Transcript Design and Fabrication of Radiation Cell

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,
Ithaca, NY 14850, USA
Presentation Outline
• Motivation
• Review theory of betavoltaic cell
– Potential loss mechanisms
– Comparison of materials options and predicted
efficiencies
• Results to date
• Conclusion
Beta-Voltaic Battery
Motivation
Emax
(keV)
Emean
(keV)
Activity
(Gbq/g)
Half
Life
(Years)
Mean
Penetration
Depth in SiC
(um)
Power
Density
(mWcm-3)
Comments
• Long half lives of β-radiation sources.
• Low energy sources are relatively
Nickel 63
67
benign
17
2190
100
3
400
Energy
below
damage
threshold
200
Energy
below
damage
threshold
– Small penetration depths
Tritium 3
18.6
5.685
247000
12.32
0.5
• Significant power density in source
Applications
• Low accessibility sensor nodes
• On-chip power source for MEMS
• Standby power for cell-phones
• Pacemaker power supply
Basic Operation
High energy -particle E0
e-
e-
ee-
Recombination
Optical /Acoustic
phonons
Optical/Acoustic
phonons
Ec
EFn
EFp
Recombination
h
Ev
h
h
e-*
~Dn
~Dp
V
J  J 0 exp(
)  Jsc
nVth
Comparison with AA Battery
Type
Power
mW
Total
Energy
mWh
Volume
cm3
Weight
g
Total Energy
Density
mWh/g
Lithium AA
Battery
~1
(1.5V)
4350
7.9
14.5
300
Beta Voltaic
1 cm2
footprint
~.1
(2V)
10512
.025
.08
131400
Interaction of Hot Electrons with
Semiconductors*
Kinetic Energy
• Electron-hole pairs
Optical Phonons
• Backscattered electrons
Impact
Ionizations
Bandgap Losses
– Elastic scattering
• Acoustic phonons
Hot Secondary
Electrons
~50meV
Thermalization Loss
e-h Pairs in
Equilibrium
• Secondary electrons
• Optical Phonons
~100meV
* From Klein. C.A. JAP 39 p.2029
Energy Bookkeeping
• Important energy loss mechanisms accounted for
by defining effective e-h pair creation energy:
E=Eg+<Ek>+<ER>
– E= 8.4eV for 4H SiC- energy independent
• Backscattering losses accounted for by subtracting
percentage η from incident electron energy E0
• Carrier multiplication achieved (1-η)E0/E
Beta-voltaic Operation
RSe
I
ries
Voc
Ig
RL
qV
I  I Sat (e kT  1)
oad
I Sat 
qDp pno
Lp

qDnn po
Ln
Isc
V
Fill factor
Voc=nkT/q ln(Isc/Isat)
Prediction for Mature Materials
Open Circuit Voltage Tritium 1
2.5
Open Circuit Voltage (V)
4H SiC
2
Jsc
Voc  nVth ln( )
J0
GaN
GaP
1.5
1
GaAs
Si
0.5
Predicted Voc (n=1)
Predicted Voc (n=2)
Experimentally realized Voc
0
1
1.5
2
2.5
3
3.5
4
Bandgap (eV)
1. Backscattering and fill factor effects included
with 100% CCE.
Prediction for Mature Materials
Efficiency Tritium 1
25
4H SiC
VocJsc
Efficiency  FF
Vtritium J Tritium
20
Efficiency (%)
GaN
15
GaP
Si
GaAs
FF 
10
5
V peak J peak
VocJsc
Predicted efficiency (n=1)
Predicted efficiency (n=2)
Experimentally realized efficiency
0
1
1.5
2
2.5
3
3.5
4
Bandgap (eV)
1. Backscattering and fill factor effects included
with 100% CCE.
Prediction for Mature Materials
Power Density Tritium
3.5
4H SiC
GaN
2
Power Density P (W/cm )
3
P  FF  Voc  Jsc
 Vpeak J peak
2.5
GaP
2
Si
GaAs
1.5
1
Predicted Power Density (n=1)
Predicted power density (n=2)
Experimentally realized power density
0.5
0
1
1.5
2
2.5
Bandgap (eV)
3
3.5
4
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
GaN
3.4
3.5
1.5x107
1000
300
1.3
3C-SiC
2.36
1.5
2.5x107
<800
<320
5.0
4H-SiC
3.2
3-5
2x107
<900
<120
4.9
6H-SiC
3.0
3-5
2.5x107
<400
<90
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
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%.
Betavoltaic Cell Design Considerations
• Absorption depth of electrons
– Bethe range ~E01.6 = 3µm@17keV
– Determines junction width and depth
• Backscattering of electrons from high Z-contact
• Self absorption in source
– Not considered here
Materials are grown at Cornell in a VEECO D180
SiC rotating disc multi-wafer reactor
• Growth Temperature-1600°C
• Rotation-1000 rpm
• Growth Pressure 50-300 torr
4H SiC Deep Junction PN Diode I-V Characteristics
1
High resistance
contact
2
Current Density (A/cm )
0.01
0.0001
10
-6
10
-8
Forward active region-used
to extract J0
Noise
10
-10
10
-12
10
-14
10
-16
10
-18
10
-20
J0=10-17A/cm2
n=2
0
1
2
3
4
5
Voltage (V)
•
•
•
Junction depth is 0.5 μm.
J0=10-17A/cm2, n=2
J0=10-24A/cm2 with n=2 available commercially-achievable.
6
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
17 kV electrons
from SEM
Probe inside SEM
Annealed contacts
-
pn diode
P substrate
V
Voltmeter
+
Collection of Charge
1.6
20
Open Circuit Voltage Voc (V)
Ni-63 Voc
1.2
15
1
Ni-63
0.8
10
0.6
0.4
5
Efficiency SEM 17kV
0.2
Power Conversion Efficiency (%)
Voc SEM 17kV
1.4
Ni-63 Efficiency
0
-12
10
0
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
Illumination Current Density at 17 kV (J
beam
-5
)
•Efficiency up to 14% for high current
density with no edge recombination
Current for 500 um diode (A)
Irradiation with Ni-63
8 10
-11
6 10
-11
4 10
-11
2 10
-11
Efficiency= 6%
Voc=0.72V
0
“FF”=0.52
-11
-2 10
-11
-4 10
-11
-6 10
0
0.2
0.4
0.6
Voltage (V)
0.8
1
Irradiation with Ni-63
• Power conversion efficiency of 6% and Voc=0.72V
• 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.
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 <0.1V 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!
Top view
Tritiated water
2x radiation
penetration
depth
n- epitaxial layer
Thin p type
diffused contact
layer
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.