Betavoltaic devices are p-n/p-i-n junction diodes that use the kinetic energy of beta (electron) particles emitted by beta isotopes to create electron-hole pairs and generate electrical power in a similar way as photovoltaic devices use photons energy to generate electrical power. Unlike photovoltaic devices (solar cells), betavoltaic devices can generate electrical power day and night continuously for decades (12 years with 3H (tritium) and 100 years with Ni63 (nickel-63)), enabling new capabilities that are not possible with photovoltaic devices or current state of the art chemical batteries. New capabilities include decade-long continuous power for unattended sensor, tagging/tracking devices, and other electronics placed in remote areas (underwater, polar region, space, etc) where change/charge of batteries is highly inconvenient or impossible. It is expected that wide band gap semiconductors like GaN with an energy gap of 3.4 eV may provide a better performance in terms of output power and stability under beta radiations. However, GaN semiconductor technology is still maturing in terms of growth and fabrication techniques. InGaP has a moderately wide bandgap of 1.86 eV, but it is well advanced in terms of crystal growth and fabrication techniques. Therefore, our study and research focused on the development (design, fabrication, evaluation) and comparison of a wide band gap (GaN) and a moderately high band gap InGaP, that are considered very promising in betavoltaic applications.
Betavoltaic devices were fabricated on three GaN p-i-n structures with different i-layer thicknesses (600 nm, 700 nm and 1 µm). Two GaN p-i-n structures were grown on top of a sapphire substrate and the third GaN structure was grown on top of a bulk GaN substrate. InGaP devices were fabricated on an InGaP n-i-p structure grown on top of a gallium arsenide (GaAs) substrate. Devices were characterized using current - voltage (IV) measurements in the dark, using a UV light source, and under an electron beam stimulus to mimic their performance under real beta isotopes. Dark IV measurements confirmed good quality diodes with low leakage currents, and IV curves under the UV light (365nm, 3.40 eV) source showed a clear photo-response. IV curves under the electron beam irradiation at 16 KeV (average energy emission of Ni63 beta source at 250 mCi/cm2) resulted in the output powers of 3.01 µW/cm2 with an efficiency of 12.63 % for the InGaP device, and 3.32 µW/cm2 with an efficiency of 13.2% for the GaN device.
InGaP and GaN devices were also exposed under a 4.5 MeV alpha beam to determine their suitability for an alphavoltaic power source (Direct energy conversion). Both InGaP and GaN devices showed degradation in their MPPs under the direct alpha beam exposure. We also investigated an indirect alpha-photovoltaic (APV) power source by employing ZnS phosphor as an intermediate layer to limit this degradation. This ZnS layer absorbs all the alpha energy and converts it into photons to create EHPs in the semiconductor device to generate electrical output power via indirect energy conversion. We determined that even though APV approach prevented radiation damage in the semiconductor device but the degradation rate of ZnS phosphor is faster compared to the degradation rate of GaN and InGaP devices under direct alpha beam exposure
