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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 Advanced Materials and Devices Applications (AMDA) Department of Electrical and Computer Engineering Cornell University, Ithaca, NY 14850, USA. Presentation Outline.
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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
Presentation Outline • Motivation • Review theory of betavoltaic cell • Potential loss mechanisms • Comparison of materials options and predicted efficiencies • Results to date • Conclusion
Motivation • Long half lives of β-radiation sources. • Low energy sources are relatively benign • Small penetration depths • Significant power density in source
Applications • Low accessibility sensor nodes • On-chip power source for MEMS • Standby power for cell-phones • Pacemaker power supply
e- h h h Basic Operation High energy -particle E0 Optical /Acoustic phonons e- e- e- Optical/Acoustic phonons Recombination Ec EFn EFp Recombination Ev e-* ~Dp ~Dn
Kinetic Energy Optical Phonons Impact Ionizations Bandgap Losses Hot Secondary Electrons Thermalization Loss e-h Pairs in Equilibrium 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
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
RSeries I Voc V Ig RLoad Fill factor Isc Beta-voltaic Operation Voc=nkT/q ln(Isc/Isat)
Prediction for Mature MaterialsOpen Circuit Voltage Tritium 1 1. Backscattering and fill factor effects included with 100% CCE.
Prediction for Mature MaterialsEfficiency Tritium 1 1. Backscattering and fill factor effects included with 100% CCE.
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
High resistance contact Forward active region-used to extract J0 Noise J0=10-17A/cm2 n=2 4H SiC Deep Junction PN Diode I-V Characteristics • Junction depth is 0.5 μm. • J0=10-17A/cm2, n=2 • J0=10-24A/cm2 withn=2 available commercially-achievable.
Probe inside SEM 17 kV electrons from SEM Annealed contacts - pn diode V Voltmeter P substrate + 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
Collection of Charge • Efficiency up to 14% for high current density with no edge recombination
Efficiency= 6% Voc=0.72V “FF”=0.52 Irradiation with Ni-63
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% vs0.22% in Si • Estimated power 1 μW/cm2 • New shallow junction 0.25 µm expected to show unpassivated efficiency of ~20% with power density of ~2μW/cm2-useful!
Top view Tritiated water Thin p type diffused contact layer 2x radiation penetration depth n- epitaxial 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.
