Second Harmonic Spectroscopy Study of Silicon

Nanocrystals Embedded in SiO2

Junwei Wei, Adrian Wirth, Michael C. DownerPhysics Department, the University of Texas at Austin

““““Si lasers start to take shape”

Si nanocrystals have properties & applications different from those of bulk Si

Observation of optical gain in Si

nanocrystals embedded in SiO2Pavesi et al., Nature 408, 440 (2000)

Those interesting properties originate at Si NC/SiO2 interfaces

SHG has a reputation for being interface-specific

Interface and bulk contributions to SHG from

planar surfaces are never sperated with full rigor

SiO2

Si

Interface dipole SHG

Bulk quadrupolar SHG

J.E. Sipe et. al., Phys. Rev. B 35, 1129 (1987)

Empirical separation of surface & bulk contribution is usually based on:

1. Azimuthal anisotropy 2. Spectroscopy study 3. Interface modification

Similar bulk/interface ambiguity in SHG from

Si NCs must be distinguished empiricallyMochan et. al., Phys. Rev. B 68, 085318 (2003)

mµ1

TEM ImagesSi ions

C. W. White et al., NIM B 141, 228 (1998) - ORNL

glass

substrate

The samples are prepared by Si Ion implantation into SiO2

• Multi-energy implant (35-500 keV) yields uniform NC density1

2 • Samples annealed @ 1100 C / 1 hr in Ar + H2 to precipitate NC formation

substrate

4

350 400 450 500 550 600

0

2000

4000

6000

In

ten

sit

y (

arb

. u

n.)

Raman Shift (cm-1)

Raman Spectra Photoluminescence excitation spectra

2.5 3.0 3.5 4.0 4.5 5.00.0

0.3

0.6

0.9

1.2

PL

E In

ten

sit

y (

arb

. u

nit

s)

Photon energy (eV)

5 nm Si NCs3 nm Si NCs

As-Implanted

c-Si

2.5 3.0 3.5 4.0 4.5 5.00.0

0.3

0.6

0.9

1.2

20

30

40

50

εε εε 22 22

Spectroscopic elllipsometry (SE) shows modified

c-Si E1 and E2 critical points in the Si NCs

c-SiE1

E2• Bulk CPs E1 and E2 preserved,

with E2 dominating the spectra

and E1 greatly suppressed

• Appearance of peak around 3.9 eV

5 nm and 3 nm Si NCs:

1 2 3 4 5 6

0

10

20εε εε

Photon Energy (eV)

eV

• No bulk CPs E1 and E2• Similar but blue-shifted

shape to a-Si

As-Implanted:

• SE spectra provide a comparison for SHG spectroscopy

• Measured ɛ1,2 determine the Fresnel factors used in SHG analysis

Cross-Polarized 2-Beam-SHG (XP2-SHG) enhances the

signal from Si NCs by enhancing the field gradient

EEPeff

vvv∇⋅∆= '

0

2

~w

EEEvv

∇⋅ αλ

SinEE

EE 21~vv

∇⋅

XP2-SHGSingle-beam SHG

2ω

L. Sun, et al, Opt. Lett., 30, 2287 (2005)

P. Figliozzi, L. Sun, et al, Phy. Rev. Lett., 94, 047401(2005)

SiO2

NCs

r E 1

2ωk1

r E 2

k2

αs θ p

SiO2

NCs

r E 1

2ωk1

r E 2

p

20 )(~ αλ

Sinw

tEnhancemenIntensity

Oriel

PMT

PMTdetection bandwidth

∆λ=10 nm

ARC 300i

spectrometer

trigger pulse

pre-amp

gated counter

CH2CH1

XP2-SHG Measurement Setup

Oriel

spectrometer

NOPA

signalBS1 BS2

L1

L2

spectrometer

translation stage

step motor

L3

L4

0ϕ

delay stage

BBO

sample

WP

L5

Periscope

L6

FiltersFilters

from 800 to 520 nm

SHG spectra show strong interface resonance

and modified c-Si critical points in the Si NCsSE measurement

PLE measurement

1 2 3 4 5

0

20

40

εε εε 22 22

Photon energy (eV)

1 . 2

PL

E I

nte

ns

ity

(a

rb.

un

its

)

4

No

rmalized

SH

G |Γ|Γ |Γ|Γ

nc|/Γ

|/Γ

|/Γ

|/Γ

g

4

15

20

25

≤1 nm 3 nm 5 nm

Raman measurement

2 . 5 3 . 0 3 . 5 4 . 0 4 . 5 5 .00 . 0

0 . 3

0 . 6

0 . 9

1 . 2

PL

E I

nte

ns

ity

(a

rb.

un

its

)

P h o t o n e n e r g y ( e V )

350 400 450 500 550 600

Inte

nsity (

arb

. u

n.)

Raman Shift (cm -1)

3 4 50

2

No

rmalized

SH

G

3 4 50

2

3 4 50

5

10

S H P hoton E nergy (eV )

G. Erley, W. Daum, Phys. Rev. B 58, R1734 (1998)

S. Bergfeld, et al, Phys. Rev. Lett. 93,097402

(2004)N. Daldosso, et al. Phys. Rev. B 68, 085327 (2003)

A. Sakko, et al. Phys. Rev. B 81,205317 (2010)

Quadrupolar SHG appears to be selectively

sensitive to nano-interface structure (in close

analogy to dipolar SHG of planar interfaces)

Conclusion

� SHG, complemented with SE, PLE and Raman, has been applied

to study Si NCs to help elucidate the unique structure of the NCs

� The unique sensitivity of SHG spectral structure and amplitude

suggest SHG is uniquely sensitive to nano-interfacial structure

Future directions: 1. Pump-probe XP2-SHG for dynamics study2. Free-standing Si NCs

Research group

Robert Ehlert, Ming Lei, Loucas

Loumakos, Adrienne Prem,

Aaron Roberts

Funding

National Science Foundation

Grant DMR-0706227

Robert Welch Foundation

Grant F-1038

Acknowledgements

2. Free-standing Si NCs