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4 Epitaxial of III-Nitride LED Materials
4.1.1 3-D Growth Mode (Volmer-Weber Mode)
If the lattice mismatch between the substrate and the deposited film is relatively
large, or the temperature of the substrate is relatively low, the atoms reaching the
surface of the substrate are first aggregated into a core at a location that is relatively
easy to nucleate. The atoms that reach the substrate later gather around the core,
causing the core to grow up and form a thin film. Films formed in a 3-D growth
mode generally exhibit polycrystalline morphology. Low temperature GaN nucleation layers deposited on sapphire substrates generally prefer a 3-D growth mode.
The 3-D growth mode can be divided into the following four stages.
(a) Nucleation stage: the source gas that reaches the surface of the substrate by the
transport of the carrier gas is decomposed, and the Ga and N atoms are transported and adsorbed on the surface of the substrate. They gather at a place where
nucleation is easy. During the physical adsorption process, this part of GaN
nucleation is not easily decomposed. Other atoms that undergo chemisorption
are discharged with the carrier gas from the reaction chamber.
(b) Nucleus growth into small islands: The nucleation sites adsorbed on the surface
of the substrate, where the atoms deposited on the substrate, are relatively easily
to adsorb. These cores continue to grow into large grains and become islands
one by one.
(c) The islands are connected into a grid: As the deposited atoms increase, the
nucleus grows and the adjacent islands come into contact (so called coalescence). The islands which are contacted with each other, can release a part of
the energy due to the coalescence of the interface and reduce the surface energy.
The islands that are in contact with each other tend to be connected to each other
to form a grid.
(d) Grid forming a film: As the deposited atoms increase, the meshes heal each
other and form a film.
4.1.2 2-D Growth Mode (Frank-Vander Merwe Mode)
The 2-D growth mode generally occurs when the homoepitaxial or substrate temperature is relatively high, but the atomic deposition rate is relatively low. The atoms
deposited on the surface of the substrate are adsorbed to the surface of the substrate
during transport on the surface of the substrate to form a two-dimensional core. This
requires that the surface of the substrate be relatively flat, free of defects or defective
outcrops, and that the surface energy of the substrate is consistent with the surface
energy of the film. This is difficult to achieve when GaN film is epitaxially grown on
sapphire substrate.
4 Epitaxial of III-Nitride LED Materials
4.1.1 3-D Growth Mode (Volmer-Weber Mode)
If the lattice mismatch between the substrate and the deposited film is relatively
large, or the temperature of the substrate is relatively low, the atoms reaching the
surface of the substrate are first aggregated into a core at a location that is relatively
easy to nucleate. The atoms that reach the substrate later gather around the core,
causing the core to grow up and form a thin film. Films formed in a 3-D growth
mode generally exhibit polycrystalline morphology. Low temperature GaN nucleation layers deposited on sapphire substrates generally prefer a 3-D growth mode.
The 3-D growth mode can be divided into the following four stages.
(a) Nucleation stage: the source gas that reaches the surface of the substrate by the
transport of the carrier gas is decomposed, and the Ga and N atoms are transported and adsorbed on the surface of the substrate. They gather at a place where
nucleation is easy. During the physical adsorption process, this part of GaN
nucleation is not easily decomposed. Other atoms that undergo chemisorption
are discharged with the carrier gas from the reaction chamber.
(b) Nucleus growth into small islands: The nucleation sites adsorbed on the surface
of the substrate, where the atoms deposited on the substrate, are relatively easily
to adsorb. These cores continue to grow into large grains and become islands
one by one.
(c) The islands are connected into a grid: As the deposited atoms increase, the
nucleus grows and the adjacent islands come into contact (so called coalescence). The islands which are contacted with each other, can release a part of
the energy due to the coalescence of the interface and reduce the surface energy.
The islands that are in contact with each other tend to be connected to each other
to form a grid.
(d) Grid forming a film: As the deposited atoms increase, the meshes heal each
other and form a film.
4.1.2 2-D Growth Mode (Frank-Vander Merwe Mode)
The 2-D growth mode generally occurs when the homoepitaxial or substrate temperature is relatively high, but the atomic deposition rate is relatively low. The atoms
deposited on the surface of the substrate are adsorbed to the surface of the substrate
during transport on the surface of the substrate to form a two-dimensional core. This
requires that the surface of the substrate be relatively flat, free of defects or defective
outcrops, and that the surface energy of the substrate is consistent with the surface
energy of the film. This is difficult to achieve when GaN film is epitaxially grown on
sapphire substrate.