Beta-gallium oxide (β-Ga₂O₃), a wide-bandgap semiconductor, has gained significant attention for high-power/high-temperature devices and deep-UV sensors due to its 4.8 eV bandgap. Nanowires, with their high surface-to-volume ratio, offer enhanced sensitivity for chemical and light detection compared to thin films. This research addresses the need for improved UV photodetectors (PDs) for various civil, military, environmental, and industrial applications demanding operation in harsh environments. β-Ga₂O₃ is an ideal candidate for visible-blind UV sensors due to its chemical stability and wide bandgap, and nanowire structures further enhance features such as responsivity and light absorption through effects like localized surface plasmons. This study focuses on a simple, inexpensive method to fabricate high-performance β-Ga₂O₃ nanowire UV photodetectors.

Existing literature highlights the advantages of β-Ga₂O₃ for UV detection, particularly in nanowire form. Studies have demonstrated enhanced responsivity and light absorption in nanowire-based UV photodetectors. The use of metal nanoparticles, such as those of silver, as catalysts or surface modifiers has been explored to further enhance performance. Previous research has reported on various growth methods and their effects on the optical and electrical properties of β-Ga₂O₃ nanowires, including bandgap variations. This work builds upon these findings, aiming to optimize a simple, cost-effective growth method and investigate the impact of a silver catalyst.

The study utilized a simple thermal oxidation method at 1000°C to grow β-Ga₂O₃ nanowires on quartz substrates. Quartz substrates were cleaned using standard procedures before a 5 nm silver thin film was deposited on some samples using a sputtering system. Samples were then placed in a quartz crucible within a tube furnace, exposed to 800°C or 1000°C for 60 min in a nitrogen flow with trace oxygen. After growth, electrical contacts were patterned using a shadow mask and sputtering of 5 nm Cr and 50 nm Au, or a gold mesh was used. Characterizations included scanning electron microscopy (SEM) for morphology, X-ray diffraction (XRD) for crystal structure, UV-Vis spectrophotometry for optical bandgap determination, secondary ion mass spectrometry (SIMS) for compositional depth profiling, scanning transmission electron microscopy (STEM) and high-resolution transmission electron microscopy (HRTEM) for nanowire structure and elemental mapping, and current-voltage (I-V) measurements under UV illumination for electrical characterization.

SEM imaging revealed that the Ag catalyst significantly enhanced nanowire growth rate and density. The nanowires grown with the Ag catalyst were longer, denser, and thinner (120-160 nm diameter) compared to those grown without (150-270 nm). XRD confirmed the nanowires were polycrystalline β-Ga₂O₃. The optical bandgap was determined to be 4.6 eV for Ag-free nanowires and 4.4 eV for those grown with the Ag catalyst, consistent with literature values but showing a slight shift due to the presence of silver. SIMS depth profiling indicated a non-uniform distribution of Ag within the nanowire forest, with higher concentration closer to the quartz surface. STEM and HRTEM analysis confirmed the presence of Ag nanoparticles (5-10 nm) at the interface of the β-Ga₂O₃ nanowires grown with the Ag catalyst. Electrical characterization using Au/β-Ga₂O₃/Au metal-semiconductor-metal (MSM) photoconductors demonstrated a significant enhancement in photocurrent with the Ag catalyst, achieving a photo-to-dark current ratio of 38.3 at 10 V compared to 4.55 for the Ag-free sample. The photoresponse was also faster with the Ag catalyst. Responsivity measurements showed improved performance with Ag across a broad UV range, with a solar-UV rejection ratio of approximately 50 (compared to 100 for the Ag-free sample). The enhanced performance is attributed to the surface plasmonic effect of Ag nanoparticles.

The results demonstrate that the simple thermal oxidation method, particularly when using a silver catalyst, effectively produces high-quality β-Ga₂O₃ nanowires suitable for UV detection. The silver catalyst significantly improves nanowire growth and enhances photocurrent response by several orders of magnitude. The presence of Ag nanoparticles induces localized surface plasmon resonance, leading to enhanced light absorption and carrier generation. The slightly lower bandgap observed in the Ag-catalyzed samples might be due to the interaction between silver and the Ga₂O₃ nanowires. The observed enhancement in photocurrent is consistent with the increased light absorption and improved carrier transport facilitated by the Ag nanoparticles. The differences in dark current between sputtered and mesh gold contacts highlight the importance of minimizing contact damage. The faster rise time observed in the Ag-catalyzed samples is attributed to enhanced carrier transport, while the similar fall times suggest that the recombination processes are not significantly affected by the presence of silver.

This study successfully demonstrates a simple and scalable method for producing high-performance β-Ga₂O₃ nanowire UV photodetectors. The use of a silver catalyst significantly enhances the growth process and improves the device performance, resulting in a substantially increased photo-to-dark current ratio and faster response time. Future work could investigate different catalyst materials and concentrations, explore the optimization of nanowire dimensions and densities, and investigate the long-term stability and reliability of these devices.

The study's characterization primarily focused on ensembles of nanowires, limiting the detailed investigation of individual nanowire properties. The SIMS analysis provided aggregate data due to instrumental limitations. The precise mechanisms underlying the observed bandgap shift and the effects of the Ag catalyst on carrier recombination require further investigation. The long-term stability and reliability of the devices over extended periods of operation were not assessed.