SHG Spectroscopy

•Clean surfaces•Clean surfaces•Oxidation•SOI wafer

SHG set-uppScan regions

Idler: 730-1050 nm

1000- 1400 nm

Signal: 680-550 nm

600 500600-500 nm

Ti:Sapphire:

700-1000 nm

SHG from GaAs

Bergfeld, Daum, PRL 90, 2915

Spectroscopy on Si/SiO interfacesSpectroscopy on Si/SiO₂ interfaces

3.4 eV: E₀/E₁ transition at Γ-point4 2 eV: E transition at Γ point4.2 eV: E₂ transition at Γ-point3.6 eV: Interface transition

Si(100) Si/Ge SiOSi(100)‐Si/Ge‐SiO₂

Growth of strained layer

SHG Spectroscopy

8

10

Si(111)7x7

Interband transitionsSurface states below 3.4 eV

4

4

6

8

Clean

arb.

uni

ts) p to p

2

3

4

0

2

10 L O2

SH

G (a

0

1

2

U /S

U2

ergy

(eV

)

4

2,0 2,5 3,0 3,5 4,0 4,5

units

)

Cleanp to s

2

-1

0

S3

S2

U1/S1En

2

SH

G (a

rb. Clean

10 L O

-3

-2

MK Γ

2,0 2,5 3,0 3,5 4,0 4,5 5,00

S

SH Photon Energy (eV)

10 L O2

2.8 eV (1.4 eV): 1ω S2→U12.0/2.4 eV: 2ω S3→U13 4 eV: 2ω E

Surface states3.4 eV: 2ω E14.4 eV: 2ω E2

Pump-probe:Dynamics of surface statesDynamics of surface states

3

4

1

2U2

gy (

eV)

-1

0

S2

U1/S1Ene

rg

-3

-2

S3

MK Γ

Saturation and recovery of unoccupied ad-atom stateby pump pulse.Intra band ∼100 fs, interband (surface)∼1-2 psSurface states to bulk ∼100 psHöfer et al. PRB 73 245305 (2006)

SH resonances

∑⎭⎬⎫

⎩⎨⎧

+∝ mmnns

ifif)2(

)½2()exp(

)½()exp()2( φφωχ

10

∑⎭⎬

⎩⎨ +−+−mn

mmnn

s ii, )½2()½()(

γωωγωωχ

7

8

9

Clean p to p polarization

Resonances:

2.05 eV SS 1ω resonance3 06 V SS 2

5

6

7

arb.

uni

ts] 3.06 eV SS 2ω resonance

3.34 eV E1 2ω resonance4.34 eV E2 2ω resonance

3

4

SHG

[a

1ω or 2ω resonances?

0

1

2

10 L oxygen Constructive / destructiveinterference

1.5 2 2.5 3 3.5 4 4.5 50

SH Photon Energy [eV]

SH resonances

8

9

p to p polarization

Resonances:

2.15 eV SS 1ω resonance

6

7

8

its]

p to p polarization 2.15 eV SS 1ω resonance3.06 eV SS 2ω resonance3.34 eV E1 2ω resonance3.55 eV Si-O 2ω resonance

4

5

G [a

rb. u

n

10 L oxygen

Clean

4.34 eV E2 2ω resonance

2

3SH New resonance from Si-O bondsat the interface

1.5 2 2.5 3 3.5 4 4.5 50

1

SH Photon Energy [eV]

Daum et al. PRL 93 097402

gy [ ]

Ge(111) – linear response

Lowest direct transition very weakSpin-orbit splitting of 2 1-eV transition ~0 2 eVSpin orbit splitting of 2.1 eV transition 0.2 eV

Ge(111) spectra ‐ comparison

2.1-2.4 eV transitions: spin-orbit or interface states?2 8-3 0 eV transitions: sensitive to surface treatment2.8 3.0 eV transitions: sensitive to surface treatment.

Ge(111) spectra ‐ comparison

2.1-2.4 eV transitions: spin-orbit or interface states?2.8-3.0 eV transitions: sensitive to surface treatment.Additional peak at 3.1 eV?What happens below 2.0 eV?

Second harmonic generationSecond harmonic generation spectroscopy on SOI wafersp py

Kjeld PedersenThomas Garm PedersenAalborg University

Denmark

Sn nanocrystals in Si

1,0

1,2Sn in SiB lk l

0,6

0,8

nits

)

Si d i lSi epilayer

0,8

1,0

Si epilayer Snunits

)

Bulk sample

0,4

,

HG

(arb

. u Si device layer

Si waferOxide

0,4

0,6Si wafer

Si epilayer Sn

HG

(arb

. u

700 800 900 1000 1100 1200 1300 14000,0

0,2SH

700 800 900 1000 1100 1200 13000,0

0,2SH

Pump Wavelength (nm)

Samples from Arne Nylandsted

700 800 900 1000 1100 1200 1300

Pump Wavelength (nm)

Conclusion: Don’t use SOI wafers for structures for optical characterization(Unless you want to probe the SOI interfaces)

SOI – SHG from interfaces1

380 nm Si400 nm SiO2

0,1

Log scale!

b. u

nits

) Si device layerOxide

~380 nm~400 nm

SH

G (a

rb

Si handling wafer

700 800 900 1000 1100 1200 1300 1400

0,01

Pump Wavelength (nm)Pump Wavelength (nm)

SHG oscillationsLinear properties

SHGSHG sources

Can the oscillations be used to isolate SHG from buried interfaces?Can the oscillations be used to isolate SHG from buried interfaces?

SOI wafers

• Fast IC’s – transistor insulation

• CMOS

• MEMS

• Si Photonics

• ….

Si device layer

Oxide

~200 nm

~200 nm

Si handling wafer

SHG measurements on SOI interfaces

B. Jun, IEEE Trans Nucl Sci, 51, 3231B. Jun, Appl. Phys. Lett. 85, 3095N. Tolk, Microelectronic Engineering 84, 2089

Linear propertiesEllipsometryEllipsometry

0,8 SOI waferFit to eliipsometric data:209 nm Si410 nm SiOan

d s 2)

0,0

0,4 410 nm SiO2

eter

s (s

1 a

-0,4

c pa

ram

e

-0,8

psom

etri

400 500 600 700 800-1,2E

lli

Wavelength (nm)

Linear propertiesReflection multilayer structureReflection – multilayer structure

1 2 3 4

Si SiSiO₂

1,0Linear reflection    ‐data from Palik

0 4

0,6

0,8 θ=30oec

tion

θ=40o

600 800 1000 1200 1400 16000,0

0,2

0,4

600 800 1000 1200 1400 1600

Ref

le

600 800 1000 1200 1400 1600 600 800 1000 1200 1400 1600

0 4

0,6

0,8

θ=50o

ectio

n θ=60o

600 800 1000 1200 1400 16000,0

0,2

0,4

400 600 800 1000 1200 1400 1600

Ref

le

600 800 1000 1200 1400 1600 400 600 800 1000 1200 1400 1600

0,6

0,8

θ=70o

ectio

n

θ=30o

Fit to data: dSi=203 nm doxide=402 nm

0,0

0,2

0,4

Ref

le

θ=70o

600 800 1000 1200 1400 1600,

Wavelength (nm)600 800 1000 1200 1400 1600

Wavelength (nm)

SHG set-upScan regions

Idler: 720-1400 nm

Signal: 680-500 nm

Excitations: 1-5 eV

Refractive index ‐ absorption

7

2ωω Si

5

6

7 ω Si

ex

3

4

5

nr

ve In

de

1

2

3

niRef

ract

i

200 400 600 800 1000 1200 1400 1600 1800

0

1 iR

Wavelength (nm)

Transparent to pump light in whole regionTransparent to pump light in whole regionStrong absorption of SHG for pump shorter than 800 nm

12

SHG spectra

10

11

12

E2

SOI wafer, 200 nm Si, 400 nm SiO2

8

9

Interface

2

Eunits

)

70o

60o

5

6

7 E1

G (a

rb. u

60 50o

40o

30o

3

4

SH

G 30o

0

1

2

500 600 700 800 900 1000 1100 1200 1300 14000

Pump Wavelength(nm)

Resonances at critical points + oscillations

SHG spectra10

SOI wafer, 200 nm Si, 400 nm SiO2

1

70o

60o

50oLog scale!

1

SH

G

40o

30o

0,1

Interface

E2 E1

500 600 700 800 900 1000 1100 1200 1300 1400

0,01

Pump Wavelength(nm)Pump Wavelength(nm)

SHG sources

Sum of radiations from dipole sheets

ω ω 2ω

Interface contributions

Bulk contributions

Rotational anisotropyBulk contributionBulk contribution

1,0

0,8ts

)

0,6

p to parb.

uni

t

0 2

0,4p pλp=750 nmθ=60o

SH

G (a

0 45 90 135 180 225 270 315 3600,0

0,2y=(0.9+0.1*cos(4*pi/180*col(A)))^2

S

0 45 90 135 180 225 270 315 360

Rotational Angle (deg.)

Bulk contribution ~10%

Effect of linear reflections1st interface1st interface

1

R( )

R(2ω) + Min at 550 nm+ Min at 1000 nm

0,1

R(ω)

θ=50o

on

+ Peak at 750-800 nm

÷ Min at 620 nm

0,01Ref

lect

io Min at 620 nm

0,01SHGR

500 600 700 800 900 1000 1100 1200 13001E-3

Wavelength (nm)Wavelength (nm)

SHG Si/SiO₂ interface response

Natural oxide

W. Daum, PRB 59, 2915

Si(100) interface resonance

oxide

( )

Depends on interface formation60 nm thermal oxide

Interface resonance part of responsefunction in near IR (eg. 800 nm)

105 nm dry/wet/dry+annealing+annealing

Si(111)/oxide interface

8

10

Si(111)7x7

4

6

8

Clean

arb.

uni

ts) p to p

2 0 2 5 3 0 3 5 4 0 4 50

2

10 L O2

SH

G (a

4

2.0 2.5 3.0 3.5 4.0 4.5

b. u

nits

)

Cleanp to s

2

SH

G (a

rb10 L O2

2.0 2.5 3.0 3.5 4.0 4.5 5.00

SH Photon Energy (eV)

Model – SHG from dipole sheets

1

zSipe’s model:J. Opt. Soc. Am. B4, 481 (1987)Phys. Rev. B35, 1129 (1987)

2

••

y ( )

Field in medium 1 from source in medium i

iP

zi

••

n

∑ += EE

Other models: Bethune, J. Opt. Soc. Am. B6, 910

∑=i

iEE 1,1

, p ,Yeganeh, Phys. Rev. B46, 1603Wierenga, Physica B204, 281

SHG model (continued)

)()()2( )2( ωωχω kjijki EEP =

I fEEl∑= ll if φχ )2( )exp(InterfaceEEl ,,: 21∑ −−

=l ll iγωω

χ 22)(

2

xyzzzzzxxeff aaaaa ∂+∂+∂++= 54321 γξχ

BulkAnisotropic

B lk

SurfaceIsotropic

BulkIsotropic Depends on angle of incidence

Interface contributions

10

8

9χ(2) constant θ=50o Only 1st interface

for λ<850 nm

6

7 Exp 1st interface3rd interfaceun

its)

Peak at 900 nm:2nd and 3rd interface

3

4

53rd interface 2nd interface

G (a

rb.

1

2

3

SH

G

500 600 700 800 900 1000 1100 1200 13000

1

Wavelength (nm)

Fit to experiments10 SHG model: 3 interfaces

θ=50o 10θ=30o

1 2nd and 3rd interface

b. u

nits

)

1

b. u

nits

)

0,1

SH

G (a

rb

0,01

0,1

SH

G (a

r

500 600 700 800 900 1000 1100 1200 1300

0,01

Wavelength (nm)

500 600 700 800 900 1000 1100 1200 1300Wavelength (nm)Wavelength (nm)

S ² f b th l d f ll 3 i t fd l1st

Same χ² for both angles and for all 3 interfaces

No bulk contribution

Si device layerOxide

Si handling wafer

2nd

3rdg

Looking for buried interfaces

0,8

1,0 1st interface 2nd 3rd

dSi=200 nmdSiO =400 nms)

0,8

1,0

dSi=100 nmdSiO =200 nm)

0,4

0,6

SiO2

χ(2)=1

G (a

rb. u

nits

0,4

0,6

SiO2

χ(2)=1

G (a

rb. u

nits

)

500 600 700 800 900 1000 1100 1200 1300 14000,0

0,2SH

G

Wa elength (nm) 500 600 700 800 900 1000 1100 1200 1300 14000,0

0,2SH

G

Wavelength (nm) 500 600 700 800 900 1000 1100 1200 1300 1400

Wavelength (nm)

0,8

1,0 1st interface 2nd 3rd

dSi=300 nmdSiO2

=300 nm)

We can find a wavelength to test 3rd interface

0,4

0,6

2

χ(2)=1

G (a

rb. u

nits

)

2nd interface is difficult to reach

1 t

500 600 700 800 900 1000 1100 1200 1300 14000,0

0,2SH

G

Si device layerOxide

1st

2nd

Wavelength (nm) Si handling wafer 3rd

Conclusions

• Oscillations in SHG from SOI wafer– Multiple reflections in linear fieldp

– Multiple reflections in SH field

Variations in χ² of Si– Variations in χ² of Si

• Only first interface for λ<800 nm• Wavelength for2nd and 3rd interface can be foundfound– λp ,θ