70
4 Epitaxial of III-Nitride LED Materials
4.6.3 Roots of GaN Stress on SiC Substrates
Epitaxial GaN layer on SiC substrate will subject to tensile strain during cooling
due to the inherent defects of SiC substrate and the difference of thermal expansion
coefficient of 33.1%. If the stress accumulation reaches a certain level, it will cause
the GaN epitaxial layer to crack. This will seriously affect the performance and
reliability of the device.
In order to solve the tensile strain and cracking problem, AlN or AlGaN is
commonly used as a buffer layer to modulate the stress in the GaN epitaxial layer.
Since the lattice parameter of AlN is smaller than that of GaN, the GaN epitaxial
layer on AlN is inevitably subjected to compressive stress. The tensile stress between
the SiC substrate and the GaN epitaxial layer is shielded by the compressive stress
provided by AlN.
We discuss the stress in the GaN epitaxial layer in two cases. One case is that
the GaN material is directly epitaxially grown on the SiC substrate or a very thin
AlN buffer layer. Since the lattice mismatch between GaN and SiC substrates is only
3.4%, the critical thickness of GaN on SiC substrates is only a few tens of nanometers
thick. At the growth temperature of GaN, the GaN epitaxial layer is mainly subjected
to compressive stress from the SiC substrate. As the thickness of GaN increases,
the compressive stress introduced by lattice mismatch also accumulates. When the
thickness of GaN exceeds its critical thickness on the SiC substrate, the stress in the
GaN epitaxial layer will be released by the formation of dislocations. Heying’ studies
have shown that stresses due to lattice mismatch during growth will be relaxed in the
manner of producing edge dislocations [59]. This is also the reason why the (002)
dichroic diffraction of GaN or AlN epitaxial layer grown on the SiC substrate has a
small full width at half maximum (FWHM), while the (102) diffraction has a large
FWHM. During the cooling process, a difference in thermal expansion coefficient
of 33.1% between GaN and SiC substrate will cause a large tensile stress in the
GaN epitaxial layer. However, since the compressive stress generated by the lattice
mismatch during growth has been relaxed by dislocations and the like, the tensile
stress generated during the cooling process cannot be shielded, and thus cracking will
be difficult to avoid. Moreover, the more dislocations in the GaN epitaxial layer are,
the larger the compressive stress through dislocation relaxation is. Also, the more the
tensile stress introduced during the cooling process will increase, the more severe
the cracking will be.
Another case is to use an AlN or AlGaN buffer layer [60]. When employed as a
buffer layer, the mismatch between SiC and GaN will relax through the AlN layer.
During epitaxial growth, compressive stress is generated between SiC and AlN due
to lattice mismatch. At the interface, the mismatch stress is released by the threedimensional growth mode of the AlN buffer layer. Therefore, the compressive stress
in the GaN epitaxial layer will mainly come from the AlN buffer layer [50]. The
compressive stress generated by epitaxial growth of GaN on AlN will be divided
into two steps for relaxation [51]: 70% relaxation at the AlN/GaN interface; the
remaining stress decreases exponentially with increasing GaN thickness [61]. If the
4 Epitaxial of III-Nitride LED Materials
4.6.3 Roots of GaN Stress on SiC Substrates
Epitaxial GaN layer on SiC substrate will subject to tensile strain during cooling
due to the inherent defects of SiC substrate and the difference of thermal expansion
coefficient of 33.1%. If the stress accumulation reaches a certain level, it will cause
the GaN epitaxial layer to crack. This will seriously affect the performance and
reliability of the device.
In order to solve the tensile strain and cracking problem, AlN or AlGaN is
commonly used as a buffer layer to modulate the stress in the GaN epitaxial layer.
Since the lattice parameter of AlN is smaller than that of GaN, the GaN epitaxial
layer on AlN is inevitably subjected to compressive stress. The tensile stress between
the SiC substrate and the GaN epitaxial layer is shielded by the compressive stress
provided by AlN.
We discuss the stress in the GaN epitaxial layer in two cases. One case is that
the GaN material is directly epitaxially grown on the SiC substrate or a very thin
AlN buffer layer. Since the lattice mismatch between GaN and SiC substrates is only
3.4%, the critical thickness of GaN on SiC substrates is only a few tens of nanometers
thick. At the growth temperature of GaN, the GaN epitaxial layer is mainly subjected
to compressive stress from the SiC substrate. As the thickness of GaN increases,
the compressive stress introduced by lattice mismatch also accumulates. When the
thickness of GaN exceeds its critical thickness on the SiC substrate, the stress in the
GaN epitaxial layer will be released by the formation of dislocations. Heying’ studies
have shown that stresses due to lattice mismatch during growth will be relaxed in the
manner of producing edge dislocations [59]. This is also the reason why the (002)
dichroic diffraction of GaN or AlN epitaxial layer grown on the SiC substrate has a
small full width at half maximum (FWHM), while the (102) diffraction has a large
FWHM. During the cooling process, a difference in thermal expansion coefficient
of 33.1% between GaN and SiC substrate will cause a large tensile stress in the
GaN epitaxial layer. However, since the compressive stress generated by the lattice
mismatch during growth has been relaxed by dislocations and the like, the tensile
stress generated during the cooling process cannot be shielded, and thus cracking will
be difficult to avoid. Moreover, the more dislocations in the GaN epitaxial layer are,
the larger the compressive stress through dislocation relaxation is. Also, the more the
tensile stress introduced during the cooling process will increase, the more severe
the cracking will be.
Another case is to use an AlN or AlGaN buffer layer [60]. When employed as a
buffer layer, the mismatch between SiC and GaN will relax through the AlN layer.
During epitaxial growth, compressive stress is generated between SiC and AlN due
to lattice mismatch. At the interface, the mismatch stress is released by the threedimensional growth mode of the AlN buffer layer. Therefore, the compressive stress
in the GaN epitaxial layer will mainly come from the AlN buffer layer [50]. The
compressive stress generated by epitaxial growth of GaN on AlN will be divided
into two steps for relaxation [51]: 70% relaxation at the AlN/GaN interface; the
remaining stress decreases exponentially with increasing GaN thickness [61]. If the