48
4 Epitaxial of III-Nitride LED Materials
However, the surface of the substrate is prone to large temperature gradients and
growth concentration gradients, which tends to cause depletion of reactants downstream that is not favorable to the uniform epitaxial wafer. The development of the
vertical growth chamber starts relatively late. Usually, the substrate is placed on the
rotatable table below the growth chamber, which makes it easier to obtain high-speed
rotation of the substrate. The uniformity of the grown GaN is better, but the substrate
is more susceptible to the GaN particles and by-products from the top above the
wafer. This can contaminate and affect the quality of the epitaxial wafer.
HVPE equipment is relatively simple and easy to maintain. It offers fast growth
rate, where the growth rate can exceed 300 μm/h. It is also easy to obtain uniform and
large size GaN thick film. High-quality GaN substrate can be obtained by laser stripping and other technologies. Moreover, as the thickness (or the reaction) increases,
the quality of the material can be continuously improved. Combined with the lateral
epitaxy technique, the dislocation density can be reduced to the order of 10
5 cm
−2
[20]. Its disadvantage is that it is difficult to precisely control the film thickness and
composition. It is also difficult to grow multi-nitride materials and heterostructures
(such as quantum wells, superlattices). The reaction gas has a strong corrosive effect
on the growth equipment, which can limit the improvement in material purity. In
addition, the by-product NH 4 Cl is easy to block the pipeline due to the serious parasitic reaction. For example, polycrystalline GaN can easily grow on the quartz tube
and is difficult to remove, which reduces the lifetime of the Ga boat and the quartz
tube.
4.4 Two-Step Growth Method for MOCVD Grown Nitride
Materials
Due to the large lattice mismatch and thermal mismatch between GaN and sapphire,
GaN grown directly on the sapphire substrate generally has a poor crystal quality and a
rough surface. Special process must be employed in order to grow GaN material with
a flat surface and high quality on a sapphire substrate. Similar to the growth of GaAs
on a Si substrate, the two-step growth method is a typical process for GaN/sapphire
MOCVD growth techniques. Since the growth process is accompanied by a change
in growth mode, many phenomena can be explained by the surface dynamics of the
film growth. Here, the principle is introduced, and the general procedure for growing
GaN by the two-step method is introduced in combination with the kinetic principle.
4.4.1 Surface Dynamics for Film Growth
In epitaxial growth, since the vacuum deposition process is in a thermodynamic equilibrium state, nucleation and growth of the material is a dynamic process. Because
4 Epitaxial of III-Nitride LED Materials
However, the surface of the substrate is prone to large temperature gradients and
growth concentration gradients, which tends to cause depletion of reactants downstream that is not favorable to the uniform epitaxial wafer. The development of the
vertical growth chamber starts relatively late. Usually, the substrate is placed on the
rotatable table below the growth chamber, which makes it easier to obtain high-speed
rotation of the substrate. The uniformity of the grown GaN is better, but the substrate
is more susceptible to the GaN particles and by-products from the top above the
wafer. This can contaminate and affect the quality of the epitaxial wafer.
HVPE equipment is relatively simple and easy to maintain. It offers fast growth
rate, where the growth rate can exceed 300 μm/h. It is also easy to obtain uniform and
large size GaN thick film. High-quality GaN substrate can be obtained by laser stripping and other technologies. Moreover, as the thickness (or the reaction) increases,
the quality of the material can be continuously improved. Combined with the lateral
epitaxy technique, the dislocation density can be reduced to the order of 10
5 cm
−2
[20]. Its disadvantage is that it is difficult to precisely control the film thickness and
composition. It is also difficult to grow multi-nitride materials and heterostructures
(such as quantum wells, superlattices). The reaction gas has a strong corrosive effect
on the growth equipment, which can limit the improvement in material purity. In
addition, the by-product NH 4 Cl is easy to block the pipeline due to the serious parasitic reaction. For example, polycrystalline GaN can easily grow on the quartz tube
and is difficult to remove, which reduces the lifetime of the Ga boat and the quartz
tube.
4.4 Two-Step Growth Method for MOCVD Grown Nitride
Materials
Due to the large lattice mismatch and thermal mismatch between GaN and sapphire,
GaN grown directly on the sapphire substrate generally has a poor crystal quality and a
rough surface. Special process must be employed in order to grow GaN material with
a flat surface and high quality on a sapphire substrate. Similar to the growth of GaAs
on a Si substrate, the two-step growth method is a typical process for GaN/sapphire
MOCVD growth techniques. Since the growth process is accompanied by a change
in growth mode, many phenomena can be explained by the surface dynamics of the
film growth. Here, the principle is introduced, and the general procedure for growing
GaN by the two-step method is introduced in combination with the kinetic principle.
4.4.1 Surface Dynamics for Film Growth
In epitaxial growth, since the vacuum deposition process is in a thermodynamic equilibrium state, nucleation and growth of the material is a dynamic process. Because