Design and Fabrication of a 4H SiC Betavoltaic Cell M.V.S. Chandrashekhar, C.I. Thomas, Hui Li, M.G. Spencer and Amit Lal Advanced Materials and Devices Applications (AMDA) Department of Electrical and Computer Engineering Cornell University,

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Design and Fabrication of a 4H SiC Betavoltaic
CellM.V.S. Chandrashekhar, C.I. Thomas, Hui
Li, M.G. Spencer and Amit LalAdvanced
Materials and Devices Applications (AMDA)
Department of Electrical and Computer
EngineeringCornell University, Ithaca, NY
14850, USA
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Presentation Outline

• Motivation
• Review theory of betavoltaic cell
• Potential loss mechanisms
• Comparison of materials options and predicted
  efficiencies
• Results to date
• Conclusion

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Beta-Voltaic Battery
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Motivation

• Long half lives of ß-radiation sources.
• Low energy sources are relatively benign
• Small penetration depths
• Significant power density in source

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Applications

• Low accessibility sensor nodes
• On-chip power source for MEMS
• Standby power for cell-phones
• Pacemaker power supply

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Basic Operation
High energy ?-particle E0
Optical /Acoustic phonons
e-
e-
e-
Optical/Acoustic phonons
Recombination
Ec
EFn
EFp
Recombination
Ev
e-
Dp
Dn
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Comparison with AA Battery
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Interaction of Hot Electrons with Semiconductors

• Electron-hole pairs
• Secondary electrons
• Backscattered electrons
• Elastic scattering
• Acoustic phonons 50meV
• Optical Phonons 100meV

From Klein. C.A. JAP 39 p.2029
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Energy Bookkeeping

• Important energy loss mechanisms accounted for by
  defining effective e-h pair creation energy
• EEgltEkgtltERgt
• E 8.4eV for 4H SiC- energy independent
• Backscattering losses accounted for by
  subtracting percentage ? from incident electron
  energy E0
• Carrier multiplication achieved (1-?)E0/E

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Beta-voltaic Operation
VocnkT/q ln(Isc/Isat)
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Prediction for Mature MaterialsOpen Circuit
Voltage Tritium 1
1. Backscattering and fill factor effects
included with 100 CCE.
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Prediction for Mature MaterialsEfficiency
Tritium 1
1. Backscattering and fill factor effects
included with 100 CCE.
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Prediction for Mature MaterialsPower Density
Tritium
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Why SiC ?

• Property
• Band gap (eV)
• Breakdown field for 1017cm-3 (MV/cm)
• Saturated Electron Drift (cm/s)
• Electron mobility (cm2/Vs)
• Hole mobility (cm2/Vs)
• Thermal Conductivity (W/cmK)

Si 1.1 0.6 107 1350 450 1.5
GaAs 1.42 0.65 1x107 6000 330 0.46
4H-SiC 3.2 3-5 2x107 lt900 lt120 4.9
3C-SiC 2.36 1.5 2.5x107 lt800 lt320 5.0
GaN 3.4 3.5 1.5x107 1000 300 1.3
6H-SiC 3.0 3-5 2.5x107 lt400 lt90 4.9

• SiC Beta Voltaic Cell are promising for nano-watt
  power generation

High electric breakdown field High saturated
electron velocity High thermal conductivity
Suited for high temperature, high power, high
frequency, high radiation environment
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4H SiC as Cell Material

• 4H SiC is ideal material owing to its large
  bandgap (3.3eV)
• Low realizable leakage current-substrates
• 4H SiC is extremely radiation hard
• Low Z-elements
• Minimal loss from backscattering.
• Significant progress in SiC radiation detectors
  with charge collection efficiencies (CCE) close
  to 100.

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Betavoltaic Cell Design Considerations

• Absorption depth of electrons
• Bethe range E01.6 3µm_at_17keV
• Determines junction width and depth
• Backscattering of electrons from high Z-contact
• Self absorption in source
• Not considered here

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Materials are grown at Cornell in a VEECO D180
SiC rotating disc multi-wafer reactor

• Growth Temperature-1600C
• Rotation-1000 rpm
• Growth Pressure 50-300 torr

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4H SiC Deep Junction PN Diode I-V Characteristics

• Junction depth is 0.5 µm.
• J010-17A/cm2, n2
• J010-24A/cm2 with n2 available
  commercially-achievable.

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Evaluation of Radiation Cell in SEM

• 17 kV electron beam to simulate Ni-63 source
• Magnification changes current density
• Lowest incident current density 0.3 nA/cm2.
• higher than Ni-63 source - 6 pA/cm2
• comparable to tritium source 2 nA/cm2

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Collection of Charge

• Efficiency up to 14 for high current density
  with no edge recombination

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Irradiation with Ni-63
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Irradiation with Ni-63

• Power conversion efficiency of 6 and Voc0.72V
• Limited by fill factor and edge recombination.
• Better fill factor 75 at higher
  currents-contacts
• Equivalent corrected efficiency 15- approaches
  predicted value.
• Enhanced current multiplication compared to
  monochromatic electron illumination 2400
• Ni-63 irradiated output stable after ten days of
  continuous monitoring.

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Irradiation with Tritium

• Under Tritium illumination Jsc 1.2 µA/cm2
  observed in deeper junction 0.5 µm
• 96 µA/Ci vs 20 µA/Ci in Si
• Voc 1V vs lt0.1V in Si
• Unpassivated efficiency of 10 vs 0.22 in Si
• Estimated power 1 µW/cm2
• New shallow junction 0.25 µm expected to show
  unpassivated efficiency of 20 with power
  density of 2 µW/cm2-useful!

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Top view
Tritiated water
Thin p type diffused contact layer
2x radiation penetration depth
n- epitaxial layer
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Conclusion

• Efficiency of 6 demonstrated for shallow
  junction under Ni-63 illumination.
• Highest efficiency of 10 and power density 1.0
  µW/cm2 observed under Tritium illumination.
• Efficiency limited by edge recombination and poor
  fill factor from poor contacts
• Can scale to 0.4 mW/cm2 for single layer by
  utilizing high aspect ratio structures.