Simulation and optimization of current generation in gallium phosphide nanowire betavoltaic devices

The geometry of a gallium phosphide nanowire (NW) array has been optimized for maximum current generation in a betavoltaic (BV) device. The energy capture efficiency for various device geometries with different radioisotope source compounds was calculated in GEANT4. A validation of GEANT4 for BV device simulation was performed by comparing a model output with the available bulk semiconductor BV performance data, followed by predictions of the performance of NW-based devices. The pitch and the diameter of the NWs were found to have the most significant impact on the β-generated current density, with the optimum diameter-to-pitch ratios ranging from 0.55 to 0.8, depending on the source. The energy capture efficiency improved when low energy beta (β) emitters were used. For devices utilizing ⁶³Ni source compounds, the β-generated current densities approached 0.95 μA cm⁻², representing an improvement by a factor as high as 5.8 compared to planar devices. In the case of ³H source compounds, the generated current density was 3.05 μA cm⁻², a factor of 15.5 larger than comparable planar devices. However, NW devices utilizing sources with a higher decay energy, such as ¹⁴⁷Pm, did not demonstrate any improvements over planar geometries. Using the results for optimum NW geometries, NW-based or other nanostructured devices could be made to surpass the present commercial BV batteries.