Fluence

1
Fluencedependent lifetime variations in neutron
irradiated MCZ Si measured by microwave probed
photoconductivity and dynamic grating techniques
E.Gaubas, A.Kadys, A.Uleckas, and J.Vaitkus
Vilnius university, Institute of Materials
Science and Applied Research, Sauletekio av. 10,
LT-10223, Vilnius, Lithuania

• Outline
• Objectives of investigations
• Setup of MWR and DG experiments
• Fluence and heat-treatment dependent lifetime
  variations
• Characteristics of lifetime cross-sectional
  profiles
• Summary

2

• Objectives of investigation
• Direct measurements of recombination lifetime
• ? combined investigations of of MWR and DG
  applied in the range of the highest fluence
• Control of possible anneal of defects
• ? heat treatments 80C 5 min, 30 min and 24 h,-
  WODEAN standard
• - Cross-sectional scans within wafer depth to
  control of oxygen out-diffusion

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Measurement techniques and instruments
Microwave probed photoconductivity (MW-PCD) in
MW reflection mode (MWR)
Dynamic gratings (DG)
K.Jarasiunas, J.Vaitkus, E.Gaubas, et al. IEEE
Journ. QE, QE-22, (1986) 1298.
??(t)
The microwave probed photoconductivity (MW-PCD)
technique is based on the direct measurements of
the carrier decay transients by employing MW
absorption by excess free carriers. Carriers are
photoexcited by 1062 nm light generated by pulsed
(700 ps) laser and probed by 22 GHz cw microwave
probe.
Diffraction efficiency (?I-1/I0) on light
induced dynamic grating is a measure ? ? (?N)2
of excess carrier density, while its variations
in time ?(t) ? exp(-2t/?G) by changing a grating
spacing (?) enable one to evaluate directly the
parameters of grating erase 1/?G  1/?R  1/?D 
through carrier recombination (?R) and diffusion?
D  ?2/(4?2D) with D as a carrier diffusion
coefficient.
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Direct measurements of recombination lifetime by
MWR

• ?R ? ?tU exp(-1)
• ?R ? gexc?Rs/gexc?RL (UMWRslt2 ns /UMWRLgt5 ns)

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Direct measurements of recombination lifetime by
DG
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Recombination lifetime in wafer and diode samples
measured by MWR
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Lifetime under heat treatments at 80C for 5 min,
30 min and 24 h
Lifetime variation with neutron irradiation
fluence in MCZ Si 61,62,63,64, 54,56,58, and 60
wafers for the as-received and heat treated
material
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Cross-sectional scans within wafer depth
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SUMMARY

• Lifetime decreases from few ?s to about of 200
  ps with enhancement of neutron irradiation
  fluence ranging from 1012 to 3?1016 n/cm2, as
  measured directly by exploiting microwave probed
  photoconductivity transients and verified by
  dynamic grating technique.
• Lifetime values are nearly the same for wafer
  and diode samples.
• Small increase of lifetime values under
  annealing can be implied.
• Lifetime values are nearly invariable within
  wafer thickness, as determined from the lifetime
  cross-sectional scans within wafer depth.
• Investigation of the lifetime temperature
  variations in the range of more than RT is
  anticipated, to determine the activation factors
  for the dominant recombination centers. However,
  annealing and instability of defects would be a
  problem for precise extraction of these
  parameters.

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Thank You for attention!