84
5 InGaN/GaN Multiple Quantum Wells Materials …
increases, resulting in a decrease in the electron and hole transition level. Then
E E− f ield (L,a well ) changes to negative, as a result of transition energy level caused
by the polarization field.
5.3.2 Effect on Luminous Intensity
The polarized electric field bends the energy band in the active region, and the
electrons and holes are spatially separated. This reduces the wave function overlap
of the carriers, and finally reduces the radiation recombination probability. Due to the
spatial separation of the carrier wave function caused by the polarization field, the
increase of the thickness of the well layer further promotes the decrease of the electron
and hole wave function overlap probability. The radiation combination intensity of
the InGaN quantum well will decrease. The electroluminescence intensity of the
InGaN/GaN multiple quantum wells LED epitaxial wafer varies with the width of
the well. It can be seen that as the well width increases, the peak wavelength has a
blue-shift and the electroluminescence intensity decreases.
5.4 Carrier Localization in InGaN/GaN Multiple Quantum
Wells
Due to lattice mismatch, InGaN/GaN multiple quantum wells generate large stresses
during growth, resulting in high screw dislocation density. Even so, the internal
quantum efficiency of InGaN LEDs can be very high (the internal quantum efficiency of blue LEDs exceeds 80%, and that of green LEDs can reach about 60%),
which is in contrast to carriers in InGaN/GaN multiple quantum wells that has a lot to
do with carriers localization in InGaN/GaN multiple quantum wells [22]. The ideal
band structure should be flat, but the presence of point defects and changes in growth
parameters [23] can cause potential fluctuations, which in turn make carriers easily
confined to the minimum value of potential energy. The phenomenon is called the
localization effect [24–26]. The reason for the formation of localization effects vary
widely, but it can be summarized into three types: quantum well width fluctuations,
alloy fluctuations [27], and In clustering (indium clustering). The effect of localization of carriers on the luminescence properties of InGaN/GaN multiple quantum
wells can be analyzed by temperature-dependent photoluminescence [28].
As shown in Fig. 5.3, when the temperature is 10 K, the carriers are randomly
distributed at the minimum point of potential energy. As the temperature increases
from 10 to 70 K, the carriers in the weakly localized states are activated by heat,
and then they are relaxed to strongly localized states by a Hopping mechanism.
The energy (E p ) corresponding to the luminescence peak appears red-shift, and the
carriers are released to the lower-energy tail state which are concentrated in the
5 InGaN/GaN Multiple Quantum Wells Materials …
increases, resulting in a decrease in the electron and hole transition level. Then
E E− f ield (L,a well ) changes to negative, as a result of transition energy level caused
by the polarization field.
5.3.2 Effect on Luminous Intensity
The polarized electric field bends the energy band in the active region, and the
electrons and holes are spatially separated. This reduces the wave function overlap
of the carriers, and finally reduces the radiation recombination probability. Due to the
spatial separation of the carrier wave function caused by the polarization field, the
increase of the thickness of the well layer further promotes the decrease of the electron
and hole wave function overlap probability. The radiation combination intensity of
the InGaN quantum well will decrease. The electroluminescence intensity of the
InGaN/GaN multiple quantum wells LED epitaxial wafer varies with the width of
the well. It can be seen that as the well width increases, the peak wavelength has a
blue-shift and the electroluminescence intensity decreases.
5.4 Carrier Localization in InGaN/GaN Multiple Quantum
Wells
Due to lattice mismatch, InGaN/GaN multiple quantum wells generate large stresses
during growth, resulting in high screw dislocation density. Even so, the internal
quantum efficiency of InGaN LEDs can be very high (the internal quantum efficiency of blue LEDs exceeds 80%, and that of green LEDs can reach about 60%),
which is in contrast to carriers in InGaN/GaN multiple quantum wells that has a lot to
do with carriers localization in InGaN/GaN multiple quantum wells [22]. The ideal
band structure should be flat, but the presence of point defects and changes in growth
parameters [23] can cause potential fluctuations, which in turn make carriers easily
confined to the minimum value of potential energy. The phenomenon is called the
localization effect [24–26]. The reason for the formation of localization effects vary
widely, but it can be summarized into three types: quantum well width fluctuations,
alloy fluctuations [27], and In clustering (indium clustering). The effect of localization of carriers on the luminescence properties of InGaN/GaN multiple quantum
wells can be analyzed by temperature-dependent photoluminescence [28].
As shown in Fig. 5.3, when the temperature is 10 K, the carriers are randomly
distributed at the minimum point of potential energy. As the temperature increases
from 10 to 70 K, the carriers in the weakly localized states are activated by heat,
and then they are relaxed to strongly localized states by a Hopping mechanism.
The energy (E p ) corresponding to the luminescence peak appears red-shift, and the
carriers are released to the lower-energy tail state which are concentrated in the