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Design and Fabrication of a 4H SiC Betavoltaic Cell M.V.S. Chandrashekhar, C.I. Thomas, Hui Li, M.G. Spencer and Amit Lal 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

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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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Long half lives of β-radiation sources. Low energy sources are relatively benign –Small penetration depths Significant power density in source Motivation E max (keV) E mean (keV) Activity (Gbq/g) Half Life (Years) Mean Penetration Depth in SiC (um) Power Density (mWcm -3 ) Comments Nickel Energy below damage threshold Tritium Energy below damage threshold

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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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~D n ~D p EcEc EvEv e-e- e-e- e-e- High energy -particle E 0 e -* Optical /Acoustic phonons E Fn E Fp h h h Optical/Acoustic phonons e-e- Recombination Basic Operation

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Comparison with AA Battery TypePower mW Total Energy mWh Volume cm 3 Weight g Total Energy Density mWh/g Lithium AA Battery ~1 (1.5V) Beta Voltaic 1 cm 2 footprint ~.1 (2V)

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* From Klein. C.A. JAP 39 p.2029 Electron-hole pairs Secondary electrons Backscattered electrons –Elastic scattering Acoustic phonons ~50meV Optical Phonons ~100meV Interaction of Hot Electrons with Semiconductors* Kinetic Energy Impact Ionizations Optical Phonons Hot Secondary Electrons Bandgap Losses e-h Pairs in Equilibrium Thermalization Loss

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Important energy loss mechanisms accounted for by defining effective e-h pair creation energy: E=E g + + –E= 8.4eV for 4H SiC- energy independent Backscattering losses accounted for by subtracting percentage η from incident electron energy E 0 Carrier multiplication achieved (1-η)E 0 /E Energy Bookkeeping

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Beta-voltaic Operation IgIg R Se ries R L oad I V V oc I sc Fill factor Voc=nkT/q ln(Isc/Isat)

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Prediction for Mature Materials Open Circuit Voltage Tritium 1 1. Backscattering and fill factor effects included with 100% CCE.

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Prediction for Mature Materials Efficiency Tritium 1 1. Backscattering and fill factor effects included with 100% CCE.

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Prediction for Mature Materials Power Density Tritium

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Why SiC ? Property Band gap (eV) Breakdown field for cm -3 (MV/cm) Saturated Electron Drift (cm/s) Electron mobility (cm 2 /Vs) Hole mobility (cm 2 /Vs) Thermal Conductivity (W/cmK) Si GaAs x H-SiC x10 7 <900 < C-SiC x10 7 <800 < 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 GaN x H-SiC x10 7 <400 <90 4.9

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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%. 4H SiC as Cell Material

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Betavoltaic Cell Design Considerations Absorption depth of electrons –Bethe range ~E = –Determines junction width and depth Backscattering of electrons from high Z-contact Self absorption in source –Not considered here

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Growth Temperature-1600°C Rotation-1000 rpm Growth Pressure torr Materials are grown at Cornell in a VEECO D180 SiC rotating disc multi-wafer reactor

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High resistance contact Forward active region-used to extract J 0 Noise J 0 = A/cm 2 n=2 4H SiC Deep Junction PN Diode I-V Characteristics Junction depth is 0.5 μm. J 0 = A/cm 2, n=2 J 0 = A/cm 2 with n=2 available commercially-achievable.

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Evaluation of Radiation Cell in SEM 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/cm 2. –higher than Ni-63 source - 6 pA/cm 2 –comparable to tritium source ~2 nA/cm 2 Annealed contacts Voltmeter V kV electrons from SEM P substrate Probe inside SEM pn diode

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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 “FF”=0.52 Efficiency= 6% Voc=0.72V

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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.

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Irradiation with Tritium Under Tritium illumination Jsc= 1.2 μA/cm 2 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/cm 2 New shallow junction 0.25 µm expected to show unpassivated efficiency of ~20% with power density of ~2 μW/cm 2 -useful!

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

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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/cm 2 observed under Tritium illumination. Efficiency limited by edge recombination and poor “fill factor” from poor contacts Can scale to ~0.4 mW/cm 2 for single layer by utilizing high aspect ratio structures.

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