7.2 Internal Quantum Efficiency Improvement Technology
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layer and the substrate. The epitaxial layer growth process will produce dislocation
defects. (2) There is thermal mismatch between the epitaxial layer and the substrate.
The epitaxial layer is subjected to a temperature change after the epitaxial growth.
The growth process can generate thermal stress, resulting in the formation of defects,
cracks, problems such as bending of the wafer. (3) Crystal polarity is different, which
often causes structural defects such as anti-phase domains. (4) The crystal quality of
the substrate directly affects the crystal quality of the epitaxial layer. The dislocations
in substrate may extend, multiplicate and re-distribute during the epitaxy growth
process.
Due to the above problems, the nitride device based on the heteroepitaxial epitaxy
cannot achieve its intended excellent performance. In other words, its performance
is severely restricted. In order to improve the crystal quality of the epitaxial layer,
researchers have developed different material growth technologies such as low
temperature buffer layer and lateral epitaxial overgrown (LEO) technology [7]. These
techniques improve the crystal quality of the epitaxial layer in certain degree, but
increase the epitaxial growth time and process complexity.
Nitride homoepitaxial uses single crystal GaN as the substrate, also known as
freestanding GaN substrate. The freestanding GaN substrate is rapidly grown into a
thick GaN film on a sapphire or other material substrate by a hydride vapor phase
epitaxy (HVPE) technique. The substrate is then removed by mechanical polishing or
laser technology to form a GaN quasi-substrate. Since the dislocation density of the
epitaxial GaN layer by HVPE method is reduced as the thickness of the epitaxial layer
is increased [8], the crystal quality can be improved as long as the GaN film thickness
reaches a certain value. Homogeneous epitaxy can overcome the unfavorable factors
caused by heteroepitaxial growth. The epitaxial layer is completely lattice-matched
and thermally matched with the substrate, which improves the crystal quality of
the epitaxial layer and improves the quantum efficiency of the device. The GaN
substrate can be made into a conductive substrate, which is convenient for preparing
a vertical structure chip with excellent current spreading performance. GaN epitaxial
technique based on freestanding GaN substrate is an important direction for future
development.
The freestanding GaN substrate has a Ga polar surface and an N polar surface.
The N polar surface is chemically active and easily corroded by acid. However, since
the surface energy of the N polar surface is relatively large, the epitaxial GaN tends
to form a hexagonal pyramid and a rough surface. The Ga polar surface is easy to
form a flat surface due to the relatively small surface energy. Studies have shown that
the Ga polar surface is easier to achieve p-type doping. Therefore, the homo-epitaxy
generally selects the Ga polar plane as the substrate surface of the epitaxial layer.
However, since the Ga polar surface is chemically stable and is not easily corroded
by acid, the Ga polar surface is usually obtained by mechanical polishing.
At present, the crystal quality and surface quality of GaN freestanding substrates
are not very good, which influents the quality of epitaxial materials. Figure 7.4 is a
topography of Ga polar GaN surface. Many scratches can be seen on the substrate
surface. The ICP etching and patterning processes are used to treat the substrate
surface. In Fig. 7.5 dislocation photo of GaN epitaxial layer grown on freestanding
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layer and the substrate. The epitaxial layer growth process will produce dislocation
defects. (2) There is thermal mismatch between the epitaxial layer and the substrate.
The epitaxial layer is subjected to a temperature change after the epitaxial growth.
The growth process can generate thermal stress, resulting in the formation of defects,
cracks, problems such as bending of the wafer. (3) Crystal polarity is different, which
often causes structural defects such as anti-phase domains. (4) The crystal quality of
the substrate directly affects the crystal quality of the epitaxial layer. The dislocations
in substrate may extend, multiplicate and re-distribute during the epitaxy growth
process.
Due to the above problems, the nitride device based on the heteroepitaxial epitaxy
cannot achieve its intended excellent performance. In other words, its performance
is severely restricted. In order to improve the crystal quality of the epitaxial layer,
researchers have developed different material growth technologies such as low
temperature buffer layer and lateral epitaxial overgrown (LEO) technology [7]. These
techniques improve the crystal quality of the epitaxial layer in certain degree, but
increase the epitaxial growth time and process complexity.
Nitride homoepitaxial uses single crystal GaN as the substrate, also known as
freestanding GaN substrate. The freestanding GaN substrate is rapidly grown into a
thick GaN film on a sapphire or other material substrate by a hydride vapor phase
epitaxy (HVPE) technique. The substrate is then removed by mechanical polishing or
laser technology to form a GaN quasi-substrate. Since the dislocation density of the
epitaxial GaN layer by HVPE method is reduced as the thickness of the epitaxial layer
is increased [8], the crystal quality can be improved as long as the GaN film thickness
reaches a certain value. Homogeneous epitaxy can overcome the unfavorable factors
caused by heteroepitaxial growth. The epitaxial layer is completely lattice-matched
and thermally matched with the substrate, which improves the crystal quality of
the epitaxial layer and improves the quantum efficiency of the device. The GaN
substrate can be made into a conductive substrate, which is convenient for preparing
a vertical structure chip with excellent current spreading performance. GaN epitaxial
technique based on freestanding GaN substrate is an important direction for future
development.
The freestanding GaN substrate has a Ga polar surface and an N polar surface.
The N polar surface is chemically active and easily corroded by acid. However, since
the surface energy of the N polar surface is relatively large, the epitaxial GaN tends
to form a hexagonal pyramid and a rough surface. The Ga polar surface is easy to
form a flat surface due to the relatively small surface energy. Studies have shown that
the Ga polar surface is easier to achieve p-type doping. Therefore, the homo-epitaxy
generally selects the Ga polar plane as the substrate surface of the epitaxial layer.
However, since the Ga polar surface is chemically stable and is not easily corroded
by acid, the Ga polar surface is usually obtained by mechanical polishing.
At present, the crystal quality and surface quality of GaN freestanding substrates
are not very good, which influents the quality of epitaxial materials. Figure 7.4 is a
topography of Ga polar GaN surface. Many scratches can be seen on the substrate
surface. The ICP etching and patterning processes are used to treat the substrate
surface. In Fig. 7.5 dislocation photo of GaN epitaxial layer grown on freestanding