Micro- and Nanostructure Analysis of Vapor-Phase-Grown AlN on Face-to-Face Annealed Sputtered AlN/Nanopatterned Sapphire Substrate Templates.

Micro- and nanostructures in vapor-phase-grown AlN on face-to-face annealed sputtered AlN (FFA Sp-AlN) templates formed on nanopatterned sapphire substrates (NPSS) were comprehensively analyzed using transmission electron microscopy. The comparison between metal-organic vapor-phase epitaxy-grown AlN/FFA Sp-AlN/hole-type NPSS (Sample MOH) and hydride vapor-phase epitaxy-grown AlN/FFA Sp-AlN/cone-type NPSS (Sample HVC) showed apparent differences in the morphology of dislocation propagation, presence of voids, shape of polarity inversion boundaries, and crystal structure on the slope region of NPSS. Notably, cross-sectional and plan-view observations revealed that the quality of FFA Sp-AlN significantly affects the threading dislocation density in the vapor-phase-grown layer. At the slope region of the AlN/NPSS interface, γ-AlON was observed in the MOH sample, while highly misaligned AlN grains were observed in the HVC sample. These characteristic crystal structures affect the occurrence of dislocations via different mechanisms in each sample. This study provides practical information for strategically controlling the micro- and nanostructures formed in AlN/NPSS structures for high-performance AlGaN-based deep-ultraviolet emitters. Supplementary Information: The online version contains supplementary material available at 10.1007/s11664-023-10348-3.

Molecular beam homoepitaxy of N-polar AlN: Enabling role of aluminum-assisted surface cleaning.

N-polar aluminum nitride (AlN) is an important building block for next-generation high-power radio frequency electronics. We report successful homoepitaxial growth of N-polar AlN by molecular beam epitaxy (MBE) on large-area, cost-effective N-polar AlN templates. Direct growth without any in situ surface cleaning leads to films with inverted Al polarity. It is found that Al-assisted cleaning before growth enables the epitaxial film to maintain N-polarity. The grown N-polar AlN epilayer with its smooth, pit-free surface duplicates the structural quality of the substrate, as evidenced by a clean and smooth growth interface with no noticeable extended defects generation. Near band-edge photoluminescence peaks are observed at room temperature on samples with MBE-grown layers but not on the bare AlN templates, implying the suppression of nonradiative recombination centers in the epitaxial N-polar AlN.

Rare UV-resistant cells in clonal populations of Escherichia coli.

Water disinfection is one of the most important applications of ultraviolet light-emitting diodes (UV-LEDs), though bacterial regrowth remains a serious problem. In this study, we showed that UV-resistant cells, though rare, exist in an Escherichia coli clonal population. The UV-resistance of stationary phase cells was higher than that of exponential phase cells. Regrowth cell populations showed identical UV sensitivity before and after UV treatment, indicating that UV resistance is not acquired genetically, but is generated stochastically. The characteristics of these UV-resistant cells are similar to those of non-heritable antibiotic-resistant cells, termed persisters. The induction of persister formation increased the number of viable cells after UV treatment. The toxin-antitoxin system gene hipA (high persistence A) is a key factor in persister cell formation. We observed that hipA was strongly expressed in the stationary phase cells, while regrowth cells after UV treatment lost hipA expression, suggesting that the regrowth cells lost their persistence. Compared to UV batch radiation, we demonstrated that intermittent UV irradiation, which included the induction of regrowth between UV treatments, significantly reduced the number of viable E. coli cells.

Transcriptome analysis of molecular response to UVC irradiation in zebrafish embryos.

Ultraviolet (UV) rays can be both harmful and beneficial to humans. This study aimed to investigate the toxicity and safety of ultraviolet C (UVC) exposure in living organisms and the corresponding biodefense molecular mechanisms. Zebrafish embryos, at an early developmental stage (5-6 h post-fertilization), were irradiated with increasing UVC dosages using high-efficiency deep-ultraviolet light-emitting diodes (278 nm). Morphological phenotypes including survival rate, hatching rate, heart rate, and malformation rate were evaluated. Compared to un-irradiated controls, all zebrafish embryos exposed to 4.5 mJ/cm2 UVC survived and showed no significant difference in hatching and heart rate. However, 7.5 mJ/cm2 of UVC irradiation caused a significantly decreased survival rate (37.5%) and an increased malformation rate (81.8%). Therefore, 4.5 mJ/cm2 was chosen as the limit dosage that the internal biodefense system of zebrafish embryos can protect against UVC radiation. Transcriptome analysis (RNA sequencing) performed on 3 min and 3 days post-irradiation embryos (4.5 mJ/cm2) revealed the molecular mechanisms underlying the response of zebrafish embryos to irradiation. The embryos quickly responded to UVC-induced stress by activating the p53 signaling pathway. In addition, after 3 days of recuperation, the embryos showed activation of signal transducer and activator of transcription (STAT) signaling pathway. To our knowledge, this is the first study to evaluate the toxicological effects and the molecular mechanism of biodefense in zebrafish embryos upon 278 nm UVC irradiation.

Improvements in structural and optical properties of wafer-scale hexagonal boron nitride film by post-growth annealing.

Remarkable improvements in both structural and optical properties of wafer-scale hexagonal boron nitride (h-BN) films grown by metal-organic chemical vapor deposition (MOCVD) enabled by high-temperature post-growth annealing is presented. The enhanced crystallinity and homogeneity of the MOCVD-grown h-BN films grown at 1050 °C is attributed to the solid-state atomic rearrangement during the thermal annealing at 1600 °C. In addition, the appearance of the photoluminescence by excitonic transitions as well as enlarged optical band gap were observed for the post-annealed h-BN films as direct consequences of the microstructural improvement. The post-growth annealing is a very promising strategy to overcome limited crystallinity of h-BN films grown by typical MOCVD systems while maintaining their advantage of multiple wafer scalability for practical applications towards two-dimensional electronics and optoelectronics.