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12 Novel Nitride LED Technology
“Top-down” method:
In 2005, Tao-Hung Hsueh et al. of Taiwan Jiaotong University reported that the
InGaN/GaN MQW nanorods could be obtained by RIE-ICP etching, where contractive Ni sphere formed by rapid annealing at 850 °C under N 2 was used as a mask.
The PL results show that the peak position of the InGaN/GaN MQW nanorod is
446.8 nm, which is 5.1 nm lower than the quantum well of the bulk material. They
believe that the blue shift is the result of strain relief and quantum confinement effects
in MQW. At the same time, the light emission intensity of the nanorod is 5 times that
of the bulk material, which is ascribed to the decrease of the piezoelectric field, the
increase of the superposition of electron and hole wave functions, and the increase of
the radiation recombination rate. Seung Hwan Kim et al. also found that the nanorod
structure acted as photonic waveguide and scattering center, which can effectively
alleviate the effect of total reflection in the device. J. Bai et al. used the same method
to verify that the PL intensity of nanorod LEDs is increased by 8 times and the IQE
by 50% compared with conventional planar LEDs [7].
In 2011, Qiming Li et al. reported a two-step method to prepare nanorods using a
silica sphere as a mask on a planar LED. A tapered structure was firstly etched
by plasma, and then was selectively wet etched using KOH to mitigate surface
mechanical damage caused by dry etching. A regular “flashlight-like” dislocationfree nanorod LED was obtained as shown in Fig. 12.3. Liang-Yi Chen et al. of Taiwan
University also used a spin-coating method with silica nanospheres as a mask, and
then etched the nanorods by ICP-RIE. In order to reduce defects caused by mechanical damage and avoid short circuits, they deposited 100 nm thick SiO 2 layer at
surface of the nanorod by PECVD. This can effectively reduce the leakage [8].
However, using top-down methods also has some drawbacks. A significant portion
of the GaN material is etched away, which no longer contributes to the generation of
light and adds additional cost for the bottom-up approach during device fabrication.
In addition, the increased defects and leakage current are also problems that need to
be addressed.
“Bottom up” approach:
There are two main bottom-up methods. One is self-organized growth such as
catalyst-assisted self-organized growth and catalyst-free self-organized growth. The
other is selective area growth [5].
➀ The catalyst-assisted self-organized growth
Catalyst assisted GaN nanorods self-organized growth method is also referred to as
the VLS (Vapor-Liquid-Solid) synthesis since such a process uses gaseous reactant
where the catalyst is in a liquid state during the reaction and the final reaction product
is solid. The growth mechanism of catalytic reaction growth method based on the
gas-liquid-solid (VLS) [6], as shown in Fig. 12.4, was proposed by Wagner in the
study of a large number of single crystal whiskers grown in 1960. Since the diameter
of the nanowire is determined by the diameter of the particles of the catalyst, the
length of the nanowire can be controlled by the reaction time. This method gives
12 Novel Nitride LED Technology
“Top-down” method:
In 2005, Tao-Hung Hsueh et al. of Taiwan Jiaotong University reported that the
InGaN/GaN MQW nanorods could be obtained by RIE-ICP etching, where contractive Ni sphere formed by rapid annealing at 850 °C under N 2 was used as a mask.
The PL results show that the peak position of the InGaN/GaN MQW nanorod is
446.8 nm, which is 5.1 nm lower than the quantum well of the bulk material. They
believe that the blue shift is the result of strain relief and quantum confinement effects
in MQW. At the same time, the light emission intensity of the nanorod is 5 times that
of the bulk material, which is ascribed to the decrease of the piezoelectric field, the
increase of the superposition of electron and hole wave functions, and the increase of
the radiation recombination rate. Seung Hwan Kim et al. also found that the nanorod
structure acted as photonic waveguide and scattering center, which can effectively
alleviate the effect of total reflection in the device. J. Bai et al. used the same method
to verify that the PL intensity of nanorod LEDs is increased by 8 times and the IQE
by 50% compared with conventional planar LEDs [7].
In 2011, Qiming Li et al. reported a two-step method to prepare nanorods using a
silica sphere as a mask on a planar LED. A tapered structure was firstly etched
by plasma, and then was selectively wet etched using KOH to mitigate surface
mechanical damage caused by dry etching. A regular “flashlight-like” dislocationfree nanorod LED was obtained as shown in Fig. 12.3. Liang-Yi Chen et al. of Taiwan
University also used a spin-coating method with silica nanospheres as a mask, and
then etched the nanorods by ICP-RIE. In order to reduce defects caused by mechanical damage and avoid short circuits, they deposited 100 nm thick SiO 2 layer at
surface of the nanorod by PECVD. This can effectively reduce the leakage [8].
However, using top-down methods also has some drawbacks. A significant portion
of the GaN material is etched away, which no longer contributes to the generation of
light and adds additional cost for the bottom-up approach during device fabrication.
In addition, the increased defects and leakage current are also problems that need to
be addressed.
“Bottom up” approach:
There are two main bottom-up methods. One is self-organized growth such as
catalyst-assisted self-organized growth and catalyst-free self-organized growth. The
other is selective area growth [5].
➀ The catalyst-assisted self-organized growth
Catalyst assisted GaN nanorods self-organized growth method is also referred to as
the VLS (Vapor-Liquid-Solid) synthesis since such a process uses gaseous reactant
where the catalyst is in a liquid state during the reaction and the final reaction product
is solid. The growth mechanism of catalytic reaction growth method based on the
gas-liquid-solid (VLS) [6], as shown in Fig. 12.4, was proposed by Wagner in the
study of a large number of single crystal whiskers grown in 1960. Since the diameter
of the nanowire is determined by the diameter of the particles of the catalyst, the
length of the nanowire can be controlled by the reaction time. This method gives