Epitaxial growth
Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where the overlayer is in registry with the substrate. In other words, there must be one or more preferred orientations of the overlayer with respect to the substrate for this to be termed epitaxial growth. The overlayer is called an epitaxial film or epitaxial layer. The growth temperature must be lower than melting point of substrate and epitaxial material.
The LED process can be distinguish three processes; epitaxial growth, chip fabrication and packaging. The attached images shows the crystal structure of GaN on sapphire (Al2O3). The substrate generally used is a sapphire substrate because GaN substrate demands high cost. The GaN grown on sapphire substrate generates defects cause by lattice mismatch between sapphire and GaN lattice constant.
The defects known as a non-radiative center which decrease LED efficiency. Epitaxial growth techniques are used to minimize defects in the crystal is one of the major factor for the high power LED.
[SAPPHIRE C-PLANE]
Color of LED by epitaxial growth
The color of LED is decided by the alloy composition (MQW). The ratio of Indium and Gallium in the MQW makes a proper band-gap. The band gap energy decides wavelength of light from MQW. InGaN LED makes from near UV to green light. AlGaInP LED can emit Amber to Red color. Generally White LED is made of a blue chip and phosphors.Different color light has different emission energy, shorter wavelength (Violet) has the higher emission energy and Longer wavelength (Red) has lower emission energy.
Color | Wavelength (nm) | Color | Wavelength (nm) |
| Violet | 400~430 | Amber | 590~595 |
| Blue | 430~480 | Orange | 600~615 |
| Green | 490~530 | Orange-Red | 620~640 |
| Yellow | 550~580 | Red | 645~700 |
EPI Growth Mechanism
MOCVD(Metal Organic Chemical Vapor Deposition) is the main epitaxial growth system. Using metalorganic and hydride sources, many compound semiconductor layers such as GaN, P/N-GaN, InGaN, AlGaN, AlInGaN, AlGaInP can be grown.
The image shows the mechanism of growing GaN compound semiconductor by the MOCVD method.
[The mechanism of growing GaN compound semiconductor by the MOCVD method.]
