64
4 Epitaxial of III-Nitride LED Materials
4.5.2.2 Influence of Roughing Layer V/III Ratio
In addition to increasing the pressure of the roughened layer, it is also possible to
reduce the merge speed of the island by reducing the V/III ratio of the roughened
layer. This is because the low V/III ratio makes the growth of nucleation islands more
inclined to 3D growth. While for high V/III ratio, there will be more NH 3 . This will
increase the viscosity of the airflow and make the MO source difficult to pass through
the boundary layer. It therefore leads to small and dense nucleation islands, makes
the high temperature growth island merge faster, reduces the lateral growth during
the merger process, and increases the dislocation density and deteriorates the crystal
quality. In comparison with the in situ monitoring curves under different roughing
layer V/III conditions shown in Fig. 4.17, it can be seen that the reflectivity recovery
becomes slower as the V/III ratio decreases. This will allow the nucleation islands
to grow fully. Such a process is beneficial to enhance the lateral growth mode in the
post-high temperature growth [38, 39].
Hall measurement is carried out for the intrinsic GaN samples grown under
different V/III ratio conditions for the roughened layer as shown in Fig. 4.18. It
is found that the background carrier concentration of the samples with different
V/III ratios is similar, but the mobility increases rapidly with the decrease of V/III
ratio. It can be seen that the low V/III ratio for the roughening layer can reduce the
island merging speed, make the island growth fully, enhance the lateral growth of
the post-high temperature GaN, reduce the dislocation density and the amount of
implanted impurities, and hence improve the carrier mobility.
The surface morphology of the intrinsic GaN samples with roughened layer of
different V/III ratios is also studied. It can be seen that the surface morphology of the
sample with a low V/III ratio is better. This is because the low roughening layer V/III
ratio can reduce the merging speed of the island, which results in a two-dimensionally
growth mode for the subsequent high-temperature layer deposition and therefore a
better surface topography.
4.6 Epitaxial Technology of High Quality GaN on SiC
Substrate
4.6.1 Basic Properties of SiC
SiC is a Group IV–IV binary compound semiconductor and is the only solid
compound comprised Group IV elements from the Periodic Table of the Elements.
SiC is consisting of two elements Si and C. Each atom is surrounded by four heterogeneous atoms, which forms tetrahedral units by oriented strong tetrahedral SP 3 bonds
(Fig. 4.19) [40]. The bond length between Si–C atoms is 1.89 Å, and the bond length
between Si–Si or C–C is 3.08 Å. The SiC crystals are formed by interconnecting
these tetrahedrons at the corners. The SiC crystal has particular strong ionic covalent
4 Epitaxial of III-Nitride LED Materials
4.5.2.2 Influence of Roughing Layer V/III Ratio
In addition to increasing the pressure of the roughened layer, it is also possible to
reduce the merge speed of the island by reducing the V/III ratio of the roughened
layer. This is because the low V/III ratio makes the growth of nucleation islands more
inclined to 3D growth. While for high V/III ratio, there will be more NH 3 . This will
increase the viscosity of the airflow and make the MO source difficult to pass through
the boundary layer. It therefore leads to small and dense nucleation islands, makes
the high temperature growth island merge faster, reduces the lateral growth during
the merger process, and increases the dislocation density and deteriorates the crystal
quality. In comparison with the in situ monitoring curves under different roughing
layer V/III conditions shown in Fig. 4.17, it can be seen that the reflectivity recovery
becomes slower as the V/III ratio decreases. This will allow the nucleation islands
to grow fully. Such a process is beneficial to enhance the lateral growth mode in the
post-high temperature growth [38, 39].
Hall measurement is carried out for the intrinsic GaN samples grown under
different V/III ratio conditions for the roughened layer as shown in Fig. 4.18. It
is found that the background carrier concentration of the samples with different
V/III ratios is similar, but the mobility increases rapidly with the decrease of V/III
ratio. It can be seen that the low V/III ratio for the roughening layer can reduce the
island merging speed, make the island growth fully, enhance the lateral growth of
the post-high temperature GaN, reduce the dislocation density and the amount of
implanted impurities, and hence improve the carrier mobility.
The surface morphology of the intrinsic GaN samples with roughened layer of
different V/III ratios is also studied. It can be seen that the surface morphology of the
sample with a low V/III ratio is better. This is because the low roughening layer V/III
ratio can reduce the merging speed of the island, which results in a two-dimensionally
growth mode for the subsequent high-temperature layer deposition and therefore a
better surface topography.
4.6 Epitaxial Technology of High Quality GaN on SiC
Substrate
4.6.1 Basic Properties of SiC
SiC is a Group IV–IV binary compound semiconductor and is the only solid
compound comprised Group IV elements from the Periodic Table of the Elements.
SiC is consisting of two elements Si and C. Each atom is surrounded by four heterogeneous atoms, which forms tetrahedral units by oriented strong tetrahedral SP 3 bonds
(Fig. 4.19) [40]. The bond length between Si–C atoms is 1.89 Å, and the bond length
between Si–Si or C–C is 3.08 Å. The SiC crystals are formed by interconnecting
these tetrahedrons at the corners. The SiC crystal has particular strong ionic covalent