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4 Epitaxial of III-Nitride LED Materials
4.2 Substrate for Epitaxial Growth of III-Nitride LEDs
(Sapphire/Si/SiC/LiAlO 2 /GaN)
Since GaN materials lack a suitable homogeneous substrate, GaN-based LEDs are
generally heteroepitaxially grown on substrates such as sapphire, silicon carbide
(SiC), silicon (Si), gallium arsenide (GaAs), and the like. Among them, the most
widely used substrate is sapphire which has become the material of choice for
commercial application of various LED epitaxial factories. Since 2010, the demand
for sapphire substrates has increased tremendously.
There is a large lattice mismatch and thermal expansion mismatch between the
sapphire substrate and the GaN-based epitaxial material. The GaN material epitaxially grown on the sapphire substrate has dislocations in the order of 10
8 cm
−2 .
However, sapphire substrates have incomparable advantages over other heterogeneous substrates, and may be replaced by other substrates in the future although the
sapphire is still currently the most suitable substrate for epitaxial GaN-based LEDs.
Below we briefly introduce the advantages and disadvantages of various substrates.
Si substrate: The wide application of Si substrate in the current microelectronics
industry is the basis for the development of integrated circuit technology. At the same
time, the development of Si substrates is also the most mature, and commercial 8-inch
and 12-inch silicon wafers have emerged. However, the epitaxial GaN material on
the Si substrate has a large number of misfit dislocations. The performance of GaNbased LEDs on Si is currently not as good as the LEDs on the sapphire substrate.
However, the cost advantage of Si substrate and its wide application in the integrated
circuit industry make Si substrate attractive over other substrates. Therefore, the
epitaxial growth of GaN-based LED on Si substrate has also been investigated by
many national research institutes and the companies. Recently, the GaN-based LEDs
on Si substrates released by Bridgelux in the United States have reached 160 lm/w
at 350 mA [1], reaching certain level on sapphire substrates. Although it has not
been put into large-scale commercial applications, its investment in research and
development is very large. It is expected that GaN-based LEDs on Si substrates will
be further developed in the near future.
SiC substrate: SiC substrate has relatively small thermal expansion and lattice
mismatch with GaN. It is an ideal material for epitaxial GaN-based LED. However,
the fabrication of SiC substrates is very difficult and expensive. Furthermore, largescale applications of SiC substrate for LEDs have not been demonstrated yet. Cree
in the United States has made good progress in the processing technology of SiC
substrates. Its GaN epitaxial growth on SiC substrates has been widely used. In 2014,
they announced that GaN-based LEDs on SiC substrates have been reached 308 lm/w
at 350 mA [2]. It is the highest index of GaN-based LEDs at present. Cree’s success
in development of SiC substrates has also pointed out a way for other companies:
if the production of SiC substrates can be greatly improved, the price of the LEDs
can be significantly reduced. The use of SiC as the substrate for GaN-based LEDs is
undoubtedly more advantageous than sapphire.
4 Epitaxial of III-Nitride LED Materials
4.2 Substrate for Epitaxial Growth of III-Nitride LEDs
(Sapphire/Si/SiC/LiAlO 2 /GaN)
Since GaN materials lack a suitable homogeneous substrate, GaN-based LEDs are
generally heteroepitaxially grown on substrates such as sapphire, silicon carbide
(SiC), silicon (Si), gallium arsenide (GaAs), and the like. Among them, the most
widely used substrate is sapphire which has become the material of choice for
commercial application of various LED epitaxial factories. Since 2010, the demand
for sapphire substrates has increased tremendously.
There is a large lattice mismatch and thermal expansion mismatch between the
sapphire substrate and the GaN-based epitaxial material. The GaN material epitaxially grown on the sapphire substrate has dislocations in the order of 10
8 cm
−2 .
However, sapphire substrates have incomparable advantages over other heterogeneous substrates, and may be replaced by other substrates in the future although the
sapphire is still currently the most suitable substrate for epitaxial GaN-based LEDs.
Below we briefly introduce the advantages and disadvantages of various substrates.
Si substrate: The wide application of Si substrate in the current microelectronics
industry is the basis for the development of integrated circuit technology. At the same
time, the development of Si substrates is also the most mature, and commercial 8-inch
and 12-inch silicon wafers have emerged. However, the epitaxial GaN material on
the Si substrate has a large number of misfit dislocations. The performance of GaNbased LEDs on Si is currently not as good as the LEDs on the sapphire substrate.
However, the cost advantage of Si substrate and its wide application in the integrated
circuit industry make Si substrate attractive over other substrates. Therefore, the
epitaxial growth of GaN-based LED on Si substrate has also been investigated by
many national research institutes and the companies. Recently, the GaN-based LEDs
on Si substrates released by Bridgelux in the United States have reached 160 lm/w
at 350 mA [1], reaching certain level on sapphire substrates. Although it has not
been put into large-scale commercial applications, its investment in research and
development is very large. It is expected that GaN-based LEDs on Si substrates will
be further developed in the near future.
SiC substrate: SiC substrate has relatively small thermal expansion and lattice
mismatch with GaN. It is an ideal material for epitaxial GaN-based LED. However,
the fabrication of SiC substrates is very difficult and expensive. Furthermore, largescale applications of SiC substrate for LEDs have not been demonstrated yet. Cree
in the United States has made good progress in the processing technology of SiC
substrates. Its GaN epitaxial growth on SiC substrates has been widely used. In 2014,
they announced that GaN-based LEDs on SiC substrates have been reached 308 lm/w
at 350 mA [2]. It is the highest index of GaN-based LEDs at present. Cree’s success
in development of SiC substrates has also pointed out a way for other companies:
if the production of SiC substrates can be greatly improved, the price of the LEDs
can be significantly reduced. The use of SiC as the substrate for GaN-based LEDs is
undoubtedly more advantageous than sapphire.