© 2019 Elsevier Ltd n-GaN/p-GaAs heterojunctions were fabricated by plasma-assisted molecular beam epitaxy (PA-MBE). The fabrication was carried out by the growth of n-type cubic GaN on p-type GaAs(100) substrates, followed by Ohmic contact deposition using conventional e-beam and rapid thermal annealing. With the n-GaN/p-GaAs heterojunction we have obtained photovoltaic effect and demonstrated that this effect strongly depends of the GaN nucleating layer on the GaAs surface. Likewise, we found that the decrease of hexagonal-stable phase and planar defects inclusions in the cubic-metastable GaN decreases the leakage current, and therefore, increases the open circuit voltage (Voc). Furthermore, the increase in crystal quality also increases the External quantum efficiency (EQE) at longer wavelengths due to the reduction of surface recombination velocity at the GaN/GaAs interface. In addition, the heterojunctions were tested as radiation detectors, demonstrating that they could be a candidate for alpha particle detectors if the thickness of the absorber layer is increased.