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Abstract

III-nitrides are well-known and highly promising semiconductor materials that have been widely used in electronic and optoelectronic devices, including light-emitting diodes, laser diodes, high-electron-mobility transistors, and photodetectors. Beyond these established applications, III-nitrides have also emerged as promising materials for photoelectrochemical (PEC) devices, which are attractive for solar-driven production of clean energy and chemicals. In addition, PEC devices have recently attracted interest as photodetectors operating in aqueous environments, giving rise to a new class of devices known as PEC photodetectors (PEC-PDs).

This thesis focuses on developing III-nitride materials for PEC applications. First, PEC-PDs have been investigated. Although significant progress has been made in III-nitride dual-wavelength-distinguishable PEC-PDs, most previously reported devices have been limited to the ultraviolet (UV) range. However, the highest transparency window in aqueous environments lies in the blue region, making dual-wavelength detection in the long-UV and blue ranges particularly important. Moreover, while self-powered PEC-PDs have previously been demonstrated, they have been limited to single-wavelength operation. Motivated by this, we first demonstrate a dual-wavelength distinguishable PEC-PD capable of detecting both long-UV and blue light. We have then identified the main barrier to self-powered operation in dual-wavelength-distinguishable PEC-PDs: the built-in electric field at the interface of the p-n heterojunction. Through proper band engineering using a tunnel junction, this limitation was overcome, enabling the first self-powered dual-wavelength-distinguishable PEC-PD. High responsivities in the mA/W range and ultrafast response times of less than 10 ms are achieved under self-powered operation.

This thesis also demonstrates, for the first time, the potential of emerging Sc-III-nitrides for PEC applications. Although Sc-III-nitrides are mainly known for their ferroelectric properties, Sc incorporation also shifts both the conduction and valence band edges upward while only slightly changing the bandgap. This provides higher-energy electrons without sacrificing light absorption, which is advantageous for photocatalytic reduction reactions. Based on this, ScGaN nanowires were grown by molecular beam epitaxy and demonstrated significantly enhanced photocatalytic CO2 reduction performance compared with GaN. Finally, to further expand PEC applications of Sc-III-nitrides, particularly for reduction reactions, p-type doping of ScGaN nanowires was investigated. For the first time, p-type doping was achieved in ScGaN nanowires and confirmed by multiple characterization techniques, with doping concentrations in the range of 10^18–10^19 cm^-3.