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12 Novel Nitride LED Technology
12.2.3 Advantages and Research Status of Quantum Dot
Light-Emitting Diodes
The unique optical properties of quantum dots can be applied to light-emitting diodes
to improve the performance of light-emitting diodes.
Compared to conventional quantum well LED, LED with nitride quantum dot
based active region has many advantages. First, it can reduce the formation of dislocations. When the quantum dots are formed, the surface energy is enhanced and the
strain energy is reduced. The stress can be also released to reduce the formation of
dislocations. The reduction of dislocations not only reduces dislocation-related nonradiative recombination, but also improves LED efficiency. It can also reduce the
leakage of device, improve the antistatic capability of LED, reduce the light decay
of LED, and improve the reliability and stability of LED. Second, the polarization
effect can be attenuated. The polarization electric field caused by the polarization
charge can cause the spatial separation of electron and holes in the quantum well,
which reduces the radiation recombination rate. At the same time, the energy band
bending caused by the polarization electric field is not conducive to carrier transport, which increases the LED operating voltage, the electron leakage current, and
drop of efficiency at the large injected current (Droop). The reduction in strain in the
quantum dots helps to reduce the polarization effect and improve the LED performance. Third, the strong local effect of quantum dots on carriers is beneficial to
increase the overlap of wave functions of electron and hole, thereby enhancing the
radiation recombination rate. At the same time, the lateral limitation of quantum dots
also reduces the lateral diffusion of carriers and the recombination resulted from the
dislocations. In addition, wide spectral emission and spectral modulation are easily
achieved using quantum dots. Quantum dots exhibit discrete sub-levels similar to
those of a single atom. When a higher carrier density is injected, the carrier will
continue to fill the excited state due to the energy level filling effect after filling the
ground state. Thus, the multi-level luminescence can cause the broadening of the
luminescence spectrum, depending on the energy level (or density of states) distribution of the quantum dots and the filling level of the carriers. In addition, due to the
random position and size of the island at the beginning of the quantum dot formation
from the two-dimensional to three-dimensional growth mode, the size and composition of the self-organized quantum dots are generally not uniform, which also leads
to a certain uneven spread of luminescence spectrum. Due to the strong quantum
confinement effect, the sub-level distribution of the quantum dots can be adjusted in
a large range by adjusting the size of them, which further adjusts the luminescence
spectrum of the quantum dots. In summary, quantum dot LEDs have many advantages over quantum well LEDs, and have potentials in application of improving LED
luminescence performance and regulating LED spectroscopy.
The preparation, structure and optical properties of quantum dots and related
device research have received great attention in GaAs and InP material systems,
and have achieved fruitful research results. However, research on nitride quantum
dots based materials and devices are still in its infancy. At present, research in this
12 Novel Nitride LED Technology
12.2.3 Advantages and Research Status of Quantum Dot
Light-Emitting Diodes
The unique optical properties of quantum dots can be applied to light-emitting diodes
to improve the performance of light-emitting diodes.
Compared to conventional quantum well LED, LED with nitride quantum dot
based active region has many advantages. First, it can reduce the formation of dislocations. When the quantum dots are formed, the surface energy is enhanced and the
strain energy is reduced. The stress can be also released to reduce the formation of
dislocations. The reduction of dislocations not only reduces dislocation-related nonradiative recombination, but also improves LED efficiency. It can also reduce the
leakage of device, improve the antistatic capability of LED, reduce the light decay
of LED, and improve the reliability and stability of LED. Second, the polarization
effect can be attenuated. The polarization electric field caused by the polarization
charge can cause the spatial separation of electron and holes in the quantum well,
which reduces the radiation recombination rate. At the same time, the energy band
bending caused by the polarization electric field is not conducive to carrier transport, which increases the LED operating voltage, the electron leakage current, and
drop of efficiency at the large injected current (Droop). The reduction in strain in the
quantum dots helps to reduce the polarization effect and improve the LED performance. Third, the strong local effect of quantum dots on carriers is beneficial to
increase the overlap of wave functions of electron and hole, thereby enhancing the
radiation recombination rate. At the same time, the lateral limitation of quantum dots
also reduces the lateral diffusion of carriers and the recombination resulted from the
dislocations. In addition, wide spectral emission and spectral modulation are easily
achieved using quantum dots. Quantum dots exhibit discrete sub-levels similar to
those of a single atom. When a higher carrier density is injected, the carrier will
continue to fill the excited state due to the energy level filling effect after filling the
ground state. Thus, the multi-level luminescence can cause the broadening of the
luminescence spectrum, depending on the energy level (or density of states) distribution of the quantum dots and the filling level of the carriers. In addition, due to the
random position and size of the island at the beginning of the quantum dot formation
from the two-dimensional to three-dimensional growth mode, the size and composition of the self-organized quantum dots are generally not uniform, which also leads
to a certain uneven spread of luminescence spectrum. Due to the strong quantum
confinement effect, the sub-level distribution of the quantum dots can be adjusted in
a large range by adjusting the size of them, which further adjusts the luminescence
spectrum of the quantum dots. In summary, quantum dot LEDs have many advantages over quantum well LEDs, and have potentials in application of improving LED
luminescence performance and regulating LED spectroscopy.
The preparation, structure and optical properties of quantum dots and related
device research have received great attention in GaAs and InP material systems,
and have achieved fruitful research results. However, research on nitride quantum
dots based materials and devices are still in its infancy. At present, research in this