Two 400 um diameter Al_0.52In_0.48P p⁺-i-n⁺ mesa photodiodes (6 um i layer) were fabricated from a wafer grown by metalorganic vapour phase epitaxy (MOVPE) and then studied at temperatures from 140 °C to -20 °C for the development of temperature tolerant ⁵⁵Fe x-ray photovoltaic and ⁶³Ni betavoltaic microbatteries. The Al_0.52In_0.48P epitaxial layers are the thickest so far reported for this emerging application. At each temperature, the performances of the Al_0.52In_0.48P detectors were analysed in dark conditions, as well as under the illumination of a 182 MBq ⁵⁵Fe radioisotope x-ray source and a 185 MBq ⁶³Ni radioisotope source. An open circuit voltage as high as 1.13 V was found for both the Al_0.52In_0.48P x-ray photovoltaic cells at -20 °C; whilst an open circuit voltage of 0.47 V was found for the best ⁶³Ni betavoltaic cell, at the same temperature. Maximum output powers of 1.44 and 1.36 pW were obtained from the two x-ray photovoltaic cells at -20 °C; combining the output powers of these two Al_0.52In_0.48P x-ray photovoltaic cells, a total maximum output power as high as 2.8 pW could be obtained at -20 °C. Maximum output powers of 0.18 pW and 0.13 pW were instead extracted from the two betavoltaic cells at -20 °C, these could lead to a total maximum output power as high as 0.3 pW at -20 °C. Conversion efficiencies of 2.2% and 0.06% were found, respectively, for the best Al_0.52In_0.48P x-ray photovoltaic and betavoltaic cells at -20 °C. With respect to previously reported Al_0.52In_0.48P x-ray photovoltaic cells with thinner i layers, the 6 um Al_0.52In_0.48P x-ray photovoltaic cells had higher short circuit current, open circuit voltage, maximum output power, and conversion efficiency. The 6 um Al_0.52In_0.48P betavoltaic cells instead presented similar performances to previously analysed Al_0.52In_0.48P betavoltaic cells.

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