Gallium oxide-based photodetectors for water quality monitoring

We present an approach to water quality monitoring using gallium oxide (Ga2O3) ultra-wide bandgap semiconductors. Nitrates, dissolved organic carbon and suspended solid concentrations are three commonly measured water quality parameters, that display optical absorption ranging from the deep ultraviolet to the visible region. This broad spectral region poses a challenge for accurate and efficient (simultaneous) measurement of absorption/extinction arising from varying concentrations of these parameters, because silicon (Si), the classical detector material has poor performance across this optical region. To overcome these limitations, we propose the use of ultra-wide bandgap semiconductors to trace changes in optical absorption from varying water compositions by measuring photocurrent response at different wavelengths. Here we use α-phase Ga2O3 as a suitable material to measure a broad photocurrent response ranging from 200 nm to 465 nm. The photocurrent response consisted of three well-defined regions inherently linked to the rich electronic landscape of the material. Region (i) (200-250 nm) corresponds to band-to-band excitation of charge carriers, aligning well with the absorption characteristics of nitrates. Region (ii) (250-350 nm) corresponds to band tail related transitions, allowing a photocurrent response to dissolved organic carbon concentrations. Finally, we utilise defect-mediated transitions in Region (iii) (350-465 nm) to monitor suspended solid concentrations. It was observed here that the sensitivity of the photocurrent response to changing water composition strongly depends on the excitation wavelength, where 225 nm, 260 nm, and 465 nm excitation yielded (for our setup) the best results for the monitoring of nitrates, dissolved organic carbon and suspended solid concentrations, respectively.