Low temperature Silicon RF-PECVD epitaxy for thin film...
Transcript of 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
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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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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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Thin Film c-Si: literature overview
Bottom up: epitaxy and lift off from wafer
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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)
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).
Thin Film c-Si: literature overview
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)
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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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
rf-PECVD (13,56 MHz) for a-Si materials deposition
ARCAM
ARCAM 200
Cluster tool
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Equipment update
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15cm15cm
ARCAM reactor
rf-PECVD (13,56 MHz) for a-Si materials deposition
ARCAM
ARCAM 200
Cluster tool
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Outline
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►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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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
Epitaxy up to 6 µm, no breakdown observed so far !
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Silicon epitaxy proof
► 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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Chemical texturation
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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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Chemical texturation
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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
►
■
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►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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