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28 February 2011 Nanofabrication of GaN surfaces by ultrashort laser pulses
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Proceedings Volume 7996, Fundamentals of Laser-Assisted Micro- and Nanotechnologies 2010; 79960G (2011)
Event: Fundamentals of Laser Assisted Micro- and Nanotechnologies 2010, 2010, St. Petersburg, Russian Federation
Wet-chemical-assisted femtosecond (fs) laser ablation has been developed for high-efficiency, high-quality nanofabrication of gallium nitride (GaN) surface. In this technique, an fs laser beam is focused on the surface of a single-crystal GaN substrate immersed in 35% hydrochloric (HCl) acid by a high-numerical-aperture objective lens. This method produced higher quality ablation craters with sharper edges, smoother surfaces, and less residual debris than conventional fs-laser ablation in air followed by etching in HCl (two-step processing method). This is presumably due to a photochemically or photothermally assisted chemical reaction associated with the HCl acid solution. Using the second harmonic of an fs laser (λ = 387 nm), nanocraters with diameters as small as 320 nm (full width at half maximum) were formed at a GaN substrate surface. Assuming that this process is based on two-photon absorption, we theoretically calculated the diameters of ablation craters. The calculation results agree well with the experimental results, particularly in the low-energy region. Multiscan irradiation was performed to fabricate a two-dimensional (2D) array of nanoholes with a high aspect ratio of ~1.6, which is much higher than that obtained using single-pulse irradiation. The aspect ratio was found to depend on the refractive index of the medium above the GaN substrate. The fabricated 2D array of nanoholes on a GaN surface could be used as a 2D photonic crystal for enhancing the light extraction efficiency of GaN-based blue LEDs.
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Seisuke Nakashima, Koji Sugioka, Takuma Ito, Hiroshi Takai, and Katsumi Midorikawa "Nanofabrication of GaN surfaces by ultrashort laser pulses", Proc. SPIE 7996, Fundamentals of Laser-Assisted Micro- and Nanotechnologies 2010, 79960G (28 February 2011);

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