1 Introduction
3
band structure of crystals. This phenomenon occurs in InGaN/GaN and AlGaN/GaN
quantum wells. Such a phenomenon is often called quantum confined Stark effect
[10]. The built-in electric field tilts the conduction band and valence band edges,
which separates the wave functions of electrons and holes in quantum wells. The
band bending reduces the spatial overlap of carrier wave functions, the radiation
recombination probability, and consequently the luminescence efficiency of LED.
In order to effectively reduce the influence of polarized electric field and quantum
confinement Stark effect on the luminescence efficiency of LED, it is necessary to
control and reduce the total strain in the crystal. Commonly used methods include
epitaxy on heterogeneous substrates with specific crystal surfaces (such as non-polar
or semi-polar surfaces), optimization of quantum well structure, fabrication of micronano graphical substrates and nano-pillar LED, etc.[9, 11]. In addition, in order to
inject holes into the quantum well region more effectively in p-type GaN, electronic barrier layer, strain compensation technology and gradient growth technology
are usually adopted. These methods can effectively improve the internal quantum
efficiency of LED [12].
There are three major technological routes for LED chips: front-loading, flipflop and vertical structure. Chip technology has an important impact on improving
the efficiency of electric injection and light extraction of LED devices. In order to
improve the injection efficiency of LED and obtain low working voltage devices, the
ohmic contact problem between electrode material and GaN material must be solved.
Ohmic contact fabrication of p-type GaN has always been a difficult problem because
the current growth technology cannot obtain p-type GaN with high hole concentration [13]. The work function of p-type GaN is large and there is no equivalent metal
material. In order to achieve low specific contact resistivity and high optical transmittance, transparent conductive oxide films, metal nanowires and graphene were
used as ohmic contact layers [14]. In addition, in order to further improve the injection efficiency of LED and solve the current aggregation effect, current diffusion
layer, current barrier layer and other technologies are usually used. Chip technology
can not only improve the injection efficiency, but also improve its light extraction
efficiency. Because the refractive index difference between GaN material (refractive
index 2.5) and air (refractive index 1) is large, the light emitted from the active region
cannot be emitted from the interface because the critical angle of total reflection is too
small (only 23.5°), resulting in low light extraction efficiency (only 4%). The light
that cannot be emitted will propagate repeatedly in the dielectric material until all the
light energy is dissipated into thermal energy. This can consequently affect the device
performance [15]. In view of this, a series of technologies has been adopted, such
as graphical substrates, surface roughening, mirror technology, photonic crystals,
nano-column structure and surface plasmon polaritons, etc.[16, 17].
LED packaging technology directly affects the reliability of devices. Due to the
uniqueness of the nitride LED, the reliability of LED devices is mainly affected by:
heteroepitaxy growth of substrates, materials and compound semiconductors; passivation of p-type doping and doping elements of GaN; metallization of electrodes
and ohmic contact; current, temperature and static electricity in device storage and
working environment. A large number of studies have shown that for LED light
3
band structure of crystals. This phenomenon occurs in InGaN/GaN and AlGaN/GaN
quantum wells. Such a phenomenon is often called quantum confined Stark effect
[10]. The built-in electric field tilts the conduction band and valence band edges,
which separates the wave functions of electrons and holes in quantum wells. The
band bending reduces the spatial overlap of carrier wave functions, the radiation
recombination probability, and consequently the luminescence efficiency of LED.
In order to effectively reduce the influence of polarized electric field and quantum
confinement Stark effect on the luminescence efficiency of LED, it is necessary to
control and reduce the total strain in the crystal. Commonly used methods include
epitaxy on heterogeneous substrates with specific crystal surfaces (such as non-polar
or semi-polar surfaces), optimization of quantum well structure, fabrication of micronano graphical substrates and nano-pillar LED, etc.[9, 11]. In addition, in order to
inject holes into the quantum well region more effectively in p-type GaN, electronic barrier layer, strain compensation technology and gradient growth technology
are usually adopted. These methods can effectively improve the internal quantum
efficiency of LED [12].
There are three major technological routes for LED chips: front-loading, flipflop and vertical structure. Chip technology has an important impact on improving
the efficiency of electric injection and light extraction of LED devices. In order to
improve the injection efficiency of LED and obtain low working voltage devices, the
ohmic contact problem between electrode material and GaN material must be solved.
Ohmic contact fabrication of p-type GaN has always been a difficult problem because
the current growth technology cannot obtain p-type GaN with high hole concentration [13]. The work function of p-type GaN is large and there is no equivalent metal
material. In order to achieve low specific contact resistivity and high optical transmittance, transparent conductive oxide films, metal nanowires and graphene were
used as ohmic contact layers [14]. In addition, in order to further improve the injection efficiency of LED and solve the current aggregation effect, current diffusion
layer, current barrier layer and other technologies are usually used. Chip technology
can not only improve the injection efficiency, but also improve its light extraction
efficiency. Because the refractive index difference between GaN material (refractive
index 2.5) and air (refractive index 1) is large, the light emitted from the active region
cannot be emitted from the interface because the critical angle of total reflection is too
small (only 23.5°), resulting in low light extraction efficiency (only 4%). The light
that cannot be emitted will propagate repeatedly in the dielectric material until all the
light energy is dissipated into thermal energy. This can consequently affect the device
performance [15]. In view of this, a series of technologies has been adopted, such
as graphical substrates, surface roughening, mirror technology, photonic crystals,
nano-column structure and surface plasmon polaritons, etc.[16, 17].
LED packaging technology directly affects the reliability of devices. Due to the
uniqueness of the nitride LED, the reliability of LED devices is mainly affected by:
heteroepitaxy growth of substrates, materials and compound semiconductors; passivation of p-type doping and doping elements of GaN; metallization of electrodes
and ohmic contact; current, temperature and static electricity in device storage and
working environment. A large number of studies have shown that for LED light