Investigations using 532nm Nanosecond Lasers Lujia Xu, Klaus Weber, - - PowerPoint PPT Presentation

Lujia Xu, Klaus Weber, Andreas Fell, Evan Franklin, Ziv Hameiri

Understanding Laser Doping: Investigations using 532nm Nanosecond Lasers

Outline

• Motivation
• Dopant measurement by Scanning Electron Microscopy
• Characterisation of dielectric window edge regions
• Impact of laser parameters / dielectrics on recombination

in laser doped regions

2

Motivation

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All laser doped IBC cell concept

Localized base contact: n++ laser doped / opened Localized pn-junction: p++ laser doped / opened n-type wafer High quality surface passivation

Motivation

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Simulated efficiency potential (Quokka)

First all-laser processed cell batch

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Laser processing (532nm DPSS)

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Sample Laser head Optical (mirrors, beam expander, collimation lens, and etc.) Scanning mirror F-theta lens x-y-movable and rotatable stage

Laser doping

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• Small, inhomogeneously doped areas
• Rough surfaces
• How to characterise?
• EBIC / SIMS?
• SEM can be used to image dopant density under certain conditions

p diffusion / n substrate n diffusion / p substrate

SEM Doping Contrast – Laser doped regions

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p doping / n substrate n doping / p substrate

• L. Xu, et al., "Secondary electron microscopy dopant contrast imaging (SEMDCI) for laser doping"

IEEE Journal of Photovoltaics, vol. 3, pp. 762-768, 2013.

Comparison – SEM DCI / EBIC

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• L. Xu, et al, "Comparison between SEMDCI and EBIC for Laser Doping of Crystalline Silicon"

SiliconPV 2014, 's-Hertogenbosch, The Netherlands, 2014.

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Comparison – SEM DCI / EBIC

Can SEM DCI give doping concentration?

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• Establish empirical relationship (diffused samples)
• Apply to laser (excimer) doped samples
• Compare with ECV profiles

Laser doping – (active) doping density maps

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0.5 1 1.5 2 2.5 3 x 10

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2000 4000 0.5 1 1.5 2 2.5 3 x 10

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2000 4000 1000 2000 3000 4000 1000 1E17 1E18 1E19 1E20 1E21 nm nm nm

Concentration color bar (cm-3) (a) 40kHz 15uJ 125nm (b) 40kHz 17.5uJ 500nm (c) Boron diffusion

SEM DCI – Surface profile characterisation

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SEM DCI Optical microscope image

Application of SEM DCI

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How to check the edge region? Single step doping process – what happens at the dielectric edges?

Application of SEM DCI

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• 1. Use dielectric as etch mask, then use SEM

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Application of SEM DCI

• 2. Direct imaging with dielectric film still present

2000 4000 6000 8000 10000 12000 14000 1000 2000 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 500 1000 1500

Dielectric film

nm nm

Laser doping region 40kHz 17.5uJ 1000nm 40kHz 15uJ 125nm

Experiment: laser doping process

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Dielectric SOD Silicon

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Application of SEM DCI

SiO2 19nm Si Si3N4 60/170nm

• L. Xu at el., "The impact of SiO2/SiNx stack thickness on laser doping of silicon solar cell"

IEEE Journal of Photovoltaics, vol. 4, pp. 594-600, 2014.

• 10000
• 5000

5000 10000 15000 125 250 500 125 250 500 1000 250 500 1000 2000 12.5 15 17.5

Gap distance (µm) Pulse distance (nm) Pulse energy (µJ)

Thin Thick

15 10 5

• 5
• 10

Assessing damage from laser doping

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What really matters in the end? J0 depends on

• 3-D doping profile
• Dopant source
• Laser properties
• Dielectric
• Surface roughness
• Etc…
• Defects
• Dopant source
• Laser properties
• Dielectric
• Surface roughness
• Etc…

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Assessing damage from laser doping

Simple experiment to focus on laser-dielectric interactions

Dielectric B Diffusion Silicon

PL Measurement test structure

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Unprocessed, B diffused, unpassivated Processed, B diffused, unpassivated Alignment mark

4mm 4mm Laser processed region Pitch distance 30µm

PL Measurements - calibration

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0.4 0.5 0.6 0.7 0.8 0.9 1.0 20 40 60 80 100

0.7 0.8 0.9 1.0 1 2 3 4 5 6 7 8 9 10

Normalized J0e Normalized PL

Lowest background J0e1407.3 fA/cm

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Second lowest background J0e1783.5 fA/cm

2

Average background J0e 2236.6 fA/cm

2

Highest background J0e 2544.4 fA/cm

2

Results – no dielectric

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1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 200 400 600 800 1000 1200 1400 1600

0.25m Rsh 1 m Rsh 2.5 m Rsh

Sheet resistance (฀) Pulse energy (J)

1 10 100

0.25m Joen 1 m Joen 2.5 m Joen 20 m Joen

Normalized J0e 20ns

4 5 6 7 8 9 10 11 12 13 14 15 16 200 400 600 800 1000

0.25m Rsh 1 m Rsh 2.5 m Rsh

Sheet resistance (฀) Pulse energy (J)

1 10 100

0.25m Joen 1 m Joen 2.5 m Joen 20 m Joen

Normalized J0e 400ns

• L. Xu et al., "The influence of thermal effects and dielectric films on the electronic quality of p+-doped silicon processed by

nanosecond laser" IEEE Journal of Photovoltaics, vol. PP, pp. 1-8, 2014.

Results – evaporation threshold

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2 4 6 8 10 12 14 16 18

100 400

Evaporation threshold

Pulse energy (J) Pulse duration (ns)

0.25m parameter window 1m parameter window 2.5m parameter window

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Results – with dielectric films

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1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 0.1 1 10 100 1000

0.25m Bare 1m Bare 2.5m Bare 20m Bare 0.25m SiO2 1m SiO2 2.5m SiO2 20m SiO2 0.25m SiNxA 1m SiNxA 2.5m SiNxA 20m SiNxA

Normalized J0e Pulse energy (J)

4 5 6 7 8 9 10 11 12 13 14 15 16 0.1 1 10 100 1000

0.25m Bare 1m Bare 2.5m Bare 20m Bare 0.25m SiO2 1m SiO2 2.5m SiO2 20m SiO2 0.25m SiNxA 1m SiNxA 2.5m SiNxA 20m SiNxA

Normalized J0e Pulse energy (J)

20ns 400ns

Conclusions

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• Single step laser doping processes for IBC are very challenging
• SEM DCI is a convenient tool to assess laser doping near dielectric edges
• Under the right conditions, good doping is achieved under dielectric
• Laser induced damage silicon damage can be kept to acceptable levels
• More work is needed to understand dielectric edge regions / dopant precursors

Acknowledgements

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ANU Laser team Colleagues at ISE and ISFH (S. Kluska, B. Fleischmann, S. Hopmann, K. Bothe, B. Lim) Australian Renewable Energy Agency (3-GER002 “High quality laser doping for solar cells through improved characterisation techniques”)