Design optimization of GaAs betavoltaic batteries

GaAs junctions are designed and fabricated for betavoltaic batteries. The design is optimized according to the characteristics of GaAs interface states and the diffusion length in the depletion region of GaAs carriers. Under an illumination of 10 mCi cm^{-2 63}Ni, the open circuit voltage of the optimized batteries is about ∼0.3 V. It is found that the GaAs interface states induce depletion layers on P-type GaAs surfaces. The depletion layer along the P⁺PN⁺ junction edge isolates the perimeter surface from the bulk junction, which tends to significantly reduce the battery dark current and leads to a high open circuit voltage. The short circuit current density of the optimized junction is about 28 nA cm^-2, which indicates a carrier diffusion length of less than 1 μm. The overall results show that multi-layer P⁺PN⁺ junctions are the preferred structures for GaAs betavoltaic battery design.