plenary 25th pvsec - kessels

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Plenary presentation about Al2O3 given at the 25th European PVSEC meeting in Valancia, September 2010 - Erwin Kessels

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Silicon surface passivation by Al2O3 thin films : 

Silicon surface passivation by Al2O3 thin films Erwin Kessels Gijs Dingemans, Magda Mandoc Department of Applied Physics w.m.m.kessels@tue.nl

Outline – Si surface passivation by Al2O3 : 

Outline – Si surface passivation by Al2O3 Introduction What are the properties of Al2O3? What makes Al2O3 so interesting? On the passivation mechanism Field-effect or chemical passivation - What is more important? What are the underlying microscopic mechanisms? Deposition methods What are the choices for film synthesis? Do different methods yield different (key) properties? Upscaling Market push or pull? Precursor quality & high-volume-manufacturing equipment? /Applied Physics - Erwin Kessels 1/16

Al2O3 thin film properties : 

Al2O3 thin film properties Al2O3 c-Si SiOx 1-2 nm Can be synthesized by vapor-phase deposition methods (ALD, PECVD, PVD) Wide band gap amorphous dielectric, fully transparent for visible light Refractive index (at 2 eV) n = ~1.65 High thermal stability (Tcrystallization >800 ºC) and good stability against UV radiation Can contain H, mostly as –OH groups When deposited on H-terminated c-Si: 1-2 nm interfacial SiOx is present between Al2O3 and Si TEM J.A. Woollam Co. /Applied Physics - Erwin Kessels 2/16

(Record) efficiencies for c-Si solar cells with atomic-layer-deposited (ALD) Al2O3 : 

(Record) efficiencies for c-Si solar cells with atomic-layer-deposited (ALD) Al2O3 p-type Si base material Excellent rear surface passivation, no parasitic shunting Efficiency: 20.6% (now: 21.5%) Schmidt et al., Prog. Photovolt. Res. Appl. 16, 461 (2008). Saint-Cast et al., IEEE Electron Device Lett. 31, 695 (2010) n-type Si base material Excellent p+ emitter passivation because of negative fixed charge Efficiency: 23.2% (now: 23.9%) Benick et al., Appl. Phys. Lett. 92, 253504 (2008). Benick et al., 35th IEEE PVSC (2010), in press. Rear surface passivation Passivated Emitter and Rear Cell (PERC) B-diffussed front emitter passivation Passivated Emitter Rear Locally (PERL) /Applied Physics - Erwin Kessels 3/16

Chemical passivation vs. field-effect passivation: defect density Dit and fixed charge density Qf : 

Chemical passivation vs. field-effect passivation: defect density Dit and fixed charge density Qf C-V measurements Corona charging experiments /Applied Physics - Erwin Kessels 4/16 Dingemans et al., 35th IEEE PVSC (2010), in press.; Electrochem. Solid State Lett. (2010), accepted Al2O3 is unique as it contains a very high density of negative fixed charge Qf Defect density Dit is low (comparable to other state-of-the-art materials)

On the chemical passivation induced by Al2O3 : 

On the chemical passivation induced by Al2O3 Si/Al2O3 interface is basically Si/SiO2-like due to 1-2 nm interfacial SiOx Si dangling-bond-type center Pb0 is prominent defect (SiSi) Defects “disappear” upon annealing at 400 ºC in N2 Hydrogen is released from the Al2O3 film and diffuses to the Si interface to passivate dangling bond defects Electrically-detected magnetic resonance SIMS on deuterated Al2O3/SiO2/Si stack Dingemans et al., Appl. Phys. Lett (2010), accepted. /Applied Physics - Erwin Kessels 5/16

Deposition of Al2O3 films : 

Deposition of Al2O3 films Pyrolysis Al(O-iPr)3 Atomic layer deposition (ALD) Thermal ALD – Al(CH3)3 & H2O Thermal ALD – Al(CH3)3 & O3 Plasma ALD - Al(CH3)3 & O2 plasma Plasma-enhanced CVD Al(CH3)3 & O2 or CO2 Physical vapor deposition (PVD) Al-target & Ar-O2 plasma Hezel & Jaeger 1989 IMEC & TU/e 2005 Tokyo Tech 2008 ANU 2009 /Applied Physics - Erwin Kessels 6/16

Plasma ALD of Al2O3 from Al(CH3)3 & O2 plasma : 

/Applied Physics - Erwin Kessels Plasma ALD of Al2O3 from Al(CH3)3 & O2 plasma ALD of Al2O3 from Al(CH3)3 and H2O ALD is an CVD-like process in which films are deposited by repeating cycles each yielding a submonolayer of film and with excellent uniformity & conformality 1 ALD cycle consists of 4 steps: 1) Precursor [Al(CH3)3] exposure 2) Reactor purge 3) Reactant [H2O/O3/O2 plasma] exposure 4) Reactor purge ~1 Å/cycle 7/16 Van Hemmen et al, J. Electrochem. Soc. 154, G165 (2007).

Plasma ALD Al2O3 vs. thermal ALD Al2O3:Annealing & firing : 

Plasma ALD Al2O3 vs. thermal ALD Al2O3:Annealing & firing After anneal at 400 ºC After firing at ~800 ºC 5 nm thermal and plasma ALD Al2O3 /Applied Physics - Erwin Kessels 8/16 lifetime mapping by μ-PCD Dingemans et al., J. Appl. Phys. 106, 114109 (2009) Dingemans et al., Phys. Status Solidi RRL 4, 10 (2010) Optimum anneal temperature is within the range 350 – 450 ºC Films can be ultrathin (~5 nm) and are still sufficiently firing stable As-dep.

Plasma ALD Al2O3 vs. thermal ALD Al2O3:Thickness dependence & fixed charge density : 

Plasma ALD Al2O3 vs. thermal ALD Al2O3:Thickness dependence & fixed charge density Passivation decreases for <5 nm (plasma ALD) and <10 nm (thermal ALD) Fixed charge density Qf is lower for thermal ALD than for plasma ALD Al2O3 Fixed charge density Qf is located at the Si/SiOx –Al2O3 interface /Applied Physics - Erwin Kessels 9/16 Presentations 2AO.2.4 & 2CV.3.68 Dingemans et al., 35th IEEE PVSC (2010), in press.; Electrochem. Solid State Lett. (2010), accepted.

Plasma ALD Al2O3 vs. thermal ALD Al2O3:Field-effect vs. chemical passivation : 

Plasma ALD Al2O3 vs. thermal ALD Al2O3:Field-effect vs. chemical passivation Fixed charge density Qf is lower for thermal-H2O ALD than for plasma ALD Al2O3 (Very) good chemical passivation for (as-deposited) thermal-H2O ALD Al2O3 Thermal-O3 ALD Al2O3 has similar (excellent) properties than plasma ALD Al2O3 /Applied Physics - Erwin Kessels 10/16 Fixed charge density Qf and interface state density Dit from C-V measurements Presentations 2AO.2.4 & 2CV.3.68 Dingemans et al., 35th IEEE PVSC (2010), in press.; Electrochem. Solid State Lett. (2010), accepted. Plasma ALD 4.2×1012 ~1013 5.8×1012 9.6×1010 Thermal ALD-H2O 1.3×1011 2.9×1011 2.5×1012 1.2×1011 Qf (cm-2) Qf (cm-2) Dit (eV-1cm-2) Dit (eV-1cm-2) Thermal ALD-O3 5.3×1012 ~1013 3.4×1012 1.0×1011 Before anneal After anneal

PECVD of Al2O3 from Al(CH3)3 and O2 : 

PECVD of Al2O3 from Al(CH3)3 and O2 Dingemans et al., Electrochem. Solid State Lett. 13, H76 (2010) Also PECVD Al2O3 provide excellent passivation, even at a rate of 30 nm/min PECVD Al2O3 contains a high negative fixed charge density (Qf = 6.5x1012 cm-2) Passivation by Al2O3 is very robust and does not require very high quality films /Applied Physics - Erwin Kessels 11/16 0.8 cm/s 2.9 cm/s 14 cm/s

Precursor quality and precursor alternatives : 

Precursor quality and precursor alternatives Visual presentation 2CV.3.72 Lower purity “solar grade” Al(CH3)3 provides similar high level of surface passivation as semiconductor grade Al(CH3)3 Non-pyrophoric Al-precursors provide good and “safe” alternative for Al(CH3)3 /Applied Physics - Erwin Kessels 12/16 Al(CH3)2(OiPr) – “DMAI” Non-pyrophoric Al(CH3)3 – “TMA” Solar grade Seff,max = 7.2 cm/s Seff,max = 3.4 cm/s

High-volume manufacturing equipment : 

High-volume manufacturing equipment Batch ALD www.solaytec.com www.levitech.nl www.asm.com /Applied Physics - Erwin Kessels 13/16 www.beneq.com In-line spatial ALD In-line PECVD www.roth-rau.de Visit their booths at the exhibition

High-throughput ALD of Al2O3In-line spatial ALD (Levitrack) : 

High-throughput ALD of Al2O3In-line spatial ALD (Levitrack) /Applied Physics - Erwin Kessels 14/16 Levitrack: wafers are floating in a linear, atmospheric N2 gas track Al(CH3)3 and H2O injection from single side and separated by N2 curtain Single side deposition and only at the wafers, no etch steps necessary Relatively small gas flows as gap is 150 m Speed of ~0.2 m/s leads to throughput of 3600 wafers/hour Cesar et al., 35th IEEE PVSC (2010), in press.

High-throughput ALD of Al2O3In-line spatial ALD (Levitrack) : 

High-throughput ALD of Al2O3In-line spatial ALD (Levitrack) /Applied Physics - Erwin Kessels 15/16 High–throughput ALD yields similar high level of passivation as plasma ALD Al2O3 Passivation decreases slightly for < 10 nm as typical for thermal ALD Al2O3 High–throughput ALD Al2O3 (single layer) is sufficiently thermally stable against firing at ~850 ºC Cesar et al., 35th IEEE PVSC (2010), in press.

Summary – Si surface passivation by Al2O3 : 

Summary – Si surface passivation by Al2O3 Al2O3 is a transparent, highly-stable, negative-charge dielectric Al2O3 provides unique solutions for solar cells: rear surface passivation of p-type Si and p-type emitter passivation of n-type Si Al2O3 leads to excellent chemical passivation (passivation of Si dangling bonds; Dit ~1011 cm-2eV-1), also when used in film stacks Al2O3 can also induce an unique high level of field-effect passivation by negative fixed charges (Qf up to 1013 cm-2) Al2O3 thickness < 10 nm, processing <425 ºC, sufficient firing stable Excellent passivation can be obtained by various deposition methods providing choice between more chemical (thermal ALD-H2O) or more field-effect passivation (thermal ALD-O3, plasma ALD & PEVCD)) HVM equipment (ALD, PECVD, …) and processes under development (Sufficiently) low cost-of-ownership, e.g. solar grade Al(CH3)3 and non-pyrophoric precursors can be used /Applied Physics - Erwin Kessels 16/16 Proceedings & literature : www.phys.tue.nl/pmp w.m.m.kessels@tue.nl

Acknowledgments : 

Acknowledgments /Applied Physics - Erwin Kessels Dr. Ernst Granneman, Jaap Beijersbergen, Pascal Vermont Dr. Jan Schmidt Dr. Jan Benick, Dr. Stefan Glunz Dr. Peter Engelhart, Stefan Bordihn, Dr. Jörg Müller, Bernd Hintze, Dr. Robert Seguin, Dr. Verena Mertens, Dr. David Rychtarik Christophe Lachaud, Nicolas Blasco, Alain Madec Dr. Dieter Pierreux James Hilfiker Prof. Marco Fanciulli MDM Lab TU/e Nick Terlinden Jan-Pieter van Delft Martijn Adams Cristian van Helvoirt Dr. Bram Hoex Prof. Richard van de Sanden Proceedings & literature : www.phys.tue.nl/pmp w.m.m.kessels@tue.nl

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