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7 III-Nitride LED Quantum Efficiency Improvement Technology
weakens the electron energy and reduces its mobility in the vertical direction.
However, a very thick InGaN electron-emitting layer with a CART structure will
produce mismatch dislocations in the InGaN electron-emitting layer and lead to
the mismatch of InGaN and GaN in the MQW active region. These dislocations
increase non-radiative recombination and leakage current [15]. Thus, researchers
have proposed double step multi-quantum well LED, i.e., the use of a low In composition of GaN/InGaN MQW active layer as an electron-emitting region. The In composition of the InGaN is increased linearly along the direction of the active region. Such
a design can further improve luminous efficiency of the LEDs [16, 17].
7.2.3 Active Region Doping
In nitride LEDs, the hole injection rate and migration ability are an important issue
that limits the luminous efficiency of LEDs. By changing the structure of the quantum
barrier in the LED and directly doping in the GaN barrier can improve holes distribution and transport. For example, localization effect can be enhanced and the carrier
concentration will be increased by lightly Si doping in a multi-quantum well. It can
also enhance the lateral spread of current, while heavily doped Si can shield the
polarized electric field to some extent [18].
In addition, Mg doping is performed at the quantum barrier position of the multiple
quantum well. More holes can be generated after activation by annealing. Holes are
injected into the quantum well region to shield the polarization charge generated by
the lattice constant mismatch. Such a scheme can effectively suppress the influence
of the polarization electric field and enhance the light output power of the LED. By
comparing the Mg-doped and undoped samples, it is apparent from the photocurrent
test that the polarization field of the sample after Mg doping is significantly weakened
[19]. However, doping can efficiently change wavelength of LEDs [20].
7.2.4 Electronic Barrier Layer
As mentioned above, the electron trapping efficiency and hole injection efficiency of
the multiple quantum well active region are very low. This makes it easy for electrons
to overcome the limitation of the quantum well to achieve non-radiative recombination in the p-region with the hole, especially at high current density. Therefore,
reducing the electron leakage rate is an effective way to improve the luminous efficiency of LEDs. Figure 7.8 shows several carrier recombination processes in LED,
i.e. indirect (Shockley-Read-Hall, SRH) recombination, the spontaneous radiative
recombination and Auger recombination (Auger), and the leakage to the p-region
non-radiative recombination.
Component gradient electronic barrier: In order to reduce the overshoot of the
electron, electron blocking layer (EBL) is inserted between the LED multi-quantum
7 III-Nitride LED Quantum Efficiency Improvement Technology
weakens the electron energy and reduces its mobility in the vertical direction.
However, a very thick InGaN electron-emitting layer with a CART structure will
produce mismatch dislocations in the InGaN electron-emitting layer and lead to
the mismatch of InGaN and GaN in the MQW active region. These dislocations
increase non-radiative recombination and leakage current [15]. Thus, researchers
have proposed double step multi-quantum well LED, i.e., the use of a low In composition of GaN/InGaN MQW active layer as an electron-emitting region. The In composition of the InGaN is increased linearly along the direction of the active region. Such
a design can further improve luminous efficiency of the LEDs [16, 17].
7.2.3 Active Region Doping
In nitride LEDs, the hole injection rate and migration ability are an important issue
that limits the luminous efficiency of LEDs. By changing the structure of the quantum
barrier in the LED and directly doping in the GaN barrier can improve holes distribution and transport. For example, localization effect can be enhanced and the carrier
concentration will be increased by lightly Si doping in a multi-quantum well. It can
also enhance the lateral spread of current, while heavily doped Si can shield the
polarized electric field to some extent [18].
In addition, Mg doping is performed at the quantum barrier position of the multiple
quantum well. More holes can be generated after activation by annealing. Holes are
injected into the quantum well region to shield the polarization charge generated by
the lattice constant mismatch. Such a scheme can effectively suppress the influence
of the polarization electric field and enhance the light output power of the LED. By
comparing the Mg-doped and undoped samples, it is apparent from the photocurrent
test that the polarization field of the sample after Mg doping is significantly weakened
[19]. However, doping can efficiently change wavelength of LEDs [20].
7.2.4 Electronic Barrier Layer
As mentioned above, the electron trapping efficiency and hole injection efficiency of
the multiple quantum well active region are very low. This makes it easy for electrons
to overcome the limitation of the quantum well to achieve non-radiative recombination in the p-region with the hole, especially at high current density. Therefore,
reducing the electron leakage rate is an effective way to improve the luminous efficiency of LEDs. Figure 7.8 shows several carrier recombination processes in LED,
i.e. indirect (Shockley-Read-Hall, SRH) recombination, the spontaneous radiative
recombination and Auger recombination (Auger), and the leakage to the p-region
non-radiative recombination.
Component gradient electronic barrier: In order to reduce the overshoot of the
electron, electron blocking layer (EBL) is inserted between the LED multi-quantum