S. Battiato, V. Zannier, U. P. Gomes, D. Ercolani and L. Sorba

NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza S. Silvestro 12, 56127

Pisa, Italy

Heterogenous nucleation of catalyst-free InAs NWs on silicon

Outline

400 nm

Introduction

Semiconductor nanowires: definition and applications

Catalyst-free growth of semiconductor NWs

Nucleation and density control

Conclusions

Semiconductor nanowires (NWs)

Novel properties: - high surface/volume ratio - carrier confinement - defect-free growth Integration of III-V NWs on silicon substrate key research

1D growth

Quasi-one dimentional crystals

III-V NWs on Si

FETs

LEDs

Solar Cells

Lasers

Chemical Beam Epitaxy

CBE for III-V semiconductor growth Metal-organic precursors: Group III : TMIn, TEGa, TMAl Group V : TBAs, TBP, TDMASb, TMSb

n-doping: TBSe

- Low impurity conc. thanks to the UHV

- Monolayer control

- High flexibility in material combination

Advantages +++

Ultra High Vacuum (UHV) chamber (base pressure: 10-9 Torr)

Growth mechanisms of semiconductor NWs

Gold-assisted growth: Vapour-liquid-solid (VLS) mechanism

200 nm

Au-catalyzed growth Au

× The incorporation of metal catalyst can be detrimental for the applications

Sputtering NW growth

Substrate 200 nm

Absence of foreign metal contamination

Integration of III-V NWs and silicon

InAs NWs

Catalyst-free growth:

InAs NWs

2’ BOE etch +10’’ DI rinse + dry N2

III-V Growth

100 nm

Ar or SiO2

Sputtering

2’ BOE etch +10’’ DI rinse + dry N2

100 nm

Catalyst-free growth of InAs NWs on Si (111) substrate

Substrate preparation

U. P. Gomes, D. Ercolani, V. Zannier, S. Battiato, E. Ubyivovk, V. Mikhailovskii,

Y. Murata, S.Heun, F. Beltram and L. Sorba, Nanotechnology , 28, 065603 (2016).

HRTEM analysis shows defect-free

crystalline order of the sputtered Silicon

substrates after growth.

Amorphization is unlikely to be the source of

nucleation sites.

AFM scans show comparable surface

roughness.

Roughness is unlikely to be the source of

nucleation sites

Catalyst-free and growth of InAs NWs on silicon substrate

Nature of nucleation sites

Sputtering probably creates defects

Defects serve as nucleation sites

Arsenic

Growth time

Tem

per

atu

re

Indium

Arsenic

Growth time

Tem

per

atu

re

Indium

High Radial growth

Low axial growth

High Axial growth

Low radial growth

LT Nucleation stage

HT Growth stage

200 nm 200 nm 200 nm

• LT enhances nucleation but NWs grow only radially

• Two-step temperature growth

• LT nucleation stage to control NW density

• HT stage to enhance anisotropic NW growth

380-400°C

380-400°C

480-500°C

15’ 30’ 45’

100 nm 100 nm 100 nm

10’ 20’ 80’

Catalyst-free growth of InAs NWs on silicon substrate

U. P. Gomes et al., Nanotechnology, 26, 415604 (2015)

Growth protocol

Annealing

Annealing

200 nm 200 nm

Sputtering

III-V Growth

U.P. Gomes et al. Nanotechnology, 2015, 26, 415604.

SiO2 sputtering Argon sputtering Non-sputtered Si (111)

400 nm

100 nm

<111> <112>

InAs NWs islands

Catalyst-free growth of InAs NWs on silicon substrate

Morphological characterization

RF magnetron

SiO2 sputtering

0 400 800 12000

25

50

75

tsputter

(s)

N (

m-2

)

0 200 400 600 8000

25

50

75

N (

m-2

)

Vsputter

(V)

0 30 60 900.0

0.2

0.4

0.6

N (

m-2

)

Vsputter (V)

0 400 800 12000.0

0.5

1.0

1.5

2.0

N (

m-2

)

tsputter

(s)

ICP Argon sputtering

Catalyst-free and growth of InAs NWs on silicon substrate

Effect of sputtering parameters on crystal density and yield

Crystal density increases with tsputter and Vsputter (as long as no etching /amorphization occurs)

Yield = InAs NW density/total crystal density

3 sec 6 sec

200 sec 1000 sec

400 nm

0 500 1000

0.25

0.50

0.75

1.00Y

tsputter

(s)

0 500 10000.0

0.5

1.0

Y

Vsputter (V)

Yield independent of sputtering parameters (~ 50 %)

Catalyst-free and growth of InAs NWs on silicon substrate

Effect of sputtering parameters on NW Yield

Yield vs sputtering

350 385 420 455 490

0

10

20

30

40

50 SiO2 sputtering tsputter=60 s,Vsputter=520 V

SiO2 sputtering tsputter=3 s,Vsputter=450 V Argon sputtering tsputter=600 s,Vsputter=35 V

N (

m-2

)

Tgrowth (oC)

15 16 17 18

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

lo

g10N

(

m-2)

1/KT

DE=0.23±0.05 eV

DE=1.43±0.30 eV

Catalyst-free and growth of InAs NWs on silicon substrate

Effect of growth parameters on crystal density and yield

• InAs crystal density decreases with Tgrowth

• High Vsputter and tsputter : low ΔE (Arrhenius plot)

• Maximum yield at 400°C

• NW density in the range of 1-30 NWs/µm2

Tgrowth=400°C, tgrowth=15 mins Tann=790°C, tgrowth=15 mins

• Annealing reorders the Si (111) surface

• Defect density decreases and crystal density decreases

• Yield increases with annealing temperature

• Highest yield of NWs at 790°C

350 385 420 455 490

0.0

0.2

0.4

0.6

Y

Tgrowth (o

C)480 560 640 720 800

0.0

0.2

0.4

0.6

T

ann (oC)

Y

0 40 80 120

0.0

0.2

0.4

0.6

0.8 Argon sputtering

tsputter

=600 s,Vsputter

=35 V

(

m-2

)

tgrowth (min)0 20 40 60 80

0

10

20

30

40

50 SiO2 sputtering

tsputter

=60 s,Vsputter

=520 V

(

m-2

)

tgrowth (min)

1000 nm

200 nm

200 nm

15 min 60 min 135 min

Catalyst-free and growth of InAs NWs on silicon substrate

Nucleation mechanism vs growth time

Conclusions

InAs nanowires have been synthesized by Chemical Beam Epitaxy technique using

catalyst-free approach.

We assessed the role of substrate preparation, showing that InAs crystals nucleate

on sputtered Si(111) surfaces while no nucleation occurs on non-sputtered Si(111)

surfaces.

We showed that the silicon surface can be obtained by modifying in situ growth

and ex situ sputtering parameters, allowing us to achieve a good control of the InAs

NWs density.

Although the nucleation of parasitic islands could not be completely inhibited, the

yield of NWs could be increased to about 0.5 by proper choice of growth and

annealing temperatures.

Acknowledgments

• Lucia Sorba • Valentina Zannier • Umesh Prasad Gomes • Daniele Ercolani • Fabio Beltram • Francesca Rossi (IMEM-CNR)

Thank you for your attention!