Download The critical electric field

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
Motivation
System for generation and detection of THz radiation
Experimental Results and Discussions
Summary
What is Terahertz Radiation (THz or T-ray) ?
THz Gap
Terahertz region : 0.1 ~ 30 THz
1 THz = 1012 Hz ~ 300 µm ~ 4.1 meV ~ 47.6 K
Application 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
TeraView.Ltd
(2001 UK) => http://www.teraview.com
medical imaging and diagnosis :
cancer (oncology) , cosmetics , oral healthcare
pharmaceutical applications :
drug discovery & formulation , proteomics
security
non-destructive testing
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
physical 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 ;
Repetition rate : 82 MHz;
Pulse power ~ 8.0 nJ
Pulse width : 80 fs;
Semiconductor
crystal
Laser pulse
THz pulse ETHz(t, W)
Dt
E THz ( t ) 
Voltage
source
reflected optical beam
& THz pulse
THz pulse
Et
E1
q1
qo
optical beam
E2
q2
J ( t )
t
(1) laser pulse + semiconductor
  E g
(2) create transient photocurrent
J( t )  n ( t )eE b
(3) far field THz radiation
E THz ( t ) 
J ( t )
t
DI
L
 D  n3r41 ETHz
I

detector
s
[1,-1,0]
ZnTe
DI
p
Wollaston
polarizer
/4 plate
probe beam
pellicle
E
E
polarizer
System for generation and detection of THz radiation
Ti:Sapphire pulse laser (Tsunami, Spectro-Physics)
Power : 700 mw (max);
Wavelength : 790 nm ;
Repetition rate : 82 MHz;
Pulse power ~ 8.0 nJ
Pulse width : 80 fs;
Porbe beam pulse
signal
THz pulse
t=t0
t
t
t=t1
t=t2
Intensity (a.u)
t=t2
t=t0
t=t1
0
2
4
Time delay (ps)
6
t
Time-domain THz spectroscopy
FFT of THz spectroscopy
GaAs
Amplitude
Intensity (a.u)
GaAs
0
2
4
6
Time delay (ps)
8
10
12
0
1
Frequency (THz)
2
3
System for generation and detection of THz radiation
Ti:Sapphire pulse laser (Tsunami, Spectro-Physics)
Power : 700 mw (max);
Wavelength : 790 nm ;
Repetition rate : 82 MHz;
Pulse power ~ 8.0 nJ
Pulse width : 80 fs;
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.
ETHz (t ) 
J n ph (t )

eEloc
t
t
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 Sampling
1. 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 Structures
In0.52Al0.48As SIN+
GaAs SIN+
In0.52Al0.48As (100)
Thickness d
GaAs (100)
Thickness d
In0.52Al0.48As (100)
1μm
Si-doped 1*1018cm-3
GaAs (100)
1μm
n-doped 1*1018cm-3
InP (100)
Semi-insulated
d = 200, 120, 50, 20 nm
GaAs (100)
Semi-insulated
d = 100 nm
Time domain THz radiation spectrum:
(a)
Amplitude (arb.units)
1.5
GaAs wafer
+
GaAs SIN d=100nm
+
InAlAs SIN d=104nm
+
InAlAs SIN d=200nm
1.0
0.5
0.0
-0.5
-1.0
0
2
4
6
8
10
time delay (ps)
Frequency domain THz radiation (FFT) spectrum:
Phase (rad)
50
Amplitude (arb. unit)
3.0
2.5
40
30
20
10
2.0
0
0.0
1.5
0.5
1.0
1.5
2.0
2.5
3.0
Frequency (THz)
1.0
GaAs wafer
+
GaAs SIN d=100nm
+
InAlAs SIN d=104nm
+
InAlAs SIN d=200nm
0.5
0.0
0.0
0.5
1.0
1.5
2.0
2.5
Frequency (THz)
3.0
3.5
4.0
Intensities of THz radiation from InAlAs SIN+ structures with various
intrinsic layer thicknesses d :
2.5
GaAs SIN +
etching from 100 nm
0.2
40
60
80
100
1.5
0.0
1.0
Top layer thickness (nm)
InAlAs SIN +
0.5
Built-in electric field (kV/cm)
0.4
2.0
THz amplitude (nA)
0.6
Surface field of different SIN Structure
GaAs (Etched from 100 nm)
InAlAs ( As Grown )
InAlAs ( Etched from 200 nm )
350
THz amplitude (nA)
0.8
20
+
1.0
300
250
200
150
100
etching from 200 nm
as grown
50
0.0
-200
-100
0
100
Top layer thickness (nm)
200
-200
-100
0
100
Intrinsic Layer thickness d (nm)
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 !
200
On the other hand, the number of photo-excited free charged carriers
can be estimated as function of the intrinsic layer thickness d by
(1  R ) ds
n ph (d) 
I 0 exp( x )dx
 cos q 0
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
S  2 0 r / eN
where  r 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.
We have :
2.5
GaAs SIN +
etching from 100 nm
0.6
0.4
0.2
20
40
60
80
100
2.0
1.5
0.0
1.0
Top layer thickness (nm)
InAlAs SIN +
0.5
etching from 200 nm
as grown
0.0
-200
-100
0
100
200
Top layer thickness (nm)
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.
THz amplitude (nA)
0.8
THz amplitude (nA)
1.0
Let’s come back to the equation:
ETHz (t ) 
J n ph (t )

eEloc
t
t
In the instantaneous photo-excited case:
t c (~1ps)
Carrier life time
>> laser pulse duration (~80fs)
n ph (t )  n phe t /t c
1
ETHz (t )  ( )n ph (t )eEloc
tc
ETHz (0)  n ph
The THz amplitude:
2.5
GaAs SIN +
etching from 100 nm
0.6
0.4
0.2
20
40
60
80
100
2.0
1.5
0.0
1.0
Top layer thickness (nm)
InAlAs SIN +
0.5
etching from 200 nm
as grown
0.0
-200
-100
0
100
Top layer thickness (nm)
200
THz amplitude (nA)
0.8
THz amplitude (nA)
1.0
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 field
In 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 : depends on the energy difference between the Γ to L
valley (intervalley threshold, L valley offset ) in the semiconductor.
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/cm
Solid State Electron. 43 (1999) 403 :
InAlAs : ΔE = 430meV, Ec ~ 47 kV/cm (estimated)
The surface fields in our samples exceed
their corresponding critical electric fields
In0.52Al0.48As SIN+
d (nm)
Field (kV/cm)
+
Surface field of different SIN Structure
GaAs (Etched from 100 nm)
InAlAs ( As Grown )
InAlAs ( Etched from 200 nm )
Built-in electric field (kV/cm)
350
300
250
200
47.25
120
53.33
50
122.90
20
255.30
200
150
GaAs SIN+
100
d (nm)
50
-200
-100
0
100
Intrinsic Layer thickness d (nm)
Field (kV/cm)
200
100
61.15
All the surface fields are larger than their corresponding critical fields, therefore;
the amplitudes of THz are independent of the surface field.
These results have been published in APL 87,121107 (2005).
GaAs SIN+
THz Amplitude v.s. Thickness
16
GaAs (100)
Thickness d
GaAs (100)
Semi-insulated
12
THz Amplitude (nA)
GaAs (100)
1μm
n-doped 1*1018cm-3
14
10
8
6
4
2
0
-4000
-2000
0
2000
4000
Thickness(Å)
d = 800 nm
6000
8000
THz
Carrier
12
8
10
6
8
6
4
THz Amplitude and Carriers
v.s.
Thickness
4
2
2
0
0
-4000
-2000
0
2000
4000
6000
8000
Thickness (Å)
THz
Field
16
300
THz Amplitude and Field
v.s.
Thickness
12
250
10
200
8
150
6
100
4
50
2
0
0
-4000
-2000
0
2000
Thickness(Å)
4000
6000
8000
Field (kV/cm)
14
THz Amplitude (nA)
THz Amplitude (nA)
14
10
8
16
12
Carrier Number (10 )
18
14
n E
THz Amplitude and
v.s.
Thickness
Carrier Field
THz
14
THz Amplitude (nA)
20
10
15
8
6
10
4
5
2
0
0
-4000
-2000
0
2000
Thickness(Å)
4000
6000
8000
9
12
CarrierField (10 kV/cm)
25
n  Eeffective
THz Amplitude and
v.s.
Thickness
Carrier Number Effective Field
25
9
14
 kV/cm)
THz
16
12
15
8
6
10
4
5
2
0
0
-4000
-2000
0
2000
4000
Thickness(Å)
6000
8000
Carrier Number
THz Amplitude (nA)
10
 Effective Field (10
20
Summary
•
THz radiation from series of GaAs and InAlAs SIN+ structures without
external 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.
References
1. X. C. Zhang and D. H. Auston: J. Appl. Phys. 71 (1992) 326.
2. K. Liu, A. Krotkus, K. Bertulis, J. Z. Xu and X. C. Zhang: J. Appl. Phys. 94 (2003) 3651.
3. P. Gu, M. Tani, S. Kono and K. Sakai: J. Appl. Phys. 91 (2002) 5533.
4. M. B. Johnston, D. M. Whittaker, A. Corchia, A. G. Davies and E. H. Linfield: Phys. Rev. B 65
(2002) 165301.
5. J. S. Hwang, S. L. Tyan, W. Y. Chou, M. L. Lee, D. Weyburne and Z. Hang: Appl. Phys. Lett. 64
(1994) 3314.
6. J. S. Hwang, W. C. Hwang, Z. P. Yang and G. S. Chang: Appl. Phys. Lett. 75 (1999) 2467.
7. J. S. Hwang, W. Y. Chou and M. C. Hung, J. S. Wang and H. H. Lin: J. Appl. Phys. 82 (1997) 3888.
8. Q. Wu and X. C. Zhang: Appl. Phys. Lett. 68 (1996) 1604.
9. Q. Wu and X. C. Zhang: Appl. Phys. Lett. 70 (1997) 1784.
10. Q. Wu and X. C. Zhang: Appl. Phys. Lett. 71 (1997) 1285.
11. J. N. Heyman, N. Coates and A. Reinhardt: Appl. Phys. Lett. 83 (2003) 5476.
12. A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss and W. H. Knox: Appl. Phys. Lett. 74 (1999)
1516.
13. A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss and W. H. Knox: Phys. Rev. Lett. 82 (1999)
5140.
14. R. Dittrich and W. Schroeder: Solid State Electron. 43 (1999) 403.
15. S. M. Sze: Semiconductor Device Physics and Technology (Wiley, New York, 1985).
The End.
Thanks for your attention !
ZnTe Crystal
ETHz

Z(001)

Ep
Y(010)
X(100)
Kp , KTHz
(110)
DI ( ,  )  I p
n3 ETHz r41L
2c
Ip
n
r41
L
(cos  sin 2  2 sin  cos 2 )
Probe beam intensity
Refraction index of ZnTe
Electro-optical coefficient of ZnTe
Thickness of ZnTe
DI ( ,  )  I p
n3 ETHz r41L
2c
(cos  sin 2  2 sin  cos 2 )
 
    900
8
10
Eprobe // ETHz
6
Eprobe  ETHz
8
6
Intensity (arb. units)
Intensity (arb. units)
4
2
0
-2
-4
4
2
0
-2
-4
-6
-6
-8
-8
-10
0
90
180
Azimuthal angle (degrees)
270
360
0
90
180
Azimuthal angle (degrees)
270
360
vphonon= 5.3 THz
E= 89 V/cm
f > 40 THz; t < 30 fs
Eg= 2.2 eV
ZnTe
e = 11; ng = 3.2
r41 = 4 pm/V
vg(800 nm) = vp(150 μm)
Visible pulse experiences different THz induced refractive-index
Change for different polarizations
Phase matching condition Dk=0, optical group velocity = THz phase velocity
Dk  k op  THz   k op   k THz 
 k op  THz   k op  k THz 
 THz 


THz 

THz
 k
k THz  

 THz

  
THz 
op


dn


 THz  nopt   opt opt |opt nTHz 
d


Lc 
c

Dk
THz nopt
c
dnopt
  opt
|opt nTHz
d
k ()  n () / c
Spectra absorptionα(ω) (abs.vs.frequency)
Refractive index n(ω) (time delay vs. frequency