The annealing of sputtered AlN films with different thicknesses grown on sapphire in nitrogen ambient was investigated. In the annealing, two AlN films on sapphire were overlapped “face-to-face” to ...suppress the thermal decomposition of the AlN films. The sputtered AlN films with small grains consisted of columnar structure were initially aligned with (0002) orientation but became slightly inclined with increasing film thickness resulting in the formation of a two-layer structure. After annealing, films became a single crystalline layer regardless of the film thickness, and their crystallinity markedly improved after annealing at 1600–1700°C. The full widths at half maximum of the (0002)- and (101¯2)-plane X-ray rocking curves were improved to 49 and 287 arcsec, respectively, owing to the annihilation of domain boundaries in the sputtered AlN films, which concurrently increased the compressive stress in the films.
•The “face-to-face” annealing of sputtered AlN films on sapphire in nitrogen ambient was investigated.•Crystallinity of annealed AlN films markedly improved after annealing at 1600–1700°C.•The FWHM of the (0002)- and (102)-plane X-ray rocking curves were improved to 49 and 287″, respectively.
•Improvement mechanism of sputtered AlN films by high-temperature was investigated.•A two-layer structure with different sizes and strain was in sputtered AlN films.•Columnar structure split into ...small domains and coalesced during annealing.•The polarity switched from N- to Al-polar in the growth direction owing to O2.
The improvement mechanism of sputtered AlN films by high temperature annealing in nitrogen ambient has been investigated. Sputtered AlN films were annealed at 1100–1700 °C and their microstructures were observed by scanning transmission electron microscopy. A two-layer structure consisting of columnar domains with different sizes was observed in the as-grown sputtered AlN films. The first layer with 10 nm thickness located at the AlN/sapphire interface, comprised columnar domains with diameters of nm order and was compressively strained owing to the lattice mismatch between AlN and sapphire. The diameter of columnar domains in the other layer was approximately 25 nm. The columnar domains split into irregularly shaped and coalesced at an elevated annealing temperature, resulting in improved crystal quality. When the annealing temperature was 1700 °C, the domain boundaries in AlN films were almost annihilated and the full width at half maximum of the (0 0 0 2)- and (1 0 −1 2)-plane X-ray rocking curves were improved to as low as 49 and 310 arcsec, respectively. The polarity switched from N-polar to Al-polar after about 4–10 AlN layers in the growth direction. The oxygen element content slightly increased at the polarity inversion boundary, which may have caused the polarity to switch.
•N-polar face-to-face annealed sputtered AlN was fabricated with Al sputtering target.•The controllability of the polarity by changing the sputtering target was demonstrated.•With increasing AlN film ...thickness, a reduction in the TDDs of N-polar FFA Sp-AlN was observed.
N-polar face-to-face annealed sputtered AlN (FFA Sp-AlN) was fabricated by sputtering with an Al metal target and high-temperature annealing in a face-to-face configuration. The polarities of the FFA Sp-AlN samples fabricated with different sputtering targets (i.e., Al metal or sintered AlN targets) were checked by KOH etching and cross-sectional scanning transmission electron microscope images. As a result, samples sputtered with a sintered AlN target and an Al metal target resulted in Al-polar and N-polar FFA Sp-AlN, respectively. Then, we fabricated FFA Sp-AlN with N-polar AlN on the top-most layers (N-polar FFA Sp-AlN) with different total film thicknesses. The threading dislocation densities (TDDs) of N-polar FFA Sp-AlN were estimated from the full width at half maximum values of the X-ray rocking curves. Consequently, the TDD of N-polar FFA Sp-AlN decreased with increasing AlN film thickness, which was the same trend as that of Al-polar FFA Sp-AlN. The minimum TDD of 1.7 × 108 cm−2 was obtained from N-polar FFA Sp-AlN with a total thickness of 730 nm.
Combination of sputter deposition and high-temperature annealing is a promising technique for preparing AlN templates with a low threading dislocation density (TDD) at a lower film thickness compared ...to those prepared by the conventional metalorganic vapor phase epitaxy. However, cracking of AlN films during annealing is a critical issue. In this study, we controlled the residual stress of the sputter-deposited AlN films by modifying the sputtering conditions. Consequently, the occurrence of cracking was effectively suppressed. By optimizing the fabricating conditions, a TDD of 2.07 × 108 cm−2 was achieved for the AlN template with a thickness of 480 nm.
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.
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•The toxicity and safety of UVC irradiation in living organisms were investigated.•Zebrafish embryos at the early developmental stage (5–6 h post-fertilization) survived after 4.5 mJ/cm2 of UVC irradiation.•Zebrafish embryos respond quickly to UVC-induced stress via activated p53 signaling pathway.•Zebrafish embryos showed activated STAT signaling pathway after three days of recovery from UVC irradiation.
•Strain-relaxation of annealed sputtered AlN film on c-plane sapphire was revealed.•Sputtered AlN films before annealing have different strains and tilt mosaics.•The AlN film after annealing consists ...of a layer that have a compressive strain.•After MOVPE growth, the compressive strain was inherited.
The strain relaxation of sputter-deposited AlN (sp-AlN) films on c-plane sapphire substrates before and after face-to-face annealing (FFA) was evaluated. After FFA, the AlN films consisted of a layer with compressive strain and an extremely low dislocation density, which was ascribed to solid-state growth during FFA. In addition, the crystallinity was further improved after the homoepitaxial growth of AlN by metal-organic vapor phase expitaxy (MOVPE). The full widths at half maximum of the X-ray rocking curves of AlN (0 0 0 2) and (1 0 −1 2) for the MOVPE-grown AlN layer on the FFA-sp-AlN film were 15 and 240 arcsec, respectively. The surface morphology of the MOVPE-grown AlN layer on the FFA-sp-AlN film was covered with an atomically flat step-and-terrace structure. The compressive strain of the MOVPE-grown AlN layer was inherited from the underlying FFA-sp-AlN film.
This study comprehensively investigates the properties of metalorganic vapor phase epitaxy (MOVPE)‐grown AlN films on high‐quality face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) templates on ...sapphire substrates, which are highly important to control the surface morphology for various applications, such as UV light‐emitting diodes and laser diodes. The conditions of thermal cleaning and AlN growth by MOVPE are investigated to remove numerous small islands on as‐annealed FFA Sp‐AlN. Subsequent to thermal cleaning in H2 + NH3 at 1300 °C, MOVPE growth is performed with varying NH3 flow rate and growth temperature (Tg) under a constant pressure and group‐III flow rate. An atomically flat surface with an atomic step‐and‐terrace structure is obtained at a growth rate of ≈1.0 μm h−1 and a Tg of 1300 °C. Transmission electron microscopy images and secondary‐ion mass spectrometry reveal low dislocation densities and impurity concentrations. Finally, the effects of compressive strain in FFA Sp‐AlN on the lattice constant and curvature of the MOVPE‐grown AlN film on FFA Sp‐AlN are investigated. The compressive strain of the AlN film, which is carried over from FFA Sp‐AlN, can prevent crack formation but leads to a large wafer curvature after cooling down from the Tg.
Properties of a face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) template on a sapphire substrate and metalorganic vapor phase epitaxy (MOVPE)‐grown AlN films on FFA Sp‐AlN templates are comprehensively investigated. As a result, the AlN films on FFA Sp‐AlN templates exhibit superior properties than those on the conventional AlN templates directly MOVPE‐grown on sapphire substrates.
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.
•Rare UV-resistant cells exist in an E. coli clonal population.•The characters of these UV-resistant cells are similar to persister cells.•Intermittent UV irradiation significantly reduced viable E. coli cells.
The annealing of an AlN buffer layer in a carbon-saturated N2-CO gas on a sapphire substrate was investigated. The crystal quality of the buffer layer was significantly improved by annealing at ...1650-1700 °C. An AlN buffer layer with a thickness of 300 nm was grown by metalorganic vapor phase epitaxy (MOVPE), and was annealed at 1700 °C for 1 h. We fabricated a 2-µm-thick AlN layer on the annealed AlN buffer layer by MOVPE. The full widths at half maximum of the (0002)- and ()-plane X-ray rocking curves were 16 and 154 arcsec, respectively, and the threading dislocation density was 4.7 × 108 cm−2.
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