4.4 Two-Step Growth Method for MOCVD Grown Nitride Materials
53
(4) After the buffer layer is grown, the temperature is raised from a low temperature
of 535 °C to a high temperature of 1080 °C. This section is generally referred to
as an annealing stage of the buffer layer. During this procedure, the amorphous
buffer layer is recrystallized so that the reflectance is slightly increased at the
beginning stage. However, when the temperature is raised above about 850 °C,
the GaN nucleation island begins to coarsen, resulting in a rapid decline in
reflectance. Its AFM image is shown in Fig. 4.7b.
(5) The roughening layer, which is the growth stage of the GaN nucleation island,
is a three-dimensional growth. At high temperatures, high-density nucleation
islands gradually grow. As large nucleation islands gradually absorb nearby
atoms, such a process makes the islands larger and sparse distribution. It is
therefore that the reflectivity of this segment is very low. The large and sparse
nucleation island can reduce the dislocation density. The roughening speed can
be promoted by elevating the pressure and lowering the V/III ratio, making
the roughened island larger and sparse. The AFM image of the island’s merger
process is shown in Fig. 4.7c, d.
(6) The recovery layer and the high temperature layer are the lateral and vertical
growth of the island at the nucleation sites. The lateral growth causes the nucleation islands to gradually merge into a quasi-two-dimensional plane. Then, hightemperature growth of the GaN epitaxial layer can be performed on this basis
with a two-dimensional growth mode to obtain a high-quality GaN material.
The AFM image of its surface topography is shown in Fig. 4.7e, f.
Among them, steps 1–5 are the initial stages of GaN two-step epitaxial growth,
providing a basis for high temperature epitaxial growth listed in step 6).
Figure 4.7 shows the surface morphology of the buffer layer obtained by the
method of growth interruption at different growth stages. The surface topography
corresponds to the reflectance curve of Fig. 4.6 respectively. Observation of the
surface topography at each stage helps us to better understand the changing process
of buffer layer.
The initial growth conditions have a great impact on the growth mode of the
subsequent high temperature epitaxial layer. If the initial growth conditions are not
suitable, the surface of the material is rough and the crystal quality deteriorates
sharply. Therefore, it is quite necessary to optimize the growth conditions of the
initial stage for the GaN epitaxy.
4.5 Influence of Growth Conditions on Epitaxial Layer
Quality of Group III Nitride Materials
The initial growth stage includes high-temperature pretreatment, surface nitridation, buffer layer growth, buffer layer recrystallization, and roughening layer growth
during the two-step GaN growth process, which is the basis for the growth of hightemperature GaN materials. In particular, the growth parameters of the buffer layer
53
(4) After the buffer layer is grown, the temperature is raised from a low temperature
of 535 °C to a high temperature of 1080 °C. This section is generally referred to
as an annealing stage of the buffer layer. During this procedure, the amorphous
buffer layer is recrystallized so that the reflectance is slightly increased at the
beginning stage. However, when the temperature is raised above about 850 °C,
the GaN nucleation island begins to coarsen, resulting in a rapid decline in
reflectance. Its AFM image is shown in Fig. 4.7b.
(5) The roughening layer, which is the growth stage of the GaN nucleation island,
is a three-dimensional growth. At high temperatures, high-density nucleation
islands gradually grow. As large nucleation islands gradually absorb nearby
atoms, such a process makes the islands larger and sparse distribution. It is
therefore that the reflectivity of this segment is very low. The large and sparse
nucleation island can reduce the dislocation density. The roughening speed can
be promoted by elevating the pressure and lowering the V/III ratio, making
the roughened island larger and sparse. The AFM image of the island’s merger
process is shown in Fig. 4.7c, d.
(6) The recovery layer and the high temperature layer are the lateral and vertical
growth of the island at the nucleation sites. The lateral growth causes the nucleation islands to gradually merge into a quasi-two-dimensional plane. Then, hightemperature growth of the GaN epitaxial layer can be performed on this basis
with a two-dimensional growth mode to obtain a high-quality GaN material.
The AFM image of its surface topography is shown in Fig. 4.7e, f.
Among them, steps 1–5 are the initial stages of GaN two-step epitaxial growth,
providing a basis for high temperature epitaxial growth listed in step 6).
Figure 4.7 shows the surface morphology of the buffer layer obtained by the
method of growth interruption at different growth stages. The surface topography
corresponds to the reflectance curve of Fig. 4.6 respectively. Observation of the
surface topography at each stage helps us to better understand the changing process
of buffer layer.
The initial growth conditions have a great impact on the growth mode of the
subsequent high temperature epitaxial layer. If the initial growth conditions are not
suitable, the surface of the material is rough and the crystal quality deteriorates
sharply. Therefore, it is quite necessary to optimize the growth conditions of the
initial stage for the GaN epitaxy.
4.5 Influence of Growth Conditions on Epitaxial Layer
Quality of Group III Nitride Materials
The initial growth stage includes high-temperature pretreatment, surface nitridation, buffer layer growth, buffer layer recrystallization, and roughening layer growth
during the two-step GaN growth process, which is the basis for the growth of hightemperature GaN materials. In particular, the growth parameters of the buffer layer