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7 III-Nitride LED Quantum Efficiency Improvement Technology
7.3.5 Photonic Crystal
A photonic crystal is a novel micro-structured material in which the dielectric
constant changes periodically with space. One of the characteristics of a photonic
crystal is that it has a photonic band gap (PBG) and does not allow electromagnetic wave propagation in the band gap. Distribution of the photonic crystal will
be influenced by dielectric permittivity difference between the lattice structure and
the filling factor. The presence of photonic band gaps allows photonic crystals to
suppress spontaneous emission well. Spontaneous radiation is suppressed when the
frequency of the spontaneously radiated light of the atom falls in the PBG. Another
feature of photonic crystals is the localization of the photons. It is closely connected
to the defect level in the photonic crystal. If the symmetry of the photonic crystal is
changed or impurities and defects are introduced therein, a corresponding defect state
is generated in the photonic band gap. Frequency coincides with the defect states of
photons will be restricted at the position of the defect. Once deviated from the defect
states the intensity of photons will decay rapidly. The characteristics of the localized
photon are mainly determined by the type of defects, including point defects, line
defects and surface defects. Line defects are similar to optical waveguides. Light can
only propagate along line defects. Plane defects function like a full reflection mirror,
capable of reflecting incident light from any direction, where the reflection rate can
reach 100%.
There are two main mechanisms for improving the luminous efficiency of LEDs
by using photonic crystals. (l) The periodic scattering of light on the surface of the
photonic crystal structure causes the light that should be totally internally reflected
to escape the LED, thereby improving the luminous efficiency of the LED. (2) The
photonic crystal structure forms a special energy band. By adjusting the parameters
of the photonic crystal, the LED emission wavelength may fall within the photonic
band gap range, thereby suppressing the light emitted from the side surface. The
LED light can be extracted within the propagation modes. Both effects have different
characteristics. The first effect overcomes the total internal reflection by means of
light scattering, similar to surface roughening techniques. The second effect is mainly
to extract the propagation mode originally confined within the material by means of
the photonic band gap. The second effect can also be applied to different illuminating
regions and light extraction regions by adjusting the structural parameters of the
photonic crystal. Figures 7.17 and 7.18 are p-GaN surface of the photonic crystal
and the n-GaN implanted photonic crystal.
7.4 Current Injection Efficiency Improvement Technology
The main methods to improve the efficiency of current injection efficiency are current
spreading technology, current blocking layer technology, and optimized electrode
contact.
7 III-Nitride LED Quantum Efficiency Improvement Technology
7.3.5 Photonic Crystal
A photonic crystal is a novel micro-structured material in which the dielectric
constant changes periodically with space. One of the characteristics of a photonic
crystal is that it has a photonic band gap (PBG) and does not allow electromagnetic wave propagation in the band gap. Distribution of the photonic crystal will
be influenced by dielectric permittivity difference between the lattice structure and
the filling factor. The presence of photonic band gaps allows photonic crystals to
suppress spontaneous emission well. Spontaneous radiation is suppressed when the
frequency of the spontaneously radiated light of the atom falls in the PBG. Another
feature of photonic crystals is the localization of the photons. It is closely connected
to the defect level in the photonic crystal. If the symmetry of the photonic crystal is
changed or impurities and defects are introduced therein, a corresponding defect state
is generated in the photonic band gap. Frequency coincides with the defect states of
photons will be restricted at the position of the defect. Once deviated from the defect
states the intensity of photons will decay rapidly. The characteristics of the localized
photon are mainly determined by the type of defects, including point defects, line
defects and surface defects. Line defects are similar to optical waveguides. Light can
only propagate along line defects. Plane defects function like a full reflection mirror,
capable of reflecting incident light from any direction, where the reflection rate can
reach 100%.
There are two main mechanisms for improving the luminous efficiency of LEDs
by using photonic crystals. (l) The periodic scattering of light on the surface of the
photonic crystal structure causes the light that should be totally internally reflected
to escape the LED, thereby improving the luminous efficiency of the LED. (2) The
photonic crystal structure forms a special energy band. By adjusting the parameters
of the photonic crystal, the LED emission wavelength may fall within the photonic
band gap range, thereby suppressing the light emitted from the side surface. The
LED light can be extracted within the propagation modes. Both effects have different
characteristics. The first effect overcomes the total internal reflection by means of
light scattering, similar to surface roughening techniques. The second effect is mainly
to extract the propagation mode originally confined within the material by means of
the photonic band gap. The second effect can also be applied to different illuminating
regions and light extraction regions by adjusting the structural parameters of the
photonic crystal. Figures 7.17 and 7.18 are p-GaN surface of the photonic crystal
and the n-GaN implanted photonic crystal.
7.4 Current Injection Efficiency Improvement Technology
The main methods to improve the efficiency of current injection efficiency are current
spreading technology, current blocking layer technology, and optimized electrode
contact.