Gallium nitride LEDs are useful but suffer from a significant drawback: Their external quantum efficiency is low because the refractive index of GaN, which is 2.5, is very different from that of air, 1.0. Putting GaN on a sapphire substrate (refractive index 1.76) lessens the difference and increases the efficiency. Texturing the sapphire’s back side provides a further boost.Now a group of researchers from National Sun Yat-Sen University in Kaohsiung, Taiwan, has demonstrated a technique that produces a Fresnel microlens on the sapphire back side in a single step. The method, or variations of it, could improve the performance of GaN LEDs.Scanning electron photographs show Fresnel microlenses milled into the back side of a sapphire substrate with a focused ion beam at dose densities of 7.0 × 1017 ions/cm2 (a), 2.3 × 1018 ions/cm2 (b) and 3.5 × 1018 ions/cm2 (c). A close-up (d) shows more surface detail of the microlens in (b). Researchers used an array of Fresnel microlenses to increase the optical power output of a GaN blue LED (e). Reprinted with permission of the American Institute of Physics.The group used focused ion beam milling, in which a stream of gallium ions strikes a surface, milling it away at the point of impact. By moving the point, features can be carved into a surface. Although any structure can be built, in theory, the researchers chose to create Fresnel microlenses. A Fresnel lens, unlike other refractive focusing elements, is nearly flat, making it easier to integrate with other optical components. It also requires less milling than other possible optics.Peaks and valleysThe scientists fabricated a blue GaN LED, depositing the thin-film device on a sapphire substrate. They used a focused ion beam system from FEI Corp. of Hillsboro, Ore., moving the milling spot in an annular pattern while varying the beam energy to create a Fresnel microlens on the back side of the substrate (see figure). The resulting structure consisted of a series of peaks where the original material remained. The peaks were interspersed with valleys where the material had been removed. The depth of the valleys was less than 0.5 μm, and the width of the entire lens was about 30 μm. The researchers repeated the fabrication process, leading to a microlens array that stepped across the entire LED. They measured the result using a silicon photodiode from Otsuka Electronics of Osaka, Japan, comparing the output of an LED incorporating the microlens array to one without the lenses. They found the spectra to be about the same for the two, with a peak output around 460 nm. For an injection current of 20 mA, they found devices with the Fresnel microlens to be about 1.68 times as optically intense as those without.The investigators noted that focused ion beam milling allows the construction of complicated Fresnel microlenses without masks or other processing steps. The technique enables alterations in structure design, with a change in the program controlling the focused ion beam.The microlens array can increase the LED emission area and make the optical output power more uniform. Plans include fabricating a Fresnel lens on the LED side to enhance the output further. Other studies also are planned, noted professor of electrical engineering Ming-Kwei Lee.“The coupling efficiency between the LED and fiber and a package suitable for this kind of LED will be studied,” he said.Applied Physics Letters, July 30, 2007, Vol. 91, 051111.