It's All About The Chemistry:

    Blue LEDs have been getting much attention over the past few years because their production had researchers stumped. How did they finally produce blue LEDs? This article will discuss some of the interesting details of blue diode fabrication.

    InGaN films with a high amount of indium are needed to produce blue LEDs. Gallium nitride platelets can be used but is difficult to fabricate. Some other choices are sapphire, aluminum nitride (AIN) and silicon carbide (SiC). The major reason sapphire would be abandoned is the dislocation density created by a large lattice mismatch when using gallium nitride. AlN is more desirable because it matches the crystalline structure of GaN. One of the advantages of using SiC is it draws heat from the PN junction at ten times the rate of sapphire, and SiC's natural cleave plane can channel energy from a blue emitting device. Despite this, sapphire is used simply because it is widely available, needs no special equipment to use, and needs no complex cleaning procedures before growth is begun.

    The problem with the lattice mismatch occurs between the substrate and active layers of the PN junction, but lateral epitaxial overgrowth is the desired method to minimize these dislocation defects. Lateral epitaxial overgrowth is a process where parts of the sapphire substrate are masked and then GaN is grown on top of it. Ultraviolet photolithography and wet chemical etching are used to partially remove the mask before the GaN can be grown. (However, wet etching is slow but good for reducing density).

    After the correct chemicals have been determined and the epitaxial process has been done, crystals must be grown so the blue diode can finally be used. The first step is making the buffer layer. It is grown after the substrate wafers are etched in buffered HG for one minute. This takes place at 900 degrees C using the Metal Organic Chemical Vapor Deposition (MOCVD) process. The GaN starts to come together through a nucleation stage of epitaxial growth. The third step is geometric selection, then the small nuclides fuse to form an island. Then, the islands expand to cover the buffer area. Lastly, they form crystalline planes. Once this is made, it is doped into the P/N junction.

     The brightest LEDs are either single or multiple quantum well structures and consists of a low temperature GaN or AlN buffer layer on a c-plane sapphire or silicon-doped GaN layers. Blue LEDs are III-V nitride products that are based on either a SQW or MQW structure, which vastly improves the efficiency and color purity. The width of the spectrum for these lamps is narrower than the Zn-Si co-doped LEDs before them, which makes much better color saturation. With this technology, there are now solid state LED colors that span the entire spectrum.