5.5 Green LED and Non-polar, Semi-polar LED
89
and m-plane SiC [33]. Due to the obvious anisotropy of material properties in all
in-plane directions, stacking faults are also an important type of extensional defects
in non-polar and semi-polar materials in addition to dislocations [34].
5.5.3 Research Progress on Semi-polar and Non-polar LEDs
In 2000, Waltereit et al. [35] first reported their non-polar GaN results on Nature, and
obtained a non-polar m-plane AlGaN/GaN MQWs structure on a LiAlO 2 substrate,
showing the removal of polarization effects and thus opening up the new study of
non-polar GaN. In the following years, the internationally renowned GaN research
institutions, including the University of California at Santa Barbara (UCSB), the
University of South Carolina, the University of Nagoya, Japan, the Japanese Roma,
and the US Cree are actively engaged in non-polar research on GaN. They have
grown (11-20) a-plane GaN on r-plane sapphire, or (1-100) m-plane GaN on LiAlO 2
and m-SiC substrates. However, in the experiment, it was found that the growth
window of non-polar GaN is narrow, and the In is difficult to be incorporated in the
non-polar InGaN materials. In 2005, Sharma et al. [36] proposed semi-polar GaN
which can reduce the polarization electric field, and obtained the first semi-polar
green LED with crystal orientation along the (10-13) direction. Semi-polar GaN is
a non-polar surface with a growth plane between the c-axis and the non-polar plane,
which is oblique to the c-axis at an angle, including (11-22), (10-13), (1-101) and
(20-21), etc. Semi-polar materials can greatly reduce the polarization electric field.
In 2008, Gühne et al. [37] prepared (11-22) green LEDs on m-plane sapphire. As
the current increased, the EL peak wavelength did not move, which proved that the
effect of polarization effect was basically eliminated. Since the In doping in the InGaN
material grown along the semi-polar direction is easier, the growth window is also
wider than the non-polarity. Thus, the semi-polar InGaN material can be extended
to emit longer wavelengths. However, semi-polar GaN grown directly on m-plane,
r-plane sapphire, MgAl 2 O 4 , etc. by MOCVD technology has not yet achieved the
same results as traditional common growth process such as GaN because of the late
start, although it has certain advantages over non-polar GaN growth. There are still
a series of problems such as surface roughness, stacking faults and dislocations, etc.
[38, 39], which causes the subsequent growth of semi-polar green LED structures to
be greatly affected.
At present, the research on semi-polar LEDs in the world is studied mainly by
UCSB in US, Kyoto University of Japan, and the famous city university. Better
device results are obtained on the GaN substrate. Among them, the research team
led by “Father of Blue LED” Nakamura from UCSB has been the leader in the field
of non-polar and semi-polar GaN. They have effectively combined homoepitaxy
and semi-polar growth to reduce polarization effects on the basis of previous work.
In addition, a GaN substrate grown by HVPE (Mitsubishi Chemical) was used for
beveling to obtain semi-polar GaN materials with a variety of crystal orientations,
followed by homoepitaxial semi-polar LEDs. Breakthroughs have been made along
89
and m-plane SiC [33]. Due to the obvious anisotropy of material properties in all
in-plane directions, stacking faults are also an important type of extensional defects
in non-polar and semi-polar materials in addition to dislocations [34].
5.5.3 Research Progress on Semi-polar and Non-polar LEDs
In 2000, Waltereit et al. [35] first reported their non-polar GaN results on Nature, and
obtained a non-polar m-plane AlGaN/GaN MQWs structure on a LiAlO 2 substrate,
showing the removal of polarization effects and thus opening up the new study of
non-polar GaN. In the following years, the internationally renowned GaN research
institutions, including the University of California at Santa Barbara (UCSB), the
University of South Carolina, the University of Nagoya, Japan, the Japanese Roma,
and the US Cree are actively engaged in non-polar research on GaN. They have
grown (11-20) a-plane GaN on r-plane sapphire, or (1-100) m-plane GaN on LiAlO 2
and m-SiC substrates. However, in the experiment, it was found that the growth
window of non-polar GaN is narrow, and the In is difficult to be incorporated in the
non-polar InGaN materials. In 2005, Sharma et al. [36] proposed semi-polar GaN
which can reduce the polarization electric field, and obtained the first semi-polar
green LED with crystal orientation along the (10-13) direction. Semi-polar GaN is
a non-polar surface with a growth plane between the c-axis and the non-polar plane,
which is oblique to the c-axis at an angle, including (11-22), (10-13), (1-101) and
(20-21), etc. Semi-polar materials can greatly reduce the polarization electric field.
In 2008, Gühne et al. [37] prepared (11-22) green LEDs on m-plane sapphire. As
the current increased, the EL peak wavelength did not move, which proved that the
effect of polarization effect was basically eliminated. Since the In doping in the InGaN
material grown along the semi-polar direction is easier, the growth window is also
wider than the non-polarity. Thus, the semi-polar InGaN material can be extended
to emit longer wavelengths. However, semi-polar GaN grown directly on m-plane,
r-plane sapphire, MgAl 2 O 4 , etc. by MOCVD technology has not yet achieved the
same results as traditional common growth process such as GaN because of the late
start, although it has certain advantages over non-polar GaN growth. There are still
a series of problems such as surface roughness, stacking faults and dislocations, etc.
[38, 39], which causes the subsequent growth of semi-polar green LED structures to
be greatly affected.
At present, the research on semi-polar LEDs in the world is studied mainly by
UCSB in US, Kyoto University of Japan, and the famous city university. Better
device results are obtained on the GaN substrate. Among them, the research team
led by “Father of Blue LED” Nakamura from UCSB has been the leader in the field
of non-polar and semi-polar GaN. They have effectively combined homoepitaxy
and semi-polar growth to reduce polarization effects on the basis of previous work.
In addition, a GaN substrate grown by HVPE (Mitsubishi Chemical) was used for
beveling to obtain semi-polar GaN materials with a variety of crystal orientations,
followed by homoepitaxial semi-polar LEDs. Breakthroughs have been made along