High deposition rate nanocrystalline silicon with enhanced homogeneity

1
High deposition rate nanocrystalline silicon with
enhanced homogeneity

• A. D. Verkerk, J.K. Rath and R.E.I.
  SchroppUtrecht University, The
  NetherlandsNanophotonics Physics of Devices
• ICANS23 August 25, 2009

2
Introduction

• Hydrogenated nanocrystalline silicon (nc-SiH)
• VHF PECVD - 60 MHz
• High deposition rate (rd) 4.5 nm/s
• Solar cells with 6.4 efficiency
• Comparison between high and low rd
• Low rd High rd
• rd 0.5 nm/s 4.5 nm/s
• pressure 5 mbar 9 mbar
• SiH4 flow 3 sccm 20 sccm
• H2 flow 84 sccm 300 sccm
• VHF power 20 W 350 W

3
Outline

• Goal
• Crystalline fraction in low and high deposition
  rate
• Mechanisms for instability
• Length scales and time scales for back diffusion
• Stabilisation time for back diffusion effects
• Methods of homogeneity improvement
• Results

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Goal

• High efficiency solar cells need homogeneous
  phase composition
• The growth-direction homogeneity of high
  deposition rate (rd) nc-SiH needs to be improved

5
Development of crystalline fraction
Intrinsic films deposited onnc-SiH seed layer
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Possible explanations

• Surface diffusion length of silicon shorter at
  high rd
• Very high depletion gives stronger start-up
  effects
• Back diffusion of silane from outside plasma
  volume

7
Diffusion length

• Calculate diffusion length of Ar in H2
• Diffusion coefficient D 1/p
• Residence time in plasma tres,p p and
  1/flowtotal
• Diffusion length is
• Low rd High rd
• rd 0.5 nm/s 4.5 nm/s
• pressure 5 mbar 9 mbar
• total flow 87 sccm 320 sccm
• Residence time 0.39 s 0.19 s
• Diffusion constant 240 cm2s-1 133 cm2s-1
• Diffusion length 19 cm 10 cm

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Possible explanations

• Surface diffusion length of silicon shorter at
  high rd
• Very high depletion gives stronger start-up
  effects
• Back diffusion of silane from outside plasma
  volume

POSSIBLE
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Stabilisation time for silane back diffusion

• Depends on residence time in inactive region
• Depends on amount of unused silane depletion
• Solution can be written
• Background initial mixture is important

N SiH4 concentration S Change due to source
flow G Change due to deposition tres,r Reactor
residence time
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Calculation of SiH4 concentration (high rd)
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Compare with low rd regime

• Stabilisation time depends on flow and pressure
• High rd (4.5 nm/s) stabilizes in 7 seconds
• Compare Low rd (0.5 nm/s) stabilizes in 15
  seconds
• Diffusion length and stabilisation time
  comparable
• Stabilisation affects thicker section of the film
  in high rd

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Methods of homogeneity improvement

• After deposition start
• Profiling of hydrogen flow
• Profiling of silane flow
• Profiling of VHF power
• Before deposition start
• Tuning of background mixture
• Start with hydrogen plasma
• Features of hydrogen plasma start
• Hydrogen background gas mixture
• Increased surface mobility by atomic H abundance

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Effect of 5-second hydrogen plasma start
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Further tuning of the crystalline development
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Discussion and Conclusions

• Possible increasing back diffusion at lower
  depletion
• Stabilisation time comparable between high and
  low rd
• Diffusion length comparable between high and low
  rd
• Still incubation layer is much thicker in high rd
• Hydrogen start effectively reduces incubation
  layer
• Depletion profiling can be used for fine-tuning

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Acknowledgement

• Thanks to
• Martin Huijzer for depositions
• Wim Goedheer for fruitful discussions
• This project is undertaken with financial support
  from the Ministry of Economic Affairs of The
  Netherlands program EOS (Subsidy Energy
  Research)
• Thank you for your attention!