New Insights into the Role of Hydrogen in the StaeblerWronski Effect in aSi:H

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New Insights into the Role of Hydrogen in the
Staebler-Wronski Effect in a-SiH
P. Craig Taylor Department of Physics Colorado
School of Mines
Amorphous and Nanocrystalline Silicon Team
Meeting April 7, 2006 San Francisco, CA
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Outline

• The Story of Silicon Dihydride (SiH2)
• NMR lineshapes
• Calculations
• Relationship to Metastable Doublet

• H and Dangling Bonds in Tritiated a-SiH
• Temperature Dependence of Creation
• Annealing
• Possible H-Related Site

• Light Soaking at 77 K

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Paired Hydrogen Bonding Sites Metastable
Site Polysilane (SiH2)n Silicon Dihydride (SiH2)
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1H NMR Lineshapes in a-SiH due to Bonded Hydrogen
Narrow line dilute or random phase
(Lorentzian) Broad line clustered phase
(Gaussian)
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NMR Lineshapes in a-SiH due to Paired Hydrogen
Site
T. Su, P. C. Taylor, G. Ganguly and D. E.
Carlson, Phys. Rev. Lett. 89, 015502-1 (2002).
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The Microscopic Picture What Stabilizes the
Dangling Bond?
2.3 Å
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Stabilization of the Staebler-Wronski Effect in
a-SiH

• Paired hydrogen site appears only after light
  soaking and anneals at elevated temperature
• Lineshape indicates site is a doublet
• Two hydrogen atoms are 2.3 Å apart
• Density of paired hydrogen sites is consistent
  with the density of dangling bonds from ESR
• Much different T1 and much longer T1d for paired
  H site indicates that it is decoupled from bonded
  H

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Lineshape Fitting of Silicon Dihydride 1H NMR
Doublet

• Assume SiH2 with distribution of r(H-H) centered
  at 1.8 Å with a width (FWHM) of 0.5 Å and minimal
  isotropic broadening of 1 kHz (FWHM).

D. Bobela et al. W06.2 This meeting, Wednesday
morning
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Lineshape Fitting of Silicon Dihydride 1H NMR
Doublet

• Assume unique SiH2 with r(H-H) 1.8 Å and
  isotropic broadening of 12 kHz (FWHM) to account
  for randomly occurring nearby H

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Title
Table 1. Summary of (SiH2)n concentration
measured from NMR and IR. The reported value is
the percent concentration relative to the total
hydrogen concentration observable by NMR and IR.
Min refers to the minimum concentration
estimate, within experimental uncertainty.
Details of the growth conditions are also
provided.
 
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Possible SiH2 Sites in a-SiH
T. A. Abtew, D. A. Drabold, and P. C. Taylor,
Appl. Phys. Lett. 86, 241916 (2005).
Silicon dangling bond
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Calculated H-H Separation in SiH2
T. A. Abtew, D. A. Drabold, and P. C. Taylor,
Appl. Physl Lett. 86, 241916 (2005).
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Summary of SiH2 Sites in a-SiH

• Significant SiH2 in most, if not all, films
• Calculations yield H-H distances too large
• What is metastable paired hydrogen site?

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Hydrogen and Dangling Bonds in Tritiated a-SiH
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Growth of ESR and PDS in Tritiated a-SiH at
Different Temperatures
Defects expected from 3H decays
t1/3
Stretched Exponential with ß 0.6
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Growth of ESR in Tritiated a-SiH device
quality sample
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ESR in Tritiated a-SiH high defect density
sample
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ESR in Tritiated a-SiH device quality sample
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Summary

• In tritiated a-SiH defects created more slowly
  than expected from 3He production.
• Growth is dispersive and saturates at values
  similar to those produced optically
  (Staebler-Wronski effect).
• Most defects are neutral.
• At room temperature and above there is
  significant annealing of dangling bonds.
• No apparent saturation at 77 K at long times.
• New H-related ESR tentatively identified.
• Optically induced ESR at 77 K also appears not to
  saturate.

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Major Conclusions

• More Silicon Dihydride Sites in Device Quality
  Films than Previously Assumed
• Relationship of Silicon Dihydride to Metastable H
  Doublet Sites Unclear
• Dangling Bond Creation in Tritiated a-SiH
  Yielding Interesting Results (Possible H related
  ESR Site)
• Light Soaking at77 K for Long Times Initiated

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