Radio-voltaic cell is a kind of nuclear micro-battery, directly converting ionizing radiations (alpha, beta or gamma) emitted by long-life radioisotopes into electric energy using semiconductor transducers. With the rapid development of nuclear science and technology, radio-voltaic cells have emerged as ideal energy sources for unattended miniaturized systems used in terrestrial, deep sea, and space environments, owing to their advantages such as small size, expected long service life, output stability, high environmental adaptability, and no need for external energy input or maintenance. In general, radio-voltaic cells can be divided into beta-voltaic cells and alpha-voltaic cells according to the type of radioisotope. The beta-voltaic cells convert the energy of β particles into electricity, and they can realize long lifetime. However, there are some limitations such as low upper limit of output power and unsatisfactory power conversion efficiency owing to the energy self-absorption of low-energy β radioisotopes (³H, ⁶³Ni, etc.) and poor energy deposition rate in transducer for high-energy β radioisotopes (⁹⁰Sr-⁹⁰Y, ¹⁴⁷Pm, etc.). The emitted energy of α radioisotopes is much higher than that of β radioisotopes, and hence α radioisotopes can significantly improve the energy deposition rate in the transducer. Therefore, the development of alpha-voltaic cells is an important direction to break through the bottleneck of beta-voltaic cells.

However, the PCE of alpha-voltaic cells is much lower than the theoretical value, and the lifetime is still inadequate for applications due to the poor radiation resistance of the semiconductor transducers. Limited by the insufficient radiation tolerance of most of the mature traditional semiconductors, Si, Ge and GaAs cannot maintain the long-term stable operation of AV cells. Thus, transducers based on wide bandgap semiconductors with ultrahigh radiation tolerance and excellent stability under harsh environments are being actively pursued. At present, silicon carbide (SiC) and diamond transducers with PN junction, Schottky junction and PIN junction have been fabricated and used for several AV cell prototypes. With the rapid improvement of gallium nitride (GaN) technology, GaN is also being considered as a valuable transducer material for radio-voltaic cells because of its large displacement energy (45 eV for Ga and 109 eV for N), high stopping power for α-particles and excellent electrical properties, which is expected to achieve a strong radiation tolerance and superior output performance. Furthermore, the price of GaN transducers on heteroepitaxial substrates is significantly lower than the electronic-grade single-crystal diamond. In recent years, GaN beta voltaic cells have been widely investigated by several research groups, attaining valuable experimental and theoretical results. However, GaN alpha voltaic cells have not been reported yet, as well as the quantitative evaluation of service life is still unclear.

In this study, an alpha-voltaic cell based on a GaN transducer with PIN structure is designed and investigated for the first time, and the mechanism of improving the output characteristics of GaN alpha-voltaic cell have been revealed. We find that isoelectronic aluminium-doping is an effective way for boosting the performance of the gallium nitride transducer by decreasing the unintentional doping concentration, deep trap concentration, and dislocation density in the GaN epilayer. The isoelectronic aluminium-doped cell demonstrates a large depletion region of 1.89 μm and a charge collection efficiency of 61.6% at 0 V bias, resulting in a high PCE of 4.51%, comparable to the best GaN beta voltaic cells, as well as the highest among reported alpha voltaic cells. In addition, performance degradation of GaN transducer after He ions irradiation at different fluences has been systematically studied and the lifetime of GaN alpha voltaic cell has been evaluated.

Our work explores the key technologies of alpha-voltaic cells based on irradiation-resistant PIN type GaN wide bandgap semiconductor transducers. And the electrical performance calibration and stability evaluation of GaN alpha voltaic cells were carried out, which provided theoretical basis and data support for realizing the electrical performance, effective life and working stability of alpha-voltaic cells. This work demonstrates an effective way for boosting the performance of GaN alpha voltaic cells, which may promote the application of nuclear micro-batteries in extreme environments.

In summary, the research objective of high-performance micro nuclear battery technology is to realize high PCE and long life. After reviewing the literature on alpha voltaic cells, it can be concluded that even with highly anti-irradiation transducers, performance degradation caused by radiation damage cannot be avoided. Therefore, the most effective option is to improve PCE by optimizing the design and processing technology of the energy conversion transducer. In order to achieve higher PCE, the most effective method is to further improve the transducer's sensitive region width and charge collection efficiency. In the future, the PCE and lifetime of GaN alpha voltaic cell is expected to be further improved by optimizing the structure and technological process of GaN transducer. This work demonstrates an effective way for boosting the performance of GaN alpha voltaic cells, which may promote the application of nuclear micro-batteries in extreme environments.