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12 Novel Nitride LED Technology
12.1.1 Advantages of Nanorod LEDs
There are three main reasons for the bottleneck of lighting efficiency of LED with
traditional structures.
First, high-quality GaN bulk single crystal homogeneous substrates are difficult to
prepare. Current GaN-based LEDs are mainly heteroepitaxially grown on sapphire,
6H-SiC or Si substrates. However, large lattice mismatch and thermal mismatch in
heteroepitaxial growth will lead to high-density threading dislocation in the GaN
epitaxial layer, which will result in a large non-radiative recombination probability
of electrons and holes in the active region. This will reduce the internal quantum
efficiency [1].
Second, the excessive lattice mismatch (up to 11%) of InGaN/GaN multiple
quantum wells in GaN-based LEDs can also cause severe polarization effects in
InGaN. The stress-induced polarization field in the InGaN quantum well layer causes
the tilt of energy band. Then the wave function overlapping electrons and holes in
space is reduced, which is the so-called quantum confinement Stark effect (QCSE).
The probability of radiative recombination electron-hole pairs will be reduced, which
eventually lead to a decrease in the quantum efficiency of the LED [2].
Third, since the refractive index difference between the GaN material (refractive
index of 2.5) and air (refractive index of 1) is large, the light emitted from the
active region cannot be extracted from the interface due to too small critical angle of
total reflection (only 23.5°). This will result in low light extraction efficiency (only
4%). Light that cannot be extracted will repeatedly propagate through the dielectric
material until all of the light energy is dissipated into thermal energy, which also
adversely affects the performance of the device [3].
Researchers have done a lot of work in reducing dislocations, eliminating polarization and improving light extraction by using lateral epitaxy, homogenous growth,
etc. to suppress dislocations and to improve the internal quantum efficiency of LEDs.
People also use non-polar and semi-polar growth to reduce the polarization effect,
or use surface roughening, photonic crystals, graphic substrates, etc. to increase
light extraction efficiency. However, it is difficult to take care of all aspects of relevant issues, which further limit the development of high efficiency LEDs. In recent
years, nanorod LED has become the hot topic in the semiconductor lighting since
it shows many advantages. As shown in Figs. 12.1 and 12.2, it can effectively solve
the problem that the internal and external quantum efficiency of conventional planar
structure LEDs are not high [4, 5]. The specific advantages are listed as follows:
➀ The growth free energy of nanorod is relatively low. It is possible to achieve
near-defect-free growth, thereby greatly improving the crystal quality of GaN
materials;
➁ The special geometry and large specific surface area make the nanorod LED
a much larger illuminating area than the planar structure. Simultaneously, the
nano-scale scattering effect also has an anti-reflection effect, which will greatly
improve the light extraction efficiency;
12 Novel Nitride LED Technology
12.1.1 Advantages of Nanorod LEDs
There are three main reasons for the bottleneck of lighting efficiency of LED with
traditional structures.
First, high-quality GaN bulk single crystal homogeneous substrates are difficult to
prepare. Current GaN-based LEDs are mainly heteroepitaxially grown on sapphire,
6H-SiC or Si substrates. However, large lattice mismatch and thermal mismatch in
heteroepitaxial growth will lead to high-density threading dislocation in the GaN
epitaxial layer, which will result in a large non-radiative recombination probability
of electrons and holes in the active region. This will reduce the internal quantum
efficiency [1].
Second, the excessive lattice mismatch (up to 11%) of InGaN/GaN multiple
quantum wells in GaN-based LEDs can also cause severe polarization effects in
InGaN. The stress-induced polarization field in the InGaN quantum well layer causes
the tilt of energy band. Then the wave function overlapping electrons and holes in
space is reduced, which is the so-called quantum confinement Stark effect (QCSE).
The probability of radiative recombination electron-hole pairs will be reduced, which
eventually lead to a decrease in the quantum efficiency of the LED [2].
Third, since the refractive index difference between the GaN material (refractive
index of 2.5) and air (refractive index of 1) is large, the light emitted from the
active region cannot be extracted from the interface due to too small critical angle of
total reflection (only 23.5°). This will result in low light extraction efficiency (only
4%). Light that cannot be extracted will repeatedly propagate through the dielectric
material until all of the light energy is dissipated into thermal energy, which also
adversely affects the performance of the device [3].
Researchers have done a lot of work in reducing dislocations, eliminating polarization and improving light extraction by using lateral epitaxy, homogenous growth,
etc. to suppress dislocations and to improve the internal quantum efficiency of LEDs.
People also use non-polar and semi-polar growth to reduce the polarization effect,
or use surface roughening, photonic crystals, graphic substrates, etc. to increase
light extraction efficiency. However, it is difficult to take care of all aspects of relevant issues, which further limit the development of high efficiency LEDs. In recent
years, nanorod LED has become the hot topic in the semiconductor lighting since
it shows many advantages. As shown in Figs. 12.1 and 12.2, it can effectively solve
the problem that the internal and external quantum efficiency of conventional planar
structure LEDs are not high [4, 5]. The specific advantages are listed as follows:
➀ The growth free energy of nanorod is relatively low. It is possible to achieve
near-defect-free growth, thereby greatly improving the crystal quality of GaN
materials;
➁ The special geometry and large specific surface area make the nanorod LED
a much larger illuminating area than the planar structure. Simultaneously, the
nano-scale scattering effect also has an anti-reflection effect, which will greatly
improve the light extraction efficiency;