86
5 InGaN/GaN Multiple Quantum Wells Materials …
E g (T ) = E g (0) −
αT
2
β + T
−
σ
2
kT
((5.26))
where k is the Boltzmann constant and σ is the amount that characterizes the degree
of localization.
This formula can be used to characterize the energy of the local state.
5.5 Green LED and Non-polar, Semi-polar LED
At present, the external quantum efficiency of red LEDs represented by AlGaInP
and blue LEDs represented by InGaN can reach more than 80%, indicating that the
internal quantum efficiency and light extraction efficiency of red and blue LEDs can
exceed 90% by using appropriate processing methods. However, the luminescence
efficiency of InGaN-based LEDs decreases sharply with increasing wavelength of
light. The internal quantum efficiency drops below 30% when the wavelength is
520 nm. In addition, the decreased efficiency of green LEDs is also more serious
than blue and violet LEDs. As the injection current increases, the external quantum
efficiency of green LEDs decreases faster than red and blue LEDs, resulting in low
output power of green LEDs under high current. This can affect the light mixing
efficiency of high power white light. Therefore, the low-efficiency green LED has
become the bottleneck of the current development of RGB three-primary LEDs.
At present, there are two main ways to improve the internal quantum efficiency of
InGaN based green LEDs. The first method is to continuously optimize the epitaxial
growth process and structural design of the InGaN quantum wells region to minimize the occurrence of defects in the active region. For example, growth interruption
method is used to improve the quantum well interface [30]; InGaN pre-strain layer
is inserted below the multiple quantum well layer to block dislocation propagation
and reduce the stress in the quantum well [31]; the sapphire is replaced with GaN
homoepitaxial substrate for heterogeneous growth [32] to reducing quantum well
penetration dislocation density and so on. The second method is to epitaxially grow
green LEDs in the non-polar and semi-polar directions from the crystal structure so
that the grown material does not spontaneously polarize and grow in the direction
perpendicular to the polar axis [0001]. The strain can avoid the piezoelectric polarization problem, thereby eliminating the influence of the polarized electric field and
increasing the probability of radiation recombination. At present, most researchers
focus on the second approach. Depolarization around non-polar and semi-polar GaN
has become a new research topic.
5 InGaN/GaN Multiple Quantum Wells Materials …
E g (T ) = E g (0) −
αT
2
β + T
−
σ
2
kT
((5.26))
where k is the Boltzmann constant and σ is the amount that characterizes the degree
of localization.
This formula can be used to characterize the energy of the local state.
5.5 Green LED and Non-polar, Semi-polar LED
At present, the external quantum efficiency of red LEDs represented by AlGaInP
and blue LEDs represented by InGaN can reach more than 80%, indicating that the
internal quantum efficiency and light extraction efficiency of red and blue LEDs can
exceed 90% by using appropriate processing methods. However, the luminescence
efficiency of InGaN-based LEDs decreases sharply with increasing wavelength of
light. The internal quantum efficiency drops below 30% when the wavelength is
520 nm. In addition, the decreased efficiency of green LEDs is also more serious
than blue and violet LEDs. As the injection current increases, the external quantum
efficiency of green LEDs decreases faster than red and blue LEDs, resulting in low
output power of green LEDs under high current. This can affect the light mixing
efficiency of high power white light. Therefore, the low-efficiency green LED has
become the bottleneck of the current development of RGB three-primary LEDs.
At present, there are two main ways to improve the internal quantum efficiency of
InGaN based green LEDs. The first method is to continuously optimize the epitaxial
growth process and structural design of the InGaN quantum wells region to minimize the occurrence of defects in the active region. For example, growth interruption
method is used to improve the quantum well interface [30]; InGaN pre-strain layer
is inserted below the multiple quantum well layer to block dislocation propagation
and reduce the stress in the quantum well [31]; the sapphire is replaced with GaN
homoepitaxial substrate for heterogeneous growth [32] to reducing quantum well
penetration dislocation density and so on. The second method is to epitaxially grow
green LEDs in the non-polar and semi-polar directions from the crystal structure so
that the grown material does not spontaneously polarize and grow in the direction
perpendicular to the polar axis [0001]. The strain can avoid the piezoelectric polarization problem, thereby eliminating the influence of the polarized electric field and
increasing the probability of radiation recombination. At present, most researchers
focus on the second approach. Depolarization around non-polar and semi-polar GaN
has become a new research topic.