Silicon Silicon nanowires for thermoelectric harvesting nanowires for thermoelectric harvesting

G. Gadea, A. Morata, J.D. Santos, I. Domnez, C. Calaza, M. Salleras,

ggapplications: growth, integration and characterizationapplications: growth, integration and characterization

G. Gadea, A. Morata, J.D. Santos, I. Domnez, C. Calaza, M. Salleras, D. Dávila, L. Fonseca, A. Tarancón

Lille, EMRS May 2016

Thermoelectric energy harvesting with silicon nanowires

Si NWs are promising materials TE

energy harvesting because:

• Their high ZT with respect bulk

• The high availability of silicon

• The easy integrability in micro-technology

Doped Si: ZT=0.02

The easy integrability in micro technology

(which is silicon-based!)

Doped Si: ZT 0.02 @ RT

Si NWs: ZT=0.60 @ RT

P needed:10 W/cm2

2WSN node

~ µ10 W/cm2

Thermoelectric energy harvesting with silicon nanowiresIntegrating NWs in thermoelectric devices presents challenges:• Contacting dense arrays of NWs / cm2

• NWs with low contact resistance • Submitting NWs to large ∆T

For this we use a micro-machined planar thermoelectric generator (µTEG)• Submitting NWs to large ∆T

• Using technology friendly techniques

p g (µ )

• Length:10 µm

• Diameter: 100 nm

• Density: 1-5 NWs/µm2

• Epitaxial growth on Si

From D. Dávila et al. Nano Energy, 1(6), (2012) 812

GD CVD - VLS

Easy growth + integration

3

GD CVD VLSAu3+ SiH4, B2H6

Silicon nanowire morphology control

GD CVD - VLS

Au3+ SiH4, B2H6Au SiH4, B2H6

+ gold amount

0.6 mM Au

2 5 M A

ure

time

2.5 mM Au5 mM Au

+ ex

pos

0.6 2.5 5

• Increasing the gold amount/exposure time, the nanoparticlesi e increases and hile the densit decreases

[gold] (mM)

4From G. Gadea et al., Nanotechnology, 26 (2015) 195302

size increases and while the density decreases

Silicon nanowire morphology control

GD CVD – VLS

Au3+ SiH4, B2H6Au SiH4, B2H6

520 ºC 630 ºC 725 ºC

• Epitaxial <111> aligned silicon nanowires were obtained • Growth time temperature and pressure increase NW length

5From G. Gadea et al., Nanotechnology, 26 (2015) 195302

Growth time, temperature and pressure increase NW length• Low pressures and high temperatures favour 111 aligned nanowires

Integration in devices and effect of diborane flow

-5x10-5

0

R 05 B H 3120

-9x10-5

A)

R 05 sccm B2H

6= 3 1 2 0

05 sccm B2H

6

2 5 s c c m B2H

6

T = 0 5 0 º C4

-1x10-4

I (A

T = 0 5 0 º C

T = 1 0 0 º C

T = 1 5 0 º C

T = 2 0 0 º C

0 000 0 024 0 048 0 072 0 096 0 120-2x10-4

-2x10-4

R 2 5 s c c m B2H

6 = 5 2 0

Doping of ~ 10-19 cm-3

0.000 0.024 0.048 0.072 0.096 0.120

V (V) From A. Van Herwaarden, Sensors and Actuators 6 (1984) 245.

40

R (

)

NW arrayHeater circuit

60 150 300

30R

T (ºC)

1941 ppm/ºC

Integration in devices and effect of diborane flow

P m a x @ T = 1 0 0 ºC0 5 s c c m B

2H

6: 0 . 2 5 µ W

5

6 0 5 s c c m B2

H6

2 5 s c c m B2

H6

T 0 0 º C2 5 s c c m B2

H6

: 1 . 7 5 µ W

3

4

5 T = 0 5 0 º C

T = 1 0 0 º C

T = 1 5 0 º C

T = 2 0 0 º CW)

2

3

P (µ

W

0

1

0 .0 0 0 .0 5 0 .1 0V (V )

40

R (

)

NW arrayHeater circuit

70 150 300

30R

T (ºC)

1941 ppm/ºC

Single NW electrical measurements5 10-75x10 7

NW1: 2.2 µm L // 92 nm ø NW2: 10.1 µm L // 131 nm ø NW3: 14.6 µm L // 79 nm ø NW4: 30.6 µm L // 99 nm ø

0

I (A

)

R1: 3.03E4 OhmR2: 4.10E4 Ohm

-0.15 0.00 0.15-5x10-7

V (V)

R3: 1.73E5 OhmR4: 2.22E5 Ohm

V (V)2x109

2 )

S lope: 5.30E+02 Ohm x nm Intersect.: 5.70E+07 Ohm x nm 2

1 x 1 09

A (

x n

m2 Intersect.: 5.70E 07 Ohm x nm

From W.R. Thurber, J. Electrochem. Soc.

Resistivity = 5.3 ·10-3 Ohm·cmContact resistance < 03%

0 1 0 2 0 3 00

RxA

D i f 10 19 3

127 (1980) 2291.

8

Length (µm)Doping of ~ 10-19 cm-3

Single NW thermal AFM measurementsC t t AFMContact AFM TopographySEM image Thermal AFM imageThot I

Tcold Tcold Q=R·I2

The thermal afm tip can inject a

known heat Q and measure T locally

Approach curveLine scan along the NW

under the tip

9 7 0

9 7 2

rrent

(µA

)

1 .0 1 .59 6 6

9 6 8

Pro

be C

urZ S ( )Z S ca n (µ m )

9

Single NW thermal AFM measurements

310

Thotx

R = ∆T/ Q(x)Q

tip (µ

W)

Tcold Tcold

2 2

3002

kNW= 4 W/m·K

0 5 10

Z detector position (µm)

S = 620 µV/K σ = 190 Ω/cm

ZT @ RT = 0.54

1

2

3

p (µ

W)

0.4

0.6

T

Boukai 2008

-1.0 -0.5 0.0 0.5 1.00

1

Qst

ep

pos within wire (µm)20 30 40 50 60 70 80

0.0

0.2

ZT

Stranz 2012

Hochbaum 2008

BULK

10

Diameter (nm)

A. Boukai et al. Silicon nanowires as efficient thermoelectric materials. Nature, 451(2008) 7175

Stranz et al. Thermoelectric Properties of High-Doped Silicon from Room Temperature to 900 K. Journal of Electronic Materials, 42(7) (2012), 2381–2387.

A. Hochbaum et al. Enhanced thermoelectric performance of rough silicon nanowires. Nature, 451(7175), 163–7

Conclusions

• 111 Si NWs were grown. Their properties could be controlled through:

– Au catalyst deposition (selective deposition, density and size)

– CVD growth (length, doping)

• Si NWs arrays were integrated in thermoelectric microgenerators and characterized

– Seebeck coefficient was 620 µV/K

– Higher dopant flow leaded to higher power (25 µW/cm2)

• Single Si NWs were integrated in thermoelectric characterization structures

– Electrical resistivity was 5.3·10-3 Ω·cm and negligible electrical contact resistance

– Thermal conductivity was 4 W/m·K by, by means of thermal AFM along a single wire

• Combining the results a ZT of 0.54 at room temperature was obtained. ~ 25 times higher than

b lk ili lbulk silicon value

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Acknowledgements

• A. Morata • M. Salleras

• J.D. Santos

• I. Domnez

• D. Dávila

• L. Fonseca

• C. Calaza • A. Tarancón

And to IRECs NI-SOFC group

12

13

14

T air = 25 ºC

Device in harvesting mode

600

700 P device T

3

400

500

evic

e (n

W) 2

atfo

rm (º

C)

Heat sink –enhanced cold area

100

200

300

P de

1 T

pla

µTEG

50 100 150 200

0

T hot plate (ºC)

0T source = 50 -200 ºC

T hot plate ( C)

1 NW/µm2 5·10-3 Ohm·cm

R expected NW array = 1 Ω

R measured NW array 50 100 Ω

15R collector path = 200 Ω

R measured NW array 50-100 Ω

Growth of Si-Ge NWs for future integration in µTEG

From Stohr, et al. Chem. 241 , 1954, 30

500 ºC

Si-Ge allowing should lead to a drastic reduction in NW thermal conductivity

• Moderate taper, rough• All NPs nucleate, only thickwires aligned• 300 nm pSiGe layer (~ 10 kΩ)Implementation in devices is expected

to increase the power by a factor of 5-10y

400 ºC• No nucleation at all

450ºC• Some taper, rough• Only big NPs nucleate, not aligned

p y

• No pSiGe layerOnly big NPs nucleate, not aligned

• No pSiGe layer630 ºC• Rough, tapered• All NPs nucleate, ~ aligned

> 500 nm pSiGe layer• > 500 nm pSiGe layer

16