Ch Leung

Photonic crystal light-emitting diodes fabricated by microsphere lithography

Instead of using conventional electron lithography, a two-dimensional photonic crystal consisting of a hexagonal array of triangular air-holes was created on the surface of a GaN LED substrate using microsphere lithography. The microspheres self-assemble into a single-layered hexagonal-close-packed array acting as an etch mask. A significant enhancement in photoluminescence intensity was recorded from the PhC LED structure. A twofold increase in electroluminescence was observed from the PhC LED compared to an as-grown LED with identical geometry. Besides geometrical factors due to surface roughening, the dispersive nature of PhCs and diffractive properties of the PhC as a grating contribute to the enhancement of light extraction from the LED.

Polychromatic light-emitting diodes with a fluorescent nanosphere opal coating

A hexagonally close-packed array consisting of fluorescent nanospheres was coated onto short-wavelength GaN light-emitting diodes to demonstrate polychromatic white light emission. The spherical particles self-assemble into ordered three-dimensional opal structures, performing the role of color conversion to generate a polychromatic spectrum with smooth and uniform emission patterns. Different ratios of green and orange-red fluorescent nanospheres were mixed and coated onto high-extraction-efficiency micro-LEDs. Four devices with different shades of white emission were demonstrated. Device A, with a high content of orange-red nanospheres, offers the highest CRI value of 80, whereas device C with a well-balanced ratio of green and orange-red nanospheres exhibits color characteristics closest to ideal white with CIE coordinate at (0.34, 0.34). At 20xa0mA driving current, the luminous efficacy of the devices A, B, C, and D are 40.5xa0lmxa0W(-1), 57.7xa0lmxa0W(-1), 63.1xa0lmxa0W(-1), and 67.2xa0lmxa0W(-1) respectively, while the correlated color temperatures (CCTs) of the corresponding devices are 3587, 4778, 5271, andxa013u2009000xa0K.

Nanostructuring GaN using microsphere lithography

The authors report on the fabrication and characterization of nanopillar arrays on GaN substrates using the technique of microsphere lithography. Self-assembled hexagonally packed silica microsphere arrays were formed on GaN wafers by spin coating and tilting. By precision control of process parameters, a monolayer can be formed over a wide region. The silica microspheres act as a hard mask for pattern transfer of the nanostructures. After dry etching, arrays of nanopillars were formed on the surface of the wafer. The ordered nanostructures can be clearly seen in the scanning electron microscopy images, while photoluminescence measurements revealed a twofold enhancement of light emission intensity.

Nanostructures on GaN by Microsphere Lithography

The fabrication of nanopores on GaN substrates using the simple and economical technique of microsphere lithography is demonstrated. A self-assembled hexagonal microsphere array formed on GaN substrate acts as a hard-mask for the fabrication of nanostructures. In subsequent dry etch processes, arrays of holes in a nanostructure were formed on top of the LED. The structural properties of the nanopores are characterized by scanning electron microscopy (SEM), while photoluminescence (PL) measurements showed a 25% enhancement of light emission intensity, attributed to improved light extraction.