7.3 Light Extraction Efficiency Improvement Technology
131
improve the LED performance. A distributed Bragg reflection film (DBR) is a good
choice. Distributed Bragg reflectors with highly reflective nature and selectivity to
the incident spectrum are widely used in semiconductor optoelectronic devices [25].
Internal quantum efficiency of GaN-based LED has reached an optimal configuration
of 80% based on MOCVD epitaxial growth techniques and multiple quantum wells
structure [26]. However, due to insufficient reflection of the LED chip, the light
extraction efficiency of the device is reduced. This will affect the external quantum
efficiency of the device. By depositing DBR on the back side of the chip (sapphire
surface), the light reaching the back side of the chip is re-reflected. Such a scheme is
an important means to effectively improve the light extraction efficiency of the LED.
For the highly reflective DBR film used in the specific band spectral section, there
are mainly three film structures: a metal layer, a dielectric layer, or a metal plus dielectric layer. Each of the three reflectors has its own characteristics. (1) Common metal
reflective materials for metal-distributed Bragg reflection films include silver (Ag),
aluminum (Al), gold (Au), copper (Cu), etc. These materials have relatively high
reflectance, broad reflectance spectrum, and good polarization effect. The aluminum
film has a high reflectivity from the ultraviolet region to the infrared region. Furthermore, the aluminum film forms a thin layer of aluminum oxide on the surface in
the atmosphere to make the film more stable. The silver film has a high reflectivity
in the visible and infrared regions. The reflectivity is very high. The polarization
effect introduced in the operation is small. However, silver is easy to oxidize and
vulcanize. The properties of Ag are not very stable. The gold film has high reflectivity at 600 nm, and the reflectance is high in the infrared region. However, in actual
use, the metallic reflective film is rarely used alone due to the disadvantages such
as insufficient stability and hardness, and poor adhesion to the substrate. (2) The
metal-enhanced distributed Bragg reflection film: Due to the absorption of the metal
film, the actual reflectance of metal reflector cannot be maximized. Also, the metal
is soft and can be easily damaged. The adhesion to the substrate is poor as well.
Therefore, plating a pair of or pairs of dielectric layers with alternating high and low
refractive index in a pure metal film can reduce the absorption of metal and improve
the reflectivity. At the same time, it can serve as a protective layer. However, due to
the interference effect of the dielectric film, the high reflection band of the film layer
is thus narrowed. (3) All-media type distributed Bragg reflection film: The all-media
type distribution Bragg reflection film is a periodic structure with high reflectivity
which is formed by alternately forming a plurality of layers of high-low refractive
index materials. The beam has the same phase as it is reflected back from the interface of each layer λ 0 /4 thick back to the front surface, thus producing constructive
interference.
For the lateral structure LED, the higher the sapphire surface reflectance, the better
the light extraction efficiency of the device is.
After depositing a high refractive index (n 1 ) film layer with the optical thickness
of λ 0
4 on a substrate having a refractive index n s , the reflectance is increased.
For light with the center wavelength λ 0 , the admittance of the single layer film and
substrate combination is n
2
1
n s , the reflectance of normal incidence is:
131
improve the LED performance. A distributed Bragg reflection film (DBR) is a good
choice. Distributed Bragg reflectors with highly reflective nature and selectivity to
the incident spectrum are widely used in semiconductor optoelectronic devices [25].
Internal quantum efficiency of GaN-based LED has reached an optimal configuration
of 80% based on MOCVD epitaxial growth techniques and multiple quantum wells
structure [26]. However, due to insufficient reflection of the LED chip, the light
extraction efficiency of the device is reduced. This will affect the external quantum
efficiency of the device. By depositing DBR on the back side of the chip (sapphire
surface), the light reaching the back side of the chip is re-reflected. Such a scheme is
an important means to effectively improve the light extraction efficiency of the LED.
For the highly reflective DBR film used in the specific band spectral section, there
are mainly three film structures: a metal layer, a dielectric layer, or a metal plus dielectric layer. Each of the three reflectors has its own characteristics. (1) Common metal
reflective materials for metal-distributed Bragg reflection films include silver (Ag),
aluminum (Al), gold (Au), copper (Cu), etc. These materials have relatively high
reflectance, broad reflectance spectrum, and good polarization effect. The aluminum
film has a high reflectivity from the ultraviolet region to the infrared region. Furthermore, the aluminum film forms a thin layer of aluminum oxide on the surface in
the atmosphere to make the film more stable. The silver film has a high reflectivity
in the visible and infrared regions. The reflectivity is very high. The polarization
effect introduced in the operation is small. However, silver is easy to oxidize and
vulcanize. The properties of Ag are not very stable. The gold film has high reflectivity at 600 nm, and the reflectance is high in the infrared region. However, in actual
use, the metallic reflective film is rarely used alone due to the disadvantages such
as insufficient stability and hardness, and poor adhesion to the substrate. (2) The
metal-enhanced distributed Bragg reflection film: Due to the absorption of the metal
film, the actual reflectance of metal reflector cannot be maximized. Also, the metal
is soft and can be easily damaged. The adhesion to the substrate is poor as well.
Therefore, plating a pair of or pairs of dielectric layers with alternating high and low
refractive index in a pure metal film can reduce the absorption of metal and improve
the reflectivity. At the same time, it can serve as a protective layer. However, due to
the interference effect of the dielectric film, the high reflection band of the film layer
is thus narrowed. (3) All-media type distributed Bragg reflection film: The all-media
type distribution Bragg reflection film is a periodic structure with high reflectivity
which is formed by alternately forming a plurality of layers of high-low refractive
index materials. The beam has the same phase as it is reflected back from the interface of each layer λ 0 /4 thick back to the front surface, thus producing constructive
interference.
For the lateral structure LED, the higher the sapphire surface reflectance, the better
the light extraction efficiency of the device is.
After depositing a high refractive index (n 1 ) film layer with the optical thickness
of λ 0
4 on a substrate having a refractive index n s , the reflectance is increased.
For light with the center wavelength λ 0 , the admittance of the single layer film and
substrate combination is n
2
1
n s , the reflectance of normal incidence is: