4.5 Influence of Growth Conditions on Epitaxial Layer Quality …
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When the buffer layer is grown under a low pressure, the island has a small size
and a high density in the initial stage of nucleation. Therefore, the islands are rapidly
merged, and a large number of edge dislocations are generated during the merging
process. The dislocations extend along the normal direction of the substrate into
the epitaxial layer. The process is shown on the left in Fig. 4.15. While under high
pressure conditions during film growth, the island has a large size and a small density.
This makes the distance between the islands become larger, and the merging process
becomes slower. The edge dislocations generated during the merging process will
be much affected by the mixed dislocations. The density of dislocations extended
to the epitaxial layer is normally reduced due to the interaction between the edge
dislocations and mixed dislocations.
4.5.2 Effect of Rough Layer Growth Conditions
After undergoing buffer layer growth and recrystallization, the surface has formed
a nucleation islands, which provides seed crystals for post-high temperature GaN
material growth. With the growth of high-temperature GaN epitaxial materials, the
growth mode changes: First, at the GaN nucleation island, the island grows horizontally and vertically. In the lateral direction, the large islands gradually become larger
by absorbing small islands, while in the longitudinal direction, the growth rate of the
islands gradually decreases. This stage is three-dimensional growth. Subsequently,
the adjacent islands are merged to form a quasi-two-dimensional plane, which is
due to the competition between the coarsening caused by island growth and the
flattening brought about by the island merge. The growth mode at this time should
be the mixed growth mode. Finally, when a two-dimensional plane is formed, the
mismatch stress will be moderated, which is beneficial for the film to tend to grow
two-dimensionally, making the surface smooth as a mirror. The growth conditions in
the roughening stage also have a great impact on the epitaxial growth mode, which
in turn affects the material quality, so it requires to be investigated.
4.5.2.1 Influence of Rough Layer Pressure
The growth pressure of the roughened layer can also affect the size and distribution
of the island. When the pressure is small, it can only grow at the nearby nucleation
island due to the small activity of the reactants, which reduces the lateral growth
effect and thus speeds up the combination of the islands. During the formation of
islands and merging islands process, it can result in even higher density of stacking
faults, micro twin dislocations, and defects as shown in Fig. 4.16.
63
When the buffer layer is grown under a low pressure, the island has a small size
and a high density in the initial stage of nucleation. Therefore, the islands are rapidly
merged, and a large number of edge dislocations are generated during the merging
process. The dislocations extend along the normal direction of the substrate into
the epitaxial layer. The process is shown on the left in Fig. 4.15. While under high
pressure conditions during film growth, the island has a large size and a small density.
This makes the distance between the islands become larger, and the merging process
becomes slower. The edge dislocations generated during the merging process will
be much affected by the mixed dislocations. The density of dislocations extended
to the epitaxial layer is normally reduced due to the interaction between the edge
dislocations and mixed dislocations.
4.5.2 Effect of Rough Layer Growth Conditions
After undergoing buffer layer growth and recrystallization, the surface has formed
a nucleation islands, which provides seed crystals for post-high temperature GaN
material growth. With the growth of high-temperature GaN epitaxial materials, the
growth mode changes: First, at the GaN nucleation island, the island grows horizontally and vertically. In the lateral direction, the large islands gradually become larger
by absorbing small islands, while in the longitudinal direction, the growth rate of the
islands gradually decreases. This stage is three-dimensional growth. Subsequently,
the adjacent islands are merged to form a quasi-two-dimensional plane, which is
due to the competition between the coarsening caused by island growth and the
flattening brought about by the island merge. The growth mode at this time should
be the mixed growth mode. Finally, when a two-dimensional plane is formed, the
mismatch stress will be moderated, which is beneficial for the film to tend to grow
two-dimensionally, making the surface smooth as a mirror. The growth conditions in
the roughening stage also have a great impact on the epitaxial growth mode, which
in turn affects the material quality, so it requires to be investigated.
4.5.2.1 Influence of Rough Layer Pressure
The growth pressure of the roughened layer can also affect the size and distribution
of the island. When the pressure is small, it can only grow at the nearby nucleation
island due to the small activity of the reactants, which reduces the lateral growth
effect and thus speeds up the combination of the islands. During the formation of
islands and merging islands process, it can result in even higher density of stacking
faults, micro twin dislocations, and defects as shown in Fig. 4.16.