These interesting properties originate at Si NC/SiO2 interfaces. SHG has a reputation for being interface-specific

Nonlinear Spectroscopy of Si Nanocrystals & Step-Edges

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

Mike DownerUniversity of Texas at Austin

EPI-OPTICS 11Erice, SicilyJuly 20, 2010

Observation of optical gain in Si nanocrystals embedded in SiO2Pavesi et al., Nature 408, 440 (2000)

“Si lasers start to take shape”

Walters et al, Nature Mat. 4,143 (2005).

Vgate > Ve- injection

Vgate > Vh+ injection

Field-effect LED In vivo bio-sensing

Erogbogbo et al, ACS Nano.2, 873 (2008)

Live pancreatic cancer cells

17 µm

amine-terminatedmicelle-encapsulatedSi NC, a non-toxic al-ternative to CdSe NCs

These interesting properties originate at Si NC/SiO2 interfaces. SHG has a reputation for being interface-specific

Nonlinear Spectroscopy of Si Nanocrystals & Step-Edges

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

Mike DownerUniversity of Texas at Austin

step-edges

EPI-OPTICS 11Erice, SicilyJuly 20, 2010

Observation of optical gain in Si nanocrystals embedded in SiO2Pavesi et al., Nature 408, 440 (2000)

“Si lasers start to take shape”

Walters et al, Nature Mat. 4,143 (2005).

Vgate > Ve- injection

Vgate > Vh+ injection

Field-effect LED In vivo bio-sensing

Erogbogbo et al, ACS Nano.2, 873 (2008)

Live pancreatic cancer cells

17 µm

amine-terminatedmicelle-encapsulatedSi NC, a non-toxic al-ternative to CdSe NCs

nanowire

DNA bases adsorbed at vicinal SiMauricio et al., Nanoletters 3, 479 (2003)

courtesy D. R. T. Zahn, TU-Chemnitz

silicon stepped surfacesprovide templates for1D quantum wires,1

molecular electronics,2atomic-scale memory,3quantum computers4

& other nano-electronicstructures

1McChesney, Nanotech. 13, 545 (02)2Kasemo, Surf. Sci. 500, 656 (02)3Bennewitz, Nanotech. 14, 499 (02)4Ladd, Phys. Rev. Lett. 89, 017901 (02)

Co-workers

Si NCs Si step-edges Theory

Pete Figliozzi

PhD 2007

Y. Jiang

PhD 2002Liangfeng Sun

PhD 2006

Jinhee Kwon

PhD 2006

Bernardo MendozaCIO, León, México

W. Luis MochanU. Nacional Autónoma

Cuernavaca, MéxicoFinancial Support: • Robert Welch Foundation

• U.S. National Science Foundation

Adrian Wirth (MS 2007)

Robert EhlertJunwei Wei

Yongqiang An

PhD UC-Boulder

2004

Their elusive nano-interfaces make Si NCs

interesting & challenging

0.1961632725 nm

0.47982092 nm

surface atom fraction# surface atoms# atomsdiameter

c-Si

strained SiOx ?

a-SiO2

Radiative double bonds:Wolkin et al., Phys. Rev. Lett. 82, 197 (1999)

Luppi & Ossicini, Phys. Rev. B 71 (2005)

Transition layer(s):Daldosso et al., Phys. Rev. B 68, (2003)

a-Si ?

Si

Si

H

O

Si

• Buried nano-interfaces inaccessible to many surface science probes

and challenging to described theoretically (e.g. by DFT, Monte Carlo)

• Here we use multiple complementary spectroscopies

Bridge bonds:Sa’ar et al., Nano Lett. 5, 2443 (2005).

Si

SiO

Dangling bonds

(SE)

(SHG)

(XPS, Raman)(PL)

SH

G i

nte

ns

ity

Ip

p(2

)

azimuthal angle (degrees)

0 180 360

Si(001)/SiO2

E2

E1

Erley & Daum, PRB 58, R1734 (1998)

Surface & bulk contributions to SHG from planar surfaces

are never separated with full rigor...J. E. Sipe et al., Phys. Rev. B 35, 1129 (1987)

Si

SiO2

asymmetric interface

electron response

p

t

P(0)

P( )

P(2 )

Fourier decomposition

surf(2) :

E

E

p-polarized interface dipole SHG

bulk quadrupolar SHG

... but empirical separation is usually possible based on:

1. azimuthal anisotropy 2. spectrum 3. sensitivity to interface

modification

I(2 ) = |a0 + b4sin4 |2

bulksurface + bulk

SiOx/Si interfacethermal

oxide

Second-h

arm

onic

sig

nal (a

rb.

units)

thermal

oxide +

anneal in

Ar

native

oxide

Similarly, nano-interface & bulk contributions to SHG from

Si NCs are intertwined, and must be distinguished empiricallyMochan et al., Phys. Rev. B 68, 085318 (03)

single nanoparticle:

uniform nano-composite:

Pb (r ) = E2 + E E

Ps (r ) = ijk

s (a,b, f )FjFk ,

PNL

= E E

nNC[ e ( , ,a,b, f )

m ( , ,a,b, f )

q (a,b, f ) / 6]

Ps

From symmetry alone,

assuming l

single 5 nm NC

mμ1

TEM Images

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

Photoluminescenceexcitation @ 486 nm

PL spectrum is unchanged throughout the

excitation range 250 < < 500 nm

500 600 700 800 900 10001100

Int

[a

. u

.]

[nm]

Ar + H2

Ar

L pez et al., Appl. Phys. Lett. 9, 1637 (2002)

= 3nm

Si ions

• Samples annealed @ 1100 C / 1 hr in Ar or Ar + H2to precipitate NC formation2

glass substrate

One group of samples is prepared by Si ion implantation into SiO2C. W. White et al., NIM B 141, 228 (1998) - ORNL

(simplifies optical analysis)

We also fabricate Si NC samples on a benchtopby thermolysis of hydrogen silsesquioxane (HSQ)

• C. M. Hessel et al., Chem. Mater. 18, 6139 (2006); J. Phys. Chem. C 111, 6956 (2007)

XPS and Raman scatter document conversion of multiply-

coordinated a-Si clusters into 4-fold-coordinated c-Si NCsKachurin et al., Semiconductors 36, 647 (2002)

____________ , Fiz. Tekh. Poluprov. 36, 685 (2002)

Hessel, J. Chem. Phys. 112, 14247 (2008)

after Si implant

SiO2

500 C

650 C

1000 C

1150 C

Si 2p

Si(IV) Si(0)Si(II)Si(I)Si

(III)

After annealing at > 1000 C,

negligible sub-oxide is

detectable by XPS.

XPS Raman backscatter

3 nm NC-Si3 nm NC-Si

a-Si

Si-Si bonds

Anneal

300 400 500 600

c-Si

Raman shift (cm-1)

Ar 514.5 nm pump

Similar measurements by

previous investigators

Spectroscopic ellipsometry (SE) shows modified c-Si E1

and E2 critical points after annealing

as-implanted: • no E1,2 critical point features

• consistent with small a-Si clusters

after 1100 C anneal in Ar: • E1,2 peaks appear

• E1 suppressed, blue-shifted

• consistent with:

- previous SE measurements [1,2]

- ab initio calculations of optical

properties of Si NCs in SiO2 [3]

after 1100 C anneal in Ar + H2: • negligible further change

• SE appears selectively sensitive to c-Si core of Si NCs

• Measured 1,2 determine Fresnel factors used in SHG analysis

2

[1] En Naciri et al. J. Chem. Phys. 129, 184701 (2008)

[2] Cen et al., Appl. Phys. Lett. 93, 023122 (2008)

[3] Seino, Bechstedt, Kroll, Nanotech. 20, 135702 (2009) previous related SE results

E1

E2c-Si

a-Si

PL excitation spectrum demonstrates that linear absorption

occurs primarily in bulk c-Si cores, consistent with SE

suppressed

E1

E2

Photo-excited carriers cross-relax to interface states for PL

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

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

2

2E...))(2(

)2(

+EEPg

Single beam SHG XP2-SHG

Jiang et al., Appl. Phys. Lett., 78, 766 (2001)

nc silica

( ) ...)(6/)2(

+EEnP qmebnc1E

nm8002,1 =

SH signalEEP )()2(

2

0

1

w

2

1

Ar + H2

Ar

Three parameters needed to

describe XP2-SHG response

g

NC = | NC|ei

sample

z-scan

0 +1000-1000z-scan position (microns)

0

100

SH

G (

co

un

ts/s

) • Ar

• Ar + H2

Cross-Polarized 2-beam SHG (XP2-SHG) enhances signal 100

Conventional single-beam SHG is weak

0-100 +1000

104

XP

2-S

HG

(co

un

ts/s

)z-scan position (microns)

• Ar

• Ar + H2single beam

single beam

-h -h/2 h

SH

G I

nte

ntisty

(a

.u.)

Position of beam overlaph/20-h -h/2 h

SH

G I

nte

ntisty

(a

.u.)

Position of beam overlaph/20-h -h/2 h

SH

G I

nte

ntisty

(a

.u.)

Position of beam overlaph/20

Three independent z-scan measurements determine g, | NC| and

-h -h/2 h

SH

G I

nte

ntisty

(a

.u.)

Position of beam overlaph/20

glass

0z

1E

2E

2

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

Measurements (scans):

1 substrate

2 NC @ exit

3 NC @ entrance

• The peaks result from relaxation of phase mismatch when boundaries of the sample fall

within the 2-beam overlap region

• SHG signal growth in the glass is affected by phase mismatch

• An analogous enhancement underlies 3rd harmonic microscopy with focused beamsBarad, Appl. Phys. Lett. 70, 922 (1997)

• Peak heights are asymmetric because of linear absorption of SH light by NCs and

interference of SHG signals generated by NCs and silica

h/2-h/2

( )040.0159.0

060.05217.1

±=

±=g

nc

( )020.0241.0

064.0743.1

±=

±=

g

nc

= 630 nm

glass nc@ent nc@exit fit nc@ent fit nc@exit fit glass

SH

In

ten

sity [

a.u

.]

0 hh/2-hDeviation of beam overlap from sample center

-h/2

SH

In

ten

sity [

a.u

.]

glass nc@ent nc@exit fit nc@ent fit nc@exit fit glass

0 hh/2-hDeviation of beam overlap from sample center

-h/2

= 540 nm

Examples at 2 wavelengths illustrate extraction

of fitting parameters | nc|/ g and

SE determined Fresnel factors used in this analysis

SHG spectra lack E1,2 critical point resonances

• Kramers-Kronig

consistencybetween | NC|/| g|

and

HH

E1 E23 nm Si NCs

in SiO2

as-implanted:

like a-Si

SHG spectrum

of a-Si:

Erley & Daum,

Phys. Rev. B 58,

R1734 (1998)

SHG spectra lack E1,2 critical point resonances

• annealed in Ar:

enhanced NC near known

SiOx resonances

• annealed in Ar/H2:

minimal H-effect

consistent with

previous SHG

Erley & Daum,

PRB 58, 1743 (1998)

SHG spectrum of

thermally oxidized

planar Si(001) after

anneal in Ar

Second-harmonic photon energy (eV)

SH

reflection c

oeffic

ient

p-in/

p-out

s-in/p-out

SHG spectrum of

Si(111)-(1x1)-H

during oxidation

3.5 eV

Bergfeld, PRL 93,

097402 (2004)

O

HH

E1 E23 nm Si NCs

in SiO2

E1 (3.4 eV) and 3.8 eV resonances appear in SHG spectra of 5 nm diameter Si NCs

E 1

E1 E2

3.8

eV

!NC

(3nm )= 7e18 cm

"3

!NC(5nm )

= 3e18 cm"3

SHG per NC is >10× stronger from 5 nm NCSthan from 3 nm NCs

Ideal single particle SHG scaling: (5/3)6 = 21 [Dadap, PRL (1999)]

× 7/3

Spectroscopic XP2-SHG is sensitive to nc-Si/SiO2 interfacial

features* not observed by other spectroscopies

that appear in recent MD simulationsDjurabekova & Nordlund, “Atomistic simulation of the interface structure of

Si NCs embedded in amorphous silica,” Phys. Rev. B 77, 115325 (2008)

2 nm

S-SHG validates &

guides simulations

of the nc-Si/SiO2

interface

*elevated PE/atom a-Si shell

~ 10% undercoordinated bonds,

Si=O bonds also present

d = 1.3 nm NC

a-Si interior

d[nm] =

1.3

2.4

3.6

r/r0

PE

/ato

m (

eV

)

~ 10% suboxide*

a-Si

c-Si

Conclusions about Si embedded nanocrystals

< 2 nm

3 nm

5 nm

> 8 nm

XP2-SHG SE/PLE Raman XPS

emergent 3.8 eV resonance

featurelessspectrum

featurelessbackground

strong 3.8 eV

resonance

E1 resonance

broadfeaturelessspectrum

residual bkgd

E1 and E2resonances

E1 and E2resonances

broad 480 cm-1

peak

sharp 520 cm-1peak

480 cm-1 tail

480 cm-1 tail

sharp 520 cm-1

peak

SiO2

SiOx

• interface region stabilizes and thins with increasing dNC

a-Si

c-Si

Stepped (vicinal) Si surfaces are attractive

templates for nanofabrication

Non-invasive in-situ sensors that provide atomic-scale

information over the dimensions of a wafer are needed

investigation of step-enhanced

chemical reactions

atomic wires suitable for

transport

“lithography” by self

assembly of nanostructures

…

Optical metrology bridges the nano-scale & wafer-scale

STM

nano-patterned surface

nano-scale probe wafer-scale nanomanufacturing

optical

metrology

r ||r

r

r= 2

r r||r + r||

Aspnes & Studna, PRL 54, 1956 (1985)

Reflectance

Anisotropy

Spectroscopy

Si(001):6o

step-edges

Ein Ein

r Ein r Ein

r/r

[10

-3]

Photon energy (eV)1 2 3 4 5

2

-2

0

RAS cleanH2 @ steps

cyclopentene

Si(001):6o

180 270 360900

H2

SH

G i

nte

ns

ity

(a

rb.u

.)

0

azimuthal angle (deg)

1

2

steps

[110]

2cyclo-

pentene

clean

SHG @

780 nm

We combine SHG & RAS probes of

stepped Si(001) surfaces in UHV

monochromator

+ PMT

QMSsample

gas inlet valve

PEM

Xenon lamp(1.5 to 5 eV)

tun

ab

le

fs-

sou

rce

B.S.

quartz

PM

T

BG39

analyzer

strain free

window

analyzer

polarizer

SHG ports

Coherent MIRA

Ti:Sapphire oscillator

710-900nm

NOPA

520nm-780nm

PMT

RAS

SHG

• RAS & SHG track contam-

ination non-invasively

• completely reversible

We have studied 3 very different surface chemical reactionson Si(001):6o

1) H2

2) H2 + atomic H

3) H2 + C5H8 (cyclopentene)C5H8

DB step

terrace dangling bonds

step dangling bondsstep rebonds

terrace dimers

back bonds

upperterrace

lowerterrace

• ~ 1000 L H2 @ 150 C• bonds selectively to step-edge db’s• Dürr et al., Phys. Rev. B 63, 121315 (2001)

• ~ 106 L H2 (saturation dose) • passivates step AND terrace db’s, leaving steps and dimers intact• atomic H can react with step rebonds and/or terrace dimers. Which happens first?

?

?

• C5H8 bonds by 2+2 cycloaddition to terrace dimers, forming crystalline organic monolayer• step-adsorbed H may help modify & control C5H8 adsorption

H2H2 H2 H2

H2

H2

SHG/RAS Case Study #1: Dissociative adsorption of H2 at DB steps of Si(001):6o

M. Durr, U. Höfer et al., Phys. Rev B. 63, 121315 (2001)

cleanSi(001):6o

In the absence of first principles theory, Simplified Bond

Hyperpolarizability Model (SBHM) provides

SHG - RAS interpretation at the molecular bond levelPowell et al., Phys. Rev. B 65, 205320 (2002)

pj(2 )

= j|| b̂j (b̂j Ein )

2

b̂j = bond unit vector

j||= axial hyperpolarizability

• A chemical bond is the basic polarizable unit

Ej2

=eikr

r2(I k̂k̂) pj

(2 )

• Induced axial SH polarization of bond j:

• Far-field SH radiation of bond j :

Ein

s

p

• Simplifications: - transverse hyperpolarizabilities neglected

- local field corrections folded into ’s

- boundary conditions not treated rigorously

b̂j ,

Ej2

k̂

• Total far-field SH radiation:

Ej2

=eikr

r2(I k̂k̂) pj

(2 )

j

terrace dimer

0o 180o 360o

step rebond

0o 180o 360o

step dangling bond

0o 180o 360o

SH

G i

nte

nsit

y

p-in/p-out

i = 45o

b̂k ,

b̂l ,

d.bstep

dimerterrace

d.b.terrace

ab initio DFT

calculation

provides input

structure

for SBHM

analysis

back.terrace

rebondstep

backstep

SHG “dictionary” for Si(001): 6o

Multi-parameter fitting (like Wall St. investing)

requires “government regulation” based on...

Bonds that are parallel to E , charge-rich and non-centrosymmetric contribute most strongly

... Bond physics & chemistry

.. Consistency for Multiple Angles ( ) and Polarizations (MAP)

... Kramers-Kronig consistency

p-in/p-out

SH

G i

nte

ns

ity

[a

rb.u

.]

0o 180o 360o

azimuthal angle (degrees)

00o 180o 360o

azimuthal angle (degrees)

00o 180o 360o

azimuthal angle (degrees)

0

s-in/s-out s-in/p-out = 52.5o

= 45o

= 30oclean

Si(001):6oclean

Si(001):6o

clean

Si(001):6o

stepdb

= 0.4

terrace = 0.2db

terrace = -0.2dimer

e steprebond

= 0.3

m steprebond

= 0.2

0o 180o 360o

azimuthal angle (degrees)

0

p-in/p-outafter exposureto 1500 L H2at RT

SH

G i

nte

ns

ity

[a

rb.u

.]

e steprebond

= 0.3

m steprebond

= 0

stepdb

= 0

H2 terrace = 0.2

db

terrace = -0.2dimer

1

0.8

0.2

0.1

Full SBHM fits SHG data with high fidelity = 42oclean Si(001): 6

o

Si(001): 6o with H-terminated steps p-in/p-out

Strict regulation: derive RAS response from SHG data

step terrace

Fitted bond hyperpolarizability spectra j||

Elinear2 ( Î k̂k̂) j ,2

j

b̂jb̂j Ein2

*Miller’s theorem:

j||= j ,2 j , j ,

r

rr~

~~110011

linear bond polarizabilities

j ,2

spectral range

of SHG data

*R. C. Miller, Appl. Phys. Lett. 5, 17 (1964)

Ehlert, JOSA B 27,

981 (2009)

Hyperpolarizability spectra show charge transfer from step

dangling bond to 3 underlying bonds when H2dissociatively adsorbs at step-edges

clean

1000 L H2

With SHG-RAS-SBHM, we watch charge transfer

accompanying the formation of specific step-

edge chemical bonds.

SHG/RAS/SBHM Case Study #2: Reaction of atomic H with H2-saturated Si(001):6o surface

?

?

1000 L H2 × 1000

• symmetrized dimers• 2×1-reconstructed terraces• re-bonded (r) DB steps intact

Pehlke & Kratzer, Phys. Rev. B 59, 2790 (1999)

Laracuente & Whitman, Surf. Sci.545, 70 (2003)

• atomic H catalyzes conversion of r-DB to non-rebonded DB, SB and SA steps

H

top view

sideview

• 2×1 monohydride terraces convert to 1×1 dihydride, breaking dimers

2×1

1×1

Borenzstein et al., Phys. Rev. Lett.95, 117402 (2005)

clean
(2x1) full
H2
+50L
H
@300C

+
50L
H
@
300C


+
50L
H
@RT

2x1
spots

SHG: Rebonds break immediately

LEED: Dimers break later

spot
spli+ng

2x1
spotsp-in/p-out

No remaining SHG signature of rebondafter ~ 100 L atomic H

2x1
spotsgone!

dimers broken

rebondcontribution

2x1
spots

pp


step
rebond step
backbond

sp

ss

SHG-MAP shows shows decreasing rebond, in-creasing backbond, expression during H exposure

Si(001):4o 780nm 45o incidence

azimuthal angle [radians]

0 3 6

1×106 L H2+ 50 L atomic H

RAS is a “spectator” as the rebond breaks

... H2 pre-adsorption at step db’s partially pro- tects steps from reacting with C5H8

RAS-SHG-SBHM Case Study #3: monitor and control ofnanofabrication of organic monolayers on Si(001)

0 90 180 270 3600.0

0.1

0.2

0.3

0.4

clean H2 H2+Cyclopentene Cyclopentene

SH in

tens

ity [

arb.

units

]

azimuthal angle

780nm Si(001):6

SHG

Inte

nsity

[arb

. u.]

0

1

2

3

4

Hamers et al. (2000). Acc. Chem. Res. 33(9): 617-624Lu et al., Phys. Rev. B 68, 115327 (2003)

STM shows C5H8 bonds to Si=Si terrace dimers to form an ordered monolayer.

clean vicinal Si(001):6o after C5H8 adsorption

pp 300 CSHG, on the other hand, shows ...

... C5H8 reacts immediately & strongly with step rebonds of the clean Si(001):6o surface

C5H8

I. 0-D: Si NCs embedded in SiO2

• method: XP2-SHG + SE, Raman, XPS, PL

II. 1-D: step-edges of vicinal Si

Summary: Noninvasive optical spectroscopy of nano-interfaces

• method: SHG & RAS & SBHM

• importance: Si LEDs, bio-sensors

• importance: templates for molecular electronics, quantum wires & computers

Figliozzi et al., Phys. Rev. Lett. 94, 047401 (2005); Wei et al., in preparation

Kwon et al., Phys. Rev. B 73, 195330 (2006). Ehlert et al., J. Opt. Soc. Am. B 27, 981 (2009) & more in the oven.

Yu Suzuki• results: visualization of formation of step Si-H bond

and of breaking of step rebond; - control & optical monitoring of cyclopentene nano-lithography by self-assembly

• results: new SHG evidence for a-Si and SiOx nano-interfacial transition regions

END