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Nanopatterned sapphire substrates were fabricated by annealing of patterned Al thin films. Square-patterned Al thin films with the diagonal length of 600 nm, period of 1 um and height of ∼200 nm were obtained by the Laser Interference Lithography and Reactive Ion Etching. Patterned Al thin films were subsequently subjected to dual stage annealing due to the melting temperature of Al thin films (660 C). The first comprised a low temperature oxidation anneal. The hillocks formation on Al thin films was minimized with an oxidation annealing at 450 C for 24 h. The little change in the morphology of patterned Al thin films was observed at 450 C for 24 h. This was followed by a high temperature annealing to induce growth of the underlying sapphire single crystal to consume the oxide layer. The SEM results show the patterns were retained on sapphire substrates after high temperature annealing at less than 1200 C. The XRD and Raman results reveal that the orientation of island patterns by dual stage annealing of patterned Al thin films for 24 h at 450 C, and 1 h at 1000 C, was the same as that of the sapphire (0001) substrates. [ABSTRACT FROM AUTHOR]

Large-scale nanopatterned sapphire substrates were fabricated by annealing of patterned Al thin films. Patterned Al thin films were obtained by soft UV-nanoimprint lithography and reactive ion etching. The soft mold with 550-nm-wide lines separated by 250-nm space was composed of the toluene-diluted polydimethylsiloxane (PDMS) layer supported by the soft PDMS. Patterned Al thin films were subsequently subjected to dual-stage annealing due to the melting temperature of Al thin films (660°C). The first comprised a low-temperature oxidation anneal at 450°C for 24 h. This was followed by a high-temperature annealing in the range of 1,000°C and 1,200°C for 1 h to induce growth of the underlying sapphire single crystal to consume the oxide layer. The SEM results indicate that the patterns were retained on sapphire substrates after high-temperature annealing at less than 1,200°C. Finally, large-scale nanopatterned sapphire substrates were successfully fabricated by annealing of patterned Al thin films for 24 h at 450°C and 1 h at 1,000°C by soft UV-nanoimprint lithography.

We report on the theoretical and the experimental investigations of the coherent phonon dynamics in sapphire crystal using the femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) technique. The temporal chirped white-light continuum (WLC) is used for the Stokes pulse, therefore we can perform the selective excitation of the phonon modes without using a complicated laser system. The expected quantum beat phenomenon is clearly observed. The theoretical formulas consist very well with the experimental results. The dephasing times of the excited phonon modes, the wavenumber difference, and the phase shift between the simultaneously excited modes are obtained and discussed. This work opens up a way to study directly high-frequency coherent phonon dynamics in bulk crystals on a femtosecond time scale and is especially helpful for understanding the nature of coherent phonons. [ABSTRACT FROM AUTHOR]

Abstract: Sapphire single crystals grown by an improved Kyropoulos-like method are irradiated by fast neutron flux. The irradiated doses of neutron are 1018 and 1019 n/cm2. The infrared transmission spectra of sapphire were studied before and after irradiation. The irradiated samples were annealed at 200, 400, 600, 800 and 1000°C for 10min in ambient atmosphere. Positron annihilation studies have been carried out before and after neutron irradiation. The experimentally measured positron lifetime in the pristine specimen is 143ps. There were aluminum vacancies produced in sapphire crystals after neutron irradiation. The positron lifetime increased with the dose of neutron flux. A longer value τ 2 was found after annealing at 600°C, which indicated vacancies were aggregated with each other. The second long-time component τ 2 has been found to increase with the annealing temperature. There was almost no change in peak position of the CDB spectra after neutron irradiation and isothermal annealing. The chemical environment of core in sapphire did not change greatly after neutron irradiation. [Copyright &y& Elsevier]

In this paper, the technology of large size sapphire grown by SAPMAC method is elaborated and the quality of the obtained crystal has been inspected and analyzed. Results indicate that the crystal structure is complete and the FHWM value is only 11 arcsec. The defect concentration is in the 10²/cm² level, the purity is higher than 99.995%, and the transmission in 3 - 5 µm band is greater than 85%. Ultrasonic vibration machining is used to process the large size sapphire and a special grinding composite cutting tool is designed to improve its efficiency. [ABSTRACT FROM AUTHOR]

Compared with conventional anti-reflective film, an anti-reflective sub-wavelength surface structure provides an ideal choice for a sapphire optical window especially in harsh environments. However, it is still a challenge to obtain a sapphire anti-reflective surface microstructure because of its high hardness and chemical inertness. In this paper, combined with optical simulation, we proposed a facile method based on the anodic oxidation of aluminum film and following epitaxial annealing. Al thin film was deposited on a sapphire substrate by magnetron sputtering, and anodic oxidation was then performed to prepare surface pore-like structures on the Al film. Followed by two-step annealing, both the anodic oxidized coating and underlying unoxidized Al film were transformed totally into alumina. The parameters of anodic oxidation were analyzed to obtain the optimal pore-like structures for the antireflection in the mid-infrared and visible spectrum regions, respectively. Finally, the optimized surface sub-wavelength nanostructure on sapphire can increase the transmittance by 7% in the wavelength range of 3000–5000 nm and can increase 13.2% significantly for visible spectrum region, respectively. Meanwhile, the surface wettability can be also manipulated effectively. The preparation of surface pore-like sub-wavelength structure by the annealing of anodic oxidized aluminum film on sapphire is a feasible, economical and convenient approach and can find the applications for various optoelectronic fields. [ABSTRACT FROM AUTHOR]

GaN nanowires were synthesized on sapphire substrates by chemical vapor deposition. The selective growth of GaN nanowires was obtained through an N-Ga2O3 layer prepared by radio frequency magnetron sputtering. The X-ray diffraction (XRD) and Raman measurements indicated the GaN nanowires to be all indexed to the hexagonal wurtzite structure. The photoluminescence (PL) spectra were composed of a strong UV emission peak (365nm) and a weak yellow luminescence (YL) band (~600nm). The selective growth mechanism of GaN nanowires was briefly discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] N2 gas flux affects the microstructure, surface morphology and optical properties. [•] The films have better crystallization quality after incorporating properly. [•] The photoluminescence spectra include ultraviolet and green emission parts. [•] The optical band gap decreases with increasing the N2 gas flux. [ABSTRACT FROM AUTHOR]

Abstract: ZnO thin films were prepared on sapphire (0001) substrates by the dip-coating sol–gel technique. Then, ZnO thin films with thicknesses of 50nm, 150nm, 250nm, and 350nm were annealed at 800°C and with thickness of 250nm were annealed at 600°C, 700°C, 800°C, and 900°C, respectively. The effect of film thickness and annealing temperature on the structure and optical properties of sol–gel ZnO thin films on sapphire substrates were investigated by XRD, SEM, RT-PL. All the sol–gel ZnO thin films show polycrystalline hexagonal wurtzite structure and a high preferential c-axis orientation. The XRD and SEM results show that the better structural quality, high c-axis preferred orientation, uniform, compact sol–gel ZnO thin films 250nm thick were obtained when annealed at 700°C in air. Room-temperature PL spectrum of sol–gel ZnO thin films can be divided into the UV emission and the visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the visible broadband emission can be ascribed to the deep level emissions (DLE). The visible emission is suppressed with the annealing temperature of 600–700°C. By analyzing our experimental results, we concluded that the deep-level emission corresponds to oxygen vacancies (V O ). The biggest ratio of the PL intensity of UV emission to that of visible emission (I NBE/I DLE) is observed from sol–gel ZnO thin films 250nm thick annealed at 700°C. Therefore, we suggest that film thickness of 250nm and annealing temperature of 700°C are the most suitable conditions for obtaining high quality, high c-axis preferred orientation, uniform, compact sol–gel ZnO thin films with good luminescence performance. [Copyright &y& Elsevier]

Abstract: ZnO thin films were epitaxially grown on sapphire (0001) substrates by radio frequency magnetron sputtering. ZnO thin films were then annealed at different temperatures in air and in various atmospheres at 800°C, respectively. The effect of the annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films are investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL). A strong (002) diffraction peak of all ZnO thin films shows a polycrystalline hexagonal wurtzite structure and high preferential c-axis orientation. XRD and AFM results reveal that the better structural quality, relatively smaller tensile stress, smooth, uniform of ZnO thin films were obtained when annealed at 800°C in N2. Room temperature PL spectrum can be divided into the UV emission and the Visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the Visible broad band emission can be ascribed to the deep level emissions (DLE). By analyzing our experimental results, we recommend that the deep-level emission correspond to oxygen vacancy (VO) and interstitial oxygen (Oi). The biggest ratio of the PL intensity of UV emission to that of visible emission (I NBE/I DLE) is observed from ZnO thin films annealed at 800°C in N2. Therefore, we suggest that annealing temperature of 800°C and annealing atmosphere of N2 are the most suitable annealing conditions for obtaining high quality ZnO thin films with good luminescence performance. [Copyright &y& Elsevier]