Outline

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.


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



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



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


z am

plit

ud

e (n

A)

TH

z am

plit

ude

(nA

)


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


z am

plit

ude

(nA

)

TH

z am

plit

ude

(nA

)


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.

References1. 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.



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.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

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