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12 Novel Nitride LED Technology
GaN-based LEDs, faces serious challenges. The reason is that the p-type region of
the GaN-based LED must ensure enough thickness to keep sufficient hole injection
and reduce leakage current caused by defects such as dislocations. The thickness is
generally above 200 nm. This makes it hard for the metal film or particles on the
surface of the LED chip to couple with the quantum well. Despite this, a variety of
methods have been tried to fabricate the SP enhanced GaN-based LEDs.
Yang et al. [31] reduced the distance between the surface metal and the active
region by growing a 70 nm p-type GaN in the green LED, and preparing a layer
of silver nanoparticles on the surface of p-type GaN using annealing. Although
the coupling distance has exceeded the penetration depth of Ag in gallium nitride
(47 nm), LSP still has a certain degree of coupling enhancement to the quantum
well’s luminescence. The external quantum efficiency is improved by about 120%.
In order to further shorten the distance between the metal particles and the active
region, embedding metal particles in the vicinity of the quantum well region in the
process of material growth is another option. The C.Y Cho et al. fabricated a 0.6 nm
layer of Ag in the growth of p-type GaN near the blue-light quantum well, and
then continued the growth of p-type GaN after annealing. This keeps the distance
between Ag particles and the active region within 30 nm, greatly reducing the distance
between the metal and active region. Eventually, the LED’s optical power at 20 mA
is increased by 38% [32]. However, the addition of a metal layer in the middle
of the growth process reduces the GaN’s material quality and degrades the optical
and electrical properties of the device. This also increases the fabrication cycle of
the LED epitaxy. In order to embed Ag particles into the GaN without degrading
the material quality, Chu-Young Cho et al. improved the above method by locally
preparing the Ag nanoparticles and then growing a layer of silica nanodisks on the
particles. This method can improve the thermal stability of Ag and the quality of
the material. Eventually, Chu-Young Cho further increased the optical output power
by 72% using this method [33]. Although this method can prepare metal particles
close to the active region, it has many defects such as interrupting the growth process
when depositing metal, reducing the quality of GaN, and increasing the unintentional
doping of Au and Ag in GaN.
To solve this problem, C. H. Lu proposed to prepare a two-dimensional nanopore
array structure in the p-GaN layer of conventional LED epitaxial wafer using a topdown method and deposit metal particles at the hole bottom in 2012 [34]. Then
in 2013, C. C. Yang et al. prepared an electro-injected LED device with a twodimensional nanopore array structure [35]. Although this coupling method does not
have to consider many complicated issues in the material growth process, it also
causes current leakage at the hole bottom and etching damage to the material. In
2014, Z. G. Yu et al. successfully solved the hole bottom’s leakage problem by
introducing a sidewall passivation layer. He further extended the nanopore into the
quantum well to increase the coupling intensity, achieving 5 times of luminescence
gain and 4.5 times of radiation lifetime reduction [36].
So far, the preparation of SP coupling enhanced LED is still very challenging.
However, the superior properties of surface plasmons still attract people’s attention
to design new device structures to achieve this goal.
12 Novel Nitride LED Technology
GaN-based LEDs, faces serious challenges. The reason is that the p-type region of
the GaN-based LED must ensure enough thickness to keep sufficient hole injection
and reduce leakage current caused by defects such as dislocations. The thickness is
generally above 200 nm. This makes it hard for the metal film or particles on the
surface of the LED chip to couple with the quantum well. Despite this, a variety of
methods have been tried to fabricate the SP enhanced GaN-based LEDs.
Yang et al. [31] reduced the distance between the surface metal and the active
region by growing a 70 nm p-type GaN in the green LED, and preparing a layer
of silver nanoparticles on the surface of p-type GaN using annealing. Although
the coupling distance has exceeded the penetration depth of Ag in gallium nitride
(47 nm), LSP still has a certain degree of coupling enhancement to the quantum
well’s luminescence. The external quantum efficiency is improved by about 120%.
In order to further shorten the distance between the metal particles and the active
region, embedding metal particles in the vicinity of the quantum well region in the
process of material growth is another option. The C.Y Cho et al. fabricated a 0.6 nm
layer of Ag in the growth of p-type GaN near the blue-light quantum well, and
then continued the growth of p-type GaN after annealing. This keeps the distance
between Ag particles and the active region within 30 nm, greatly reducing the distance
between the metal and active region. Eventually, the LED’s optical power at 20 mA
is increased by 38% [32]. However, the addition of a metal layer in the middle
of the growth process reduces the GaN’s material quality and degrades the optical
and electrical properties of the device. This also increases the fabrication cycle of
the LED epitaxy. In order to embed Ag particles into the GaN without degrading
the material quality, Chu-Young Cho et al. improved the above method by locally
preparing the Ag nanoparticles and then growing a layer of silica nanodisks on the
particles. This method can improve the thermal stability of Ag and the quality of
the material. Eventually, Chu-Young Cho further increased the optical output power
by 72% using this method [33]. Although this method can prepare metal particles
close to the active region, it has many defects such as interrupting the growth process
when depositing metal, reducing the quality of GaN, and increasing the unintentional
doping of Au and Ag in GaN.
To solve this problem, C. H. Lu proposed to prepare a two-dimensional nanopore
array structure in the p-GaN layer of conventional LED epitaxial wafer using a topdown method and deposit metal particles at the hole bottom in 2012 [34]. Then
in 2013, C. C. Yang et al. prepared an electro-injected LED device with a twodimensional nanopore array structure [35]. Although this coupling method does not
have to consider many complicated issues in the material growth process, it also
causes current leakage at the hole bottom and etching damage to the material. In
2014, Z. G. Yu et al. successfully solved the hole bottom’s leakage problem by
introducing a sidewall passivation layer. He further extended the nanopore into the
quantum well to increase the coupling intensity, achieving 5 times of luminescence
gain and 4.5 times of radiation lifetime reduction [36].
So far, the preparation of SP coupling enhanced LED is still very challenging.
However, the superior properties of surface plasmons still attract people’s attention
to design new device structures to achieve this goal.