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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 (㎚) Color Wavelength (㎚)
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.

GaN EPI layer growth process

  1. Thermal cleaning at high temperature
  2. Buffer layer growth to relax lattice mismatch between a substrate and a GaN
  3. Growth of undoped GaN and n-type GaN (electron injection layer)
  4. Grow of the emission layer MQWs(Multi Quantum Wells) ; several periods of InGaN/GaN
  5. AlGaN growth for electron blocking
  6. Growth p-GaN to inject holes into the MQW

The image shows temperature and gas flow condition for GaN growth.

Temperature and gas flow condition for GaN growth.

Polar and (non-polar) crystal

GaN has a Wurtzite crystal structure. The Ga and N ordering depends on crystal plane. Currently c-plane sapphire is used as conventional growth of LED. The asymmetry caused by c-axis growth generates a spontaneous polarization and piezoelectric polarization is caused by lattice mismatch strain between InGaN and GaN on MQW.

The electric fields created by these polarizations disturb the recombination of electrons and holes which in turn reduce internal quantum efficiency, red-shifting wavelength, high threshold voltage.

The Nonpolar LED is considered a suitable technology for solving these issues. The Nonpolar LED can be reduce the effects of polarization. IN particular, it can be used in high power devices and long wavelength applications. Nonpolar LEDs has been intensively researched on by SSC.

※ Wurtzite structure : Among the compounds that can take the wurtzite structure are wurtzite itself, AgI, ZnO, CdS, CdSe, α-SiC, GaN, AlN, and other semiconductors. Each of the two individual atom types forms a sublattice which is HCP-type (short for "hexagonal close-pack"). When viewed altogether, the atomic positions are the same as in lonsdaleite (hexagonal diamond). Each atom is tetrahedrally coordinated. The wurtzite structure is non-centrosymmetric (i.e., lacks inversion symmetry). Due to this, wurtzite crystals can (and generally do) have properties such as piezoelectricity and pyroelectricity, which centrosymmetric crystals lack.[citation needed]
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