Strong scattering of light in Strong scattering of light in silicon nanowires formed by silicon nanowires formed by

metal-asmetal-asssisted chemical etchingisted chemical etching A. Efimova

Moscow State M.V. Lomonosov University, Physics Department

Nanomeeting2005, Minsk, 26—29 May, 2015

A. TkachevA. EliseevD. PresnovV. TimoshenkoT. NychyporukYu. Ryabchikov

Minsk, 26-29 May, 2015

Why MACE silicon nanowires?Why MACE silicon nanowires?

What for? APPLICATION

What’s different? What’s new?What’s novel? What’s peculiar?

PROPERTIESPROPERTIES LINEAR OPTICSLINEAR OPTICS

EXTREMELY LOW or EXTREMELY LOW or

HIGH REFLECTIONHIGH REFLECTION How comesHow comes??

Minsk, 26-29 May, 2015

Just a simple, reliable and cheap procedure

1. Ag nanoparticles deposition onto Si substrate in the aqueous solution of AgNO3

2. Ag nanoparticles-assisted chemical etching of Si substrates in the aqueous HF and H2O2 solution

3. Ag nanoparticles removal by rinsing in concentrated HNO3

Silicon nanowires (SiNWs)Silicon nanowires (SiNWs)First formed in the middle-1960s by vapour-liquid-solid (VLS) technique

Metal-assisted chemical etching - MACEMetal-assisted chemical etching - MACE

Minsk, 26-29 May, 2015

V. Sivakov, F. Voigt, B. Hoffmann, et al. Wet - Chemically Etched Silicon Nanowire Architectures: Formation and Properties // Nanowires - Fundamental Research – 2011, p. 45-80. V. Sivakov, S. Christiansen. Novel discovery of silicon // J. Nanoelectron. Optoelectron. 2012. V.7. N6. P. 583-590.

КК..ВВ. . БуньковБуньков, , ЛЛ..АА. . ГолованьГоловань, , КК..АА. . ГончарГончар и др.и др.. . Зависимость эффективности комбинационного рассеяния света в Зависимость эффективности комбинационного рассеяния света в ансамблях кремниевых нанонитей от длины волны возбуждения // Физика и техника полупроводников – 2013. ансамблях кремниевых нанонитей от длины волны возбуждения // Физика и техника полупроводников – 2013. Т.47. В.3, С. 329-333.Т.47. В.3, С. 329-333.

Columnar PrismaticBunched PyramidalTilted ZigzagCotton-like Flaky

Structure: Si(100)Structure: Si(100) andand SiSi(111)(111)

What are MACE SiNWs?What are MACE SiNWs?

Minsk, 26-29 May, 2015

... ... What’s peculiar?What’s peculiar?

- Variety of individual SiNW shapes Variety of individual SiNW shapes

- Variety of SiNW lateral dimensions Variety of SiNW lateral dimensions

BUTBUT

-- Dense arrays of SiNWs with comparableDense arrays of SiNWs with comparable

lateral dimension and interwire spacelateral dimension and interwire space

- Free of catalyst contamination - Free of catalyst contamination

Minsk, 26-29 May, 2015

PROPERTIES: Low reflection Low reflection at normal incidence at normal incidence Black silicon Black siliconSTRUCTURE: Pyramidal shape, thin layers Pyramidal shape, thin layers

J.-Y. Jung, Zh. Guo, S.-W. Jee, et al. A strong antireflective solar cell prepared by tapering silicon nanowires // Optics Express – 2010. V.18. N103, p.A286-A292, p.A286-A292

Bunched

Tapered

c-Sic-SiBlack silicon

Minsk, 26-29 May, 2015

PROPERTIES: High reflection High reflection in Near-InfraredSTRUCTURE: Columnar shape, thick layersColumnar shape, thick layers

A=1–T–R

6000 8000 10000 120000

20

40

60

80

100

Abso

rption (%

)Wavenumber (cm-1)

6000 8000 10000 120000

20

40

60

47%

54%

Tota

l Refl

ecta

nce

(%

)

Wavenumber (cm-1)

c-Si L = 1.3 m L = 8.0 m L = 23 m

56%E

g

2 1,8 1,6 1,4 1,2 1 0,8 Wavelength (m)

15-20% 15-20% increase in Rincrease in REntirely diffuseEntirely diffuse

6000 8000 10000 120000

20

40

60E

g

Tota

l Tra

nsm

itta

nce

(%

)

Wavenumber (cm-1)

2 1,8 1,6 1,4 1,2 1 0,8 Wavelength (m)

c-Si substratec-Si substrate

Minsk, 26-29 May, 2015

STRUCTURE: STRUCTURE: What’s peculiar?What’s peculiar?

Lateral dimensionsLateral dimensions

Columnar wires 1 m SEM

Interwire space 1 m SEM

Inner channels 100 nm TEM, SEM

Surface nanostructures 5–10 nm TEM

8

11 mm 11 mm

What are SiNWs under study?What are SiNWs under study?

Minsk, 26-29 May, 2015

Radiative transport equation&Diffusion Radiative transport equation&Diffusion approximation: Applicability approximation: Applicability

-- Radiative transport ls generated a media a media

- Light diffusion is generated before absorption - Light diffusion is generated before absorption

- Light diffusion is generated in a restricted layer - Light diffusion is generated in a restricted layer

- Scatterers are pointwise and independent - Scatterers are pointwise and independent 2

,

(3 10)

(2 )

s a

a tr

tr

s

l l

l l

L l

l a

0

0 0

(1.25 0.8)

2eff

m

n

PROPERTIES: How comes?How comes?

2

02 2( ) BB p

a

DD I z z

t z L

1

3

3

( )

Е trB

a tr a

tr s a

lD

L l

l

v

Minsk, 26-29 May, 2015

Diffusion approximation: Diffusion approximation: Diffuse reflectance. StaticsDiffuse reflectance. Statics

Types of variablesTypes of variables

1) 1) Table:Table: substratesubstrate

2) 2) Directly measured:Directly measured: LL,, RRdd

3) Obtained from the interference fringers in the effective medium spectral range: nneffeff

44) ) Calculated:Calculated: zz1,21,2((nneffeff, , substratesubstrate), ),

55) ) Estimated or got from photoacoustics: aa

Infinite media (solid line) Infinite media (solid line) and a layer of the thickness and a layer of the thickness L L in the air in the air ((dash linedash line)), , L L ==  55lltrtr

O. L. Muskens, J.G. Rivas, R.E. Algra, O. L. Muskens, J.G. Rivas, R.E. Algra, et alet al. Design of Light Scattering in Nanowire Materials for Photovoltaic Applications // . Design of Light Scattering in Nanowire Materials for Photovoltaic Applications // Nano Letters – 2008, V.8, N.9, p. 2638-2642.Nano Letters – 2008, V.8, N.9, p. 2638-2642.

2

1 2 1 22

sinh cosh

1 sinh cosh

tr tr

a a a

a a aa

L l z L l

L L Ld z z L z z L

L L LL

R

PROPERTIES: How comes?How comes?

1

1 2

3

( )

, extrapolation lengths

a tr a

tr s a

L l

l

z z

Transport mean free path to be extractedTransport mean free path to be extracted

Minsk, 26-29 May, 2015

Transport mean free path of light.Transport mean free path of light. Static resultsStatic results

Transport mean free path valueTransport mean free path valuemeets the demands of diffusion approximationmeets the demands of diffusion approximation

0,

m

1.250 0.800

1000 cm 12 m

L, m 8 24 8 24

ltr m

1 1a

4.0±0.4 6.0±0.6 2.5±0.3 3±0.3

Minsk, 26-29 May, 2015

Diffusion approximation: Diffusion approximation: Time-resolved reflection. DynamicsTime-resolved reflection. Dynamics

2

21 2

1

d a

D D

L z zL

1

1 2

2 2 211 2

21 2

1 cos23

( ) exp( )

exp( )

tr

ntr a

l zn

L z zD DtR t

l L z z L n Dt

L z z

-500 0 500 1000

-0,1

0,0

0,1

0,2

0,3

0,4

-1000 -500 0 500 1000 1500

1E-5

1E-4

1E-3

0,01

0,1

I cr (re

l.un.

)

Time (fs)

I (a

rb. units)

Time delay (fs)

2

2

1( ) lim ( ) *( τ)

T

cr in scatT

T

I E t E t dtT

Cross-correlation functionCross-correlation function

Cr4+:forsterit laser ultrashort pulses (60 fs), repetition rate 80MHz

At long timesAt long times

Minsk, 26-29 May, 2015

Transport mean free path of light:Transport mean free path of light: Static and dynamic resultsStatic and dynamic results

L = 24 m, = 1,250

m

Statics

Dynamics

6.0±0.6

5.0±0.5

Minsk, 26-29 May, 2015

ConclusionConclusion

The reflection of thick silicon nanowire layers in The reflection of thick silicon nanowire layers in

the near-infrared range below and above the the near-infrared range below and above the

band gap is well understood by assuming the band gap is well understood by assuming the

diffusion-like radiative transport in silicon diffusion-like radiative transport in silicon

nanowire array.nanowire array.

Minsk, 26-29 May, 2015

Stone Forest of ShilinStone Forest of Shilin

Thank you for your attentionThank you for your attention