low temperature silicon rf-pecvd epitaxy for thin film...

Romain Cariou

Jean-Louis Gentner & Pere Roca i Cabarrocas 14/12/2012

Low temperature Silicon RF-PECVD epitaxy for thin film crystalline

solar cells

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Outline

► Introduction ■ Motivations & literature overview

■ Potential of thin crystalline silicon solar cells

■ RF-PECVD: a versatile tool for PV materials

►Silicon epitaxy by RF-PEVCD at 175°C ■ Material properties

■ Thin film epi-Si solar cell

■ Improvement strategy

►Perspectives ■ Light management

■ Transfer to foreign substrate

■ Hetero-epitaxy

►Conclusion

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Motivations

► Low cost innovative concepts are needed for TW scale

►Material: ~ 1/3 of PV pannel cost ■ High temperature (T > 1000°C), kerf losses, etc.

►Thin wafers are difficult to handle ■ 180-150 µm wafer thickness is close to the

mechanical limit

►Thinner c-Si layer can achieved high Eff. ■ Epi match wafer based devices: 43 µm | 19.1%

certified

■ Currently at the lab scale

3

J.H. Petermann et al., Prog. in Photovoltaics 20, 1 (2012)

http://en.wikipedia.org/wiki/Solar_cell

►Crystalline silicon solar cells > 80 % market share

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Thin Film c-Si: literature overview

Top down approach: wafer thinning, exfolitation, Epi-Free

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a

b

c

aA. Wang et al.,Prog in Photovoltaics 4, 55–58 (1996) bR.A. Rao et al., 37th IEEE PVSC, 001504 –001507 (2011) cC. Trompoukis et al., Applied Physics Letters 101, 103901 (2012)

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Bottom up: epitaxy and lift off from wafer

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a

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dR.B. Bergmann et al., Sol Energ Mater 74, 213 (2002)

eJ.H. Petermann et al., Prog. in Photovoltaics 20, 1 (2012) fhttp://optics.org/indepth/3/7/3

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gP. Rosenits et al., Thin Solid Films 519, 3288 (2011) hK. Van Nieuwenhuysen et al., Thin Solid Films 518,S80 (2010)

eK. Alberi et al., Appl. Phys. Lett. 96, 073502 (2010) jR. Cariou et al., Proc. SPIE - 84700B (2012).



dR.B. Bergmann et al., Sol Energ Mater 74, 213 (2002)

eJ.H. Petermann et al., Prog. in Photovoltaics 20, 1 (2012) fhttp://optics.org/indepth/3/7/3

a

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RF-PECVD tool for thin film crystalline materials

► Target low cost solar cells, whithout compromise on efficiency, using our novel approach

► Tool: RF-PECVD (13,56 MHz) capacitively coupled reactor

► Temperature range [150-250]°C: - 3 plasma boxes - 1 vacuum chamber - no load lock - no UHV

► Industrial standard, low cost & flexible tool

► Wide range of materials:

■ a-Si:H, a-Ge:H, µc-Si, SiNx, SiOx c-Si, c-Ge

15cm15cm

ARCAM reactor

rf-PECVD (13,56 MHz) for a-Si materials deposition

High quality crystalline material for photovoltaics deposited

below 200°C !! P. Roca i Cabarrocas, J. Vac. Sci. Technol. A 9, 2331 (1991)

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Equipment update

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15cm15cm

ARCAM reactor


ARCAM

ARCAM 200

Cluster tool

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Equipment update

9

15cm15cm

ARCAM reactor


ARCAM

ARCAM 200

Cluster tool

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Outline

►

■

■

■

►Silicon epitaxy by RF-PEVCD at 175°C ■ Material properties

■ Thin film epi-Si solar cell

■ Improvement strategy

►

■

■

■

►

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Silicon epitaxy proof

►Si homoepitaxy in literature: ■ Widely used techniques: CVD, HWCVD, etc. Temp. range T=[700-1100]°C

►Evidence of Si epitaxy by RF-PECVD at 175 °C on c-Si (100) wafers by ellipsometry

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►Good agreement with optical model

►Epi fitted with 100 % c-Si

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►Si homoepitaxy in literature: ■ Widely used techniques: CVD, HWCVD, etc. Temp. range T=[700-1100]°C

►Evidence of Si epitaxy by RF-PECVD at 175 °C on c-Si (100) wafers by ellipsometry

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►Good agreement with optical model

►Epi fitted with 100 % c-Si

Epitaxy up to 6 µm, no breakdown observed so far !

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► TEM crossection of epi-layer covered by a-Si:H

► Monocrystalline quality confirmed by FFT

► Defects at c-Si|epi-Si interface due to air exposure after chemical cleaning

► Defects within the bulk material: probably passivated thanks to the high hydrogen incorporated during growth at 175 °C

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C-Si wafer

Epi-Si

Epi-Si

a-Si:H

Glue

(-1,1,1) (1,-1,1)

(1,-1,-1) (-1,1,-1)

(2,0,0)

(-2,0,0)

(-2,2,0) (2,-2,0)

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Strain engineering

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0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

Y (

µm

)

-2 0 2

X (µm)

200

400

600

800

1 000

1 200

1 400

1 600

1 800

2 000

2 200

Inte

nsi

ty (

a.u

.)

0.5 µm

D. Shahrjerdi et al., Journal of Electronic Materials 41, 494 (2012)

Biaxially compressively strained PECVD epitaxial silicon: hydrogen content

plays a key role (IBM)

Raman cross section epi-Si/c-Si intensity map – HYJ Lab Aramis

Thanks to Cynthia Takchi & Nada Habka

ITO

N+ aSi:H

Epi-Si

P++ c-Si wafer

Al

Al

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Low Temp. Epitaxy growth mechanism

►Conventionnal HWCVD requires T>500°C avoid epitaxy breakdown (NREL, Teplin et al.) ■ Main growth precursor: SiHx,Si2Hx, etc.

►By PECVD: high hydrogen content, no epitaxy break down: ■ Nanocrystals observed for sample

co-deposited on glass (pm-Si)

■ Epitaxy hapened on c-Si (100) and not (111)

P. Roca i Cabarrocas et al., (2012) MRS Proceedings, 1426 P. Roca i Cabarrocas, et al., J. Non-Cryst. Solids 358, 2000 (2012) N. Ning and H. Vach, H., J. Phys. Chem. A 114, 3297–3305 (2010)

Autocorrelation function

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Nanostructured material Silicon nanocrystals in an

amorphous matrix

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Low Temp. Epitaxy growth mechanism

►Conventionnal HWCVD requires T>500°C avoid epitaxy breakdown (NREL, Teplin et al.) ■ Main growth precursor: SiHx,Si2Hx, etc.

►By PECVD: high hydrogen content, no epitaxy break down: ■ Nanocrystals observed for sample

co-deposited on glass (pm-Si)

■ Epitaxy hapened on c-Si (100) and not (111)

P. Roca i Cabarrocas et al., (2012) MRS Proceedings, 1426 P. Roca i Cabarrocas, et al., J. Non-Cryst. Solids 358, 2000 (2012) N. Ning and H. Vach, H., J. Phys. Chem. A 114, 3297–3305 (2010)

Autocorrelation function

Growth hypothesis:

Nanocrystals/clusters that melting by impacting the (100) c-Si surface with subsequent recrystallization at the c-

Si/melted silicon interface

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Nanostructured material Silicon nanocrystals in an

amorphous matrix

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Thin film epi-Si solar cell

►Wafer equivalent solar cells: ■ PIN device with epitaxial thin film acting as an

absorber

■ n+ aSi:H emitter and P++ c-Si « dead » wafer

+R. Cariou et al., Sol. Energy Mater. Sol. Cells 95 (2011) 2260-2263

• Good quality solar cells based on this epi-layer grown at 175°C

• Proof of high quality structural and electrical properties of epi-layers

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Sample Jsc mA/cm2

Voc V

FF %

Eff %

1104132-A 18,6 527 79 7,7

1104132-A 1h-200°c

19,9 534 80 8,5

►Device with 3,4 µm epi absorber: ■ Low Voc attributed to c-Si|epi-Si

interface

■ 80 % FF achieved upon annealing

3,4 µm

J01 (A/cm2) :1.6E-11 Voc (mV) :541

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Modelisation for improving thin epi-Si cells

Starting from the extracted material parameters, we obtain:

► Increasing thickness will increase Eff and fraction of photocurrent created in epi-layer (increase dep. rate > 2 A.s-

1)

► Performance more sensitive to epi-Si|wafer interface: Voc can reach > 580 mV (in-situ plasma cleaning)

► FF of 80 achieved by reducing DB in epi-Si (annealing @ 200°C)

► 5µm cell with improvement listed above + front texturing: > 11% Eff.

► Additional back texturing: > 14% Eff.

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*P. Chatterjee et al., MRS Online Proceedings Library 426 (1996) S. Chakraborty et al., submitted to EPJPV (july 2012)

-2

-1

0

1

0.001 0.01 0.1 1 10 100 1000

En

ergy (

eV

)

Position (microns)

epitaxial SiN-

a-Si:H

P-c-Si

Dandling bonds [cm-3]

Interface defect [cm-2]

Epitaxy thickness

[µm]

Actual 1015 ≥ 1012 3.2

Target 1014 5 X 1011 ≥ 5

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Epitaxial layer characterisation

►Epitaxy below < 200°C: [0,1-1]% hydrogen content revealed by SIMS ■ Good defect passivation

19 EBIC: Sebastian Schmitt, Max planck institute for the science of light

1,2 mm

1cm2 cell

1602 µm2

►Threading dislocations density evaluation by EBIC : ~ 1.0 X 105 cm2 (HWCVD ≥ 10-4)*

*C.W. Teplin et al., Applied Physics Letters 96, 201901 (2010)

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Epitaxial layer characterisation

►Epitaxy below < 200°C: [0,1-1]% hydrogen content revealed by SIMS ■ Good defect passivation

20 EBIC: Sebastian Schmitt, Max planck institute for the science of light

1,2 mm

1cm2 cell

1602 µm2

►Threading dislocations density evaluation by EBIC : ~ 1.0 X 105 cm2 (HWCVD ≥ 10-4)*

►EBIC cross section shows charge separation at wafer|epi-Si interface ■ Epitaxy slightly n-type due to residual impurities

■ BSF is needed to separate carriers away from poor wafer/epi interface

*C.W. Teplin et al., Applied Physics Letters 96, 201901 (2010)

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Chemical texturation

► DI water (89,5mL)|IPA(4,5mL)|KOH (5,9 mL @500g.L-1) - 90°C

► (111) facetted pyramids achieved on (100) wafer: Antireflection + Light trapping effect

► However few µm size motif are needed

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A: 4 min

Epi-Si 5 µm

P++ c-Si wafer 500 µm

SEM E. Lefeuvre

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B: 7 min

A: 4 min ► Short etching time = small light trapping effect

► Longer etching = material waste & poor pyramid shapes (blue shift)

SEM E. Lefeuvre

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B: 7 min

A: 4 min Smart light trapping / AR needed for TF-cSi:

- Plasmonic, Nano-photonic, etc. - Large scale compatible (e.g. nano-imprint)

SEM E. Lefeuvre

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Factor limiting the Voc

Voc mV

Jsc mA.cm-2

FF %

Eff %

HF 537 20.3 79.2 8.0

Piranha 540 19.3 79.3 8.0

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C-Si wafer

Epi-Si

► Interface quality is not very sensitive to ex-situ cleaning process

►Need for in in-situ oxyde removal

►Or lift off to allow back side processing

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Outline

►

■

■

■

►

■

■

■

►Perspectives ■ Light management

■ Hetero-epitaxy

■ Transfer to foreign substrate

►

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►Assumptions: - c-Si, AM1.5

- Perfect antireflection

- 100 % IQE

►Single pass abs:

►X10 light trapping:

►Max light trapping*:

Asinglepass = 1 - 𝒆𝒙𝒑(−α𝒅)

AX10 = 1 - 𝒆𝒙𝒑(−𝟏𝟎. α𝒅)

Ayablo. = 1 - 𝟏

𝟏+𝟒𝒏𝟐𝜶𝒅

3µm epi Single pass

3µm epi X10 light trapping

3µm epi max light trapping

Jsc mA/cm2

20.4 35.5 37.7

𝒅

1 - A

How much current for a given thickness 𝒅 ?

26 *E. Yablonovitch and G.D. Cody, IEEE Trans. Electron Devices 29, 300 (1982) M.A. Green, Prog Photovoltaics 10, 235–241 (2002)

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Advanced design for light management

►Separately RCWA optimized double side nano-cones grating

►Front cones antireflection effect: ■ High aspect ratio: grading index from air to Si

■ Broadband reflection suppression

■ Wide angle efficiency

►Back cones light trapping effect: ■ Excitation of guided resonnances

■ Low coupling/leaking with external channels

27 K.X. Wang et al., Nano Lett. 12, 1616 (2012)

Period Radius Height

Front 500 250 710

Back 1000 475 330

►Feasibility: ■ Langmuir-Blodgett + RIE

Compatible with large scale and lift off process

►34,6 mA/cm2 should be achieved in 2 µm c-Si

2 µm epi-Si is enough to achieve solar cell with Eff > 19 %

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Silicon epitaxy transfer

► Epi-layers can be detached and transferred to foreign substrates:

■ Need mechanical weak interface as porous silicona

■ Or sacrificial layer and chemical etching

aM. Reuter et al., Sol. Energy Mater. Sol. Cells 93, 704 (2009) bM. Moreno and P. Roca i Cabarrocas, EPJ Photovoltaics 1, 10301 (2010)

► Weak interface epi-Si/c-Si created by plasma treatmentb: ■ Subsequent epi-growth

■ Metal + polymide

■ Curing + annealing

► On going work: - Anode bonding - Epitaxy on sacrificial layers - etc.

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C-S

i waf

er

Epi-

Si

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Germanium epitaxy at 175° C by RF-PECVD

29 Submitted to APL (12/2012)

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Ge epitaxy on c-Si at 175° C by RF-PECVD

► TDD confined close to interface

►Epitaxy improving with thickness

► Low TDD quasi relaxed 150 nm deposited on c-Si residual stress ~0.4%

30 Submitted to APL (12/2012)

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Multi layer stacks on III-V

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E.V. Johnson et al., Appl. Phys. Lett. 92, 103108 (2008) M. Labrune et al., EPJ Photovoltaics 3, 30303 (2012)

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Conclusion

PECVD epitaxy open path to exciting physics and promising solar cell devices

Thank you

Acknowledgements

> LPICM team

> Rosa Ruggeri - TEM > III-V Team

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low temperature silicon rf-pecvd epitaxy for thin film...

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