
nsity (
arb
. u
nits)
Azimuthal angle (degrees)
Eprobe
// ETHz
0 90 180 270 360-10
-8
-6
-4
-2
0
2
4
6
8
10
Inte
nsity (
arb
. u
nits)
Azimuthal angle (degrees)
Eprobe
ETHz
)2cossin22sin(cos2
),( 413
c
LrEnII THz
p
090
ZnTe
e = 11; ng = 3.2
vg(800 nm) = vp(150 μm)
Eg= 2.2 eVvphonon= 5.3 THz
E= 89 V/cm
f > 40 THz; t < 30 fs
r41 = 4 pm/V
Visible pulse experiences different THz induced refractive-indexChange for different polarizations
THzopt
optoptTHz
THz
THzTHz
THz
THz
THz
opTHzopTHz
THzopTHzop
n|d
dnn
kk
kkk
kkkk
opt
op
THzopt
optoptTHz
c
n|d
dnn
ckc
L
opt
Phase matching condition k=0, optical group velocity = THz phase velocity
c/)(n)(k
Spectra absorptionα(ω) (abs.vs.frequency)
Refractive index n(ω) (time delay vs. frequency