S' Dixit1,2, S' Dhar2,3, J' Rozen1,2, S' Wang3, S' T' Pantelides3,

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Radiation damage in SiO2/SiC interfaces

• S. Dixit1,2, S. Dhar2,3, J. Rozen1,2, S. Wang3,
  S. T. Pantelides3,
• D. M. Fleetwood4,3, R. Schrimpf4 and L. C.
  Feldman1,2,3
• 1Interdisciplinary Materials Science Program
• 2Vanderbilt Institute of Nanoscale Science and
  Engineering
• 3Department of Physics Astronomy
• 4Department of Electrical Engineering Computer
  Science
• Vanderbilt University, Nashville, TN - 37235

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Outline

• Objective
• Motivation
• Introduction
• Experimental
• Results
• Conclusion

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Objective

• Effect of ionizing radiation on SiO2/4H-SiC MOS
  devices.
• Comparison between as-oxidized and nitrided
  oxides.
• Comparison between positively-biased (electrons
  swept) and grounded irradiations.

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Motivation
Deep-space missions - Electronic switches and
circuits High power and high temperature
systems Concerns Weight, efficiency and
reliability
Eg 3.23 eV at RT
Wide Band Gap
4.5 Wcm-1s-1
2.0 MV cm-1
High thermal conductivity
High breakdown field strength
High power device
4H Silicon Carbide
2.0 x 107 cm s-1
High current densities
Inert
Only compound semiconductor whose native oxide is
SiO2
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Introduction

• Thermal oxidation of SiC

SiO2/SiC interface is different from
SiO2/Si Typical oxidation temperatures 1100 -
1300 C
SiO2
Transition layer
1nm
N N N N N N N N N N N N N
N
SiC
sub-oxide bonds oxycarbides free carbon
Nitrided
At Ec-E 0.1 eV, Dit 1013 cm-2 eV-1 ---gt
as-oxidized Dit 1012 cm-2 eV-1 ---gt nitrided
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Experimental
Sample preparation
n-type 4H-SiC
SiO2
SiO2 thermally grown at 1150C, 4 hrs
NO Passivation at 1175C, 2 hrs
n-type 4H-SiC
Sputter deposited Mo/Au dots
Au back contact
Sputter deposited back contact

• Irradiation
• 10 keV X-rays, RT radiation
• 31.5 krad(SiO2)/min dose rate
• Function of dose
• Function of positive bias 1.5 MV/cm, 2.3
  MV/cm

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Results
As-oxidized samples

• Positive charge buildup (like SiO2/Si) followed
  by a turnaround to net negative charge trapping
• For high doses (gt1Mrad(SiO2)), grounded samples
  show net negative charge while the biased samples
  show net positive charge (like SiO2/Si)

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Results
Nitrided samples

• Turn around also observed
• Low dose regime (lt1 Mrad) nitrided samples show
  more positive charge buildup
• Indications of Fermi level Pinning was observed

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Bandgap argument
EC
Eg 9.0 eV
SiO2
EV
Band diagram SiO2 and p-Si
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Bandgap argument
Time constant ?p(E) (1/?p?TNv)exp(E-Ev)/kT
EC
Eg 9.0 eV
SiO2
EV
Band diagram SiO2 and p-type 4H-SiC
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Results
?Vmg vs dose (Grounded samples)
Nitrided/As-oxidized comparison

• Positive charge trapping reported for the first
  time in grounded 4H-SiC capacitors for low dose
  irradiations
• At higher doses, negative charge trapping
  (consistent with previous work 1) observed due to
  creation of oxide trapped charge and deep
  interface states (Ec-E0.6 eV to mid-gap)

1 T. Chen, Z. Luo, J. D. Cressler, T. F.
Isaacs-Smith, J. R. Williams, G. Chung, and S. D.
Clark, The effects of NO passivation on the
radiation response of SiO2/4H-SiC MOS
capacitors, Solid State Electron., vol. 46, pp
2231-2235, 2002
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Results
?Vmg vs dose (Biased samples)
Nitrided/As-oxidized comparison

• Increased positive charge trapping observed in
  the case of nitrided samples under bias
• This might suggest that the nitrided samples
  have less electron traps due to NO passivation as
  compared to the as-oxidized ones at low doses

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Photo-CV results
Photo-CV (Biased and grounded capacitors)

• The hysteresis and prominent ledges are
  indicative of slow interface traps and
  near-interface (border) traps
• Density of these traps are a factor of 2 higher
  for grounded as compared to positively-biased
  case, consistent with higher negative charge
  trapping

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Conclusions

• Significantly higher negative charge trapping for
  SiO2/SiC MOS devices compared to typical SiO2/Si
• This can be attributed to
• rapid buildup of interface states analogous to
  the hole-H interaction in SiO2/Si
• wider bandgap of SiC which exposes the oxide
  related defects
• 3. Higher net positive charge buildup in the
  nitrided samples, consistent with a lower density
  of pre-existing interface and near-interface
  electron traps

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Acknowledgements
Work supported by the Air Force Office of
Scientific Research through the MURI program and
DARPA
Thank you
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Back up slides
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